Prof. Dr. Jelena Klinovaja
ContactDepartment of PhysicsUniversity of Basel Klingelbergstrasse 82 4056 Basel, Switzerland

Short Biography
Jelena Klinovaja received her Bachelor and Master degree in Applied Mathematics and Physics from the Moscow Institute of Physics and Technology (State University), Department of General and Applied Physics, in 2007 and 2009, resp. Subsequently, she joined the group of Prof. Daniel Loss at the University of Basel, where she received her PhD in Theoretical Physics in 2012 with summa cum laude. In 2013, she was awarded a threeyear Harvard Fellowship to perform independent research in the area of the theoretical quantum condensed matter physics. Klinovaja was appointed as a tenure track assistant professor at the Department of Physics at the University of Basel in 2014. In her career, she was offered several prestigious fellowships and received research prizes such as the Swiss Physical Society Prize 2013 in Condensed Matter Physics, sponsored by IBM. In 2017, she has received the prestigious Starting Grant of the European Research Council (ERC).Research Summary
Our group is interested in many aspects of the quantum theory of condensed matter systems with a special focus on topological effects and spin phenomena. We explore the physics of topological insulators, carbonbased systems (graphene, bilayer graphene, and carbon nanotubes), atomic chains, semiconducting 2DEGs, and nanowires. In our work, we not only study the properties of existing structures but also combine wellknown ingredients such as nonuniform magnetic fields, superconductivity, and spinorbit interaction to 'engineer' systems with exotic quantum properties, in particular in the presence of strong electronelectron interactions treated by quantum field theoretic methods. Part of our work is related to the physics of exotic bound states such as fractional fermions, Majorana fermions, and parafermions, particles that possess nonAbelian braid statistics and have attracted considerable attention in recent years, also due to their potential use for topological quantum computing.Open Positions
We are looking for outstanding, highly motivated, and enthusiastic graduate students and/or postdoctoral fellows. PhD Candidates need to hold a Master's (or equivalent) degree in theoretical condensed matter physics. Postdoc Candidates should have a PhD in theoretical condensed matter physics. To apply please submit the following documents per email to Prof. Jelena Klinovaja: a curriculum vitae
 a list of publications
 your academic records (Bachelor's, Master's or PhD diploma)
 a short statement of your research interests and how they relate to the work of our group
 three recommendation letters sent directly to Prof. Jelena Klinovaja
Publications
Show all abstracts.
Proximity effect in a twodimensional electron gas coupled to a thin superconducting layer
Christopher Reeg, Daniel Loss, and Jelena Klinovaja.
Beilstein Journal of Nanotechnology 9, 1263 (2018)
There have recently been several experiments studying induced superconductivity in semiconducting twodimensional electron gases that are strongly coupled to thin superconducting layers, as well as probing possible topological phases supporting Majorana bound states in such setups. We show that a large band shift is induced in the semiconductor by the superconductor in this geometry, thus making it challenging to realize a topological phase. Additionally, we show that while increasing the thickness of the superconducting layer reduces the magnitude of the band shift, it also leads to a more significant renormalization of the semiconducting material parameters and does not reduce the challenge of tuning into a topological phase.

Conductance of fractional Luttinger liquids at finite temperatures
Pavel Aseev, Daniel Loss, and Jelena Klinovaja.
arXiv:1803.07359
We study the electrical conductance in singlemode quantum wires with Rashba spinorbit inter action subjected to externally applied magnetic fields in the regime in which the ratio of spinorbit momentum to the Fermi momentum is close to an odd integer, so that a combined effect of multi electron interaction and applied magnetic field leads to a partial gap in the spectrum. We study how this partial gap manifests itself in the temperature dependence of the fractional conductance of the quantum wire. We use two complementing techniques based on bosonization: refermionization of the model at a particular value of the interaction parameter and a semiclassical approach within a dilute soliton gas approximation of the functional integral. We show how the lowtemperature fractional conductance can be affected by the finite length of the wire, by the properties of the contacts, and by a shift of the chemical potential, which takes the system away from the resonance condition. We also predict an internal resistivity caused by a dissipative coupling between gapped and gapless modes.

Rashba Sandwiches with Topological Superconducting Phases
Yanick Volpez, Daniel Loss, and Jelena Klinovaja.
Phys. Rev. B 97, 195421 (2018)
We introduce a versatile heterostructure harboring various topological superconducting phases characterized by the presence of helical, chiral, or unidirectional edge states. Changing parameters, such as an effective Zeeman field or chemical potential, one can tune between these three topolog ical phases in the same setup. Our model relies only on conventional nontopological ingredients. The bilayer setup consists of an swave superconductor sandwiched between two twodimensional electron gas layers with strong Rashba spinorbit interaction. The interplay between two different pairing mechanisms, proximity induced direct and crossed Andreev superconducting pairings, gives rise to multiple topological phases. In particular, helical edge states occur if crossed Andreev su perconducting pairing is dominant. In addition, an inplane Zeeman field leads to a 2D gapless topological phase with unidirectional edge states, which were previously predicted to exist only in noncentrosymmetric superconductors. If the Zeeman field is tilted out of the plane, the system is in a topological phase hosting chiral edge states.

Boundary spin polarization as robust signature of topological phase transition in Majorana nanowires
Marcel Serina, Daniel Loss, and Jelena Klinovaja.
arXiv:1803.00544
We show that the boundary charge and spin can be used as alternative signatures of the topological phase transition in topological models such as semiconducting nanowires with strong Rashba spinorbit interaction in the presence of a magnetic field and in proximity to an swave superconductor. We identify signatures of the topological phase transition that do not rely on the presence of Majorana zeroenergy modes and, thus, can serve as independent probes of topological properties. The boundary spin component along the magnetic field, obtained by summing contributions from all states below the Fermi level, has a pronounced peak at the topological phase transition point. Generally, such signatures can be observed at boundaries between topological and trivial sections in nanowires and are stable against disorder.

Metallization of Rashba wire by superconducting layer in the strongproximity regime
Christopher Reeg, Daniel Loss, and Jelena Klinovaja.
Phys. Rev. B 97, 165425 (2018)
Semiconducting quantum wires defined within twodimensional electron gases and strongly coupled to thin superconducting layers have been extensively explored in recent experiments as promising platforms to host Majorana bound states. We study numerically such a geometry, consisting of a quasionedimensional wire coupled to a disordered threedimensional superconducting layer. We find that, in the strongcoupling limit of a sizable proximityinduced superconducting gap, all transverse subbands of the wire are significantly shifted in energy relative to the chemical potential of the wire. For the lowest subband, this band shift is comparable in magnitude to the spacing between quantized levels that arise due to the finite thickness of the superconductor (which typically is ~500 meV for a 10nmthick layer of Aluminum); in higher subbands, the band shift is much larger. Additionally, we show that the width of the system, which is usually much larger than the thickness, and moderate disorder within the superconductor have almost no impact on the induced gap or band shift. We provide a detailed discussion of the ramifications of our results, arguing that a huge band shift and significant renormalization of semiconducting material parameters in the strongcoupling limit make it challenging to realize a topological phase in such a setup, as the strong coupling to the superconductor essentially metallizes the semiconductor. This metallization of the semiconductor can be tested experimentally through the measurement of the band shift.

Effects of nuclear spins on the transport properties of the edge of twodimensional topological insulators
ChenHsuan Hsu, Peter Stano, Jelena Klinovaja, and Daniel Loss.
Phys. Rev. B 97, 125432 (2018)
The electrons in the edge channels of twodimensional topological insulators can be described as a helical TomonagaLuttinger liquid. They couple to nuclear spins embedded in the host materials through the hyperfine interaction, and are therefore subject to elastic spinflip backscattering on the nuclear spins. We investigate the nuclearspininduced edge resistance due to such backscattering by performing a renormalizationgroup analysis. Remarkably, the effect of this backscattering mechanism is stronger in a helical edge than in nonhelical channels, which are believed to be present in the trivial regime of InAs/GaSb quantum wells. In a system with sufficiently long edges, the disordered nuclear spins lead to an edge resistance which grows exponentially upon lowering the temperature. On the other hand, electrons from the edge states mediate an anisotropic RudermanKittelKasuyaYosida nuclear spinspin interaction, which induces a spiral nuclear spin order below the transition temperature. We discuss the features of the spiral order, as well as its experimental signatures. In the ordered phase, we identify two backscattering mechanisms, due to charge impurities and magnons. The backscattering on charge impurities is allowed by the internally generated magnetic field, and leads to an Andersontype localization of the edge states. The magnonmediated backscattering results in a powerlaw resistance, which is suppressed at zero temperature. Overall, we find that in a sufficiently long edge the nuclear spins, whether ordered or not, suppress the edge conductance to zero as the temperature approaches zero.

Majorana Kramers pairs in Rashba double nanowires with interactions and disorder
Manisha Thakurathi, Pascal Simon, Ipsita Mandal, Jelena Klinovaja, and Daniel Loss.
Phys. Rev. B 97, 045415 (2018)
We analyze the effects of electronelectron interactions and disorder on a Rashba doublenanowire setup coupled to an swave superconductor, which has been recently proposed as a versatile platform to generate Kramers pairs of Majorana bound states in the absence of magnetic fields. We identify the regime of parameters for which these Kramers pairs are stable against interaction and disorder effects. We use bosonization, perturbative renormalization group, and replica techniques to derive the flow equations for various parameters of the model and evaluate the corresponding phase diagram with topological and disorderdominated phases. We confirm aforementioned results by considering a more microscopic approach which starts from the tunneling Hamiltonian between the threedimensional swave superconductor and the nanowires. We find again that the interaction drives the system into the topological phase and, as the strength of the source term coming from the tunneling Hamiltonian increases, strong electronelectron interactions are required to reach the topological phase.

Topological Phase Detection in Rashba Nanowires with a Quantum Dot
Denis Chevallier, Pawel Szumniak, Silas Hoffman, Daniel Loss, and Jelena Klinovaja.
Phys. Rev. B 97, 045404 (2018)
We study theoretically the detection of the topological phase transition occurring in Rashba nanowires with proximityinduced superconductivity using a quantum dot. The bulk states lowest in energy of such a nanowire have a spin polarization parallel or antiparallel to the applied magnetic field in the topological or trivial phase, respectively. We show that this property can be probed by the quantum dot created at the end of the nanowire by external gates. By tuning one of the two spinsplit levels of the quantum dot to be in resonance with nanowire bulk states, one can detect the spin polarization of the lowest band via transport measurement. This allows one to determine the topological phase of the Rashba nanowire independently of the presence of Majorana bound states.

Suppression of the overlap between Majorana fermions by orbital magnetic effects in semiconductingsuperconducting nanowires
Olesia Dmytruk and Jelena Klinovaja.
Phys. Rev. B 97, 155409 (2018)
We study both analytically and numerically the role of orbital effects caused by a magnetic field applied along the axis of a semiconducting Rashba nanowire in the topological regime hosting Majorana fermions. We demonstrate that the orbital effects can be effectively taken into account in a onedimensional model by shifting the chemical potential, and, thus modifying the topological criterion. We focus on the energy splitting between two Majorana fermions in a finite nanowire and find a striking interplay between orbital and Zeeman effects on this splitting. In the limit of strong spinorbit interaction, we find regimes where the amplitude of the oscillating splitting stays constant or even decays with increasing magnetic field, in stark contrast to the commonly studied case where orbital effects of the magnetic field are neglected. The period of these oscillations is found to be almost constant in many parameter regimes.

DIII Topological Superconductivity with Emergent TimeReversal Symmetry
Christopher Reeg, Constantin Schrade, Jelena Klinovaja, and Daniel Loss.
Phys. Rev. B 96, 161407 (2017)
We find a new class of topological superconductors which possess an emergent timereversal symmetry that is present only after projecting to an effective lowdimensional model. We show that a topological phase in symmetry class DIII can be realized in a noninteracting system coupled to an swave superconductor only if the physical timereversal symmetry of the system is broken, and we provide three general criteria that must be satisfied in order to have such a phase. We also provide an explicit model which realizes the class DIII topological superconductor in 1D. We show that, just as in timereversal invariant topological superconductors, the topological phase is characterized by a Kramers pair of Majorana fermions that are protected by the emergent timereversal symmetry.

Finitesize effects in a nanowire strongly coupled to a thin superconducting shell
Christopher Reeg, Daniel Loss, and Jelena Klinovaja.
Phys. Rev. B 96, 125426 (2017)
We study the proximity effect in a onedimensional nanowire strongly coupled to a finite superconductor with a characteristic size which is much shorter than its coherence length. Such geometries have become increasingly relevant in recent years in the experimental search for Majorana fermions with the development of thin epitaxial Al shells which form a very strong contact with either InAs or InSb nanowires. So far, however, no theoretical treatment of the proximity effect in these systems has accounted for the finite size of the superconducting film. We show that the finitesize effects become very detrimental when the level spacing of the superconductor greatly exceeds its energy gap. Without any finetuning of the size of the superconductor (on the scale of the Fermi wavelength), the tunneling energy scale must be larger than the level spacing in order to reach the hard gap regime which is seen ubiquitously in the experiments. However, in this regime, the large tunneling energy scale induces a large shift in the effective chemical potential of the nanowire and pushes the topological phase transition to magnetic field strengths which exceed the critical field of Al.

Magnonic topological insulators in antiferromagnets
Kouki Nakata, Se Kwon Kim, Jelena Klinovaja, and Daniel Loss.
Phys. Rev. B 96, 224414 (2017)
Extending the notion of symmetry protected topological phases to insulating antiferromagnets (AFs) described in terms of opposite magnetic dipole moments associated with the magnetic Neel order, we establish a bosonic counterpart of topological insulators in semiconductors. Making use of the AharonovCasher effect, induced by electric field gradients, we propose a magnonic analog of the quantum spin Hall effect (magnonic QSHE) for edge states that carry helical magnons. We show that such up and down magnons form the same Landau levels and perform cyclotron motion with the same frequency but propagate in opposite direction. The insulating AF becomes characterized by a topological Z_2 number consisting of the Chern integer associated with each helical magnon edge state. Focusing on the topological Hall phase for magnons, we study bulk magnon effects such as magnonic spin, thermal, Nernst, and Ettinghausen effects, as well as the thermomagnetic properties of helical magnon transport both in topologically trivial and nontrivial bulk AFs and establish the magnonic WiedemannFranz law. We show that our predictions are within experimental reach with current device and measurement techniques.

ThreeDimensional Fractional Topological Insulators in Coupled Rashba Layers
Yanick Volpez, Daniel Loss, and Jelena Klinovaja.
Phys. Rev. B 96, 085422 (2017)
We propose a model of threedimensional topological insulators consisting of weakly coupled electron and holegas layers with Rashba spinorbit interaction stacked along a given axis. We show that in the presence of strong electronelectron interactions the system realizes a fractional strong topological insulator, where the rotational symmetry and condensation energy arguments still allow us to treat the problem as quasionedimensional with bosonization techniques. We also show that if Rashba and Dresselhaus spinorbit interaction terms are equally strong, by doping the system with magnetic impurities, one can bring it into the Weyl semimetal phase.

Lowfield Topological Threshold in Majorana Double Nanowires
Constantin Schrade, Manisha Thakurathi, Christopher Reeg, Silas Hoffman, Jelena Klinovaja, and Daniel Loss.
Phys. Rev. B 96, 035306 (2017)
A hard proximityinduced superconducting gap has recently been observed in semiconductor nanowire systems at low magnetic fields. However, in the topological regime at high magnetic fields a soft gap reemerges and represents a fundamental obstacle to topologically protected quantum information processing with Majorana bound states. Here we show that this obstacle can be overcome in a setup of double Rashba nanowires which are coupled to an swave superconductor and subjected to an external magnetic field along the wires. Specifically, we demonstrate that the required field strength for the topological threshold can be significantly reduced by the destructive interference of direct and crossedAndreev pairing in this setup; precisely down to the regime in which current experimental technology allows for a hard superconducting gap. We also show that the resulting Majorana bound states exhibit sufficiently short localization lengths which makes them ideal candidates for future braiding experiments.

Spindependent coupling between quantum dots and topological quantum wires
Silas Hoffman, Denis Chevallier, Daniel Loss, and Jelena Klinovaja.
Phys. Rev. B 96, 045440 (2017)
Considering Rashba quantum wires with a proximityinduced superconducting gap as physical realizations of Majorana fermions and quantum dots, we calculate the overlap of the Majorana wave functions with the local wave functions on the dot. We determine the spindependent tunneling amplitudes between these two localized states and show that we can tune into a fully spin polarized tunneling regime by changing the distance between dot and Majorana fermion. Upon directly applying this to the tunneling model Hamiltonian, we calculate the effective magnetic field on the quantum dot flanked by two Majorana fermions. The direction of the induced magnetic field on the dot depends on the occupation of the nonlocal fermion formed from the two Majorana end states which can be used as a readout for such a Majorana qubit.

Nuclear spininduced localization of the edge states in twodimensional topological insulators
ChenHsuan Hsu, Peter Stano, Jelena Klinovaja, and Daniel Loss.
Phys. Rev. B 96, 081405 (2017)
We investigate the influence of nuclear spins on the resistance of helical edge states of twodimensional topological insulators (2DTIs). Via the hyperfine interaction, nuclear spins allow electron backscattering, otherwise forbidden by time reversal symmetry. We identify two backscattering mechanisms, depending on whether the nuclear spins are ordered or not. Their temperature dependence is distinct but both give resistance, which increases with the edge length, decreasing temperature, and increasing strength of the electronelectron interaction. Overall, we find that the nuclear spins will typically shut down the conductance of the 2DTI edges at zero temperature.

Spin and Charge Signatures of Topological Superconductivity in Rashba Nanowires
Pawel Szumniak, Denis Chevallier, Daniel Loss, and Jelena Klinovaja.
Phys. Rev. B 96, 041401(R) (2017)
We consider a Rashba nanowire with proximity gap which can be brought into the topological phase by tuning external magnetic field or chemical potential. We study spin and charge of the bulk quasiparticle states when passing through the topological transition for open and closed systems. We show, analytically and numerically, that the spin of bulk states around the topological gap reverses its sign when crossing the transition due to band inversion, independent of the presence of Majorana fermions in the system. This spin reversal can be considered as a bulk signature of topological superconductivity that can be accessed experimentally. We find a similar behaviour for the charge of the bulk quasiparticle states, also exhibiting a sign reversal at the transition. We show that these signatures are robust against random static disorder.

Destructive interference of direct and crossed Andreev pairing in a system of two nanowires coupled via an swave superconductor
Christopher R. Reeg, Jelena Klinovaja, and Daniel Loss.
Phys. Rev. B 96, 081301(R) (2017)
We consider a system of two onedimensional nanowires coupled via an swave superconducting strip, a geometry that is capable of supporting Kramers pairs of Majorana fermions. By performing an exact analytical diagonalization of a tunneling Hamiltonian describing the proximity effect (via a Bogoliubov transformation), we show that the excitation gap of the system varies periodically on the scale of the Fermi wavelength in the limit where the interwire separation is shorter than the superconducting coherence length. Comparing with the excitation gaps in similar geometries containing only direct pairing, where one wire is decoupled from the superconductor, or only crossed Andreev pairing, where each nanowire is considered as a spinpolarized edge of a quantum Hall state, we find that the gap is always reduced, by orders of magnitude in certain cases, when both types of pairing are present. Our analytical results are further supported by numerical calculations on a tightbinding lattice. Finally, we show that treating the proximity effect by integrating out the superconductor cannot reproduce the results of our exact diagonalization.

Quantum dynamics of skyrmions in chiral magnets
Christina Psaroudaki, Silas Hoffman, Jelena Klinovaja, and Daniel Loss.
Phys. Rev. X 7, 041045 (2017)
We study the quantum propagation of a skyrmion in chiral magnetic insulators by generalizing the micromagnetic equations of motion to a finite temperature path integral formalism, using field theoretic tools. Promoting the center of the skyrmion to a dynamic quantity, the fluctuations around the skyrmionic configuration give rise to a timedependent damping of the skyrmion motion. From the frequency dependence of the damping kernel, we are able to identify the skyrmion mass, thus providing a microscopic description of the kinematic properties of skyrmions. When the free energy is translationally invariant we find the skyrmion mass is finite only at finite temperature. However, if defects are present or a magnetic trap is applied, the skyrmion mass acquires a finite value, even at vanishingly small temperature. We demonstrate that a skyrmion in a confined geometry provided by a magnetic trap behaves as a massive particle owing to its quasione dimensional confinement.

Finitetemperature conductance of strongly interacting quantum wire with a nuclear spin order
Pavel Aseev, Jelena Klinovaja, and Daniel Loss.
Phys. Rev. B 95, 125440 (2017)
We study the temperature dependence of the electrical conductance of a clean strongly interacting quantum wire in the presence of a helical nuclear spin order. The nuclear spin helix opens a temperaturedependent partial gap in the electron spectrum. Using a bosonization framework we describe the gapped electron modes by sineGordonlike kinks. We predict an internal resistivity caused by an Ohmiclike friction these kinks experience via interacting with gapless excitations. As a result, the conductance rises from G=e^2/h at temperatures below the critical temperature when nuclear spins are fully polarized to G=2e^2/h at higher temperatures when the order is destroyed, featuring a relatively wide plateau in the intermediate regime. The theoretical results are compared with the experimental data for GaAs quantum wires obtained recently by Scheller et al. [Phys. Rev. Lett. 112, 066801 (2014)].

Magnonic quantum Hall effect and WiedemannFranz law
Kouki Nakata, Jelena Klinovaja, and Daniel Loss.
Phys. Rev. B 95, 125429 (2017)
Casher effect, a magnon moving in an electric field acquires a geometric phase and forms Landau levels in an electric field gradient of sawtooth form. At low temperatures, the lowest energy band being almost flat carries a Chern number associated with a Berry curvature. Appropriately defining the thermal conductance for bosons, we find that the magnon Hall conductances get quantized and show a universal thermomagnetic behavior, i.e., are independent of materials, and obey a WiedemannFranz law for magnon transport. We consider magnons with quadratic and linear (Diraclike) dispersions. Finally, we show that our predictions are within experimental reach for ferromagnets and skyrmion lattices with current device and measurement techniques.

Floquet Majorana and ParaFermions in Driven Rashba Nanowires
Manisha Thakurathi, Daniel Loss, and Jelena Klinovaja.
Phys. Rev. B 95, 155407 (2017)
We study a periodically driven nanowire with Rashbalike conduction and valence bands in the presence of a magnetic field. We identify topological regimes in which the system hosts zeroenergy Majorana fermions. We further investigate the effect of strong electronelectron interactions that give rise to parafermion zero energy modes hosted at the nanowire ends. The first setup we consider allows for topological phases by applying only static magnetic fields without the need of superconductivity. The second setup involves both superconductivity and timedependent magnetic fields and allows one to generate topological phases without finetuning of the chemical potential. Promising candidate materials are graphene nanoribbons due to their intrinsic particlehole symmetry.

Tomography of Majorana Fermions with STM Tips
Denis Chevallier and Jelena Klinovaja.
Phys. Rev. B 94, 035417 (2016)
We investigate numerically the possibility to detect the spatial profile of Majorana fermions (MFs) modeling STM tips that are made of either normal or superconducting material. In both cases, we are able to resolve the localization length and the oscillation period of the MF wavefunction. We show that the tunneling between the substrate and the tip, necessary to get the information on the wave function oscillations, has to be smaller in the case of a superconducting STM. In the strong tunneling regime, the differential conductance saturates making it more difficult to observe the exponential decay of MFs. The temperature broadening of the profile is strongly suppressed in case of the superconducting lead resulting, generally, in better resolution.

Fractional boundary charges in quantum dot arrays with density modulation
JinHong Park, Guang Yang, Jelena Klinovaja, Peter Stano, and Daniel Loss.
Phys. Rev. B 94, 075416 (2016)
We show that fractional charges can be realized at the boundaries of a linear array of tunnel coupled quantum dots in the presence of a periodically modulated onsite potential. While the charge fractionalization mechanism is similar to the one in polyacetylene, here the values of fractional charges can be tuned to arbitrary values by varying the phase of the onsite potential or the total number of dots in the array. We also find that the fractional boundary charges, unlike the ingap bound states, are stable against static random disorder. We discuss the minimum array size where fractional boundary charges can be observed.

Universal Quantum Computation with Hybrid SpinMajorana Qubits
Silas Hoffman, Constantin Schrade, Jelena Klinovaja, and Daniel Loss.
Phys. Rev. B 94, 045316 (2016)
We theoretically propose a set of universal quantum gates acting on a hybrid qubit formed by coupling a quantum dot spin qubit and Majorana fermion qubit. First, we consider a quantum dot tunnelcoupled to two topological superconductors. The effective spinMajorana exchange facilitates a hybrid CNOT gate for which either qubit can be the control or target. The second setup is a modular scalable network of topological superconductors and quantum dots. As a result of the exchange interaction between adjacent spin qubits, a CNOT gate is implemented that acts on neighboring Majorana qubits, and eliminates the necessity of interqubit braiding. In both setups the spinMajorana exchange interaction allows for a phase gate, acting on either the spin or the Majorana qubit, and for a SWAP or hybrid SWAP gate which is sufficient for universal quantum computation without projective measurements.

Majorana bound states in magnetic skyrmions
Guang Yang, Peter Stano, Jelena Klinovaja, and Daniel Loss.
Phys. Rev. B 93, 224505 (2016)
Magnetic skyrmions are highly mobile nanoscale topological spin textures. We show, both analytically and numerically, that a magnetic skyrmion of an even azimuthal winding number placed in proximity to an swave superconductor hosts a zeroenergy Majorana bound state in its core, when the exchange coupling between the itinerant electrons and the skyrmion is strong. This Majorana bound state is stabilized by the presence of a spinorbit interaction. We propose the use of a superconducting trijunction to realize nonAbelian statistics of such Majorana bound states.

Topological Phases of Inhomogeneous Superconductivity
Silas Hoffman, Jelena Klinovaja, and Daniel Loss.
Phys. Rev. B 93, 165418 (2016)
We theoretically consider the effect of a spatially periodic modulation of the superconducting order parameter on the formation of Majorana fermions induced by a onedimensional system with magnetic impurities brought into close proximity to an swave superconductor. When the magnetic exchange energy is larger than the interimpurity electron hopping we model the effective system as a chain of coupled Shiba states. While in the opposite regime, the effective system is accurately described by a quantum wire model. Upon including a spatially modulated superconducting pairing, we find, for sufficiently large magnetic exchange energy, the system is able to support a single pair of Majorana fermions with one Majorana fermion on the left end of the system and one on the right end. When the modulation of superconductivity is large compared to the magnetic exchange energy, the Shiba chain returns to a trivially gapped regime while the quantum wire enters a new topological phase capable of supporting two pairs of Majorana fermions.

Chiral and NonChiral Edge States in Quantum Hall Systems with Charge Density Modulation
Pawel Szumniak, Jelena Klinovaja, and Daniel Loss.
Phys. Rev. B 93, 245308 (2016)
We consider a system of weakly coupled wires with quantum Hall effect (QHE) and in the presence of a spatially periodic modulation of the chemical potential along the wire, equivalent to a charge density wave (CDW). We investigate the competition between the two effects which both open a gap. We show that by changing the ratio between the amplitudes of the CDW modulation and the tunneling between wires, one can switch between nontopological CDWdominated phase to topological QHEdominated phase. Both phases host edge states of chiral and nonchiral nature robust to onsite disorder. However, only in the topological phase, the edge states are immune to disorder in the phase shifts of the CDWs. We provide analytical solutions for filling factor n=1 and study numerically effects of disorder as well as present numerical results for higher filling factors.

Theory of time reversal topological superconductivity in double Rashba wires  symmetries of Cooper pair and Andreev bound states
Hiromi Ebisu, Bo Lu, Jelena Klinovaja, and Yukio Tanaka.
Prog. Theor. Exp. Phys. 083I01 (2016)
We study the system of double Rashba wires brought into the proximity to an swave superconductor. The time reversal invariant topological superconductivity is realized if the interwire pairing corresponding to crossed Andreev reflection dominates over the standard intrawire pairing. We derive the topological criterion and show that the system hosts zero energy Andreev bound states such as a Kramers pair of Majorana fermions. We classify symmetry of the Cooper pairs focusing on the four degrees of freedom, i.e., frequency, spin, spatial parity inside wires, and spatial parity between wires. The magnitude of the oddfrequency pairing is strongly enhanced in the topological state. We also explore properties of junctions occurring in such double wire systems. If one section of the junction is in the topological state and the other is in the trivial state, the energy dispersion of Andreev bound states is proportional to \pm sin(\phi), where \phi denotes the macroscopic phase difference between two sections. This behavior can be intuitively explained by the couplings of a Kramers pair of Majorana fermions and spinsinglet swave Cooper pair and can also be understood by analyzing an effective continuum model of the s+p/swave superconductor hybrid system.

From Coupled Rashba Electron and Hole Gas Layers to 3D Topological Insulators
Luka Trifunovic, Daniel Loss, and Jelena Klinovaja.
Phys. Rev. B 93, 205406 (2016)
We introduce a system of stacked twodimensional electron and hole gas layers with Rashba spin orbit interaction and show that the tunnel coupling between the layers induces a strong threedimensional (3D) topological insulator phase. At each of the twodimensional bulk boundaries we find the spectrum consisting of a single anistropic Dirac cone, which we show by analytical and numerical calculations. Our setup has a unitcell consisting of four tunnel coupled Rashba layers and presents a synthetic strong 3D topological insulator and is distinguished by its rather high experimental feasibility.

Topological Floquet Phases in Driven Coupled Rashba Nanowires
Jelena Klinovaja, Peter Stano, and Daniel Loss.
Phys. Rev. Lett. 116, 176401 (2016)
We consider periodicallydriven arrays of weakly coupled wires with conduction and valence bands of Rashba type and study the resulting Floquet states. This nonequilibrium system can be tuned into nontrivial phases such as of topological insulators, Weyl semimetals, and dispersionless zeroenergy edge mode regimes. In the presence of strong electronelectron interactions, we generalize these regimes to the fractional case, where elementary excitations have fractional charges e/m with m being an odd integer.

LongDistance Entanglement of Spin Qubits via Quantum Hall Edge States
Guang Yang, ChenHsuan Hsu, Peter Stano, Jelena Klinovaja, and Daniel Loss.
Phys. Rev. B 93, 075301 (2016)
The implementation of a functional quantum computer involves entangling and coherent manipulation of a large number of qubits. For qubits based on electron spins confined in quantum dots, which are among the most investigated solidstate qubits at present, architectural challenges are often encountered in the design of quantum circuits attempting to assemble the qubits within the very limited space available. Here, we provide a solution to such challenges based on an approach to realizing entanglement of spin qubits over long distances. We show that longrange RudermanKittelKasuyaYosida interaction of confined electron spins can be established by quantum Hall edge states, leading to an exchange coupling of spin qubits. The coupling is anisotropic and can be either Isingtype or XYtype, depending on the spin polarization of the edge state. Such a property, combined with the dependence of the electron spin susceptibility on the chirality of the edge state, can be utilized to gain valuable insights into the topological nature of various quantum Hall states.

Antiferromagnetic nuclear spin helix and topological superconductivity in $^{13}$C nanotubes
ChenHsuan Hsu, Peter Stano, Jelena Klinovaja, and Daniel Loss.
Phys. Rev B 92, 235435 (2015)
We investigate the RKKY interaction arising from the hyperfine coupling between localized nuclear spins and conduction electrons in interacting 13C carbon nanotubes. Using the Luttinger liquid formalism, we show that the RKKY interaction is sublattice dependent, consistent with the spin susceptibility calculation in noninteracting carbon nanotubes, and it leads to an antiferromagnetic nuclear spin helix in finitesize systems. The transition temperature reaches up to tens of millikelvins, due to a strong boost by a positive feedback through the Overhauser field from ordered nuclear spins. Similar to GaAs nanowires, the formation of the helical nuclear spin order gaps out half of the conduction electrons, and is therefore observable as a reduction of conductance by a factor of two in a transport experiment. The nuclear spin helix leads to a density wave combining spin and charge degrees of freedom in the electron subsystem, resulting in synthetic spinorbit interaction, which induces nontrivial topological phases. As a result, topological superconductivity with Majorana fermion bound states can be realized in the system in the presence of proximityinduced superconductivity without the need of fine tuning the chemical potential. We present the phase diagram as function of system parameters, including the pairing gaps, the gap due to the nuclear spin helix, and the Zeeman field perpendicular to the helical plane.

Supercurrent Reversal in TwoDimensional Topological Insulators
Alexander Zyuzin, Mohammad Alidoust, Jelena Klinovaja, and Daniel Loss.
Phys. Rev. B 92, 174515 (2015)
We theoretically demonstrate that a supercurrent across a twodimensional topological insulator subjected to an external magnetic field unambiguously reveals the existence of edgemode superconductivity. When the edge states of a narrow twodimensional topological insulator are hybridized, an external magnetic field can close the hybridization gap, thus driving a quantum phase transition from insulator to semimetal states of the topological insulator. Importantly, we find a sign reversal of the supercurrent at the quantum phase transition which offers a simple and experimentally feasible way to observe intrinsic properties of topological insulators including edgemode superconductivity.

Proximityinduced Josephson $\pi$Junctions in Topological Insulators
Constantin Schrade, A. A. Zyuzin, Jelena Klinovaja, and Daniel Loss.
Phys. Rev. Lett. 115, 237001 (2015)
We study two microscopic models of topological insulators in contact with an swave superconductor. In the first model the superconductor and the topological insulator are tunnel coupled via a layer of scalar and of randomly oriented spin impurities. Here, we require that spinflip tunneling dominates over spinconserving one. In the second model the tunnel coupling is realized by an array of singlelevel quantum dots with randomly oriented spins. It is shown that the tunnel region forms a $\pi$junction where the effective order parameter changes sign. Interestingly, due to the random spin orientation the effective descriptions of both models exhibit timereversal symmetry. We then discuss how the proposed $\pi$junctions support topological superconductivity without magnetic fields and can be used to generate and manipulate Kramers pairs of Majorana fermions by gates.

Probing Atomic Structure and Majorana Wavefunctions in MonoAtomic Fechains on Superconducting PbSurface
Remy Pawlak, Marcin Kisiel, Jelena Klinovaja, Tobias Meier, Shigeki Kawai, Thilo Glatzel, Daniel Loss, and Ernst Meyer.
npj Quantum Information 2, 16035 (2016)
Motivated by the striking promise of quantum computation, Majorana bound states (MBSs) in solidstate systems have attracted wide attention in recent years. In particular, the wavefunction localization of MBSs is a key feature and crucial for their future implementation as qubits. Here, we investigate the spatial and electronic characteristics of topological superconducting chains of iron atoms on the surface of Pb(110) by combining scanning tunneling microscopy (STM) and atomic force microscopy (AFM). We demonstrate that the Fe chains are monoatomic, structured in a linear fashion, and exhibit zerobias conductance peaks at their ends which we interprete as signature for a Majorana bound state. Spatially resolved conductance maps of the atomic chains reveal that the MBSs are well localized at the chain ends (below 25 nm), with two localization lengths as predicted by theory. Our observation lends strong support to use MBSs in Fe chains as qubits for quantum computing devices.

Fractional Charge and Spin States in Topological Insulator Constrictions
Jelena Klinovaja and Daniel Loss.
Phys. Rev. B 92 121410(R) (2015)
We investigate theoretically properties of twodimensional topological insulator constrictions both in the integer and fractional regimes. In the presence of a perpedicular magnetic field, the constriction functions as a spin filter with nearperfect efficiency and can be switched by electric fields only. Domain walls between different topological phases can be created in the constriction as an interface between tunneling, magnetic fields, charge density wave, or electronelectron interactions dominated regions. These domain walls host nonAbelian bound states with fractional charge and spin and result in degenerate ground states with parafermions. If a proximity gap is induced bound states give rise to an exotic Josephson current with 8\piperidiodicity.

Impurity Induced Quantum Phase Transitions and Magnetic Order in Conventional Superconductors: Competition between Bound and Quasiparticle states
Silas Hoffman, Jelena Klinovaja, Tobias Meng, and Daniel Loss.
Phys. Rev. B 92, 125422 (2015)
We theoretically study bound states generated by magnetic impurities within conventional swave superconductors, both analytically and numerically. In determining the effect of the hybridization of two such bound states on the energy spectrum as a function of magnetic exchange coupling, relative angle of magnetization, and distance between impurities, we find that quantum phase transitions can be modulated by each of these parameters. Accompanying such transitions, there is a change in the preferred spin configuration of the impurities. Although the interaction between the impurity spins is overwhelmingly dominated by the quasiparticle contribution, the ground state of the system is determined by the bound state energies. Selfconsistently calculating the superconducting order parameter, we find a discontinuity when the system undergoes a quantum phase transition as indicated by the bound state energies.

Superconducting Gap Renomalization around two Magnetic Impurities: From Shiba to Andreev Bound States
Tobias Meng, Jelena Klinovaja, Silas Hoffman, Pascal Simon, and Daniel Loss.
Phys. Rev. B 92, 064503 (2015)
We study the renormalization of the gap of an swave superconductor in the presence of two magnetic impurities. For weakly bound Shiba states, we analytically calculate the part of the gap renormalization that is sensitive to the relative orientation of the two impurity spins. For strongly exchange coupled impurities, a quantum phase transition from a subgap Shiba state to a supragap Andreev state is identified and discussed by solving the gap equation selfconsistently by numerics.

Integer and Fractional Quantum Anomalous Hall Effect in a Strip of Stripes Model
Jelena Klinovaja, Yaroslav Tserkovnyak, and Daniel Loss.
Phys. Rev. B 91, 085426 (2015)
We study the quantum anomalous Hall effect in a strip of stripes model coupled to a magnetic texture with zero total magnetization and in the presence of strong electronelectron interactions. A helical magnetization along the stripes and a spinselective coupling between the stripes gives rise to a bulk gap and chiral edge modes. Depending on the ratio between the period of the magnetic structure and the Fermi wavelength, the system can exhibit the integer or fractional quantum anomalous Hall effect. In the fractional regime, the quasiparticles have fractional charges and nontrivial Abelian braid statistics.

Majorana Fermions in Ge/Si Hole Nanowires
Franziska Maier, Jelena Klinovaja, and Daniel Loss.
Phys. Rev. B 90, 195421 (2014)
We investigate theoretically the longdistance coupling and spin exchange in an array of quantum dot spin qubits in the presence of microwaves. We find that photon assisted cotunneling is boosted at resonances between photon and energies of virtually occupied excited states and show how to make it spin selective. We identify configurations that enable fast switching and spin echo sequences for efficient and nonlocal manipulation of spin qubits. We devise configurations in which the nearresonantly boosted cotunneling provides nonlocal coupling which, up to certain limit, does not diminish with distance between the manipulated dots before it decays weakly with inverse distance.

Fast LongDistance Control of Spin Qubits by Photon Assisted Cotunneling
Peter Stano, Jelena Klinovaja, Floris R. Braakman, Lieven M. K. Vandersypen, and Daniel Loss.
Phys. Rev. B 92, 075302 (2015)
We investigate theoretically the longdistance coupling and spin exchange in an array of quantum dot spin qubits in the presence of microwaves. We find that photon assisted cotunneling is boosted at resonances between photon and energies of virtually occupied excited states and show how to make it spin selective. We identify configurations that enable fast switching and spin echo sequences for efficient and nonlocal manipulation of spin qubits. We devise configurations in which the nearresonantly boosted cotunneling provides nonlocal coupling which, up to certain limit, does not diminish with distance between the manipulated dots before it decays weakly with inverse distance.

Fermionic and Majorana Bound States in Hybrid Nanowires with NonUniform SpinOrbit Interaction
Jelena Klinovaja and Daniel Loss.
Eur. Phys. J. B 88, 62 (2015)
We study intragap bound states in the topological phase of a Rashba nanowire in the presence of a magnetic field and with nonuniform spin orbit interaction (SOI) and proximityinduced superconductivity gap. We show that fermionic bound states (FBS) can emerge inside the proximity gap. They are localized at the junction between two wire sections characterized by different directions of the SOI vectors, and they coexist with Majorana bound states (MBS) localized at the nanowire ends. The energy of the FBS is determined by the angle between the SOI vectors and the lengthscale over which the SOI changes compared to the Fermi wavelength and the localization length. We also consider doublejunctions and show that the two emerging FBSs can hybridize and form a double quantum dotlike structure inside the gap. We find explicit analytical solutions of the bound states and their energies for certain parameter regimes such as weak and strong SOI. The analytical results are confirmed and complemented by an independent numerical tightbinding model approach. Such FBS can act as quasiparticle traps and thus can have implications for topological quantum computing schemes based on braiding MBSs.

Helical nuclear spin order in a strip of stripes in the Quantum Hall regime
Tobias Meng, Peter Stano, Jelena Klinovaja, and Daniel Loss.
Eur. Phys. J. B 87, 203 (2014)
We investigate nuclear spin effects in a twodimensional electron gas in the quantum Hall regime modeled by a weakly coupled array of interacting quantum wires. We show that the presence of hyperfine interaction between electron and nuclear spins in such wires can induce a phase transition, ordering electrons and nuclear spins into a helix in each wire. Electronelectron interaction effects, pronounced within the onedimensional stripes, boost the transition temperature up to tens to hundreds of millikelvins in GaAs. We predict specific experimental signatures of the existence of nuclear spin order, for instance for the resistivity of the system at transitions between different quantum Hall plateaus.

Quantum Spin Hall Effect in Strip of Stripes Model
Jelena Klinovaja and Yaroslav Tserkovnyak.
Phys. Rev. B 90, 115426 (2014)
We consider quantum spin Hall effect in an anisotropic strip of stripes and address both integer and fractional filling factors. The first model is based on a gradient of spinorbit interaction in the direction perpendicular to the stripes. The second model is based on two weakly coupled strips with reversed dispersion relations. We demonstrate that these systems host helical modes, modes in which opposite spins propagate in opposite directions. In the integer regime, the modes carry an elementary electron charge whereas in the fractional regime they carry fractional charges, and their excitations possess anyonic braiding statistics. These simple quasionedimensional models can serve as a platform for understanding effects arising due to electronelectron correlations in topological insulators.

Kramers Pairs of Majorana Fermions and Parafermions in Fractional Topological Insulators
Jelena Klinovaja, Amir Yacoby, and Daniel Loss.
Phys. Rev. B 90, 155447 (2014)
We propose a scheme based on topological insulators to generate Kramers pairs of Majorana fermions or parafermions in the complete absence of magnetic fields. Our setup consists of two topological insulators whose edge states are brought close to an swave superconductor. The resulting proximity effect leads to an interplay between a nonlocal crossed Andreev pairing, which is dominant in the strong electronelectron interaction regime, and usual superconducting pairing, which is dominant at large separation between the two topological insulator edges. As a result, there are zeroenergy bound states localized at interfaces between spatial regions dominated by the two different types of pairing. Due to the preserved timereversal symmetry, the bound states come in Kramers pairs. If the topological insulators carry fractional edge states, the zeroenergy bound states are parafermions, otherwise, they are Majorana fermions.

Renormalization of anticrossings in interacting quantum wires with Rashba and Dresselhaus spinorbit couplings
Tobias Meng, Jelena Klinovaja, and Daniel Loss.
Phys. Rev. B 89, 205133 (2014)
We discuss how electronelectron interactions renormalize the spinorbit induced anticrossings between different subbands in ballistic quantum wires. Depending on the ratio of spinorbit coupling and subband spacing, electronelectron interactions can either increase or decrease anticrossing gaps. When the anticrossings are closing due to a special combination of Rashba and Dresselhaus spinorbit couplings, their gap approaches zero as an interaction dependent power law of the spinorbit couplings, which is a consequence of Luttinger liquid physics. Monitoring the closing of the anticrossings allows to directly measure the related renormalization group scaling dimension in an experiment. If a magnetic field is applied parallel to the spinorbit coupling direction, the anticrossings experience different renormalizations. Since this difference is entirely rooted in electronelectron interactions, unequally large anticrossings also serve as a direct signature of Luttinger liquid physics. Electronelectron interactions furthermore increase the sensitivity of conductance measurements to the presence of anticrossing.

TimeReversal Invariant Parafermions in Interacting Rashba Nanowires
Jelena Klinovaja and Daniel Loss.
Phys. Rev. B 90, 045118 (2014)
We propose a scheme to generate pairs of timereversal invariant parafermions. Our setup consists of two quantum wires with opposite Rashba spin orbit interactions coupled to an swave superconductor, in the presence of electronelectron interactions. The zeroenergy bound states localized at the wire ends arise from the interplay between two types of proximity induced superconductivity: the usual intrawire superconductivity and the interwire superconductivity due to crossed Andreev reflections. If the latter dominates, which is the case for strong electronelectron interactions, the system supports Kramers pair of parafermions. Moreover, the scheme can be extended to a twodimensional sea of timereversal invariant parafermions.

Parafermions in Interacting Nanowire Bundle
Jelena Klinovaja and Daniel Loss.
Phys. Rev. Lett. 112, 246403 (2014)
We propose a scheme to induce Z_3 parafermion modes, exotic zeroenergy bound states that possess nonAbelian statistics. We consider a minimal setup consisting of a bundle of four tunnel coupled nanowires hosting spinless electrons that interact strongly with each other. The hallmark of our setup is that it relies only on simple onedimensional wires, uniform magnetic fields, and strong interactions, but does not require the presence of superconductivity or exotic quantum Hall phases.

Transport signature of fractional Fermions in Rashba nanowires
Diego Rainis, Arijit Saha, Jelena Klinovaja, Luka Trifunovic, and Daniel Loss.
Phys. Rev. Lett. 112, 196803 (2014)
We study theoretically transport through a semiconducting nanowire (NW) in the presence of Rashba spin orbit interaction, uniform magnetic field, and spatially modulated magnetic field. The system is fully gapped, and the interplay between the spin orbit interaction and the magnetic fields leads to fractionally charged fermion (FF) bound states of JackiwRebbi type at each end of the nanowire. We investigate the transport and noise behavior of a N/NW/N system, where the wire is contacted by two normal leads (N), and we look for possible signatures that could help in the experimental detection of such states. We find that the differential conductance and the shot noise exhibit a subgap structure which fully reveals the presence of the FF state. Our predictions can be tested in standard twoterminal measurements through InSb/InAs nanowires.

Topological Superconductivity and Majorana Fermions in RKKY Systems
Jelena Klinovaja, Peter Stano, Ali Yazdani, and Daniel Loss.
Phys. Rev. Lett. 111, 186805 (2013)
We consider quasi onedimensional RKKY systems in proximity to an swave superconductor. We show that a $2k_F$peak in the spin susceptibility of the superconductor in the onedimensional limit supports helical order of localized magnetic moments via RKKY interaction, where $k_F$ is the Fermi wavevector. The magnetic helix is equivalent to a uniform magnetic field and very strong spinorbit interaction (SOI) with an effective SOI length $1/2k_F$. We find the conditions to establish such a magnetic state in atomic chains and semiconducting nanowires with magnetic atoms or nuclear spins. Generically, these systems are in a topological phase with Majorana fermions. The inherent selftuning of the helix to $2k_F$ eliminates the need to tune the chemical potential.

Correlations between Majorana fermions through a superconductor
A.A. Zyuzin, Diego Rainis, Jelena Klinovaja, and Daniel Loss.
Phys. Rev. Lett. 111, 056802 (2013)
We consider a model of ballistic quasione dimensional semiconducting wire with intrinsic spinorbit interaction placed on the surface of a bulk swave superconductor (SC), in the presence of an external magnetic field. This setup has been shown to give rise to a topological superconducting state in the wire, characterized by a pair of Majoranafermion (MF) bound states formed at the two ends of the wire. Here we demonstrate that, besides the wellknown direct overlapinduced energy splitting, the two MF bound states may hybridize via elastic correlated tunneling processes through virtual quasiparticles states in the SC, giving rise to an additional energy splitting between MF states from the same as well as from different wires.

Integer and Fractional Quantum Hall Effect in a Strip of Stripes
Jelena Klinovaja and Daniel Loss.
Eur. Phys. J. B 87, 171 (2014)
We study anisotropic stripe models of interacting electrons in the presence of magnetic fields in the quantum Hall regime with integer and fractional filling factors. The model consists of an infinite strip of finite width that contains periodically arranged stripes (forming supercells) to which the electrons are confined and between which they can hop with associated magnetic phases. The interacting electron system within the onedimensional stripes are described by Luttinger liquids and shown to give rise to charge and spin density waves that lead to periodic structures within the stripe with a reciprocal wavevector 8k_F. This wavevector gives rise to Umklapp scattering and resonant scattering that results in gaps and chiral edge states at all known integer and fractional filling factors \nu. The integer and odd denominator filling factors arise for a uniform distribution of stripes, whereas the even denominator filling factors arise for a nonuniform stripe distribution. We calculate the Hall conductance via the Streda formula and show that it is given by \sigma_H=\nu e^2/h for all filling factors. We show that the composite fermion picture follows directly from the condition of the resonant Umklapp scattering.

Spintronics in MoS_2 monolayer quantum wires
Jelena Klinovaja and Daniel Loss.
Phys. Rev. B 88, 075404 (2013)
We study analytically and numerically spin effects in MoS_2 monolayer armchair quantum wires and quantum dots. The interplay between intrinsic and Rashba spin orbit interactions induced by an electric field leads to helical modes, giving rise to spin filtering in timereversal invariant systems. The Rashba spin orbit interaction can also be generated by spatially varying magnetic fields. In this case, the system can be in a helical regime with nearly perfect spin polarization. If such a quantum wire is brought into proximity to an swave superconductor, the system can be tuned into a topological phase, resulting in midgap Majorana fermions localized at the wire ends.

Local Spin Susceptibilities of LowDimensional Electron Systems
Peter Stano, Jelena Klinovaja, Amir Yacoby, and Daniel Loss.
Phys. Rev. B 88, 045441 (2013)
We investigate, assess, and suggest possibilities for a measurement of the local spin susceptibility of a conducting lowdimensional electron system. The basic setup of the experiment we envisage is a sourceprobe one. Locally induced spin density (e.g. by a magnetized atomic force microscope tip) extends in the medium according to its spin susceptibility. The induced magnetization can be detected as a dipolar magnetic field, for instance, by an ultrasensitive nitrogenvacancy center based detector, from which the spatial structure of the spin susceptibility can be deduced. We find that onedimensional systems, such as semiconducting nanowires or carbon nanotubes, are expected to yield a measurable signal. The signal in a twodimensional electron gas is weaker, though materials with high enough $g$factor (such as InGaAs) seem promising for successful measurements.

Topological Edge States and Fractional Quantum Hall Effect from Umklapp Scattering
Jelena Klinovaja and Daniel Loss.
Phys. Rev. Lett. 111, 196401 (2013)
We study anisotropic lattice strips in the presence of a magnetic field in the quantum Hall effect regime. At specific magnetic fields, causing resonant Umklapp scattering, the system is gapped in the bulk and supports chiral edge states in close analogy to topological insulators. These gaps result in plateaus for the Hall conductivity exactly at the known fillings n/m (both positive integers and m odd) for the integer and fractional quantum Hall effect. For double strips we find topological phase transitions with phases that support midgap edge states with flat dispersion. The topological effects predicted here could be tested directly in optical lattices.

Fractional Fermions with NonAbelian Statistics
Jelena Klinovaja and Daniel Loss.
Phys. Rev. Lett. 110, 126402 (2013)
We introduce a novel class of lowdimensional topological tightbinding models that allow for bound states that are fractionally charged fermions and exhibit nonAbelian braiding statistics. The proposed model consists of a double (single) ladder of spinless (spinful) fermions in the presence of magnetic fields. We study the system analytically in the continuum limit as well as numerically in the tightbinding representation. We find a topological phase transition with a topological gap that closes and reopens as a function of system parameters and chemical potential. The topological phase is of the type BDI and carries two degenerate midgap bound states that are localized at opposite ends of the ladders. We show numerically that these bound states are robust against a wide class of perturbations.

RKKY interaction in carbon nanotubes and graphene nanoribbons
Jelena Klinovaja and Daniel Loss.
Phys. Rev. B 87, 045422 (2013)
We study RudermannKittelKasuyaYosida (RKKY) interaction in carbon nanotubes (CNTs) and graphene nanoribbons in the presence of spin orbit interactions and magnetic fields. For this we evaluate the static spin susceptibility tensor in real space in various regimes at zero temperature. In metallic CNTs the RKKY interaction depends strongly on the sublattice and, at the Dirac point, is purely ferromagnetic (antiferromagnetic) for the localized spins on the same (different) sublattice, whereas in semiconducting CNTs the spin susceptibility depends only weakly on the sublattice and is dominantly ferromagnetic. The spin orbit interactions break the SU(2) spin symmetry of the system, leading to an anisotropic RKKY interaction of Ising and MoryiaDzyaloshinsky form, besides the usual isotropic Heisenberg interaction. All these RKKY terms can be made of comparable magnitude by tuning the Fermi level close to the gap induced by the spin orbit interaction. We further calculate the spin susceptibility also at finite frequencies and thereby obtain the spin noise in real space via the fluctuationdissipation theorem.

Giant spin orbit interaction due to rotating magnetic fields in graphene nanoribbons
Jelena Klinovaja and Daniel Loss.
Phys. Rev. X 3, 011008 (2013)
We theoretically study graphene nanoribbons in the presence of spatially varying magnetic fields produced e.g. by nanomagnets. We show both analytically and numerically that an exceptionally large Rashba spin orbit interaction (SOI) of the order of 10 meV can be produced by the nonuniform magnetic field. As a consequence, helical modes exist in armchair nanoribbons that exhibit nearly perfect spin polarization and are robust against boundary defects. This paves the way to realizing spin filter devices in graphene nanoribbons in the temperature regime of a few Kelvins. If a nanoribbon in the helical regime is in proximity contact to an swave superconductor, the nanoribbon can be tuned into a topological phase sustaining Majorana fermions.

Helical States in Curved Bilayer Graphene
Jelena Klinovaja, Gerson J. Ferreira, and Daniel Loss.
Phys. Rev. B 86, 235416 (2012)
We study spin effects of quantum wires formed in bilayer graphene by electrostatic confinement. With a proper choice of the confinement direction, we show that in the presence of magnetic field, spinorbit interaction induced by curvature, and intervalley scattering, bound states emerge that are helical. The localization length of these helical states can be modulated by the gate voltage which enables the control of the tunnel coupling between two parallel wires. Allowing for proximity effect via an swave superconductor, we show that the helical modes give rise to Majorana fermions in bilayer graphene.

Transition from fractional to Majorana fermions in Rashba nanowires
Jelena Klinovaja, Peter Stano, and Daniel Loss.
Phys. Rev. Lett. 109, 236801 (2012)
We study hybrid superconductingsemiconducting nanowires in the presence of Rashba spinorbit interaction as well as helical magnetic fields. We show that the interplay between them leads to a competition of phases with two topological gaps closing and reopening, resulting in unexpected reentrance behavior. Besides the topological phase with localized Majorana fermions (MFs) we find new phases characterized by fractionally charged fermion (FF) bound states of JackiwRebbi type. The system can be fully gapped by the magnetic fields alone, giving rise to FFs that transmute into MFs upon turning on superconductivity. We find explicit analytical solutions for MF and FF bound states and determine the phase diagram numerically by determining the corresponding Wronskian null space. We show by renormalization group arguments that electronelectron interactions enhance the Zeeman gaps opened by the fields.

Towards a realistic transport modeling for a superconducting nanowire with Majorana fermions
Diego Rainis, Luka Trifunovic, Jelena Klinovaja, and Daniel Loss.
Phys. Rev. B 87, 024515 (2013)
Motivated by recent experiments searching for Majorana fermions (MFs) in hybrid semiconductingsuperconducting nanostructures and by subsequent theoretical interpretations, we consider the so far most realistic model (including disorder) and analyze its transport behavior numerically. In particular, we include in the model superconducting contacts used in the experiments to extract the current. We show that important new features emerge that are absent in simpler models, such as the enhanced visibility of the topological gap for increased spinorbit interaction. We find oscillations of the zero bias peak as function of magnetic field and explain their origin. Even taking into account all the possible (known) ingredients of the experiments and exploring many parameter regimes for MFs, we are not able to reach a satisfactory agreement with the reported data. Thus, a different physical origin for the observed zerobias peak cannot be excluded.

Exchangebased CNOT gates for singlettriplet qubits with spin orbit interaction
Jelena Klinovaja, Dimitrije Stepanenko, Bertrand I. Halperin, and Daniel Loss.
Phys. Rev. B 86, 085423 (2012)
We propose a scheme for implementing the CNOT gate over qubits encoded in a pair of electron spins in a double quantum dot. The scheme is based on exchange and spin orbit interactions and on local gradients in Zeeman fields. We find that the optimal device geometry for this implementation involves effective magnetic fields that are parallel to the symmetry axis of the spin orbit interaction. We show that the switching times for the CNOT gate can be as fast as a few nanoseconds for realistic parameter values in GaAs semiconductors. Guided by recent advances in surface codes, we also consider the perpendicular geometry. In this case, leakage errors due to spin orbit interaction occur but can be suppressed in strong magnetic fields.

Composite Majorana Fermion Wavefunctions in Nanowires
Jelena Klinovaja and Daniel Loss.
Phys. Rev. B 86, 085408 (2012)
We consider Majorana fermions (MFs) in quasionedimensional nanowire systems containing normal and superconducting sections where the topological phase based on Rashba spin orbit interaction can be tuned by magnetic fields. We derive explicit analytic solutions of the MF wavefunction in the weak and strong spin orbit interaction regimes. We find that the wavefunction for one single MF is a composite object formed by superpositions of different MF wavefunctions which have nearly disjoint supports in momentum space. These contributions are coming from the extrema of the spectrum, one centered around zero momentum and the other around the two Fermi points. As a result, the various MF wavefunctions have different localization lengths in real space and interference among them leads to pronounced oscillations of the MF probability density. For a transparent normalsuperconducting junction we find that in the topological phase the MF leaks out from the superconducting into the normal section of the wire and is delocalized over the entire normal section, in agreement with recent numerical results by Chevallier et al. (arXiv:1203.2643).

Electricfieldinduced Majorana Fermions in Armchair Carbon Nanotubes
Jelena Klinovaja, Suhas Gangadharaiah, and Daniel Loss.
Phys. Rev. Lett. 108, 196804 (2012)
We consider theoretically an armchair carbon nanotube (CNT) in the presence of an electric field and in contact with an swave superconductor. We show that the proximity effect opens up superconducting gaps in the CNT of different strengths for the exterior and interior branches of the two Dirac points. For strong proximity induced superconductivity the interior gap can be of the pwave type, while the exterior gap can be tuned by the electric field to be of the swave type. Such a setup supports a single Majorana bound state at each end of the CNT. In the case of a weak proximity induced superconductivity, the gaps in both branches are of the pwave type. However, the temperature can be chosen in such a way that the smallest gap is effectively closed. Using renormalization group techniques we show that the Majorana bound states exist even after taking into account electronelectron interactions.

Carbon nanotubes in electric and magnetic fields
Jelena Klinovaja, Manuel J. Schmidt, Bernd Braunecker, and Daniel Loss.
Phys. Rev. B 84, 085452 (2011)
We derive an effective lowenergy theory for metallic (armchair and nonarmchair) singlewall nanotubes in the presence of an electric field perpendicular to the nanotube axis, and in the presence of magnetic fields, taking into account spinorbit interactions and screening effects on the basis of a microscopic tightbinding model. The interplay between electric field and spinorbit interaction allows us to tune armchair nanotubes into a helical conductor in both Dirac valleys. Metallic nonarmchair nanotubes are gapped by the surface curvature, yet helical conduction modes can be restored in one of the valleys by a magnetic field along the nanotube axis. Furthermore, we discuss electric dipole spin resonance in carbon nanotubes, and find that the Rabi frequency shows a pronounced dependence on the momentum along the nanotube.

Helical modes in carbon nanotubes generated by strong electric fields
Jelena Klinovaja, Manuel J. Schmidt, Bernd Braunecker, and Daniel Loss.
Phys. Rev. Lett. 106, 156809 (2011)
Helical modes, conducting opposite spins in opposite directions, are shown to exist in metallic armchair nanotubes in an allelectric setup. This is a consequence of the interplay between spinorbit interaction and strong electric fields. The helical regime can also be obtained in chiral metallic nanotubes by applying an additional magnetic field. In particular, it is possible to obtain helical modes at one of the two Dirac points only, while the other one remains gapped. Starting from a tightbinding model we derive the effective lowenergy Hamiltonian and the resulting spectrum.

Spinselective Peierls transition in interacting onedimensional conductors with spinorbit interaction
Bernd Braunecker, George I. Japaridze, Jelena Klinovaja, and Daniel Loss.
Phys. Rev. B 82, 045127 (2010)
Interacting onedimensional conductors with Rashba spinorbit coupling are shown to exhibit a spinselective Peierlstype transition into a mixed spinchargedensitywave state. The transition leads to a gap for onehalf of the conducting modes, which is strongly enhanced by electronelectron interactions. The other half of the modes remains in a strongly renormalized gapless state and conducts opposite spins in opposite directions, thus providing a perfect spin filter. The transition is driven by magnetic field and by spinorbit interactions. As an example we show for semiconducting quantum wires and carbon nanotubes that the gap induced by weak magnetic fields or intrinsic spinorbit interactions can get renormalized by 1 order of magnitude up to 10  30 K.

A mixed scenario for the reconstruction of a charged helium surface
Valerii Shikin and Elena Klinovaya.
Low Temp. Phys. 36, 142 (2010)
A mixed scenario for the periodic reconstruction of a charged surface of a liquid when the liquid is close to occupancy saturation by 2D charges is discussed. It is shown that the unit cell of the periodic structure arising is a modified multicharged dimple.