Diego Rainis
ContactDepartment of PhysicsUniversity of Basel Klingelbergstrasse 82 CH4056 Basel, Switzerland

Short CV
01/2011  present  Postdoc in the group of Prof. D. Loss, University of Basel, Switzerland 
01/2008  12/2010  PhD under the supervision of Prof. R. Fazio, Scuola Normale Superiore, Pisa, Italy 
09/2005  10/2008  Master of Science in Physics, University of Pisa, Italy. Master Thesis Advisor: Prof. M. Tosi 
10/2002  09/2005  Bachelor of Science in Physics, University of Pisa, Italy. Bachelor Thesis Advisor: Prof. M. Tosi 
Research Interests
 Quantum transport in mesoscopic systems
 Transport properties of graphenebased hybrid structures
Publications
Show all abstracts.1.  Transport signature of fractional Fermions in Rashba nanowires 
Diego Rainis, Arijit Saha, Jelena Klinovaja, Luka Trifunovic, and Daniel Loss. arXiv:1309.3738
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.
 
2.  Correlations between Majorana fermions through a superconductor 
Alexander Zyuzin, Diego Rainis, Jelena Klinovaja, and Daniel Loss. Phys. Rev. Lett. 111, 056802 (2013); arXiv:1305.4187.
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.
 
3.  Towards a realistic transport modeling in a superconducting nanowire with Majorana fermions 
Diego Rainis, Luka Trifunovic, Jelena Klinovaja, and Daniel Loss. Phys. Rev. B 87, 024515 (2013); arXiv:1207.5907.
Motivated by recent experiments searching for Majorana fermions (MFs) in hybrid semiconductingsuperconducting nanostructures, we consider a realistic tightbinding model and analyze its transport behavior numerically. In particular, we take into account the presence of a superconducting contact, used in real experiments to extract the current, which is usually not included in theoretical calculations. We show that important features emerge that are absent in simpler models, such as the shift in energy of the proximity gap signal, and the enhanced visibility of the topological gap for increased spinorbit interaction. We find oscillations of the zero bias peak as a function of the magnetic field and study them analytically. We argue that many of the experimentally observed features hint at an actual spinorbit interaction larger than the one typically assumed. However, even taking into account all the known ingredients of the experiments and exploring many parameter regimes for MFs, we are not able to reach full agreement with the reported data. Thus, a different physical origin for the observed zerobias peak cannot be excluded.
 
4.  Decoherence of Majorana qubits by noisy gates 
Manuel Schmidt, Diego Rainis, and Daniel Loss. Phys. Rev. B 86, 085414 (2012); arXiv:1206.0743.
We propose and study a realistic model for the decoherence of topological qubits, based on Majorana fermions in onedimensional topological superconductors. The source of decoherence is the fluctuating charge on a capacitively coupled gate, modeled by noninteracting electrons. In this context, we clarify the role of quantum fluctuations and thermal fluctuations and find that quantum fluctuations do not lead to decoherence, while thermal fluctuations do. We explicitly calculate decay times due to thermal noise and give conditions for the gap size in the topological superconductor and the gate temperature. Based on this result, we provide simple rules for gate geometries and materials optimized for reducing the negative effect of thermal charge fluctuations on the gate.
 
5.  Majorana qubit decoherence by quasiparticle poisoning 
Diego Rainis and Daniel Loss. Phys. Rev. B 85, 174533 (2012); arXiv:1204.3326.
We consider the problem of quasiparticle poisoning in a nanowirebased realization of a Majorana qubit, where a spinorbitcoupled semiconducting wire is placed on top of a (bulk) superconductor. By making use of recent experimental data exhibiting evidence of a lowtemperature residual nonequilibrium quasiparticle population in superconductors, we show by means of analytical and numerical calculations that the dephasing time due to the tunneling of quasiparticles into the nanowire may be problematically short to allow for qubit manipulation.
 
6.  Gauge fields and interferometry in folded graphene 
Diego Rainis, Fabio Taddei, Marco Polini, Gladys Leon, Francisco Guinea, and Vladimir I. Fal'ko. Phys. Rev. B 83, 165403 (2011); arXiv:1009.0330.
Folded graphene flakes are a natural byproduct of the micromechanical
exfoliation process. In this Letter we show by a combination of analytical and
numerical methods that such systems behave as intriguing interferometers due to
the interplay between an externally applied magnetic field and the gauge field
induced by the deformations in the region of the fold.
 
7.  Blockade and Counterflow Supercurrent in excitoncondensate Josephson junctions 
Fabrizio Dolcini, Diego Rainis, Fabio Taddei, Marco Polini, Rosario Fazio, and A.H. MacDonald. Phys. Rev. Lett. 104, 027004 (2010); arXiv:0908.0478.
We demonstrate that perfect conversion between charged supercurrents in
superconductors and neutral supercurrents in electronhole pair condensates is
possible via a new Andreevlike scattering mechanism. As a result, when two
superconducting circuits are coupled through a bilayer exciton condensate, the
superflow in both layers is drastically modified. Depending on the phase biases
the supercurrents can be completely blocked or exhibit perfect drag.
 
8.  Andreev reflection in graphene nanoribbons 
Diego Rainis, Fabio Taddei, Fabrizio Dolcini, Marco Polini, and Rosario Fazio. Phys. Rev. B 79, 115131; arXiv:0806.4475.
We study Andreev reflection in graphene nanoribbon/superconductor hybrid
junctions. By using a tightbinding approach and the scattering formalism we
show that finitesize effects lead to notable differences with respect to the
bulk graphene case. At subgap voltages, conservation of pseudoparity, a quantum
number characterizing the ribbon states, yields either a suppression of Andreev
reflection when the ribbon has an even number of sites in the transverse
direction or perfect Andreev reflection when the ribbon has an odd number of
sites. In the former case the suppression of Andreev reflection induces an
insulating behavior even when the junction is biased; electron conduction can
however be restored by applying a gate voltage.
 
9.  Timedependent currentdensityfunctional theory of spincharge separation and spin drag in onedimensional ultracold Fermi gases 
Gao Xianlong, Marco Polini, Diego Rainis, M.P. Tosi, and G. Vignale. Phys. Rev. Lett. 101, 206402 (2008); arXiv:0804.1514.
Motivated by the large interest in the nonequilibrium dynamics of
lowdimensional quantum manybody systems, we present a fullymicroscopic
theoretical and numerical study of the "charge" and "spin" dynamics in a
onedimensional ultracold Fermi gas following a quench. Our approach, which is
based on timedependent currentdensityfunctional theory, is applicable well
beyond the linearresponse regime and produces both spincharge separation and
spindraginduced broadening of the spin packets.
 
10.  Spindrag relaxation time in onedimensional spinpolarized Fermi gases 
Diego Rainis, Marco Polini, M.P. Tosi, and G. Vignale. Phys. Rev. B 77, 035113 (2008); arXiv:0801.2324.
Spin propagation in systems of onedimensional interacting fermions at finite
temperature is intrinsically diffusive. The spreading rate of a spin packet is
controlled by a transport coefficient termed "spin drag" relaxation time
τ_{sd}. In this paper we present both numerical and analytical
calculations of τ_{sd} for a twocomponent spinpolarized cold Fermi
gas trapped inside a tight atomic waveguide. At low temperatures we find an
activation law for τ_{sd}, in agreement with earlier calculations of
Coulomb drag between slightly asymmetric quantum wires, but with a different
and much stronger temperature dependence of the prefactor. Our results provide
a fundamental input for microscopic timedependent spindensity functional
theory calculations of spin transport in 1D inhomogeneous systems of
interacting fermions.
