Christina Psaroudaki
ContactDepartment of PhysicsUniversity of Basel Klingelbergstrasse 82 CH4056 Basel, Switzerland

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Show all abstracts.1.  Quantum Brownian Motion of a Magnetic Skyrmion 
Christina Psaroudaki, Pavel Aseev, and Daniel Loss. arXiv:1904.09215
Within a microscopic theory, we study the quantum Brownian motion of a skyrmion in a magnetic insulator coupled to a bath of magnonlike quantum excitations. The intrinsic skyrmionbath coupling gives rise to damping terms for the skyrmion centerofmass, which remain finite down to zero temperature due to the quantum nature of the magnon bath. We show that the quantum version of the fluctuationdissipation theorem acquires a nontrivial temperature dependence. As a consequence, the skyrmion mean square displacement is finite at zero temperature and has a fast thermal activation that scales quadratically with temperature, contrary to the linear increase predicted by the classical phenomenological theory. The effects of an external oscillating drive which couples directly on the magnon bath are investigated. We generalize the standard quantum theory of dissipation and we show explicitly that additional timedependent dissipation terms are generated by the external drive. From these we emphasize a friction and a topological charge renormalization term, which are absent in the static limit. The skyrmion response function inherits the time periodicity of the driving field and it is thus enhanced and lowered over a driving cycle. Finally, we provide a generalized version of the nonequilibrium fluctuationdissipation theorem valid for weakly driven baths.
 
2.  Spin liquid fingerprints in the thermal transport of a KitaevHeisenberg ladder 
Alexandros Metavitsiadis, Christina Psaroudaki, and Wolfram Brenig. arXiv:1806.02344
We identify fingerprints of a proximate quantum spinliquid (QSL), observable by finite temperature dynamical thermal transport within a minimal version of the idealized Kitaev model on a twoleg ladder, if subjected to inevitably present Heisenberg couplings. Using exact diagonalization and quantum typicality, we uncover (i) an insulatorconductor crossover induced by fracton recombination at infinitesimal Heisenberg coupling, (ii) low and highenergy signatures of fractons, which survive far off the pure QSL point, and (iii) a nonmonotonous current lifetime versus Heisenberg couplings. Guided by perturbation theory, we find (iv) a Kitaevexchange induced “onemagnon” contribution to the dynamical heat transport in the strong Heisenberg rung limit.
 
3.  Skyrmions Driven by Intrinsic Magnons 
Christina Psaroudaki and Daniel Loss. Phys. Rev. Lett. 120, 237203 (2018)
We study the dynamics of a skyrmion in a magnetic insulating nanowire in the presence of timedependent oscillating magnetic field gradients. These ac fields act as a net driving force on the skyrmion via its own intrinsic magnetic excitations. In a microscopic quantum field theory approach we include the unavoidable coupling of the external field to the magnons, which gives rise to timedependent dissipation for the skyrmion. We demonstrate that the magnetic ac field induces a superOhmic to Ohmic crossover behavior for the skyrmion dissipation kernels with timedependent Ohmic terms. The ac driving of the magnon bath at resonance results in a unidirectional helical propagation of the skyrmion in addition to the otherwise periodic bounded motion.
 
4.  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 finitetemperature 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 defects are present or a magnetic trap is applied, the Skyrmion mass acquires a finite value proportional to the effective spin, 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 quasionedimensional confinement. An additional quantum mass term is predicted, independent of the effective spin, with an explicit temperature dependence which remains finite even at zero temperature.
 
5.  Spin and magnetothermal transport in the S = 1/2 XXZ chain 
Christina Psaroudaki and Xenophon Zotos. J. Stat. Mech. (2016) 063103
We present a temperature and magnetic field dependence study of spin transport and magnetothermal corrections to the thermal conductivity in the spin S = 1/2 integrable easyplane regime Heisenberg chain, extending an earlier analysis based on the Bethe ansatz method. We critically discuss the low temperature, weak magnetic field behavior, the effect of magnetothermal corrections in the vicinity of the critical field and their role in recent thermal conductivity experiments in 1D quantum magnets.
 
6.  Effective S=1/2 description of the S=1 chain with strong easy plane anisotropy 
Christina Psaroudaki, Jacek Herbrych, Jiannis Karadamoglou, Peter Prelovsek, Xenophon Zotos, and Nikos Papanicolaou. Phys. Rev. B 89, 224418 (2014)
We present a study of the onedimensional S=1 antiferromagnetic spin chain with large easy plane anisotropy, with special emphasis on fieldinduced quantum phase transitions. Temperature and magnetic field dependence of magnetization, specific heat, and thermal conductivity is presented using a combination of numerical methods. In addition, the original S=1 model is mapped into the lowenergy effective S=1/2 XXZ Heisenberg chain, a model which is exactly solvable using the Bethe ansatz technique. The effectiveness of the mapping is explored, and we show that all considered quantities are in qualitative, and in some cases quantitative, agreement. The thermal conductivity of the considered S=1 model is found to be strongly influenced by the underlying effective description. Furthermore, we elucidate the lowlying electron spin resonance spectrum, based on a semianalytical Bethe ansatz calculation of the effective S=1/2 model.
 
7.  Magnetic excitations in the spin1 anisotropic antiferromagnet NiCl_24SC(NH2)_2 
Christina Psaroudaki, S. A. Zvyagin, J. Krzystek, A. PaduanFilho, Xenophon Zotos, and Nikos Papanicolaou. Phys. Rev. B 85, 014412 (2012)
The spin1 anisotropic antiferromagnet NiCl_24SC(NH2)_2 exhibits a fieldinduced quantum phase transition that is formally analogous to BoseEinstein condensation. Here we present results of systematic highfield electron spin resonance (ESR) experimental and theoretical studies of this compound with a special emphasis on singleion twomagnon bound states. In order to clarify some remaining discrepancies between theory and experiment, the frequencyfield dependence of magnetic excitations in this material is reanalyzed. In particular, a more comprehensive interpretation of the experimental signature of singleion twomagnon bound states is shown to be fully consistent with theoretical results. We also clarify the structure of the ESR spectrum in the socalled intermediate phase.
 
8.  Oscillations of a BoseEinstein condensate in a rapidly contracting circular box 
StavrosTheodorakis and Christina Psaroudaki. Physics Letters A 373, 441 (2009)
A BoseEinstein condensate will evolve almost adiabatically if the number of atoms is large enough, even though the trap parameters may be changing rapidly. We demonstrate this by examining a BoseEinstein condensate in a twodimensional rapidly contracting circular box. We show that as a result of the contraction the condensate will oscillate about the instantaneous ground state. These oscillations will be small though when the number of atoms is large. Approximate analytic expressions are found for the evolving condensate wavefunction, both before and after the contraction has begun.
