arXiv subjects

Bernd Rosenow

Publications and source records attributed to Bernd Rosenow.

At least 73 records · Page 4Linked to original sources

arXiv2016

Transient Features in Charge Fractionalization, Local Equilibration and Non-equilibrium Bosonization

In quantum Hall edge states and in other one-dimensional interacting systems, charge fractionalization can occur due to the fact that an injected charge pulse decomposes into eigenmodes propagating at different velocities. If the original charge pulse has some spatial width due to injection with a given source-drain voltage, a finite time is needed until the separation between the fractionalized pulses is larger than their width. In the formalism of non-equilibrium bosonization, the above physics is reflected in the separation of initially overlapping square pulses in the effective scattering phase. When expressing the single particle Green's function as a functional determinant of counting operators containing the scattering phase, the time evolution of charge fractionalization is mathematically described by functional determinants with overlapping pulses. We develop a framework for the evaluation of such determinants, describe the system's equilibration dynamics, and compare our theoretical results with recent experimental findings.

cond-mat.str-el↗

arXiv2016

Exceptional points in anisotropic planar microcavities

Planar microcavities allow the control and manipulation of spin-polarization, manifested in phenomena like the optical spin Hall effect due to the intrinsic polarization mode splitting. Here, we study a transparent microcavity with broken rotational symmetry, realized by aligning the optical axis of a uniaxial cavity material in the cavity plane. We demonstrate that the in-plane optical anisotropy gives rise to exceptional points in the dispersion relation, which occur pair-wise, are circularly polarized, and are cores of polarization vortices. These exceptional points are a result of the non-Hermitian character of the system, and are in close relationship to singular optical axes in absorptive biaxial systems.

physics.optics↗

arXiv2016

Negative permittivity attests to local attractive interactions in bubble and stripe phases

The physics of itinerant electrons in condensed matter is by and large governed by repulsive Coulomb forces. However, rare cases exist where local attractive interactions emerge and prevail in determining the ground state of the system despite the dominant Coulomb repulsion. The most notable example is no doubt electron pairing, which leads to superconductivity and is mediated by electron phonon coupling or more intricate mechanisms such as antiferromagnetic spin order in high-temperature superconductors. The interplay of attractive and repulsive interaction components may also instigate spontaneous symmetry lowering and clustering of charges in geometric patterns such as bubbles and stripes, provided these interactions act on different length scales. In high-temperature superconductors, for instance, fluctuating stripe or nematic ordering is intertwined with superconductivity itself. Both types of attractive interaction based physics, pairing and charge ordering, are also at play in high-quality two-dimensional electron systems exposed to a quantizing perpendicular magnetic field for electrons describing larger orbits. This has been concluded indirectly from the transport behaviour when such phases form. An unambiguous manifestation of an underlying attractive interaction between the electrons has however remained elusive. Here we report the observation of a negative permittivity as an immediate consequence of local attractive interaction rather than repulsion among the electrons when bubble and stripe phases form. The implemented technique based on surface acoustic waves offers directionality and confirms the stripe phase to be a strongly anisotropic medium.

cond-mat.mes-hall↗

arXiv2016

Enhancing triplet superconductivity by the proximity to a singlet superconductor in oxide heterostructures

We show how in principle a coherent coupling between two superconductors of opposite parity can be realized in a three-layer oxide heterostructure. Due to strong intraionic spin-orbit coupling in the middle layer, singlet Cooper pairs are converted into triplet ones and vice versa. This results in a large enhancement of the triplet superconductivity, persisting well above the native triplet critical temperature.

cond-mat.supr-con↗

arXiv2016

Non-local polarization feedback in a fractional quantum Hall ferromagnet

In a quantum Hall ferromagnet, the spin polarization of the two-dimensional electron system can be dynamically transferred to nuclear spins in its vicinity through the hyperfine interaction. The resulting nuclear field typically acts back locally, modifying the local electronic Zeeman energy. Here we report a non-local effect arising from the interplay between nuclear polarization and the spatial structure of electronic domains in a $ν=2/3$ fractional quantum Hall state. In our experiments, we use a quantum point contact to locally control and probe the domain structure of different spin configurations emerging at the spin phase transition. Feedback between nuclear and electronic degrees of freedom gives rise to memristive behavior, where electronic transport through the quantum point contact depends on the history of current flow. We propose a model for this effect which suggests a novel route to studying edge states in fractional quantum Hall systems and may account for so-far unexplained oscillatory electronic-transport features observed in previous studies.

cond-mat.mes-hall↗

arXiv2016

Topological Polaritons in a Quantum Spin Hall Cavity

We study the topological structure of matter-light excitations, so called polaritons, in a quantum spin Hall insulator coupled to photonic cavity modes. We identify a topological invariant in the presence of time reversal (TR) symmetry, and demonstrate the existence of a TR-invariant topological phase. We find protected helical edge states with energies below the lower polariton branch and characteristic uncoupled excitonic states, both detectable by optical techniques. Applying a Zeeman field allows us to relate the topological index to the double coverage of the Bloch sphere by the polaritonic pseudospin.

cond-mat.mes-hall↗

arXiv2016

Backscattering in helical edge states from a magnetic impurity and Rashba disorder

Transport by helical edge states of a quantum spin Hall insulator is experimentally characterized by a weakly temperature-dependent mean free path of a few microns and by reproducible conductance oscillations, challenging proposed theoretical explanations. We consider a model where edge electrons experience spatially random Rashba spin-orbit coupling and couple to a magnetic impurity with spin S >= 1/2. In a finite bias steady state, we find for S > 1/2 an impurity induced resistance with a temperature dependence in agreement with experiments. Since backscattering is elastic, interference between different scatterers possibly explains conductance fluctuations.

cond-mat.mes-hall↗

arXiv2015

Cavity Polariton Condensate in a Disordered Environment

We report on the influence of disorder on an exciton-polariton condensate in a ZnO based bulk planar microcavity and compare experimental results with a theoretical model for a non-equilibrium condensate. Experimentally, we detect intensity fluctuations within the far-field emission pattern even at high condensate densities which indicates a significant impact of disorder. We show that these effects rely on the driven dissipative nature of the condensate and argue that they can be accounted for by spatial phase inhomogeneities induced by disorder, which occur even for increasing condensate densities realized in the regime of high excitation power. Thus, non-equilibrium effects strongly suppress the stabilization of the condensate against disorder, contrarily to what is expected for equilibrium condensates in the high density limit. Numerical simulations based on our theoretical model reproduce the experimental data.

cond-mat.mes-hall↗

arXiv2015

Thermodynamic properties of a quantum Hall anti-dot interferometer

We study quantum Hall interferometers in which the interference loop encircles a quantum anti-dot. We base our study on thermodynamic considerations, which we believe reflect the essential aspects of interference transport phenomena. We find that similar to the more conventional Fabry-Perot quantum Hall interferometers, in which the interference loop forms a quantum dot, the anti-dot interferometer is affected by the electro-static Coulomb interaction between the edge modes defining the loop. We show that in the Aharonov-Bohm regime, in which effects of fractional statistics should be visible, is easier to access in interferometers based on anti-dots than in those based on dots. We discuss the relevance of our results to recent measurements on anti-dots interferometers.

cond-mat.mes-hall↗

arXiv2015

Current Correlations from a Mesoscopic Anyon Collider

Fermions and bosons are fundamental realizations of exchange statistics, which governs the probability for two particles being close to each other spatially. Anyons in the fractional quantum Hall effect are an example for exchange statistics intermediate between bosons and fermions. We analyze a mesoscopic setup in which two dilute beams of anyons collide with each other, and relate the correlations of current fluctuations to the probability of particles excluding each other spatially. While current correlations for fermions vanish, negative correlations for anyons are a clear signature of a reduced spatial exclusion as compared to fermions.

cond-mat.mes-hall↗

arXiv2015

Disorder, synchronization and phase locking in non-equilibrium Bose-Einstein condensates

We review some theories of non-equilibrium Bose-Einstein condensates in potentials, in particular of the Bose-Einstein condensate of polaritons. We discuss such condensates, which are steady-states established through a balance of gain and loss, in the complementary limits of a double-well potential and a random disorder potential. For equilibrium condensates, the former corresponds to a Josephson junction, whereas the latter is the setting for the superfluid/Bose glass transition. We explore the non-equilibrium generalization of these phenomena, and highlight connections with mode selection and synchronization.

cond-mat.mes-hall↗

arXiv2015

Symmetry-protected topological invariant and Majorana impurity states in time-reversal-invariant superconductors

We address the question of whether individual nonmagnetic impurities can induce zero-energy states in time-reversal-invariant topological superconductors, and define a class of symmetries which guarantee the existence of such states for a specific value of the impurity strength. These symmetries allow the definition of a position-space topological Z_2 invariant, which is related to the standard bulk topological Z_2 invariant. Our general results are applied to the time-reversal-invariant p-wave phase of the doped Kitaev-Heisenberg model, where we demonstrate how a lattice of impurities can drive a topologically trivial system into the nontrivial phase.

cond-mat.supr-con↗

arXiv2015

Intermediate fixed point in a Luttinger liquid with elastic and dissipative backscattering

In a recent work [Phys. Rev. Lett. {\bf 108}, 136401 (2012)] we have addressed the problem of a Luttinger liquid with a scatterer that allows for both coherent and incoherent scattering channels. We have found that the physics associated with this model is qualitatively different from the elastic impurity setup analyzed by Kane and Fisher, and from the inelastic scattering scenario studied by Furusaki and Matveev, thus proposing a new paradigmatic picture of Luttinger liquid with an impurity. Here we present an extensive study of the renormalization group flows for this problem, the fixed point landscape, and scaling near those fixed points. Our analysis is non-perturbative in the elastic tunneling amplitudes, employing an instanton calculation in one or two of the available elastic tunneling channels. Our analysis accounts for non-trivial Klein factors, which represent anyonic or fermionic statistics. These Klein factors need to be taken into account due to the fact that higher order tunneling processes take place. In particular we find a stable fixed point, where an incoming current is split ${1 \over 2}$ - $1\over 2$ between a forward and a backward scattered beams. This intermediate fixed point, between complete backscattering and full forward scattering, is stable for the Luttinger parameter $g<1$.

cond-mat.mes-hall↗

arXiv2015

Suppression of dephasing and phase lapses in the fractional quantum Hall regime

A charge fluctuator which is electrostatically coupled to a conducting channel may fully dephase quantum transport through the latter. Here, we address the case where a quantum dot (QD), playing the role of a charge fluctuator, is tunnel-coupled to an additional channel. In the case where the latter may support fractional charge, distinct differences from the integer case arise: Abrupt phase lapses of the transmission through the conducting channel occur (which may or may not be equal to $π$). This is accompanied by a cusp-like suppression of the interferometer's visibility, yet no full dephasing. We interpret our findings in terms of the entanglement between the fluctuator and the conducting channels.

cond-mat.mes-hall↗

arXiv2014

Enhanced bulk-edge Coulomb coupling in Fractional Fabry-Perot interferometers

We study the effects of bulk-edge Coulomb coupling on quantum Hall Fabry-Perot interferometers. We find that these effects can be appreciable in devices which would not usually be associated with strong bulk-edge Coulomb coupling, provided the devices in question exhibit certain fractional plateaus. With this in mind, we analyze recent experiments at $ν=5/2$ by taking into account a tunnel coupling between localized bulk Majorana states and Majorana edge states. We find that these experimental data are consistent with the widely held view that the $ν=5/2$ state harbors Moore-Read topological order. However, experiments may have measured Coulomb effects rather than an `even-odd effect' due to non-abelian braiding.

cond-mat.str-el↗

arXiv2014

Unconventional pairing and electronic dimerization instabilities in the doped Kitaev-Heisenberg model

We study the quantum many-body instabilities of the $t -J_{\mathrm{K}} - J_{\mathrm{H}}$ Kitaev-Heisenberg Hamiltonian on the honeycomb lattice as a minimal model for a doped spin-orbit Mott insulator. This spin-$1/2$ model is believed to describe the magnetic properties of the layered transition-metal oxide Na$_2$IrO$_3$. We determine the ground-state of the system with finite charge-carrier density from the functional renormalization group (fRG) for correlated fermionic systems. To this end, we derive fRG flow-equations adapted to the lack of full spin-rotational invariance in the fermionic interactions, here represented by the highly frustrated and anisotropic Kitaev exchange term. Additionally employing a set of Ward identities for the Kitaev-Heisenberg model, the numerical solution of the flow equations suggests a rich phase diagram emerging upon doping charge carriers into the ground-state manifold ($\mathbb{Z}_2$ quantum spin liquids and magnetically ordered phases). We corroborate superconducting triplet $p$-wave instabilities driven by ferromagnetic exchange and various singlet pairing phases. For filling $δ> 1/4$, the $p$-wave pairing gives rise to a topological state with protected Majorana edge-modes. For antiferromagnetic Kitaev and ferromagnetic Heisenberg exchange we obtain bond-order instabilities at van Hove filling supported by nesting and density-of-states enhancement, yielding dimerization patterns of the electronic degrees of freedom on the honeycomb lattice. Further, our flow equations are applicable to a wider class of model Hamiltonians.

cond-mat.str-el↗

arXiv2014

Topological superconductivity in Quantum Hall-superconductor hybrid systems

We develop a scenario to engineer a topological phase with Majorana edge states based on an integer quantum Hall (QH) system proximity coupled to a superconductor (SC). Due to the vortices in the SC order parameter, the SC - QH hybrid system is described by a Bloch problem with ten unpaired momenta, corresponding to the maxima and saddle points of the SC order parameter. For external potentials respecting the symmetry of the vortex lattice, the states with unpaired momenta have degeneracies such that the system always is in a trivial phase. However, an incommensurate potential can lift the degeneracies and drive the system into a topologically nontrivial phase.

cond-mat.mes-hall↗

arXiv2014

Zeeman field induced topological phase transitions in triplet superconductors

We develop a general Ginzburg-Landau theory which describes the effect of a Zeeman field on the superconducting order parameter in triplet superconductors. Starting from Ginzburg-Landau theories that describe fully gapped time-reversal symmetric triplet superconductors, we show that the Zeeman field has dramatic effects on the topological properties of the superconductors. In particular, in the vicinity of a critical chemical potential separating two topologically distinct phases, it is possible to induce a phase transition to a topologically nontrivial phase which supports chiral edge modes. Moreover, for specific directions of the Zeeman field, we obtain nodal superconducting phases with an emerging chiral symmetry, and with Majorana flat bands at the edge. The Ginzburg-Landau theory is microscopically supported by a self-consistent mean-field theory of the doped Kitaev-Heisenberg model.

cond-mat.stat-mech↗