arXiv subjects

Yuli Lyanda-Geller

Publications and source records attributed to Yuli Lyanda-Geller.

At least 19 recordsLinked to original sources

arXiv2026

Dynamic nuclear spin polarization in the fractional quantum Hall effect spin transitions

Experiments suggest that nuclear spins play a significant role in the quantum Hall effect (QHE) near integer and fractional QHE spin transitions, but many of these phenomena still remain to be understood. Here we study theoretically the dynamic nuclear polarization (DNP) in the two-dimensional electron liquid near a quantum point contact (QPC) or a domain wall between spin polarized and unpolarized phases induced by electrostatic gating in the fractional QHE at a filling factor 2/3 and analyze the dependence of the spin transition on temperature and the magnitude of the flowing current. We demonstrate that nearly all nuclear spins in the QPC or in the domain wall can be polarized by the electric current. The Overhauser effective magnetic field from the DNP can be strong enough to induce (or modify) a phase transition between polarized and unpolarized phases. This changes the gate voltages and magnetic fields required for the spin transitions, and leads to the reconstruction of the boundary between two phases and a domain wall and a current path displacement. The spread of nuclear spin polarization and the domain wall displacement are strongly asymmetric with respect to the direction of the current flow. Equilibration due to hyperfine interactions and its role on the nuclear spin polarization, domain wall displacements and spin transitions is studied. Back and forth oscillatory transitions between polarized and unpolarized phases near a source contact are discussed. Hyperfine interactions of nuclear spins provide a route for observation and control of the parafermion zero modes that can be induced when the domain wall between the polarized and unpolarized regions is placed in the proximity of a superconductor

cond-mat.mes-hall↗

arXiv2023

Unusual quasiparticles and tunneling conductance in quantum point contacts in $ν=2/3$ fractional quantum Hall systems

Understanding topological matter in the fractional quantum Hall (FQH) effect requires identifying the nature of edge state quasiparticles. FQH edge state at the filling factor $ν=2/3$ in the spin-polarized and non-polarized phases is represented by the two modes of composite fermions (CF) with the parallel or opposite spins described by the chiral Luttinger liquids. Tunneling through a quantum point contact (QPC) in such systems between different or similar spin phases is solved exactly. With the increase of the applied voltage, the QPC conductance grows from zero and saturates at $e^2/2h$ while a weak electron tunneling between the edge modes with the same spin transforms into a backscattering carried by the charge $q=e/2$ quasiparticles. These unusual quasiparticles and conductance plateau emerge in the QPC with one or two CF modes scattering into a single mode, as occurs in these systems. We propose experiments on the applied voltage and temperature dependence of the QPC conductance and noise that can shed light on the nature of edge states and FQH transport.

cond-mat.mes-hall↗

arXiv2023

Superconducting diode effect in quasi-one-dimensional systems

The recent observations of the superconducting diode effect pose the challenge to fully understand the necessary ingredients for non-reciprocal phenomena in superconductors. In this theoretical work, we focus on the non-reciprocity of the critical current in a quasi-one-dimensional superconductor. We define the critical current as the value of the supercurrent at which the quasiparticle excitation gap closes (depairing). Once the critical current is exceeded, the quasiparticles can exchange energy with the superconducting condensate, giving rise to dissipation. Our minimal model can be microscopically derived as a low-energy limit of a Rashba spin-orbit coupled superconductor in a Zeeman field. Within the proposed model, we explore the nature of the non-reciprocal effects of the critical current both analytically and numerically. Our results quantify how system parameters such as spin-orbit coupling and quantum confinement affect the strength of the superconducting diode effect. Our theory provides a complementary description to Ginzburg-Landau theories of the effect.

cond-mat.supr-con↗

arXiv2023

Diamagnetic mechanism of critical current non-reciprocity in multilayered superconductors

Recent excitement in observation of non-reciprocal critical current (NRC) is motivated by a suggestion that "superconducting diode effect" may be an intrinsic property of non-centrosymmetric superconductors with strong spin-orbit interactions[1]. Theoretically it has been understood that linear in the Cooper pair momentum terms, caused by the Rashba spin-orbit and Zeeman interactions or, more generally, any symmetry-allowed Lifshitz invariants[2] in uniform singlet superconductors, do not contribute to the supercurrent, although the role of higher-order terms remains unclear[3, 4]. In this work we show that critical current non-reciprocity is a generic property of multilayered superconductor structures in the presence of magnetic field-generated diamagnetic currents. In the regime of an intermediate coupling between the layers, the Josephson vortices are predicted to form at high fields and currents. We report the observation of NRC in nanowires fabricated from InAs/Al heterostructures. The effect is independent of the crystallographic orientation of the wire, ruling out an intrinsic origin of NRC. Non-monotonic NRC evolution with magnetic field is consistent with the generation of diamagnetic currents and formation of the Josephson vortices. This extrinsic NRC mechanism can be used to design novel devices for superconducting circuits.

cond-mat.supr-con↗

arXiv2021

Nonlinear Quantum Electrodynamics in Dirac materials

Classical electromagnetism is linear. However, fields can polarize the vacuum Dirac sea, causing quantum nonlinear electromagnetic phenomena, e.g., scattering and splitting of photons, that occur only in very strong fields found in neutron stars or heavy ion colliders.We show that strong nonlinearity arises in Dirac materials at much lower fields $\sim 1\:\text{T}$, allowing us to explore the nonperturbative, extremely high field limit of quantum electrodynamics in solids. We explain recent experiments in a unified framework and predict a new class of nonlinear magneto-electric effects, including a magnetic enhancement of dielectric constant of insulators and a strong electric modulation of magnetization. We propose experiments and discuss the applications in novel materials.

cond-mat.other↗

arXiv2021

Transport in helical Luttinger liquids in the fractional quantum Hall regime

Domain walls in fractional quantum Hall ferromagnets are gapless helical one-dimensional channels formed at the boundaries of topologically distinct quantum Hall (QH) liquids. Naïvely, these helical domain walls (hDWs) constitute two counter-propagating chiral states with opposite spins. Coupled to an s-wave superconductor, helical channels are expected to lead to topological superconductivity with high order non-Abelian excitations. Here we investigate transport properties of hDWs in the $ν=2/3$ fractional QH regime. Experimentally we found that current carried by hDWs is substantially smaller than the prediction of the naïve model. Luttinger liquid theory of the system reveals redistribution of currents between quasiparticle charge, spin and neutral modes, and predicts the reduction of the hDW current. Inclusion of spin-non-conserving tunneling processes reconciles theory with experiment. The theory confirms emergence of spin modes required for the formation of fractional topological superconductivity.

cond-mat.mes-hall↗

arXiv2019

Spin current generation and relaxation in a quenched spin-orbit-coupled Bose-Einstein condensate

Understanding the effects of spin-orbit coupling (SOC) and many-body interactions on spin transport is important in condensed matter physics and spintronics. This topic has been intensively studied for spin carriers such as electrons but barely explored for charge-neutral bosonic quasiparticles (including their condensates), which hold promises for coherent spin transport over macroscopic distances. Here, we explore the effects of synthetic SOC (induced by optical Raman coupling) and atomic interactions on the spin transport in an atomic Bose-Einstein condensate (BEC), where the spin-dipole mode (SDM, actuated by quenching the Raman coupling) of two interacting spin components constitutes an alternating spin current. We experimentally observe that SOC significantly enhances the SDM damping while reducing the thermalization (the reduction of the condensate fraction). We also observe generation of BEC collective excitations such as shape oscillations. Our theory reveals that the SOC-modified interference, immiscibility, and interaction between the spin components can play crucial roles in spin transport.

cond-mat.quant-gas↗

arXiv2017

Parafermion supporting platform based on spin transitions in the fractional quantum Hall effect regime

We propose an experimentally-feasible system based on spin transitions in the fractional quantum Hall effect regime where parafermions, high-order non-abelian excitations, can be potentially realized. We provide a proof-of-concept experiments showing that in specially designed heterostructures spin transitions at a filling factor 2/3 can be induced electrostatically, allowing local control of polarization and on-demand formation of helical domain walls with fractionalized charge excitations, a pre-requisite ingredient for parafermions formation. We also present exact diagonalization numerical studies of domain walls formed between domains with different spin polarization in the fractional quantum Hall effect regime and show that they indeed possess electronic and magnetic structure needed for parafermion formation when coupled to an s-wave superconductor.

cond-mat.mes-hall↗

arXiv2017

Mesoscopic transport in electrostatically-defined spin-full channels in quantum Hall ferromagnets

In this work we use electrostatic control of quantum Hall ferromagnetic transitions in CdMnTe quantum wells to study electron transport through individual domain walls (DWs) induced at a specific location. These DWs are formed due to hybridization of two counter-propagating edge states with opposite spin polarization. Conduction through DWs is found to be symmetric under magnetic field direction reversal, consistent with the helical nature of these DWs. We observe that long domain walls are in the insulating regime with localization length 4 - 6~$μ$m. In shorter DWs the resistance saturates to a non-zero value at low temperatures. Mesoscopic resistance fluctuations in a magnetic field are investigated. The theoretical model of transport through impurity states within the gap induced by spin-orbit interactions agrees well with the experimental data. Helical DWs have required symmetry for the formation of synthetic p-wave superconductors. Achieved electrostatic control of a single helical domain wall is a milestone on the path to their reconfigurable network and ultimately to a demonstration of braiding of non-Abelian excitations.

cond-mat.mes-hall↗

arXiv2016

Electrostatic control of quantum Hall ferromagnetic transition, a step toward reconfigurable network of helical channels

Ferromagnetic transitions between quantum Hall states with different polarization at a fixed filling factor can be studied by varying the ratio of cyclotron and Zeeman energies in tilted magnetic field experiments. However, an ability to locally control such transitions at a fixed magnetic field would open a range of attractive applications, e.g. formation of a reconfigurable network of one-dimensional helical domain walls in a two-dimensional plane. Coupled to a superconductor, such domain walls can support non-Abelian excitation. In this article we report development of heterostructures where quantum Hall ferromagnetic (QHFm) transition can be controlled locally by electrostatic gating. A high mobility two-dimensional electron gas is formed in CdTe quantum wells with engineered placement of paramagnetic Mn impurities. Gate-induced electrostatic field shifts electron wavefunction in the growth direction and changes overlap between electrons in the quantum well and d-shell electrons on Mn, thus controlling the s-d exchange interaction and the field of the QHFm transition. The demonstrated shift of the QHFm transition at a filling factor $ν=2$ is large enough to allow full control of spin polarization at a fixed magnetic field.

cond-mat.mes-hall↗

arXiv2016

In-plane electric fields and the $ν=\frac{5}{2}$ fractional quantum Hall effect in a disk geometry

The $ν=\frac{5}{2}$ fractional quantum Hall effect is of experimental and theoretical interest due to the possible non-Abelian statistics of the excitations in the electron liquid. A small voltage difference across a sample applied in experiments to probe the system is often ignored in theoretical studies due to the Galilean invariance in the thermodynamic limit. No experimental sample, however, is Galilean invariant. In this work, we explore the effects of the probe electric fields in a disk geometry with finite thickness. We find that weak probe fields enhance the Moore-Read Pfaffian state but sufficiently strong electric fields destroy the incompressible state. In a disk geometry, the behavior of the system depends on the polarity of the applied radial field, which can potentially be observed in experiments using in a Corbino disk configuration. Our simulation also shows that the application of such a field enhances the coherence length of quasiholes propagating through the edge channels

cond-mat.mes-hall↗

arXiv2016

Majorana fermions in charge carrier hole quantum wires

Luttinger holes with strong Zeemann and spin-orbit interactions in a wire proximity-coupled to a superconductor is a promising system for observation of Majorana fermions. Earlier treatments of confined Luttinger holes in wires ignored a mutual transformation of heavy and light holes upon reflection from the heteroboundaries. This effect is crucial for Zeemann and spin-orbit coupling in the ground subband of holes with several spin-orbit terms linear in momentum. We discuss the criterion for realizing Majorana modes in charge carrier hole systems. GaAs or InSb hole wires shall exhibit stronger topological superconducting pairing compared to InSb electron systems in similar or weaker magnetic fields.

cond-mat.mes-hall↗

arXiv2016

Non-Abelian $ν=1/2$ quantum Hall state in $Γ_8$ Valence Band Hole Liquid

In search of states with non-Abelian statistics, we explore the fractional quantum Hall effect in a system of two-dimensional charge carrier holes. We propose a new method of mapping states of holes confined to a finite width quantum well in a perpendicular magnetic field to states in a spherical shell geometry. This method provides single-particle hole states used in exact diagonalization of systems with a small number of holes in the presence of Coulomb interactions. An incompressible fractional quantum Hall state emerges in a hole liquid at the half-filling of the ground state in a magnetic field in the range of fields where single-hole states cross. This state has a negligible overlap with the Halperin 331 state, but a significant overlap with the Moore-Read Pfaffian state. Excited fractional quantum Hall states for small systems have sizable overlap with non-Abelian excitations of the Moore-Read Pfaffian state.

cond-mat.mes-hall↗

arXiv2014

Phase diagram and edge states of the $ν=5/2$ fractional quantum Hall state with Landau level mixing and finite well thickness

The $ν=5/2$ fractional quantum Hall effect is a system of intense experimental and theoretical interest as its ground state may host non-abelian excitations, but the exact nature of the ground state is still undetermined. We present the results of an exact diagonalization study of an electron system in the disk configuration including the effects of Landau level (LL) mixing and the finite thickness of the quantum well confining the electrons. The degeneracy between the two leading candidates for the ground state, the Pfaffian and anti-Pfaffian, is broken by interactions with a neutralizing background, in addition to the inclusion of two- and three-body interactions via LL mixing. As a result of the neutralizing background in the disc configuration, there is a phase transition from the anti-Pfaffian to the Pfaffian as LL mixing is turned on, in stark contrast to what is observed in a spherical geometry. The LL mixing leads to an increased charge $e/4$ quasihole size. LL mixing interactions are also shown to overcome the effects of edge reconstruction. Due to finite thickness effects, these properties are enhanced dramatically. We also find that only the Pfaffian and anti-Pfaffian states continue to possess energy gaps at finite width, while gaps for compressible stripe states close.

cond-mat.mes-hall↗

arXiv2013

New topological excitations and melting transitions in quantum Hall systems

We discover a new topological excitation of two dimensional electrons in the quantum Hall regime. The strain dependence of resistivity is shown to change sign upon crossing filling-factor-specified boundaries of reentrant integer quantum Hall effect (RIQHE) states. This observation violates the known symmetry of electron bubbles thought to be responsible for the RIQHE. We demonstrate theoretically that electron bubbles become elongated in the vicinity of charge defects and form textures of finite size. Calculations confirm that texturing lowers the energy of excitations. These textures form hedgehogs (vortices) around defects having (lacking) one extra electron, resulting in striking strain-dependent resistivity that changes sign on opposite boundaries of the RIQHE. At low density these textures form an insulating Abrikosov lattice. At densities sufficient to cause the textures to overlap, their interactions are described by the XY-model and the lattice melts. This melting explains the sharp metal-insulator transition observed in finite temperature conductivity measurements.

cond-mat.mes-hall↗

arXiv2012

Pure spin decoherence in quantum dots

We uncover two microscopic physical settings with significant pure spin decoherence. First, for quantum dots (QD) electrostatically confined in two-dimensional hole gas, decoherence comes from qubit spin-orbit (SO) coupling to phonons, whose decay due to free charge carriers in contacts is described by Ohmic weighted phonon spectral function. We derive significant SO interactions affecting holes with origin and symmetry distinct from that of conventionally considered Dresselhaus and Rashba terms. In the second setting of electron or hole QDs coupled to a linear chain of atoms, decoherence is due to spin-dependent coupling to phonons, whose decay due to scattering off the free ends of a chain is described by the weighted phonon spectral function inverse proportional to frequency. The decoherence rate in both settings is linear in temperature.

cond-mat.mes-hall↗

arXiv2010

Effect of strain on stripe phases in the Quantum Hall regime

Spontaneous breaking of rotational symmetry and preferential orientation of stripe phases in the quantum Hall regime has attracted considerable experimental and theoretical effort over the last decade. We demonstrate experimentally and theoretically that the direction of high and low resistance of the two-dimensional (2D) hole gas in the quantum Hall regime can be controlled by an external strain. Depending on the sign of the in-plane shear strain, the Hartree-Fock energy of holes or electrons is minimized when the charge density wave (CDW) is oriented along [110] or [1-10] directions. We suggest that shear strains due to internal electric fields in the growth direction are responsible for the observed orientation of CDW in pristine electron and hole samples.

cond-mat.str-el↗

arXiv1998

Mesoscopic phenomena in Bose-Einstein systems: Persistent currents, population oscillations and quantal phases

Mesoscopic phenomena - including population oscillations and persistent currents driven by quantal phases - are explored theoretically in the context of multiply-connected Bose-Einstein systems composed of trapped alkali-metal gas atoms. These atomic phenomena are bosonic analogues of electronic persistent currents in normal metals and Little-Parks oscillations in superconductors.

cond-mat↗