arXiv subjects

Qing-feng Sun

Publications and source records attributed to Qing-feng Sun.

At least 19 recordsLinked to original sources

arXiv2025

Signature of gate tunable superconducting network in twisted bilayer graphene

Twisted van der Waals materials provide a tunable platform for investigating two-dimensional superconductivity and quantum phases. Using spectra-imaging scanning tunneling microscopy, we study the superconducting states in twisted bilayer graphene and track their evolution from insulating phases. Gate-dependent spectroscopic measurements reveal two distinct regimes: under-doped (ν = -2.3) and optimally doped (ν = -2.6). In the under-doped regime, partial superconductivity arises, forming a network interspersed with non-gapped regions. At optimal doping, the entire unit cell demonstrates superconductivity, with gap size modulation showing an anti-correlation with the local density of states. This gate-dependent transition from an insulating phase to a modulated superconductor uncovers an unexpected spatial hierarchy in pairing behavior and offers direct microscopic insights to constrain theories of superconductivity in moiré systems.

cond-mat.supr-con↗

arXiv2021

Chiral Interface States and Related Quantized Transport in Disordered Chern Insulators

In this Letter, we study an Anderson-localization-induced quantized transport in disordered Chern insulators (CIs). By investigating the disordered CIs with a step potential, we find that the chiral interface states emerge along the interfaces of the step potential, and the energy range for such quantized transport can be manipulated through the potential strength. Furthermore, numerical simulations on cases with a multi-step potential demonstrate that such chiral state can be spatially shifted by varying the Fermi energy, and the energy window for quantized transport is greatly enlarged. Experimentally, such chiral interface states can be realized by imposing transverse electric field, in which the energy window for quantized transport is much broader than the intrinsic band gap of the corresponding CI. These phenomena are quite universal for disordered CIs due to the direct phase transition between the CI and the normal insulator.

cond-mat.mes-hall↗

arXiv2019

Majorana zero modes by engineering topological kink states in two dimensional electron gas

Majorana zero modes (MZMs)--bearing potential applications for topological quantum computing--are verified in quasi-one-dimensional (1D) Fermion systems, including semiconductor nanowires, magnetic atomic chains, planar Josephson junctions. However, the existence of multi-bands in these systems makes the MZMs fragile to the influence of disorder. Moreover, in practical perspective, the proximity induced superconductivity may be difficult and restricted for 1D systems. Here, we propose a flexible route to realize MZMs through 1D topological kink states by engineering a 2D electron gas with antidot lattices, in which both the aforementioned issues can be avoided owing to the robustness of kink states and the intrinsically attainable superconductivity in high-dimensional systems. The MZMs are verified to be quite robust against disorders and the bending of kink states, and can be conveniently tuned by varying the Rashba spin-orbit coupling strength. Our proposal provides an experimental feasible platform for MZMs with systematic manipulability and assembleability based on the present techniques in 2D electron gas system.

cond-mat.mes-hall↗

arXiv2018

Geometric Effect on Quantum Anomalous Hall States in Magnetic Topological Insulators

An intriguing observation on the quantum anomalous Hall effect (QAHE) in magnetic topological insulators (MTIs) is the dissipative edge states, where quantized Hall resistance is accompanied by nonzero longitudinal resistance. We numerically investigate this dissipative behavior of QAHE in MTIs with a three-dimensional tight-binding model and non-equilibrium Greens function formalism. It is found that, in clean samples, the geometric mismatch between the detecting electrodes and the MTI sample leads to additional scattering in the central Hall bar, which is similar to the effect of splitting gates in the traditional Hall effect. As a result, while the Hall resistance remains quantized, the longitudinal resistance deviates from zero due to such additional scattering. It is also shown that external magnetic fields as well as disorder scattering can suppress the dissipation of the longitudinal resistance. These results are in good agreement with previous experimental observations and provide insight on the fabrication of QAHE devices.

cond-mat.mes-hall↗

arXiv2016

Effect of magnetic field on a magnetic topological insulator film with structural inversion asymmetry

The effect of magnetic field on an ultrathin magnetic topological insulator film with structural inversion asymmetry is investigated. We introduce the phase diagram, calculate the Landau-level spectrum analytically and simulate the transport behavior in Landauer-Büttiker formalism. The quantum anomalous Hall phase will survive increasing magnetic field. Due to the two spin polarized zero modes of Landau levels a nontrivial phase similar with quantum spin Hall effect can be induced by magnetic field which is protected by structural inversion symmetry. Some exotic longitudinal and Hall resistance plateaus with fractional values are also found in a six terminal Hall bar, arising from the coupling between edge states due to inverted energy band and Landau levels.

cond-mat.mes-hall↗

arXiv2016

Quantum interference in topological insulator Josephson junctions

Using non-equilibrium Green's functions, we studied numerically the transport properties of a Josephson junction, superconductor-topological insulator-superconductor hybrid system. Our numerical calculation shows first that proximity-induced superconductivity is indeed observed in the edge states of a topological insulator adjoining two superconducting leads and second that the special characteristics of topological insulators endow the edge states with an enhanced proximity effect with a superconductor but do not forbid the bulk states to do the same. In a size-dependent analysis of the local current, it was found that a few residual bulk states can lead to measurable resistance, whereas because these bulk states spread over the whole sample, their contribution to the interference pattern is insignificant when the sample size is in the micrometer range. Based on these numerical results, it is concluded that the apparent disappearance of residual bulk states in the superconducting interference process as described in Ref. [\onlinecite{HartNautrePhys2014f}] is just due to the effects of size: the contribution of the topological edge states outweighs that of the residual bulk states.

cond-mat.supr-con↗

arXiv2015

Coherent Single Spin Source based on topological insulator

We report on the injection of quantized pure spin current into quantum conductors. In particular, we propose an on demand single spin source generated by periodically varying the gate voltages of two quantum dots that are connected to a two dimensional topological insulator via tunneling barriers. Due to the nature of the helical states of the topological insulator, one or several {\it spin pair}s can be pumped out per cycle giving rise to a pure quantized alternating spin current. Depending on the phase difference between two gate voltages, this device can serve as an on demand single spin emitter or single charge emitter. Again due to the helicity of the topological insulator, the single spin emitter or charge emitter is dissipationless and immune to disorders. The proposed single spin emitter can be an important building block of future spintronic devices.

cond-mat.mes-hall↗

arXiv2015

Nonlocal transport in a hybrid two-dimensional topological insulator

We study nonlocal resistance in an H-shaped two-dimensional HgTe/CdTe quantum well consist of injector and detector, both of which can be tuned in the quantum spin Hall or metallic spin Hall regime. Because of strong spin-orbit interaction, there always exist spin Hall effect and the nonlocal resistance in HgTe/CdTe quantum well. We find that when both detector and injector are in the quantum spin Hall regime, the nonlocal resistance is quantized at $0.25\frac{h}{e^2}$, which is robust against weak disorder scattering and small magnetic field. While beyond this regime, the nonlocal resistance decreases rapidly and will be strongly suppressed by disorder and magnetic field. In the presence of strong magnetic field, the quantum spin Hall regime will be switched into the quantum Hall regime and the nonlocal resistance will disappear. The nonlocal signal and its various manifestation in different hybrid regimes originate from the special band structure of HgTe/CdTe quantum well, and can be considered as the fingerprint of the helical quantum spin Hall edge states in two-dimensional topological insulator.

cond-mat.mes-hall↗

arXiv2015

Disorder and metal-insulator transitions in Weyl semimetals

The Weyl semimetal (WSM) is a newly proposed quantum state of matter. It has Weyl nodes in bulk excitations and Fermi arcs surface states. We study the effects of disorder and localization in WSMs and find three exotic phase transitions. (I) Two Weyl nodes near the Brillouin zone boundary can be annihilated pairwise by disorder scattering, resulting in the opening of a topologically nontrivial gap and a transition from a WSM to a three-dimensional (3D) quantum anomalous Hall state. (II) When the two Weyl nodes are well separated in momentum space, the emergent bulk extended states can give rise to a direct transition from a WSM to a 3D diffusive anomalous Hall metal. (III) Two Weyl nodes can emerge near the zone center when an insulating gap closes with increasing disorder, enabling a direct transition from a normal band insulator to a WSM. We determine the phase diagram by numerically computing the localization length and the Hall conductivity, and propose that the exotic phase transitions can be realized on a photonic lattice.

cond-mat.mes-hall↗

arXiv2015

Tunable Anderson metal-insulator transition in quantum-spin Hall insulators

We numerically study disorder effects in Bernevig-Hughes-Zhang (BHZ) model, and find that Anderson transition of quantum spin-Hall insulator (QSHI) is determined by model parameters. The BHZ Hamiltonian is equivalent to two decoupled spin blocks that belong to the unitary class. In contrast to the common belief that a two-dimensional unitary system scales to an insulator except at certain critical points, we find, through calculations scaling properties of the localization length, level statistics, and participation ratio, that a possible exotic metallic phase emerges between a QSHI and a normal insulator phases in InAs/GaSb-type BHZ model. On the other hand, direct transition from a QSHI to a normal insulator is found in HgTe/CdTe-type BHZ model. Furthermore, we show that the metallic phase originates from the Berry phase and can survive both inside and outside the gap.

cond-mat.mes-hall↗

arXiv2015

Superfluidity of pure spin current in ultracold Bose gases

We study the superfluidity of a pure spin current that is a spin current without mass current. We examine two types of pure spin currents, planar and circular, in spin-1 Bose gas. For the planar current, it is usually unstable, but can be stabilized by the quadratic Zeeman effect. The circular current can be generated with spin-orbit coupling. When the spin-orbit coupling strength is weak, we find that the circular pure spin current is the ground state of the system and thus a super-flow. We discuss the experimental schemes to realize and detect a pure spin current.

cond-mat.quant-gas↗

arXiv2014

Identify the topological superconducting order in a multi-band quantum wire

How to distinguish the zero-bias peak (ZBP) caused by the Majorana fermions from that by the other effects remains a challenge in detecting the topological order of a quantum wire. In this paper we propose to distinguish the topological superconducting phase from the topologically trivial phase by making a Josephson junction of the quantum wire attached to a side lead and then measuring the tunneling conductance through it as the phase difference across the junction $ϕ$ varies. Even if the ZBPs exist in both phases, we can identify the topological superconducting phase by a conductance peak at $ϕ=π$ and a nearby butterfly pattern.

cond-mat.mes-hall↗

arXiv2014

Orbital Kondo effect in a parallel double quantum dot

We construct a theoretical model to study the orbital Kondo effect in a parallel double quantum dot (DQD). Recently, pseudospin-resolved transport spectroscopy of the orbital Kondo effect in a DQD has been experimentally reported. The experiment revealed that when interdot tunneling is ignored, there exist two and one Kondo peaks in the conductance-bias curve for the pseudospin-non-resolved and pseudospin-resolved cases, respectively. Our theoretical studies reproduce this experimental result. We also investigate the situation of all lead voltages being non-equal (the complete pseudospin-resolved case), and find that there are four Kondo peaks at most in the curve of the conductance versus the pseudospin splitting energy. When the interdot tunneling is introduced, some new Kondo peaks and dips can emerge. Besides, the pseudospin transport and the pseudospin flipping current are also studied in the DQD system. Since the pseudospin transport is much easier to be controlled and measured than the real spin transport, it can be used to study the physical phenomenon related to the spin transport.

cond-mat.mes-hall↗

arXiv2014

Revisit the spin-FET: Multiple reflections, inelastic scattering, and lateral size effects

We revisit the spin-injected field effect transistor (spin-FET) by simulating a lattice model based on recursive lattice Green's function approach. In the one-dimensional case and coherent regime, the simulated results reveal noticeable differences from the celebrated Datta-Das model, which motivate thus an improved treatment and lead to analytic and generalized result. The simulation also allows us to address inelastic scattering (using Büttiker's fictitious reservoir approach) and lateral confinement effects on the control of spins which are important issues in the spin-FET device.

cond-mat.mes-hall↗

arXiv2014

Delocalization and scaling properties of low-dimensional quasiperiodic systems

In this paper, we explore the localization transition and the scaling properties of both quasi-one-dimensional and two-dimensional quasiperiodic systems, which are constituted from coupling several Aubry-André (AA) chains along the transverse direction, in the presence of next-nearest-neighbor (NNN) hopping. The localization length, two-terminal conductance, and participation ratio are calculated within the tight-binding Hamiltonian. Our results reveal that a metal-insulator transition could be driven in these systems not only by changing the NNN hopping integral but also by the dimensionality effects. These results are general and hold by coupling distinct AA chains with various model parameters. Furthermore, we show from finite-size scaling that the transport properties of the two-dimensional quasiperiodic system can be described by a single parameter and the scaling function can reach the value 1, contrary to the scaling theory of localization of disordered systems. The underlying physical mechanism is discussed.

cond-mat.mes-hall↗

arXiv2014

Enhanced spin-polarized transport through DNA double helix by gate voltage

We report on a way to manipulate the spin transport through double-stranded DNA contacted by normal-metal electrodes. On the basis of an effective model Hamiltonian, the conductance and the spin polarization are calculated in the presence of a gate voltage by using the Landauer-Büttiker formula. Our results indicate that the spin polarization presents strong dependence on the magnitude as well as the direction of the gate voltage. The spin polarization can be significantly enhanced by tuning the gate voltage and shows oscillating behavior with increasing the DNA length.

cond-mat.mes-hall↗

arXiv2014

Contact effects in spin transport along double-helical molecules

We report on spin transport along double-helical molecular systems by considering various contact configurations and asymmetries between the two helical strands in the regime of completely coherent charge transport. Our results reveal that no spin polarization appears in two-terminal molecular devices when coupled to one-dimensional electrodes. The same holds in the case of finite-width electrodes if there is a \emph{bottleneck} of one single site in the system electrode--molecule--electrode. Then, additional dephasing is necessary to induce spin-filtering effects. In contrast, nonzero spin polarization is found in molecular devices with multiple terminals or with two finite-width electrodes, each of them connected to more than one site of the molecule. Then, the magnitude of spin polarization can be enhanced by increasing the asymmetry between the two strands. We point out that the spin-filtering effects could emerge in double-helical molecular devices at low temperature without dephasing by a proper choice of the electrode number and the connection between the molecule and the electrodes.

cond-mat.mes-hall↗

arXiv2014

The transport properties of Floquet topological superconductors at the transition from the topological phase to the Anderson localized phase

The Floquet topological superconducting state is a nonequilibrium time-periodic state hosting Majorana fermions. We study its transport properties by using the Kitaev model with time-periodic incommensurate potentials, which experiences phase transition from the Floquet topological superconducting phase to the Anderson localized phase with increasing driving strength. We study both the real time dynamics of the current and the non-analytic behavior of the tunneling conductance at the transition. Especially, we find that the tunneling conductance changes continuously at the transition, being a finite value in the presence of Floquet Majorana fermions, but dropping to zero as the Majorana fermions vanish. For a special choice of parameters, the Majorana fermions revive at larger driving strength, accompanied by the revival of conductances.

cond-mat.mes-hall↗