arXiv subjects

Qing-Feng Sun

Publications and source records attributed to Qing-Feng Sun.

At least 73 records · Page 4Linked to original sources

arXiv2024

Thermal dissipation of the quantum spin Hall edge states in HgTe/CdTe quantum well

Quantum spin Hall effect is characterized by topologically protected helical edge states. Here we study the thermal dissipation of helical edge states by considering two types of dissipation sources. The results show that the helical edge states are dissipationless for normal dissipation sources with or without Rashba spin-orbit coupling in the system, but they are dissipative for spin dissipation sources. Further studies on the energy distribution show that electrons with spin-up and spin-down are both in their own equilibrium without dissipation sources. Spin dissipation sources can couple the two subsystems together to induce voltage drop and nonequilibrium distribution, leading to thermal dissipation, while normal dissipation sources cannot. With the increase of thermal dissipation, the subsystems of electrons with spin-up and spin-down evolve from non-equilibrium finally to mutual equilibrium. In addition, the effects of disorder on thermal dissipation are also discussed. Our work provides clues to reduce thermal dissipation in the quantum spin Hall systems

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arXiv2024

Absence of edge states at armchair edges in inhomogeneously strained graphene under a pseudomagnetic field

Nonuniform strain in graphene can induce a pseudo-magnetic field (PMF) preserving time-reversal symmetry, generating pseudo-Landau levels under zero real magnetic field (MF). The different natures between PMF and real MF lead to the counterpropagating valley-polarized edge states under the PMF and unidirectionally chiral edge states under the real MF. In this work, we find, due to the valley mixing on the armchair edges, the quantum valley Hall edge states only exist at the zigzag edges but not at armchair edges in a uniaxial strained graphene, very different from the case that chiral quantum Hall edge states exist at all edges in pristine graphene under a real MF. We theoretically demonstrate it through the wave function distributions, multi-terminal transport measurements and the electron local occupations, respectively. The interface state in a p-n junction under PMF is further proposed to transport electrons between the conductive zigzag boundaries, which could be used as a valley-polarized single pole double throw switch.

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arXiv2024

Four-terminal graphene-superconductor thermal switch controlled by the superconducting phase difference

We propose a superconducting phase-controlled thermal switch based on a four-terminal graphene-superconductor system. By the coupling of two superconducting leads on a zigzag graphene nanoribbon, both the normal-transmission coefficient and the crossed-Andreev-reflection coefficient, which dominate the thermal conductivity of electrons in the graphene nanoribbon, can be well controlled simultaneously by the phase difference of the superconducting leads. As a result, the thermal conductivity of electrons in the graphene nanoribbon can be tuned and a thermal switching effect appears. Using the nonequilibrium Green's function method, we verify this thermal switching effect numerically. At ambient temperatures less than about one tenth of the superconducting transition temperature, the thermal switching ratio can exceed 2000. The performance of the thermal switch can be regulated by the ambient temperature, and doping or gating can slightly increase the thermal switching ratio. The use of narrower graphene nanoribbons and wider superconducting leads facilitates the obtaining of larger thermal switching ratios. This switching effect of electronic thermal conductance in graphene is expected to be experimentally realized and applied.

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arXiv2024

Charge and spin transport through normal lead coupled to $s$-wave superconductor and a Majorana zero mode

Zero-bias charge conductance peak (ZBCCP) is a significant symbol of Majorana zero modes (MZMs). The proximity effect of s-wave superconductor is usually demanded in the fabrication of MZMs. So in transport experiments, the system is inevitably coupled to the s-wave superconductor. Here we study how the ZBCCP is affected by coupling of the s-wave superconductor. The results show that the ZBCCP could be changed into a zero-bias valley due to the coupling of s-wave superconductor, although the conductance at the zero bias still keeps a quantized value $2e^2/h$. So it does not mean no MZM exists when no ZBCCP is experimentally observed. In addition, the spin transport is investigated. Four reflection processes (the normal reflection, spin-flip reflection, normal Andreev reflection, and equal-spin Andreev reflection) usually occur. The reflection coefficients are strongly dependent on the spin direction of the incident electron, and they may be symmetrical or Fano resonance shapes. But the spin conductance always shows a zero-bias peak with the height $e/2π$ regardless of the direction of the spin bias and the coupling strength of the s-wave superconductor. So measuring spin transport properties could be a more reliable method to judge the existence of MZMs.

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arXiv2024

Topological phase transition driven by magnetic field in one-dimensional topological superconductor rings

We study the energy spectrum and transport property of a one-dimensional Kitaev quantum ring in a threading magnetic field. It is demonstrated that the magnetic field can effectively induce topological phase transitions for the ring in the topologically nontrivial phase at the zero magnetic field. However, for the ring in the topologically trivial phase at the zero field, there is no topological phase transition, and the energy spectrum of the system is always gapped. The magnetic field can control the appearance and disappearance of Majorana zero-energy states in the Kitaev quantum ring, when one half of the ring is in the topologically nontrivial phase and the other half is in the topologically trivial phase. Furthermore, we calculate the transport properties of the ring connected by two semi-infinite leads. It is found that the resonant peaks of transmission coefficient TQT correspond to the critical points of topological phase transition. In addition, we extend our findings to a more realistic quantum ring adopting a semiconductor nanowire with high spin-orbit coupling, superconducting s-wave pairing, and Zeeman splitting, and prove that our findings are universal.

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arXiv2024

Anomalous photon-assisted tunneling in periodically driven Majorana nanowires and BCS Charge Measurement

The photon-assisted tunneling of Majorana bound states in a Majorana nanowire driven by the periodic field is studied both theoretically and numerically. We find that Majorana bound states exhibit an anomalous photon-assisted tunneling signal which is different from an ordinary fermionic state : the height of photonic sidebands is related to the degree of Majorana nonlocality. Moreover, we show that the Bardeen-Cooper-Schrieffer (BCS) charge and spin components of subgap states can be well revealed by the local conductance. Our work illuminates the effect of driving field on Majorana bound states and provides a systematic scheme to directly measure BCS information of overlapping Majorana bound states

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arXiv2024

Spin phase regulated spin Josephson supercurrent in topological superconductor

Without applied bias voltage, a superconducting phase difference can drive a charge Josephson supercurrent in a superconductor junction. In analogy, we here theoretically propose a spin phase that intrinsically generates spin Josephson supercurrent, and this spin Josephson effect is studied in a junction of superconducting nanowire (SNW). We show that spin-orbit coupling and magnetic field give rise to spin-triplet superconductivity in the SNW, thus allow the superfluid of both charge and spin. Next we introduce the concept of spin phase that can be generated by controls of spin current, magnetic field or electric field. By the analysis of pairing correlations and Ginzburg-Landau-type theory, it is shown that the spin phase makes spin-up and spin-down $S=1$ Cooper pairs get opposite phases and move oppositely in a Josephson junction, so that a dissipationless pure spin current is induced. We also derive the formula that the spin current is equal to the derivative of Andreev bound state to the spin phase, which is analogous to that of charge Josephson effect. At last, our calculation verifies the existence of spin supercurrent inside the superconducting gap in the topologically nontrivial phase. Our study provides a view on spin Josephson effect and indicates the potential combination of topological superconductivity and spintronics.

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arXiv2024

Phase diagrams and edge-state transitions in graphene with spin-orbit coupling and magnetic and pseudomagnetic fields

The quantum Hall (QH) effect, the quantum spin Hall (QSH) effect and the quantum valley Hall (QVH) effect are three peculiar topological insulating phases in graphene. They are characterized by three different types of edge states. These three effects are caused by the external magnetic field, the intrinsic spin-orbit coupling (SOC) and the strain-induced pseudomagnetic field, respectively. Here we theoretically study phase diagrams when these effects coexist and analyze how the edge states evolve between the three. We find the real magnetic field and the pseudomagnetic field will compete above the SOC energy gap while the QSH effect is almost unaffected within the SOC energy gap. The edge states transition from the QH effect or the QVH effect to the QSH effect directly relies on the arrangement of the zeroth Landau levels. Using edge states transitions, we raise a device similar to a spin field effect transistor (spin-FET) and also design a spintronics multiple-way switch.

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arXiv2024

Second-order topological corner states in zigzag graphene nanoflake with different types of edge magnetic configurations

We study the energy spectrum and energy levels of the extended Kane-Mele model with magnetic atoms on their zigzag edges. It is demonstrated that the edges of ferromagnetism or antiferromagnetism are enough to break the time-reversal symmetry and host one-dimensional gapped edge states. Thus, a second-order topological phase transition could happen, which leads to the emergence of topological in-gap zero-dimensional corner states. We also prove that the in-gap corner states are robust against corner defects and magnetic disorder. Our proposal based on the edge antiferromagnetism shows that the high-order topological states can be realized, although the net magnetization of the material is zero. In addition, we discuss the influence of spin magnetization orientation on the degeneracy and energy of in-gap corner states.

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arXiv2024

Spin Transport in Normal Metal-Ising Superconductor Junction

The combination of spin-orbit coupling and superconductivity induces unconventional spin-triplet correlation in Ising superconductors. We theoretically investigate the spin transport through a normal metal-Ising superconductor junction, showing that Ising superconductors also have the characteristic of spin superconductivity.Due to the existence of spin-triplet Cooper pairs, not only charge supercurrent but also spin supercurrent can transport in Ising superconductors.We analyze the transport process in the junction which is mainly contributed by the equal-spin Andreev reflection and spin-flip reflection, and calculate the spin conductance and the spin injection efficiency under different conditions.Our findings broaden the boundary of spin superconductivity and reveal the potential applications of Ising superconductors in spintronics, especially in controlled long-distance dissipationless spin transport.

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arXiv2024

Wavefront dislocations in graphene systems revealed by transport measurement

The wavefront dislocation is an important and ubiquitous phenomenon in wave fields. It is closely related to the phase singularity in a wave function. Some recent studies have verified that the wavefront dislocations in the local density of states (LDOS) map can well manifest the intrinsic topological characteristics in graphene and some topological systems. Different from these previous schemes, we raise a transport method to measure such wavefront dislocations in monolayer and Bernal-stacked bilayer graphene. Combining analytical analysis and numerical calculation, we find phase singularities naturally appear in the transmission coefficients between different sublattices, due to the intervalley interference on the electron propagating paths. These phase singularities could contribute wavefront dislocations in the conductance map. Additionally, in bilayer graphene, the wavefront dislocations are found to remain robust even though the tip is coupled to multiple sublattices. Biased bilayer graphene is also explored. Our scheme provides a new transport routine to explore valley-related topological properties of materials.

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arXiv2024

Engineering topologically protected zero-dimensional interface end states in antiferromagnetic heterojunction graphene nanoflakes

We investigate the energy band structure and energy levels of a heterojunction composed of two antiferromagnetic graphene nanoflakes with opposite in-plane antiferromagnetic orderings, in which the modified Kane-Mele model is employed. Before forming an antiferromagnetic graphene heterojunction, the energy gap of helical edge states in each isolated graphene nanoflake are opened by the antiferromagnetic ordering and there is no the in-gap corner state. We find that when two opposite antiferromagnetic graphenes are coupled to form a heterojunction nanoflake, topologically protected zero-dimensional in-gap states can be induced. In addition, we demonstrate that the in-gap states locate at the end of the interface and are robust against magnetic disorder, Anderson disorder, and interfacial magnetic defects. The position and number of the in-gap interface end states in the heterojunction sample can be precise quantum controlled.

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arXiv2024

Magnetization-Induced Phase Transitions on the surface of 3D Topological Insulators

From the low-energy model, the topological field theory indicates that the surface magnetization can open a surface gap in 3D topological insulators (TIs), resulting in a half-quantized Hall conductance. Here by employing the realistic lattice model, we show the occurrence of { the surface phase transitions}, accompanied with the sharp changes of the surface Chern number from $\frac{1}{2}$ to $-\frac{3}{2}$ finally to $-\frac{1}{2}$, in 3D TIs induced by surface magnetization. These surface phase transitions lead to the sudden jumps in the magneto-electric coefficient and the quantum Hall conductance, which are experimentally observable. Furthermore, we present the phase diagram that elucidates the behavior of the 3D TI surface Chern numbers under surface magnetization for different $Z_{4}$ topological numbers. Our study highlights the presence of the new phases with broken bulk-boundary correspondence and enriches understandings of the properties of TIs.

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arXiv2024

Andreev reflection in normal metal/charge-4e superconductor junctions

We investigate the Andreev reflection in a normal metal/charge-4e superconductor junction.Compare with the electron-hole conversion in normal charge-2e superconductors, here four electrons participate simultaneously, enriching the possibility of conversion ways.Using nonequilibrium Green's function method, we obtain a four-particle-type Laudauer-Büttiker formula with generalized charge-4e anomalous Green's function to describe it.We then calculate and clarify the behavior of the Andreev coefficient with various incident energy and show the conductance contributed by it.Our research makes up the blank of research for transport property of charge-4e superconductors and can be served as hallmarks for future experimental verifications.

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arXiv2024

Quarter-quantized thermal Hall effect with parity anomaly

We show that in the proximity of s-wave superconductors, the magnetic topological surface states can transform into Majorana surface state, featuring a single gapless Majorana cone with parity anomaly when the superconducting pairing gap matches the surface magnetization gap. The emergence of $N=1/2$ Majorana chiral edge current is observed at the boundaries between the gap region and the gapless region. Additionally, in systems with a single gapless Majorana cone, a quarter-quantized thermal Hall conductance appears under the dephasing. By mapping the system to a conductor-network model, we identify the appearance of 1/4 chiral heat channels as the cause of the quarter-quantized Hall thermal conductance. We observe the stability of this quarter-quantized thermal conductance under temperature variations, serving as a distinctive feature indicating the presence of a single gapless Majorana cone in the system. Our models can be experimentally realized using magnetic topological insulators or iron-based superconductors.

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arXiv2024

Dissipation and dephasing in quantum Hall interferometers

In recent years, counter-intuitive results have shown that the quantum Hall edge states with topological protection can be dissipative. In this paper, we point out that the non-equilibrium nature of edge states in quantum Hall interferometers leads to inevitable dissipation. We consider a graphene interferometer operating in the integer quantum Hall regime and simulate the inelastic scattering that causes both dissipation and dephasing in the interferometer using non-equilibrium Green's function and virtual leads. We describe the dissipation process with the numerical results of the spatial distribution of heat generation and the evolution of electron energy distribution. In addition, with the enhancement of dephasing, a competition between Aharonov-Bohm interference and topologically protected quantized Hall plateaus is observed in the oscillations and fluctuations of the Hall resistances. At a suitable dephasing strength, quantum Hall plateaus can be promoted by dephasing. Our results not only give clues for the design of dissipation-free devices but also provide a platform for studying the non-equilibrium relaxation and the dissipation mechanism of topological states.

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arXiv2024

Universal theory of tunable second-order topological corner states induced by interlayer coupling in twist bilayer Chern insulators

We propose a universal theory for tunable second-order topological corner states induced by interlayer coupling in bilayer Chern insulators with opposite Chern numbers. We demonstrate that the existence of the topological corner state is determined by the relationship between the twist angle of the bilayer Chern insulators and the normal angles of the two sides of the corner. In addition, the position of these corner states can be sensitively controlled by the twist angle, as confirmed by a rigorous analysis of edge state theory. Our findings serve as a universal theory, opening avenues for the design and realization of higher-order topological materials.

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arXiv2024

Classification of Chern Numbers Based on High-Symmetry Points

The Chern number is a crucial topological invariant for distinguishing the phases of Chern insulators. Here we find that for Chern insulators with inversion symmetry, the Chern number alone is insufficient to fully characterize their topology. Specifically, distinct topological phases can be differentiated based on skyrmions at different high-symmetry points. Interfaces between these topological phases exhibit gapless helical states, which provide counter-propagating transport channels and robust quantized transport. Additionally, we identify topological transitions that do not involve changes in the Chern number but can be characterized by transitions of skyrmions between high-symmetry points. These transitions arise due to the toroidal structure of the two-dimensional Brillouin zone, which is generally applicable to two-dimensional periodic lattice system. Our research introduces new degrees of freedom for controlling topological optical transport and deepens the understanding of Chern insulators with inversion symmetry.

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