arXiv subjects

Qing-Feng Sun

Publications and source records attributed to Qing-Feng Sun.

At least 37 records · Page 2Linked to original sources

arXiv2025

Helical Fermi Arc in Altermagnetic Weyl Semimetal

We investigate the topological properties of modified Dirac Hamiltonians with an altermagnetic mass term and reveal a novel mechanism for realizing altermagnetic Weyl semimetals. Unlike the conventional Wilson mass, the altermagnetic mass drives direct transitions between nontrivial Chern phases of opposite sign and fundamentally reshapes the band inversion surface. By extending this framework to three dimensions, we construct a minimal lattice model that hosts pairs of Weyl nodes as well as coexisting helical Fermi arcs with opposite chirality on the same surface, which is a phenomenon not found in conventional magnetic Weyl semimetals. We further propose a practical scheme to realize these phases in multilayer structures of 2-dimensional Rashba metal with engineered $d$-wave altermagnetic order. Our results deepen the theoretical understanding of mass terms in Dirac systems and provide concrete guidelines for the experimental detection and realization of altermagnetic Weyl semimetals.

cond-mat.mes-hall↗

arXiv2025

Quantum Anomalous Hall Effect in Ferromagnetic Metals

The quantum anomalous Hall (QAH) effect holds fundamental importance in topological physics and technological promise for electronics. It is generally believed that the QAH effect can only be realized in insulators. In this Letter, we theoretically demonstrate that the QAH effect can also be realized in metallic systems, representing a phase distinct from the conventional QAH phase in insulators. This phase is characterized by the coexistence of chiral edge channels and isotropic bulk conduction channels without a bulk energy gap. Notably, in a six-terminal Hall bar, our calculations show that, the quantized Hall conductivity and nonzero longitudinal conductivity can emerge due to dephasing, despite the Hall resistivity itself never becoming quantized. Furthermore, the quantized Hall conductivity exhibits remarkable robustness against disorder. Our findings not only extend the range of materials capable of hosting the QAH effect from insulators to metals, but also provide insights that may pave the way for the experimental realization of the QAH effect at elevated temperatures.

cond-mat.mes-hall↗

arXiv2025

Characterizing second-order topological insulators via entanglement topological invariant in two-dimensional systems

Higher-order topological insulators have attracted significant interest in recent years. However, identifying a universal topological invariant capable of characterizing higher-order topology remains challenging. Here, we propose a entanglement topological invariant designed to characterize secondorder topological systems. This entanglement topological invariant captures the entanglement of topological corner states under open boundary conditions by employing a bipartite entanglement entropy method. In several representative models, the entanglement topological invariant assumes a nonzero value exclusively in the presence of second-order topology, with its magnitude exactly matching the number of topologically protected corner states. Consequently, the proposed entanglement topological invariant not only provides a clear criterion for detecting higher-order topology, but also offers a quantitative measure for the related corner states. Our study establishes a universal and precise method for characterizing higher-order topological phases, opening avenues for their fundamental understanding and future investigations.

cond-mat.mes-hall↗

arXiv2025

Anomalous Magnetoresistance beyond the Jullière Model for Spin Selectivity in Chiral Molecules

The issue of anomalous high magnetoresistance, beyond the Jullière model, observed in nonmagnetic electrode-chiral molecular-ferromagnetic electrode devices has puzzled the community for a long time. Here, by considering the magnetic proximity effect which shifts the nonmagnetic-ferromagnetic interface toward chiral molecules, we show the anomalous high magnetoresistance beyond the spin polarization in ferromagnetic electrodes even in the very weak spin-orbit coupling. Our results are in excellent agreement with the experiments, demonstrating that the spin-orbit coupling plays a fundamental role in chiral-induced spin selectivity and the magnetic proximity effect can dramatically enhance the magnetoresistance. These results elucidate the interaction between chiral molecules and ferromagnetic electrodes and facilitate the design of chiral-based spintronic devices.

physics.chem-ph↗

arXiv2025

Asymmetric quantum Hall effect and diminished $ν=0$ longitudinal resistance in graphene/InSe heterostructures

We investigate quantum transport in graphene/InSe heterostructures and find major asymmetries in the longitudinal resistance ($R_{xx}$) and vanishing $R_{xx}$ peaks at high magnetic fields, particularly at the charge-neutrality point. Our Landauer-Buttiker analysis and numerical simulations show that a monotonically varying density gradient combined with a full equilibration mechanism can explain these phenomena. Our results also suggest the presence of trivial long-range chiral edge current and offer a broadly applicable way to engineer transport properties in quantum Hall systems.

cond-mat.mes-hall↗

arXiv2025

Construction of Kondo Chains by Engineering Porphyrin π-Radicals on Au(111)

Quantum manipulation of molecular radical spins provides a crucial platform for exploring emergent phenomena in many-body systems. Here, we combine surface-confined synthesis with scanning tunneling microscopy(STM)tip-induced dehydrogenation to achieve atom-precise engineering of quasi-one-dimensional porphyrin-based Kondo chains (1-7 units) on Au(111). High-resolution STS measurements and low-energy effective modeling collectively demonstrate that π-radicals at each fused-porphyrin unit form Kondo singlets screened by conduction electrons. Adjacent singlets develop direct coherent coupling via quantum-state-overlap-enabled electron tunneling. Crucially, chiral symmetry in the effective model governs zero-mode distribution-present in odd-length chains yet absent in even-length chains-which dictates pronounced odd-even quantum effects in STS spectra of finite chains. Furthermore, the number of parallel porphyrin chains non-monotonically tunes the competition between the Kondo effect and spin exchange, showing opposing trends in strength and demonstrating that both wave-function overlap and the SOMO-LUMO gap collectively govern these interactions. This work simultaneously resolves the dimensional dependence of many-body correlations in confined quantum systems and pioneers approaches for quantum-critical manipulation in molecular spin architectures.

cond-mat.mes-hall↗

arXiv2025

Equal-spin and oblique-spin crossed Andreev reflections in ferromagnet/Ising superconductor/ferromagnet junction

We study the subgap transport through a ferromagnet/Ising superconductor/ferromagnet (F/ISC/F) junction by solving the Bogoliubov-de Gennes equations. It is found that the crossed Andreev reflection (CAR) and local Andreev reflection (LAR) strongly depend on the spin polarized F, the magnetization direction, and the Ising superconducting phase. For the same magnetization directions of the two F leads, the equal-spin CAR could take place due to spin-flip mechanism induced by the Ising spin-orbit coupling and equal-spin-triplet pairing. Both equal-spin CAR and equal-spin LAR exhibit a remarkable magnetoanisotropy with period πand show oscillatory behavior with chemical potential. The equal-spin CAR is more prominent for half-metal F and double-band ISC while the normal CAR is completely suppressed. When the magnetization directions of the two F leads are different, the oblique-spin CAR occurs and its magnetoanisotropic period generally becomes 2πinstead of π. In the oblique-spin CAR process, the spins of the electron and hole are neither parallel nor antiparallel. Furthermore, the property of oblique-spin CAR is very sensitive to the spin and valley degrees of freedom. The spin and valley polarized CAR can be achieved and controlled by the chemical potentials and the magnetization directions.

cond-mat.supr-con↗

arXiv2025

Cone-dependent retro and specular Andreev reflections in AA-stacked bilayer graphene

We theoretically study the Andreev reflection (AR) in AA-stacked bilayer graphene/superconductor (AABG/SC) junction. AABG has a linear gapless energy band with two shifted Dirac cones and the electronic states are described by the cone indices. The results indicate that the property of AR strongly depends on the cone degree of freedom. In the absence of the inter-layer potential difference, only intra-cone AR and normal reflection (NR) could occur, and the inter-cone process is forbidden. By adjusting the potential, the intra-cone AR can be specular AR (SAR) in one cone and it is retro-AR (RAR) in the other cone. The existence of the inter-layer potential difference would lead to the inter-cone scattering. As a result, double ARs and double NRs can take place between the two cones. The inter-cone SAR could happen in a broad potential region. Furthermore, the inter-cone retro-NR (RNR) could happen as well. The switch between SAR and RAR, and the switch between specular NR (SNR) and RNR can be achieved by regulating the potential. Therefore, different cone carriers can be separated spatially based on the RAR and SAR. The cone-dependent Andreev conductance may be separately measured near the critical values where RAR crosses over to SAR.

cond-mat.mes-hall↗

arXiv2025

Superconducting lens and Josephson effect in AA-stacked bilayer graphene

We study the superconducting transport phenomena, involving lensing effect and supercurrent in AA-stacked bilayer graphene, which is characterized by a linear gapless band with two shifted Dirac cones. Our findings indicate that cross Andreev reflection and Josephson current occur exclusively within the intracone process, while intercone scatterings are strictly prohibited. The normal/superconductor/normal junction can act as a superconducting lens for the upper and lower cones. Depending on cone index, the transmitted electrons and holes can be focused, collimated or diverged by adjusting the gate voltages. In superconductor/normal/superconductor junction, due to interlayer coupling, the critical currents of the two cones exhibit distinct oscillation periods with junction width, leading to an irregular oscillation of the total critical current. Furthermore, the oscillations of critical currents with exchange field maintain a stable phase difference of one quarter period between the two cones. Consequently, a cone-dependent 0-πtransition is achieved in this Josephson junction.

cond-mat.supr-con↗

arXiv2025

Antiferromagnetic superlattices: anisotropic band and spin-valley valve in buckled two-dimensional materials

Antiferromagnetic superlattices (AFSL) are proposed based on the buckled hexagonal two-dimensional materials, which can be realized by the proximity effect of the periodically deposited antiferromagnets. It is found that the AF proximity effect can give rise to valley-polarized minibands and conductance, which are not held under ferromagnetic proximity. The spin degeneracy and valley degeneracy are lifted simultaneously in the presence of AF proximity and electric field. In consequence, both minibands and conductance could be spin-valley polarized completely in AFSL. The symmetry of spin-valley polarization is analysed by considering the pseudospin rotation operations and spatial inversion operations. Furthermore, AFSL also induce a highly anisotropic band structure due to the spin-orbit coupling (SOC). In particular, the group velocity parallel to the periodic direction of AFSL is greatly renormalized, while the velocity perpendicular to the periodic direction remains unaffected, contrary to that observed in graphene superlattices. With the increase of SOC, the anisotropy becomes more prominent, leading to flattened band and electron supercollimation. The direction of anisotropy can be regulated by adjusting the potential and SOC. These findings offer an alternative approach to engineering anisotropic two-dimensional materials. As an application, the AFSL may well work as a symmetry-protected spin-valley valve easily controlled by the gate voltages.

cond-mat.mes-hall↗

arXiv2025

Topologically protected magnetoresistance by quantum anomalous Hall effect

Recently, antiferromagnetic (AF) materials have attracted rapid attention, because they are considered as outstanding candidates to replace the widely used ferromagnets in the next generation of spintronics. We propose a magnetoresistance model based on the quantum anomalous Hall effect in an AF system, which is protected by the topological Chern number. By regulating the AF exchange field and an electric field, the system can be controlled between the quantum spin Hall insulator (QSHI) phases and the quantum anomalous Hall insulator (QAHI) phases. As a result, a QAHI/QSHI/QAHI junction can be formed. In the QAHI region, the spin orientation of chiral edge state can be manipulated by tuning the AF exchange field and the electric field. Therefore, the spin directions of two QAHIs in the junction can have parallel and antiparallel configurations. The conductances of two configurations offered by chiral edge states are significantly different, and this is a magnetoresistance effect that can be electrically controlled. Because of the topological invariance, the magnetoresistance plateaus are robust to the size effect and the disorder.

cond-mat.mes-hall↗

arXiv2025

Dynamical simulation of chiral induced spin-polarization and magnetization

Despite generally lacking ferromagnetic properties or strong spin-orbit coupling, electrons in chiral molecules exhibit unique spin-dependent transport behavior, known as chiral-induced spin selectivity (CISS). This phenomenon implies a profound connection between chirality and spin, and draws attention to the link between chirality and magnetism. Experiments in recent years have shown that chirality can induce spin-polarizations and magnetizations, providing fresh insights into interpreting chirality-related biochemical processes and designing nano-magnetic devices. In this paper, we present a dynamical theoretical model aimed at elucidating how charge-polarization combined with the CISS leads to spin-polarization and magnetization. Our theoretical model successfully explains the spin-polarization and magnetization observed in three types of experiments, where the charge-polarization is induced in the chiral molecules by the dispersion interaction, gate voltage, and molecular adsorption. The model simulates a clear time evolution process and provides a comprehensive theoretical framework for this field.

physics.chem-ph↗

arXiv2025

Spin-to-charge conversion modulated by chiral molecules

Molecular chirality and electron spin are intricately intertwined via the fascinating phenomenon of chiral-induced spin selectivity (CISS), which has garnered considerable attention due to its extensive potential applications. A recent experiment has revealed that chiral molecules self-assembled on the gold surface can modulate the inverse spin Hall effect, providing an alternative platform for studying the interplay between chirality and spin transport. Our study uncovers that this modulation stems from the CISS effect, which enhances spin currents of one spin orientation while suppressing those of the opposite spin orientation. We provide numerical results that are highly consistent with the experimental phenomena and further investigate the influence of various factors on this modulation. This work offers a theoretical explanation of previously unexplained experimental findings, and the underlying physical mechanism broadens current perspectives on understanding and applying CISS.

physics.chem-ph↗

arXiv2025

Orbital hybridization in graphene-based artificial atoms

Intraatomic orbital hybridization and interatomic bond formation are the two fundamental processes when real atoms are condensed to form matter. Artificial atoms mimic real atoms by demonstrating discrete energy levels attributable to quantum confinement. As such, they offer a solid-state analogue for simulating intraatomic orbital hybridization and interatomic bond formation. Signatures of interatomic bond formation has been extensively observed in various artificial atoms. However, direct evidence of the intraatomic orbital hybridization in the artificial atoms remains to be experimentally demonstrated. Here we, for the first time, realize the orbital hybridization in artificial atoms by altering the shape of the artificial atoms. The anisotropy of the confining potential gives rise to the hybridization between quasibound states with different orbital quantum numbers within the artificial atom. These hybridized orbits are directly visualized in real space in our experiment and are well reproduced by both numerical calculations and analytical derivations. Our study opens an avenue for designing artificial matter that cannot be accessed on real atoms through experiments. Moreover, the results obtained inspire the progressive control of quantum states in diverse systems.

cond-mat.mes-hall↗

arXiv2025

Atomic collapse of high-order singular potentials in graphene

Artificial atoms in graphene hosting a series of quasi-bound states can serve as an excellent platform to explore atomic collapse and become a basis to design novel graphene nanodevices. We theoretically study behaviors of massless Dirac fermions in singular potentials with a general form of 1/r^γ. Different from the Coulomb potential that demands a supercritical charge Z > Zc, a high-order singular potential (γ > 1) is found to in principle induce atomic collapse with an infinitesimal charge Z. The energies of atomic collapse states (ACSs) within these potentials are arranged roughly as a power sequence. We also show that some special ACSs can exist even above the bulk Dirac point, which cannot appear in the Coulomb potential. These findings uncover the anomalies of massless Dirac fermions in diverse charge potentials and provide guidance for further experiments and graphene nanodevice applications.

cond-mat.mes-hall↗

arXiv2025

Band bending and zero-conductance resonances controlled by edge electric fields in zigzag silicene nanoribbons

We study the band structure and transport property of a zigzag silicene nanoribbon when the electric fields are applied to the edges. It is found that a band bending could be induced and controlled by the antisymmetric edge fields, which can be understood based on the wave functions of the edge states. The highest valence band and the lowest conduction band coexist in the band bending region. With the narrowing of edge potentials, the bending increases gradually. When the edge fields become symmetric, an asymmetric band gap at the Dirac points can be obtained due to the intrinsic spin-orbit interaction, suggesting a valley polarized quantum spin Hall state. The gap could reach a maximum value rapidly and then decrease slowly as the electric fields increase. Due to the combining effect of the band bending, band selective rule, and resonant states, many zero-conductance resonances and resonance peaks appear in different regions, which could be described by the Fano resonance effect. Furthermore, the band bending and zero-conductance resonances are robust against the Hubbard interaction. The Hubbard interaction could work as a spin-dependent edge field, together with the edge electric fields, leading to a spin-dependent band gap and various quantum phases such as metal and half-metal.

cond-mat.mes-hall↗

arXiv2025

Electrical control of crossed Andreev reflection and spin-valley switch in antiferromagnet/superconductor junctions

We study the subgap transport through the antiferromagnet/superconductor (AF/S) and antiferromagnet/superconductor/antiferromagnet (AF/S/AF) junctions controlled by electric field in a generic buckled honeycomb system, such as silicene, germanene, and stanene. In the present of electric field and antiferromagnetic exchange field, the spin-valley polarized half metallic phase can be achieved in the honeycomb system due to the spin-orbit coupling, which affords an opportunity to generate the pure crossed Andreev reflection (CAR). It is found that the pure CAR can be generated without local Andreev reflection (AR) and elastic cotunneling (EC) over a wide range of electric field. A spin-valley switch effect can be realized between the pure CAR and the pure EC by adjusting the electric field. The properties of AR process and CAR process strongly depend on the spin-valley polarized states. Our results suggest that the device can implement an electrical measurement of the CAR process and spin-valley switch.

cond-mat.mes-hall↗

arXiv2025

Spin-valley polarized edge states and quantum anomalous Hall states controlled by side potential in 2D honeycomb lattices

Based on the tight-binding formalism, we study the effect of side potential on the spin and valley related electronic property of $2$D honeycomb lattices with intrinsic spin-orbit coupling, such as silicene and germanene. The side potential is composed of potential field and exchange field applied on the boundaries of the zigzag nanoribbon. It is found that the side potential could greatly affect the helical edge states with different spin indices and the spin and valley are locked to each other. By adjusting the side potential and ribbon width, the system shows quantum spin-valley Hall effect, valley polarized quantum spin Hall effect, and spin polarized quantum anomalous Hall effect. Due to the side potential and the coupling of edge states in narrow ribbon, a band gap could be opened for specific spin and the time-reversal symmetry could be broken, leading to a spin polarized quantum anomalous Hall phase. Various kinds of spin-valley polarized edge states are formed at the two boundaries. Furthermore, the spin-valley polarized insulating states can be used to realize a perfect spin-valley switch.

cond-mat.mes-hall↗