arXiv subjects

Qing-Feng Sun

Publications and source records attributed to Qing-Feng Sun.

At least 19 recordsLinked to original sources

arXiv2026

Altermagnetism-induced higher-order topological Dirac semimetals in two-dimensional nonsymmorphic systems

We propose an altermagnetism-induced route to a two-dimensional higher-order topological Dirac semimetal in a trilayer nonsymmorphic Dirac system. Interlayer hybridization preserves the nonsymmorphic bulk Dirac crossings and generates helical edge states in the trilayer phase. Introducing outer-layer $d$-wave altermagnetic order gaps the helical edge states, while only the symmetry-protected Dirac crossing at high-symmetry point $M$ remains gapless. The resulting phase hosts a surviving bulk Dirac point, gapped edge spectra, and corner states. An exact layer-space decomposition further reveals that these features arise from the coexistence of a gapless Dirac sector and a gapped $\mathbb{Z}_2$ topological sector. Our results establish altermagnetism as an effective mechanism for engineering higher-order topological Dirac semimetals in two-dimensional nonsymmorphic systems.

cond-mat.mes-hall↗

arXiv2026

Strain-tunable electronic transport in disordered borophene nanoribbons with line defects

Resonant tunneling intrinsically exists in disordered borophene nanoribbons (BNRs) with randomly distributed line defects (LDs), providing a versatile platform for transport modulation. Here, we theoretically investigate the effects of in-plane mechanical strain on the electronic transport properties of disordered BNRs coupled to metallic electrodes. Our results demonstrate that the resonant peaks in disordered BNRs of different widths remain robust against variations in electrode parameters and can be efficiently modulated via mechanical strain. Specifically, uniaxial strain significantly modulates the energies, widths, and amplitudes of these resonant peaks, and can even suppress the peaks selectively and induce new ones. In particular, the response of the resonant energies to the applied strain exhibits remarkable anisotropy, where strains along the longitudinal and transverse directions induce opposite energy shifts. Moreover, identical resonant peaks recurring in different nanoribbon widths respond uniformly to applied strain. Biaxial strain enables highly efficient conductance switching across multiple distinct levels, with the modulation frequency increasing for wider nanoribbons. Our findings establish a promising defect-strain-engineering strategy for manipulating electronic transport in borophene and other gapless materials.

cond-mat.mes-hall↗

arXiv2026

Spin-to-charge conversion driven by inverse chiral-induced spin selectivity

Chiral molecules have attracted significant multidisciplinary interest and extensive research owing to their remarkable ability to achieve charge-to-spin conversion, known as the chiral-induced spin selectivity (CISS). A recent experiment has revealed that chiral molecules also exhibit an unexpected capability for spin-to-charge conversion, referred to as the inverse CISS (ICISS), opening unprecedented avenues for the study and application of chiral molecules. Here, we propose a theoretical model, suggesting that ICISS can be understood in terms of spin-dependent electron deflection induced by the interplay between spin and chiral structure. Our numerical results are consistent with experimental observations, demonstrating that ICISS persists under strong disorder. Our model also reproduces the inverse spin Hall effect (ISHE) in this experiment. Comparative analysis indicates that ICISS exhibits a spin-to-charge conversion behavior that differs from ISHE. Our work develops a microscopic theoretical model that accounts for the experimentally observed phenomena, and may provide a useful perspective for organic spintronics.

physics.chem-ph↗

arXiv2026

Anti-higher-order topological insulators

Duality is a fundamental concept in physics that connects complementary opposites like particles and holes. Similarly, while topological states in topological insulators localize at boundaries, the existence and nature of their dual counterparts remain unexplored. Here, we introduce anti-topological states as the dual of topological states, exemplified by anti-higher-order topological insulators. Unlike higher-order topological insulators, where states localize at corners, anti-higher-order topological insulators host states along edges but absent at corners, realizing an inverted distribution of states. We demonstrate this phenomenon in a bilayer Chern insulator with opposite Chern numbers, where the band inversion surfaces enclose distinct high-symmetry points. The topological invariant distinguishing these phases is given by the topological charges enclosed by band inversion surfaces. This work establishes anti-topology as a new paradigm, opening a chapter in topological research.

cond-mat.mes-hall↗

arXiv2026

Anti-higher-order Weyl semimetal

Higher-order topology extends the bulk-boundary correspondence by enabling corner or hinge localized states. Here we identify an anti-higher-order Weyl semimetal, a three dimensional topological phase in which the conventional boundary hierarchy is reversed. Unlike conventional higher-order Weyl semimetals where bulk topology enforces hinge Fermi arcs, this phase hosts anti-hinge states, meaning the bulk topology forces states to vanish at specific hinge orientations. Using a minimal two-band model, we show how Weyl points separate the Brillouin zone into quantum anomalous Hall and anti-higher-order topological insulating regions, with the latter characterized by a band-inversion surface enclosing two distinct high-symmetry points carrying opposite topological charges. Analytical solutions for Weyl points, Berry curvature monopoles, and slice Chern numbers are derived, with numerical simulations confirming the resulting anti-hinge behavior. Our work establishes anti-higher-order topology as a dual counterpart to conventional higher-order phenomena, further extending the exploration of topological phases.

cond-mat.mes-hall↗

arXiv2026

Spin-textured orbitals in altermagnetic artificial atoms

Artificial atoms provide a versatile platform for engineering atomic-like orbitals, yet spin generally remains a passive degree of freedom in their orbital structure. Here, we introduce the concept of altermagnetic artificial atoms formed by confining electrons with momentum-dependent spin splitting. We show that altermagnetism reconstructs conventional confined orbitals into spin-textured orbitals, with spatially distinct distributions of opposite spin components. The resulting confined spectrum retains a twofold degeneracy protected by the combined $C_{4z}\mathcal{T}$ symmetry. These spin textures persist in higher-energy states, where additional radial structures combine with the characteristic angular spin pattern. Furthermore, strain resolves the degenerate orbital pairs into spin-polarized states, and continuously tunes their energy splitting. Our results establish altermagnetic artificial atoms as a route to engineering spin-dependent orbital structures in quantum-confined systems.

cond-mat.mes-hall↗

arXiv2026

Quantum anomalous Hall effect with tunable Chern numbers induced by d-wave sublattice-staggered altermagnetism

We construct a minimal spinful tight-binding model on a square lattice, where a $d$-wave sublattice-staggered altermagnetism drives the quantum anomalous Hall effect. Here the exchange field is staggered between the two sublattices, where it takes opposite signs on $A$ and $B$ described by the Pauli matrix $τ_z$. The resulting insulating phases host tunable Chern numbers $\mathcal{C}=\pm1$ and $\mathcal{C}=\pm2$, controlled by the staggered exchange strength and the sublattice-staggered potential. We determine the complete phase diagram, identify valley-resolved band inversions at the $X$ and $Y$ points in the Brillouin zone, and demonstrate chiral edge states together with quantized two-terminal conductance plateaus. Our work provides a simple route to realizing the quantum anomalous Hall effect in compensated magnets via a $d$-wave sublattice-staggered altermagnetism.

cond-mat.mes-hall↗

arXiv2026

Second-order topological insulator induced by compensated altermagnetism without bulk spin splitting

We theoretically demonstrate a second-order topological insulating phase induced by compensated altermagnetism, while keeping the bulk gap unchanged, in a two-dimensional topological insulator film. By introducing a layer-resolved out-of-plane $d$-wave altermagnetic term with opposite signs on the top and bottom layers, the system preserves $\mathcal{PT}$ symmetry and maintains spin degeneracy in the bulk bands, while simultaneously gapping the helical edge states and generating localized corner states. The resulting higher-order phase is characterized by nonzero mirror-graded winding numbers, and an effective edge theory shows that the corner states arise from Dirac mass domain walls. We further determine the phase boundaries analytically and construct the corresponding topological phase diagram, establishing a robust route to higher-order topology without bulk spin splitting.

cond-mat.mes-hall↗

arXiv2026

Engineering Two-Dimensional Hybrid-Order Topological Insulators via Trilayer Coupling

We propose an interlayer-engineering scheme to realize a two-dimensional hybrid-order topological insulator, characterized by the coexistence of first-order and second-order topological phases, in a coupled trilayer Chern system. Starting from three quantum anomalous Hall layers with Chern numbers $\mathcal{C}_{1/2/3}=+1/-1/+1$ in the decoupled limit, interlayer tunneling hybridizes their edge states into a single chiral edge mode, while simultaneously opening a gap that supports corner states. Consequently, the system exhibits the coexistence of one-dimensional chiral edge states and zero-dimensional corner states within the same bulk gap, a hallmark of the hybrid-order topology. Furthermore, we map out the topological phase diagram, and show that the hybrid-order phase is robust against mass-type disorder. Our results identify interlayer hybridization as a minimal and broadly applicable strategy for engineering coexisting edge and corner states within a topological platform.

cond-mat.mes-hall↗

arXiv2026

Unconventional Spin Valve Based on Normal Metal/Chiral Molecule/Altermagnet Junctions

Chiral molecules have attracted broad interdisciplinary interest for their ability to produce highly spin-polarized current. This phenomenon, known as the chiral-induced spin selectivity effect, holds great potential in the field of spintronics. Here, we propose to combine chiral molecules with altermagnets to construct highly efficient and tunable spin valves. Using the nonequilibrium Green's function method and the Landauer-Büttiker formula, we obtain the conductance and the magnetoresistance of a normal metal/chiral molecule/altermagnet spin valve. Our theoretical results reveal that the conductance of the spin valve can be effectively tuned by reorienting the Néel vector of the altermagnet, and the magnetoresistance of the spin valve increases with molecular length and altermagnetic anisotropy. Moreover, the magnetoresistance vanishes for achiral molecules or in the absence of molecular spin-orbit coupling. Our work paves the way for developing efficient, controllable, and stray-field-free spintronic devices.

physics.chem-ph↗

arXiv2026

Josephson effects in the spin-triplet superconductor/altermagnet/spin-triplet superconductor junctions: the detection of the intrinsic $\bf{d}$-vector

We study the Josephson effects in the spin-triplet superconductor/altermagnet/spin-triplet superconductor junctions using the Green's function method. It is found that the current-phase difference relationships in the junctions strongly depend on the direction of the $\bf{d}$-vectors in the spin-triplet superconductors and the orientation angle of the altermagnet. For the given orientation angle, the $0$-$π$ transition can be obtained when the $\bf{d}$-vector is rotated. The variations of the critical current of the junctions with the direction of the $\bf{d}$-vector, the orientation angle and the strength of altermagnetism are systematically investigated. These Josephson effects can provide the distinguishable information about the direction of the $\bf{d}$-vector. Compared to the existing research, the proposed altermagnetic Josephson junctions can effectively avoid the negative influence of the magnetic field on the $\bf{d}$-vector and can serve as a feasible scheme for the detection of the intrinsic $\bf{d}$-vector. The obtained $0$-$π$ transition in the junctions can also have potential applications in the design of quantum devices.

cond-mat.supr-con↗

arXiv2026

Current-induced re-entrant superconductivity and extreme nonreciprocal superconducting diode effect in valley-polarized systems

The superconducting diode effect (SDE) refers to the nonreciprocity of superconducting critical currents. Generally, the SDE has a positive and a negative critical currents jc+- corresponding to two opposite directions with unequal amplitudes. It is demonstrated that an extreme nonreciprocity where two critical currents can become both positive (or negative) has been observed in twisted graphene systems. In this work, we theoretically propose a possible mechanism to realize an extreme nonreciprocal SDE. Based on a simple microscopic model, we demonstrate that depairing currents required to dissolve Cooper pairs can be remodulated under the interplay between valley polarizations and applied currents. Near the disappearance of the superconductivity, the remodulation is shown to induce extreme nonreciprocity and also the current-induced re-entrant superconductivity where the system has two different critical current intervals. Our study may provide new horizons for understanding the coexistence of superconductivity and spontaneous valley polarizations, and pave a way for designing SDE with 100% efficiency

cond-mat.supr-con↗

arXiv2026

Majorana modes in helical altermagnet without net magnetism and spin-orbit coupling

We propose a scheme to realize topological superconductor and Majorana bound states (MBSs) in a one-dimensional metal nanowire on the surface of a helical altermagnet and in proximity to an s-wave superconductor, removing the requirement of conventional spin-orbit coupling and net magnetism. Through gauge transformation, we demonstrate that the helical frame naturally induces spin-momentum locking while the altermagnetism breaks time-reversal symmetry. The topological superconducting phase is well tuned by chemical potential, altermagnet strength, and helical frequency. Besides, our transport calculation results reveal quantized conductance signatures: a 2e2/h zero-bias peak at nanowire ends and a 4e2/h tunneling conductance at the domain wall of nanowires with opposite chirality, detected via metal lead and scanning tunneling microscopy, respectively. Our research offers new perspectives on finding MBSs.

cond-mat.supr-con↗

arXiv2026

Spin transport in a normal meta-altermagnetic superconducting nanowire junction

Spin triplet superconductors are considered a promising platform for dissipationless spin transport, where spin currents are carried by spin triplet Cooper pairs. In this paper, we propose that the spin triplet superconductivity and spin supercurrent can be engineered in an altermagnetic superconducting nanowire, where a one-dimensional nanowire is placed on the surface of an s-wave superconductor and in proximity to the altermagnet. Using the nonequilibrium Green's function method, we demonstrate a nonzero equal spin Andreev reflection coefficient at the normal metal-altermagnetic superconducting nanowire interface, thereby verifying the injection of spin triplet Cooper pairs. Furthermore, we systematically investigate the spin transport properties in this hybrid system under a spin bias. Our results demonstrate that these properties can be effectively tuned by the chemical potential and spin bias orientation. Our proposal provides a pathway toward realizing dissipationless spin transport.

cond-mat.supr-con↗

arXiv2026

Helimagnetic Josephson diode effect

We study the Josephson diode effect in the one-dimensional superconductor/helimagnet/superconductor junctions using the Green's function method. For the spin-singlet $s$-wave pairing in superconductors, it is found that the necessary conditions for the Josephson diode effect are the nonzero chemical potential and the conical magnetic configuration in the helimagnet. The diode efficiency is strongly dependent on the chemical potential, chirality, tilt angle and exchange coupling in the helimagnet. The high efficiency close to $40\%$ can be obtained for specific parameter values. The sign of the diode efficiency can be tuned by changing the chirality, tilt angle, exchange coupling and chemical potential. The dependence of the diode efficiency on the number of supercells in the helimagnet is also investigated. The characteristics of the supercurrent nonreciprocity and diode efficiency in the junctions are clarified through the symmetry analysis and the energy band calculations. The diode effect for the spin-triplet $p$-wave pairing in superconductors is also discussed and the nonzero chemical potential is no longer a necessary condition for the Josephson diode effect due to the equal-spin Cooper pair-mediated transport in the $p$-wave junctions. These results provide a scheme for the Josephson diode effect without spin-orbit coupling, which possesses the potential applications in the design of dissipationless electronic devices.

cond-mat.supr-con↗

arXiv2026

Quantum anomalous Hall effect in chiral semimetals

The quantum anomalous Hall (QAH) effect is conventionally understood to exist only in Chern insulators, while a recent study has shown that ferromagnetic metals can also host the QAH effect. Between insulators and metals, we demonstrate that QAH can persist even in a chiral semimetal, where conduction and valence bands touch at zero energy. Transport calculations demonstrate that the Hall conductivity of such a system can be quantized in the presence of dephasing. Interestingly, its longitudinal conductivity remains finite and exhibits semimetallic behavior, in contrast to Chern insulators. This unusual transport behavior originates from the quantization of the Berry curvature integral over occupied states and the semimetallic band structure. This chiral semimetal can transition into a Chern insulator, accompanied by the vanishing of longitudinal conductivity and a reduction of the intrinsic length scale of the Hall response. Our results extend the concept of QAH and uncover the semimetallic QAH transport signatures.

cond-mat.mes-hall↗

arXiv2026

Robust quantized thermal conductance of Majorana floating edge bands in d-wave superconductors

We propose and characterize a new class of Majorana boundary states, i.e., floating Majorana edge bands (FMEBs), which emerge in two-dimensional (2D) superconductors that break time-reversal symmetry yet host helical-like transport. In contrast to conventional chiral or helical edge modes, FMEBs form isolated, momentum-separated counterpropagating Majorana modes detached from the bulk continuum. We identify a minimal mechanism for their emergence via anisotropic Wilson masses in a two-band Bogoliubov-de Gennes (BdG) model, and demonstrate their microscopic realization in a quantum anomalous Hall (QAH) insulator proximitized by a $d$-wave superconductor. Using nonequilibrium Green's function (NEGF) simulations, we uncover clear transport fingerprints: a quantized total thermal conductance in two-terminal devices, and a robust half-quantized plateau in four-terminal geometries that cleanly distinguishes FMEBs from chiral $\mathcal{N}= \pm 2$ QAH phases. This thermal response remains remarkably stable under finite temperature, moderate long-range disorder, and finite chemical potential. Our findings establish FMEBs as an experimentally accessible route toward helical-like Majorana transport in systems without time-reversal symmetry, with direct implications for topological quantum computation.

cond-mat.mes-hall↗

arXiv2026

Engineering chiral-induced spin selectivity in an artificial topological quantum well

Chiral-induced spin selectivity (CISS) is a striking phenomenon in which spin-unpolarized electrons become spin-polarized after traversing a chiral medium. Theoretical studies have shown that spin-orbit coupling, geometric chirality, and dephasing act cooperatively for this effect to emerge. Inspired by this, we demonstrate a solid-state realization of CISS in an engineered InAs/GaSb quantum well where geometric chirality and dephasing can be introduced controllably. Introducing a chiral structure produces a clear spin polarization whose sign reverses when the chirality is flipped, and whose magnitude grows systematically with the number of dephasing electrodes, while achiral configurations exhibit no spin selectivity. The polarization remains robust even under strong Anderson disorder, showing that the engineered chiral structures provides an intrinsically stable route to spin-selective transport. These results establish a solid-state platform in the topological quantum well system for controllably generating the CISS effect.

cond-mat.mes-hall↗