arXiv subjects

Chang-An Li

Publications and source records attributed to Chang-An Li.

At least 19 recordsLinked to original sources

arXiv2026

Phase Transitions in Disordered Altermagnetic Josephson Junctions

Altermagnetic Josephson junctions (AMJJs) can host unconventional $π$ phase and $φ$ phase despite vanishing net magnetizations. Whether these phases are stable against disorder existing in real materials remains an open question. Here, we investigate impact of disorder on exotic phases in two-dimensional AMJJs consisting of two conventional superconductors mediated by a $d$-wave altermagnet. We show that disorder is able to drive phase transitions from the exotic $π$ phase to conventional $0$ phase, accompanied by a substantial suppression of critical current. This behavior is attributed to modifications of the tunneling Cooper-pair phase shift and superconducting decoherence. Remarkably, the anomalous $φ$ phase is highly fragile in presence of disorder and can be driven to either a $π$ phase or $0$ phase in a nonreciprocal way. Across such transitions, the first harmonic of current-phase relation changes its sign, while the higher-order harmonics are rapidly suppressed. Our findings reveal the crucial role of disorder in tailoring distinct phases of AMJJs and shed new light on their potential functionalities.

cond-mat.supr-con↗

arXiv2026

Altermagnetism Induced Bogoliubov Fermi Surfaces Form Topological Superconductivity

We propose a novel type of topological superconductivity based on Bogoliubov Fermi surfaces (BFSs) in an altermagnetic topological insulator proximitized by an s-wave superconductor. The 3D altermagnetic topological insulator is characterized by zero-energy surface states in bulk nodal-ring phases and anisotropically shifted surface Dirac cones in topological insulating phases. The altermagnetic order in combination with superconductivity gives rise to highly anisotropic superconducting gaps with crystal-facet-dependent BFSs at the physical boundaries. These particular BFSs provide distinct platforms to realize topological superconductivity. We propose a quasi-1D nanowire in which the anisotropic BFSs experience topological phase transitions due to quantum confinement leading to Majorana zero modes (MZMs) at its ends. We further consider vortex phase transitions in the superconducting altermagnetic topological insulators. Remarkably, we find that the altermagnetic order allows us to transit between two distinct type of MZMs, one type is located at the vortex line, while the other type is located at the physical boundaries. Our work paves a new avenue utilizing altermagnetism-induced BFSs to engineer topological superconductivity through crystal anisotropy and quantum confinement.

cond-mat.supr-con↗

arXiv2026

Hidden topology and strong quantum metric bounds in trivial systems

The quantum metric integral (QMI) in two-dimensional (2D) systems is conventionally bounded from below by the Chern number. For systems with zero Chern number or identically vanishing Berry curvature, however, this bound becomes trivial and provides no useful geometric constraints. Here, we develop a dimension-reduction framework that decomposes the 2D QMI into lower-dimensional components in a nested-loop way. With this method, we establish a nonzero lower bound on the QMI arising from one-dimensional topological obstructions even when the conventional 2D topology is trivial. We explicitly demonstrate this mechanism in a tilted 2D Su-Schrieffer-Heeger model and an anisotropic Wilson-Dirac model with chiral symmetry. The resulting lower bounds of QMI are determined by the quantized Wannier bands along two different directions. We further investigate the quantum geometry in higher-order topological phases following the same strategy. By introducing Wannier-band basis obtained from the nested Wilson loop, we demonstrate that the Wannier-band QMI is bounded from below by the higher-order topological invariant, e.g. the quadrupole moment in Benalcazar-Bernevig-Hughes model. Our results establish nonzero lower bounds on QMI from a dimension-reduction framework, thereby generalizing the fundamental relation between quantum geometry and topology.

cond-mat.mes-hall↗

arXiv2026

First-Order Topological FFLO Transition and Superconducting Diode Sign Reversal in Altermagnetic Nanowires

Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state conventionally emerges via a second-order phase transition driven by finite magnetization. Here we show that a spin-orbit-coupled nanowire proximitized to $d$-wave altermagnets -- with zero net magnetization -- can realize topological FFLO states through a first-order transition, marked by a sharp sign-reversing superconducting diode effect. The altermagnetic field generates band-resolved competing pairing channels, giving rise to a double-valley free energy landscape whose global minimum switches discontinuously. It consequently leads to a first-order topological FFLO transition with simultaneous jumps in the Cooper pairing amplitude and finite center-of-mass momentum. Remarkably, this discontinuous topological reconfiguration substantially enhances the diode efficiency and drives a characteristic sharp sign reversal across the transition. The mechanism of such exotic phenomena is captured by Ginzburg--Landau theory. Our results provide a field-free altermagnetic route to topological FFLO states and identify their direct transport fingerprint.

cond-mat.supr-con↗

arXiv2026

Regularized universal topological markers for Dirac systems

Topological markers provide an efficient and powerful characterization of topological features of many systems, especially when the translation symmetry is broken. Recently, a universal topological marker applicable in different symmetry classes of topological systems is proposed. However, it suffers from irregular behaviors at the boundary and its connection to other topological indexes remains elusive. In this work, we construct regularized universal topological markers that apply to Dirac systems by utilizing position operators that are compatible with periodic boundary conditions. The regularized markers eliminate the obstructive boundary irregularities successfully and give rise to the desired global topological invariants, such as the Chern number, consistently when integrated over all the lattice sites. Furthermore, the regularized form allows us to establish an explicit connection between the markers and some other known topological indices in two dimensions. For instance, it turns out to be equivalent to the Bott index in classes A, D, and C and equivalent to the spin Chern number in classes DIII and AII. We further examine the utility and stability of this marker in disordered scenarios. We find that its variance shows peaks at the phase boundaries, which promotes it as a useful indicator for detecting disorder-induced topological phase transitions.

cond-mat.other↗

arXiv2026

Altermagnetism and its induced higher-order topology on the Lieb lattice

Altermagnetism (AM) has brought renewed attention to the Lieb lattice. Here, we broaden the scope of altermagnetic models on the Lieb lattice by using a general scheme based on spin clusters. We design various altermagnetic models with d- and g-wave on the Lieb lattice, and investigate its interplay with spin-orbit coupling. While the altermagnetic unit cell reconstructs the topological edge states in the strip geometry and leads to the emergence of Dirac points, the in-plane magnetic moments of AM can induce gaps at these points. In an open square geometry, corner modes emerge within these gaps, realizing higher-order topological states. We further verify that the induction of higher-order topology is applicable to all altermagnetic configurations constructed here on the Lieb lattice, and is most pronounced for AM by comparing with the other types of magnetism such as ferromagnetism and ferrimagnetism. Our results highlight the exotic properties of AM, and suggest its potential applications in engineering topological quantum states.

cond-mat.str-el↗

arXiv2026

Marginal Metals and Kosterlitz-Thouless Type Phase Transition in Disordered Altermagnets

Altermagnetism, a recently discovered magnetic phase characterized by spin-split bands without net magnetization, has emerged as promising platform for novel physics and potential applications. However, its stability against disorder-ubiquitous in real materials-remains poorly understood. Here, we study the electron localization properties of two-dimensional $d$-wave altermagnets subject to disorder. Remarkably, we discover a disorder-driven phase transition from a marginal metallic phase to an insulator, which falls into the Kosterlitz-Thouless class. We demonstrate this by strong numerical evidence and propose an interpretation in terms of vortex-antivortex pairs in the disorder-induced local in-plane spin magnetization. Moreover, we show that the characteristic spin anisotropy of altermagnets persists but gradually fades away across the transition. These changes directly affect the spin splitting features that are detectable in angle-resolved photoemission spectroscopy and tunneling magnetoconductance. Our findings provide a new perspective on recent experimental observations of altermagnetism in candidate materials.

cond-mat.mes-hall↗

arXiv2025

Exceptional Andreev spectrum and supercurrent in p-wave non-Hermitian Josephson junctions

We investigate the spectrum of Andreev bound states and supercurrent in a $p$-wave non-Hermitian Josephson junction (NHJJ) in one dimension. The studied NHJJ is composed of two topological $p$-wave superconductors connected by a non-Hermitian dissipative junction. Starting from the effective non-Hermitian Bogoliubov-de Gennes bulk Hamiltonian, we find that a pair of exceptional points emerge in the complex spectrum of Andreev quasi-bound states. The two exceptional points with zero energy locate symmetrically with respect to Josephson phase difference $ϕ=π$, at which a Majorana zero mode persists. Notably, the exceptional points descend from a pair of Majorana zero modes after turning on the non-Hermiticity and are topologically protected. By analyzing the non-Hermitian scattering process at the junction, we explicitly demonstrate the loss of quasiparticles through the decay of scattering amplitude probabilities. Furthermore, we obtain the supercurrent directly by the inelastic Andreev reflection amplitudes, which provides a more intuitive interpretation of transport properties in NHJJs. The supercurrent varies continuously as a function of $ϕ$ across the exceptional points. No enhancement of critical current is observed. We also generalize our analysis to a mixed $s$-$p$ wave NHJJ. Our results provide new insights on transport properties of Josephson junctions in presence of Majorana zero modes, exceptional points, and non-Hermiticity.

cond-mat.supr-con↗

arXiv2025

Random-Flux-Induced Transition Sequence between Weak and Strong Topological Phases with Anisotropic Localization Properties

We demonstrate that random flux is able to drive nontrivial topological phase transitions, in particular between weak topological insulators (WTIs) and Chern insulators (CIs), illustrated on an anisotropic Wilson-Dirac model in two dimensions. Remarkably, an intriguing topological transition sequence WTIs$\rightarrow$CIs$\rightarrow$WTIs occurs with the reentrance to a WTI but of different weak topology, which is unattainable with chemical potential disorder. The involvement of anisotropy and weak topology in such a transition gives rise to emergent quasi-critical points, where eigen states are extended in one spatial direction but localized in the other one. This new quantum criticality lies outside the conventional quantum Hall universality class. We provide a comprehensive characterization of the random-flux-induced phase transitions and quantum criticality from both bulk and boundary perspectives. Our results describe a qualitatively new disorder effect based on the interplay of random flux with topological phases of matter.

cond-mat.mes-hall↗

arXiv2025

Tunable second harmornic in altermagnetic Josephson junctions

We study the influence of external electric and Zeeman fields on the Josephson effect in a planar superconductor/altermagnet/superconductor junction. Remarkably, we find that the current-phase relation (CPR) can be forward or backward skewed due to a pronounced second harmonic term. It decisively depends on the altermagnetic field strength. This second harmonic can be measured directly using double SQUID devices. The CPR skewness can be effectively manipulated by electric gating. Moreover, we identify two additional impacts of external electric and magnetic fields on the Josephson current: (i) Fields can induce 0-$π$ transitions. (ii) Fields can substantially enhance the critical current. This enhancement is surprising since supercurrents are typically suppressed by magnetic fields.

cond-mat.supr-con↗

arXiv2024

Inner non-Hermitian skin effect on Bethe lattice

We investigate the non-Hermitian Su-Schrieffer-Heeger (SSH) model on Bethe lattice, revealing a novel localization phenomenon coined inner non-Hermitian skin effect. This effect is featured by the localization of all eigenstates within the bulk of the lattice, diverging from the conventional skin effect observed in general non-Hermitian systems. The analytical treatment of the model demonstrates that the Hamiltonian can be decoupled into a series of one-dimensional chains, with one end fixed at the bottom boundary while the other ends positioned at varying generations within the bulk. This configuration leads to the emergence of the inner non-Hermitian skin effect, which is further validated by performing circuit simulations. Our findings provide new insights into the interplay between non-Hermitian physics and the self-similar structure on Bethe lattice.

cond-mat.mes-hall↗

arXiv2024

Non-Hermitian Quantum Fractals

The first quantum fractal discovered in physics is the Hofstadter butterfly. It stems from large external magnetic fields. We discover instead a new class of non-Hermitian quantum fractals (NHQFs) emerging in coupled Hatano-Nelson models on a tree lattice in absence of any fields. Based on analytic solutions, we are able to rigorously identify the self-similar recursive structures in energy spectrum and wave functions. We prove that the complex spectrum of NHQFs bears a resemblance to the Mandelbrot set in fractal theory. The self-similarity of NHQFs is rooted in the interplay between the iterative lattice configuration and non-Hermiticity. Moreover, we show that NHQFs exist in generalized non-Hermitian systems with iterative lattice structures. Our findings open a new avenue for investigating quantum fractals in non-Hermitian systems.

cond-mat.mes-hall↗

arXiv2024

Anomalous Andreev Spectrum and Transport in Non-Hermitian Josephson Junctions

We propose a phase-biased non-Hermitian Josephson junction (NHJJ) composed of two superconductors mediated by a short non-Hermitian link. Such a NHJJ is described by an effective non-Hermitian Hamiltonian derived based on the Lindblad formalism in the weak coupling regime. By solving the Bogoliubov-de Gennes equation, we find that its Andreev spectrum as a function of phase difference exhibits Josephson gaps, i.e., finite phase windows with no Andreev (quasi)bound states. The complex Andreev spectrum and the presence of Josephson gaps constitute particular spectral features of the NHJJ. Moreover, we propose complex supercurrents arising from inelastic Cooper pair tunneling to characterize the anomalous transport in the NHJJ. Additional numerical simulations complement our analytical predictions. We demonstrate that the Josephson effect is strongly affected by non-Hermitian physics.

cond-mat.mes-hall↗

arXiv2024

Helical Topological Superconducting Pairing at Finite Excitation Energies

We propose helical topological superconductivity away from the Fermi surface in three-dimensional time-reversal-symmetric odd-parity multiband superconductors. In these systems, pairing between electrons originating from different bands is responsible for the corresponding topological phase transition. Consequently, a pair of helical topological Dirac surface states emerges at finite excitation energies. These helical Dirac surface states are tunable in energy by chemical potential and strength of band-splitting. They are protected by time-reversal symmetry combined with crystalline two-fold rotation symmetry. We suggest concrete materials in which this phenomenon could be observed.

cond-mat.mes-hall↗

arXiv2023

Klein-bottle quadrupole insulators and Dirac semimetals

The Benalcazar-Bernevig-Hughes (BBH) quadrupole insulator model is a cornerstone model for higher-order topological phases. It requires π-flux threading through each plaquette of the two-dimensional Su-Schrieffer-Heeger model. Recent studies showed that particular π-flux patterns can modify the fundamental domain of momentum space from the shape of a torus to a Klein bottle with emerging topological phases. By designing different π-flux patterns, we propose two types of Klein-bottle BBH models. These models show rich topological phases, including Klein-bottle quadrupole insulators and Dirac semimetals. The phase with nontrivial Klein-bottle topology shows twined edge modes at open boundaries. These edge modes can further support second-order topology, yielding a quadrupole insulator. Remarkably, both models are robust against flux perturbations. Moreover, we show that different π-flux patterns dramatically affect the phase diagram of the Klein-bottle BBH models. Going beyond the original BBH model, Dirac semimetal phases emerge in Klein-bottle BBH models featured by the coexistence of twined edge modes and bulk Dirac points.

cond-mat.mes-hall↗

arXiv2023

Enhancement of Second-Order Non-Hermitian Skin Effect by Magnetic Fields

The non-Hermitian skin effect is a unique phenomenon in which an extensive number of eigenstates are localized at the boundaries of a non-Hermitian system. Recent studies show that the non-Hermitian skin effect is significantly suppressed by magnetic fields. In contrast, we demonstrate that the second-order skin effect (SOSE) is robust and can even be enhanced by magnetic fields. Remarkably, SOSE can also be induced by magnetic fields from a trivial non-Hermitian system that does not experience any skin effect at zero field. These properties are intimately related to to the persistence and emergence of topological line gaps in the complex energy spectrum in presence of magnetic fields. Moreover, we show that a magnetic field can drive a non-Hermitian system from a hybrid skin effect, where the first-order skin effect and SOSE coexist, to pure SOSE. Our results describe a qualitatively new magnetic field behavior of the non-Hermitian skin effect.

cond-mat.mes-hall↗

arXiv2023

Nonlinear planar magnetotransport due to tilted Dirac cones in topological materials

Nonlinear planar magnetotransport is ubiquitous in topological HgTe structures, both in tensile (topological insulator) or compressively strained layers (Weyl semimetal phase). We show that the common reason for the nonlinear planar magnetotransport is the presence of tilted Dirac cones combined with the formation of charge puddles. The origin of the tilted Dirac cones is the mix of the Zeeman term due to the in-plane magnetic field and quadratic contributions to the dispersion relation. We develop a network model that mimics transport of tilted Dirac fermions in the landscape of charge puddles. The model captures the essential features of the experimental data. It should be relevant for nonlinear planar magnetotransport in a variety of topological and small band gap materials.

cond-mat.mes-hall↗

arXiv2023

Hybrid higher-order skin-topological effect in hyperbolic lattices

We investigate the non-Hermitian Haldane model on hyperbolic $\{8, 3\}$ and $\{12, 3\}$ lattices, and showcase its intriguing topological properties in the simultaneous presence of non-Hermitian effect and hyperbolic geometry. From bulk descriptions of the system, we calculate the real space non-Hermitian Chern numbers by generalizing the method from its Hermitian counterpart and present corresponding phase diagram of the model. For boundaries, we find that skin-topological modes appear in the range of the bulk energy gap under certain boundary conditions, which can be explained by an effective one-dimensional zigzag chain model mapped from hyperbolic lattice boundary. Remarkably, these skin-topological modes are localized at specific corners of the boundary, constituting a hybrid higher-order skin-topological effect on hyperbolic lattices.

cond-mat.mes-hall↗