arXiv subjects

Longwen Zhou

Publications and source records attributed to Longwen Zhou.

At least 19 recordsLinked to original sources

arXiv2026

Non-Hermitian topological Euler insulators

Topological Euler insulators emerge in multiband systems with real Bloch Hamiltonians and wavefunctions. Their fragile topologies are characterized by the Euler class of degenerate bands and protected by the $PT$ or $C_2T$ symmetry in two dimensions, which go beyond the tenfold $K$-theory classification of topological matter. In this work, we extend the conception of topological Euler insulators to non-Hermitian systems and propose a theoretical framework to unlock their nontrivial Euler topology. Focusing on two-dimensional, three-band non-Hermitian lattice models with symmetric Hamiltonians, we formulate a comprehensive description of their topological Euler bands, entanglement spectrum and bulk-boundary correspondence. Three typical models of non-Hermitian Euler insulators are constructed and investigated explicitly to illustrate our theory. Unique topological phase transitions and anomalous edge-band overlaps with non-Hermitian origins are further identified. Our study establishes the presence of topological Euler bands in non-Hermitian systems and unveils their intriguing physical characteristics, thereby broadening the existing territory of topological matter in non-Hermitian open systems.

cond-mat.mes-hall↗

arXiv2026

$\mathcal{PT}$ and anti-$\mathcal{PT}$ phase transitions in a trimerized Su--Schrieffer--Heeger chain with nonreciprocal Rashba spin-orbit coupling

We theoretically investigate a one-dimensional trimerized Su--Schrieffer--Heeger chain with three sublattices per unit cell subjected to a nonreciprocal Rashba spin-orbit coupling. Invoking a spin-flip symmetry, the non-Hermitian Hamiltonian decomposes into two independent spin sectors, enabling a spin-resolved analysis of non-Hermitian skin effects and system symmetries. We identify a rich phase diagram consisting of four bulk phases and two edge-state phases. The bulk phases include fully $\mathcal{PT}$-unbroken (real-spectrum) and fully anti-$\mathcal{PT}$-unbroken (imaginary-spectrum) regimes, as well as two mixed phases where one band remains on the real or imaginary axis while the other two form complex-conjugate pairs. The two edge-state phases correspond to topological edge modes with either $\mathcal{PT}$-unbroken (real) or anti-$\mathcal{PT}$-unbroken (imaginary) energies. Using non-Bloch band theory and Cardano's method, we derive closed-form expressions for phase boundaries and establish the bulk-edge correspondence for each spin sector. Calculations of Berry phase and directional inverse participation ratios confirm our analytical predictions. Our results provide a minimal platform for realizing spin-resolved non-Hermitian topology and edge-selective symmetry preservation, in which the bulk and edge states belong to distinct symmetry classes.

cond-mat.mes-hall↗

arXiv2026

Lateral Shift as a Control Knob for Localization Transitions in a Quasiperiodic Ladder

This work reports rich localization-delocalization transitions in a quasiperiodic ladder, of which the two legs are subject to the same quasiperiodic onsite potential but can be shifted laterally relative to each other. It is found that the lateral shift between the two legs effectively generates a magnetic flux in the reciprocal momentum space. The lateral shift thus offers a control knob, allowing us to access and simulate rich phenomena including magnetic-flux-enhanced localization, magnetic-flux-suppressed localization, and magnetic-flux-induced reentrant localization transitions. The underlying physical mechanisms as well as the phase boundaries separating localized, mixed, and extended phases are both qualitatively and quantitatively understood, based on a band-structure analysis that employs a commensurate approximation to the quasiperiodic potential, requiring only unit cells of small to modest sizes. Our work provides a highly tunable platform for exploring localization physics with promising applications such as quantum switching, and a broadly applicable approach for understanding localization-delocalization transitions in quasiperiodic systems.

cond-mat.dis-nn↗

arXiv2026

Topological metal-insulator transitions in one-dimensional non-Hermitian quasicrystals: beyond PT-symmetry

One-dimensional non-Hermitian quasicrystals with parity and time-reversal (PT) symmetry can simultaneously exhibit localization-delocalization transition, topological phase transition, and PT-symmetry-breaking transition. This motivates this work to investigate how the absence of PT symmetry impacts topological metal-insulator transitions in non-Hermitian quasicrystals. We propose a non-Hermitian quasiperiodic model that generally does not preserve PT symmetry and demonstrate that, in most parameter regions, such a system supports triple phase transitions that encompass localization, topology, and degeneracy-breaking. The system may also exhibit a particular type of localization-delocalization transition analogous to the Hermitian case, namely, without activating topological phase transitions or degeneracy-breaking transitions. Our work extends the topological metal-insulator transitions previously studied in PT-symmetric systems to a more general class of non-Hermitian setting, and further reveals that non-Hermitian systems can host distinct types of localization behavior.

cond-mat.dis-nn↗

arXiv2026

Topology and edge modes surviving criticality in non-Hermitian Floquet systems

The discovery of critical points that can host quantized nonlocal order parameters and degenerate edge modes relocate the study of symmetry-protected topological phases (SPTs) to gapless regions. In this letter, we reveal gapless SPTs (gSPTs) in systems tuned out-of-equilibrium by periodic drivings and non-Hermitian couplings. Focusing on one-dimensional models with sublattice symmetry, we introduce winding numbers by applying the Cauchy's argument principle to generalized Brillouin zone (GBZ), yielding unified topological characterizations and bulk-edge correspondence in both gapped phases and at gapless critical points. The theory is demonstrated in a broad class of Floquet bipartite lattices, unveiling unique topological criticality of non-Hermitian Floquet origin. Our findings identify gSPTs in driven open systems and uncover robust topological edge modes at phase transitions beyond equilibrium.

cond-mat.mes-hall↗

arXiv2025

Coupled-wire construction of non-Abelian higher-order topological phases

Non-Abelian topological charges (NATCs), characterized by their noncommutative algebra, offer a framework for describing multigap topological phases beyond conventional Abelian invariants. While higher-order topological phases (HOTPs) host boundary states at corners or hinges, their characterization has largely relied on Abelian invariants such as winding and Chern numbers. Here, we propose a coupled-wire scheme of constructing non-Abelian HOTPs and analyze a non-Abelian second-order topological insulator as its minimal model. The resulting Hamiltonian supports hybridized corner modes, protected by parity-time-reversal plus sublattice symmetries and described by a topological vector that unites a non-Abelian quaternion charge with an Abelian winding number. Corner states emerge only when both invariants are nontrivial, whereas weak topological edge states of non-Abelian origins arise when the quaternion charge is nontrivial, enriching the bulk-edge-corner correspondence. The system further exhibits both non-Abelian and Abelian topological phase transitions, providing a unified platform that bridges these two distinct topological classes. Our work extends the understanding of HOTPs into non-Abelian regimes and suggests feasible experimental realizations in synthetic quantum systems, such as photonic or acoustic metamaterials.

cond-mat.mes-hall↗

arXiv2025

Topological characterization of phase transitions and critical edge states in one-dimensional non-Hermitian systems with sublattice symmetry

Critical edge states appear at the bulk gap closing points of topological transitions. Their emergence signify the existence of topologically nontrivial critical points, whose descriptions fall outside the scope of gapped topological matter. In this work, we reveal and characterize topological critical points and critical edge states in non-Hermitian systems. By applying the Cauchy's argument principle to two characteristic functions of a non-Hermitian Hamiltonian, we obtain a pair of winding numbers, whose combination yields a complete description of gapped and gapless topological phases in one-dimensional, two-band non-Hermitian systems with sublattice symmetry. Focusing on a broad class of non-Hermitian Su-Schrieffer-Heeger chains, we demonstrate the applicability of our theory for characterizing gapless symmetry-protected topological phases, topologically distinct critical points, phase transitions along non-Hermitian phase boundaries and their associated topological edge modes. Our findings not only generalize the concepts of topologically nontrivial critical points and critical edge modes to non-Hermitian setups, but also yield additional insights for analyzing topological transitions and bulk-edge correspondence in open systems.

cond-mat.mes-hall↗

arXiv2025

Floquet M\"obius topological insulators

M\"obius topological insulators have dispersive edge bands with M\"obius twists in momentum space, which are protected by the combination of chiral and $Z_2$-projective translational symmetries. In this work, we reveal a unique type of M\"obius topological insulator, whose edge bands could twist around the quasienergy $\pi$ of a periodically driven system and are thus of Floquet origin. By applying time-periodic quenches to an experimentally realized M\"obius insulator model, we obtain interconnected M\"obius edge bands around zero and $\pi$ quasienergies, which can coexist with a gapped or gapless bulk. These M\"obius bands are topologically characterized by a pair of generalized winding numbers, which are integer-quantized due to an emergent chiral symmetry at a high-symmetry point in momentum space. Numerical investigations of the quasienergy and entanglement spectra provide consistent evidence for the presence of such M\"obius topological phases. A protocol based on the adiabatic switching of edge-band populations is further introduced to dynamically characterize the topology of Floquet M\"obius edge bands. Our findings thus extend the scope of M\"obius topological phases to nonequilibrium settings and unveil a unique class of M\"obius-twisted topological edge states without static counterparts.

cond-mat.mes-hall↗

arXiv2025

Floquet non-Abelian topological charges and edge states

Non-Abelian topological insulators are characterized by matrix-valued, non-commuting topological charges with regard to more than one energy gap. Their descriptions go beyond the conventional topological band theory, in which an additive integer is endowed separately with each (degenerate group of) energy band(s). In this work, we reveal that Floquet (time-periodic) driving could not only enrich the topology and phase transitions of non-Abelian topological matter, but also induce bulk-edge correspondence unique to nonequilibrium setups. Using a one-dimensional (1D), three-band model as an illustrative example, we demonstrate that Floquet driving could reshuffle the phase diagram of the non-driven system, yielding both gapped and gapless Floquet band structures with non-Abelian topological charges. Moreover, by dynamically tuning the anomalous Floquet $\pi$-quasienergy gap, non-Abelian topological transitions inaccessible to static systems could arise, leading to much more complicated relations between non-Abelian topological charges and Floquet edge states. These discoveries put forth the periodic driving as a powerful scheme of engineering non-Abelian topological phases (NATPs) and incubating unique non-Abelian band topology beyond equilibrium.

cond-mat.mes-hall↗

arXiv2025

Observing the exponential growth of the eigenmodes in the absence of coalescence for a non-Hermitian circuit with an unavoidable inductor dissipation

We investigate, both experimentally and theoretically, the eigenmodes of an electronic circuit in which gain and loss $RLC$ resonators are coupled through a capacitor. Due to the unavoidable magnetic loss in the inductors, we find that the eigenmode coalescence no longer emerges in contrast to the conventional non-Hermitian systems with the spontaneous $\cal{PT}$-symmetry breaking. In particular, we find a transition from the exponential decay to exponential growth in the amplitude of the periodic voltage oscillations of the resonators. The transition occurs near the exceptional points of the non-Hermitian circuit without considering the dissipations in inductors. We introduce a small resistor of three orders of magnitude smaller than that of the $RLC$ resonators to mimic the energy dissipation in inductors and numerically solve the equivalent non-Hermitian Schr{\" o}dinger equation. The numerical results can well reproduce experimental observations. Our above findings unambiguously indicate that the exponential growth behavior beyond the exceptional points is robust against some unavoidable dissipative perturbations.

quant-ph↗

arXiv2025

Gapless higher-order topology and corner states in Floquet systems

Higher-order topological phases (HOTPs) possess localized and symmetry-protected eigenmodes at corners and along hinges in two and three dimensional lattices. The numbers of these topological boundary modes will undergo quantized changes at the critical points between different HOTPs. In this work, we reveal unique higher-order topology induced by time-periodic driving at the critical points of topological phase transitions, which has no equilibrium counterparts and also goes beyond the description of gapped topological matter. Using an alternately coupled Creutz ladder and its Floquet-driven descendants as illustrative examples, we analytically characterize and numerically demonstrate the zero and $\pi$ corner modes that could emerge at the critical points between different Floquet HOTPs. Moreover, we propose a unified scheme of bulk-corner correspondence for both gapless and gapped Floquet HOTPs protected by chiral symmetry in two dimensions. Our work reveals the possibility of corner modes surviving topological transitions in Floquet systems and initializes the study of higher-order Floquet topology at quantum criticality.

quant-ph↗

arXiv2024

Two-body interaction induced phase transitions and intermediate phases in nonreciprocal non-Hermitian quasicrystals

Non-Hermitian phenomena, such as exceptional points, non-Hermitian skin effects, and topologically nontrivial phases have attracted continued attention. In this work, we reveal how interactions and nonreciprocal hopping could collectively influence the behavior of two interacting bosons on quasiperiodic lattices. Focusing on the Bose-Hubbard model with Aubry-Andr\'e-Harper quasiperiodic modulations and hopping asymmetry, we discover that interactions could enlarge the localization transition point of the noninteracting system into an intermediate mobility edge phase, in which localized doublons formed by bosonic pairs can coexist with delocalized states. Under the open boundary condition, the bosonic doublons could further show non-Hermitian skin effects, realizing doublon condensation at the edges, and their direction of skin-localization can be flexibly tuned by the hopping parameters. A framework is developed to characterize the spectral, localization, and topological transitions accompanying these phenomena. Our work advances the understanding of localization and topological phases in non-Hermitian systems, particularly in relation to multiparticle interactions.

cond-mat.dis-nn↗

arXiv2024

Topological edge states at Floquet quantum criticality

Topologically protected edge states exactly at topological phase boundaries challenge the conventional belief that topological states must be associated with a bulk energy gap. Because periodically driven (Floquet) systems host unusually intricate topological phase boundaries, topological edge states can be prolific at such Floquet quantum criticality. Working on a class of chiral-symmetric, Floquet-driven Majorana fermion chains, we analytically and computationally show that the precise boundaries between different Floquet topological gapped phases can accommodate topological edge modes, including the so-called Majorana $\pi$ modes. We also identify a general bulk-edge correspondence formula to predict and understand the emergence of topological edge modes at Floquet quantum criticality. Of direct interest to quantum simulation experiments, our results break new grounds for studies of nonequilibrium topological phases of matter undergoing topological phase transitions.

cond-mat.stat-mech↗

arXiv2024

Quantum geometry and geometric entanglement entropy of one-dimensional Floquet topological matter

The geometry of quantum states could offer indispensable insights for characterizing the topological properties, phase transitions and entanglement nature of many-body systems. In this work, we reveal the quantum geometry and the associated entanglement entropy (EE) of Floquet topological states in one-dimensional periodically driven systems. The quantum metric tensors of Floquet states are found to show non-analytic signatures at topological phase transition points. Away from the transition points, the bipartite geometric EE of Floquet states exhibits an area-law scaling vs the system size, which holds for a Floquet band at any filling fractions. For a uniformly filled Floquet band, the EE further becomes purely quantum geometric. At phase transition points, the geometric EE scales logarithmically with the system size and displays cusps in the nearby parameter ranges. These discoveries are demonstrated by investigating typical Floquet models including periodically driven spin chains, Floquet topological insulators and superconductors. Our findings uncover the rich quantum geometries of Floquet states, unveiling the geometric origin of EE for gapped Floquet topological phases, and introducing information-theoretic means of depicting topological transitions in Floquet systems.

quant-ph↗

arXiv2024

Multiple topological transitions and spectral singularities in non-Hermitian Floquet systems

The interplay between Floquet driving and non-Hermitian gain/loss could give rise to intriguing phenomena including topological funneling of light, edge-state delocalization, anomalous topological transitions and Floquet non-Hermitian skin effects. In this work, we uncover two unique phenomena in Floquet systems caused by gain and loss. First, multiple topological transitions from anomalous Floquet second-order topological insulators to anomalous Floquet first-order topological insulators and then to normal insulators can be induced by gain and loss. Interestingly, the resulting anomalous Floquet insulators further carry hybrid skin-topological boundary modes, which could either be fully localized or localized to different edges at different time slices and traversing along all edges in a single driving period. The topological phase transitions are also shown to be detectable through studies of transmission properties in the setting of coupled ring resonators. Second, gain and loss are found to induce singularities in the Floquet spectral, around which anomalous transmissions at flat quasienergy bands are predicted. These discoveries not only enhanced our understanding of topological matter and phase transitions in driven non-Hermitian systems, but also promoted their experimental realizations in optical and acoustic settings.

physics.app-ph↗

arXiv2024

Entanglement phase transitions in non-Hermitian Floquet systems

The competition between unitary time-evolution and quantum measurements could induce phase transitions in the entanglement characteristics of quantum many-body dynamics. In this work, we reveal such entanglement transitions in the context of non-Hermitian Floquet systems. Focusing on noninteracting fermions in a representative bipartite lattice with balanced gain/loss and under time-periodic quenches, we uncover rich patterns of entanglement transitions due to the interplay between driving and non-Hermitian effects. Specially, we find that the monotonic increase of quenched hopping amplitude could flip the system between volume-law and area-law entangled Floquet phases, yielding alternated entanglement transitions. Meanwhile, the raise of gain/loss strength could trigger area-law to volume-law reentrant transitions in the scaling behavior of steady-state entanglement entropy, which are abnormal and highly unexpected in non-driven systems. Connections between entanglement transitions and parity-time-reversal (PT) transitions in Floquet spectra are further established. Our findings not only build a foundation for exploring entanglement phase transitions in Floquet non-Hermitian setups, but also provide efficient means to engineer and control such transitions by driving fields.

quant-ph↗

arXiv2024

Entanglement phase transitions in non-Hermitian Kitaev chains

The intricate interplay between unitary evolution and projective measurements could induce entanglement phase transitions in the nonequilibrium dynamics of quantum many-particle systems. In this work, we uncover loss-induced entanglement transitions in non-Hermitian topological superconductors. In prototypical Kitaev chains with local particle losses and varying hopping and pairing ranges, the bipartite entanglement entropy of steady states is found to scale logarithmically versus the system size in topologically nontrivial phases and become independent of the system size in the trivial phase. Notably, the scaling coefficients of log-law entangled phases are distinguishable when the underlying system resides in different topological phases. Log-law to log-law and log-law to area-law entanglement phase transitions are further identified when the system switches between different topological phases and goes from a topologically nontrivial to a trivial phase, respectively. These findings not only establish the relationships among spectral, topological and entanglement properties in a class of non-Hermitian topological superconductors, but also provide an efficient means to dynamically reveal their distinctive topological features.

quant-ph↗

arXiv2024

Non-Abelian generalization of non-Hermitian quasicrystal: PT-symmetry breaking, localization, entanglement and topological transitions

Non-Hermitian quasicrystal forms a unique class of matter with symmetry-breaking, localization and topological transitions induced by gain and loss or nonreciprocal effects. In this work, we introduce a non-Abelian generalization of the non-Hermitian quasicrystal, in which the interplay between non-Hermitian effects and non-Abelian quasiperiodic potentials create mobility edges and rich transitions among extended, critical and localized phases. These generic features are demonstrated by investigating three non-Abelian variants of the non-Hermitian Aubry-André-Harper model. A unified characterization is given to their spectrum, localization, entanglement and topological properties. Our findings thus add new members to the family of non-Hermitian quasicrystal and uncover unique physics that can be triggered by non-Abelian effects in non-Hermitian systems.

quant-ph↗