arXiv subjects

Hai-Qing Lin

Publications and source records attributed to Hai-Qing Lin.

At least 19 recordsLinked to original sources

arXiv2026

Topological Tricritical Ising Universality Class in One Dimension

Quantum critical points can host symmetry-protected topological edge modes even when the bulk is gapless, giving rise to symmetry-enriched universality classes beyond the conventional Landau--Ginzburg--Wilson paradigm. Here we show that the tricritical Ising (TCI) critical point---described by a paradigmatic conformal field theory (CFT) with emergent supersymmetry---admits a topologically nontrivial, symmetry-enriched realization, which we term the topological TCI. We construct this critical point in the cluster O'Brien--Fendley spin chain by applying a symmetry-protected topological entangler to the original O'Brien--Fendley model. The two realizations therefore share the same bulk TCI CFT. Under open boundary conditions, however, the original model exhibits the free conformal boundary condition, whereas the cluster model exhibits a spontaneously fixed boundary condition: an infinitesimal boundary field can select one of the fixed boundary states, yielding spontaneous boundary magnetization. We further show that symmetry enrichment and boundary renormalization-group flow provide two physically distinct routes to the spontaneously fixed boundary condition. Whereas the free and spontaneously fixed boundary conditions of the original model can be interconverted by tuning a symmetry-preserving boundary term, no such conversion occurs for the cluster model along the boundary deformations studied here, even in the strong-deformation limit. Although they share the same conventional boundary CFT data, including the boundary operator content and boundary $g$-function, the two realizations of the spontaneously fixed boundary condition are distinguished by different time-reversal charges of the disorder field....

cond-mat.str-el↗

arXiv2026

Quantum criticality from spectral collapse in the two-photon Rabi model

Spectral collapse in the two-photon quantum Rabi model (tpQRM) has long been regarded as incompatible with quantum criticality due to the absence of a vanishing excitation gap. We show that, in the anisotropic tpQRM, spectral collapse constitutes a genuine continuous quantum phase transition governed by a single soft mode. The excitation gap within the same parity closes as $\epsilon_{sp} \sim |g - g_c|^{z\nu}$ with $z\nu = 1/2$, placing the system in the same universality class as the standard QRM, while the gap between different parities reflects symmetry-induced level splitting rather than a critical excitation. This soft mode defines a unique energy scale that controls both equilibrium and nonequilibrium properties, including macroscopic observables, quantum Fisher information, and Kibble-Zurek dynamics. These results establish spectral collapse as an experimentally accessible realization of quantum criticality in a few-body system and demonstrate that universality is fully determined by the soft-mode structure rather than by microscopic details.

quant-ph↗

arXiv2026

Tomonaga-Luttinger liquid and charge-density wave in a quasi-one-dimensional material

In one-dimensional (1D) electron systems, the Fermi liquid state breaks down due either to electron interactions, which results in a Tomonaga-Luttinger liquid (TLL) state, or to Peierls instability, which leads to an insulating charge-density-wave (CDW) phase. In general, these two phenomena are mutually exclusive, and their coexistence remains elusive in real materials. Here, we report the discovery of a new quasi-1D material, Cs$_{1-\delta}$Cr$_3$S$_3$, which unexpectedly exhibits coexistence of the antithetical CDW and TLL states. The CDW state is evidenced by the intra-unit-cell dimerization, and the opening of an optical band gap of $\sim$250 meV. Meanwhile, TLL behaviour is unambiguously demonstrated by the measurements of electrical transport and angle-resolved photoemission spectroscopy, which reveal a power-law scaling with temperature, bias voltage and electron energy. Band structure calculations reveal isolated, linearly dispersive, 1D bands around the Fermi level. For the dimerized CDW phase, the 1D Fermi-surface sheets located at the boundary of the Brillouin zone are gapped from intra-unit-cell bond symmetry breaking. Experimentally, subtle Cs vacancies shift the Fermi level into the linearly dispersive valence band, enabling the observation of TLL behaviour without interrupting the CDW order. This work establishes Cs$_{1-\delta}$Cr$_3$S$_3$ as a rare material platform in which the antagonistic Fermi-liquid instabilities coexist and intertwine, opening new avenues for studying emergent quantum phenomena in 1D systems.

cond-mat.str-el↗

arXiv2026

Dimensionality tuning of heavy-fermion states in ultrathin CeSi2 films

Dimensionality tuning is an important method to modify the electronic states of quantum materials. However, the mechanism of such tuning in heavy fermion systems and its connection with transport properties remain largely unexplored. Here by combining molecular beam epitaxy (MBE), in-situ angle-resolved photoemission spectroscopy (ARPES) and transport measurements, we study the electronic states of the heavy-fermion compound CeSi2 as a function of film thickness. In three dimensional thick films, our measurements reveal a dispersive Kondo peak at the Fermi level (EF) and satellite peaks originating from crystal electric field (CEF) excitations, characteristic of heavy fermion systems. For two-dimensional ultrathin films, the CEF satellites are largely suppressed while the ground-state Kondo peak at EF remains strong, although it develops at lower temperatures. Simultaneously, the maximum temperature Tmax of the magnetic resistivity, \r{ho}m(T), changes from ~100 K in thick films to ~35 K in ultrathin films. This can be attributed to the dimensionality driven reduction of CEF excitations during the Kondo process, in good agreement with spectroscopic results. Our work provides direct insight to understand the quantum confinement effects on strongly correlated 4f-electron systems and opens up new opportunities to explore emergent phenomena in two-dimensional heavy-fermion materials.

cond-mat.str-el↗

arXiv2026

Topological Quantum Criticality in Quasiperiodic Ising Chain

Topological classifications of quantum critical systems have recently attracted growing interest, as they go beyond the traditional paradigms of condensed matter and statistical physics. However, such classifications remain largely unexplored at critical points in aperiodic environments, particularly under quasiperiodic modulations. In this Letter, we uncover a novel class of topological quasiperiodic fixed points that are intermediate between the clean and infinite-randomness limits. By exactly solving the quasiperiodic cluster-Ising chain, we unambiguously demonstrate that all phase boundaries separating quasiperiodically modulated phases are governed by a new family of topological Ising-like fixed points unique to strongly modulated quasiperiodic systems: Despite exhibiting indistinguishable bulk critical properties, these fixed points host robust topological edge degeneracies and are therefore topologically distinct from previously recognized quasiperiodic universality classes, as further supported by complementary lattice simulations.

cond-mat.stat-mech↗

arXiv2026

Two-dimensional Intrinsic Janus Structures: Design Principle and Anomalous Nonlinear Optics

Two-dimensional Janus structures have garnered rapidly growing attention across multidisciplinary fields. However, despite extensive theoretical and experimental efforts, a principle for designing intrinsic Janus materials remains elusive. Here, we propose a first-principles alloy theory based on cluster expansion, incorporating a strong repulsive interaction of a cation-mediated anion-pair cluster and refined short-range cluster-cluster competitions, to unravel the formation mechanism of intrinsic Janus structures with a distorted 1T phase among numerous competing phases. Our theory not only explains why intrinsic Janus structures are accidentally observed in RhSeCl and BiTeI which are composed of alloyed elements from different groups, but also accurately predicts a wide range of 1T-like intrinsic Janus materials that are ready for synthesis. Intriguingly, as demonstrated in the case of RhSeCl, we reveal that intrinsic Janus materials can exhibit anomalous second-harmonic generation (SHG) with a distinct quantum geometric effect, originating from strong lattice and chemical-potential mirror asymmetry. Furthermore, a novel skin effect unexpectedly emerges in finite-thickness RhSeCl, accompanied by a hidden SHG effect within the bulk region. Our theory paves the way for the ab initio design of intrinsic Janus materials, significantly accelerating progress in Janus science.

cond-mat.mtrl-sci↗

arXiv2026

Topological physics in quantum critical systems

Topology forms a cornerstone in modern condensed matter and statistical physics, offering a new framework to classify the phases and phase transitions beyond the traditional Landau paradigm. However, it is widely believed that topological properties are destroyed when the bulk energy gap closes, making it highly nontrivial to consider topology in gapless quantum critical systems. To address these challenges, recent advancements have sought to generalize the notion of topology to systems without a bulk energy gap, including quantum critical points and critical phases, collectively referred to as gapless symmetry-protected topological states. Extending topology to gapless quantum critical systems challenges the traditional belief in condensed matter physics that topological edge states are typically tied to the presence of a bulk energy gap. Furthermore, it suggests that topology plays a crucial role in classifying quantum phase transitions even if they belong to the same universality class, fundamentally enriching the textbook understanding of phase transitions. Given its importance, here we give a pedagogical review of the current progress of topological physics in quantum critical systems. We introduce the topological properties of quantum critical points and generalize them to stable critical phases, both for noninteracting and interacting systems. Additionally, we discuss further generalizations and future directions, including higher dimensions, nonequilibrium phase transitions, and realizations in modern experiments.

cond-mat.str-el↗

arXiv2025

Unveiling the Phase Diagram and Nonlinear Optical Responses of a Twisted Kitaev Chain

Detecting Kitaev interactions in real materials remains challenge, as conventional experimental techniques often have difficulty distinguishing fractionalized excitations from other normal contributions. Terahertz two-dimensional coherent spectroscopy (2DCS) offers a novel approach for probing many-body phenomena, such as exotic excitations in quantum magnets. Motivated by recent experiments on CoNb$_2$O$_6$ and the development of the terahertz spectroscopy in Kitaev quantum spin liquid, we proposed a twisted Kitaev model for CoNb$_2$O$_6$ and determined the precise twist angle according to experimental specific-heat phase diagram. With this calibrated model, we found that non-rephasing diagonal and rephasing anti-diagonal signals appear in the 2DCS nonlinear response. The $x$ and $y$ components of the spin superexchange interactions split the rephasing signals into a grid of discrete peaks. We further demonstrate that the diagonal and the discrete rephasing signals primarily originate from two-spinon and four-spinon excitation processes based on numerical projection method. These findings indicate that even weak Kitaev interactions in quantum materials can be effectively detected via two-dimensional coherent spectroscopy .

cond-mat.str-el↗

arXiv2025

Charge stripe and superconductivity tuned by interlayer interaction in a sign-problem-free bilayer extended Hubbard model

Competing orders represent a central challenge in understanding strongly correlated systems. In this work, we employ projector quantum Monte Carlo simulations to study a sign-problem-free bilayer extended Hubbard model. In this model, a charge stripe phase, characterized by a peak at momentum $k_x=2\pi\delta$ is induced by highly anisotropic interlayer spin-exchange coupling $J_z$, and strongly suppressed upon introducing the spin-flip term $J_\bot$; in contrast, $J_\perp$ favors the emergence of interlayer pairing superconductivity. We further demonstrate that the anisotropy of the interlayer spin-exchange directly governs the competition between these two phases, while the on-site interaction $U$ plays a complex role in tuning both the charge stripe and superconductivity. Our work identifies the key factors driving charge stripe formation, highlights the sensitivity of both the charge stripe and superconducting phases to interaction parameters, and thereby provides valuable insights into competing orders in strongly correlated systems.

cond-mat.supr-con↗

arXiv2025

Quantum Quench Dynamics in an Exactly Solvable Two-Dimensional Non-Fermi Liquid System

Understanding the behavior of non-Fermi liquids (NFLs) is an important topic in condensed matter physics. Here we introduce an exactly solvable multi-orbital model based on iron oxypnictides and the Hatsugai-Kohmoto model, and provide exact investigations of the 2D NFLs nonequilibrium physics present in this model. Our results reveal fundamental departures from Fermi liquids and prior NFLs in the well-know SYK model: anomalous short-time scaling $-\tau^2 \ln \tau$, $O(\tau) \sim \tau^2$; long-time scaling $ \tau^{-1}, \tau^{-1/2}, \ln \tau / \tau$; a strange critical behavior in the steady-state phase diagram. Our asymptotic results and dynamical critical behavior offer new insights into the orbital-related dynamical physics of 2D NFLs.

cond-mat.str-el↗

arXiv2025

Emerging kinetic-exchange for the enhanced metallic ferromagnetism in CrGeTe$_3$ under pressure

The microscopic origin of ferromagnetism in correlated materials remains heavily debated, particularly for the competing mechanisms governing insulating versus metallic phases. In this work, we theoretically study the electronic structure evolution of CrGeTe$_{3}$ under pressure and provide a consistent explanation to three unique features of this system, i.e. the semiconducting ferromagnetism at low pressure, the metallic ferromagnetism at high pressure, and the enhanced Curie temperature in the metallic phase. We propose that it is the reduced electronic correlation and enhanced $d$-$p$ hybridization that universally drive the continuous evolution of CrGeTe$_{3}$ under pressure and glue the three distinct experimental observations. Central to our discovery is the dual role of metallicity -- it simultaneously establishes kinetically driven exchange via $d$-$p$ hybridization and enables Stoner-type magnetic instability, with the contribution also from the residual super-exchange. Our analyses reveal that {\it intraband} excitations dominate the pressure-enhanced $\omega_p^2$ and $T_c$ correlation. These findings establish $d$-$p$ hybridization and electronic correlation as the bridge between localized and itinerant magnetism, at least, in CrGeTe$_{3}$.

cond-mat.str-el↗

arXiv2025

Resilient cluster Mott states in layered Nb$_3$Cl$_8$ against pressure-induced symmetry breaking

In this work, by combining density-functional theory (DFT) with dynamical mean-field calculations (DMFT), we compare the crystal and electronic structures of the prototype cluster Mott insulator Nb$_{3}$Cl$_8$ at ambient and high-pressure. We explain the finite but significantly reduced charge gap experimentally observed at $P=9.7$ GPa. We reveal a local symmetry breaking of the Nb$_{3}$ trimer under pressure, reducing its symmetry from $C_{3v}$ to $C_{s}$. This leads to a strong bandwidth enhancement and a lift of band degeneracy. Crucially, despite the significant change of band details, the cluster Mott insulating state is robust against local symmetry breaking. We show that the experimental observed gap under pressure is still a cluster Mott gap and its reduced value stems from both increased bandwidth and reduced Coulomb interactions under pressure. Our study provides the first systematic theoretical elucidation of how pressure-induced symmetry breaking dictates the cluster Mott state, establishing a robust foundation for understanding the intricate relationship between symmetry, local/non-local correlations, and emergent quantum states in correlated cluster compounds.

cond-mat.str-el↗

arXiv2025

Robust cross-chain surface interstitial electronic states and doping-enhanced superconductivity in monolayer $M_2$N ($M$= Ti, Zr, Hf) electrides

The exploration of electrides holds great promise for advancing both fundamental physics and chemistry, owing to their unique characteristics arising from loosely bound interstitial anionic electrons. Here we report a class of cross-chain electrides, distinguished by two distinct anionic electron subchannels forming alternating chains in real space. Through structural symmetry analysis and first-principles calculations, we identify two-dimensional $M_2$N ($M$ = Ti, Zr, Hf) materials as prototypical systems exhibiting these unique features. The anionic electron channels on the upper and lower surfaces of these materials display a vertically alternating pattern, with their projected bands revealing momentum-dependent splitting behavior in the reciprocal space, protected by a crystal symmetry operation $\mathcal{O}$. Notably, the cross-chain electride characteristic in the $M_2$N monoalyers is independent of the layer number and remains robust on the upper and lower surfaces of layered structures, presenting a pronounced and robust surface interstitial electronic state. Additionally, we have explored the superconductivity of these systems, and found that both Ti$_2$N and Zr$_2$N are intrinsic superconductors with superconducting transition temperatures below 1.0 K. Further results show that appropriate hole doping can significantly enhance their superconducting transition temperatures and can induce the Hf$_2$N monolayer to exhibit superconductivity. Our findings provide valuable insights into the design and tuning of novel electrides with enhanced superconducting properties, offering another pathway for deeply understanding the interplay between electride behavior and superconductivity in novel materials.

cond-mat.supr-con↗

arXiv2025

Dual role of stripe phase on superconducting correlation in a bilayer square lattice

While the stripe phase has been observed not only in monolayer cuprates but also in bilayer cuprates, research on its behavior in bilayer cuprates has been limited. Using constrained path quantum Monte Carlo, we explore the effect of stripes on the bilayer square lattice. We find the system exhibits short-range antiferromagnetism, which is enhanced by stripes and is strongest when the electron density of the interstriped rows reaches half-filling. The hole doping concentration plays a crucial role in the interaction between stripes and superconductivity. The $d$-wave pairing is enhanced by stripe potential $V_0$ at the hole doping $\delta_h=1/4$, whereas it is suppressed by stripe potential $V_0$ at the hole doping $\delta_h=1/8$. We elucidate this phenomenon through an analysis of the magnetism of the interstriped rows. Furthermore, the effective $d$-wave pairing is stronger in the bilayer model compared to the monolayer model when stripes are introduced on the square lattice. Overall, our unbiased numerical simulations provide a further understanding of the crossed bilayer square lattice model.

cond-mat.str-el↗

arXiv2025

Coplanar order induced by emergent frustration

Traditional frustration arises from the conflict between the spin alignments due to the geometry or the nature of the interactions. Here, we demonstrate a novel form of frustration, dubbed ``emergent frustration'', which is induced by the symmetry that emerges at the phase transition point of a quantum spin model devoid of geometric frustration. We study the two-dimensional bipartite chequerboard $J$-$Q$ model, which hosts the antiferromagnetic (AFM) state to the plaquette-singlet solid state (PSS) phase transition detected in the Shastry-Sutherland compound SrCu$_2({\rm BO}_3)_2$. By analyzing the scaling behavior of the R\'enyi entanglement entropy with smooth boundaries at the transition point, we observe an unexpected scaling behavior, which indicates that the number of Goldstone modes is five. We explain this by proposing a novel scenario in which the system is described by an effective quantum rotor Hamiltonian with a three-sublattice geometry that frustrates collinear order while supporting coplanar order. Such a three-sublattice geometry arises from the emergent symmetry of coexisting orders, which may also occur at the AFM-PSS transition point of SrCu$_2({\rm BO}_3)_2$. Therefore, experimental investigations are warranted.

cond-mat.str-el↗

arXiv2025

Topological Phase Transition and Geometrical Frustration in Fourier Photonic Simulator

XY models with continuous spin orientation play a pivotal role in understanding topological phase transitions and emergent frustration phenomena, such as superconducting and superfluid phase transitions. However, the complex energy landscapes arising from frustrated lattice geometries and competing spin interactions make these models computationally intractable. To address this challenge, we design a programmable photonic spin simulator capable of emulating XY models with tunable lattice geometries and spin couplings, allowing systematic exploration of their statistical behavior. We experimentally observe the Berezinskii-Kosterlitz-Thouless (BKT) transition in a square-lattice XY model with nearest-neighbor interactions, accurately determining its critical temperature. Expanding to frustrated systems, we implement the approach in triangular and honeycomb lattices, uncovering sophisticated phase transitions and frustration effects, which are consistent with theoretical predictions. This versatile platform opens avenues for probing unexplored XY model phenomena across diverse geometries and interaction schemes, with potential applications in solving complex optimization and machine learning problems.

physics.optics↗

arXiv2025

Deconfined criticality as intrinsically gapless topological state in one dimension

Deconfined criticality and gapless topological states have recently attracted growing attention, as both phenomena go beyond the traditional Landau paradigm. However, the deep connection between these two critical states, particularly in lattice realization, remains insufficiently explored. In this Letter, we reveal that certain deconfined criticality can be regarded as an intrinsically gapless topological state without gapped counterparts in a one dimensional lattice model. Using a combination of field-theoretic arguments and large-scale numerical simulations, we establish the global phase diagram of the model, which features deconfined critical lines separating two distinct spontaneous symmetry breaking ordered phases. More importantly, we unambiguously demonstrate that the mixed anomaly inherent to deconfined criticality enforces topologically robust edge modes near the boundary, providing a general mechanism by which deconfined criticality manifests as a gapless topological state. Our findings not only offer a new perspective on deconfined criticality but also deepen our understanding of gapless topological phases of matter.

cond-mat.str-el↗

arXiv2025

Quantum entanglement of fermionic symmetry-enriched quantum critical points in one dimension

Quantum entanglement can be an effective diagnostic tool for probing topological phases protected by global symmetries. Recently, the notion of nontrivial topology in critical systems has been proposed and is attracting growing attention. In this work, as a concrete example, we explore the quantum entanglement properties of fermionic symmetry-enriched quantum critical points by constructing exactly solvable models based on stacked multiple Kitaev chains. We first analytically establish the global phase diagram using entanglement entropy and reveal three topologically distinct gapped phases with different winding numbers, along with three topologically distinct transition lines separating them. Importantly, we unambiguously demonstrate that two transition lines exhibit fundamentally different topological properties despite sharing the same central charge. Specifically, they display nontrivial topological degeneracy in the entanglement spectrum under periodic boundary conditions, thereby generalizing the Li-Haldane bulk-boundary correspondence to a broader class of fermionic symmetry-enriched criticality. Additionally, we identify a novel Lifshitz multicritical point at the intersection of the three transition lines, which also exhibits nontrivial topological degeneracy. This work provides a valuable reference for investigating gapless topological phases of matter from the perspective of quantum entanglement.

cond-mat.str-el↗