arXiv subjects

Hai-Qing Lin

Publications and source records attributed to Hai-Qing Lin.

At least 37 records · Page 2Linked to original sources

arXiv2025

Superradiant phase transitions in the quantum Rabi model: Overcoming the no-go theorem through anisotropy

Although the superradiant phase transition (SRPT) is prohibited in the paradigmatic quantum Rabi model due to the no-go theorem caused by the $\mathbf{A}^2$ term, we demonstrate two distinct types of SRPTs emerging from the normal phase in the anisotropic quantum Rabi model. A discontinuous phase transition between the two types of superradiant phases also emerges in the presence of a strong $\mathbf{A}^2$ term. Additionally, a rich phase diagram featuring a triple point, which connects first- and second-order phase transitions, is derived analytically and confirmed through numerical diagonalization at large effective system sizes. Finally, distinct critical behavior at the triple point is revealed and contrasted with that of a single continuous SRPT. This work may open a new avenue for observing SRPTs in their intrinsic form without altering the $\mathbf{A}^2$ term, while also offering a practical platform for exploring rich quantum phenomena.

quant-ph↗

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 $δ_h=1/4$, whereas it is suppressed by stripe potential $V_0$ at the hole doping $δ_h=1/8$. 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 Rényi 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

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

Altermagnetic Ground State in Distorted Kagome Metal CsCr$_3$Sb$_5$

The CsCr$_3$Sb$_5$ exhibits superconductivity in close proximity to a density-wave (DW) like ground state at ambient pressure\cite{Liu:2024aa}, however details of the DW is still elusive. Using first-principles density-functional calculations, we found its ground state to be a $4\times2$ altermagnetic spin-density-wave (SDW) at ambient pressure, with an averaged effective moment of $\sim$1.7$μ_B$/Cr. The magnetic long range order is coupled to the lattice, generating 4$a_0$ structural modulation. Multiple competing SDW phases are present and energetically close, suggesting strong magnetic fluctuation at finite temperature. The electronic states near Fermi level are dominated by Cr-3$d$ orbitals, and the kagome flat bands are closer to the Fermi level than those in the $A$V$_3$Sb$_5$ family in paramagnetic state. When external pressure is applied, the energy differences between competing orders and structural modulations are suppressed. Yet, the magnetic fluctuation remains present and important even at high pressure because the high-symmetry kagome lattice is unstable in nonmagnetic phase up to 30 GPa. Our results suggest the crucial role of magnetism to stabilize the crystal structure, under both ambient and high pressure.

cond-mat.supr-con↗

arXiv2025

Exact Spectral Function of One-Dimensional Bose Gases

Exactly solved models provide rigorous understanding of many-body phenomena in strongly correlated systems. In this article, we report a breakthrough in uncovering universal many-body correlated properties of quantum integrable Lieb-Liniger model. We calculate exactly the dynamical correlation functions by computing the form factor through a newly developed method, by which we are capable of calculating all possible "relative excitations" over the ground state or a finite temperature state at a high precision. Consequently, full spectral functions obtained for the model manifests the unique power-law singularity behaviour at the spectral threshold, confirming the validity of nonlinear Luttinger liquid theory. Our method advances the theory of dynamical correlation functions with high precision towards the thermodynamic limit, and is capable of benchmarking experimental observation of such novel correlated properties.

cond-mat.quant-gas↗

arXiv2025

One-body dynamical correlation function of Lieb-Liniger model at finite temperature

The dynamical correlated properties of one-dimensional (1D) Bose gases provide profound understanding of novel physics emergent from collective excitations, for instance, the breakdown of off-diagonal long-range order, and the establishment of Tomonaga-Luttinger liquid theory. However, due to the nonperturbative nature of 1D many-body systems, the exact evaluation of correlation functions is notoriously difficult. Here, by means of a form factor approach based on an algebraic Bethe ansatz and numerics, we present a thorough study on the one-body dynamical correlation function (1BDCF) of the Lieb-Liniger model at finite temperature. The influence of thermal fluctuation and interaction on the behavior of 1BDCF has been demonstrated and analyzed from various perspectives, including the spectral distribution, the line shape at fixed momentum, and the corresponding static correlations.

cond-mat.quant-gas↗

arXiv2025

Flat-Band Enhanced Antiferromagnetic Fluctuations and Superconductivity in Pressurized CsCr$_3$Sb$_5$

The spin dynamics and electronic orders of the kagome system at different filling levels stand as an intriguing subject in condensed matter physics. By first-principles calculations and random phase approximation analyses, we investigate the spin fluctuations and superconducting instabilities in kagome phase of CsCr$_3$Sb$_5$ under high pressure. At the filling level slightly below the kagome flat bands, our calculations reveal strong antiferromagnetic spin fluctuations in CsCr$_3$Sb$_5$, together with a leading $s_{\pm}$-wave and a competing ($d_{xy}$, $d_{x^2-y^2}$)-wave superconducting order. Unlike the general intuition that the flat bands are closely related to the ferromagnetic correlations, here we propose a sublattice-momentum-coupling-driven mechanism for the antiferromagnetic fluctuations enhanced from the unoccupied flat bands. The mechanism is generally applicable to kagome systems where the Fermi level intersects near the flat bands, offering a new perspective for future studies of geometrically frustrated systems.

cond-mat.supr-con↗

arXiv2025

Exploring nontrivial topology at quantum criticality in a superconducting processor

The discovery of nontrivial topology in quantum critical states has introduced a new paradigm for classifying quantum phase transitions and challenges the conventional belief that topological phases are typically associated with a bulk energy gap. However, realizing and characterizing such topologically nontrivial quantum critical states with large particle numbers remains an outstanding experimental challenge in statistical and condensed matter physics. Programmable quantum processors can directly prepare and manipulate exotic quantum many-body states, offering a powerful path for exploring the physics behind these states. Here, we present an experimental exploration of the critical cluster Ising model by preparing its low-lying critical states on a superconducting processor with up to $100$ qubits. We develop an efficient method to probe the boundary $g$-function based on prepared low-energy states, which allows us to uniquely identify the nontrivial topology of the critical systems under study. Furthermore, by adapting the entanglement Hamiltonian tomography technique, we recognize two-fold topological degeneracy in the entanglement spectrum under periodic boundary condition, experimentally verifying the universal bulk-boundary correspondence in topological critical systems. Our results demonstrate the low-lying critical states as useful quantum resources for investigating the interplay between topology and quantum criticality.

quant-ph↗

arXiv2025

Gapless Symmetry-Protected Topological States in Measurement-Only Circuits

Measurement-only quantum circuits offer a versatile platform for realizing intriguing quantum phases of matter. However, gapless symmetry-protected topological (gSPT) states remain insufficiently explored in these settings. In this Letter, we generalize the notion of gSPT to the critical steady state by investigating measurement-only circuits. Using large-scale Clifford circuit simulations, we investigate the steady-state phase diagram across several families of measurement-only circuits that exhibit topological nontrivial edge states at criticality. In the Ising cluster circuits, we uncover a symmetry-enriched non-unitary critical point, termed symmetry-enriched percolation, characterized by both topologically nontrivial edge states and string operator. Additionally, we demonstrate the realization of a steady-state gSPT phase in a $\mathbb Z_4$ circuit model. This phase features topological edge modes and persists within steady-state critical phases under symmetry-preserving perturbations. Furthermore, we provide a unified theoretical framework by mapping the system to the Majorana loop model, offering deeper insights into the underlying mechanisms.

cond-mat.str-el↗

arXiv2024

Flat bands and superconductivity induced by periodic strain in monolayer graphene

Superconductivity in single-layer graphene has attracted considerable interest. Here, using the determinant quantum Monte Carlo method, we study transitions of superconductivity and magnetism in a monolayer graphene with a special periodic strain. Consistent with experiments, the deformation accumulates a series of flat bands, whose robustness under interaction is verified through electron localization in real space. During the reconstruction of the band structure, the superconductivity appears in flat band range with next-nearest neighbor $d+id$ pairing symmetry dominating other modes and is accompanied by ferromagnetism caused by symmetry breaking. We also demonstrate that the strain-induced symmetry breaking would accumulate an energy-gap antiferromagnetic insulating phase at half filling even under the limitation of not strong enough interaction, which shows its potential as a platform that exhibits strongly correlated phenomena.

cond-mat.str-el↗

arXiv2024

Charge Stripe Manipulation of Superconducting Pairing Symmetry Transition

Charge stripes have been widely observed in many different types of unconventional superconductors, holding varying periods ($\mathcal{P}$) and intensities. However, a general understanding on the interplay between charge stripes and superconducting properties is still incomplete. Here, using large-scale unbiased numerical simulations on a general inhomogeneous Hubbard model, we discover that the charge-stripe period $\mathcal{P}$, which is variable in different real material systems, could dictate the pairing symmetries -- $d$ wave for $\mathcal{P} \ge 4$, $s$ and $d$ waves for $\mathcal{P} \le 3$. In the latter, tuning hole doping and charge-stripe amplitude can trigger a $d$-$s$ wave transition and magnetic-correlation shift, where the $d$-wave state converts to a pairing-density wave state, competing with the $s$ wave. These interesting phenomena arise from an unusual stripe-induced selection rule of pairing symmetries around on-stripe region and within inter-stripe region, giving rise to a critical point of $\mathcal{P}=3$ for the phase transition. In general, our findings offer new insights into the differences in the superconducting pairing mechanisms across many $\mathcal{P}$-dependent superconducting systems, highlighting the decisive role of charge stripe.

cond-mat.str-el↗

arXiv2024

Microscopic origin of quantum supersonic phenomenon in one dimension

Using the Bethe ansatz (BA), we rigorously obtain non-equilibrium dynamics of an impurity with a large initial momentum $Q$ in the one-dimensional (1D) interacting bosonic medium. We show that magnon and exciton-like states obtained from the BA equations drastically determine the oscillation nature of the quantum flutter with the periodicity given by $τ_{\rm QF} = 2π/(|\varepsilon_{\rm c}(0)|- |\varepsilon_{\rm s}(0)|)$. Where the charge and spin dressed energies $\varepsilon_{\rm c,s}(0)$ are precisely given by the thermodynamical BA equations. While we further find a persistent revival dynamics of the impurity with a larger periodicity $τ_{L} = L/\left(v_{\rm c}(Q-k^*)-v_{\rm s}(k^*)\right)$ than $τ_{\rm QF}$, manifesting a quantum reflection induced by the periodic boundary conditions of a finite length $L$, here $v_{\rm c,s}$ are the sound velocities of charge and spin excitations, respectively, and $k^*$ is a characteristic momentum of the impurity to the Fermi point. Finally, we study the application of such a magnon impurity as a quantum resource for measuring the gravitational force.

cond-mat.quant-gas↗

arXiv2024

Phase Transitions in the Anisotropic Dicke-Stark Model with $A$-square terms

The superradiant phase transition (SRPT) is forbidden in the standard isotropic Dicke model due to the so-called no-go theorem induced by A-square term. In the framework of the Dicke model, we demonstrate that SRPTs can occur at both zero and finite temperatures if we intrinsically tune the rotating wave and count-rotating atom-cavity coupling independently, and/or introduce the nonlinear Stark coupling terms, thus overcoming the no-go theorem. The phase transitions in this so-called anisotropic Dicke-Stark model share the same universality class with the original Dicke model. The critical coupling strength of this model decreases with the isotropic constant gradually, but can be driven to zero quickly with the strong nonlinear Stark coupling. We believe that we have proposed a feasible scheme to observe the SRPT in the future solid-state experiments.

quant-ph↗

arXiv2024

Gifts from long-range interaction: Emergent gapless topological behaviors in quantum spin chain

Topology in condensed matter physics is typically associated with a bulk energy gap. However, recent research has shifted focus to topological phases without a bulk energy gap, exhibiting nontrivial gapless topological behaviors. In this letter, we explore a cluster Ising chain with long-range antiferromagnetic interactions that decay as a power law with the distance. Using complementary numerical and analytical techniques, we demonstrate that long-range interactions can unambiguously induce an algebraic topological phase and a topological Gaussian universality, both of which exhibit nontrivial gapless topological behaviors. Our study not only provides a platform to investigate the fundamental physics of quantum many-body systems but also offers a novel route toward searching for gapless topological phases in realistic quantum simulators.

cond-mat.str-el↗

arXiv2024

Ac$_3$Ni$_2$O$_7$ and La$_2$$Ae$Ni$_2$O$_6$F ($Ae$ = Sr, Ba): Benchmark Materials for Bilayer Nickelate Superconductivity

We theoretically propose Ac$_3$Ni$_2$O$_7$, La$_2$BaNi$_2$O$_6$F, and La$_2$SrNi$_2$O$_6$F compounds to be benchmark materials for bilayer nickelate superconductivity. The stable phase of Ac$_3$Ni$_2$O$_7$ and La$_2$BaNi$_2$O$_6$F are found to be $I4/mmm$ without the lattice distortion caused by octahedra rotation at ambient pressure, where as the lattice distortion in La$_2$SrNi$_2$O$_6$F can be suppressed with relatively small external pressure of 4 GPa. The magnetism, electronic structure and spin susceptibilities of Ac$_3$Ni$_2$O$_7$ are extremely close to those of La$_3$Ni$_2$O$_7$ at 30 GPa. The ground state of La$_2$BaNi$_2$O$_6$F and La$_2$SrNi$_2$O$_6$F are antiferromagnetically coupled checkerboard bilayer with sizable magnetic moment on Ni. In addition, the inter-layer coupling $J_{\perp}$ between Ni-bilayers in La$_2$BaNi$_2$O$_6$F or La$_2$SrNi$_2$O$_6$F is only $\sim$ 1/10 of that in Ac$_3$Ni$_2$O$_7$ or La$_3$Ni$_2$O$_7$ at 30 GPa. We argue that these compounds may serve as superconducting candidates at ambient pressure and can be employed to testify theoretical proposals for bilayer nickelate superconductivity.

cond-mat.supr-con↗

arXiv2024

Low-lying excited states quantum entanglement and continuous quantum phase transitions: The criticality of a one-dimensional deconfined critical point

From the perspective of low-lying excited states, we study the deconfined quantum critical point (DQCP) in a one-dimensional quantum spin chain by means of the entanglement entropy and fidelity. Our results show that there is a close connection between the reconstruction of low-lying excitation spectra and the DQCP. The precise position of the critical point and its continuous nature is indicated by the singular behavior of the entanglement and fidelity of the first-excited state. Furthermore, compared with the Berezinskii-Kosterlitz-Thouless type phase transitions, which also go beyond the scope of Landau-Ginzburg-Wilson paradigm, we attempt to reveal the essence of different types of symmetries on both sides of the DQPT from different manifestations of entanglement singularity.

cond-mat.str-el↗

arXiv2024

Universal entanglement spectrum in gapless symmetry protected topological states

Quantum entanglement marks a definitive feature of topological states. However, the entanglement spectrum remains insufficiently explored for topological states without a bulk energy gap. Using a combination of field theory and numerical techniques, we accurately calculate and analyze the entanglement spectrum of gapless symmetry protected topological states in one dimension. We highlight that the universal entanglement spectrum not only encodes the nontrivial edge degeneracy, generalizing the Li-Haldane conjecture to gapless topological states, but also contains the operator content of the underlying boundary conformal field theory. This implies that the bulk wave function can act as a fingerprint of both quantum criticality and topology in gapless symmetry protected topological states. We also identify a symmetry enriched conformal boundary condition that goes beyond the conventional conformal boundary condition.

cond-mat.str-el↗