arXiv subjects

Hong-Gang Luo

Publications and source records attributed to Hong-Gang Luo.

At least 19 recordsLinked to original sources

arXiv2026

Field-amplified readouts of weak altermagnetic exchange in MnF$_2$

MnF$_2$, the textbook two-sublattice antiferromagnet, has reemerged as a prototypical altermagnet, yet the sublattice-odd exchange that defines this identity remains under active debate: it enters the magnon splitting only in quadrature with the dipole--dipole interaction, its magnitude suppressed and its sign erased. An overdetermined first-principles total-energy mapping resolves this scale as a seventh-neighbor imbalance $\delta{J_7}\simeq+8~\mu$eV. The resulting Hamiltonian, with the dipole--dipole interaction included explicitly, reproduces the low-energy gap and the visible finite-momentum splitting. A longitudinal field $B\parallel c$ then acts as a linear amplifier of the hidden scale, opening two signed, field-linear readouts. The first is the compensation field $B^\ast(\mathbf Q)$, the position of minimum splitting, which is equal and opposite at the rotation-related partner momenta: a shift from zero field is itself evidence of a finite imbalance, its side gives the sign, and its magnitude, $|B^\ast|\simeq0.34$~T here, gives the scale. The second is the fixed-field contrast of the partner splittings, $\simeq0.14$~meV at $1$~T, six times the zero-field excess: a sign check from just two spectra. Both readouts survive a $0.12$~meV energy resolution, and the construction carries over to any easy-axis collinear altermagnet, bringing $\mu$eV altermagnetic exchange within present instrumental reach.

cond-mat.str-el↗

arXiv2026

Spin Splitter without Spin-Split Bands: A Reconfigurable Altermagnetic Texture

The altermagnetic spin-splitter effect converts an electric field into a transverse pure spin current, with no net magnetization and no charge-Hall counterpart. In established materials this function is tied to crystal-fixed spin-split bands that lock the polarization axis to the lattice. We show that the noncoplanar counter-spiral ground state of a frustrated honeycomb magnet instead carries the altermagnetic operation through a $\mathbf Q$-locked helicity mirror $g$. The mirror selects the spin-current polarization and forbids the perpendicular one, while an antitranslation $\Theta$ forbids even-parity spin splitting. Band splitting and spin-splitter response therefore rest on different symmetry elements. Either element alone enforces the charge-Hall zero---a redundancy absent from other spin--orbit-free noncollinear routes---and a charge Hall appears only when both elements are removed. Hole doping then realizes a \emph{spin splitter without spin-split bands}---the symmetry-allowed odd-parity residual below $2\times10^{-7}$ of the hopping $t$ at the Fermi level---with $\sigma_H^{(s_y)}=0.082\,e^2/h$ without spin--orbit coupling and with zero charge Hall response. Selecting among the three degenerate $\mathbf{Q}$ orientations rotates the polarization axis in exact $120^\circ$ steps at fixed magnitude and charge-Hall zero; the selection rules persist in a $32$-site cell accessible to programmable photonic and circuit lattices.

cond-mat.str-el↗

arXiv2026

Ground-state phase diagram and route to supersolidity in a two-component extended Bose-Hubbard model

We investigate the ground-state phase diagram of a two-component extended Bose-Hubbard model recently realized with dipolar excitons, using the projected entangled-pair states. For the experimentally relevant parameter regime, checkerboard, Mott-insulating, superfluid, and vacuum phases are identified. These phases exhibit orbital-selective character, wherein the two components occupy different quantum states, but we find no evidence for a supersolid phase. The absence of supersolidity is attributed to the strongly interaction-dominated microscopic energy scales, which severely restrict the superfluid regime. Guided by the supersolid mechanism, we further explore a nearby parameter regime with enhanced hopping of one component and identify an orbital-selective supersolid phase. Further finite bond-dimension and unit-cell analyses establish the robustness of this phase. Our results clarify the zero-temperature phase structure of the dipolar-exciton platform and provide a possible route toward realizing supersolidity in this setting.

cond-mat.quant-gas↗

arXiv2026

Generalized Holstein-Primakoff transformation with optimizable bosonic truncation

Spin-wave theory provides a quasiparticle description of excitation spectra in quantum spin systems. In this theory, the spin operators are usually bosonized by the Holstein-Primakoff transformation or the Dyson-Maleev transformation. In practical calculations, the bosonized Hamiltonian has to be truncated, and thus the resulting excitation spectra depend on the choice of bosonic representation. Here, we introduce a generalized Holstein-Primakoff transformation that continuously interpolates between the conventional Holstein-Primakoff and Dyson-Maleev transformations through a single parameter. The formulation naturally extends to the SU(N) algebra and provides a flexible framework for optimizing bosonic truncations beyond harmonic order. As an application, we investigate the spin-1 bilinear-biquadratic model on the square lattice. By combining the generalized Holstein-Primakoff transformation with continuous similarity transformations, we obtain excitation spectra in excellent agreement with tensor-network calculations. Our results demonstrate that optimizing the bosonic representation substantially reduces truncation errors and provides quantitatively reliable descriptions of both quasiparticle dispersions and multi-boson continua.

cond-mat.str-el↗

arXiv2026

Field-Selected Topological Buffering in a Disordered Skyrmion Crystal

Quenched disorder can disrupt crystalline order without immediately destroying the topology of its constituent textures, but the relation between these processes in skyrmion crystals remains unclear. Using large-scale simulations of a triangular-lattice chiral magnet with random DM interactions, we show that the magnetic field selects between two disordering routes. At high fields, global translational coherence is lost at a weak-disorder scale, while sixfold bond-orientational order survives to a larger disorder strength and the total topological charge remains nearly locked up to a substantially larger scale. The resulting interval defines a topological buffer containing a Bragg-glass- like skyrmion regime followed by a skyrmion-glass regime. Finite-size scaling, spatial correlations, defect statistics, and spin autocorrelations support their distinct structural and glassy character. At lower fields, bond-orientational disordering nearly coincides with topological reconstruction, eliminating the skyrmion-glass window and contracting the buffer. These results identify the magnetic field as a control knob for separating crystalline disordering from topological-charge loss and establish topological buffering as a mechanism by which topological textures can remain robust in structurally disordered media.

cond-mat.str-el↗

arXiv2026

Beyond-adiabatic flat Chern bands from a double-helix skyrmion crystal

A central challenge in flat-band engineering is suppressing kinetic energy without sacrificing Berry curvature. We show that a double-helix skyrmion crystal (DHSKX)--two sublattice-resolved skyrmion textures locked at opposite helicities, obtained here as the classical ground state of a frustrated honeycomb spin model--provides such a route under double exchange. The key mechanism is a single real-space organization, phase clustering: the $\pi$-locked helicities expel the wave function's phase winding from the skyrmion cores, and the magnetic $C_3$ symmetry pins it into three phase-locked clusters whose distributed destructive interference cancels net transport while preserving the Berry curvature. Ordinary skyrmion crystals, even with the same symmetry, do not develop this organization. Phase clustering yields isolated flat $|C| = 1$ Chern bands over broad coupling windows, one of which surpasses the adiabatic reference in quantum geometry at intermediate coupling. In this beyond-adiabatic window, band-projected exact diagonalization gives finite-size evidence consistent with $\nu = 1/3$ Laughlin-type fractional-Chern-insulator physics; the same texture also hosts a higher-Chern ($C = -2$) flat band. Built from site-resolved complex hoppings alone, the DHSKX architecture is directly programmable in topolectric, acoustic, and photonic platforms.

cond-mat.str-el↗

arXiv2026

Interaction-Split Edge Spectral Flow in an Integer Hubbard Thouless Pump

We show that local Hubbard correlations can split the open-boundary spectral flow of an otherwise integer Thouless pump. Using density-matrix renormalization group calculations for a period-three Hubbard chain with a sliding onsite-repulsion modulation, we identify at filling $\rho=2/3$ a correlated insulating pump with many-body Chern number $C=+2$, corresponding to two units of charge transported per cycle. A spin-degenerate Hartree/Aubry--Andr\'e--Harper reference model realizes the same integer response through simultaneous edge spectral flow in two independent spin channels. In the full Hubbard model, however, this integer bulk pump is implemented differently at an open boundary: the edge flow separates into two boundary events, and the intervening low-energy state is a nearly charge-neutral triplet edge excitation. Thus the quantized bulk charge transport remains integer, but local correlations reorganize its boundary realization into separated charged and neutral spinful sectors.

cond-mat.str-el↗

arXiv2026

The antiferromagnetic Chern insulator phase in the Kane-Mele-Hubbard model

The emergence of the antiferromagnetic (AFM) Chern insulator (AFCI) phase in the Kane-Mele-Hubbard (KMH) model with a finite sublattice potential is investigated. The AFCI, characterized by AFM correlations coexisting with quantized Hall conductance, has long raised the question of whether it can exist in the KMH model that respects time-reversal symmetry (TRS). Using exact diagonalization, we analyze the excitation gap, anisotropic AFM correlations along the $z$ axis and in the $xy$ plane, and the fidelity susceptibility under twisted boundary conditions, all of which provide consistent evidence for the AFCI phase. In particular, our numerical evaluation on the (spin) Chern number reveals a breakdown of adiabatic continuity in the twist-angle space, indicating an instability toward TRS breaking driven by Hubbard-induced AFM perturbations. A modified computational scheme is further proposed, which yields a robust quantized Chern number $C=1$ within this phase.

cond-mat.str-el↗

arXiv2026

Nonlinearity-Induced Thouless Pumping in Quasiperiodic Lattices

Nonlinear Thouless pumping has been established in periodic lattices; its counterpart in quasiperiodic lattices remains unexplored. Here, we show a nonlinear topological pumping of gap solitons in quasiperiodic lattices where the local nonlinear self-consistent potentials lead to a lattice potential reconstruction; as a result, an emergent topological structure induced by this local reconstruction governs the dynamics of the gap solitons. This enables solitons to adiabatically occupy a single topological band, realizing quasi-quantized Thouless pumping. In addition, the intrinsic lattice perturbations disrupt this band occupation, which drives solitons into a non-quantized drifting regime. However, even in this regime, we also find that the soliton transport is constrained by the topological properties of a critical rational approximant. Tuning nonlinearity or lattice scaling reveals a controllable switching among topological pumping, drifting, and localization. Our work uncovers a mechanism for nonlinearity-induced topological behavior in complex lattice potentials.

quant-ph↗

arXiv2025

Quantum Mpemba Effect Induced by Non-Markovian Exceptional Points

Quantum Mpemba effect describes an anomalous phenomenon of accelerated relaxation which is of fundamental interest in the field of nonequilibrium thermodynamics. Conventional theories on this phenomenon strongly rely on the Born-Markovian approximation resulting in a Lindblad-type master equation whose evolution is governed by a Liouvillian superoperator. It has been demonstrated that exceptional points of the Liouvillian superoperator can induce the Mpemba effect in Markovian regimes. Moving beyond this Markovian limit, we here propose a mechanism for realizing the quantum Mpemba effect in a general non-Markovian relaxation process by means of non-Markovian exceptional points. We verify the feasibility of this mechanism within a dissipative quantum harmonic oscillator model, which is exactly solvable and experimentally practical. Providing new insight into the interesting non-equilibrium dynamics, our work paves a way to accelerate the transfer of energy and information in quantum systems.

quant-ph↗

arXiv2025

On the Origin of the Hidden Symmetry in the Asymmetric Quantum Rabi Model

The introduction of an asymmetric term into the quantum Rabi model generally lifts energy-level degeneracies. However, when the asymmetry parameter takes specific multiples of the bosonic mode frequency, level degeneracies reappear$-$a phenomenon referred to as the hidden symmetry in the asymmetric quantum Rabi model. Identifying the origin of this hidden symmetry and its explicit operator form constitutes two central tasks in studying this system. Here, we investigate the origin of this hidden symmetry using the method of two successive diagonalizations, with a focus on physics in the regime where the ratio between the two-level splitting $\Delta$ and the mode frequency $\omega$ satisfies $\Delta/\omega \gg 1$. We find that the hidden symmetry stems from energy-level matching within the asymmetric double-well potential, a picture strongly supported by the wavefunctions of both the ground and excited states. Moreover, the emergence of an excited-state quantum phase transition is identified and qualitatively discussed, which arises from the breaking and restoration of this hidden symmetry across different coupling regimes. Our results provide deeper insight into the physics of the asymmetric quantum Rabi model, particularly in the previously less-explored strong-coupling regime where $\Delta/\omega \gg 1$.

quant-ph↗

arXiv2025

Practical algorithm for simulating thermal pure quantum states

The development of novel quantum many-body computational algorithms relies on robust benchmarking. However, generating such benchmarks is often hindered by the massive computational resources required for exact diagonalization or quantum Monte Carlo simulations, particularly at finite temperatures. In this work, we propose a new algorithm for obtaining thermal pure quantum states, which allows efficient computation of both mechanical and thermodynamic properties at finite temperatures. We implement this algorithm in our open-source C++ template library, Physica. Combining the improved algorithm with state-of-the-art software engineering, our implementation achieves high performance and numerical stability. As an example, we demonstrate that for the $4 \times 4$ Hubbard model, our method runs approximately $10^3$ times faster than $\mathcal{H}\Phi$ 3.5.2. Moreover, the accessible temperature range is extended down to $\beta = 32$ across arbitrary doping levels. These advances significantly push forward the frontiers of benchmarking for quantum many-body systems.

cond-mat.str-el↗

arXiv2025

Emergent supercounterfluid and quantum phase diagram of two-component interacting bosons in one-dimensional optical lattice

Motivated by a recent experiment that realizes nearest-neighbor dipolar couplings in an optical lattice [C. Lagoin, $\textit{et al.}$, Nature $\textbf{609}$, 485 (2022)], we study a one-dimensional version of the two-component extended Bose-Hubbard model via the density-matrix renormalization group method. By using the nearest-neighbor and on-site interaction parameters from the experiment, we start by mapping the quantum phase diagram in the hopping parameters $t_{A}\mbox{-}t_{B}$ plane with boson densities $\rho_{A}=\rho_{B}=1/2$. In addition to the density wave phase reported in the experiment, we find several regimes of superfluidity when one or two hopping parameters are large enough, and interestingly there is a supercounterfluid phase at moderate and comparable hopping parameters. The universality classes of these phase transitions are analyzed from the correlation functions, excitation gaps, and entanglement entropy. In particular, a Berezinskii-Kosterlitz-Thouless type is recognized several gapped-to-gapless transitions. In addition, we also study the quantum phase transitions when varying $\rho_{B}$ from 0 to 1 while keeping $\rho_A = 1/2$. We identify a supersolid phase in a wide range of $1/2<\rho_B<1$. Our work paves the way for realizing exotic many-body phases in cold atom experiments upon proper tuning of experimental parameters.

cond-mat.quant-gas↗

arXiv2025

Energy Dispersion, Superconductivity and Magnetic Fluctuations in Stacked Altermagnetism Materials

Recently, altermagnetism (AM) has emerged as a new category of magnetism, alongside conventional antiferromagnetism (AFM) and ferromagnetism (FM). In an AM, superconductivity (SC) is faced with a dilemma that the spin-polarized bands, induced by the broken time reversal (T ) symmetry, dominantly supports spin-triplet pairing. In contrast, AM spin fluctuations routinely facilitate spin-singlet pairing as in AFM. Consequently, unconventional SC is either absent or weak in AM materials. Here, we propose that stacking 2D AM materials could resolve this dilemma. Stacked 2D materials have yielded a variety of new electronic properties by altering the symmetries inherent in the monolayer. In a 2D anisotropic Hubbard model, we investigate the general energy dispersions of both single-layer and stacked AM materials. We demonstrate that AM sheet stacking can alter the original symmetries, consequently affecting the energy dispersion. The interlayer magnetic coupling enhances the low q magnetic fluctuations. T symmetry is restored in the AA stacking with an antiferromagnetic interlayer coupling, and then both the energy dispersion and pairing interaction are in favor of spin-singlet SC. The ferromagnetic interlayer coupling in the AB stacking not only recovers T symmetry but also supports spin-triplet pairing. It is further anticipated that twisted bilayer AM sheets could exhibit additional novel electronic properties, including topology, flat bands, and collective excitations. Our work illustrates that stacking sheets of AM materials could open up a unique research domain in exploring novel quantum phenomena and offer a fertile ground for potential electronic applications.

cond-mat.supr-con↗

arXiv2025

Altermagnetism and beyond in the $t$-$t^\prime$-$\delta$ Fermi-Hubbard model

In this work, we revisit the phase diagram of the $t$-$t^\prime$-$\delta$ Fermi-Hubbard model on the square lattice to gain a more comprehensive understanding of this correlated model at half filling. This model has recently become a prominent topic of research because it hosts altermagnetic phases. Using mean-field analysis, we identify four metallic phases and two insulating phases with nontrivial magnetic orders at an intermediate value of $\delta = 0.5$, presenting a rich ground-state phase diagram in the $U$-$t^\prime$ plane. We also highlight the distinct features of the Fermi surface topology for each metallic phase. To go beyond the mean-field theory, we employ the density-matrix renormalization group method to simulate the ground state numerically. The phase boundaries are determined from the discontinuities and peaks in the entanglement entropy and magnetizations. In addition to the phases identified in the mean-field theory, we find a valence-bond solid state in a narrow intermediate-$t'$ region. Our work offers a firm step forward in understanding the complex behaviors of correlated electrons in the $t$-$t^\prime$-$\delta$ Hubbard model over a large parameter space.

cond-mat.str-el↗

arXiv2025

Weak electronic correlations in the cobalt oxychalcogenide superconductor Na2CoSe2O

Motivated by the newly discovered Co-based superconductor Na2CoSe2O, we performed systematic calculations of its electronic band structures using the density functional theory (DFT) plus the dynamical mean-field theory (DMFT) approaches. Our comparative studies reveal weakly correlated and itinerant nature of the Co-3d electrons and show no sign of fluctuating local moments as expected in many other unconventional superconductors, although the Co eg orbitals are close to half filling. These suggest that Na2CoSe2O is a normal paramagnetic metal and its superconductivity might not be of strongly correlated nature, contrary to the initial speculation. We suggest future investigations of electron-phonon interactions to clarify its pairing mechanism.

cond-mat.str-el↗

arXiv2024

Low-temperature Quantum Metrology Enhanced by Strong Couplings

Equilibrium probes have been widely used in various noisy quantum metrology schemes. However, such an equilibrium-probe-based metrology scenario severely suffers from the low-temperature-error divergence problem in the weak-coupling regime. To circumvent this limit, we propose a strategy to eliminate the error-divergence problem by utilizing the strong coupling effects, which can be captured by the reaction-coordinate mapping. The strong couplings induce a noncanonical equilibrium state and greatly enhance the metrology performance. It is found that our metrology precision behaves as a polynomial-type scaling relation, which suggests the reduction of temperature can be used as a resource to improve the metrology performance. Our result is sharply contrary to that of the weakcoupling case, in which the metrology precision exponentially decays as the temperature decreases. Paving a way to realize a high-precision noisy quantum metrology at low temperatures, our result reveals the importance of the non-Markovianity in quantum technologies.

quant-ph↗

arXiv2024

Quantum dynamics in a spin-1/2 square lattice $J_{1}$-$J_{2}$-$\delta$ altermagnet

A key feature of the newly discovered altermagnet is that its spin degeneracy is lifted, although it has an antiferromagnetic order and zero net magnetization. In this work, we investigate a frustrated spin-1/2 $J_1$-$J_2$-$\delta$ Heisenberg model on the square lattice by the tensor network methodin combination with the linear spin-wave theory, with our focus on both the magnon excitations and longitudinal excitations.For a small $J_2$ and a finite range of $\delta$ we demonstrate that such a model hosts an altermagnetic ground state. Its magnon spectrum is split into two branches and the largest splitting occurs at $\left(\pm\pi/2, \pm\pi/2\right)$ in the Brillouin zone. The magnitudes of splitting in the two magnon modes are equal with respect to the case of $\delta=0$. Dynamical spin structure factors show that the low-energy peak in the longitudinal spectral weight around $(\pi/2, \pi/2)$ is also split, and thus the relative positions of the magnon modes and longitudinal modes in energy may change in the presence of a finite $\delta$. These findings demonstrate that the altermagnets harbor more complex quantum dynamics than the conventional collinear antiferromagnets.

cond-mat.str-el↗