arXiv subjects

Weiguo Yin

Publications and source records attributed to Weiguo Yin.

At least 19 recordsLinked to original sources

arXiv2026

A Human-AI Theorem Connecting Spontaneous and Field-Induced Mechanisms of Collective Behavior in One Dimension

Can an artificial intelligence (AI) generate a scientific hypothesis outside a human collaborator's active hypothesis space (AHS), and can human-AI research be organized to make such breakthroughs more likely? We document such a case while proving a theorem that connects two basic organizing mechanisms of statistical physics: collective behavior arising in zero field from competing interactions and that induced or controlled by an external field. A zero-field $O(n)$-vector open chain with arbitrary inhomogeneous nearest- and next-nearest-neighbor interaction functions $U_i(S_i\cdot{S}_{i+1})$ and $V_i(S_i\cdot{S}_{i+2})$ is microscopically, via a temperature-independent mapping at the Hamiltonian level, equivalent to a simpler $O(n)$ open chain with nearest-neighbor interaction $V_i( \sigma_i\cdot \sigma_{i+1})$ and axial single-spin potential $U_i(\sigma_i^z)$ for every integer $n\ge1$ and every system size $L\ge1$. The homogeneous linear specialization maps the foundational frustrated $J_1$-$J_2$ model onto the canonical $J$-$h$ field model---with $n=1,2,3$ being the Ising, XY, and Heisenberg classical spin models, respectively. An analogous theorem holds when the continuous $O(n)$ spins are replaced by the $q$-state Potts spins with the standard Potts interaction, implying a closed-form exact solution of the $J_1$-$J_2$ Potts open chain for every $q\ge2$ and every $L\ge1$. The emergence of the theorems from sustained human-AI collaboration suggests that involving AI throughout a systematic research program may incubate autonomous scientific breakthroughs.

cond-mat.stat-mech↗

arXiv2026

Data-efficient surrogate modeling of spectral functions using Gaussian processes: An application to the $t$-$t'$-$t''$-$J$ model

Spectral functions encode key many-body information but are costly to compute with high fidelity. Machine-learning surrogates have emerged as a powerful alternative, yet many approaches require large training datasets. We develop a data-efficient surrogate for spectral functions using the $t$-$t'$-$t''$-$J$ model, which describes the motion of a hole in a quantum antiferromagnet. Using $\sim$ 10$^5$ self-consistent Born approximation-based spectra from Lee, Carbone and Yin (Phys. Rev. B 107, 205132 (2023)), we train a deep-kernel Gaussian process surrogate model with sparse variational inference (DKL-SVGP) using only 10% of the available training spectra. We benchmark against feed-forward neural networks (FFNN) trained on the same reduced subset and on the full dataset. The proposed DKL-SVGP model consistently outperforms the reduced-data FFNN and, despite using only 10% of the training spectra, achieves spectrum-wise errors within the same order-of-magnitude as the full-data FFNN baseline. Worst-tail diagnostics show improved fidelity on difficult spectra, while peak-level analysis indicates that DKL-SVGP recovers dominant peak heights with comparable accuracy and improves peak-location agreement under a matched-peak evaluation that mitigates rare peak-swapping cases. Overall, these results highlight GP-based surrogates as a competitive and data-efficient approach for spectral-function prediction in scarce-data regimes.

cond-mat.str-el↗

arXiv2026

Site-decorated model for unconventional frustrated magnets: Ultranarrow phase crossover and two-dimensional spin reversal transition

The site-decorated Ising model is introduced to advance the understanding and experimental realization of the recently discovered one-dimensional (1D) finite-temperature ultranarrow phase crossover in an external magnetic field, while mitigating the geometric complexities of traditional bond-decorated models. The unconventional frustration and physics are clarified by exactly mapping the 1D site-decorated Ising model in a magnetic field onto a zero-field bond-decorated $J_1$-$J_2$ Ising model with conventional geometrical frustration. Furthermore, although higher-dimensional Ising models in an external field remain unsolved exactly, an exact solution for a spin-reversal transition -- driven by an exotic, hidden half-ice, half-fire state induced by site decoration -- is derived. This transition, triggered by a slight variation in temperature or magnetic field -- without changing its direction -- even in the weak-field limit, offers a promising route toward energy-efficient applications such as data storage and processing. The results suggest that site decoration offers an avenue for materials and device design, particularly in systems such as mixed $d$-$f$ compounds, optical lattices, and neural networks, calling for further studies with site-decorated Heisenberg models. In addition, the site-decorated model offers a rigorous test ground for artificial intelligence (AI) in science, as the analytic derivation of the present results was not only validated but also improved by a general-purpose large language model, inspiring the use of AI as scientific discoverer.

cond-mat.stat-mech↗

arXiv2025

Competing magnetic phases in Cr$_{3+\delta}$Te$_4$ are spatially segregated

Cr$_{1+x}$Te$_2$ is a self-intercalated vdW system that is of current interest for its room-temperature FM phases and tunable topological properties. Early NPD measurements on the monoclinic phase Cr$_3$Te$_4$ ($x=0.5$) presented evidence for competing FM and AFM phases. Here we apply neutron diffraction to a single crystal of Cr$_{3+\delta}$Te$_4$ with $\delta=-0.10$ and discover that it consists of two distinct monoclinic phases, one with FM order below $T_{\rm C} \approx 321$ K and another that develops AFM order below $T_{\rm N} \approx 86$ K. In contrast, we find that a crystal with $\delta=-0.26$ exhibits only FM order. The single-crystal analysis is complemented by results obtained with NPD, XPD, and TEM measurements on the $\delta=-0.10$ composition. From observations of spontaneous magnetostriction of opposite sign at $T_{\rm C}$ and $T_{\rm N}$, along with the TEM evidence for both monoclinic phases in a single thin ($\approx$ 100 nm) grain, we conclude that the two phases must have a fine-grained ($\lesssim$ 100 nm) intergrowth character, as might occur from high-temperature spinodal decomposition during the growth process. Calculations of the relaxed lattice structures for the FM and AFM phases with DFT provide a rationalization of the observed spontaneous magnetostrictions. Correlations between the magnitude and orientation of the magnetic moments with lattice parameter variation demonstrate that the magnetic orders are sensitive to strain, thus explaining why magnetic ordering temperatures and anisotropies can be different between bulk and thin-film samples, when the latter are subject to epitaxial strain. Our results point to the need to investigate the supposed coexistence FM and AFM phases reported elsewhere in the Cr$_{1+x}$Te$_2$ system, such as in the Cr$_5$Te$_8$ phase ($x=0.25$).

cond-mat.str-el↗

arXiv2025

Metamagnetic Transition in Low-Dimensional Site-Decorated Quantum Heisenberg Ferrimagnets

The prohibition of finite-temperature phase transition in one-dimensional (1D) Ising models and 1D/2D quantum Heisenberg models with short-range interactions fundamentally constrains the application potentials of low-dimensional magnetic materials. Recently, ultranarrow phase crossover (UNPC), which can approach a transition at a desirable finite temperature $T_0$ arbitrarily closely, was discovered in 1D decorated Ising chains and ladders. Here we present a theoretical study of similarly decorated, yet much more challenging, quantum Heisenberg ferrimagnets in a magnetic field, which features ferromagnetic backbone exchange $J$, antiferromagnetic site-decoration coupling $J_{AF}$, and different magnetic moments for the backbone and decorating spins $μ_aS_a<μ_bS_b$. We exactly solved the model in the large $J$ limit -- as a central-macrospin model -- and found two finite-temperature second-order transitions; just above $T_{c2}$ a ``half-ice, half-fire'' regime appears. Finite-$J$ weak-field results follow from an effective-field mapping, suggesting the emergence of UNPC at finite $T_0$ in 2D square lattices thanks to its exponentially strong initial magnetic susceptibility $χ_0\propto e^{4πS_a^2 J/T_0}$, though less likely in 1D chains where $χ_0\propto J/T_0$. These results may shed light on new technological applications of low-dimensional quantum spin systems and attract experimental and computational tests.

cond-mat.str-el↗

arXiv2025

Exact solution of the frustrated Potts model with next-nearest-neighbor interactions in one dimension via AI bootstrapping

The one-dimensional (1D) $J_1$-$J_2$ $q$-state Potts model is solved exactly for arbitrary $q$ by analytically block-diagonalizing the original $q^2\times q^2$ transfer matrix into a simple $2\times 2$ maximally symmetric subspace, based on using OpenAI's reasoning model o3-mini-high to exactly solve the $q=3$ case. Furthermore, by matching relevant subspaces, we map the Potts model onto a simpler effective 1D $q$-state Potts model, where $J_2$ acts as the nearest-neighbor interaction and $J_1$ as an effective magnetic field, nontrivially generalizing a 56-year-old theorem previously limited to the simplest case ($q=2$, the Ising model). Our exact results provide insights to phenomena such as atomic or electronic order stacking in layered materials and the emergence of dome-shaped phases in complex phase diagrams. This work is anticipated to fuel both research in 1D frustrated magnets for recently discovered finite-temperature application potentials and the fast moving topic area of AI in science.

cond-mat.stat-mech↗

arXiv2025

EuAuSb: An odd-parity helical variation on altermagnetism

EuAuSb is a triangular-lattice Dirac semimetal in which a topological Hall effect has been observed to develop in association with a magnetically-ordered phase. Our single-crystal neutron diffraction measurements have identified an incommensurate helical order in which individual ferromagnetic Eu$^{2+}$ layers rotate in-plane by $\sim$120$^{\circ}$ from one layer to the next. An in-plane magnetic field distorts the incommensurate order, eventually leading to a first order transition to a state that is approximately commensurate and that is continuously polarized as the bulk magnetization approaches saturation. From an analysis of the magnetic diffraction intensities versus field, we find evidence for a dip in the ordered in-plane moment at the same field where the topological Hall effect is a maximum, and we propose that this is due to field-induced quantum spin fluctuations. Our electronic structure calculations yield exchange constants compatible with the helical order and show that the bands near the Fermi level lose their spin degeneracy via a mechanism similar to that in the collinear altermagnets. We find that, unlike the even symmetry seen in the altermagnets, the spin-splitting in EuAuSb has odd-wave symmetry similar to that recently found in a number of coplanar magnetic materials.

cond-mat.str-el↗

arXiv2025

Single crystalline orthorhombic GdAlGe as a rare earth magnetic Dirac nodal-line metal

Crystal engineering is a method for discovering new quantum materials and phases, which may be achieved by external pressure or strain. Chemical pressure is unique in that it generates internal pressure perpetually to the lattice. As an example, GdAlSi from the rare-earth ($R$) $R$Al$X$ ($X =$ Si or Ge) family of Weyl semimetals is considered. Replacing Si with the larger isovalent element Ge creates sufficiently large chemical pressure to induce a structural transition from the tetragonal structure of GdAlSi, compatible with a Weyl semimetallic state, to an orthorhombic phase in GdAlGe, resulting in an inversion-symmetry-protected nodal-line metal. We find that GdAlGe hosts an antiferromagnetic ground state with two successive orderings, at $T_\mathrm{N1}$ = 35 K and $T_\mathrm{N2}$ = 30 K. In-plane isothermal magnetization shows a magnetic field induced metamagnetic transition at 6.2 T for 2 K. Furthermore, electron-hole compensation gives rise to a large magnetoresistance of $\sim 100\%$ at 2 K and 14 T. Angle-resolved photoemission spectroscopy measurements and density functional theory calculations reveal a Dirac-like linear band dispersion over an exceptionally large energy range of $\sim$ 1.5 eV with a high Fermi velocity of $\sim 10^6$ m/s, a rare feature not observed in any magnetic topological materials.

cond-mat.mtrl-sci↗

arXiv2025

Half-ice, half-fire-driven ultranarrow phase crossover in one-dimensional decorated $q$-state Potts ferrimagnets: An AI-co-led exploration

OpenAI's reasoning model o3-mini-high was used to carry out an exact analytic study of one-dimensional ferrimagnetic site- and bond-decorated $q$-state Potts models. We demonstrate that the finite-temperature ultranarrow phase crossover (UNPC), driven by a hidden "half-ice, half-fire" state recently discovered in the $q=2$ case (Ising model), persists for $q>2$. Moreover, we identify unique features for $q>2$, including the dome structure in the field-temperature phase diagram, and for large $q$ a secondary high-temperature UNPC to the fully disordered paramagnetic state. As the UNPC quickly approaches a genuine transition by enhancing $J$, the interaction between the backbone spins, two distinct behaviors emerge: In the site-decorated Potts model, $T_0$ is independent of $J$ and thus remains unchanged (Type-I UNPC), and in the bond-decorated Potts model with $q > 2$, $T_0$ depends on $J$ and quickly shifts toward a finite temperature as $J$ increases (Type-II UNPC). These results establish a versatile framework for engineering controlled fast state-flipping switches in low-dimensional systems. Our nine-dan artificial intelligence (AI)-contribution framework assigns AI the meritorious status of AI-co-led discovery in this work.

cond-mat.stat-mech↗

arXiv2024

Phase Switch Driven by the Hidden Half-Ice, Half-Fire State in a Ferrimagnet

The notion of "half fire, half ice" was recently introduced to describe an exotic macroscopic ground-state degeneracy emerging in a ferrimagnet under the critical magnetic field, in which the "hot" spins are fully disordered on the sublattice with smaller magnetic moments and the "cold" spins are fully ordered on the sublattice with larger magnetic moments. Here we further point out that this state has a twin named "half ice, half fire" in which the hot and cold spins switch positions. The new state is an excited state -- thus hidden in the ground-state phase diagram -- and is robust with respect to the interactions that destroy the half fire, half ice state. We demonstrate with exact results how this hidden state can drive phase switching at desirable finite temperature, even for the one-dimensional Ising model where phase transition at finite temperature is forbidden. We suggest that our findings may open a new door to the understanding and controlling of phase competition and transition in unconventional frustrated systems.

cond-mat.stat-mech↗

arXiv2024

Machine learning approach for vibronically renormalized electronic band structures

We present a machine learning (ML) method for efficient computation of vibrational thermal expectation values of physical properties from first principles. Our approach is based on the non-perturbative frozen phonon formulation in which stochastic Monte Carlo algorithm is employed to sample configurations of nuclei in a supercell at finite temperatures based on a first-principles phonon model. A deep-learning neural network is trained to accurately predict physical properties associated with sampled phonon configurations, thus bypassing the time-consuming {\em ab initio} calculations. To incorporate the point-group symmetry of the electronic system into the ML model, group-theoretical methods are used to develop a symmetry-invariant descriptor for phonon configurations in the supercell. We apply our ML approach to compute the temperature dependent electronic energy gap of silicon based on density functional theory (DFT). We show that, with less than a hundred DFT calculations for training the neural network model, an order of magnitude larger number of sampling can be achieved for the computation of the vibrational thermal expectation values. Our work highlights the promising potential of ML techniques for finite temperature first-principles electronic structure methods.

cond-mat.mtrl-sci↗

arXiv2024

Paradigm for approaching the forbidden phase transition in the one-dimensional Ising model at fixed finite temperature: Single chain in a magnetic field

In a previous paper [Weiguo Yin, Phys. Rev. Res. 6, 013331 (2024)], the forbidden spontaneous phase transition in the one-dimensional Ising model was found to be approachable arbitrarily closely in decorated ladders by ultranarrow phase crossover (UNPC) at a given finite temperature $T_0$ with the crossover width $2δT$ reduced exponentially, which resembles a genuine first-order transition with large latent heat. Here, I reveal that the forbidden phase transition can be approached at fixed $T_0$ as well in decorated single-chain Ising models in the presence of a magnetic field, in which $T_0$ is determined by the interactions involving only the decorated parts and the magnetic field, while $2δT$ is independently, exponentially reduced ($δT=0$ means a genuine transition) by restoring the ferromagnetic interaction between the ordinary spins on the chain backbone -- which was neglected in the previous studies of pseudotransition -- thus manifesting that this asymptoticity to the forbidden transition is essentially the buildup of coherence in preformed crossover of local states. Furthermore, I show that the UNPC can be realized even in the absence of the conventional geometric frustration because the magnetic field itself can induce previously unnoticed hidden spin frustration. These findings make the doors wide open to the engineering and utilization of UNPC as a new paradigm for exploring exotic phenomena and 1D device applications.

cond-mat.stat-mech↗

arXiv2024

On the origin of metal-insulator transitions in the parent compounds of ruthenium-pnictide superconductors

We study the interplay of the structural phase transition, flat electronic band dispersion, and metal-to-insulator transition (MIT) in the parent compounds of the Ru-pnictide superconductors by using first-principles calculations. Our electron and phonon calculations reveal that Ru(P,As) undergo MIT accompanied by orthorhombic to monoclinic distortion at low temperature, but RuSb stays orthorhombic and metallic in agreement with the experimental findings. We find that although small monoclinic distortion can remove the van Hove singularity at the Fermi level, it does not immediately gap out the Fermi surface and a large value of monoclinic distortion is necessary for a clear MIT suggesting the possibility of an intermediate pseudogapped monoclinic metallic phase. Furthermore, we predict a light-induced two-step insulator-to-metal and structural transitions in the monoclinic phases of RuP and RuAs, which can be tested in future ultrafast pump-probe experiments as an alternative ideal play ground to VO$_2$.

cond-mat.mtrl-sci↗

arXiv2023

Electronic structure, magnetic and transport properties of antiferromagnetic Weyl semimetal GdAlSi

We report the topological electronic structure, magnetic, and magnetotransport properties of a noncentrosymmetric compound GdAlSi. Magnetic susceptibility shows an antiferromagnetic transition at $T_\mathrm{N}$ = 32 K. In-plane isothermal magnetization exhibits an unusual hysteresis behavior at higher magnetic field, rather than near zero field. Moreover, the hysteresis behavior is asymmetric under positive and negative magnetic fields. First-principles calculations were performed on various magnetic configurations, revealing that the antiferromagnetic state is the ground state, and the spiral antiferromagnetic state is a close competing state. The calculations also reveal that GdAlSi hosts multiple Weyl points near the Fermi energy. The band structure measured by angle-resolved photoemission spectroscopy (ARPES) shows relatively good agreement with the theory, with the possibility of Weyl nodes slightly above the Fermi energy. Within the magnetic ordered state, we observe an exceptionally large anomalous Hall conductivity (AHC) of ~ 1310 $Ω^{-1}$cm$^{-1}$ at 2 K. Interestingly, the anomalous Hall effect persists up to room temperature with a significant value of AHC (~ 155 $Ω^{-1}$cm$^{-1}$). Our analysis indicates that the large AHC originates from the Berry curvature associated with the multiple pairs of Weyl points near Fermi energy.

cond-mat.str-el↗

arXiv2023

A paradigm of spontaneous marginal phase transition at finite temperature in one-dimensional ladder Ising models

The Ising model describes collective behaviors such as phase transitions and critical phenomena in various physical, biological, economical, and social systems. It is well-known that spontaneous phase transition at finite temperature does not exist in the Ising model with short-range interactions in one dimension. Yet, little is known about whether this forbidden phase transition can be approached arbitrarily closely -- at fixed finite temperature. To describe such asymptoticity, here I introduce the notion of marginal phase transition (MPT) and use symmetry analysis of the transfer matrix to reveal the existence of spontaneous MPT at fixed finite temperature $T_0$ in one class of one-dimensional Ising models on decorated two-leg ladders, in which $T_0$ is determined solely by on-rung interactions and decorations, while the crossover width $2δT$ is independently, exponentially reduced ($δT = 0$ means a genuine phase transition) by on-leg interactions and decorations. These findings establish a simple ideal paradigm for realizing an infinite number of one-dimensional Ising systems with spontaneous MPT, which would be characterized in routine lab measurements as a genuine first-order phase transition with large latent heat thanks to the ultra-narrow $δT$ (say less than one nano-kelvin), paving a way to push the limit in our understanding of phase transitions and the dynamical actions of frustration arbitrarily close to the forbidden regime.

cond-mat.stat-mech↗

arXiv2023

Verwey transition as evolution from electronic nematicity to trimerons via electron-phonon coupling

Understanding the driving mechanisms behind metal-insulator transitions (MITs) is a critical step towards controlling material's properties. Since the proposal of charge-order-induced MIT in magnetite Fe3O4 in 1939 by Verwey, the nature of the charge order and its role in the transition have remained elusive-a longstanding challenge in the studies of complex oxides. Recently, a trimeron order was discovered in the low-temperature monoclinic structure of Fe3O4; however, the expected transition entropy change in forming trimeron at the Verwey transition is greater than the observed value, which arises a reexamination of the ground state in the high-temperature phase. Here we use electron diffraction to unveil that a nematic charge order on particular Fe sites emerges in the high-temperature cubic structure of bulk Fe3O4, and that upon cooling, a competitive intertwining of charge and lattice orders leads to the emergence of the Verwey transition. Moreover, MeV ultrafast electron diffraction (UED) provides a dynamic measure of the strong coupling between photoexcited electrons and the X3 phonon modes. Our findings discover a new type of electronic nematicity in correlated materials and offer novel insights into the Verwey transition mechanism in Fe3O4 via the electron-phonon coupling.

cond-mat.str-el↗

arXiv2023

Machine-learning the spectral function of a hole in a quantum antiferromagnet

Understanding charge motion in a background of interacting quantum spins is a fundamental problem in quantum many-body physics. The most extensively studied model for this problem is the so-called $t$-$t'$-$t''$-$J$ model, where the determination of the parameter $t'$ in the context of cuprate superconductors is challenging. Here we present a theoretical study of the spectral functions of a mobile hole in the $t$-$t'$-$t''$-$J$ model using two machine learning techniques: K-nearest Neighbors regression (KNN) and a feed-forward neural network (FFNN). We employ the self-consistent Born approximation to generate a dataset of about $1.3 \times 10^5$ spectral functions. We show that for the forward problem, both methods allow for the accurate and efficient prediction of spectral functions, allowing for e.g. rapid searches through parameter space. Furthermore, we find that for the inverse problem (inferring Hamiltonian parameters from spectra), the FFNN can, but the KNN cannot, accurately predict the model parameters using merely the density-of-state. Our results suggest that it may be possible to use deep learning methods to predict materials parameters from experimentally measured spectral functions.

cond-mat.str-el↗

arXiv2022

Thermal transport properties of IrSbSe

We report a thermal transport study of IrSbSe, which crystallizes in a noncentrosymmetric cubic structure with the $P2_13$ space group and shows a narrow-gap semiconducting behavior. The large discrepancy between the activation energy for conductivity [$E_ρ$ = 128(2) meV] and for thermopower [$E_S$ = 17.7(9) meV] from 200 to 300 K indicates the polaronic transport mechanism. Electrical resistivity varies as $exp(T_0/T)^{1/4}$ and thermopower varies as $T^{1/2}$ at low temperatures, indicating that it evolves into the Mott's variable-range hopping dominant conduction with decreasing temperature. IrSbSe shows relatively low value of thermal conductivity ($\sim$ 1.65 W/K$\cdot$m) and thermopower of about 0.24 mV/K around 100 K, yet poor electrical conductivity. On the other hand, high vacancy defect concentration on both Ir and Sb atomic sites of up to 15\%, suggests high defect tolerance and points to possibility of future improvement of carrier density by chemical substitution or defect optimization.

cond-mat.str-el↗