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Physics Condensed Matter Strongly Correlated Electrons

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Friday 14 August

Strongly Correlated Electrons

Antiferroquadrupolar Order in Altermagnetic CoF$_2$

Daniel Halliday, Laura Pöysti, Chung Xu et al. (+5 more)

Altermagnets host non-relativistic spin-split electronic states whose microscopic origin is theoretically linked to a hidden charge order, yet experimental studies of this charge order are limited. We therefore investigate charge order within CoF$_2$, a $d$-wave altermagnetic compound with a rutile crystal structure and $\Gamma$-point antiferromagnetism. Combining resonant elastic X-ray scattering, symmetry analysis and ab initio calculations, we directly observe charge ordering and identify it as antiferroquadrupolar in nature. Via electronic structure calculations, we show that the experimentally observed antiferroquadrupolar order gives rise to the characteristic altermagnetic spin-splitting, thereby establishing empirical evidence for the decomposition of the altermagnetic order parameter into magnetic and charge degrees of freedom. We hence demonstrate that antiferroquadrupolar order is the microscopic origin of altermagnetism in CoF$_2$, with implications to the wider family of rutile altermagnets. Furthermore, our approach is applicable to studying altermagnetism in general, having demonstrated that resonant elastic X-ray scattering can serve as a direct probe of the charge multipoles that underpin spin-split electronic states in these materials.
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All authors Daniel Halliday, Laura Pöysti, Chung Xu, Daniel A. Mayoh, Didier Wermeille, Dharmalingam Prabhakaran, David R. Bowler, Roger D. Johnson

Strongly Correlated Electrons

Electronic structure and magnetic correlations in the epitaxially strained bilayer nickelate La$_3$Ni$_2$O$_{7}$

I. V. Leonov

Using the DFT+dynamical mean-field theory method we study the effects of electron-electron correlations and epitaxial strain of the crystal structure on the normal-state electronic structure, quasiparticle band renormalizations, Fermi surface, and magnetic correlations of the bilayer Ruddlesden-Popper nickelate La$_3$Ni$_2$O$_7$ (LNO). Our results exhibit a remarkable orbital-selective renormalization and strong incoherence of the Ni $3d$ bands, pointing to the proximity of the Ni $x^2-y^2$ and $3z^2-r^2$ states to orbital-selective localization. The electronic properties of LNO show a high sensitivity to the in-plane strain. We note that both a tensile and a moderate compressive strain (up to about $-2$%) yield a significant enhancement of magnetic correlations compared to the unstrained LNO. Under a large compressive strain of about $-4$%, we observe a Lifshitz transition characterized by the disappearance of the $\gamma$ Fermi surface sheet, which is associated with a nearly fully occupied, shallow flat-band of the bonding Ni $3z^2-r^2$ orbital character. As a result, we observe a sharp decrease of magnetic correlations, implying suppression of superconductivity. Overall, our results support the picture of spin- and change-density-wave stripe instability driven by the Fermi surface nesting in LNO. Our results suggest that both pressure and strain can effectively tune (suppress or enhance) spin-change-density-wave ordering, giving rise to enhanced spin fluctuations.
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Strongly Correlated Electrons · Cross-listed from cond-mat.mes-hall

Interaction driven charge transfer transitions in closely spaced graphene double layers

Kenneth A. Lin, Unmesh Ghorai, Kenji Watanabe et al. (+3 more)

Charge transfer between two conductors is conventionally viewed as a single-particle process governed by electrostatics and band alignment. Using tunneling spectroscopy, we show that charge transfer in closely spaced graphene double layer quantum Hall ferromagnets instead proceeds through a sequence of interaction driven phase transitions governed by the competition between capacitive charging and Coulomb exchange interactions. A comparison of experimental data and theoretical calculations identifies spectroscopic signatures of the interaction driven charge transfer transitions, and reveals that this charge transfer reconstructs the quasiparticle spectrum. While intralayer exchange favors abrupt transfer of entire spin-valley subbands between the layers, interlayer exchange stabilizes coherent intermediate phases that enable gradual charge transfer. Our results establish interlayer tunneling as a powerful probe of interacting electronic systems whose quasiparticle spectrum is itself bias dependent.
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All authors Kenneth A. Lin, Unmesh Ghorai, Kenji Watanabe, Takashi Taniguchi, Emanuel Tutuc, Rafi Bistritzer

Strongly Correlated Electrons · Cross-listed from cond-mat.mes-hall

Skyrmion Fractional Chern Insulator: An Intrinsically Multiband Route to Fractionalization in Rhombohedral Graphene

Julian May-Mann, Tixuan Tan, Patrick J. Ledwith et al. (+2 more)

We propose an unconventional microscopic origin for the fractional quantum anomalous Hall (FQAH) effect in rhombohedral graphene moiré superlattices: skyrmion fractionalization. We view the state at filling $\nu<1$ as a metal of skyrmion vacancies, charge $+e$ objects formed by removing layer-pseudospin skyrmions from the interaction-generated skyrmion lattice Chern insulator at $\nu=1$. These vacancies are intrinsically multiband degrees of freedom, absent in single Chern band-projected studies. Building on a recently proposed ideal limit, we first develop an effective field theory showing that skyrmion vacancies can themselves fractionalize, thereby inducing charge fractionalization. Focusing on $\nu=\frac{2}{3}$, we then construct explicit variational trial wavefunctions for the resulting skyrmion fractional Chern insulator and provide numerical evidence, together with general arguments, showing that this process is energetically favored. Our results establish a realistic route to the FQAH that does not rely on a partially filled Chern band, but instead arises from fractionalization of collective pseudospin textures.
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All authors Julian May-Mann, Tixuan Tan, Patrick J. Ledwith, Zhengyan Darius Shi, Trithep Devakul

Strongly Correlated Electrons

Chiral spin liquid and chiral antiferromagnetism in half-filled moiré Hubbard model: possible applications to twisted bilayer TMDs

Chuyi Tuo and Hong Yao

Twisted transition metal dichalcogenides offer an exceptionally tunable moiré platform for studying correlation physics beyond conventional condensed matter systems. In particular, the intriguing interplay between the displacement field and the twist angle remains to be fully resolved. In this paper, we use large-scale density matrix renormalization group simulations to study the minimal moiré Hubbard model on a triangular lattice at half-filling, where the displacement field effect is captured by a spin-dependent staggered flux. We find that the displacement field significantly enriches the triangular Hubbard phase diagram in several qualitative ways. It rapidly destabilizes the chiral spin liquid phase beyond a narrow weak-field regime, induces pronounced chiral correlations in the strong-coupling $120^\circ$-antiferromagnetic phase, and stabilizes incommensurate spin-density wave phases at weaker coupling. We further find signatures of a continuous transition between the chiral spin liquid and chiral antiferromagnetic phases at a finite displacement field, potentially driven by spinon condensation. Our results uncover rich displacement-field-driven many-body physics and provide useful guidance for future experiments in moiré superlattice systems.
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Strongly Correlated Electrons

Universal Signature of Hundness and Its Quantification

Dongwook Kim, Ina Park, Bo Gyu Jang et al. (+1 more)

Hund's coupling $J$ induces fundamentally different correlation effects from Hubbard $U$. This leads to a violation of the Brinkman–Rice scenario and the emergence of a Janus-faced phase owing to its low-energy effectiveness, in which band renormalization is confined below a characteristic energy scale. We propose a quantitative framework to capture low-energy effectiveness through two correlation factors: $z_L$ for low-energy quasiparticle renormalization and $z_H$ for high-energy charge fluctuations, newly introduced in this study. The discrepancy between $z_L$ and $z_H$ reflects the Hund character of the correlation. By establishing a one-to-one correspondence between correlation factors and the spin and charge susceptibilities, we identify the spin-degree-of-freedom effectiveness as the microscopic origin of low-energy effectiveness. Our framework, validated across multiorbital models and real materials, provides a universal and quantitative measure of Hundness.
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All authors Dongwook Kim, Ina Park, Bo Gyu Jang, Ji Hoon Shim

Strongly Correlated Electrons · Cross-listed from cond-mat.stat-mech

Linear response across interaction regimes in two-dimensional ferromagnets

Aaron Müller, Pavel E. Dolgirev, Oleksii Malyshev et al. (+1 more)

Recent discoveries of two-dimensional (2D) ferromagnets have stimulated intense interest in understanding and controlling their spin transport properties. A central microscopic feature of these systems is that exchange-driven magnon–magnon interactions are strongly momentum dependent: low-momentum magnons interact weakly, while high-momentum ones can scatter strongly and exhibit collective hydrodynamic behavior. Understanding transport in such systems therefore requires a microscopic description capable of capturing ballistic and hydrodynamic regimes on equal footing. The natural framework is the quantum Boltzmann equation (QBE), whose solution is notoriously difficult because of the multidimensional collision integrals. Here, we develop a method based on an efficient representation of distribution functions as sums of Gaussians, which renders the collision integrals tractable. This approach enables accurate solution of the linearized QBE and computation of momentum- and frequency-resolved linear response in 2D ferromagnets across a broad range of temperatures and magnetic fields. In particular, we resolve a temperature-driven crossover from a ballistic regime dominated by weakly interacting low-momentum magnons to a collective hydrodynamic regime governed by strongly interacting high-momentum modes. Applying this method to monolayer CrCl$_3$, we obtain good agreement with recent nitrogen-vacancy-center dephasing experiments that reported anomalous magnetic noise consistent with magnon sound. More broadly, our work establishes a general framework for computing momentum- and frequency-resolved linear response in interacting 2D quantum systems describable within quantum Boltzmann kinetics.
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All authors Aaron Müller, Pavel E. Dolgirev, Oleksii Malyshev, Eugene Demler

Strongly Correlated Electrons

A Fixed Universal Determinant is Variationally Complete for Continuum Fermions

Giuseppe Carleo and Riccardo Rossi

How many Slater determinants does an accurate variational description of interacting fermions require? Exact expansions in a finite basis need combinatorially many, and state-of-the-art fermionic neural quantum states stack growing numbers of them. We prove that, in the norms that govern variational calculations, at most two are needed, independently of the number of particles and of the target accuracy. A single universal Slater determinant-specified in advance, independent of both the system and the state-multiplied by a smooth bosonic wave function approximates any fermionic wave function in up to three spatial dimensions in the first-order Sobolev norm, which controls the variational energy. Reaching the second-order Sobolev norm-for Coulomb interactions, the domain of the Hamiltonian, which bounds the variance of the local energy at the core of variational Monte Carlo-requires at most one additional fixed determinant, and only in three dimensions. Antisymmetry therefore costs at most two universal determinants and no expressiveness: generalized Slater-Jastrow neural quantum states are variationally complete.
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Strongly Correlated Electrons · Cross-listed from physics.comp-ph

Learning Spin Hamiltonians from Terahertz Two-Dimensional Coherent Spectroscopy

Martin Mootz, Chuankun Huang, Liang Luo et al. (+2 more)

Effective Hamiltonians connect microscopic interactions to measurable collective behavior in quantum materials, but determining their parameters directly from experiment remains a challenging inverse problem. We introduce a supervised machine-learning framework that infers Hamiltonian parameters from nonlinear terahertz two-dimensional coherent spectra. A calibrated forward model generates spectra from candidate Hamiltonians, a common preprocessing pipeline maps simulated and experimental spectra into the same representation, and a neural network learns the inverse map from spectral fingerprints to microscopic parameters. We demonstrate the approach for rare-earth orthoferrites using a two-sublattice Landau–Lifshitz–Gilbert spin model with exchange, Dzyaloshinskii–Moriya interaction, anisotropies, and damping. Synthetic benchmarks show that nonlinear spectra encode parameters beyond those fixed by the linear response, with inference accuracy tracking the physical spectral sensitivity and robustness against noise improved by using multiple inter-pulse delays. Applied to experimental THz-2DCS data from Sm$_{0.4}$Er$_{0.6}$FeO$_3$, the inferred parameters yield physically reasonable forward simulations, while remaining discrepancies identify limitations of the reduced model. These results establish THz-2DCS as a data-rich platform for effective-Hamiltonian inference and model refinement, enabling experimentally driven identification of microscopic interactions while providing a foundation for understanding, predicting, and ultimately controlling the emergent properties of quantum materials.
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All authors Martin Mootz, Chuankun Huang, Liang Luo, Jigang Wang, Yong-Xin Yao

Strongly Correlated Electrons

Tuning crystal-fields by He-irradiation and orbital Widom line in SrVO$_3$ films

Matthias Pickem, Karsten Held and Jan M. Tomczak

Helium-ion irradiation of epitaxial SrVO$_3$/SrTiO$_3$ films causes a metal-insulator transition, so far attributed to a Mott localization driven by a reduced kinetic energy. Using density-functional theory plus dynamical mean-field theory, we show that the driving mechanism is instead the crystal-field splitting generated by the irradiation-induced tetragonal expansion, not the interaction-to-bandwidth ratio. The resulting $c$-axis vs. temperature phase diagram mirrors that of the one-band Hubbard model, but its critical end-point spawns an orbital Widom line, rooted in an anomalous compressibility of orbital, rather than charge occupation. At an effectively quarter filling, superexchange-like processes favor orbital over magnetic long-range order. Our results semi-quantitatively reproduce the fluence-dependent spectral gaps and transition thresholds reported experimentally, establishing ion implantation as a route to chemically expand correlated materials.
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Strongly Correlated Electrons · Cross-listed from cond-mat.mtrl-sci

Dispersive-phonon-driven room-temperature Ni1+-Ni2+ polaron hopping in spin-charge coupled rutile niobate

Gourab Roy, Mohit Kumar, Sayan Ghosh et al. (+4 more)

Understanding how lattice dynamics mediate polaron hopping is essential for designing multifunctional correlated oxides. Here, we demonstrate room-temperature dispersive phonon excitations and elucidate the Ni1+-Ni2+ polaron-hopping mechanism and the presence of rare spin-charge-phonon coupling even in a magnetically short-range-ordered state in rutile niobate, a rare room-temperature magnetodielectric system. We reveal room-temperature dispersive phonon excitations using inelastic neutron scattering (INS), complemented by machine-learning-based phonon calculations, to establish the microscopic origin of the polaron-hopping mechanism. Experimental evidence of dispersive phonon-driven polaron hopping is scarce. INS measurements show significant dispersive phonon excitations at 21, 33, and 47 meV, implying collective lattice dynamics that enable delocalized polaron propagation via coupled charge-spin-phonon interactions. Dispersive phonons couple to charge carriers and promote correlated NiO6 lattice distortions, facilitating delocalized polaron hopping. Low-energy magnetic excitations at 4 and 8 meV indicate the presence of local short-range magnetic correlations or spin-orbit-coupling-induced anisotropy in deformed NiO6 octahedra, which are thoroughly discussed. Machine-learning phonon calculations replicate the experimentally observed phonon excitations and demonstrate lattice instability, which is compatible with dynamic local distortions caused by polaron production. These findings provide microscopic evidence for a coupled charge-spin-phonon mechanism that mediates polaron hopping in rutile oxide systems.
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All authors Gourab Roy, Mohit Kumar, Sayan Ghosh, Ekta Kushwaha, Manh Duc Le, Devashibhai T. Adroja, Tathamay Basu

Strongly Correlated Electrons · Cross-listed from cond-mat.mtrl-sci

Microscopic investigation of spin dynamics in the single-chain magnet Sr4Mn2CoO9

G. Roy, S. Ghosh, M. Kumar et al. (+8 more)

One-dimensional single-chain magnets offer a unique platform for studying the interplay of crystal-field effects, exchange interactions, and lattice dynamics. Here, we investigate spin excitations in Sr4Mn2CoO9 using inelastic neutron scattering (INS) and theoretical modelling. INS reveals two low-energy magnetic excitations at 4 and 7 meV from Mn-Co-Mn spin chains, alongside higher-energy crystal-electric-field (CEF) excitations from two crystallographically inequivalent Co2+ sites. Interestingly, these spin excitations persist at room temperature, demonstrating dynamic magnetic correlations in the absence of long-range order. Furthermore, the crystal-field modelling, based on Stevens operator formalism, reproduces well the CEF spectra, establishing Ising-like Kramers ground-state doublets with strong uniaxial magnetic anisotropy for both Co2+ ions. In addition, the spin wave simulation using SpinW reproduces the spin excitation spectrum and reveals microscopic exchange interactions in two non-interacting Mn-Co-Mn spin chains. Finally, machine-learning lattice-dynamics calculations confirm the phonon spectrum and spin-phonon coupling. By projecting the exchange Hamiltonian onto CEF ground-state doublets, we estimate exchange-induced splittings matching the observed excitations. Thus, our results elucidate low-energy spin dynamics arising from combined crystal-field anisotropy and exchange interactions, with the persistent low-energy excitation providing a microscopic pathway for thermally activated spin relaxation. Furthermore, this work delivers a unified microscopic understanding of the interplay between crystal-field effects, magnetic exchange, and lattice dynamics in Sr4Mn2CoO9, advancing insights into spin dynamics in low-dimensional transition-metal oxides.
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All authors G. Roy, S. Ghosh, M. Kumar, E. Kushwaha, J. Sannigrahi, V. Caignaert, W. Prellier, D. T. Adroja, D. Voneshen, V. Hardy, T. Basu

Strongly Correlated Electrons

Topological phases and quantum criticality from $SU(2)$ Chern-Simons-matter theories

Yunchao Hao, Yingcheng Li, Kangle Li et al. (+1 more)

Motivated by recent numerical studies where various $SU(2)$ Chern-Simons-matter theories emerge, we analytically study topological phases and quantum criticality in two-dimensional systems described by such theories. First, we classify $SU(2)_k$ topological orders in all lattice spin systems with a $p4\times SO(3)$ symmetry, where $k$ is an arbitrary nonzero integer. We find that for each odd $k$, the topological order can emerge in systems with an arbitrary Lieb-Schultz-Mattis (LSM) anomaly, and the symmetry cannot permute anyons. If the system has a nontrivial (respectively, trivial) LSM anomaly, then there is exactly one (respectively, nine) symmetry-enriched topological (SET) phases. On the other hand, $SU(2)_k$ topological order with any even $k$ can only emerge in systems with a trivial LSM anomaly. If $k\notin\{6, 10, 14, \cdots\}$, the symmetry cannot permute anyons, and there are 16 SET phases. If $k\in\{6, 10, 14, \cdots\}$, there are 4 different ways how the symmetry can permute anyons, and there are 64 SET phases. Next, we analyze the $SU(2)_k$ Chern-Simons theories coupled to $N_f$ flavors of gapless matter fields that can be either bosonic or fermionic. For both types of theories, we consider a joint large-$N_f$ and large-$k$ limit with $N_f/k$ fixed, and compute the scaling dimensions of the bilinear operators of the bosons or fermions to the order of $1/N_f$. These results sharpen our understanding of these emergent exotic topological phases and quantum criticality, and provide useful guidance to explore them further.
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All authors Yunchao Hao, Yingcheng Li, Kangle Li, Liujun Zou

Strongly Correlated Electrons

Quantum Geometric Kondo Cloud

Grant Z. X. Yang and K. T. Law

A magnetic impurity embedded in a metal is collectively screened by Fermi-surface quasiparticles into a many-body spin-singlet ground state, forming a Kondo cloud of size $\xi_{\rm K}\sim\hbar v_F/(k_B T_{\rm K})$. This kinematic picture collapses in flat bands, where $v_F=0$ and the hierarchy of dispersive energy shells is absent. Here we show that the missing organizing principle is quantum geometry. A magnetic impurity coupled to an isolated flat band selects a single active bath mode: a coherent superposition of flat-band Bloch states weighted by the hybridization factor $v(\mathbf{k})$, while all orthogonal flat-band modes remain dark. The resulting flat-band Kondo problem is a quantum geometric molecule, with an algebraic Kondo scale set by the total projected hybridization strength rather than a logarithmic-renormalization scale. In real space, the impurity-bath spin correlation defines a quantum geometric Kondo cloud. Its cloud-size tensor admits a gauge-invariant decomposition into a hybridization-weighted quantum metric, a dressed Berry-connection covariance, and a positive hybridization-gradient term, yielding the lower bound $\xi_{\rm K}^2\geq \sum_{\mathbf{k}}\rho(\mathbf{k}){\rm Tr}\,g(\mathbf{k})$, where $\rho(\mathbf{k})=|v(\mathbf{k})|^2/\sum_{\mathbf{k}}|v(\mathbf{k})|^2$. Our result reveals that, in flat bands, Kondo screening is governed by the quantum geometry and interference structure of the impurity-selected Bloch wave packet, rather than Fermi-surface kinematics.
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Strongly Correlated Electrons

Observation of in-plane anomalous Nernst effect

Tadashi Yoneda, Shinichi Nishihaya, Markus Kriener et al. (+7 more)

The Nernst effect, which enables the conversion of a heat current into a transverse voltage under magnetic field or spin magnetization, holds significant promise for energy harvesting and thermal management in future electronics. However, the conventional Nernst effect is fundamentally constrained by the orthogonality requirement that the applied field or spontaneous magnetization must be perpendicular to the plane defined by the temperature gradient and the induced voltage. Here we report that symmetry-tailored ultrathin films of a prototypical ferromagnetic oxide exhibit anomalous Nernst effect arising from intrinsic coupling to spontaneous in-plane spin magnetization. Systematic magnetothermoelectric measurements under spherical rotations of the magnetic field reveal that a pronounced Nernst signal, comparable in magnitude to the out-of-plane response, emerges robustly associated with out-of-plane orbital magnetization. Our findings demonstrate that the anomalous Nernst effect is no longer limited by the orthogonality condition, opening new opportunities for more flexible designs of magnetothermoelectric materials and devices.
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All authors Tadashi Yoneda, Shinichi Nishihaya, Markus Kriener, Haruto Kaminakamura, Ming-Chun Jiang, Naohiro Tezuka, Yoshiya Murakami, Ryotaro Arita, Hiroaki Ishizuka, Masaki Uchida

Strongly Correlated Electrons

Periodicity-driven revision of the phase diagram of the generalized Baxter-Wu model with asymmetric complex couplings

Yuting Wang, Ye Ling, Haihong Li et al. (+1 more)

The conventional self-dual lines of the generalized Baxter-Wu (GBW) model with asymmetric complex couplings are known to be $\sinh(2K)=\pm \cos(2\phi)$, where $K$ and $\phi$ are the real and imaginary parts of the coupling. We demonstrate that these lines are incomplete: the periodicity of the partition function, encoded in the cosine factor of the bundled Boltzmann weight, generates additional self-dual lines $\sinh(2K)=\pm \sin(2\phi)$. Guided by the complete set of self-dual candidates, we perform Monte Carlo simulations using brute-force reweighting (Metropolis) and the Wang-Landau methods. Simulations indicate that the self-dual lines at the partition-function minima $\phi_{\mathcal{Z}_{\min}}=(2n+1)\pi/8$ constitute a critical threshold. They are genuine critical boundaries for $|K| \ge \frac{1}{2}\operatorname{arsinh}(\cos(\pi/4)) \approx 0.32924$, while for smaller $|K|$ they are not. At $\phi_{\mathcal{Z}_{\min}}$, the sign problem is most severe and finite-size scaling corrections are largest; the local peak observed below the phase boundary in the temperature scan is thus a finite-size artifact, not a genuine new phase. We further clarify the capability and limitations of the average sign and its derivatives for detecting phase transitions. In particular, the negative peak of the average sign at $\phi_{\mathcal{Z}_{\min}}$ does not correspond to a genuine phase transition. We also evaluate the Wang-Landau method, which, despite formally circumventing the sign problem, still faces the exponential barrier.
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All authors Yuting Wang, Ye Ling, Haihong Li, Yuhai Liu

Strongly Correlated Electrons · Cross-listed from cond-mat.mes-hall

Thermal effect on the anomaly-induced electromechanical response in gapped graphene

A. Sedrakyan and K. Ziegler

Mechanical deformation of gapped graphene can act on Dirac quasiparticles as an emergent gauge field. When this deformation field couples to the same current as the electromagnetic field, the parity anomaly produces a mixed electromechanical Chern-Simons response: a phonon electric field drives a transverse electrical current, and a phonon magnetic field binds charge. Previous zero-temperature results predict a sharp change in the response when the chemical potential crosses the band edge. We show that finite temperature replaces this sharp feature by a universal smooth crossover controlled only by the ratios of temperature, gap, and chemical potential. The response remains almost quantized in the insulating regime, is rounded over a gate window of order temperature near the band edge, and approaches the doped Berry-curvature result with a controlled Sommerfeld correction. We apply the result to two experimentally useful drives: a traveling flexural wave, which produces a transverse second-harmonic current, and a dynamic phonon mixed with a static ripple, which produces a fundamental-frequency signal. The same gate-temperature line shape controls both signals. This gives a direct way to separate the anomaly-induced current from ordinary electromechanical backgrounds and to extract the effective gap and electronic temperature in graphene devices.
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Strongly Correlated Electrons

Anomalous Orbital Reconstruction Controlled by Interorbital Correlations and Hund's Coupling

Yu Ni, Lvjin Wang, Jinfeng Wang et al. (+3 more)

The microscopic origin of anomalous orbital polarization in a class of low-dimensional correlated oxides remains unresolved due to the competition among crystal-field effects, electronic correlations, and orbital-dependent dimensionality. Using dynamical mean-field theory, we investigate a two orbital Hubbard model with orbital-dependent dimensionality and reveal the mechanism for anomalous orbital polarization through orbital reconstruction beyond the bare crystal-field picture. We identify interorbital Coulomb interaction-induced charge competition as a key microscopic mechanism responsible for the orbital redistribution, which leads first to an orbital-polarized correlated metal and subsequently to an orbital-polarized Mott insulator. We further find that Hund's coupling acts as a filling-dependent regulator of orbital reconstruction. It weakens the correlation-induced orbital redistribution at quarter filling by competing with interorbital charge fluctuations, while at half filling it completely suppresses the orbital-polarized state by stabilizing high-spin orbital-balanced configurations. These results provide a unified picture of correlation driven orbital reconstruction and highlight the relevance of interorbital interactions and Hund's coupling for understanding orbital phenomena in low-dimensional transition-metal oxides.
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All authors Yu Ni, Lvjin Wang, Jinfeng Wang, Yunkun Niu, Zhaoming Fu, Yun Song

Strongly Correlated Electrons

Posterior Inference of Hamiltonian Parameters from RIXS Spectroscopy

Samuel Klein, Thomas M. Linker, Louis Conreux et al. (+14 more)

We present the first application of simulation-based inference to resonant inelastic X-ray scattering spectroscopy. Using truncated marginal neural ratio estimation to efficiently restrict the prior and conditional flow matching as the joint density estimator, we infer full posteriors with a modest simulation budget for two Ni$^{2+}$ compounds—NiPS$_3$ as a representative covalent case and K$_2$NiF$_4$ as a more atomic one. We demonstrate that a vision transformer encoder whose tokenization matches the physical layout of the RIXS map yields better-covered and sharper posteriors than generic image encoders. Applying the validated method to experimental NiPS$_3$ and K$_2$NiF$_4$ data, we recover a joint posterior that reveals parameter correlations invisible to point estimators, and a posterior predictive distribution that closely matches the observed spectrum. The amortized posterior unlocks a class of analyses not previously available to the field such as nuisance-marginalized uncertainty quantification, multi-measurement posterior fusion and active experimental design.
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All authors Samuel Klein, Thomas M. Linker, Louis Conreux, Daniel Ratner, Apurva Mehta, Makoto Tachibana, Jiemin Li, Jonathan Pelliciari, Valentina Bisogni, Wei He, Xiangpeng Luo, Mark P. M. Dean, Marton K. Lajer, Michael Kagan, Joshua J. Turner, Yongqiang Cheng, Sean Gasiorowski

Thursday 13 August

Strongly Correlated Electrons · Cross-listed from cond-mat.mes-hall

Trimer Thouless Pump: Topology, Symmetries, and Multigap Structure

Rittwik Chatterjee

Topological charge pumping is paradigmatically understood through two-band systems such as the Rice-Mele model, which are intrinsically restricted to a single independent pumping channel. In this work, we introduce the Trimer Thouless Pump (TTP), a minimal three-band generalization that exhibits genuinely multigap topological transport. By subjecting a one-dimensional three-site lattice to cyclic adiabatic modulations of its hopping amplitudes and antisymmetric onsite potentials, we explore a topological regime characterized by two independent bulk gaps. We show that the quantized charge transport is driven by highly localized Berry curvature hotspots corresponding to effective two-level Dirac avoided crossings on the parameter torus. Crucially, we demonstrate that the middle energy band acts as a geometric mediator: it facilitates the exchange of quantized Berry flux between the upper and lower bands while maintaining a net zero Chern number itself. This bulk topology is corroborated by the spectral flow of boundary-localized edge states traversing multiple gaps. Furthermore, we map the topological phase diagram as a function of central-site detuning, illustrating a band-selective transfer of topological invariants across discrete phase transitions. Finally, we propose a concrete experimental protocol to realize the TTP and observe its multigap charge transport using ultracold atoms in phase-controlled optical superlattices.
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Strongly Correlated Electrons

Systematic extinctions in inelastic neutron scattering from molecular spin clusters

Shadan Ghassemi Tabrizi

Inelastic neutron scattering on a single crystal resolves how the intensity of a magnetic transition of a molecular spin cluster varies with the momentum-transfer vector $\mathbf{Q}$. The point group fixes that dependence completely when a single symmetry species mediates the transition and the local spin operators contain only one occurrence of that species. Functions for such universal $\mathbf{Q}$ dependences have been tabulated. As we show here, even when these conditions are not fulfilled, the point group still fixes, at a given geometry, momentum transfers at which the intensity vanishes for every Hamiltonian that has the symmetry of the cluster. We call such momentum transfers extinctions and derive an equation whose every zero is an extinction, built from the symmetry species of the two levels and from the positions and scattering amplitudes of the magnetic sites. The extinctions follow in closed form in two cases: when each orbit of symmetry-equivalent sites contributes a single occurrence of the mediating species, and when the site phases recur under a subgroup up to one common factor.
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Strongly Correlated Electrons

Imaginary-time correlations in time-sliced stochastic series expansion

Ryan Flynn, Adam Iaizzi, Sibin Yang et al. (+2 more)

Combined with numerical analytic continuation techniques, quantum Monte Carlo (QMC) methods enable the extraction of real-frequency dynamical properties from imaginary-time correlation functions. However, the efficient computation of imaginary-time correlation functions by QMC simulations can (depending on the particular model used) be challenging, particularly for operators that are off-diagonal in the computational basis. In this work, we present an efficient and general algorithm within the stochastic series expansion (SSE) framework for evaluating imaginary-time correlation functions of both diagonal and off-diagonal operators. The algorithm builds on a discrete imaginary-time slicing of the SSE operator string, which provides correlation functions on a grid of well-defined imaginary-time points with no discretization error. For off-diagonal operators, we derive estimators that integrate directly into the existing SSE directed-loop or cluster updating schemes, introducing only minimal computational overhead. We benchmark the method on the one-dimensional transverse-field Ising model (sampling with cluster updates) and XXZ spin chain (using directed-loop sampling), demonstrating excellent agreement (with only statistical errors) with exact diagonalization of small systems. We also study larger systems to demonstrate efficiency.
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All authors Ryan Flynn, Adam Iaizzi, Sibin Yang, Ying-Jer Kao, Anders W. Sandvik

Strongly Correlated Electrons

Emergent Symmetry-Protected Topological Phases via Polyakov Confinement in Quantum Spin Systems

Li-Wei He, Shun-Li Yu and Jian-Xin Li

A main theme in modern condensed matter physics is the emergence of fractionalized excitations and gauge structures from quantum spin systems. However, understanding how these exotic degrees of freedom reconfine into new phases of matter remains a fundamental challenge. In this work, we demonstrate that Polyakov's confinement mechanism-a foundation of compact gauge theory-can serve as a dynamical engine to transform a Dirac spin liquid into a symmetry-protected topological (SPT) state. Starting with a Dirac spin liquid with spin-dependent gauge fluxes, we show that the single-occupancy constraint inherent to the physical Hilbert space triggers monopole condensation, dynamically confining bulk spinons while preserving gapless edge modes-realizing a spinon analog of the quantum spin Hall effect. Using a large-scale variational Monte Carlo simulation on a triangular antiferromagnet with an additional Dzyaloshinskii-Moriya interaction, we provide microscopic evidence for this confined SPT phase, including a characteristic area law for the Wilson loop and vanishing topological entanglement entropy. Furthermore, we identify a measurable spin-pump response under magnetic fields, which directly encodes the Berry curvature of the parent Dirac cones. Our results reveal a previously unexplored pathway to SPT physics, bridging the fields of gauge theory, quantum magnetism, and topological matter.
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Strongly Correlated Electrons

Accelerating a Strong-Coupling Non-Equilibrium Steady-State Impurity Solver using (Quantics) Tensor Trains

Bastian Schindler and Martin Eckstein

Including higher order diagrammatic corrections to the strong-coupling expansion is mainly limited by the evaluation of high-dimensional, time-ordered integrals. In this work we present and compare four different parametrizations of the integrands in order to obtain a low-rank (quantics) tensor-train representation using tensor cross interpolation. Particular emphasis is placed on a quantics time-difference formulation in which the required retarded convolutions are performed directly in quantics tensor-train form. Using controlled Gaussian benchmarks, we analyze the accuracy, bond dimensions, and computational scaling of the different approaches. We then validate the most promising formulations in self-consistent equilibrium and nonequilibrium DMFT calculations and demonstrate calculations up to the third order in the strong-coupling expansion. Finally, we extend the solver to impurity models with retarded density-density interactions and apply it within nonequilibrium extended DMFT. Our results show that tensor cross interpolation substantially reduces the cost of evaluating higher-order diagrams and provides a controlled, systematically improvable framework for nonequilibrium quantum impurity calculations.
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Strongly Correlated Electrons

Magnetism in antiperovskite (Li$_2$M)ChO (M = Fe, Mn, Co; Ch = S, Se) diluted magnets with fixed 1/3 filling: the key role of magnetic anisotropy

Jieyuan Zheng, Frederik L. Carstens, Lennart Singer et al. (+7 more)

We report the magnetic properties of a series of lithium-rich antiperovskites (Li$_2M$)$Ch$O ($M$ = Fe, Co, Mn and $Ch$ = Se, S) where transition metal and lithium ions are randomly distributed on the X-sites of the X$_3$BA structure, thereby forming a strongly diluted magnetic sublattice. Our study hence enables us to investigate the evolution of magnetic order at fixed 1/3-filling – which is in the vicinity but slightly above the percolation threshold – upon variation of the spin size, the magnetic anisotropy, and the orbital configuration. The data imply the absence of a distinct Curie-Weiss behavior up to 350~K but show rather large and weakly temperature-dependent magnetic susceptibility. We observe clear signatures of long-range antiferromagnetic order evolving in the 1/3-filled and strongly diluted magnetic X-site lattice with increasing Néel temperatures from $T_{\rm{N}}\simeq 30$~K in (Li$_2$Mn)$Ch$O to $\simeq 50$~K in (Li$_2$Fe)$Ch$O and $70-90$~K in (Li$_2$Co)$Ch$O. Except for $M$ = Co, the chalcogenide has no sizable effect on $T_{\rm N}$. We conclude significant magnetic coupling and short-range magnetic correlations at well above $T_{\rm N}$ which is in line with the observation of a broad electron spin resonance signal at room temperature. The actual ordering temperatures are strongly diminished by magnetic dilution. While structural parameters such as the tolerance factor and bonding angles do not strongly affect $T_{\rm N}$, a key parameter is the magnetic anisotropy of the transition metals.
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All authors Jieyuan Zheng, Frederik L. Carstens, Lennart Singer, Mohammed A.A. Mohamed, Luca Bischof, Alexey Alfonsov, Jan Arneth, Silke Hampel, Nico Gräßler, Rüdiger Klingeler

Strongly Correlated Electrons

Topology and Quantum-Spin-Classical-Spin Crossover of the Gapped Kondo Effect

David Krüger and Michael Potthoff

The gapped Kondo effect describes the screening of an $S=\frac12$ impurity spin locally coupled via an antiferromagnetic exchange interaction to a conduction-electron system exhibiting a finite hard gap. Using a combination of a Lanczos transformation and a self-consistent configuration-interaction scheme, we numerically investigate the local phase diagram. Furthermore, we show that the different phases can be characterized by several topological invariants: the conventional momentum-space Chern number of the underlying two-dimensional host system, corresponding to a Chern insulator; the B-space Chern number, defined by coupling the impurity spin to a fictitious local magnetic field $\boldsymbol B$, in the limit $B \to 0$; and the S-space Chern number, defined for a classical impurity spin, i.e., a vector of fixed length. The classical-spin limit is obtained for $B \to \infty$. By varying the field strength, we can therefore continuously interpolate between quantum-impurity-spin and classical-impurity-spin Hamiltonians and investigate whether the corresponding phase diagrams are likewise continuously connected. The gapped underscreened Kondo effect is studied for an impurity spin $S>\frac12$ as well as in the classical-spin limit approached via $B \to \infty$ or $S \to \infty$. Different variants of scattering theory are employed to interpret the resulting phases. Finally, the gapped two-channel overscreened Kondo effect, realized by coupling a quantum impurity spin equally to the local electron spins of both orbitals within a unit cell, is shown to be characterized by spontaneous particle-hole symmetry breaking. This leads to a highly nontrivial quantum-classical phase diagram.
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Strongly Correlated Electrons · Cross-listed from cond-mat.mes-hall

Observation of Time-Domain Braiding of Non-Abelian Anyons at $\nu = 5/2$ State

Tomer Alkalay, Jinhong Park, Minseong Oh et al. (+7 more)

Unlike elementary particles, which obey either bosonic or fermionic exchange statistics, certain quasiparticles, known as anyons, are predicted to exhibit Abelian or non-Abelian braiding statistics. While braiding Abelian anyons modifies the wavefunction by a 'statistical phase', braiding non-Abelian anyons implements a unitary transformation of the state within a degenerate subspace of states. Experimental evidence of non-Abelian braiding has thus far remained elusive. Here, we report a 'time-domain braiding' signature of non-Abelian anyons in the $\nu = 5/2$ fractional quantum Hall state, by extending our previously demonstrated approach with Abelian anyons at $\nu = 1/3$. Our approach is based on measurements of the current fluctuations arising from weak partitioning of a highly dilute one-dimensional edge mode. We independently probe the partition noise of the downstream charged mode and also that of the upstream neutral mode. These independent measurements agree with our theoretical predictions for 'time-domain braiding' of the downstream Abelian and the upstream non-Abelian anyons, respectively, in the 'particle-hole Pfaffian' topological order. Together, these results provide evidence for the presence of non-Abelian anyons.
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All authors Tomer Alkalay, Jinhong Park, Minseong Oh, Changki Hong, Jun-Young M. Lee, Priya Tiwari, Tapas Senapati, Vladimir Umansky, Moty Heiblum, H.-S. Sim

Strongly Correlated Electrons

Correlation versus Causation in Quantum Criticality

Conrad Wichmann, Ryan Thorngren and Ruben Verresen

Correlation functions $\langle O_1(x) O_2(0) \rangle$ reveal scaling dimensions through spatial decay. We instead consider static susceptibility, the change in $\langle O_1(x) \rangle$ from perturbing the Hamiltonian by $O_2(0)$, which we term causation for short. In a conformal field theory (CFT), dimensional analysis predicts decay of $|x|^{-2\Delta}$ for correlation and $|x|^{-2\Delta+1}$ for causation. Yet we find causation can decay up to fifteen additional orders in $x$ through a general mechanism, which we trace to time-derivative fields being unable to contribute to static response. In higher-dimensional CFTs, this mechanism ensures leading causation arises from primaries, even when descendants dominate correlation, which we leverage with DMRG to identify a previously unresolved corner primary of $\Delta \approx 8.8$ and a heavy magnetic line defect primary of $\Delta \approx 4.6$ in the $(2+1)$D critical Ising model. Moreover, the same mechanism governs edge-mode localization in $(1+1)$D gapless symmetry-protected topological phases, explaining previously observed anomalously small edge-mode splittings and guiding our construction of spin chains with splittings as small as $1/L^{18}$ and $1/L^{25}$.
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Wednesday 12 August

Strongly Correlated Electrons

Dipolar-driven mean-field criticality in the ferrimagnet Eu$_2$MnSi$_2$O$_7$

Masahiro Kawamata, Maxim Avdeev and Yusuke Nambu

We report mean-field critical behavior in Eu$_2$MnSi$_2$O$_7$, a melilite-type ferrimagnet with spin-only Eu$^{2+}$ and Mn$^{2+}$ moments and negligible orbital contributions. Magnetization measurements combined with neutron powder diffraction reveal critical exponents close to the mean-field values, indicating that long-range dipolar interactions govern the asymptotic critical behavior in this insulating ferrimagnet. The refined magnetic structure, described by the magnetic space group $P2_12_1^\prime2^\prime$, exhibits a tilted ferrimagnetic configuration driven by the Dzyaloshinskii-Moriya interaction, reflecting the noncentrosymmetric nature of the lattice. These results extend the applicability of mean-field theory to complex insulating magnets and establish Eu$_2$MnSi$_2$O$_7$ as a platform for exploring ferrimagnetism and long-range interactions. To our knowledge, this is the first insulating ferrimagnet in which dipolar interactions drive mean-field criticality.
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Strongly Correlated Electrons · Cross-listed from cond-mat.mes-hall

Thermal Hall tomography of chiral superconductivity in rhombohedral graphene

Kumar Ghosh

A chiral superconductor carries chiral Majorana modes along its edges, and a single integer, the Bogoliubov–de Gennes Chern number, counts them. Thirty years of candidate materials have not yielded a measurement of that integer, because the magnetic signatures usually invoked are not topologically protected. Rhombohedral graphene makes the question both urgent and answerable: magnetic imaging resolves rewritable time-reversal-breaking domains inside the superconducting phase, while quantum oscillations reveal a normal state too intricate to reconstruct pocket by pocket. We show that the low-temperature thermal Hall conductance returns the integer directly, with no such reconstruction. For band-projected pairing it equals the pairing-vortex winding enclosed by the occupied regions of momentum space. Splitting the intravalley Hamiltonian into symmetric and antisymmetric parts isolates the trigonal warping and finite Cooper pair momentum of the real material: the antisymmetric part is topologically inert, direct Chern calculations across $525$ parameter points show the invariant preserved, and one inequality marks where a Bogoliubov Fermi surface removes quantization. The plateau $\kappa_{xy}/T=(\pi^2k_B^2/6h)\,C_{\rm BdG}$ then reads out the integer, its sign reverses with the imaged domain, a written domain wall should carry $2|C_{\rm BdG}|$ Majorana channels, and the thermometry required already resolves single thermal quanta in encapsulated graphene at millikelvin temperatures.
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