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Physics Condensed Matter Strongly Correlated Electrons

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Monday 24 August

Strongly Correlated Electrons

Interaction-mediated Co-existence of Altermagnetism and Topology

Jasmin Bedow, Nitin Kaushal and Marcel Franz

We study an extended Hubbard model on Lieb lattice at electron filling 2 and 4, and demonstrate that the interactions lead to the simultaneous emergence of altermagnetic order and a topological gap. Using unrestricted Hartree-Fock theory, we evaluate both the altermagnetic and topological order parameters and show that they co-exist with an associated quantum spin Hall effect in a large region of the phase diagram, which we further support using exact diagonalization. Moreover, we demonstrate that inversion-symmetry breaking terms can also mediate second-order topological phases with associated corner modes.
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Strongly Correlated Electrons

A DQMC study of the spectral and conductive properties of the two-dimensional Holstein model

J. Neuhaus, B. Cohen-Stead, N. Mannella et al. (+1 more)

We study the dynamical properties of the two-dimensional square lattice Holstein Hamiltonian using numerically exact determinant quantum Monte Carlo simulations. In particular, we report systematic calculations of the model's single-particle spectral function and optical conductivity over a range of phonon energies, electron-phonon coupling strengths, carrier concentrations, and temperatures. In doing so, we map the evolution of the system from a dressed metallic phase to a (bi)polaron liquid/insulator to a charge-density-wave insulator as the carrier concentration is tuned from dilute values to half-filling. We corroborate these results using several equal-time correlation measurements and related proxy quantities reconstructed from time-displaced correlation measurements. This paper is also accompanied by an extensive open data set, covering over 16,640 unique parameter values.
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All authors J. Neuhaus, B. Cohen-Stead, N. Mannella, S. Johnston

Strongly Correlated Electrons

Anomalous stabilization of excitons by metallic proximity

Jeongkeun Song, Uksam Choi, Shan Lin et al. (+6 more)

Metallic environments are generally expected to suppress excitons through strong dielectric screening, yet their influence can differ in composite systems where metallic and insulating regions coexist. Here, we investigate excitonic states in PdxCu1-xCrO2 thin films across a percolation-driven metal-insulator transition. Optical spectroscopy and many-body GW calculations show that CuCrO2 hosts strongly bound excitons with a binding energy of about 489 meV. With increasing Pd substitution, the system approaches an insulator-to-metal transition near x = 0.5, consistent with the site-percolation threshold of a triangular lattice. In the pre-percolation regime, the excitonic resonance redshifts by 241 meV while the Tanguy continuum onset remains nearly unchanged, consistent with a substantial increase in exciton binding energy before metallization. An image-charge-based excitonic hydrogen model shows that isolated metallic regions can enhance electron-hole binding through image-charge interactions, whereas conventional screening is recovered once a continuous metallic network forms. Although this model provides a possible interpretation of the observed excitonic evolution, an alternative scenario in which metallic and excitonic responses originate from electronically distinct states and evolve independently cannot be excluded. These results reveal unusual metallic-excitonic coexistence near a percolation-driven metal-insulator transition and suggest nanoscale metallic proximity as a possible route for modifying excitonic interactions.
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All authors Jeongkeun Song, Uksam Choi, Shan Lin, Du Li, Baekjune Kang, Li Yang, Ambrose Seo, Changhee Sohn, Ho Nyung Lee

Strongly Correlated Electrons · Cross-listed from quant-ph

Quantum Monte Carlo in the Age of Many-Body Quantum Information

Yi-Ming Ding, Bin-Bin Mao and Zheng Yan

Quantum Monte Carlo (QMC) methods are among the central numerical tools for studying strongly correlated quantum many-body systems, particularly in higher dimensions. As quantum information has introduced new information-theoretic perspectives and diagnostics into many-body physics, QMC methods have accordingly been extended beyond the measurement of conventional linear observables. This review summarizes recent progress in adapting QMC to many-body quantum-information, focusing on qubit or spin-$1/2$ systems as a concrete setting while keeping the discussion broadly applicable to qudit and bosonic systems. We present a unified perspective on the extraction of nonlinear diagnostics, including entanglement entropies and entanglement spectra, Rényi negativities for mixed-state entanglement, stabilizer entropies for quantum magic, and decoherence-driven phenomena such as the interplay between imaginary-time evolution and decoherence and strong-to-weak spontaneous symmetry breaking.
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Strongly Correlated Electrons

Quantics tensor cross interpolation for high-order strong-coupling expansions

Kanghyeon Kim, Lei Geng, Philipp Werner et al. (+1 more)

Real-time impurity solvers enable the study of transport phenomena and the description of nonequilibrium lattice systems within the framework of dynamical mean-field theory (DMFT). They also provide direct access to the spectral functions of both equilibrium and nonequilibrium systems. A widely used approach is the self-consistent strong-coupling expansion, whose lowest-order implementation corresponds to the non-crossing approximation. Higher-order implementations, however, are computationally demanding because the number of diagram topologies grows factorially with expansion order, while the evaluation of self-energies and Green's functions requires increasingly high-dimensional integrations. Here, we demonstrate that the latter challenge can be mitigated by employing quantics tensor cross interpolation in a variable-separated framework. Compared with the previously used scale-separated approach, the new scheme yields substantially lower bond dimensions and capacitates self-consistent steady-state DMFT calculations up to fourth order. We illustrate its performance with representative results for both equilibrium and photo-doped systems. In addition, we analyze the convergence of the strong-coupling expansion in the challenging noninteracting limit by computing diagrams up to sixth order. At this order, the onset of the asymptotic regime of the strong-coupling expansion becomes apparent, which allows the application of extrapolation techniques.
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All authors Kanghyeon Kim, Lei Geng, Philipp Werner, Aaram J. Kim

Strongly Correlated Electrons

Interplay of Magnetic Order, Structural Stability, and Orbital Ordering in BaFe2X3 (X = S and Se)

Kunihiko Yamauchi, Takuya Aoyama and Kenya Ohgushi

BaFe2X3 (X= S and Se) are quasi-one-dimensional Mott insulators with a ladder structure that exhibit Stripe- and Block-type antiferromagnetic order, respectively. The ladder arrangement of Fe atoms and strong electron correlations give rise to rich magnetic and orbital phenomena, including pressure-induced superconductivity and orbital-selective electronic states. Both compounds also show resistivity anomalies above the Neel temperature, suggesting that orbital degrees of freedom play an important role in their electronic properties. To clarify the interplay among magnetic order, structural stability, orbital ordering, and transport properties, we performed resistivity measurements and first-principles calculations for BaFe2S3 and BaFe2Se3, systematically comparing candidate magnetic and crystal structures. We find that the magnetic configuration strongly influences the stable crystal structure, orbital ordering, and transport anisotropy, providing a microscopic understanding of the contrasting electronic properties of the two compounds.
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Strongly Correlated Electrons · Cross-listed from physics.chem-ph

Importance-Reweighted Fock-Space Variational Monte Carlo

Zheng Che

Fock-space variational Monte Carlo (FS-VMC) evaluates variational quantities by sampling discrete many-body configurations. In molecular applications, concentrated Born distributions can hinder Markov-chain mixing, while Monte Carlo estimators can also exhibit large variance. We introduce importance-reweighted FS-VMC (IR-FS-VMC), which combines Metropolis–Hastings sampling, an evaluation kernel, and self-normalized importance sampling to estimate the Born-distribution energy, gradient, and stochastic reconfiguration (SR) matrix. Controlled comparisons on equilibrium H2O and Fe2S2 show that the acceptance rate and local-energy variance can respond differently to the sampling and estimation procedure. A single production protocol yields accurate variational energies for H2O dissociation, 36-site hydrogen lattices, and Fe2S2 and Fe4S4 active spaces without system-specific sampling parameters.
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Strongly Correlated Electrons · Cross-listed from cond-mat.supr-con

Revised symmetry rule and intrinsically time-reversal symmetry breaking pairing in multi-orbital superconductors

Chang-Youn Moon

We investigate the basic symmetry rule for the particle permutation in superconducting (SC) pairing states by examining the numerical solution of the linearized Eliashberg equation for Sr$_2$RuO$_4$. We find that the general multi-band, frequency-dependent SC gap function does not simply transform to itself up to the minus sign with either orbital ($\hat{O}$) or frequency ($\hat{T}$) exchange between two pairing electrons, contradicting the common assumption which has been used without verification. It originates from the fact that paring interactions are not invariant under the $\hat{O}$ or $\hat{T}$ operation, and is demonstrated to be essential to correctly interpret and understand multi-band SC states. One of unique properties implied by our newly found symmetry rule is the possibility of the complex eigenvalue in the linearized gap equation where the corresponding gap functions $\Delta$ always have real and imaginary components both non-zero in real frequencies, inherently breaking the time-reversal symmetry. Our numerical results suggest the possibility of this unique pairing in a real material, Sr$_2$RuO$_4$, for which the Hundness of the material is found to play a key role. The revised symmetry rule not only leads us to more comprehensive understanding of the known SC states, but also opens new possibilities into exotic and unique states.
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Strongly Correlated Electrons · Cross-listed from quant-ph

Deterministic Preparation of Arbitrary Spin Eigenfunctions

Wenxuan Tao, Jianan Wang and Fen Zuo

Quantum states with conserved total spins, or spin eigenfunctions, are important for studying quantum chemistry and quantum manybody physics problems. A typical class of spin eigenfunctions are Dicke states, which attain maximal spins. While we already have many efficient quantum algorithms to prepare Dicke states, it is not yet clear if we could do so for arbitrary spin eigenfunctions deterministically. Generalizing Bärtschi and Eidenbenz's elegant algorithms for Dicke state preparation, we successfully prepare arbitrary spin eigenfunctions characterized by branching paths and binary spin trees. As a byproduct, we also develop the corresponding classical algorithms to reconstruct all these spin states.
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Strongly Correlated Electrons

Role of d-electron density of states in the quantum size effect of Pt-Ni and Pt-Pd nanoparticles

S. Kitagawa, T. Ihara, Y. Kinoshita et al. (+3 more)

We investigated the quantum size effect (QSE) in bimetallic Pt$_{1-x}$Pd$_x$ and Pt$_{1-x}$Ni$_x$ nanoparticles, using $^{195}$Pt nuclear magnetic resonance measurements. The temperature and size dependencies of the anomaly in the nuclear spin-lattice relaxation rate divided by temperature $1/T_1T$ in the Pt$_{1-x}$Pd$_x$ nanoparticles suggest similar electron states between Pt and Pd atoms and are well understood by the QSE. The temperature and composition variations of $1/T_1T$ and Knight shift reveal a systematic increase in the density of states and reduction of the characteristic energy scale $T^*$ with increasing Ni content, consistent with the Kubo gap $\delta_{\mathrm{Kubo}}$. In contrast to Pt$_{1-x}$Cu$_x$ nanoparticles where the QSE is suppressed, the Pt$_{1-x}$Ni$_x$ nanoparticles exhibit clear signatures of quantum energy discretization. This discrepancy highlights the essential role of $d$-electrons in the manifestation of the QSE. Furthermore, analysis of the modified Korringa parameter $K(\alpha)$ suggests enhanced ferromagnetic correlations with increasing Ni concentration, approaching a ferromagnetic quantum critical regime. These results provide experimental evidence that $d$-electron density of states plays a crucial role in the manifestation of the QSE in the nanoparticles formed by the metallic $d$-electron atoms.
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All authors S. Kitagawa, T. Ihara, Y. Kinoshita, K. Ishida, K. Kusada, H. Kitagawa

Sunday 23 August

Strongly Correlated Electrons

Anomalous dynamical energy flows via nonlinear phononics in spin-Peierls chains

Jakob Dolgner, Dirk Manske, James K. Freericks et al. (+1 more)

We investigate the nonequilibrium spin-phonon dynamics and energy cascades in a strongly dimerized spin-Peierls chain using a multi-tiered nonlinear phononics architecture. To bypass linear selection rules prohibiting the direct excitation of the Raman-active dimerization mode, a terahertz laser drives an infrared mode that nonlinearly couples to the Raman lattice displacement, subsequently modulating the magnetic exchange. Employing a bond-operator formalism with a second-order cumulant expansion of the Lindblad master equation, we show that maximum energy transfer into the magnetic sector is governed by a dynamical impedance-matching condition rather than the unperturbed triplon density of states. We find that the sustained energy input of continuous-wave driving builds high excitation densities that severely back-act on the lattice, overdamping the primary phonon and smearing magnetic features via power broadening. Conversely, the small time-integrated energy of a pulse keeps the response perturbative, terminating before back-action accumulates and preserving sharp Fano-like quantum interferences. These insights establish limits for controlling dynamic magnetic states without quenching the driving lattice modes.
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All authors Jakob Dolgner, Dirk Manske, James K. Freericks, Mohsen Yarmohammadi

Strongly Correlated Electrons

Strain-driven spin-flop transition and collapse of the giant magnon gap in the bilayer iridate Sr$_3$Ir$_2$O$_7$

Choong H. Kim

The bilayer iridate Sr$_3$Ir$_2$O$_7$ is a $c$-axis collinear antiferromagnet, held there by a giant interlayer pseudodipolar anisotropy, whereas single-layer Sr$_2$IrO$_4$ cants in the $ab$ plane. We show from first principles that biaxial compression of a few percent ($\varepsilon_c\approx-2.4\%$) flops the easy axis of Sr$_3$Ir$_2$O$_7$ into the plane. A magnetic model Hamiltonian built from Wannier functions with no fitted parameter—reproducing the giant magnon gap of the bulk, so far known only from fits to experiment—identifies the mechanism. Compression collapses the interlayer exchange channel, whose straight Ir–O–Ir path weakens as the bent in-plane path strengthens. Hund's exchange sets the scale of the anisotropy and, beyond $J/U\approx0.15$, removes the collinear state altogether. The flop is not a rigid rotation—the ordered moments of the two states cross at $\varepsilon_c$—and it carries a stark fingerprint, in that the giant easy-axis magnon gap collapses to a gapless, Goldstone-like spectrum. Compressively strained films thus sit on a metamagnetic phase boundary ending in a zero-temperature bicritical point, a charge-neutral handle on spin–orbit-entangled order.
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Strongly Correlated Electrons · Cross-listed from cond-mat.mtrl-sci

A test drive for exchange-correlation functionals on noncollinear magnets: Mn$_3$Ir, Mn$_3$Ge, NiS$_2$, and YMnO$_3$

Marie-Therese Huebsch, Martijn Marsman, Jacques K. Desmarais et al. (+2 more)

A first-principles description of noncollinear magnets is challenging, and the practical behavior of genuinely noncollinear exchange-correlation approximations is largely untested. Spin-current density-functional theory (SCDFT) admits noncollinear approximations constrained by local U(1)$\times$SU(2) gauge invariance, an exact condition unavailable in the more restrictive framework of spin-DFT (SDFT). Using the Vienna ab initio simulation package (VASP), we compare two recently developed noncollinear SCDFT functionals, NCMSCAN and LFNCBR89-NCCS, against locally collinear SDFT extensions of four common functionals (PZ, PW92, PBE, and SCAN) for several noncollinear $d$-electron systems. The key finding is that among all tested functionals, only the SCDFT functionals recover the spin texture of the experimental ground state of NiS$_2$, and they do so at a cost comparable to standard semi-local approximations. Yet the two SCDFT functionals differ in equilibrium volumes, band gaps, and on-site magnetic moments. Overall, this survey illustrates the usefulness of SCDFT in capturing noncollinear physics with the tested approximations.
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All authors Marie-Therese Huebsch, Martijn Marsman, Jacques K. Desmarais, Stefano Pittalis, Fabien Tran

Strongly Correlated Electrons · Cross-listed from quant-ph

Laughlin quasihole geometry from a single snapshot ensemble

Kaushlendra Kumar

Can a probability distribution measured in one basis determine complex quantum geometry? The answer is affirmative for a lattice Laughlin quasihole. The exact occupation law at one generic reference position, together with the known analytic quasihole factor, fixes the complete complex Gram kernel. A finite snapshot ensemble estimates this kernel without preparing another member of the family, and thereby determines finite-distance overlaps, Bargmann phases, the quantum metric, and the Berry curvature. The same analytic structure confines the family to an exact projective subspace whose dimension grows at most linearly with particle number. Exact enumeration of a Nielsen-Cirac-Sierra state demonstrates finite-shot reconstruction of both the metric and a geometric phase. Moreover, a Rényi-2 divergence sets the statistical range of the reconstruction while, at nondegenerate points, occupation readout also attains the local single-copy multiparameter information bound.
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Strongly Correlated Electrons

Anomalous metallic states at magnetic interfaces in an antiferromagnetic topological insulator candidate DyPtBi with ferroquadrupolar order

Kentaro Ueda, Hiraku Saito, Zen Ogame et al. (+6 more)

Antiferromagnetic topological insulators provide a fertile platform where symmetry-breaking magnetic order is intertwined with topological electronic states. In particular, magnetic domain walls have attracted much attention, as they can be easily controlled by external fields as in general magnets, and moreover, host nontrivial electronic states distinct from those in bulks and sample surfaces. Here, we report a new antiferromagnetic topological insulator candidate DyPtBi, which hosts conductive magnetic domain walls controllable by uniaxial stress and magnetic field. We find that the resistivity exhibits abrupt increase upon the magnetic and structural phase transition. Concomitantly, the transverse ultrasonic mode shows remarkable softening of 6 %, indicating that the Dy 4f ferroquadrupolar order plays a vital role in the phase transition. Furthermore, we reveal by neutron experiments that applying compressive uniaxial stress aligns the magnetic domain state, leading to the strong resistivity enhancement of 14 % while eliminating the conductive magnetic domain walls. These findings demonstrate that DyPtBi exhibits topological electronic states entangled with multipolar degrees of freedom, providing a promising route for in situ control of topological properties.
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All authors Kentaro Ueda, Hiraku Saito, Zen Ogame, Isao Ishii, Ruo Hibino, Tatsuya Yanagisawa, Shinichi Itoh, Taro Nakajima, Yoshinori Tokura

Saturday 22 August

Strongly Correlated Electrons

Pair Filters in an Extended Hubbard Model at Resonance

Ensuo Ma and Zhi Song

We investigate the dynamics of bound pairs in an extended Hubbard model at resonance. We show that a single fermion and a singlet bound pair possess identical dispersion relations and scatter off each other. In contrast, a neighboring doublon pair is dynamically pinned in the strong-interaction regime. Remarkably, a single fermion and a singlet bound pair exhibit fundamentally different scattering behaviors when encountering a pinned doublon pair acting as a scattering center. While the singlet bound pair undergoes perfect transmission, the single fermion is completely reflected. These results demonstrate that a neighboring doublon pair functions as an efficient filter that separates bound pairs from single particles. Numerical simulations fully support the analytical predictions. Our findings provide a dynamical mechanism for generating and manipulating bound-pair states.
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Strongly Correlated Electrons · Cross-listed from cond-mat.quant-gas

Breakdown of Aharonov-Bohm cage in Rydberg synthetic lattices: the roles of inhomogeneity and long-range exchange

Li Pan, Xinlu Chen, Hong Zhang et al. (+1 more)

While the interaction-induced breakdown of Aharonov-Bohm (AB) cage is typically attributed to uniform bound-pair transport, systems with inhomogeneous exchange interactions realized with Rydberg synthetic lattices exhibit more complex dynamics. Employing the evolution-path symmetry (EPS) framework developed recently, we analyze the two-particle dynamics via path interference in Fock space. We find that a homogeneous nearest-neighbor exchange interaction cannot break the AB cage, regardless of whether the long-range exchange interaction is present or not. In contrast, we demonstrate that inhomogeneous nearest-neighbor exchange interaction breaks the destructive-interference EPS, and lifts the degeneracy of many-body compact localized states, thereby generating non-local dispersive eigenstates. Consequently, the initial state gains a non-zero overlap with these dispersive states, enabling delocalized transport. Furthermore, while long-range exchange interaction alone preserves the AB cage, its coupling with nearest-neighbor inhomogeneous exchange interaction opens non-canceling pathways that alter the diffusion profile. Our work connects microscopic path interference with macroscopic spectral reorganization, offering an analytical understanding of the mechanism underlying exchange-interaction-induced transport in Rydberg synthetic lattices.
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All authors Li Pan, Xinlu Chen, Hong Zhang, Jian-Song Pan

Friday 21 August

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

A high-entropy form of $R$Mn$_6$Sn$_6$ with distinct magnetotransport regimes correlated to different magnetic structures

Kyle W. Fruhling, Jonathan Gaudet, William D. Ratcliff et al. (+9 more)

The kagome $R$Mn$_6$Sn$_6$ material family has attracted significant attention as high-temperature metallic magnets with a host of different magnetic orderings and anisotropy. Theoretical studies point to the rare-earth ($R$) as the determining factor for both the direction of magnetic anisotropy and the type of magnetic ordering in a given compound. This motivates studying high-entropy forms of $R$Mn$_6$Sn$_6$ to examine how the interplay of several rare-earth elements leads to different magnetic states in a single crystal. Here, we present a rare-earth mix of Tb, Dy, Ho, Er, Tm, and Lu that produces phase transitions from a paramagnet to an easy-plane ferrimagnet (FiM) below $T_{\text{C}}$ = 380 K, then to a FiM easy-axis state at $T_{\text{SR1}}$ = 207 K, to a canted FiM ground state below $T_{\text{SR2}}$ = 79 K. This behavior is consistent with previously reported high-entropy $R$Mn$_6$Sn$_6$ compounds; however, uniquely, the rare-earth mix studied here exhibits a broad transition from easy-plane to easy-axis anisotropy from 270 K to 170 K, and reveals a nonmonotonic magnetoresistance. Using neutron scattering data, we found that both observations correlate with an incommensurate modulated contribution to the spin state due to competing rare-earth interactions. This magnetoresistive behavior and the correlated spin structures underscore the potential for rare-earth engineering of magnetism.
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All authors Kyle W. Fruhling, Jonathan Gaudet, William D. Ratcliff, Jonathan S. White, Siddharth Nandanwar, Noah J. Fau, Gregory T. McCandless, Enrique O. González Delgado, Julia Y. Chan, Oksana Zaharko, Michael A. Susner, Fazel Tafti

Strongly Correlated Electrons · Cross-listed from quant-ph

Many-body ergodicity breaking from wavefunction snapshots

Riccardo Andreoni, Devendra Singh Bhakuni, Marcello Dalmonte et al. (+2 more)

Modern quantum experiments can probe many-body wavefunctions through projective measurements, providing snapshots of individual many-body configurations. A fundamental question is whether the intrinsic structure of their measurement distributions can reveal quantum dynamics beyond predefined observables. Here, we address this question at the many-body ergodicity-breaking transition using two complementary characteristics of nonequilibrium wavefunction snapshots: their binary intrinsic dimension (BID) and the topology of wavefunction networks constructed from Hamming distances. We show that the critical point is characterized by an extensive but submaximal BID, and scale-free network connectivity with highly connected hubs. These signatures distinguish the ergodic, critical, and nonergodic regimes, and provide experimentally accessible probes of ergodicity breaking from projective measurement snapshots.
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All authors Riccardo Andreoni, Devendra Singh Bhakuni, Marcello Dalmonte, Lev Vidmar, Miroslav Hopjan

Strongly Correlated Electrons · Cross-listed from cond-mat.other

Theory of the Spinon-Mediated Witness Spin Glass in Herbertsmithite

Mitikorn Wood-Thanan and Felix Flicker

Herbertsmithite is a prototypical candidate quantum spin liquid (QSL), believed to feature a long-range entangled ground state and deconfined fractionalised spinon excitations. Confirmation of these properties has been hindered by the presence of magnetic impurities (spin-1/2 Cu2+ spins substituted onto non-magnetic Zn2+ sites). Recently these impurities were reconceptualised as 'witnesses' of the QSL, inheriting long-range interactions and entanglement mediated by the QSL spinons, leading to spin glass formation amongst witnesses below 260 mK. Here we present a full theoretical account of this idea. Despite having only one free parameter (the witness-kagome spin coupling), our model captures the full range of experimental data, including: the formation of a spin glass amongst witnesses; the frequency and temperature dependence of the magnetic noise; a sharp peak in the DC magnetic susceptibility as a function of temperature; and the static neutron scattering structure factor at 2 K. Both candidate QSLs (Z2 and U(1)) give similar agreement with all data; however, our model predicts a qualitative difference between the Z2 and U(1) neutron scattering intensities below 260 mK, providing a long-sought definitive test to distinguish the two cases. We also predict phase diagrams as a function of temperature and witness concentration, finding a phase transition to different long-range ordered witness states for Z2 and U(1) at high concentration.
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Strongly Correlated Electrons

Ultralow-Field Triplon Condensation in a Spin-Ladder Magnet

Ankit Labh, Ross H. Colman, Jakub Šebesta et al. (+12 more)

We realise the first ultralow-field Bose-Einstein condensation of triplons in a spin-ladder magnet, uncovering a quantum critical point at only $\mu_0 H_{c1}=0.17$ T in Henmilite ($\mathrm{Ca_2Cu(OH)_4[B(OH)_4]_2}$). Unlike dimer magnets, a ladder retains extended one-dimensional correlations in its gapped parent state, making this limit strongly fluctuation dominated. Thermodynamic, magnetoelastic, $\mu$SR, and neutron-diffraction measurements overturn the previous assignment of zero-field antiferromagnetic order, establishing a quantum-disordered coupled-ladder parent state with persistent low-energy dynamics. The weak low-temperature anomaly instead marks a gap-controlled crossover from the correlated ladder regime into the activated quantum-disordered state. These measurements further reveal an exceptionally asymmetric ordered dome extending to $\mu_0 H_{c2}\simeq 8.2$ T. Quantum Monte Carlo simulations for the relevant spin Hamiltonian place Henmilite just on the gapped side of the zero-field ladder-ordering instability, naturally accounting for the strong separation between the exchange and residual-gap scales and the tiny critical field. Our findings extend ultralow-field triplon condensation beyond the dimer paradigm and establish Henmilite as a platform for controlled tuning across quantum criticality in a fluctuation-dominated spin ladder.
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All authors Ankit Labh, Ross H. Colman, Jakub Šebesta, Noah Oefele, Elsa Lhotel, Adam Berlie, Paul Steffens, Oksana Zaharko, Pascal Manuel, Iurii Kibalin, Philipp Gegenwart, Dominik Legut, Johanna K. Jochum, Alexander A. Tsirlin, Petr Čermák

Strongly Correlated Electrons

Neural quantum states in condensed matter: advances, best practices, and prospects

Jonas B. Rigo, Björn J. Wurst, Rajah Nutakki et al. (+2 more)

Neural quantum states provide flexible variational representations of quantum many-body wave functions by combining neural-network parametrizations with Monte Carlo sampling. In this perspective, we review recent advances in their application to condensed-matter systems, focusing on frustrated quantum magnets, interacting lattice fermions, and non-equilibrium dynamics. We discuss the architectures, symmetry constraints, optimization methods, and sampling strategies underlying state-of-the-art calculations, and summarize practical guidelines for reliable simulations. We also examine the principal remaining challenges, including learning non-trivial sign and phase structures, controlling variational bias, enforcing physical symmetries, scaling optimization to large networks, and achieving stable real-time evolution. Finally, we outline promising directions in which neural quantum states may extend the reach of classical simulations of strongly correlated quantum matter.
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All authors Jonas B. Rigo, Björn J. Wurst, Rajah Nutakki, Markus Schmitt, Dante Kennes

Strongly Correlated Electrons

Two distinct gap structures in the mid-infrared optical conductivity of the Hubbard model

Dongwook Kim and Karsten Held

Mid-infrared (MIR) optical conductivities in cuprate superconductors show universal features. In this study, we demonstrate that these originate from two distinct gaps: the pseudogap mediated by antiferromagnetic spin fluctuations and the Mott-Hubbard gap. The MIR spectra from these two gaps show a characteristic and distinct shape and doping dependence, and can thus be distinguished. Specifically, the Mott-Hubbard correlations, while rendering a peak in the visible spectrum for small dopings, yield a broad optical response starting in the MIR when a “waterfall” develops in the one-particle spectrum at larger dopings. The numerically and analytically determined doping-dependence of both gaps consistently reproduce experiments, and provide a unified microscopic understanding of the cuprate MIR optical conductivity.
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Strongly Correlated Electrons

Emergent Vibronic Spectral Hierarchy in a Kagome Flat-Band Insulator

Jun Shu, Jun Shen, Yanmin Zhang et al. (+17 more)

Electron-phonon coupling is usually understood in terms of electronic quasiparticles interacting with dispersive lattice vibrations. Much less is known about the complementary limit in which the relevant phonon mode is itself localized or weakly dispersive. Here we investigate this regime in the kagome compound Rb$_{2}$Ni$_{3}$S$_{4}$, which undergoes an unconventional insulating transition near $T^{*} \approx$ 260-280~K. Combining polarization-resolved Raman spectroscopy with temperature-dependent x-ray diffraction, scanning tunneling microscopy, and electrical, thermal, and magnetic measurements, we show that the transition involves electronic localization without a conventional structural or magnetic order parameter. Raman spectra reveal a giant Franck-Condon progression associated with a nearly dispersionless 333.7~cm$^{-1}$ phonon, decorated by an equally spaced comb-like fine structure with a characteristic spacing of 40.6~cm$^{-1}$. The comb spacing is insensitive to magnetic field, whereas its spectral weight is strongly field tunable. Rather than treating either hierarchy alone as pure phonon effect, we interpret their nested coexistence as evidence for a strongly coupled electron-vibrational manifold involving a localized lattice coordinate. These results identify dispersionless phonons as an active route to vibronic correlations in solids and suggest that such electron-vibrational self-trapping is closely associated with the insulating phase of Rb$_{2}$Ni$_{3}$S$_{4}$.
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All authors Jun Shu, Jun Shen, Yanmin Zhang, Hong Du, Qingsong Wang, Zeyuan Wang, Bin Wang, Zeliang Xu, Dengjing Wang, Hengfu Lin, Zunming Lu, Lei Qin, Jie Yuan, Jinbo Peng, Zhida Song, Fedor V Kusmartsev, Anna Kusmartseva, Kui Jin, Ruidan Zhong, Ge He

Strongly Correlated Electrons

Berry-Curvature Effect in Anomalous Transport

V.P.Mineev

A theory of the Berry-phase effect in anomalous transport in ferromagnets driven by statistical forces such as the gradient of temperature or chemical potential has been developed by Di Xiao et al in Phys. Rev. Lett. 97, 026603 (2006) based on a field-dependent density of states generating an additional contribution to the current density. We present an alternative derivation of the Berry- phase effect in anomalous transport in altermagnets and noncentrosymmertric metals based on quasiclassic matrix kinetic equation. Along with anomalous Hall effect an anomalous heat flow and anomalous thermalelectric currents are derived.
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Strongly Correlated Electrons · Cross-listed from cond-mat.quant-gas

Pair size versus coherence length and quantum geometry in the spin-orbit coupled BCS-BEC crossover

R. N. Kalkan and M. Iskin

We investigate the interplay between pairing correlations and quantum geometry in the spin-orbit-coupled BCS-BEC crossover. Working in the helicity basis, we derive the exact pair-size tensor for the two-body bound states and show that it separates into intraband and quantum-geometric interband contributions, with the latter determined by the quantum metric. We extend this decomposition to two distinct many-body length scales at zero temperature: the Cooper-pair size, obtained from mean-field BCS theory, and the coherence length, obtained from the Gaussian fluctuation theory. Although these quantities almost coincide in the BCS regime, they describe distinct physical properties away from it, with the pair size characterizing the internal extent of a pair and the coherence length characterizing the long-wavelength response of the order parameter. We evaluate both quantities for three-dimensional Rashba, three-dimensional Weyl, and two-dimensional Rashba spin-orbit coupling models. We find that the pair size decreases monotonically with increasing spin-orbit coupling as the pair localizes, whereas the coherence length develops a pronounced minimum in the crossover regime before growing again in the BEC regime as the composite bosons become weakly interacting. This minimum is tied to the collapse of the noninteracting Fermi surface onto the ring or sphere of helicity-band minima. The quantum-geometric contributions reach up to nearly one third of the pair size and about 40% of the coherence length, tracking these same changes in the underlying helicity Fermi-surface topology.
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Strongly Correlated Electrons

Doping-driven Mott transition from steady-state density functional theory

David Jacob and Stefan Kurth

We describe the doping-driven Mott transition in the Hubbard model within the framework of steady-state density functional theory, or i-DFT. In order to access the many-body spectral function in i-DFT, an approximation to the exchange-correlation (xc) bias at arbitrary density and current is required. By making use of Fermi-liquid theory, we derive conditions on the xc bias of i-DFT in terms of the quasiparticle weight, thus establishing a clear connection between i-DFT and Fermi-liquid theory. The Fermi liquid conditions are then employed to guide the construction of an approximation to the xc bias functional. Numerical results obtained with this functional demonstrate that the doping-driven Mott transition can indeed be captured with i-DFT. More generally, our i-DFT approach to calculate spectral functions establishes an explicit form for the functional relationship between the many-body spectral function and the ground state electronic density.
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Strongly Correlated Electrons · Cross-listed from quant-ph

A Classification of Translation-Invariant Quantum Codes in Any Dimension

Andrew Li and Dominic J. Williamson

Quantum error-correcting codes with two-dimensional translation invariance are known to be equivalent to copies of the two-dimensional toric code. Such a simple classification is not possible for quantum codes with higher dimensional translation invariance due to the existence of multiple types of toric codes and infinite families of fracton codes. Here, we focus on D-dimensional translation-invariant quantum codes based on length-D chain complexes. This includes multivariate multicycle codes where the number of variables equals the number of cycles. We show that such codes are equivalent to copies of D-dimensional toric codes. This directly generalizes the classification result for two-dimensional translation-invariant codes.
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Strongly Correlated Electrons · Cross-listed from cond-mat.mtrl-sci

Quasi-one-dimensional topological band structure and van Hove singularities in monolayer TaIrTe$_4$ from laser $\mu$-ARPES

Honey Boban, Tanguy Prongué, Amarjyoti Choudhury et al. (+9 more)

Recent transport experiments reported a quantum spin Hall insulator phase in monolayer 1T-TaIrTe$_4$ gated away from charge neutrality. This phase is not predicted by band structure calculations and has been attributed to an electronic instability induced by strong correlations at a putative van Hove singularity. Here, we investigate the electronic structure of exfoliated monolayer 1T-TaIrTe$_4$ using micro-focus laser angle resolved photoemission. We find a strongly anisotropic band structure susceptible to density wave instabilities. Our data further reveal a saddle point singularity in the density of states. However, we find that the saddle point lies at a density for which transport experiments found no anomalies. Moreover, the quasiparticle line widths suggest weak electron correlations only. This points to a secondary role of van Hove singularities and electron correlations in the transport phase diagram of monolayer 1T-TaIrTe$_4$.
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All authors Honey Boban, Tanguy Prongué, Amarjyoti Choudhury, Dario Marchiani, Andrés Bareño, Felix Eder, Enrico Giannini, Fabian O. von Rohr, Alberto F. Morpurgo, Marco Gibertini, Anna Tamai, Felix Baumberger

Strongly Correlated Electrons · Cross-listed from quant-ph

Spin models with critical ground space degeneracy from Lie algebra relations

Andras Molnar, Efekan Kökcü, Norbert Schuch et al. (+1 more)

We introduce an $\mathrm{SO}(3)$-symmetric spin model derived from the algebraic structure of $\mathfrak{so}(3)$: It is obtained as the nearest-neighbor parent Hamiltonian of a Matrix Product State (MPS) built from the generators of the Lie algebra $\mathfrak{so}(3)$ and the identity matrix, and therefore encodes the quadratic relations of the Lie algebra. We characterize the ground space structure of the resulting model and show that for open boundary conditions (OBC), it exhibits a quadratic ground space degeneracy, given by one irreducible representation (irrep) of each odd dimension (integer spin). For periodic boundary conditions (PBC), it has a linear ground space degeneracy, consisting of a singlet — namely, the MPS underlying the model — and a ferromagnet (the irrep with maximal spin), and thus exhibits gapless excitations. We also generalize the construction and the analysis of the OBC ground space to $\mathrm{SO}(d)$ and other compact simple Lie groups. The MPS on which the model is based is an injective MPS, and therefore, its 3-site parent Hamiltonian has a unique ground space and is gapped. The observed critical behavior thus has its origin in the small parent Hamiltonian considered. The model's algebraic ground space scaling thus provides an example distinct from that reported in (Schuch et al., 2025, arXiv:2503.10767), where it was shown that small parent Hamiltonians of injective MPS generically have unique ground states, and examples with an exponential ground space degeneracy were given.
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All authors Andras Molnar, Efekan Kökcü, Norbert Schuch, Bojko N. Bakalov