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Physics Condensed Matter Strongly Correlated Electrons

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Tuesday 11 August

Strongly Correlated Electrons

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

Bin Xi, Jie Lu, Qiang Luo et al. (+4 more)

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

Strongly Correlated Electrons · Cross-listed from cond-mat.quant-gas

Composite-Boson Ansatz for Fractional Quantum Hall Manifolds of Lattice Bosons at Generic Fillings $\nu<1/2$

Umut Can Turhan, A. Levent Subaş\i and Rifat Onur Umucal{\i}lar

Hofstadter systems provide a lattice route to fractional quantum Hall physics, but their low-energy manifolds often lack simple wave-function descriptions. Here we present a composite-boson ansatz for a broad family of finite-size lattice-split Laughlin-quasihole manifolds in the low-flux Hofstadter–Bose–Hubbard model on a torus at generic fillings \(\nu<1/2\). Attaching two vortices to each boson yields reduced-flux orbitals from which we construct a many-body trial basis. Its translation-resolved rank reproduces the expected low-energy-manifold dimension and provides a composite-boson interpretation of the established quasihole counting previously inferred from generalized exclusion rules and thin-torus arguments. For smaller systems, diagonalization within the lowest-band-projected ansatz span closely reproduces the exact projected spectrum, while variational Monte Carlo extends the rank and energetic analysis to larger systems for which explicit subspace construction becomes costly. To probe the fractional quantum Hall character of these manifolds, we calculate their many-body Chern-numbers and show that localized added-flux excitations exhibit quasihole-like behavior, with fractional density depletion and an Aharonov–Bohm-free braiding phase that both track the effective filling. Together, these results establish a microscopic composite-boson framework for organizing a broad family of sub-half-filled fractional quantum Hall manifolds of lattice bosons.
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Strongly Correlated Electrons

Quantum Mechanism of Piezomagnetism in Higher-Spin Altermagnets

Daisuke Yamamoto and Makoto Naka

We investigate piezomagnetism in higher-spin altermagnets with easy-plane single-ion anisotropy using a flavor-wave approach. We show that quantum fluctuations of higher-spin collective modes provide a microscopic origin of the piezomagnetic response. For integer spins, the relevant branch softens and evolves into a Higgs-like amplitude mode on approaching the large-$D$ transition, endowing the excitation with a sizable dipolar component and producing a pronounced enhancement of piezomagnetism. By contrast, in half-integer systems the higher-spin branches are progressively separated from the low-energy dipolar sector as the anisotropy increases, which suppresses their contribution to the response. This integer-half-integer contrast directly links macroscopic piezomagnetism to the low-energy fate of multipolar excitations. Our results establish piezomagnetism as a probe of higher-spin quantum dynamics and identify higher-spin altermagnets as a promising setting for quantum magnetoelastic responses.
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Strongly Correlated Electrons

Carrier-tunable RKKY magnetism in a crystalline magnet

Xiang S.W. Huang, Bruno K. Saika, Satoshi Hamao et al. (+10 more)

In itinerant magnets governed by the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction, the exchange coupling depends on both the moment-moment distance $r$ and the Fermi wavevector $k_{\mathrm{F}}$, yet in bulk synthesis the two are tightly coupled: a change in composition typically alters both. Here, we use a thin-film approach to tune these two variables independently in a single crystalline host. Using molecular-beam epitaxy (MBE), we stabilize either $2\times2\mathrm{R}0^\circ$ Cr$_{1/4}$NbSe$_2$ or $\sqrt{3}\times\sqrt{3}\mathrm{R}30^\circ$ Cr$_{1/3}$NbSe$_2$ within the same NbSe$_2$ host through separate growth windows. Controlled post-growth annealing performed across a series of temperatures then modifies the carrier density while leaving the Cr superstructure intact below a structural-transition threshold. The two as-grown phases are distinct in electronic structure, magnetic ground state, and transport. Along this annealing series, the Hall response evolves systematically while the magnetic response changes in a structurally insensitive manner, with ferromagnetic order emerging within the same $\sqrt{3}\times\sqrt{3}\mathrm{R}30^\circ$ structural class only above a critical annealing temperature, experimentally disentangling carrier density and moment geometry. The Hall magnitude and sign evolution point to a low-carrier-density system in which $k_{\mathrm{F}}$ is susceptible to modest external tuning. Cr-NbSe$_2$ thus realizes carrier-sensitive RKKY magnetism in a single crystalline host, within an MBE-plus-annealing approach extensible across the intercalated transition-metal dichalcogenide family.
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All authors Xiang S.W. Huang, Bruno K. Saika, Satoshi Hamao, Yuki Majima, Yuki Settai, Hideki Matsuoka, Yuki M. Itahashi, Masato Sakano, Taro Nakajima, Shinichiro Seki, Yoshihiro Iwasa, Kyoko Ishizaka, Masaki Nakano

Strongly Correlated Electrons

Role of Goldstone mode in nonequilibrium insulator under DC electric field

Xi Chen and Jong E. Han

Measurements of resistive breakdown in electronic systems under a DC electric field have shown that the threshold fields for the insulator-to-metal transition are significantly lower than predicted by single-electron excitation scenarios, such as the Landau-Zener theory. In this work, we propose an alternate mechanism of destabilizing ordered insulators under a DC electric field by fluctuations of the order parameters through the Goldstone mode excitation. The low-energy bosonic excitations receive energy from accelerated electrons and thus destroy the spontaneous symmetry breaking. Using the Keldysh Gree's function formalism, we numerically confirm that the Goldstone mode remains well-defined in the nonequilibrium steady state, while its nonequilibrium excitations are sensitive to the electric field. The effective temperature of the Goldstone mode increases much more rapidly than the electronic effective temperature, with the bosonic threshold field significantly smaller than the electronic one, which suggests that collective phase dynamics may further reduce the transition field to the experimental range via a purely electronic mechanism.
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Strongly Correlated Electrons · Cross-listed from quant-ph

Symmetrized Block-Product Periodic Marginals in Infinite Translation-Invariant Quantum Chains

Xiao Zeng, Kaiyan Yang, Lingxia Zhang et al. (+1 more)

We study local marginals in one-dimensional translation-invariant quantum systems that may hide finite-period structure. Given an $n$-site reduced density matrix, we ask whether it can be obtained by repeating a finite $p$-site block state along the chain and averaging over the $p$ lattice translations. This defines a symmetrized block-product periodic marginal problem, which provides a route both to diagnosing hidden periodic order from local data and to upper bounding ground-state energy densities of infinite translation-invariant local Hamiltonians. We develop two complementary methods. The first is a semidefinite-programming relaxation based on block permutation symmetry and positive partial transpose constraints, which outer-approximates the convex hull of such marginals and yields certified infeasibility tests. The second is a symmetrized matrix product state ansatz, which constructs explicit block-product periodic states and gives variational upper bounds. We benchmark the framework on the Majumdar-Ghosh model, transverse-field Ising, XX, XXZ, and contextuality-related spin models. The results show that the method captures the expected finite-period structure in exactly solvable cases and gives systematically improving variational energies as the period and bond dimension increase. We also formulate a periodic-NPA relaxation for translation-invariant contextuality witnesses and recover the known quantum limits in the tested examples.
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All authors Xiao Zeng, Kaiyan Yang, Lingxia Zhang, Zizhu Wang

Strongly Correlated Electrons

Observation geometry for uncertainty-aware Hamiltonian inference and experimental design in quantum magnets

Roy Liu, Venugopal Ranganathan, David Dahlbom et al. (+12 more)

Determining microscopic interactions from spectroscopic and scattering measurements is central to understanding quantum materials, yet it often remains unclear which interactions can be reliably revealed by the available experimental data and how additional experimental modalities should be designed to resolve the remaining ambiguities. Here we present an artificial intelligence-enabled framework for uncertainty-aware Hamiltonian inference and adaptive experimental design. By combining Hamiltonian-conditioned neural surrogates with Bayesian inference and observation geometry, the framework characterizes how measurements constrain Hamiltonian parameter space, quantifies the identifiability of microscopic interactions, and propagates posterior uncertainty directly in the physical Hamiltonian parameter space rather than an abstract learned representation. Using multimodal powder and single-crystal inelastic neutron scattering measurements of the quantum magnet NiPS$_{3}$, we demonstrate physically interpretable Hamiltonian inference, modality-aware uncertainty quantification, and adaptive experimental design. The framework provides a general strategy for uncertainty-aware microscopic characterization and multimodal experimental design across quantum materials.
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All authors Roy Liu, Venugopal Ranganathan, David Dahlbom, Shizhou Xu, Tianyu Zhang, Yuan Ni, Daniel M. Pajerowski, Garrett Granroth, Thomas Strohmer, Matthew B. Stone, Andrew F. May, Mark D. Lumsden, Joshua J. Turner, Yongqiang Cheng, Zhantao Chen

Monday 10 August

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

Helimagnetism from competing intra- and interchain interactions in CrBr$_2$ and CrI$_2$

John A. Schneeloch, Matthew B. Stone, Zachary Morgan et al. (+1 more)

CrBr$_2$ and CrI$_2$ are promising platforms for studying the effect of dimensionality on magnetic order, having been isolated in a 3-, 2-, and 1-dimensional form as bulk crystals, monolayers, and individual chains encapsulated in carbon nanotubes. However, the interactions that give rise to their helimagnetic order are unknown. Via inelastic neutron scattering on single crystals, we have determined the exchange interactions of these compounds from the spin wave dispersions, finding that the helimagnetic order arises primarily from competing antiferromagnetic interactions between intrachain and interchain nearest-neighbors. Single-ion anisotropy is substantial, modulating the helical spin rotation and gapping the inelastic intensity at points of branch crossings. As temperature increases, long-range order vanishes but intralayer correlations remain detectable up to at least 50 K.
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All authors John A. Schneeloch, Matthew B. Stone, Zachary Morgan, Despina Louca

Strongly Correlated Electrons

Some features of high-temperature superconductivity on flat bands

V.R. Shaginyan, A.Z. Msezane and S.A. Artamonov

In this letter, we examine how the presence of flat band leads to the formation of a high-temperature superconductor even in the case of repulsive pairing interactions. We also show that in the case of flat bands, the high-temperature superconducting state deforms the flat band, tilting it and making the effective mass finite. As a result, neither the superfluid weight nor the supercurrent disappear. Our results are in good agreement with experimental data.
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Strongly Correlated Electrons

Orbital-selective oxygen holes in cuprate ladders beyond the Zhang-Rice paradigm

Chengyun Hua, Tianran Chen, Isaac C. Ownby et al. (+13 more)

The electronic structure of the spin-ladder cuprate Sr14Cu24O41 challenges the presumed universality of the Zhang-Rice singlet (ZRS) framework and models based exclusively on Cu-O hybridized orbitals. Combining polarization-dependent resonant soft X-ray scattering at the O K-edge with inelastic neutron scattering, we show that doped holes in the Cu2O3 ladders localize predominantly in planar non-bonding O 2pz (p{\pi}) orbitals of rung oxygen sites rather than forming conventional ZRS states. Polarization-resolved RSXS uniquely identifies this orbital assignment, while lattice and magnetic excitations reveal its coupled consequences, establishing a unified microscopic picture that excludes the conventional {\sigma}-bonded singlet. This oxygen-sublattice charge order produces an anomalous diagonal stretching phonon and explains the absence of incommensurate magnetic fluctuations and anomalous magnon splitting. These findings motivate a reassessment of hole pairing in ladder cuprates and the sufficiency of copper-centric models for cuprate superconductors.
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All authors Chengyun Hua, Tianran Chen, Isaac C. Ownby, Colin L. Sarkis, Garrett Granoth, Masaaki Matsuda, Jiaqiang Yan, Ho Nyung Lee, Jeongkeun Song, Yuya Shinohara, Masatomo Uehara, Jun Akimitsu, Oleksandr Prokhnenko, Eugen Weschke, Takeshi Egami, D. Alan Tennant

Strongly Correlated Electrons · Cross-listed from quant-ph

Coupled-Layer Codes: Beyond Quantum Product Constructions

Shuyu Zhang, Tzu-Chieh Wei and Nathanan Tantivasadakarn

Product codes are an important class of quantum error-correcting codes constructed from multiple input codes, which can give rise to asymptotically good quantum low-density parity check codes. In previous work, we showed how the product between two codes can be physically implemented by coupling layers of the first code using checks of the second code. In this work, we further unify product code constructions with coupled-layer constructions of phases of matter by introducing coupled-layer codes. The essential strategy is to condense general excitations created by Pauli operators among multiple decoupled layers of the first code. The condensation is specified by an excitation algebra, which encodes the excitations, along with an algebra-preserving map. This coupling between layers generalizes the notion of gauging in physics as well as the mapping cone in homological algebra, and can be used to produce non-CSS codes. As examples, we show how coupled-layer codes reproduce the X-cube and Chamon models. We further generalize the balanced product code by allowing a unitary transformation in addition to a group action by free permutation and describe its corresponding coupled-layer construction. In particular, we show how balancing by a ZX-duality can reproduce non-CSS codes such as the fermionic toric code and the 3-fermion Walker-Wang model in 3D.
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Strongly Correlated Electrons · Cross-listed from cond-mat.mtrl-sci

Cell Natural Orbitals in Quantum Materials

Harshitra Mahalingam, Nishchhal Verma, Daniel Muñoz-Segovia et al. (+1 more)

Understanding correlated quantum matter starts with an accurate model of the single-particle states that interact at low energies: their dispersion, band geometry, orbital content and charge density. In many cases, notably the topological bands of moire materials, it is not straightforward to find a real-space description with a few local orbitals that accomplishes this task. Here we provide a systematic way to identify the local degrees of freedom that best capture the band geometry and charge density of any chosen set of bands. We use the unit-cell one-particle reduced density matrix (UC-1pRDM), obtained by restricting the projector onto the target bands to a single unit cell. Its eigenstates, which we call cell natural orbitals (CNOs), form a local, symmetric basis uniquely determined by the Bloch wavefunctions and the choice of real-space partition. Their eigenvalues measure the occupation of each CNO in the target bands, quantifying entanglement across unit-cell boundaries and the importance of multi-orbital character. A set of CNOs that maximizes total spectral weight and reproduces the target band symmetries provides optimal trial states for Wannierization. We exemplify this by constructing a lattice model for twisted bilayer WSe$_2$ that tracks the orbital content across twist angles.
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All authors Harshitra Mahalingam, Nishchhal Verma, Daniel Muñoz-Segovia, Raquel Queiroz

Strongly Correlated Electrons

Cell Natural Orbitals in Interacting Topological Bands

Nishchhal Verma, Harshitra Mahalingam, Daniel Muñoz-Segovia et al. (+1 more)

Topological bands exhibit obstruction to exponentially localized and symmetric Wannier functions, challenging the standard paradigm of representing projected interactions in terms of local orbitals with finite range. To faithfully capture the form factors and quantum geometry of topological bands we introduce a singular-value decomposition of the band-projected density form factors, enabling a geometry-based truncation scheme of the Hilbert space, exposing an intrinsic hierarchy on band-projected interactions that is determined by the underlying wavefunctions. This decomposition is most naturally described in terms of Cell Natural Orbitals (CNOs), as the eigenstates of the unit-cell reduced one-particle density matrix, whose occupation provide a measure of the minimal orbital complexity required to faithfully represent the band wavefunctions overlaps. The CNO decomposition identifies systematically the minimal number of local orbitals needed to reproduce short-ranged interactions while resolving the hierarchy of interaction strengths across CNO channels. Applied to magic-angle twisted bilayer graphene in the chiral limit, we find that the dominant CNO is centered at the AA site, resembling the $f$-fermion of the heavy-fermion model. The subdominant CNO channels carry progressively weaker interaction matrix elements, allowing them to be treated at the static mean-field level, while the dominant channel requires a dynamical self-energy. The formalism illustrates how variations of charge density within the unit cell generate momentum dependence in the CNO envelope function and, consequently, dispersion in the single-particle spectral function. More broadly, our results establish CNOs as a geometry-informed bridge between band topology and real-space correlations, providing a systematic framework for analyzing interactions and emergent phases in quantum materials.
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All authors Nishchhal Verma, Harshitra Mahalingam, Daniel Muñoz-Segovia, Raquel Queiroz

Strongly Correlated Electrons · Cross-listed from quant-ph

A Quantum Coherence Microscope in the Hubbard Regime

Lin Su, Michal Szurek, Alec Douglas et al. (+2 more)

Quantum coherence underlies collective quantum phenomena and emerging quantum technologies. Quantum gas microscopes have transformed quantum simulation by providing projective snapshots of many-body states with single-atom resolution, but spatially resolved measurements of off-diagonal correlations have remained elusive. Here, using the Talbot effect, we introduce a quantum coherence microscope that maps off-diagonal correlations onto site-resolved density signals with near-single-site resolution. We use this technique to locally probe the superfluid-Mott transition in a layer of a three-dimensional optical lattice and to measure coherence beyond nearest neighbors in an engineered potential landscape. By mapping off-diagonal correlations onto density signals through controlled Talbot evolution, this work opens new possibilities for accessing observables beyond the density basis through tailored matter-wave evolution and recapture.
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All authors Lin Su, Michal Szurek, Alec Douglas, Ceren B. Dag, Markus Greiner

Strongly Correlated Electrons · Cross-listed from hep-th

The quadratic growth of Krylov spread complexity in the BTZ black hole

Aranya Bhattacharya, Mario Flory, Michal P. Heller et al. (+2 more)

The boundary quantity that captures the growth of black-hole interiors quantified by holographic complexity remains unknown beyond 2d dilaton gravity. We provide a critical analysis of a partition-function construction of Krylov spread complexity for thermofield-double states that provides a dimension-independent boundary reconstruction from semiclassical holographic partition functions, while developing the present dynamical and bulk construction for the BTZ saddle. In the double-scaled Sachdev-Ye-Kitaev model, where exact and semiclassical results can be compared, we show that the classical limit is reliable only when taken after the complexity has been reconstructed; taking this limit at the level of individual Lanczos coefficients discards essential information. Applying the construction to a large-central-charge two-dimensional conformal field theory above the Hawking-Page temperature dual to a Bañados-Teitelboim-Zanelli black hole, we find an intermediate departure from early-time quadratic growth followed by behavior compatible with a return toward asymptotically quadratic growth, rather than the linear late-time behavior of the volume and the standard finite-functional complexity = anything class. We then match this boundary behavior to a generalized complexity = anything bulk object built from an infinite series of extrinsic-curvature invariants. The construction provides a systematic route from black-hole thermodynamics to Krylov dynamics and can naturally be extended to higher-dimensional holographic black holes.
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All authors Aranya Bhattacharya, Mario Flory, Michal P. Heller, Emiliano Rizza, Tim Schuhmann

Strongly Correlated Electrons · Cross-listed from quant-ph

Decoupling 2D translation-invariant topological CSS codes

Yifei Wang, Zhongyi Ni, Mingxin He et al. (+2 more)

Two-dimensional translation-invariant topological CSS codes on qubits are known to be locally equivalent, after coarse-graining, to stacks of toric codes. However, existing constructions generally break more translation symmetry than is required to remove anyon-permuting translations, leaving open whether any further obstruction exists. We prove that no such obstruction occurs: after passing to the maximal anyon-preserving superlattice, every such code admits a local unitary decoupling into toric codes and product states. We further provide an efficient algorithm for explicitly constructing the decoupling map, together with bounds on the required supercell size and operator spreading. The decoupling requires no additional ancillas in generic cases and extends to finite systems with suitable boundary conditions.
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All authors Yifei Wang, Zhongyi Ni, Mingxin He, Jinguo Liu, Yingfei Gu

Strongly Correlated Electrons · Cross-listed from quant-ph

From Koszul-Complex Stabilizer Models to Superselection Profiles: Topological Rigidity and Nonsplit Extensions

Hao Song

Koszul-complex stabilizer models unify the toric-code hierarchy and bivariate-bicycle codes in one homological framework. To study translation-symmetry-enriched topological (SET) phases in these models and, more generally, in translation-invariant Calderbank-Shor-Steane (CSS) codes with stabilizer maps $(\varphi_X,\varphi_Z)$, we introduce the associated topological superselection profile $\mathscr S_\sigma:=\tau_{\geq1}\mathbf R\!\operatorname{Hom}_R(\overline{\operatorname{coker}\varphi_\sigma},R),\ \sigma=X,Z$. Its cohomology layers $E_\sigma^\ell:=H^\ell(\mathscr S_\sigma)\cong_R\operatorname{Ext}_R^\ell(\overline{\operatorname{coker}\varphi_\sigma},R)$ encode sectors, fusion, and translation action. When finite, the $\ell=1,2,\ldots$ layers describe pointlike, looplike, and higher-dimensional excitations; $\mathscr S_\sigma$ retains inter-layer gluing. For regular Koszul models, we compute $E_\sigma^\ell$ explicitly, find $\mathscr S_\sigma$ single-layer, and obtain finite-size $Z$-logicals from $\operatorname{Tor}$. Single-layer means that at most one positive-degree layer $E_\sigma^\ell$ is nonzero. Our matrix-level Schanuel-lemma method proves rigidity: under a finite-resolution hypothesis, the nonzero layer and degree uniquely determine a topological CSS code's translation SET order at the stable translation-invariant Clifford level. For prime qudits the hypothesis is automatic, and a finite layer's translation kernel gives the exact minimal coarse graining to a toric-code stack. We realize admissible single-layer data. Beyond this regime, split and nonsplit extensions of 3D qubit toric codes yield eight models sharing all identical $E_\sigma^\ell$ layers with trivial translation action but having distinct size-dependent ground-state degeneracies, hence distinct translation SET phases. Thus inter-layer gluing carries SET data invisible to individual layers.
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Strongly Correlated Electrons

Tunable Multiband Geometry and Fractional Phases in Higher Vortexable Systems

Xiaohan Wan, Siddhartha Sarkar, Ting Cao et al. (+3 more)

Higher vortexability is often viewed as a route to topological flat bands with higher-Landau-level-like quantum geometry. Here we emphasize a complementary perspective: it provides a tunable multiband structure in which wave function geometry can be varied continuously while the band dispersion, degeneracy, and topology remain fixed. We perform systematic exact-diagonalization studies of many-body phases in fractionally filled higher vortexable moiré systems, retaining the full flat-band Hilbert space rather than projecting onto a single band. The multiband treatment reveals a cascade of Abelian and non-Abelian phases at zero magnetic field, including integer and fractional exciton insulators, Abelian fractional Chern insulators, Moore-Read and Read-Rezayi states. At fixed filling, different phases are connected through transitions or crossovers driven solely by changes in wave function geometry, highlighting quantum geometry itself as a direct tuning parameter between competing topological states. At fillings associated with Moore-Read and Read-Rezayi states, our calculations show that interband mixing shifts the optimal quantum geometry regime without suppressing non-Abelian topological order under screened Coulomb interaction. Our results establish higher vortexable moiré bands as a tunable platform for exploring geometry-driven multiband topological phases at zero magnetic field.
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All authors Xiaohan Wan, Siddhartha Sarkar, Ting Cao, Mark Rudner, Di Xiao, Kai Sun

Strongly Correlated Electrons

Anomalous temperature dependence of polaron mobility in a nonlinear double-well potential: unbiased X-propagator approach

Stefano Ragni, Osor S. Barišić, Naoto Nagaosa et al. (+1 more)

We develop an unbiased X-propagator method for calculating finite-temperature optical conductivity $\sigma(\omega)$ and dc mobility $\mu$ for arbitrary nonlinear electron-phonon interaction. We apply it to a polaron coupled to a double-well lattice potential, a minimal model for strongly anharmonic polar materials. At moderate coupling, the mobility exhibits three temperature regimes associated with confinement within one well, thermal competition with the barrier, and barrier-insensitive high-temperature dynamics. This sequence produces a concave temperature dependence of the mobility that is absent in conventional linear-coupling polaron models. At strong coupling, the mobility becomes nonmonotonic, and its temperature evolution is reflected in a characteristic redistribution of optical spectral weight. For parameters relevant to SrTiO$_3$, our results qualitatively reproduce both the anomalous concave mobility and the onset of violation of the Mott-Ioffe-Regel limit, thereby supporting nonlinear coupling to a soft anharmonic lattice mode as a microscopic mechanism for anomalous transport in dilute polar metals.
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All authors Stefano Ragni, Osor S. Barišić, Naoto Nagaosa, Andrey S. Mishchenko

Strongly Correlated Electrons · Cross-listed from quant-ph

Quantized Stabilizer-Rényi Boundary Response across Fermionic SPT Transitions

M. A. Rajabpour

Open boundaries host symmetry-protected Majorana modes, yet their imprint on the stabilizer Rényi entropy is obscured by a nonuniversal volume law. At Rényi index $\alpha=1/2$, we isolate a bulk-subtracted boundary response in free-fermion BDI chains using exact finite-open-chain relations, large-$L$ Pfaffian evaluations, and direct positive-weight checks. Across a mass inversion, this response approaches $|\Delta\omega|\ln 2$, where $\Delta\omega$ is the change in winding number. The response survives primitive bulk deformations, the tested local boundary perturbations, and broken bulk duality. Thus, for the families studied here, the boundary response counts the Majorana channels that change the fermionic SPT index.
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Strongly Correlated Electrons

Robust spin pseudogap and spin-charge separation in the $\sigma t$-$J$ model

Zheng-Yuan Yue, Jia-Xin Zhang and Zheng-Cheng Gu

The $\sigma t$-$J$ model, obtained from the usual $t$-$J$ model by flipping the sign of the spin-down hopping term, has been proposed to eliminate the strong interference between doped holes and the spin background, known as the phase string effect. In this work, we investigate the finite-temperature properties of the $\sigma t$-$J$ model using infinite projected entangled-pair state (iPEPS) algorithms, in comparison with the $t$-$J$ model and its easy-plane variants. We show that these models share similar spin-pseudogap thermodynamics, with a maximum in the spin susceptibility at $T^* \sim J$ and a broad specific-heat peak. Tensor network renormalization (TNR) further provides strong evidences for a Berezinskii-Kosterlitz-Thouless (BKT) transition in the spin sector of the $\sigma t$-$J$ model at a lower temperature $T_\text{BKT} < T^*$ due to its reduced $\mathrm{U}(1)$ spin-rotation symmetry. Crucially, in contrast to the $t$-$J$ model and to its easy-plane variants that also exhibit BKT transitions, these signatures remain robust upon doping: the doped holes preserve and even enhance the antiferromagnetic correlations in the $xy$ spin plane, leading to a weak doping dependence of both $T^*$ and $T_\text{BKT}$. Finally, we develop a slave-fermion mean-field theory for the $\sigma t$-$J$ model, whose projective symmetry group (PSG) is selected based on numerically determined hopping and pairing correlations, and show that it explains the robust spin pseudogap upon doping.
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Strongly Correlated Electrons

Uniaxial stress effects on magnetic and electric properties of the ground state in a centrosymmetric magnetic skyrmion host Gd$_2$PdSi$_3$

Hiraku Saito, Naoki Kobayashi, Takuro Kawasaki et al. (+5 more)

We investigate effects of uniaxial stress to magnetic orders and electrical resistivity of the centrosymmetric magnetic skyrmion compound Gd$_2$PdSi$_3$, which has a hexagonal crystal structure composed of triangular lattice layers of magnetic Gd$^{3+}$ ions. This compound is known to exhibit the triple-$q$ magnetic skyrmion lattice phase with a giant topological Hall effect in the first field induced phase [T. Kurumaji $\textit{et al}$. Science $\textbf{365}$, 914-918 (2019)]. In contrast to the established picture of the field-induced phase, the ground state of this system still remains to be studied. Although previous studies reported the existence of the incommensurate magnetic modulations described by a magnetic modulation wave vector ${\bf q}=(q,0,0)$ where $q\sim 0.14$ and its equivalents, it is still unclear whether the magnetic structure is a single-$q$ structure or a multiple-$q$ structure. In the present study, we performed magnetization, resistivity and neutron diffraction measurements with a compressive uniaxial stress applied perpendicular to the $c$ axis. The observed data revealed that the system did not exhibit anisotropic magnetic and electric properties expected from a single-$q$ magnetic order, suggesting that the magnetic ground state of this system is a multi-$q$ magnetic order.
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All authors Hiraku Saito, Naoki Kobayashi, Takuro Kawasaki, Tatsuya Nakamura, Akiko Kikkawa, Yasujiro Taguchi, Yoshinori Tokura, Taro Nakajima

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

A spin-bond theory unifying non-relativistic spin splitting and emergent spin-orbit textures

S. Allende and R. M. Otxoa

Magnetic order with vanishing net magnetization can produce non-relativistic spin-split bands, broadly categorized into even-parity altermagnets and odd-parity \(p\)-wave magnets. Here, we introduce a spin-bond theory that unifies these seemingly distinct phenomena into a single algebraic framework. We demonstrate that non-relativistic spin textures are fundamentally governed by two components of the electronic bond: unitary spin phases and Hermitian spin amplitudes. The unitary sector generates odd-parity p-wave and emergent spin-orbit-like textures, while the Hermitian sector generates even-parity spin fields, including the uniform \(\Gamma\)-split and bond-structured altermagnetic limits. Beyond unifying known phases, our theory uncovers a mixed non-commuting regime that emerges when the unitary and Hermitian sectors fail to commute, revealing an underlying non-commuting spin-bond structure. This regime generates a non-coplanar spin texture characterized by an even-in-momentum transverse spin polarization, providing a direct spectroscopic fingerprint for spin- and angle-resolved photoemission spectroscopy. Furthermore, we establish that this synthetic spin-orbit coupling can be dynamically tuned by geometrically controlling the non-commutation of the bond sectors. By providing a microscopic foundation for such tuning, our theory paves the way for advanced applications, including field-free spin qubits.
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Strongly Correlated Electrons

Spin Polarization of a Two-Dimensional Electron Liquid

G. A. Nikolaev, M. E. Sergeev, I. V. Kukushkin et al. (+1 more)

We show experimentally that the longitudinal resistance of a strongly correlated two-dimensional electron system provides a quantitative measure of its spin polarization. Using electrically detected electron spin resonance to independently calibrate the spin state, we establish a parameter-free relation between magnetotransport and spin polarization, allowing the latter to be determined from transport alone. This approach enables the magnetic state of the electron liquid to be mapped across a broad range of carrier densities and magnetic fields.
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All authors G. A. Nikolaev, M. E. Sergeev, I. V. Kukushkin, A. V. Shchepetilnikov

Strongly Correlated Electrons · Cross-listed from quant-ph

Autoregressive Projective Quantum Monte Carlo: From a Hermitian to a Non-Hermitian Perspective

Lavoisier Wah, Remmy Zen and Flore K. Kunst

Accurately determining the ground-state properties of quantum many-body systems remains a central challenge. In this work, we introduce an autoregressive projective quantum Monte Carlo (PQMC) framework that leverages recurrent neural networks (RNNs) to guide the stochastic dynamics. By incorporating autoregressive sampling into PQMC, we demonstrate substantial improvements in accuracy compared to standard unguided PQMC, while retaining polynomial computational cost. We benchmark our approach against conventional variational RNN ansätze and find that the autoregressive PQMC consistently achieves lower energies and higher fidelity, regardless of system size or whether the Hamiltonian is Hermitian or non-Hermitian. Our results highlight the versatility and power of neural-guided PQMC methods, paving the way for promising scalable simulations of low-energy states in complex quantum many-body systems.
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Strongly Correlated Electrons

Geometrical approach and topological electron density in the $p_x + ip_y$ superconductor

Karyn Le Hur, Kabir Narayanan and Óscar Mañas Chambert

We present an analysis on the geometrical and physical nature of the $p_x + ip_y$ superconductor on the square lattice, with an emphasis on the topological phase transition at half-filling. We develop a local topological marker from specific Dirac points within the Brillouin zone, which is introduced via the addition of two one-dimensional (1D) $\mathbb{Z}$ $(\mathbb{Z}_2$) invariants defined on the Bloch sphere. We relate this topological marker to the electron spectral function integrated on frequency through the local momentum-resolved electron density, which may be measured via Angle Resolved Photoemission Spectroscopy (ARPES), and show that it remains well-protected including temperature effects. Integrating on a small area around a specific point in momentum space associated to the measure uncertainty, this also reveals the Van Hove logarithmic profile of the density of states in the derivative of the local marker while preserving the topological information. Topological transitions correspond to a protected semi-metal. We analyse the real space representation of this topological marker from correlation functions. We present physical responses such as the topological superfluid density.
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Strongly Correlated Electrons · Cross-listed from cond-mat.mtrl-sci

Magnetotransport evolution and nonlinear Hall effect in altermagnetic MnTe

Wei Zhou, Zhifeng Xue, Yunxing Li et al. (+6 more)

Hexagonal MnTe is a prototypical semiconducting altermagnet whose properties are heavily influenced by intrinsic disorder, yet how the resulting diverse transport regimes shape its magnetotransport behavior remains to be clarified alongside the role of relativistic spin-orbit coupling (SOC). Here, we present a systematic study of the anisotropic magnetoresistance (AMR), planar Hall effect (PHE), and nonlinear transport in MnTe bulk single crystals. Below the Néel temperature (TN ~ 304 K), the emergence of high-order harmonics in AMR and PHE within the high-temperature metallic regime reveals the interplay of magnetic order, crystalline symmetry, and SOC. At relatively lower temperatures, the disappearance of higher-order symmetries coincides with a transport crossover into the hopping conduction regime, suggesting that carrier localization diminishes the transport sensitivity to the Fermi-surface topology. In addition, we detect distinct second-order nonlinear Hall signals, providing evidence for a macroscopic inversion-asymmetric response in altermagnetic MnTe. Extending the investigations into the localized regime provides key insights into the subtle role of disorder and SOC in macroscopic charge transport. Our work thus underscores the necessity of exploring magnetotransport across diverse conducting regimes to comprehensively understand altermagnetic properties.
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All authors Wei Zhou, Zhifeng Xue, Yunxing Li, Nannan Tang, Ye Tao, Dingyong Zhong, Jiawei Luo, Donghui Guo, Huichao Wang

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

Gradient-based optimization of non-Abelian fractional quantum states in patterned superlattices

Yifei Guan, Lichen Yu, Zizhuang Liu et al. (+2 more)

The realization of fractional Chern insulator (FCI) states in moiré heterostructures has attracted intense interest in the study of correlated topological states. So far, most experimentally realized FCI states may be interpreted as lattice analogues of Abelian fractional quantum Hall (FQH) states. Realizing non-Abelian FCI states is an important challenge in the field. Patterned dielectric superlattices provide a versatile platform for engineering topological flat bands. Such systems offer substantial structural flexibility and tunability, because their lattice patterns, periods, and other structural parameters can all be designed and fabricated. Here, we provide a gradient-based optimization workflow to design non-Abelian fractional states in patterned bilayer graphene superlattices. The experimentally relevant structural parameters of the superlattice devices are gradient-optimized to favor a flat Chern band with quantum-geometric properties reminiscent of those of the first excited Landau level. Exact diagonalization calculations at 1/2 filling of the optimized Chern band naturally yield non-Abelian FCIs. We apply this workflow to triangular, honeycomb, and kagome patterned superlattices and find robust non-Abelian FCIs over a large region of the parameter space spanned by superlattice constant and vertical potential drop. Our work thus establishes an experimentally feasible framework for exploring non-Abelian FCIs in realistic patterned-superlattice devices.
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All authors Yifei Guan, Lichen Yu, Zizhuang Liu, Xin Lu, Jianpeng Liu

Sunday 9 August

Strongly Correlated Electrons

Nonequilibrium dynamics of doped Chern ferromagnets: a case study for false vacuum decay

Kilian Kuhlbrodt, Olivier Huber, Oliver Breach et al. (+7 more)

Even though metastable false vacuum decay is ubiquitous in physics, its underlying dynamics are still not well understood. Dissipative state preparation in moiré quantum materials provides an exceptional setting for exploring this physics since it allows the possibility of generating exotic quantum states that are not the ground state of the system Hamiltonian. Motivated by recent experiments demonstrating steady-state optical orientation of the spin-valley degree of freedom of holes, here we investigate dynamics of itinerant and Chern ferromagnets in the presence of an opposing magnetic field. Optical pumping using a circularly polarized Laguerre-Gauss beam allows us to deterministically prepare a true vacuum bubble embedded inside a metastable state. Depending on its initial size controlled by the pump power, we observe that the bubble collapses or expands due to an interplay between domain wall and bulk dynamics. For external magnetic fields comparable to the coercive field of ferromagnetism, we observe up to two-orders-of-magnitude prolongation of the spin polarization decay time at commensurate fillings corresponding to integer and fractional Chern insulator states. Our experiments reveal that the nonequilibrium dynamics of the ferromagnetic domains is substantially more sensitive to the precise filling factor around Chern insulator states than standard transport or optical measurements.
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All authors Kilian Kuhlbrodt, Olivier Huber, Oliver Breach, Weijie Li, Xiaodong Xu, Kenji Watanabe, Takashi Taniguchi, Martin Kroner, Siddharth A. Parameswaran, Atac Imamoglu

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

Relaxation-driven flat bands and topology in moiré transition metal dichalcogenide heterobilayers

Mitchell Luskin, Max Geier, Liang Fu et al. (+1 more)

Moiré transition metal dichalcogenide (TMD) heterobilayers are commonly modeled by a continuum theory that yields topologically trivial bands, in contrast to their homobilayer counterparts which host topological bands and fractional Chern insulators (FCI). We show this conclusion is an artifact of neglecting the pseudomagnetic field generated by lattice relaxation, an effect intrinsic to every moiré material. We develop a continuum model that resolves relaxation into three channels: a modified moiré potential with higher Fourier harmonics, a pseudoelectric (scalar deformation) potential, and a pseudomagnetic (vector) potential. Using WSe$_2$/WS$_2$ as a prototype, we find that the pseudomagnetic field alone gaps the third and fourth valence bands with Chern numbers $\pm 1$ over a broad range of twist angle and lattice mismatch, while the moiré potential correction and pseudoelectric potential narrow the bandwidth and enhance the bandgaps, which survive many-body interactions using neural-network variational Monte Carlo calculations. Relaxation also smoothens the Berry curvature and quantum metric relative to the rigid model, moving the band closer to the ideal Chern limit, beneficial for the quantum anomalous Hall effect, FCI states, and flat-band superconductivity when filled to higher bands. Our work establishes a new framework that connects first-principles calculations, through the continuum model, to many-body observables. Using this framework, we show moiré heterobilayers as a new class of topological materials whose topology is driven entirely by intrinsic lattice relaxation.
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All authors Mitchell Luskin, Max Geier, Liang Fu, Ziyan Zhu