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Physics Condensed Matter Strongly Correlated Electrons

cond-mat.str-el Watch Strongly Correlated Electrons

Monday 14 September

Strongly Correlated Electrons · Cross-listed from quant-ph

Low-Depth Initial-State Preparation for Ground-State Energy Estimation of Two-Dimensional Strongly Correlated Systems

Ryo Watanabe and Keisuke Fujii

Quantum algorithms for ground-state energy estimation require initial states with non-negligible fidelity to the ground state. Guided by classical simulations, we design shallow circuits for the two-dimensional half-filled Hubbard model by first preparing an approximate Heisenberg ground state and then applying charge-fluctuation gates derived directly from the Schrieffer-Wolff (SW) generator. For the Heisenberg model, we demonstrate effective parameter transfer from a $4\times4$ lattice to lattices up to $10\times10$ without further optimization. We obtain promising lower bounds on the ground-state fidelity using tensor-network simulations, variational Monte Carlo reference states, and estimates of the ground-state energy and singlet gap. Exact $4\times4$ calculations show that the SW gates substantially improve the Hubbard ground-state fidelity of the embedded Heisenberg state. The successful Heisenberg parameter transfer supports the use of these small-lattice results to design shallow Hubbard circuits on larger lattices beyond the reach of classical simulations. For a $10\times10$ lattice, the estimated preparation cost is on the order of $10^5$ $T$ gates, including Clifford+$T$ synthesis, which is well within the megaquop regime. These results offer a route to low-cost initial-state preparation for ground-starongly correlated systems.
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Strongly Correlated Electrons

Engineering Competing Fractional Topological States in a BHZ Superlattice

Ke-Chen Liu and Chen Cheng

Scalar superlattices offer a route from simple topological bands to fractionalized states, while multicomponent systems raise the additional question of which fractional order is energetically selected. We show that a scalar superlattice applied to the Qi-Wu-Zhang (QWZ) Chern-insulator model produces an isolated C = +1 miniband with strongly improved Berry-curvature and quantum metric uniformity. Exact diagonalization of the projected interaction identifies a 1/3 fractional Chern insulator in this miniband. Restoring the time-reversed partner to form the Bernevig-Hughes-Zhang (BHZ) superlattice yields, at total filling 2/3, a balanced fractional quantum spin Hall (FQSH)-like state and a fully polarized 2/3 fractional Chern insulator. Their competition separates two aspects of fractional-state control: the finite-momentum intercomponent coupling governs the stability of the balanced FQSH-like state, whereas a uniform pseudospin anisotropy shifts the relative energies of conserved sectors and can switch the global ground state between the balanced and polarized states. These results establish a minimal lattice setting that connects topological-miniband reconstruction, fractional-state formation, and fractional-state selection, illustrating how distinct levels of control can be combined to navigate competing fractional topological states.
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Strongly Correlated Electrons

Twinning of domains and spin anisotropy in K$_5$Fe$_4$Ag$_6$Te$_{10}$

Jiayu Guo, Hengyang Zhong, Dongsheng Yuan et al. (+12 more)

The Fe-based superconductors are derived from metallic parent compounds with nematic and stripe magnetic orders, which lead to two types of magnetic domains. Recently it was found that K$_5$Fe$_4$Ag$_6$Te$_{10}$ (KFAT), an Fe-based semiconductor, exhibits similar nematic and stripe magnetic orders, and is thus an analogue to the Fe-based superconductors in the limit of localized electrons. In this work, the superstructure and magnetic domains of KFAT are elucidated by fully mapping the reciprocal space using time-of-flight single crystal neutron diffraction. In KFAT, Fe and Ag atoms order to form a $\sqrt{5}\times\sqrt{5}$ superstructure containing $2\times2$ Fe blocks, which leads to two superstructure domains with identical main Bragg peaks but distinct superstructure peaks. Below $T_{\rm N}\approx35$~K, magnetic and nematic orders break in-plane rotational symmetry of the tetragonal $\sqrt{5}\times\sqrt{5}$ superstructure, and further give rise to two magnetic domains. These four equally populated domains account for the complex scattering pattern observed in our time-of-flight elastic neutron scattering measurements. Using polarized neutron scattering, we demonstrate a prominent spin anisotropy with an easy-plane spanned by the $c$-axis and the intra-block antiferromagnetic Fe-Fe bond direction. Such an anisotropy at ${\bf q}\neq0$ persists well above $T_{\rm N}$, accounts for the in-plane ${\bf q}=0$ magnetic anisotropy observed in uniaxial-strained KFAT, and offers an indicator for discovering similar piezomagnetic effects in other materials.
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All authors Jiayu Guo, Hengyang Zhong, Dongsheng Yuan, Xuejuan Gui, Youzhe Chen, Nathan Giles-Donovan, Naomi Kawamura, Masaaki Matsuda, Yaohua Liu, Feng Ye, Rongyan Chen, Robert J. Birgeneau, Xingye Lu, Jincheng Wang, Yu Song

Strongly Correlated Electrons · Cross-listed from quant-ph

Variational Real-Time Dynamics on Reduced Operator Manifolds

Aeishah Ameera Anuar, PV Sriluckshmy, Riccardo Rossi et al. (+1 more)

Accurate real-time simulation of correlated quantum systems remains challenging for both classical methods and near-term quantum hardware. We introduce operator-projected variational quantum real-time evolution (OVQRTE), which updates a parameterized circuit by enforcing the Ehrenfest equations for a selected set of observables. OVQRTE requires only expectation-value measurements, while the choice of operator set enables a systematic trade-off between accuracy and measurement cost, substantially reducing quantum-resource requirements relative to existing variational real-time-evolution algorithms. After implementing OVQRTE dynamics of Heisenberg model on a simulator, we benchmark the algorithm for the Anderson impurity models on the IQM Emerald superconducting processor using up to 24 qubits. We further use OVQRTE to sample computational-basis states for quantum-selected configuration interaction (QSCI), enabling the calculation of the ground-state energy and density of states within a self-consistent ghost-Gutzwiller Ansatz (gGut) embedding loop. Our results establish OVQRTE as a promising approach for investigating correlated condensed-matter systems on near-term quantum hardware.
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All authors Aeishah Ameera Anuar, PV Sriluckshmy, Riccardo Rossi, Fedor Simkovic

Strongly Correlated Electrons

Band-Saddle-Point Engineering in Mixed $A$-site $A$V$_3$Sb$_5$ Kagome Metals

Maxim Wenzel, Alexander A. Tsirlin, Andrea N. Capa Salinas et al. (+4 more)

$A$V$_3$Sb$_5$ compounds ($A$ = K, Rb, Cs) offer an experimental platform for probing the physics of kagome metals near ideal Van Hove filling. Here, we use optical spectroscopy on $A$V$_3$Sb$_5$ with mixed $A$-site compositions to reveal the effect of the alkali metal on the low-energy electronic structure. Supported by density-functional-theory calculations, we identify a band-saddle-point inversion as a chemical consequence of Cs substitution, revealing that $A$-site composition controls the orbital character of Van Hove singularities, Fermi surface topology, and electron-phonon interactions while leaving the kagome layer intact.
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All authors Maxim Wenzel, Alexander A. Tsirlin, Andrea N. Capa Salinas, Brenden R. Ortiz, Stephen D. Wilson, Ece Uykur, Martin Dressel

Strongly Correlated Electrons

Non-Abelian chiral spin liquid in a spin-$1$ antiferromagnet on the square lattice

Xiao-Tian Zhang, Yuan Yang, D. N. Sheng et al. (+1 more)

Non-Abelian chiral spin liquids (CSLs) host non-Abelian anyon excitations and are promising platforms for topological quantum computation. However, realizing non-Abelian CSLs in short-range interacting spin systems remains elusive. Here, we study a spin-$1$ square-lattice model with the first- and second-neighbor bilinear ($J_1,J_2$) and biquadratic interactions ($K_1,K_2$), as well as the three-spin scalar chiral coupling $J_{\chi}$. Inspired by the evidence of the Moore-Read state in exact diagonalization and infinite Projected Entangled Pair States calculations, we fix $J_1=1.0$, $J_2/J_1=0.623$, $K_1/J_1=-0.176$, and obtain a quantum phase diagram for $0 \leq K_2/J_1 \leq 0.4$ and $0.3 \leq J_{\chi}/J_1 \leq 0.6$ by using the density matrix renormalization group (DMRG) calculations. We identify a stripe antiferromagnetic phase, a Néel antiferromagnetic phase, a magnetically ordered chiral spin state phase, and a non-Abelian CSL phase emerging near the boundaries of the ordered phases. The unbiased DMRG results provide strong evidence for the Moore-Read state, including the three topological sectors, the quantized spin Chern number $C=1$, and the level counting of low-lying entanglement spectra which is consistent with the chiral SU(2)$_2$ conformal field theory. Our results may shed new light on searching for non-Abelian CSLs in other higher-spin ($S > 1/2$) systems with short-range couplings.
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All authors Xiao-Tian Zhang, Yuan Yang, D. N. Sheng, Shou-Shu Gong

Strongly Correlated Electrons

Observation of complex orbital order in MnV2O4

Chihaya Koyama, Shunsuke Kitou, Taishun Manjo et al. (+8 more)

Orbital ordering in vanadium spinel oxides with a geometrically frustrated pyrochlore structure has been a subject of controversy, owing to competing theoretical models and the absence of direct experimental evidence. Here we combine high-precision single-crystal synchrotron x-ray diffraction with core differential fourier synthesis to visualize the valence electron density (VED) of the orbital-ordered ground state in real space. By carefully investigating multiple-scattering artifacts, we identify the low-temperature structure as belonging to the I41/amd space group. The reconstructed VED around the V sites reveals an orbital-ordered state distinct from both previously proposed real- and complex-orbital models. Our results resolve the long-standing controversy in MnV2O4 and establish a route to identifying orbital states in frustrated spin-orbital systems.
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All authors Chihaya Koyama, Shunsuke Kitou, Taishun Manjo, Yuiga Nakamura, Takeshi Hara, Naoyuki Katayama, Takuro Katsufuji, Ryotaro Arita, Yusuke Nomura, Hiroshi Sawa, Taka-hisa Arima

Strongly Correlated Electrons

Heisenberg-like critical behavior in the quasi-two-dimensional metallic ferromagnet LaCrSb3

Qianhui Mao, Haotian Deng, Bin Chen et al. (+1 more)

LaCrSb3 is a material exhibiting both quasi-two-dimensional spin fluctuations and three-dimensional magnetic interaction characteristics. By measuring the isothermal magnetization of single-crystals and conducting a systematic critical behavior analysis, we clarify the critical properties of its ferromagnetic phase transition and the intrinsic magnetic interaction mechanism. Based on high-precision isothermal magnetization data measured in the vicinity of the critical point, the Curie temperature for the ferromagnetic-paramagnetic phase transition is determined to be TC = 126 K, with the critical exponents obtained as $\beta$ = 0.376, $\gamma$ = 1.417 and $\delta$ = 4.76 via the self-consistent iterative method based on the Arrott-Noakes equation. The reliability of these critical exponents is verified by the Widom scaling law, the magnetic state scaling equation and other analyses. A comparison with theoretical models demonstrates that the critical behavior of the magnetic phase transition in this system basically belongs to the universality class of the three-dimensional Heisenberg model. This conclusion is further confirmed by the distance-dependent decay behavior of the exchange interaction J(r), revealing the dominant role of isotropic direct exchange interactions in this system. Finally, drawing on research findings of other quasi-two-dimensional magnetic materials, this work proposes that LaCrSb3 may exhibit finite-temperature magnetic order in the two-dimensional limit, thereby possessing important theoretical research significance and promising practical application prospects.
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All authors Qianhui Mao, Haotian Deng, Bin Chen, Jinhu Yang

Sunday 13 September

Strongly Correlated Electrons

Quantum Hall Ferromagnetism in a Cavity Vacuum

Ceren B. Dag, Luis Brey, Ganpathy Murthy et al. (+1 more)

We uncover a continuous phase transition in a quantum Hall ferromagnet (QHF) at filling factor $\nu=1$, driven by vacuum fluctuations of a cavity. Our analysis starts with a Landau level projection in dipole gauge, where we find the states to be well-represented by a tensor product of the electronic and photonic degrees of freedom. Through analytic spin wave calculations, mean-field theory and density matrix renormalization group (DMRG) simulations, we show that for a spatially antisymmetric cavity field, the uniform QHF state is stable only for weak light-matter coupling and gives way to states of inhomogeneous electron density above a critical coupling. These states involve "flanks" of uniform QHF fluids, separated by a "compact core" of doubly occupied orbitals with the core size being the order parameter, which we dub as "compact-core phases". While the fully spin polarized electronic states are product states, entanglement builds up between the uniform QHF flanks across the compact core in the $S_z=0$ magnetic sector, motivating an ansatz for the compact-core electronic states. The transition boundary is exactly derived for product electronic states in terms of matter and cavity parameters, and numerically confirmed by DMRG. We also study the thin-cylinder limit near the critical point where quantum fluctuations are enhanced, and the many-body excited states in both phases by focusing on the entanglement spectrum degeneracies. Remarkably, the photon number is found to probe the order parameter of the transition, providing a possible experimental signature of the electronic transition and the compact-core states. Our study offers a rare example of a phase of electrons stabilized solely by coupling to the enhanced vacuum fluctuations of a cavity mode.
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All authors Ceren B. Dag, Luis Brey, Ganpathy Murthy, H.A. Fertig

Strongly Correlated Electrons

The Propagating Nature of Single- and Multimagnons in LaCrO$_3$

Masoud Lazemi, Fabian J. Mohammad, Ellen M. Kiens et al. (+6 more)

The dispersion of an excitation is commonly used to infer its microscopic character: dispersive modes are associated with propagation, whereas nondispersive features are often interpreted as localized. This distinction becomes less straightforward for composite excitations, whose spectral maxima can remain nearly dispersionless even when they are built from propagating quasiparticles. Here we use momentum- and temperature-dependent resonant inelastic X-ray scattering (RIXS) to reveal the origin of multimagnon excitations in antiferromagnetic LaCrO$_3$. Whereas the single magnon disperses strongly, the two- and three-magnon features remain nearly flat. On approaching the Neel temperature, thermal population of the magnon band produces an anti-Stokes branch and opens an additional three-magnon channel involving the creation of two magnons and annihilation of one. Its energy and momentum dependence are reproduced by a thermally weighted multimagnon joint density of states calculations. These results establish thermal population as a means of exposing the propagating constituents hidden within nondispersive composite excitations, and position RIXS as a probe of multimagnon dynamics and, ultimately, magnon-magnon interactions.
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All authors Masoud Lazemi, Fabian J. Mohammad, Ellen M. Kiens, Yorick A. Birkhölzer, Paras Mehta, Emma van der Minne, Stefano Agrestini, Frank M.F. de Groot, Hebatalla Elnaggar

Strongly Correlated Electrons

Rhombohedral Multilayer Graphene as a $p$-Wave Magnet

Rintaro Eto and Johannes Knolle

We propose rhombohedral multilayer graphene in an applied in-plane magnetic field as a highly tunable platform for nonrelativistic collinear $p$-wave magnetism. The orbital coupling to the magnetic field breaks time-reversal symmetry and, together with interaction-driven layer antiferromagnetism, generates an odd-in-momentum spin splitting without relying on spin-orbit coupling. Using a minimal low-energy effective theory, we show that the resulting $p$-wave spin splitting is strongly enhanced with increasing layer number. This enhancement originates from the surface-localized nature of the low-energy states and their layer-dependent orbital coupling to the in-plane field. The Zeeman coupling only weakly perturbs this predominantly orbital-field-induced mechanism. We further estimate stability of the $p$-wave magnetism under hole-doping and distinct transport signature using a more realistic Hubbard model. Our results establish orbital coupling to an external magnetic field as a controllable route to odd-parity spin splitting in collinear magnets.
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Strongly Correlated Electrons

Finite-temperature quantum Monte Carlo study of Dirac quantum criticality: Algorithmic advances and comprehensive analysis

Hou-Min Du, Qian Sun and Yuan-Yao He

We present a systematic study of the quantum phase transition from a Dirac semimetal to an antiferromagnetic Mott insulator in the honeycomb-lattice Hubbard model, using finite-temperature auxiliary-field quantum Monte Carlo (AFQMC) simulations. Given the dynamical critical exponent $z=1$ at criticality, we scale the inverse temperature $\beta$ with the linear system size $L$, i.e., $\beta t=L$ (with $t$ as the hopping amplitude), to approach the thermodynamic and zero-temperature limits simultaneously. On the algorithmic front, we generalize the fast Fourier transform in path propagation to multi-sublattice systems and employ the delayed update technique in configuration updates to accelerate the AFQMC simulations. These advances enable us to access unprecedented system sizes of up to $3528$ sites (with $L=42$ and $\beta t = 42$). We additionally use twist-averaged boundary conditions to reduce the finite-size effects in computed observables. On the physical side, we determine the Dirac quantum criticality via finite-size scaling analysis of the spin correlations and off-diagonal single-particle Green's function, and locate the critical point at $U_c/t=3.731(4)$ with the critical exponents $\nu=1.097(7)$, $\eta_{\phi}=0.72(2)$ and $\eta_{\psi}=0.155(6)$. We further corroborate the critical point by examining several relevant quantities, including the quasiparticle weight, Fermi-liquid parameter, charge compressibility, $U$-derivative of double occupancy, and fidelity susceptibility. Our work establishes an independent benchmark for the chiral Heisenberg Gross-Neveu-Yukawa criticality in the honeycomb Hubbard model, offering a complementary perspective to previous ground-state simulations.
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Strongly Correlated Electrons

Kagome lattice candidate URhSn: Measured Bragg diffraction patterns and theory for hexagonal and trigonal Sohncke phases

S. W. Lovesey and D. D. Khalyavin

Ternary intermetallics use a ZrNiAl-type chemical structure known as a kagome system. Complexity observed in the phase diagrams of these compounds is attributed to the geometric frustration. The 5f-electron compound URhSn exhibits two phase transitions at 16 K and 54 K. No super-lattice reflections in neutron powder diffraction patterns are detected in the intermediate range of temperatures (16 K - 54 K). Recent resonant x-ray diffraction patterns for uranium ions in the intermediate phase suggest a chiral structure and a trigonal Sohncke space group. It is firmly established that a simple hexagonal space group describes the parent structure at room temperature. A ferroic order parameter that includes hexadecapoles is shown to characterize the trigonal Sohncke phase. Resonant x-ray diffraction by this phase is fully investigated with our symmetry informed theory. It repairs symmetry in the original data analysis by using a grey group, and it lifts an unjustified spatial condition on uranium positions. The theory uses axial atomic multipoles that satisfy sum-rules established in the first instance for x-ray dichroic signals. A confrontation between observed and calculated diffraction patterns lends support to a trigonal Sohncke phase, and exposes grounds for additional experimental work on an enigmatic compound.
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Strongly Correlated Electrons

Reservoir-conditioned virtual returns generate random Liouvillian skin localization in a reciprocal Mott insulator

Y. T. Wang, X. Z. Zhang and W. M. Liu

Directional dissipation can concentrate relaxation modes in space, but connecting this accumulation to controlled microscopic processes in correlated matter requires separating virtual charge motion from spin coherence. We study this connection in a half-filled Hubbard chain with reciprocal hopping and number-conserving, direction-selective returns of virtual doublon–hole defects. Eliminating charge defects and then Mott coherences yields asymmetric spin-exchange rates under an explicit separation of timescales, with full short-chain dynamics supporting the reduction on finite exchange times. Balanced random returns concentrate the stationary population and several low-lying right modes at sample-selected positions rather than a predetermined edge, producing random Liouvillian skin localization despite zero end-to-end logarithmic rate bias. The accumulated rate imbalance determines an exact finite-density hard-core stationary state and a one-particle activated relaxation scale governed by random barriers. This connection between locally calibrated return rates, many-body stationary weights and slow relaxation offers a means of controlling transport in constrained open quantum matter without changing the reciprocal Hamiltonian.
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Strongly Correlated Electrons

Vestigial altermagnetism

Peng Rao and Johannes Knolle

Interaction effects and fluctuations govern the phase diagram of systems with multiple competing phases. In particular, as individual phases are destroyed by increasing temperature, fluctuations can still produce a composite `vestigial' order given by a product of individual order parameters. Here, we describe vestigial Altermagnetism (AM) from the thermal melting of antiferromagnetic (AFM) and orbital orders (OO) on the square lattice. The coexistence of AFM and OO can give rise to conventional AM with ordering wave vector $\mathbf{Q}=(\pi,\pi)$. We show that a distinct `vestigial AM' phase with $\mathbf{Q}=0$ can arise from thermal melting of AFM and OO, and investigate its generic phase diagram. We find that the transition from the high-temperature disordered phase into the vestigial AM phase can be either first or second order, depending on the AFM and OO bare susceptibilities and their interactions. We then investigate the unique experimental signatures by calculating the electron spectral function for a minimal two-orbital model. Despite the fact that the vestigial AM order parameter does not couple to electrons directly, fluctuations dominate the self-energy corrections giving rise to a spin- and orbital-dependent quasi-particle lifetime with the AM symmetry.
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Strongly Correlated Electrons

Emergence of Subdominant Quadrupolar Order in the 5$d^2$ Multipolar Mott Insulator Ba$_2$CaOsO$_6$

Ilija K. Nikolov, Giuseppe Allodi, Ginevra Corsale et al. (+5 more)

Relativistic $5d$ Mott insulators host a rich variety of exotic hidden multipolar order, whose microscopic nature remains heavily debated. A broken local point-symmetry (BLPS) phase followed by antiferromagnetism was previously observed in the doped compound Ba$_2$Na$_{0.1}$Ca$_{0.9}$OsO$_6$, where mixed Os valence places the system toward the $5d^2$ end of the $5d^1$-$5d^2$ regime. In contrast, pristine $5d^2$ Ba$_2$CaOsO$_6$ has shown a single octupolar transition with average cubic symmetry, raising the question of whether BLPS arises from chemical disorder in the doped systems. Here, we discover the BLPS phase in pristine Ba$_2$CaOsO$_6$ emerging at $T^* \approx 50$ K, precisely coincident with the octupolar transition. Beyond standard ${}^{43}$Ca nuclear magnetic resonance (NMR) spectral analysis, our spin-echo nutation measurements detect a finite electric field gradient, demonstrating spontaneous symmetry breaking despite the cubic average structure. While our NMR data support the previously identified octupolar order, the BLPS phase reveals a subdominant quadrupolar contribution, establishing that the hidden order in Ba$_2$CaOsO$_6$ is intrinsically multicomponent. Such mixed multipolar states lie beyond conventional theoretical descriptions of the $5d^2$ ground state. Our results establish local symmetry breaking as an intrinsic property of the clean $5d^2$ limit and provide a new constraint on microscopic theories of strongly spin-orbit-coupled Mott insulators.
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All authors Ilija K. Nikolov, Giuseppe Allodi, Ginevra Corsale, Anshu Kataria, Cierra Foster, Patrick Woodward, Samuele Sanna, Vesna F. Mitrović

Saturday 12 September

Strongly Correlated Electrons

Haldane-Holstein model at fractional filling: Route to bosonic fractional Chern insulator and quantum anomalous Hall crystal

Zezhu Wei, Ang-Kun Wu, Di Xiao et al. (+1 more)

Electron-phonon coupling is generally expected to suppress band topology, driving a topological band insulator into a trivial phase. Here we show that, using the prototypical Haldane-Holstein model at partial filling of a topological band, strong electron-phonon coupling can instead stabilize topological phases of matter. The electron-phonon coupling plays a double role: it generates the longer-range interactions that correlate the carriers and, in the attractive channel, also binds them into bosonic pairs. For spinful electrons, it pairs opposite spins into charge-$2e$ bipolarons that form a bosonic fractional Chern insulator (FCI) at filling $\nu=1/2$ with charge-$e$ semionic excitations. Tuning the band topology and the coupling strength maps out a rich phase diagram containing this bosonic FCI, superconductors condensed at the $M$ and $K$ points, and several charge-ordered solids. For spin-polarized electrons, strong coupling instead stabilizes a $C=2$ quantum anomalous Hall crystal (QAHC) at $\nu=1/3$. Electron-phonon coupling thus emerges as a route to, rather than an obstruction against, topology in partially filled Chern bands.
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All authors Zezhu Wei, Ang-Kun Wu, Di Xiao, Shi-Zeng Lin

Strongly Correlated Electrons · Cross-listed from math-ph

Exact admissibility radii for the lattice topological charge of quantum spin textures

Raúl Sánchez Galán and Robert Wieser

How much can a magnetic field change before a quantum spin texture loses its skyrmion charge? For a triangulated spin-$s$ texture on $N$ quantum sites, we determine the smallest change in its local moments that makes the reconstructed topological charge ill-defined, allowing unequal error bounds and fixed boundary moments. Combined with spectral perturbation theory, this geometric threshold yields explicit magnetic-field intervals that preserve the ground-state charge, scaling as $1/N$ when the gap and geometric margin have positive limits. We apply the bounds to finite spin-$1/2$ systems with exchange and Dzyaloshinskii-Moriya interactions. In a rigorously verified seven-spin example the charge changes at a single certified field, while the excitation gap and all local spin polarizations remain bounded away from zero; there the geometric margin vanishes linearly.
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Strongly Correlated Electrons

Exact solutions of hidden free fermion models

Mingchen Zheng, Balázs Pozsgay and Shu Chen

Hidden-free-fermion (HFF) Hamiltonians exhibit free-fermion spectra despite lacking an explicit quadratic representation, yet their algebraic provenance and periodic-boundary behavior remain open questions. Here we establish that a broad class of HFF models arises from distinct Lax operator realizations of a twisted multistate Perk–Schultz \(R\) matrix. Its reflected degeneration generates a closed finite hierarchy of mutually commuting transfer matrices, whose functional relations determine the complete many-body spectrum. As a decisive example, we solve the inhomogeneous free-fermions-in-disguise (FFD) chain, which exhibits HFF, nested HFF, or interacting spectra, depending on its coupling. The homogeneous FFD chain with both one-wrap and periodic boundary conditions exhibits an emergent cubic finite-size gap while retaining the exact \(z=3/2\) bulk behavior. The construction extends to higher-state and face-type realizations, providing a common algebraic framework for coupling-dependent spectral organization and multiscale behavior in interacting quantum systems.
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Strongly Correlated Electrons

Bilinear magnon–exciton coupling in biased ferromagnetic electron–hole bilayers

Pieter M. Gunnink

The hybridization of magnons and excitons would combine magnetic and optical degrees of freedom in a single composite quasiparticle. Such a hybridization is however difficult to achieve, because of their inherent energy mismatch. We propose that in biased bipolar ferromagnetic electron–hole bilayers the excitons and magnons can be brought into resonance, with the exciton energy lowered through the voltage bias to match the magnon energies. We demonstrate the linear hybridization of magnons and spin-flip excitons in this regime, starting from the microscopic exchange interactions between electrons and localized magnetic moments. We show that further increasing the gate voltage softens the hybrid magnon–exciton mode and realizes a magnon–exciton condensate, which manifests in both the magnon and exciton sectors and is associated with spin-superfluid transport. Ferromagnetic electron–hole bilayers therefore provide a new platform for the study of composite magnon–exciton quasiparticles and the realization of spinful condensates and associated spin superfluidity.
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Strongly Correlated Electrons · Cross-listed from quant-ph

Shot-based variational simulation of the toric code phase transition: Energy, global entanglement, and noise resilience

Hayede Zarei and Mohammad Hossein Zarei

We investigate the capability of variational quantum circuits in detecting a topological phase transition within the toric code model subjected to a uniform magnetic field. The detection is carried out via shot-based computations of both the ground state energy and the global entanglement. We begin by employing well-established renormalization techniques to construct a quantum circuit that accurately represents the pure toric code under periodic boundary condition. This circuit is subsequently parameterized to enable a shot-based variational simulation of the ground state in the presence of the magnetic field. Our results demonstrate that even for the minimal lattice sizes and a finite number of measurement shots, the ground state energy exhibits a distinct singularity at the transition point which can be reliably approximated through finite-size scaling analysis. Furthermore, we introduce a secondary variational circuit designed to compute the global entanglement of the generated ground state. We observe that both global entanglement and its conditional counterpart display the expected qualitative behavior at the transition point, consistent with the topological nature of the phase transition. To evaluate the practical viability of our variational approach on real quantum hardware, we systematically examine the detrimental effects of gate noise on the analysis. We find that the topological phase exhibits a notable robustness against noise, manifesting as a linear decrease in the transition point with increasing noise strength.
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Strongly Correlated Electrons

Layer- and Orbital-Selective Mott Physics Driving Dimensional Crossover in Nickelate Superlattices

Minjae Kim, Byungmin Sohn and Sangjae Lee

The design of quantum materials to manipulate dimensionality and electronic correlations is a central focus in the exploration of emergent phenomena. We propose a new correlated quantum material platform for realizing a layer- and orbital-selective Mott transition (LOSMT) in a nickelate heterostructure by combining bilayer and monolayer LaNiO$_3$ separated by a monolayer LaAlO$_3$ spacer under compressive strain from the LaAlO$_3$ substrate. The LOSMT emerges from the layer-dependent dimensional confinement of the apical Ni($d$)-$z^2$ orbital and strain-induced lifting of the orbital degeneracy. The monolayer LaNiO$_3$ exhibits an orbital-selective Mott insulating phase with a Mott gap in the Ni($d$)-$z^2$ orbital, whereas the bilayer LaNiO$_3$ exhibits a multi-orbital metallic phase, triggering the orbital-selective Mott transition upon cooling $via$ inter-layer hybridization of the Ni($d$)-$z^2$ orbitals. Calculations within the density functional theory plus dynamical mean-field theory (DFT+DMFT) framework reveal that the LOSMT drives a dimensional crossover by enabling out-of-plane electronic transport through the metallization of the localized apical Ni($d$)-$z^2$ orbital, while the planar Ni($d$)-$x^2-y^2$ orbital remains itinerant across the transition. The emergence of the LOSMT provides a new pathway for engineering correlated quantum materials toward unconventional superconductivity and Mott-transistor functionalities.
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Strongly Correlated Electrons

Probing the anomalous symmetry-breaking in kagome material CsV3Sb5 via third-order nonlinearity

Zheng Dai, Fengyi Guo, Shuai Zhang et al. (+7 more)

The kagome material has rapidly established itself as a research frontier in condensed matter physics, owing to its distinctive geometric structure and the rich array of unconventional physical phenomena. In the kagome AV3Sb5 (A = K, Rb, Cs) family, the charge-ordered state exhibits a remarkable characteristic, i.e., anomalous symmetry-breaking, which is tied to the topological nature of the electronic band structure. Here, we report the third-order nonlinear longitudinal and Hall responses that persist stably up to room temperature in the kagome material CsV3Sb5. Notably, the nonlinear responses demonstrate significant enhancement below the charge density wave (~ 77 K) order and anomalous symmetry-breaking (~ 39 K) state. The scaling analysis indicates that the third-order nonlinear transport is governed jointly by quantum geometric contribution and extrinsic scattering. This study realizes a giant third-order nonlinear response and provides a distinct method to detect anomalous symmetry-breaking in CsV3Sb5.
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All authors Zheng Dai, Fengyi Guo, Shuai Zhang, Xiubing Li, Congcong Li, Yufan Ju, Ziqi Wang, Bin Cheng, Zhe Ying, Fengqi Song

Friday 11 September

Strongly Correlated Electrons · Cross-listed from quant-ph

An entropic characterization of Haag duality

Ruizhi Liu and Lauritz van Luijk

Motivated by Haag duality in quantum spin systems, we show that Haag duality for a pair of commuting factors generated by increasing sequences of finite-dimensional matrix algebras is equivalent to the asymptotic vanishing of a suitable conditional mutual information. For spin systems, let an annulus separate a finite region from the exterior. Haag duality then holds for a region $A$ if and only if the mutual information of the inner region and the exterior, conditioned on the part of $A$ inside the annulus, vanishes as the outer radius becomes large. As a corollary, assuming a strict area law with subleading corrections, Haag duality holds for single cones, and it holds for unions of two or more disjoint cones precisely when the topological entanglement entropy vanishes. For translation-invariant pure states on spin chains, half-chain Haag duality is equivalent to vanishing entropy density.
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Strongly Correlated Electrons · Cross-listed from cond-mat.supr-con

Spatial instability of a homogeneous superconductor after an interaction quench

Medha Roy Choudhury, Sankha Subhra Bakshi, Pinaki Majumdar et al. (+1 more)

We investigate the quantum dynamics of a conventional superconductor, described by the attrac- tive Hubbard model, driven far from equilibrium by quenching the Hubbard attraction. We evolve the full density matrix of the system in time using the Heisenberg equation of motion, tracking the spatiotemporal dynamics of the pairing and density fields. We use very weakly inhomogeneous initial conditions to probe the stability of the homogeneous dynamics to spatial perturbations. At initial times after the quench the pairing and density fields remain homogeneous in space while fluctuating in time. However, depending on the quench strength the spatially uniform dynamics can become unstable beyond a threshold time, with emergence of spatial textures. The onset time for the instability depends on the strength of the quench and the initial inhomogeneity. At long times the system evolves towards a state with significantly reduced spatial heterogeneity. The time dependence of the global superconducting order parameter suggests two broad categories of re- sponse to a quench. One of these involves strongly inhomogeneous pairing and bond currents in the nonequilibrium superconductor.
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All authors Medha Roy Choudhury, Sankha Subhra Bakshi, Pinaki Majumdar, Amit Ghosal

Strongly Correlated Electrons

Tripartite entanglement in chaotic eigenstates

Junjia Zhang, Ramanjit Sohal and Shinsei Ryu

It is now well-established that classical statistical mechanics emerges from the entanglement structure of quantum chaotic systems, as quantified by the (subsystem) eigenstate thermalization hypothesis (ETH). While this statement rests on the well-studied bipartite entanglement of the eigenstates of such systems, recent years have shown that many-body states are often characterized by multipartite entanglement, raising the question: what are the universal multipartite entanglement features of chaotic eigenstates? We answer this question in part by studying a tripartite generalization of the ergodic bipartition ansatz through the lens of two tripartite entanglement probes, the Rényi tripartite multi-entropy and the Rényi reflected entropy. We derive leading-order analytical expressions for the ensemble averages of both quantities using a saddle-point analysis. Beyond these measures, we obtain the average of the Rényi-2 reflected density matrix, $\rho_{AA^*}^{(2)}$,which controls the Rényi-2 reflected entropy. We show that $\rho_{AA^*}^{(2)}$ in a chaotic eigenstate is well-described by this ensemble average which, unlike an ordinary reduced density matrix, takes three qualitatively distinct forms across subsystem-size regimes, which may be viewed as a tripartite generalization of subsystem ETH. We benchmark our analytical predictions against exact diagonalization of the mixed-field Ising model.
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Strongly Correlated Electrons · Cross-listed from cond-mat.supr-con

Topological superconductivity in an altermagnet-superconductor heterostructure

Michael Liudeng, Hrishikesh Patel, Marcel Franz et al. (+1 more)

Fully gapped spin-triplet superconductors offer a natural setting for topological edge transport and Majorana excitations. Here we propose a heterostructure that interfaces altermagnetic monolayer V$_2$Se$_2$O with a conventional $s$-wave superconductor. Starting from an ab-initio description of V$_2$Se$_2$O, we construct a low-energy model of its spin-split Fermi surface and study the superconducting instability of the coupled stack. We find that the momentum-dependent exchange splitting of the altermagnet strongly favors equal-spin pairing and converts the proximity-induced order into a fully gapped $p$-wave state. Over a broad range of interface parameters, the resulting Bogoliubov-de Gennes bands carry a nonzero Chern number, and slab spectra exhibit chiral Majorana edge modes traversing the bulk gap. We further quantify the inverse proximity effect and show how the induced gap evolves across a finite multilayer stack. These results establish a route to topological triplet superconductivity by interfacing a conventional superconductor with an experimentally available altermagnetic semiconductor, and provide practical guidance for interface fabrication and experimental detection.
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All authors Michael Liudeng, Hrishikesh Patel, Marcel Franz, Niclas Heinsdorf

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

Visualizing impurity-driven scattering phase textures in EuCd2As2

Raquel Sánchez-Barquilla, Rafael Pineda Medina, Pablo García Talavera et al. (+11 more)

Understanding how disorder modifies electronic states in magnetic semiconductors is important for controlling spin-dependent transport and topological responses. Here we use scanning tunneling microscopy to visualize scattering phase textures in EuCd2As2. By isolating a single surface wavevector we reconstruct spatial phase maps of the local density of states and identify phase dislocations characterized by 2pi winding around impurity sites. These phase singularities emerge systematically within charge puddles generated by Eu interstitials and their positions evolve with bias voltage. We show that their spatial structure is consistent with interference between multiple scattering channels, including contributions from spin-orbit coupling. We provide a model which reproduces phase dislocations and relates the decay of the phase gradient to the relative strength of spin-orbit and scalar scattering. Our results establish a route to access the phase of electronic scattering in real space and study the role of local disorder and spin-orbit interactions in shaping electronic states in quantum materials.
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All authors Raquel Sánchez-Barquilla, Rafael Pineda Medina, Pablo García Talavera, Adrian Valadkhani, Edwin Herrera, Brinda Kuthanazhi, Lin-Lin Wang, Sergey L. Bud'ko, Paul C. Canfield, Roser Valentí, Isabel Guillamón, William J. Herrera, Alfredo Levy Yeyati, Hermann Suderow

Strongly Correlated Electrons

Effective theory of the hidden-order pseudogap phase in a doped antiferromagnet

Gaia De Paciani, Gesa Dünnweber, Johannes Poersch et al. (+3 more)

The microscopic origin of the pseudogap phase constitutes a longstanding puzzle related to the emergence of high-temperature superconductivity in cuprate materials. In this work, we develop an effective model for doped antiferromagnets in terms of fluctuating stripes, or string-like domain walls, which obscure the antiferromagnetic order of the spin background. The open ends of such domain walls of the N'eel order are treated as vortices in the resulting lattice gauge theory. We numerically evaluate the phase diagram by classical Monte Carlo simulations, using percolation-based geometric order parameters to diagnose hidden N'eel order. At high temperatures, we identify a BKT-type crossover in which the domain wall ends become deconfined. We interpret this as the $T^*$ crossover from the hidden order regime to the paramagnetic metal above. At low temperatures, we identify stripe instabilities. Predictions of our effective model can be tested in ultracold fermion quantum simulators.
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All authors Gaia De Paciani, Gesa Dünnweber, Johannes Poersch, Simon M. Linsel, Henning Schlömer, Fabian Grusdt

Strongly Correlated Electrons · Cross-listed from hep-th

Integrable Chiral Quantum Field Theories with Spacetime-Dependent Interactions

Pradip Kattel

We construct a class of integrable chiral fermionic quantum field theories with interactions that depend on both space and time. The construction starts from a unitary, regular, difference-form two-body scattering matrix satisfying braiding unitarity and the Yang–Baxter equation. Each freely moving right- or left-moving particle carries a characteristic coordinate that remains constant along its trajectory, and the scattering matrix depends on differences of these coordinates. The many-body amplitudes form a discrete connection over the coordinate-ordering sectors. The Yang–Baxter equation gives local path independence, while periodic winding on a spatial circle produces quantum Knizhnik–Zamolodchikov (qKZ) difference equations expressing global flatness. Once a reference amplitude satisfying the exchange and qKZ constraints is specified, these transport equations determine the exact wavefunction in every fixed-particle-number sector. Independent reparametrizations of the right- and left-moving characteristics give couplings that depend on both space and time. For the chiral $SU(2)$ Gross–Neveu realization, we solve the qKZ equations for this full characteristic family using off-shell Bethe vectors. The resulting amplitudes are written in the characteristic labels, and their pullbacks give exact many-body wavefunctions for the corresponding space-time-dependent couplings. The two-body amplitude is a ratio of Gamma functions that resums the phase shifts from successive scattering processes. Equal-slope affine maps give a spatially homogeneous specialization in which the coupling depends only on time, and the Bethe ansatz with constant coupling is recovered when the qKZ step vanishes.
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