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Physics Condensed Matter Strongly Correlated Electrons

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Wednesday 19 August

Strongly Correlated Electrons · Cross-listed from quant-ph

Dissipation-enhanced scrambling in the SYK model coupled to a lossy cavity

Pietro Pelliconi, Bastien Lapierre and Shinsei Ryu

We study the Yukawa-Sachdev-Ye-Kitaev model, a disordered model of $N$ Majorana fermions and $R=\gamma N$ bosons in which the bosons are linearly coupled to independent realizations of SYK $p$-body interactions, in the presence of dissipation, modeled by a Lindblad master equation. Motivated by recent proposals for implementing SYK models in quantum simulators, we focus on bosonic leakage at rate $\kappa$. Initializing the system in the steady state, we analyze the late-time fermionic relaxation rate and the Lyapunov exponent, solving the large-$N$ theory both numerically and for $p$ large, finding a rich landscape of dynamical behaviors. Most notably, the Lyapunov exponent remains positive for every value of $\kappa$ and, for $p>2$, can even grow as $\kappa$ increases. The QED case $p=2$, which lies between the fully chaotic regime $p>2$ and the integrable case $p=1$, exhibits special features. We also identify a critical value of the boson-to-fermion ratio $\gamma_c \approx 2/p^2$ separating distinct dynamical regimes.
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Strongly Correlated Electrons · Cross-listed from hep-th

Holographic Local Operator Quenches with Conserved Momentum and Spin

Pawel Caputa, Pedro Castellini Grand, Justin R. David et al. (+1 more)

We investigate the holographic dictionary relating point particles carrying longitudinal momentum or angular momentum in asymptotically AdS$_3$ spacetimes to suitably regulated, time-evolved states created by local primary operators in the dual two-dimensional conformal field theory (2D CFT). We find that asymmetric left/right Euclidean smearing of local operators produces states carrying momentum, and the corresponding bulk excitation is a particle with conserved momentum. We compute the energy density and entanglement entropy in these states and in their dual back-reacted geometries, finding exact agreement between the CFT and gravity descriptions. We further extend this correspondence to particles with intrinsic spin, whose CFT duals are primary operators with unequal holomorphic and anti-holomorphic scaling dimensions. We again find a precise match between CFT and holographic calculations of energy densities and entanglement entropies. Finally, we explore applications of these setups beyond holography by deriving the evolution of Rényi entropies in 2D rational CFTs and introducing a new class of local quantum quench protocols with conserved longitudinal or angular momentum.
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All authors Pawel Caputa, Pedro Castellini Grand, Justin R. David, Rahul Metya

Strongly Correlated Electrons

Unconventional bond- and current-density waves on hexagonal lattices

Andras Szabo, Hannes Braun and Laura Classen

Charge-density wave (CDW) orders are conventionally described as modulations of on-site charge at an ordering wave vector $\boldsymbol{Q}$ with symmetry-related wave vectors typically forming a multicomponent order-parameter manifold. Recent developments significantly broadened this phenomenology to bond and loop-current density waves, which possess nontrivial, potentially symmetry-breaking textures within the unit cell in addition to their spatial modulation at $\boldsymbol{Q}$. Such textures arise from particle-hole condensates with nonzero angular momentum, analogous to unconventional superconductivity, and their symmetries are described by the little group $G_{\boldsymbol{Q}}$. In this work, we develop a framework for unconventional CDW phases that simultaneously incorporates the local symmetries described by little group and the presence of multiple symmetry-related ordering wave vectors, also known as the star. The resulting multicomponent order parameter transforms under representations of the full space group induced from irreducible representations of $G_{\boldsymbol{Q}}$. We apply this framework to two-dimensional lattices with sixfold symmetry and ordering wave vectors along high-symmetry lines, and derive the corresponding Landau free energies. As a microscopic example, we demonstrate the emergence of unconventional bond and loop-current orders from electronic interactions on the triangular lattice within the random phase approximation, and determine their ground states by microscopically evaluating the relevant coefficients in the free energy. Our framework provides a systematic route to describing unconventional modulated phases and can be readily extended to more complex lattices.
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Strongly Correlated Electrons

Chiral bosonic mean-field Ansatz and spin dynamics in spin-1 Kitaev magnets

Daiki Sasamoto, Arnaud Ralko, Jaime Merino et al. (+1 more)

The Kitaev model is a paradigmatic system for realizing quantum spin liquids, but its higher-spin extensions are not exactly solvable, and their spin dynamics is less well understood than in the spin-1/2 case. In this work, we reexamine a previously introduced triplet-pairing $\phi_t = \pi/2$ phase pattern for the antiferromagnetic $S = 1$ Kitaev model and extend the analysis to weak symmetric off-diagonal exchanges $\Gamma$ and $\Gamma'$. Using a bond-operator formulation of Schwinger-boson mean-field theory, we calculate the dynamical spin structure factor for the triplet 0-flux and triplet $\pi/2$-flux Ansätze with a spin-correlation scheme appropriate for Kitaev interactions. In the pure Kitaev limit, the $\pi/2$-flux Ansatz yields a flatter spectrum than the 0-flux Ansatz. The real-space spin correlations show that the $\pi/2$-flux Ansatz suppresses longer-distance correlations more strongly than the 0-flux Ansatz, yielding a correlation pattern closer to the short-ranged form expected in the Kitaev limit. This comparison shows that the flatness of $S(\boldsymbol{q}, \omega)$ is tied to short-ranged spin correlations and is therefore an important consistency check, although it is not, by itself, a diagnostic of time-reversal-symmetry breaking. We then study weak off-diagonal exchanges along $\Gamma' = \Gamma$ near the pure Kitaev limit, taking the same-sign relation from analyses of candidate spin-1 Kitaev materials. Gapped solutions are obtained within the constrained $\pi/2$-flux manifold, and the spectra share the qualitative energy- and momentum-space features found by finite-size exact diagonalization. Taken together, these results support the triplet $\pi/2$-flux chiral bosonic Ansatz as a useful mean-field description of spin dynamics near the antiferromagnetic $S = 1$ Kitaev limit with weak off-diagonal exchanges.
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All authors Daiki Sasamoto, Arnaud Ralko, Jaime Merino, Joji Nasu

Strongly Correlated Electrons · Cross-listed from cond-mat.supr-con

Ultrasonic attenuation in inhomogeneous superconductors

Aman Sardwal, Andreas Kreisel and Shantanu Mukherjee

Subgap bound states can produce pronounced features in the density of states without necessarily giving rise to a corresponding signal in ultrasonic attenuation. We show that the coherence factors act as a filter for the contribution of bound states to ultrasonic attenuation. In a phase-biased superconductor–normal–superconductor junction, the Andreev bound states produce a resonance in the attenuation when the phonon frequency matches the phase-dependent separation between the subgap levels. The situation is different at a $(110)$ grain boundary in a $d_{x^2-y^2}$ superconductor. There, nonmagnetic impurity-induced bound states remain weak in the attenuation despite their large spectral weight, because the relevant coherence factors nearly cancel. When local magnetism develops near the boundary, this cancellation is removed, the bound states become spin split, and a clear attenuation peak appears, which we argue is governed by a process involving emission of phonons by pair of quasiparticles at low temperatures. We obtain these results using a real-space Bogoliubov–de Gennes formulation of ultrasonic attenuation and benchmark the method against the known low-temperature behavior of homogeneous $s$- and $d$-wave superconductors. The results show that ultrasound can distinguish bound states with similar spectral signatures but different symmetry and spin structure.
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Strongly Correlated Electrons · Cross-listed from math.AG

Equations of Tree Tensor Network Varieties

Serkan Hoşten, Niharika Chakrabarty Paul, Otto T. P. Schmidt et al. (+1 more)

We show that tree tensor network varieties, including tensor train varieties, are general Markov models associated to spaced trees. This allows us to prove that the prime ideals of these varieties are generated by minors of matrix flattenings. In the case of tensor train varieties, we discuss whether these minors form a Gröbner basis and provide a combinatorial method to compute the degree for order $3$ tensor trains.
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All authors Serkan Hoşten, Niharika Chakrabarty Paul, Otto T. P. Schmidt, Dmitry Skurt

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

Anti-spin Laue groups: classification of anti-altermagnets and their representative minimal models

Colin Lange, Rodrigo Jaeschke-Ubiergo, Alexander Mook et al. (+1 more)

Anti-altermagnets exhibit odd-parity nonrelativistic spin splitting, yet unlike even-parity altermagnets, their momentum-space symmetries lack a reduced classification analogous to spin Laue groups. Here, we introduce anti-spin Laue groups, organized into three distinct classes, and identify 21 groups describing the odd-parity partial wave character of this unconventional class. Together with the 10 spin Laue groups of altermagnets, they complete the classification of nonrelativistic unconventional magnets with collinear momentum-space spin polarization. Anti-spin Laue groups also provide a many-to-one reduction of spin space (point) groups by retaining only their action on the collinear momentum-space spin polarization, thereby directly encoding the symmetry-enforced nodal spin-splitting character. Based on this we develop a systematic model-construction algorithm yielding minimal, material-oriented four-band models. This framework places odd- and even-parity unconventional magnets on equal footing within a unified momentum-space symmetry description.
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All authors Colin Lange, Rodrigo Jaeschke-Ubiergo, Alexander Mook, Jairo Sinova

Strongly Correlated Electrons · Cross-listed from cond-mat.supr-con

Emergent BEC mechanism in flat-band superconductors

V. Berger, I.S. Tupitsyn, B. Currie et al. (+3 more)

The formation of bound bosonic pairs of fermions, followed by their Bose-Einstein (quasi) condensation (BEC), is a foundational mechanism of superconductivity. At low filling, flat-band superconductivity is well captured by this mechanism provided the flat band is separated from the occupied lower band by an energy gap. However, particularly high $T_c$ values are anticipated when the non-interacting flat and lower bands touch—as in the prototypical attractive Lieb-lattice model studied here—invalidating the conventional picture: while interactions might protect the bound state by opening a gap, no small parameter guarantees the separation of the bound-state energy from this gap or occupied-band excitations, leaving the pair's fate uncertain. Based on a controlled-precision numerical protocol—which we demonstrate to be essential in this fundamentally non-perturbative problem—we show that the BEC mechanism, underpinned by an interaction-induced gap, is generically robust and remarkably efficient: fermions doped into the flat band form bound pairs within this gap with an anomalously light effective mass, enabling an exceptionally high $T_c$.
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All authors V. Berger, I.S. Tupitsyn, B. Currie, B.V. Svistunov, E. Kozik, N.V. Prokof'ev

Strongly Correlated Electrons

Sign-problem-resilient singular-value probe in determinant quantum Monte Carlo

Wen Chen and Rubem Mondaini

The sign problem limits determinant quantum Monte Carlo studies of strongly correlated fermion systems. In the spin-channel Hubbard-Stratonovich decoupling, spin correlations are exactly related to auxiliary-field correlations. This relation implies that an antiferromagnetic transition reorganizes auxiliary-field configurations and thereby changes the statistical structure of the resulting fermion matrices. We use the adjacent gap ratio of low-lying singular values of the space-time fermion matrix to probe interaction-driven transitions in two half-filled honeycomb-lattice Hubbard models. In the sign-free honeycomb Hubbard model, the statistic tracks the established transition from a Dirac semimetal to an antiferromagnetic Mott insulator. In the complex-weight Haldane-Hubbard model, the transition-sensitive feature remains visible in the phase-quenched reference ensemble and occurs near previous estimates of the transition. Moreover, phase reweighting only weakly modifies the gap ratio over the regimes investigated, despite the rapid suppression of the average phase. These results establish singular-value statistics as a sign-problem-resilient probe of interaction-driven transitions in determinant quantum Monte Carlo.
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Strongly Correlated Electrons · Cross-listed from quant-ph

Interface Capacity and Architectural Replenishment Determine Entanglement-Generation Speed in Quantum Networks

Shi-Ju Ran

We show that entanglement-generation speed across a fixed network interface is governed by two distinct resources: the entangling capacity of the interface itself and the ability of the surrounding architecture to replenish it with fresh degrees of freedom. For fermionic Gaussian dynamics, we derive the coefficient-sharp bound $\sum_k|\dot\theta_k|\leq\frac12\|K_{AB}\|_*$ on the collective speed of the canonical entanglement angles. Explicit Ising-chain rematching trajectories saturate this bound, thereby certifying exact minimum interaction times under the stated control model. Beyond the Gaussian setting, exhaustive optimization of the complete $N=8$ tree–tree family shows that, at fixed interface capacity, first-layer entanglement, connectedness, and edge budget, the saturation depth is exactly classified by rooted architecture. With higher-resolution $x$-only control, variational entanglement-enhancing-field (VEEF) optimization reaches the numerically resolved fast-$X$ optimum in a two-channel benchmark. Across all 21 symmetry-reduced rooted orbits, a pre-specified two-time VEEF growth diagnostic recovers the complete replenishment partition directly from optimized dynamics. Interface capacity therefore sets how much entangling flux is available, whereas architecture determines whether fresh degrees of freedom can continually replenish the interface and sustain repeated use of that capacity.
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Strongly Correlated Electrons · Cross-listed from cond-mat.supr-con

Predicting critical temperature in quantum simulators for high-$T_c$ superconductivity: the matrix product state plus mean field approach

Thomas Köhler and Adrian Kantian

Quantum simulation based on ultra cold atomic lattice gases is one of the most promising platforms to investigate high-$T_c$ superconductivity beyond the limited capabilities of quantum many body numerics on classical computers. Yet, despite enormous progress since the field's inception, realizing a high-$T_c$ superconducting state still remains out of reach. The present work lays the groundwork to purpose the recently proposed, and already partly realized, mixed-dimensional (mixD) models, towards this end. These systems offer the proven capability to realize very high pairing energies while retaining appreciable mobility of pairs. We specifically investigate the potential of 2D mixD-models with anisotropic tunneling, using the matrix product state plus mean field theory (MPS+MF) for fermions, and show that these models may enter a high-$T_c$ superconducting phase. These simulations in turn are based on a comprehensive characterization of the 1D mixD-systems, which are the sub-units of which the 2D system is comprized. In this, we cover the range of currently experimentally relevant system sizes, and establish practical heuristics to determine when finite\hyp size effects preclude the use of a 1D mixD-system to build the 2D ones.
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Strongly Correlated Electrons

Transitions and Critical Divergences in Periodically Hopping Modulated Su-Schrieffer-Heeger Chains

Surajit Mandal and Satyaki Kar

We use a curvature renormalization group (CRG) approach to study the topological phase transitions in a Su-Schrieffer-Heeger chain and its extensions coming from periodic hopping modulations. A curvature function is defined in terms of system parameters near high-symmetry points where the divergence of this function at critical points, in analogy to usual phase transitions, signals a topological phase transition. According to this theory, the phase transition line for the two-site Su-Schrieffer-Heeger (SSH) model is visible at the critical line \Delta = 0 where the curvature function diverges. Our study involves this model and also the modulated one with periodicity of four lattice spacing where the curvature function not only diverges at the topological phase transition point (Dirac-like) |\Delta/t| = \sqrt(2) but also shows faster divergence at the non-topological gapless point \Delta = 0. We further notice faster divergence of correlation length for \Delta -> 0 as compared to that for the |\Delta/t| -> \sqrt(2) resulting in two different sets of critical exponents making them lie in different universality classes. The edge state exhibits very slow decay into the bulk near the \Delta = 0 point while a much quicker decay from edge into bulk is discernible around the |\Delta/t| = \sqrt(2) point. We also continue similar analysis for a SSH model with hopping periodicity of eight lattice spacing.
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Strongly Correlated Electrons

Hubb_DMFT and Wan2mb_DMFT: Continuous-Time Quantum Monte Carlo Solvers for Single- and Multi-Orbital Hubbard Model

A. A. Katanin

We present two related customized software packages, Hubb_DMFT and Wan2mb_DMFT, designed to solve the Dynamical Mean-Field Theory (DMFT) equations for strongly correlated electron systems. Hubb_DMFT is adjusted for the single-band Hubbard model, providing a fast way to calculate the local self-energy, as well as the two-particle fermion and triangular fermion-boson charge and spin vertices, while Wan2mb_DMFT extends this capability to realistic multi-orbital systems, directly interfacing Wannier tight-binding Hamiltonians with many-body solvers. Both codes are based on iQIST v.0.7 impurity solver and utilize a modified and internally integrated Continuous-Time Quantum Monte Carlo (CT-QMC) core with the hybridization expansion (CT-HYB) framework and improved self-energy and vertex estimators for density-density interaction, ensuring numerically exact solutions for quantum impurity problems.
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Strongly Correlated Electrons · Cross-listed from quant-ph

Good Stabilizer Codes from Shallow Clifford Circuits with Random Matchings

Emile Anand, Elia Gorokhovsky, Jennifer Hritz et al. (+1 more)

Encoding quantum information with low circuit overhead is a fundamental challenge in fault-tolerant quantum computation. Random circuits provide a natural mechanism for rapidly spreading logical information through simple gates applied in parallel. Brown and Fawzi showed that random Clifford circuits on two-qubit Clifford gates provide such encoders that achieve the quantum Gilbert-Varshamov rate-distance tradeoff with depth $O(\log^3 n)$. We show that the same asymptotic tradeoff is attained in optimal $O(\log n)$ depth under a gate distribution with a more restricted support. For every fixed $\delta>0$ and sufficiently large $n$, if $\frac kn < 1 - H(\frac{d}{n}) - \frac{d}{n}\log_2 3 - \delta$, we can construct random circuits of depth $O(\log n)$ which define, with high probability, an $[n,k]$ stabilizer code of distance at least $d+1$, which matches the $\Omega(\log n)$ light-cone lower bound for linear distance encoders. Our ensemble employs a random matching circuit architecture consisting of $T$ independent permutation-invariant layers. In each layer, the qubits are paired up by a uniformly random perfect matching, and a random independent two-qubit Clifford gate is applied to each pair. The gate distribution need not be uniform over, or even have full support on, the two-qubit Clifford group; rather, we allow for very general distributions on Clifford gates satisfying three regularity conditions. In particular, the construction can be implemented using $n/2$ CNOT gates on randomly matched pairs in each layer, with parallel one-qubit Clifford twirls. These regularity conditions allow us to reduce the second-moment dynamics of our random circuits to a reversible Markov chain on binary support strings. We establish logarithmic hitting-time bounds for this Markov chain and comparisons of its stationary distribution to prove the coding properties of the circuits.
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All authors Emile Anand, Elia Gorokhovsky, Jennifer Hritz, Jingtong Sun

Strongly Correlated Electrons

High-field fate of the Kitaev quantum spin liquid in $\alpha$-RuCl$_3$

K. Imamura, R. Ohno, Y. C. Tsuzuki et al. (+15 more)

Kitaev quantum spin liquids (KQSLs) host fractionalized excitations described by itinerant Majorana quasiparticles and gapped $Z_2$ fluxes (visons), providing a platform for emergent topological matter. Whether such a state survives under strong magnetic fields, however, remains an open question. The layered honeycomb magnet $\alpha$-RuCl$_3$ is a leading candidate material: an in-plane field of $\sim$ 7 T suppresses antiferromagnetic order and induces a quantum-disordered phase exhibiting signatures consistent with Majorana excitations, including an anomalous thermal Hall effect and field-angle-dependent specific heat. At higher fields, the magnetization approaches saturation, suggesting a transition to a spin-polarized state, yet the microscopic evolution between these limits remains unresolved. Here we report high-field specific heat measurements up to 24 T that reveal a distinct crossover at $\mu_0H^*\approx$15 T, beyond which the perturbative Kitaev description breaks down. Above $H^*$, the characteristic six-fold angular modulation of the specific heat collapses and the excitation gap deviates from the predicted $H^3$ scaling. Meanwhile, the gap decreases with increasing field and the in-plane magnetization anisotropy persists up to $\sim$ 24 T, both in sharp contrast to a trivial spin-polarized state, indicating that KQSL signatures are preserved even at $\sim$ 90 % of magnetization saturation. These results reveal that the KQSL in $\alpha$-RuCl$_3$ extends well beyond the perturbative window, persisting as a nonperturbative regime in which the Majorana and vison energy scales merge, before eventually giving way to spin polarization. This thermodynamic roadmap provides a basis for understanding how fractionalized phases evolve under strong magnetic fields.
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All authors K. Imamura, R. Ohno, Y. C. Tsuzuki, Y. Akui, R. Namba, K. Ishihara, M. Akaki, M. Kimata, N. Kurita, H. Tanaka, N. Kimura, S. Imajo, A. Matsuo, K. Kindo, Y. Matsuda, K. Hashimoto, Y. Mizukami, T. Shibauchi

Strongly Correlated Electrons

Universal thermal breakdown of polaron coherence in one, two, and three dimensions

Jeet Shannigrahi, Janez Bonča and Mona Berciu

How a polaron loses its quasiparticle coherence with increasing temperature is a long-standing open problem. Holstein addressed it in 1959 only in the extreme antiadiabatic, strong-coupling limit, while more recent numerically exact approaches are largely restricted to one dimension. Here we solve this problem on square and simple cubic lattices across the weak-, intermediate-, and strong-coupling regimes. We show that the polaron effective mass $m^*$ and inverse lifetime $1/\tau$ increase monotonically with temperature until, at $T \sim 0.5\,\Omega$, the quasiparticle peak dissolves into a broad incoherent thermal continuum. The phonon frequency $\Omega$ therefore defines a universal coherence scale, independent of dimensionality and coupling strength, validating Holstein's prediction far beyond the regime in which it was derived. These results follow from a finite-temperature generalization of the Momentum Average (MA) approximation, yielding a closed-form, diagrammatically derived self-energy that is asymptotically exact in the strong-coupling limit at all temperatures. Benchmark comparisons demonstrate excellent quantitative agreement of the resulting 1D spectral functions with the numerically exact Variational Exact Diagonalization-Finite-Temperature Lanczos Method (VED-FTLM) and finite-$T$ Density Matrix Renormalization Group (DMRG).
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Strongly Correlated Electrons

Extracting Chern numbers from the equilibrium transport noise of multiband bosons

Zhi-Wei Wang and Samuel L. Braunstein

We show that the equilibrium transport noise of a multiband bosonic system carries a measurable imprint of its Chern numbers, and we give the protocol that extracts them. For electrons the topological part of the Berry curvature is absent from the transport noise: over a filled band it yields the quantized anomalous Hall conductivity, yet at a Fermi surface the Chern number is invisible to the fluctuations of the Hall current. That cancellation is an accident of the Fermi surface: magnons and phonons populate every band thermally, each with its own Chern number, so the harmonic sector of the Hodge decomposition is not a single constant, and a variance does not annihilate it. What survives is the occupation-weighted dispersion of Chern numbers. That current noise sees band geometry is known, and at zero temperature the antisymmetric part of the noise sum rule already returns a Chern number; the new element is an object with no zero-temperature analogue, existing only when several bands are populated and weighted differently. We derive it from a fluctuating Boltzmann equation and resolve the energy-magnetization subtraction at the level of fluctuations: the subtracted term is a curl, contributing exactly zero to the current a transport measurement records, and no magnetization auto-correlation enters the antisymmetric cross-spectrum at any wavevector. Measurement uses an equilibrium sum rule on that cross-spectrum, with no drive and hence no suppression: its frequency-integrated form cannot in practice separate topological from geometric content, for a rank reason, whereas the frequency-resolved form does, at accessible resolution. The protocol is specified for Cu(1,3-bdc), with calibration and robustness characterized. A driven alternative lies twelve orders below the equilibrium floor. Two independent implementations agree on every convention-free quantity.
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Strongly Correlated Electrons · Cross-listed from quant-ph

Integer Linear Programming Decoder for Abelian and Non-Abelian Topological Codes

Dian Jing, Aubrey Zhang, Liang Jiang et al. (+1 more)

Topological orders (TOs) are widely used as quantum error-correcting codes, with anyon excitations serving as error syndromes. For certain Abelian TOs, decoding can be performed by independently matching particle-antiparticle pairs of each species. However, matching-based decoders cannot handle more general fusion rules in either Abelian or non-Abelian TOs, nor account for noise that correlates different anyon species. While clustering decoders are more broadly applicable, they typically neglect anyon data and fusion properties, leading to poor performance in practice. In this work, we introduce a fundamentally different decoder for arbitrary TOs based on integer linear programming (ILP). The ILP formulation linearizes the error-correction problem through the introduction of auxiliary variables and encodes fusion rules as linear constraints. Classical optimization then identifies the minimum-weight error configuration. As concrete examples, we determine error-correction thresholds for three TOs: the Abelian $\mathbb{Z}_2$ TO under depolarizing noise, where charge and flux errors are correlated; the Abelian $\mathbb{Z}_3$ TO, which does not admit a pairwise matching decoder; and the non-Abelian $D_4$ TO under noise channels that generate all anyon species. We demonstrate the versatility of the ILP decoder by showing a clear performance advantage over most existing decoders in all three cases. We further extend the method to incorporate noisy syndrome measurements and propose a just-in-time variant for continuous error correction. Our results establish ILP as a natural framework for handling correlated errors and general anyon fusion rules, and as a powerful and flexible general-purpose decoder for incoherent anyon noise in arbitrary TOs, with applications to fault-tolerant quantum computation.
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All authors Dian Jing, Aubrey Zhang, Liang Jiang, Ruben Verresen

Tuesday 18 August

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

Localized Excitons and Exciton-Phonon Coupling in Antiferromagnetic AgCrP2S6

Jessica McDivitt, Dimitar Pashov, Mark van Schilfgaarde et al. (+4 more)

AgCrP2S6 combines a low-symmetry thiophosphate framework with an antiferromagnetic Cr sublattice and nonmagnetic Ag sites, providing a setting in which covalency and magnetic localization compete in the low-energy optical response. We combine single-crystal x-ray diffraction, Raman spectroscopy, lattice-dynamical calculations, and self-consistent vertex corrected Feynman diagrammatic many-body approaches to determine the structural, electronic, and excitonic properties of bulk AgCrP2S6. The material remains monoclinic between 100 and 300 K, with additional Ag-site disorder resolved at low temperature, and the optimized structure is dynamically stable. The many-body calculations yield a reduced quasiparticle gap relative to Cr trihalides, but the lowest excitons remain predominantly local: the calculated low-energy (~1.4 eV) excitation cluster is best described as a weakly bright, strongly anisotropic Frenkel exciton with dominant onsite d-d character and substantial ligand-assisted d-p admixture. Symmetry analysis in the C2h setting identifies this state as predominantly Bu and constrains its leading exciton-phonon coupling channels to Ag diagonal renormalization and Bg-mediated bright-dark mixing. These results place AgCrP2S6 in a localized excitonic regime in which the stronger p-d hybridization, relative to the more ionic Cr trihalides, narrows the quasiparticle gap, while the antiferromagnetic exchange and Ag-site dilution disfavor the intersite coherence associated with strongly delocalized low-energy excitons.
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All authors Jessica McDivitt, Dimitar Pashov, Mark van Schilfgaarde, Justin C. Johnson, Jeffrey L. Blackburn, Anna A. Berseneva, Swagata Acharya

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

Optical Voltage Profiling of 2D Semiconductors via Proximal Exciton Sensing

Ha-Leem Kim, Hyungbin Lim, Yuanyi Yang et al. (+6 more)

High contact resistances in atomically thin semiconductors often mask intrinsic electrical transport properties, particularly at low carrier densities where exotic correlated states emerge. We introduce optical voltage profiling, a noninvasive wide-field technique that replaces local voltage probes with a proximal monolayer MoSe$_2$ exciton sensor. Isolated by thin hexagonal boron nitride, this sensor converts the target's local electrostatic potential into spatially resolved modulations of exciton reflectance. Through pixel-wise in situ calibration, these signals yield quantitative two-dimensional voltage maps of an actively biased semiconductor device. Using this method, we demonstrate the carrier-density-driven metal-insulator transition in bilayer MoSe$_2$ and obtain channel resistances below 1 k$\Omega$ despite M$\Omega$-scale two-terminal resistances in the metallic region. The optically derived resistance exhibits a metal-insulator crossover near the resistance quantum $h/e^2$, and the voltage maps and reconstructed local conductivity reveal pronounced spatial heterogeneity in both insulating and metallic regimes. Beyond resolving channel resistance under high contact-resistance conditions, the technique provides spatially resolved access to microscopic transport heterogeneity in functional van der Waals devices.
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All authors Ha-Leem Kim, Hyungbin Lim, Yuanyi Yang, Ruishi Qi, Ruichen Xia, Can Uzundal, Takashi Taniguchi, Kenji Watanabe, Feng Wang

Strongly Correlated Electrons

Rare-earth spin textures and a route to electronic inhomogeneity in $Pr_2Ir_2O_7$

Kyle G Sherman and Michael J Lawler

The pyrochlore iridate $Pr_2Ir_2O_7$ remains metallic at low temperatures where its family members insulate. Recently, scanning tunneling spectroscopy experiments have revealed an inhomogeneous mixture of Kondo-screened and Kondo-destroyed regions when its surface is produced by cleaving at room temperature while it remains uniform when cleaved at low temperatures. We ask whether the frustrated praseodymium spin texture seeds this inhomogeneity. To study this, we produce Monte-Carlo-sampled Pr spin-ice configurations, presumed to evolve slowly in time, and couple through a local Kondo exchange $J_K$ to an eight-band Hartree-Fock model of the Ir electrons. We further find the charge gap of the all-in-all-out state common in the other family members closes smoothly with the coupling $J_K$, and the recently proposed monopole-rich “jellyfish” textures further suppress the insulating behavior by $\Delta J_{Kc}\approx 0.03\,t$. Additionally, scanning a small cluster across a large Pr surface yields a synthetic tunneling map that fractures into islands within a sea. Although our classical simulable theory omits the Kondo singlet, the spatial modulation of the Fermi-level density of states it produces is, through the exponential Doniach sensitivity of the Kondo temperature, sufficient to tip the local balance between screening and magnetic order; a frustration-driven route to the observed Kondo/Kondo-destroyed inhomogeneity.
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Strongly Correlated Electrons

Finite-temperature Green's function cluster expansion from thermofield doubles: Breakdown of the polaron picture

M. R. Carbone, S. Fomichev, B. Kloss et al. (+4 more)

We introduce a method, numerically exact in principle, for computing the momentum- and frequency-resolved single-particle Green's function of a polaron at finite temperature. The method, which we refer to as the finite-temperature Green's function cluster expansion, combines two ingredients: the generalized Green's function cluster expansion, a numerically exact extension of the momentum average family of methods that solves the polaron problem at zero temperature through a hierarchy of equations of motion for restricted phonon cloud configurations; and the thermofield double formalism, which maps the thermal trace onto a pure-state expectation value over a doubled Hilbert space. The resulting equations of motion have the same algebraic structure as those of the multi-boson zero-temperature theory, with the temperature entering through a Bogoliubov-type mixing angle that controls the coupling to a set of fictitious bath bosons. We implement the method in our open-source software package and benchmark it on the one-dimensional Holstein polaron, computing spectral functions, dispersions, lifetimes, and effective masses across coupling regimes and temperatures up to $T/\Omega \sim 1$. Where finite-temperature density matrix renormalization group results are available, we find quantitative agreement at affordable computational cost. The method recovers momentum-resolved spectra directly in frequency space, with no time evolution or analytic continuation. We also discuss the practical costs of the approach. In particular, since the doubled phonon Hilbert space has a non-trivial configuration structure in which real and fictitious clouds compete, convergence in the corresponding cloud parameters requires care.
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All authors M. R. Carbone, S. Fomichev, B. Kloss, A. J. Millis, M. Berciu, D. R. Reichman, J. Sous

Strongly Correlated Electrons · Cross-listed from hep-lat

Searching for symmetric mass generation with staggered fermions in four dimensions

Nouman Butt, Simon Catterall, Gwen Hartshaw et al. (+1 more)

We conduct numerical simulations to map out the phase diagram and critical behavior of a lattice Higgs model composed of two massless staggered fermion fields forming a doublet under a global $SU(2)$ and coupled to a scalar field in the adjoint representation of the group. The scalar action consists of a potential comprising quadratic and quartic terms and a scalar kinetic term. At fixed quartic coupling we explore a two-dimensional parameter space finding a massless symmetric phase at weak coupling and a massive symmetric phase (SMG phase) at strong coupling. An intermediate anti-ferromagnetic phase separates these two regimes. These results are consistent with leading order weak and strong coupling expansions. We find that the critical lines bounding the intermediate phase merge at a unique point where all fermion bilinear condensates vanish but fermion susceptibilities diverge as non-trivial powers of the lattice size. We conjecture that this merged point corresponds to a multicritical point and may describe a phase consisting of a condensate of certain topological defects.
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All authors Nouman Butt, Simon Catterall, Gwen Hartshaw, Anna Hasenfratz

Strongly Correlated Electrons

Chern insulator boundary criticality

Benjamin Moy and Eduardo Fradkin

We investigate signatures of chirality at Chern insulator transitions in the presence of a boundary. The transition between a trivial insulator and a Chern insulator with Chern number $C=1$ is described by a massless Dirac fermion whose parity anomaly gives a critical Hall conductivity $\sigma_{xy}=\frac{1}{2}\frac{e^2}{h}$. Using a Dirac mass domain wall construction, we show that the chiral edge mode delocalizes into the bulk at criticality, but the boundary fermion correlation function retains a chiral structure and acquires the scaling dimension of the bulk fermion. We compute current correlation functions and demonstrate that the anomaly of the bulk Hall response is matched by delocalized chiral modes near the boundary. Using only the residual conformal symmetry of a (2+1)d conformal field theory (CFT) in a half-space, we identify parity-odd terms in current and energy-momentum tensor correlation functions that encode these modes and determine their electromagnetic and gravitational anomaly coefficients. Our analysis therefore applies to general time-reversal breaking (2+1)d CFTs, beyond the free Dirac transition. In particular, the analysis of the gravitational anomaly is also applicable to the free Majorana CFT governing the transition between a trivial superconductor and a topological superconductor. We also extend our results to more general Chern number changing transitions and to the transition between a (3+1)d topological insulator and a trivial insulator.
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Strongly Correlated Electrons · Cross-listed from cond-mat.dis-nn

Quantum Geometric Tensor Preconditioning for Stable Training of Recurrent Neural Quantum States

Adil Attar, Amine M. Aboussalah and Mohamed Hibat-Allah

Neural Quantum States (NQS) provide a powerful neural network-based variational framework for representing many-body wave functions and solving for ground states. Recurrent Neural Networks (RNNs) are particularly promising owing to their relatively low computational cost and their autoregressive property, which enables perfect sampling. Recently, RNNs have been reported to be unstable under curvature-based optimizers such as the minimum-step stochastic reconfiguration (minSR) method. In this paper, we address this perceived limitation and show that minSR can be stabilized through simple regularization techniques, enabling robust training of RNN-based NQS with only a few samples. Our approach outperforms the Adam optimizer on the one-dimensional transverse-field Ising model and the one-dimensional cluster state, and provides competitive results on the two-dimensional Heisenberg and $J_1-J_2$ models. This work offers a promising pathway for using modern optimization techniques with autoregressive NQS to address open questions in quantum simulation.
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Strongly Correlated Electrons · Cross-listed from cond-mat.supr-con

Entropy mapping under uniaxial pressure utilizing the elastocaloric effect

Zhenhai Hu, You-Sheng Li, Aleksei V. Frolov et al. (+8 more)

Uniaxial pressure is a powerful tuning parameter for quantum materials, but conventional thermodynamic probes such as specific heat are difficult to realize in the constrained geometries of strain apparatus. We develop a quantitative analysis framework for a.c. elastocaloric effect measurements that enable the reconstruction of the absolute entropy and hence specific heat across complex phase diagrams. The absolute accuracy is achieved by combining measurements in the strong coupling regime at low frequencies with high signal-to-noise measurements in the quasi-adiabatic regime at high frequencies. Applying the approach to the correlated superconductor Sr$_2$RuO$_4$, we obtain an absolute entropy map across the phase diagram including across phase transitions deep into the superconducting state. We demonstrate that from such data one can derive the absolute specific heat which is currently not possible through other approaches. This data reinforces the finding that the quenching of entropy within the superconductor Sr$_2$RuO$_4$ is strongest at the critical strain consistent with the superconducting gap being maximized at the Van Hove singularity (VHs). Furthermore, we demonstrate that, although $\Delta c /(\gamma T) $ does increase at the VH strain, this increase is much weaker than previously inferred from more indirect caloric experiments.
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All authors Zhenhai Hu, You-Sheng Li, Aleksei V. Frolov, Fabian Jerzembeck, Manuel Brando, Naoki Kikugawa, Dmitry A. Sokolov, Hilary M. L. Noad, Andrew P. Mackenzie, Michael Nicklas, Andreas W. Rost

Strongly Correlated Electrons · Cross-listed from cond-mat.supr-con

Bridging ambient- and high-pressure superconductivity in La$_2$LnNi$_2$O$_7$ films

Motoki Osada, Chieko Terakura, Shusaku Imajo et al. (+9 more)

The discovery of high critical-temperature $T_{\mathrm{c}}$ superconductivity near 80 K in bilayer nickelates under high pressure has sparked extensive studies. While superconductivity exceeding 40 K was subsequently discovered at ambient pressure in compressively strained films, the relationship between ambient- and high-pressure regimes remains an open question. Here we present a systematic investigation of superconductivity in compressively strained La$_2$LnNi$_2$O$_7$ films (Ln = lanthanides) at ambient and high pressures. The normal-state resistivity at ambient pressure, revealed by suppressing superconductivity with magnetic fields of 59 T, tends toward $T^2$ behaviour. Under high pressure in a cubic-anvil cell, $T_{\mathrm{c}}$ was enhanced from 41-42 K at ambient pressure to 67-73 K at 16 GPa. On the other hand, lattice compression induced by Ln substitution, which may mimic effects of pressure, lowers $T_{\mathrm{c}}$. In both cases, $T_{\mathrm{c}}$ correlates with the evolution of normal-state transport between $T^2$ and $T$-linear behaviour, offering insight into the interplay between lattice structure and superconductivity in bilayer nickelates.
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All authors Motoki Osada, Chieko Terakura, Shusaku Imajo, Jean-Baptiste Morée, Akiko Kikkawa, Masamichi Nakajima, Hsiao-Yi Chen, Yusuke Nomura, Koichi Kindo, Ryotaro Arita, Yoshinori Tokura, Atsushi Tsukazaki

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

Wavelength-Resolved Control of Photovoltaic Screening and Defect-Mediated Doping in Photo-Ferroelectric/Graphene Devices

Krishna Prasad Maity, Mohd Uvais, Jean-Francois Dayen et al. (+3 more)

Ferroelectrics enable large charge doping of two-dimensional overlayers, but the coexistence of switching and nonswitching charge dynamics complicate electro-optical analysis. Here, we investigate the optoelectronic response of a ferroelectric/graphene device under 365 and 530 nm illumination, disentangling effects on ferroelectric dipole alignment from extrinsic current pathways. Graphene acts as a high-gain sensor, amplifying subtle polarization dynamics into a pronounced resistance difference. By resolving switching and nonswitching channels in dark and illuminated conditions, we reveal a competition between photovoltaic charge screening and defect-assisted excitation that governs device electrostatics. Above-band gap illumination generates free carriers that induce leaky ferroelectric hysteresis and suppress the graphene resistance ratio between opposite remanent polarization states from 290% to 15% due to dynamic photovoltaic charge screening. In contrast, 530 nm illumination primarily induces charge redistribution in ferroelectrics via defect-state excitation, leading to a significantly weaker suppression of the resistance variation of graphene. These results establish practical guidelines for selecting photon energy and intensity to either preserve remanent polarization while tuning channel doping or deliberately reconfigure polarization through optical programming.
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All authors Krishna Prasad Maity, Mohd Uvais, Jean-Francois Dayen, Bernard Doudin, Roman Gumeniuk, Bohdan Kundys

Strongly Correlated Electrons

Gauge-constrained Spinon Complexes Near Deconfined Quantum Criticality

Zhi-Yao Ning, Xue-Feng Zhang, Naoki Kawashima et al. (+1 more)

Quantum magnets provide a microscopic platform for studying confinement and gauge-constrained structures mediated by emergent gauge fields. We investigate confined spinon complexes in the columnar valence-bond-solid (VBS) phase near deconfined quantum criticality using pinned-spin defects and quantum Monte Carlo simulations. The pinned spins act as static spinon sources with controlled positions, spin projections, and VBS vorticities, enabling measurement of defect energies and direct real-space visualization of the associated VBS domain-wall strings. For matched spinon-antispinon sources, the excitation energy saturates beyond a characteristic separation as one extended dipole reorganizes into two shorter neutral dipoles, providing energetic and real-space evidence of string breaking. We further show that domain-wall connectivity is governed by lattice-scale VBS phase offsets in addition to vorticity and spin-projection neutrality. Compatible multi-pin patterns generate connected four-spinon complexes and extended domain-wall networks that retain their global connectivity under local distortions. These results establish pinned-spin defects as a controlled tool for assembling and resolving multi-spinon structures and their confining strings in a VBS phase.
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All authors Zhi-Yao Ning, Xue-Feng Zhang, Naoki Kawashima, Jun Takahashi

Strongly Correlated Electrons

Inducing metal-insulator transition via disorder in correlated kagome systems

Qingzhuo Duan, Hongdao Zhuge, Zixuan JIa et al. (+1 more)

The metal-insulator transition is often accompanied by fascinating quantum phenomena, including superconducting domes, antiferromagnetic phase transitions, and quantum spin liquids. Concurrently, kagome materials are predominantly metallic, necessitating the realization of insulating states to fully exploit their significant potential in logic and optoelectronic device applications. To address this, we investigate the electronic transport and magnetic properties in correlated kagome systems with hopping disorder using the determinant quantum Monte Carlo method. Through comprehensive analysis of the kinetic energy, dc conductivity, and density of states at the Fermi level, we demonstrate that the cooperative interplay between hopping disorder and electron correlations promotes electron localization. Within the insulator, an increase in the disorder level reduces the Coulomb interaction required for the Mott transition. Additionally, while disorder partially suppresses antiferromagnetic ordering, it remains insufficient to induce a complete magnetic transition. Finally, we summarize two schematic regions distinguishing between antiferromagnetic metal, correlated Anderson insulator, and disordered Mott insulator. Our study advances the understanding of metal-insulator transition in kagome systems by disorder and provides actionable insights for experimental control of these transitions.
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All authors Qingzhuo Duan, Hongdao Zhuge, Zixuan JIa, Tianxing Ma