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Physics Condensed Matter Strongly Correlated Electrons

cond-mat.str-el Watch Strongly Correlated Electrons

Wednesday 16 September

Strongly Correlated Electrons · Cross-listed from hep-th

Deep learning emergent spacetime from fermionic spectral functions in holography

Koji Hashimoto, Hyun-Sik Jeong, Keun-Young Kim et al. (+2 more)

We present a physics-informed machine learning framework based on Neural Ordinary Differential Equations that solves the holographic inverse problem: reconstructing the bulk spacetime and gauge field of a charged AdS black hole directly from boundary fermionic spectral functions. Encoding the UV asymptotics, horizon regularity, and zero temperature extremality as hard constraints in the neural network architecture, our framework reliably reconstructs the extremal Reissner-Nordström AdS geometry across three quantum critical regimes set by the $U(1)$ probe charge—non-Fermi liquid, marginal Fermi liquid (strange metal), and Fermi-liquid-like states—and can jointly infer the probe charge itself to sub-percent accuracy. Relaxing the near-AdS boundary constraint uncovers a geometrical degeneracy: bulk profiles that differ throughout the radial direction but share the same near-horizon $AdS_2 \times \mathbb{R}^2$ data reproduce identical spectral functions near the Fermi surface. This isospectral non-uniqueness is precisely the bulk degeneracy expected on general holographic grounds at zero temperature, and its spontaneous emergence across independent training runs shows that the network isolates the IR CFT universality rather than overfitting a single UV completion.
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All authors Koji Hashimoto, Hyun-Sik Jeong, Keun-Young Kim, Daichi Takeda, Kwan Yun

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

Complex magnetic properties of EuAgAs single crystals

Karolina Kowalczyk, Kamila Komędera, Janusz Przewoźnik et al. (+6 more)

EuAgAs is an antiferromagnetic topological material exhibiting intriguing magnetic behavior. We investigate its structural, magnetic, and local electronic properties using X ray diffraction, Mössbauer spectroscopy, dc magnetization, ac susceptibility, and heat capacity measurements. The results confirm antiferromagnetic ordering below $T_\text{N}$ and reveal pronounced magnetic anisotropy and several field induced metamagnetic transitions. We construct the magnetic phase diagram of EuAgAs, identifying several distinct magnetic regions. The field and temperature dependent behavior observed suggests a noncollinear magnetic structure in the low field regime. The sequence of field induced transitions resembles that observed in centrosymmetric rare earth compounds hosting skyrmion phases, suggesting that competing magnetic interactions may play an important role in stabilizing the observed magnetic states.
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All authors Karolina Kowalczyk, Kamila Komędera, Janusz Przewoźnik, Łukasz Gondek, Czesław Kapusta, Wojciech Tabiś, Michał Babij, Lan Maria Tran, Damian Rybicki

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

Direct observation of phonon mode with an antisymmetric Raman tensor

Shangfei Wu, Shang Ren, Kai Du et al. (+4 more)

Phonon angular momentum, characterized by circular or elliptical motion of atoms, plays a decisive role in diverse phenomena, ranging from the phonon Hall effect, phonon magnetic moment, Einstein-de Haas effect, Weyl phonons, and driven chiral phonons. The phonon modes with an antisymmetric Raman tensor, which carry an intrinsic phonon angular momentum in a pseudovector symmetry channel, have been overlooked in the past in the chiral phonon community. Here, we report the detection of this type of phonon mode using polarization-resolved Raman spectroscopy. We observe a phonon at 266cm$^{-1}$ in the antiferromagnetic chiral phase of Cr$_2$O$_3$ in the $A_2$-pseudovector symmetry channel. This $A_2$ mode's frequency and the elliptical motion are captured by first-principles phonon calculations, which treat both lattice and spins on an equal footing as 'slow' degrees of freedom. This mode gets Raman activity due to the presence of Cr vacancies in Cr$_2$O$_3$ that locally break the twofold rotational symmetry perpendicular to the threefold axis. Our results provide a new way to detect the novel symmetry-forbidden $A_2$ phonon modes with an antisymmetric Raman tensor in a pseudovector symmetry channel.
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All authors Shangfei Wu, Shang Ren, Kai Du, Xianghan Xu, Sang-Wook Cheong, David Vanderbilt, Girsh Blumberg

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

Magnetically coupled charge-transport crossover and giant negative magnetoresistance in iodine-incorporated Cr$_2$Se$_3$

Bikash Das, Shibnath Mandal, Kapildeb Dolui et al. (+12 more)

We report the synthesis and comprehensive investigation of iodine-incorporated $Cr_2Se_3$, a non-van der Waals quasi-two-dimensional magnetic material, using structural, magnetic, transport, spectroscopic, and first-principles methods. Magnetization and electron spin resonance measurements reveal an antiferromagnetically ordered state below $T_N \approx 52$ K. At higher temperatures, a second, broader anomaly emerges near $T^* \approx 150$ K, coinciding with a shallow minimum in the temperature-dependent resistivity that resembles a metal-to-insulator-like crossover. Hall measurements indicate predominantly hole-like conduction at high temperatures ($\geq$ 150 K), while the nonlinear Hall response below T*, together with an anomaly in the third-harmonic electrical signal, suggests the emergence of mobility-dependent multichannel and spatially inhomogeneous transport. A large, non-saturating negative magnetoresistance reaching approximately -78% at 25 K and 12 T further demonstrates strong coupling between charge transport and the magnetic state. Temperature-dependent Raman spectroscopy reveals no symmetry-changing structural transition near T*, whereas angle resolved photoemission spectroscopy (ARPES) measurements show no major reconstruction of the electronic structure occupied across the crossover. Taken together, these results identify the high-temperature anomaly as a broad magnetically coupled transport crossover arising from the interplay of short-range magnetic correlations, chemical disorder, and redistribution among competing conduction channels. These findings demonstrate that anion incorporation provides an effective route to tuning the coupled electronic and magnetic properties of non-layered Cr$_2$Se$_3$, establishing this system as a promising platform for investigating correlated transport and emergent spintronic functionalities in transition-metal chalcogenides.
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All authors Bikash Das, Shibnath Mandal, Kapildeb Dolui, Oleksi Laguta, Sambit Choudhury, Suvadip Masanta, Tanima Kundu, Jorge Andres Navarro Giraldo, Alexey Barinov, Shibabrata Nandi, Kai Rossnagel, Sanjoy Kr Mahatha, Rajib Mondal, Petr Neugebauer, Subhadeep Datta

Tuesday 15 September

Strongly Correlated Electrons · Cross-listed from quant-ph

Suzuki-Trotter Decompositions and other Methods for Quantum Time Evolution

Johann Ostmeyer

(Suzuki-)Trotter decompositions, splitting methods, (Lie) product formulae... The most common numerical methods for the time evolution of quantum systems come with many names. And they are used practically everywhere with applications ranging from the solution of classical equations of motion and various Monte Carlo simulations to the real and imaginary time evolution on classical as well as quantum computers. Here we review the state of the art of said methods, focussing especially on the progress made over the last few years. We highlight recently discovered efficient time evolution algorithms and explain how best to use them in practice. A central part of this work is the estimation of error bounds that has improved greatly within the past decade. The relevance of time evolution methods for quantum computing is discussed with a focus on noisy hardware. Finally, a comprehensive overview of generalisations, related methods and alternatives to Trotterization is provided. This includes time-dependent Hamiltonian dynamics, processed methods, multi-product formulae, symplectic integrators, TDVP for tensor networks, quantum signal processing, Crouch-Grossman methods and more. The overall perspective in this work is that of a theoretical physicist. All mathematical proofs as well as some technical details are omitted for easier readability. Instead, this review serves as a hands-on guide and, of course, as a starting point for references that provide further details.
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Strongly Correlated Electrons

Assessing the Reliability of Anomalous Hall Conductivity Extraction in GdAlSi

Anil Kumar, Debapratim Pal, Sudhan Koirala et al. (+6 more)

We examine the different methods of extracting the anomalous Hall conductivity using SrRuO$_3$ and GdAlSi. For SrRuO$_3$, where the ordinary Hall background is well defined and the magnetoconductance is small, subtracting the ordinary Hall contribution either before or after conversion from resistivity to conductivity yields nearly identical anomalous Hall conductivities. In GdAlSi, in contrast, the transverse response does not exhibit clear saturation, and both low- and high-field regions can appear approximately linear. We show that different interpretations of these linear regions as the ordinary Hall background, together with the treatment of the measured longitudinal resistance in the resistivity-to-conductivity conversion, can produce completely different estimates of the anomalous Hall conductivity. We also examine whether the nonlinear Hall response can instead be described within an ordinary Hall framework without invoking an anomalous contribution. Our analysis provides practical cautions for extracting excess Hall contributions in quantum materials.
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All authors Anil Kumar, Debapratim Pal, Sudhan Koirala, Muhammad Adnan, Youngsang Ji, Prakash Regmi, Bailey S. Bouley, Ludi Miao, Yun Suk Eo

Strongly Correlated Electrons

What does "instant thermalization" in large-$q$ SYK models mean?

Alexander Osterkorn and Jan C. Louw

Motivated by the Planckian thermalization rate $\Gamma \sim T$ observed in the two-body interacting SYK model, we study thermalization in the $q/2$-body case. Previous studies of such systems have established the notion of instantaneous thermalization to leading order in $1/q$. It was conjectured that the thermalization may still be Planckian but with a divergent rate $\Gamma \sim T q$, explaining the “instantaneous” part. For an analytically and numerically tractable system, we calculate the effective temperatures after a quench at time $t = 0$ and indeed find a Planckian rate, albeit with the unexpected decaying behavior $\Gamma \sim T q^{-1} $. This is contrasted with the behavior of the causal Green's function $\mathcal{G}(t_1, t_2)$, which instantly acquires a thermal form in the two-time plane block $t_1, t_2 > 0$. The resulting picture is that instant thermalization is a meaningful concept only for this block, while the off-diagonal blocks $t_1 \cdot t_2 < 0$ are inherently non-thermal at any $q$. Since the effective temperature is obtained from correlations spanning all blocks, it inherits a finite rate set by the off-diagonal. We illustrate this directly by studying the non-thermal correlations' time dependence.
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Strongly Correlated Electrons · Cross-listed from cond-mat.mes-hall

Visualizing Berry curvature in a Floquet-Chern insulator

Younsik Kim, Rishi Acharya, Hannah E. Aguirre et al. (+3 more)

Circularly polarized light can open a topological gap in a gapless Dirac system, creating a Floquet-Chern insulator. However, its Berry curvature has not been directly resolved. Here we use circular-dichroism time- and angle-resolved photoemission spectroscopy (Tr-ARPES) to map the light-induced Berry curvature of Floquet-Bloch states on the surface of Bi$_2$Se$_3$. By comparing spectra for opposite helicities of the mid-infrared pump, we isolate the induced geometric response. We observe alternating-sign Berry-curvature features at Floquet gaps that repeat at intervals set by the pump photon energy and redistribute with pump field strength in agreement with calculations for a periodically driven Dirac Hamiltonian. Both the induced Berry-curvature and the Floquet Dirac gap decay faster than the replica intensity, showing that spectral replicas can persist even after coherent Floquet hybridization weakens. These results establish momentum-resolved Berry-curvature mapping as a direct probe of light-induced topology and coherent Floquet band formation.
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All authors Younsik Kim, Rishi Acharya, Hannah E. Aguirre, Lucca Figari, Daniel P. Shoemaker, Fahad Mahmood

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

Nonlinear electron-phonon interactions from first principles

Zhenbang Dai and Feliciano Giustino

Electron-phonon interactions underpin a variety of phenomena, ranging from transport and superconductivity to polarons and ultrafast carrier dynamics. Despite being one of the most intensely studied subjects in condensed matter physics, research on electron-phonon physics mostly focused on linear, first-order couplings. Second- and higher-order nonlinear couplings are commonly ignored because their calculations are too demanding and we lack computational frameworks that can provide both diagonal and off-diagonal coupling matrix elements. In this work, we report a theory and computational method for computing nonlinear electron-phonon interactions of any order in real materials. Our approach combines the advantages of unit-cell calculations of electron wavefunctions and supercell calculations of phonon perturbations, is systematically improvable, can be used with either semilocal or nonlocal exchange-correlation functionals, and is amenable to Wannier-Fourier interpolation. As a first proof of concept, we illustrate this method by computing second-order electron-phonon coupling matrix elements in diamond, lithium fluoride, and graphite as representative nonpolar semiconductors, polar semiconductors, and metals, respectively. Furthermore, we generalize the ab initio polaron equations to second-order electron-phonon couplings, and we show that second-order couplings are essential to achieve quantitative accuracy in polaron formation energies and hopping barriers. The present methodology will find application in the study of all properties and phenomena that are currently being investigated within the linear electron-phonon coupling approximation, from phonon-mediated superconductivity to excited-states dynamics, both in harmonic and anharmonic systems.
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Strongly Correlated Electrons

La substitution studies on the heavy-fermion superconductor CeRh$_2$As$_2$

Sushma Lakshmi Ravi Sankar, Manuel Brando, Jochen Wosnitza et al. (+1 more)

CeRh$_2$As$_2$ has been receiving considerable attention due to its unusual two-phase superconductivity. The superconducting (SC) phase appears at $T_{\mathrm{c}}$ = 0.35 K in an ordered state (phase I) of the Ce-4$f$ moments, which develops below $T_{\mathrm{0}}$ = 0.55 K. The microscopic nature of phase I has not been fully established yet. We report a single-crystal study of the effect of La substitution on these low-temperature phases in Ce$_{1-\textit{x}}$La$_\textit{x}$Rh$_2$As$_2$ up to $x$ = 0.1. The lattice parameters increase monotonically with $x$, corresponding to an effective negative pressure of approximately -0.3 GPa for $x$ = 0.1. While the Ce$^{3+}$ local valence state and the non-Fermi-liquid behavior are preserved, the resistivity coherence maximum $T^{*}_{\mathrm{max}}$ $\approx$ 45 K in the pristine sample shifts to lower temperatures with increasing $x$, indicating a suppression of the Kondo energy scale upon lattice expansion. On the other hand, both $T_{\mathrm{c}}$ and $T_{\mathrm{0}}$ are rapidly suppressed to below 0.1 K for $x$ $>$ 0.05. Notably, such a rapid decrease of $T_{\mathrm{c}}$ under moderate negative pressure is unexpected, but is rather consistent with substitution-induced disorder effect which leads to strong pair breaking in unconventional superconductors. The observed fragility of phase I under both pressure and disorder may imply its itinerant origin.
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All authors Sushma Lakshmi Ravi Sankar, Manuel Brando, Jochen Wosnitza, Seunghyun Khim

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

Development of a Physics-Informed Neural Framework, MEOWN, for Rapid Prediction of Muon Stopping Sites in Crystalline Materials, for understanding Quantum Magnet employing Muon Spectroscopy

A. Pandey, K. Sharma, S. Ghosh et al. (+2 more)

Muon-spin rotation/relaxation is one of the most powerful microscopic probes for understanding magnetic order, spin dynamics, superconductivity, and complex magnetic phases in emerging quantum materials. Quantitative interpretation depends critically on the accurate identification of the muon stopping site, a problem traditionally addressed using density functional theory-based structural relaxation. However, conventional DFT approaches are computationally demanding, time-consuming, and require extensive calculations. Here, we develop MEOWN (Muon Engine for Optimized Weighted Networks), a machine-learning-based, physics-informed computational framework that combines a Polarizable Unperturbed Electrostatic Potential (P-UEP) model with machine-learning-guided optimization and symmetry-driven relaxation to predict energetically favorable muon stopping sites in crystalline solids. MEOWN explicitly incorporates electrostatic interactions, electronic screening, polarization effects, and zero-point motion into the learning workflow, providing physically interpretable predictions with substantially reduced computational cost while maintaining physical consistency and scientific rigor. To validate the framework, we investigated several well-established benchmark materials, including MnSi, CoF2, CaF2, LiF, and NaF. The predicted muon stopping sites and dipolar fields are in close agreement with previously reported DFT+mu results, demonstrating the reliability and transferability of the approach across chemically diverse systems. Notably, MEOWN predicts the equilibrium muon stopping site within a few minutes, offering a significant speedup over conventional DFT-based calculations. This manuscript presents the theoretical foundations, computational methodology, and validation of MEOWN as a general-purpose software for rapid and reliable muon stopping-site prediction in crystalline materials.
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All authors A. Pandey, K. Sharma, S. Ghosh, G. Roy, T. Basu

Strongly Correlated Electrons

Hartree-Fock Density-Matrix Renormalization Group for Very Long Chains

Steven R. White

We formulate Hartree–Fock as a density-matrix renormalization group (DMRG) sweep of a single Slater determinant, assuming a density–density electron–electron interaction. We compress the occupied state and long-range interaction separately, reducing each sweep step to a small local Fock problem. Exchange is retained throughout. Uniform antiferromagnetic hydrogen chains provide an almost ideal setting for this approach, while retaining the Coulomb interaction and its $1/r$ tail. We initialize and converge a chain of 100,000 electrons in 1.8 million spatial basis functions in about two hours on a 64-GB Mac mini, using less than a third of its memory. The work per sweep is nearly proportional to chain length, and the number of sweeps needed for convergence changes little from 1000 to 100,000 atoms. DMRG's compression of states and environments thus makes large HF calculations with long-range Hartree and exchange practical on an ordinary desktop computer.
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Strongly Correlated Electrons · Cross-listed from cond-mat.mtrl-sci

Orbital-resolved superexchange and topological magnon bands in MXene Fe$_2$C

Qiang Gao, Wei Wang, Shaojin Qi et al. (+5 more)

Magnetic MXenes are promising candidates for future spintronics due to their intriguing properties. Nevertheless, the topological properties of magnon bands and superexchange mechanism remain relatively underexplored. In this work, we have calculated the spin wave dispersions for MXene Fe$_2$C using the linear spin wave method, with the exchange coupling parameters obtained from first-principles calculations. Owing to the staggered stacking of two Fe sub-lattices, the interlayer exchange coupling lifts the degeneracy of the ferromagnetic magnon modes. A Dirac point is identified in the magnon bands at the $K$ point. The topological properties of magnon bands, including Berry curvature, valley Chern number and edge states, are further computed by means of a two-band model. We also derive an effective Hamiltonian to explain the magnonic topology, which is protected by the $C_{3v}$ rotational symmetry. To understand the microscopic origin of the exchange couplings that govern the magnon bands, we then construct a refined model integrating perturbation theory and the tight-binding approach. The ferromagnetic superexchange coupling is mediated by the virtual hopping between the d orbitals of two distinct Fe sites via a pair of orthogonal p orbitals at the bridging C site. In detail, the intralayer exchange coupling is governed by the in-plane $p_x$ and $p_y$ orbitals, while the out-of-plane $p_z$ orbital is responsible for the interlayer exchange coupling. Our results establish a direct link between the orbital-resolved anisotropic superexchange and topological properties of the magnon bands in magnetic MXene Fe$_2$C, providing theoretical guidance for future spintronics applications.
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All authors Qiang Gao, Wei Wang, Shaojin Qi, Guimei Shi, Anbang Guo, Xin Jin, Chen Shen, Xing-Qiu Chen

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

Entropy-driven transitions between extended integer and fractional quantum Hall regimes

Kyung-Su Kim and Steven A. Kivelson

Electronic states with coexisting Wigner-crystal order can sometimes exhibit a quantum Hall effect over a finite range of electron densities—i.e., exhibit “extended” quantum Hall (QH) plateaus in the absence of disorder. Such an extended quantum Hall state can then compete with other QH states over the same density range, allowing a first-order thermal transition between them. Here, we analyze several settings in which the entropy associated with Goldstone modes (e.g., magnons or phonons) or soft gapped modes (e.g., magnetoroton) drives a finite-temperature transition between competing QH regimes. Applying this framework to moiré rhombohedral graphene, we argue that a soft magnetoroton in a fractional quantum anomalous Hall state provides a plausible bulk mechanism for the observed thermal evolution from an extended integer QH to a fractional QH regime.
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Monday 14 September

Strongly Correlated Electrons

Analytical solutions to the non-equilibrium Green's functions in large-$q$ SYK models

Jan C. Louw

It is known that the Sachdev-Ye-Kitaev (SYK) model is exactly solvable at leading order in $1/q$; specifically, the instantaneously thermal time block after the quench is known in closed form. Thus far the remaining (inherently out of equilibrium) blocks have only been numerically accessible. Here we provide their analytical solutions for a quench between non-commuting $q/2$-body SYK Hamiltonians. We obtain the Green's functions in closed form for every time block. We extract a simple relation between the pre- and post-quench energies $\epsilon_1 \propto \epsilon_0$ leading to an exact temperature update rule. Via a Cauchy-Schwarz inequality, we show that heating is inevitable for the closed system; hence, the second law emerges geometrically. The solution is used in a companion paper to address what “instantaneous thermalization” means at finite $q$.
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Strongly Correlated Electrons · Cross-listed from cond-mat.mes-hall

An improved auxiliary boson method for quantum dots with finite Coulomb interaction U

Juan Pablo Ramos-Andrade, Patricia Almeida, Ana María Calle et al. (+1 more)

In this work, we present an improved version of the formulation of the four auxiliary bosons originally proposed by Kotliar and Ruckenstein, adapted to describe a quantum dot in the Kondo regime with finite electron-electron interaction U. The proposed refinement provides a more accurate treatment of strong electronic correlations while preserving the analytical tractability of the original approach, thereby enabling a reliable description of transport properties in nanoscale systems. Benchmark calculations show good agreement with the results of the numerical renormalization group, supporting the validity of the improved formulation. Our approach therefore provides an efficient and robust framework for investigating transport through strongly correlated quantum dots.
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All authors Juan Pablo Ramos-Andrade, Patricia Almeida, Ana María Calle, Gustavo Lara

Strongly Correlated Electrons · Cross-listed from hep-th

Persistent Spontaneous Time-Reversal Breaking

Michael Smolkin and Lev Yung

We provide an analytic realization of spontaneous parity and time-reversal breaking without thermal restoration in a local, unitary, ultraviolet-complete quantum field theory in $2+1$ dimensions with finitely many fields. The theory is defined by a relevant Yukawa deformation of a product of the critical biconical vector model and free massless Dirac fermions. This deformation induces a renormalization group flow whose infrared endpoint is a decoupled product of the critical vector model and the conformal Gross-Neveu-Yukawa model. At zero temperature, the vacuum preserves both symmetries, whereas upon heating, the theory undergoes an inverse thermal transition at a finite critical temperature. Above this temperature, a fermion mass, odd under parity and time reversal, is dynamically generated, and the broken phase persists to arbitrarily high temperatures.
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Strongly Correlated Electrons

Quantum Critical Solids

Subham Dutta Chowdhury, Leo Radzihovsky and Luca V. Delacrétaz

We study the fate of quantum solids with a vanishing velocity of transverse phonons. Such “floppy” solids exhibit a cubic elastic nonlinearity that is relevant in spatial dimensions $d<4$ and drives the system to a strongly-coupled Lifshitz quantum critical point in $d=3$, that we analyze in an $\epsilon = 4-d$ expansion. The associated phase transition is generically first-order, preempting the system's full access to the strongly-coupled fixed point. Nevertheless, the critical data of this quantum critical point is accessible within a Ginzburg region in the vicinity of the transition. This cubic nonlinearity vanishes exactly in $d=2$, where the leading interactions are instead quartic and are marginal, leading to weak logarithmic singularities. We discuss possible realizations of this “crystal Lifshitz criticality” in quantum materials.
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Strongly Correlated Electrons · Cross-listed from cond-mat.supr-con

Mode-selective phonon effects on magnetism and superconductivity in trilayer nickelates

C. Alexander Baum, Jun Zhan, Congcong Le et al. (+2 more)

The discovery of high-$T_c$ superconductivity in multilayer nickelates has stimulated intense interest, yet the microscopic origins of their correlated states remain debated. Recently the observed isotope effect of the magnetic state in trilayer nickelates highlights the important role of lattice vibrations in shaping these systems. Here, we investigate the impact of electron-phonon coupling (EPC) on magnetism and superconductivity in trilayer nickelates using ab initio and functional renormalization group calculations. We identify out-of-plane oxygen breathing modes with pronounced EPC and construct symmetry-resolved, layer-dependent couplings. Their relative phases across the three layers generate distinct retarded interactions, producing mode-selective enhancement or suppression of spin-density-wave order and $s_\pm$-wave superconductivity. This selectivity arises from screening-mediated spin-phonon interplay across a hierarchy of energy scales: EPC modifies charge screening at higher energies, reshapes magnetic fluctuations at lower energies, and ultimately influences pairing. Selected modes yield negative isotope coefficients for the magnetic state, qualitatively consistent with experiments. Our results establish how interlayer phonons modify electronically driven order and provide a microscopic framework for understanding isotope effects in trilayer nickelates.
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All authors C. Alexander Baum, Jun Zhan, Congcong Le, Ronny Thomale, Xianxin Wu

Strongly Correlated Electrons · Cross-listed from quant-ph

Designs without disorder: unitary $k$-designs from a single-site bulk defect

Samudra Sur and Pratik Nandy

We show that an $O(1)$ local defect is sufficient to generate approximate unitary $k$-designs from an otherwise integrable quantum many-body system. Using a two-Pauli-kick (2PK) protocol with temporal sampling, we demonstrate this mechanism in a fixed-magnetization sector of deterministic XXZ spin chains with a single integrability-breaking bulk defect. We derive an analytical selection rule for the Pauli kicks and show that symmetry-preserving strings with a specific domain-wall count, fixed by the couplings, govern design formation. An exhaustive search over all admissible Pauli strings identifies a broad family of kicks that produce designs in the chaotic regime, whereas no such kicks exist in the integrable limit, including for boundary defects. We further show that the design-forming mechanism remains effective for weak bulk defects as the system size increases.
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Strongly Correlated Electrons · Cross-listed from cond-mat.supr-con

Optical and magnetic signatures of drive-enhanced coherence in phase-disordered superconducting bilayers

Duilio De Santis, Sambuddha Chattopadhyay, Marios H. Michael et al. (+4 more)

Recent experiments demonstrating superconducting-like characteristics in materials above their equilibrium transition temperatures following strong sub-picosecond optical excitation have stimulated theoretical interest in phase-disordered superconductors subject to a periodically modulated superfluid stiffness. Previous analyses of the single-layer optical conductivity have shown that such a drive can enhance the low-frequency $1/\omega$ response, even when the period-averaged stiffness remains unchanged. We extend these studies to bilayer systems, such as the YBa$_2$Cu$_3$O$_{6+x}$ (YBCO) cuprate, by considering drives with different symmetries. Specifically, we investigate a layer-symmetric drive, in which the two planes stiffen and soften simultaneously, and a layer-antisymmetric drive, in which one plane stiffens while the other softens. Both drives enhance correlations of the layer-averaged phase and generate comparable superconducting-like optical conductivities. Only the symmetric drive, however, strengthens relative-phase locking and the associated counterflow response to a magnetic field parallel to the planes. Thus, an enhanced $1/\omega$ optical conductivity does not by itself imply enhanced magnetic screening. In view of applications to driven YBCO, where superconducting-like in-plane optics and transient diamagnetism have both been reported, our result provides a symmetry diagnostic: within a phase-synchronization mechanism, the experimentally active low-frequency stiffness modulation must contain a substantial layer-symmetric component.
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All authors Duilio De Santis, Sambuddha Chattopadhyay, Marios H. Michael, Andrea Cavalleri, Gil Refael, Patrick A. Lee, Eugene A. Demler

Strongly Correlated Electrons · Cross-listed from hep-th

Exact group invariant scar towers in two dimensional gauge theories

João Barata, Kiryl Pakrouski and Andrey V. Sadofyev

We construct exact many-body scar towers in two dimensional gauge theories with two flavors of massless fundamental fermions. For oppositely charged fermions, the scar subspace is generated by a gauge-neutral $\eta$-pairing operator, and admits a purely algebraic construction. Exact diagonalization methods reveal anomalously low entanglement and long-range pair correlations in the scar states. Charge conjugation maps the $\eta$-tower to a vector-flavor polarization sector of the equal charge model, yielding an interpretation as a coherent flavor mode with Josephson-like phase dynamics. Finite fermion masses mix the protected pair with an orthogonal channel and destroy the exact tower. The identified scar tower can be algebraically realized in a large family of lattice field theories, revealing a universal character of these states. Our results provide an analytical construction of invariant scar subspaces in a gauge theory with a nontrivial continuum interpretation.
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Strongly Correlated Electrons · Cross-listed from hep-th

Single-Instance Observables in the Sachdev-Ye-Kitaev Model

Brian Swingle

We present a diagrammatic expansion for thermal expectation values of low-weight Majorana strings in a fixed realization of the $N$-fermion Sachdev–Ye–Kitaev (SYK) model. At each order in the expansion, the observable is expressed as a finite sum of polynomials in the couplings multiplied by temperature-dependent kernels built from the large-$N$ melonic propagator. At fixed $\beta J$, the expansion is organized in powers of $N^{-1/2}$, and each order can be efficiently evaluated with classical resources polynomial in $N$. The expansion also extends to correlations at arbitrary imaginary times and sufficiently short real times. For the four-body SYK model, we show that the diagrammatic predictions give an excellent account of exact diagonalization results at sizes up to $N=24$. We also apply the method to the Maldacena–Qi (MQ) model formed from two identical copies of a fixed SYK realization. This enables the identification of a nearby SYK-like Hamiltonian whose thermofield double state has higher fidelity with the Maldacena–Qi ground state than the thermofield double of the original SYK Hamiltonian.
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Strongly Correlated Electrons · Cross-listed from quant-ph

Large-scale quantum simulations of dissipative spin-1/2 Heisenberg chains

João C. Getelina, Andrew Cox, Muhammad Asaduzzaman et al. (+4 more)

A quantum many-body system coupled to an environment relaxes to a nonequilibrium steady state that can sustain order with no equilibrium counterpart. Computing such steady states is harder than closed-system dynamics as the density matrix problem squares the Hilbert-space dimension, and no free energy selects the steady state. The dissipative spin-1/2 Heisenberg chain is a benchmark example for nonequilibrium steady state physics; various methods have each calculated its phase diagram but do not agree, and a controlled determination at large system size has remained out of reach. Here we simulate the Lindblad dynamics of chains of up to 50 sites on the superconducting processor ibm_kingston – 100 simultaneously active qubits at up to 1700 entangling-gate depths – realizing the dissipation via Stinespring dilation. The system's dissipative evolution is a self-correcting mechanism that effectively erases errors, so hardware noise enters only as a weak competing dissipator. We measure static structure factors and resolve ferromagnetic, antiferromagnetic, spin-density-wave, and paramagnetic steady states, mapping the phase diagram with 117 quantum hardware data points across the $J_x$–$J_y$ plane. We uncover a rich non-equilibrium phase diagram of ordered phases with only remnants of the mean-field order, and where sharp transitions give way to the crossovers expected in one dimension. We also find the existence of an incipient (Trotter-induced) spin density wave phase, highlighting the potential of controlled Trotterization as a tool to engineer various magnetic phases in dissipative spin systems. Our quantum simulations largely settle the lingering uncertainty regarding the correct phase diagram of this benchmark system. Moreover, they show that quantum computers are now a feasible tool for addressing scientific questions involving dissipative quantum systems.
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All authors João C. Getelina, Andrew Cox, Muhammad Asaduzzaman, Omar Alsheikh, Ryan S. Bennink, James K. Freericks, Alexander F. Kemper

Strongly Correlated Electrons · Cross-listed from hep-lat

Continuous Hasenbusch transport towards gauge diffusion with fermions

Akio Tomiya

Incorporating dynamical fermions is a central challenge for diffusion samplers of lattice gauge theories. We propose an analytic pseudofermion sampler based on continuous Hasenbusch transport as a component for gauge diffusion. The construction uses shifted linear solves and a finite-path correction, requiring neither explicit fermion determinant evaluation nor a learned pseudofermion model. We demonstrate the coupling in the two-flavour Schwinger model without neural networks, obtaining corrected physical observables compatible with independent references. We also show why accurate covariance transport can leave large weight fluctuations, tracing them to the backward transition density. This analysis leads to a correction based on the Wilson operator trace that reduces log-weight variance without changing the generated fields or increasing the number of Dirac operator applications. The predicted improvement is verified on previously unused gauge backgrounds. These results provide an analytic option for incorporating fermions in gauge diffusion and a guide to controlling its correction weights.
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Strongly Correlated Electrons · Cross-listed from cond-mat.supr-con

Stripe-Like Superconducting Enhancement and Coexisting Magnetic Texture in an Infinite Layer Nickelate

Ryan Laing, Dung Vu, Jacob Pfund et al. (+7 more)

Despite their promise as structural and electronic analogs to cuprates, nickelate thin films consistently exhibit broadened superconducting transitions in many experiments that remain poorly understood. Local measurements are ideal for revealing the presence of defects or competing phases, which may cause this transition broadening through a phase-separation. Here, we use scanning SQUID microscopy to investigate local superfluid density and magnetic texture of optimally doped $\mathrm{Nd}_{1-x}\mathrm{Eu}_{x}\mathrm{NiO}_{2}$ (NENO) (x=0.25) grown via molecular beam epitaxy. We observed a weakly-magnetic texture coexisting with superconductivity. Spatially resolved susceptibility imaging reveals a highly non-uniform superconducting state, characterized by a robust stripe-like enhancement pattern appearing near the phase transition. By resolving the mesoscopic landscape of electronic and magnetic inhomogeneities, these findings suggest that a competing magnetic phase is a primary contributor to the unusually broad superconducting transitions of these optimally doped NENO samples. Understanding the uncovered superconducting enhancement may provide a path to raising superconducting temperatures in these materials.
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All authors Ryan Laing, Dung Vu, Jacob Pfund, Wenzheng Wei, Frederick J. Walker, Haiyan Tan, Pavel Volkov, Menka Jain, Charles Ahn, Ilya Sochnikov

Strongly Correlated Electrons

Charge order before superconductivity in the doped kagome Dirac spin liquid

Yasir Iqbal

Doping a quantum spin liquid is expected to produce a superconductor. We show that for the Dirac spin liquid on the kagome lattice the doped charge orders first. The charge is carried by a chargon doublet whose two gauge components are degenerate on the square and triangular lattices, and the kagome lattice, as the line graph of the honeycomb lattice, splits them. One component disperses and condenses in four valleys. The other has flat lowest bands and stays empty over a window in the bare mass whose width, $(\sqrt6-1)t$, follows from the band structure alone. Within that window, in the mean-field chargon theory constructed from the projective symmetry group of the spin liquid, the doped state forms charge and loop-current crystals with the electromagnetic U(1) unbroken and no superconductivity. Pairing requires a gapped descendant of the spin liquid as the parent and then takes the form of a pair-density wave, with no uniform $d+id$ channel at leading order and none at any order for the crystals at the $M$ point. The predicted charge order is odd under the twofold rotation and has no cubic invariant, which distinguishes it from the charge order of the kagome metals.
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Strongly Correlated Electrons

Fractional vortices in a spin-isotropic spiral spin liquid

Cecilie Glittum, Han Yan and Johannes Reuther

Spiral spin liquids are magnetic states whose classical ground-state manifold consists of planar incommensurate spin spirals with wave vectors lying on a continuous ring or surface in reciprocal space. The resulting subextensive degeneracy suppresses magnetic long-range order and gives rise to liquid-like behavior. Despite numerous material realizations and theoretical investigations, the structure of low-temperature spin configurations of spin-isotropic Heisenberg spiral spin liquids has remained poorly understood. Here, we classify and characterize the classical topological defects supported by these systems. We uncover a rich family of vortex types involving concerted windings of spin directions, spiral-plane normals, and wave-vector orientations, yielding a $\mathbb{Z} \times \mathbb{Z}_2$ classification. Remarkably, the elementary defects are half-vortices carrying fractional $2\pi$ windings in both spin and momentum space and obey fusion rules resembling to those of Ising anyons. Large-scale classical simulations of a square-lattice spiral spin liquid reveal that these vortices are dense in the spiral-spin-liquid regime, and bind tightly below an order-by-disorder phase transition, eventually fusing to vacuum.
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Strongly Correlated Electrons

Chiral Antiferromagnetism from Momentum-Space Resonance in a 2D Semiconductor

R. Okuma, T. Ikenobe, Y. Fujisawa et al. (+28 more)

Understanding the principles governing the emergence of chiral quantum phases is a fundamental challenge, not only for uncovering new mechanisms of quantum-state formation but also for realizing giant electronic responses and transport phenomena arising from chirality and topology. While Fermi-surface instabilities in metals can stabilize complex ordered states through multiple competing scattering channels, their microscopic origin is often obscured by the complexity of the underlying electronic structure, limiting the development of general microscopic design principles. Here, we introduce a complementary strategy based on the simplicity of semiconductor band extrema. Using the layered van der Waals semiconductor GdGaI, whose low-energy electronic structure consists of simple electron and hole valleys, we discover the spontaneous emergence of an intertwined chiral triple-$q$ antiferromagnetic state accompanied by a cooperative reconstruction of the electron-hole band edges, beyond the conventional expectation of a single-$q$ ground state. This collective reconstruction generates substantial momentum-space Berry curvature, giving rise to a pronounced spontaneous anomalous Hall effect despite the semiconducting character and negligible net magnetization. Remarkably, this chiral state is realized within an atomically well-defined ($\approx2a$), topologically nontrivial magnetic texture, showing that such collective quantum states can emerge at an exceptionally small length scale from a simple two-dimensional magnetic semiconductor. More broadly, our results introduce a remarkably simple design concept for chiral quantum matter: using simple semiconductor band extrema as building blocks for resonance-like interplay in momentum space, providing a route to Berry curvature, topological transport, and emergent quantum phases.
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All authors R. Okuma, T. Ikenobe, Y. Fujisawa, K. Yamagami, H. C. H. Wu, T. Nakamura, Y. Ihara, H. Ishikawa, H. Suwa, H. Ishizuka, Y. Akagi, T. Kaneko, C. H. Hsu, Y. Obata, N. Tomoda, M. Dronova, K. Nagasawa, H. Saito, D. Ueta, H. Sagayama, J. Yamaura, M. Arita, K. Yogendra, S. Ideta, K. Kindo, T. Nakajima, S. J. Blundell, T. Kondo, K. Shimada, Y. Okamoto, Y. Okada

Strongly Correlated Electrons

Hidden Magnetic Complexity Within a Simple van der Waals Ferromagnet Ce$_2$Te$_5$

Rajesh Tripathi, D. T. Adroja, C. Ritter et al. (+10 more)

Ce$_2$Te$_5$ is a layered $f$-electron van der Waals magnet in which reduced dimensionality and inequivalent Ce sites give rise to competing magnetic interactions. We investigate its magnetic ground state using muon spin relaxation ($\mu$SR), neutron powder diffraction (NPD), and inelastic neutron scattering (INS). Zero-field $\mu$SR reveals an onset of static magnetism below $T_{\mathrm{C}} = 5.0(1)$~K, followed by an additional anomaly in the internal field at $T_{\mathrm{2}} = 2.3(2)$~K, consistent with features observed in bulk thermodynamic and transport measurements. In contrast, NPD data collected between $0.05-8$~K reveal a single long-range ordered magnetic phase below $T_{\mathrm C}$, with the magnetic Bragg intensities vanishing at $T_{\mathrm C}$ with no evidence for additional structural or magnetic phase transitions down to base temperature. The ordered state is characterized by a commensurate propagation vector $\mathbf{k}=(0,0,0)$ and ferromagnetic alignment of Ce moments along the crystallographic $b$ axis. Remarkably, only one of the two crystallographically distinct Ce sites carries an ordered moment of $\sim 0.85(3)~\mu_{\mathrm B}$ per Ce at 0.05~K. INS measurements establish the crystal electric field (CEF) energy scale of Ce$^{3+}$, revealing low-lying excitations at 7.94 and 21.46~meV and a Kramers doublet ground state with strong single-ion anisotropy. These results demonstrate that Ce$_2$Te$_5$ undergoes a single symmetry-breaking magnetic transition, while additional low-temperature anomalies reflect subtle modifications of the ordered state driven by competing interactions and CEF effects.
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All authors Rajesh Tripathi, D. T. Adroja, C. Ritter, Leandro Liborio, Manuel dos Santos Dias, Leon Petit, D. Khalyavin, Yu Liu, T. Shiroka, M. Aouane, S. Langridge, S. Patil, Eric D. Bauer