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Physics Condensed Matter Strongly Correlated Electrons

cond-mat.str-el Watch Strongly Correlated Electrons

Thursday 3 September

Strongly Correlated Electrons

Resolving the Magnetic Ground-State Controversy in RuO2 through A Flat Magnetic Energy Landscape

Tianxiao Liang, Fanhan Kong, Jijun Zhao et al. (+1 more)

Rutile RuO2 is a prominent candidate for altermagnetism, yet its magnetic ground state remains highly controversial, with experiments reporting either a nonmagnetic state or altermagnetic order. Here, we develop a generalized environment-dependent spin-lattice framework that unifies localized Heisenberg exchange, itinerant Stoner magnetism via Landau spin fluctuations, and spin-orbit-coupling-mediated spin-lattice interactions. Parameterized from a high-throughput first-principles database using machine-learning and solved by large-scale Monte Carlo simulations, the framework reveals an exceptionally flat magnetic energy landscape in RuO2, where the nonmagnetic state lies nearly degenerate with multiple altermagnetic configurations. We find that material perturbations, exemplified by intrinsic defects, select distinct magnetic ground states primarily by modifying the localized Heisenberg exchange, with perturbation-induced itinerant Stoner polarization provides an essential secondary contribution. Spin-orbit coupling controls the orientation and stability of the Néel vector, but does not determine the emergence of long-range magnetic order. These results provide a unified explanation for the conflicting experimental observations and establish a general microscopic framework for understanding how material perturbations select competing magnetic ground states in systems with nearly flat magnetic energy landscapes.
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All authors Tianxiao Liang, Fanhan Kong, Jijun Zhao, Xue Jiang

Strongly Correlated Electrons

Moiré-induced altermagnetism from nonmagnetic constituents

Jingtian Shi, Maxim Khodas and Ivar Martin

We propose a mechanism for nonmagnetic materials to develop altermagnetic order by moiré interference with nonmagnetic substrate, which is driven by structural relaxation and spontaneous twirls in moiré domain walls of lattice-mismatched moiré square lattices. When doped with one electron per moiré domain, a correlated insulating gap is opened by electron interaction. Depending on the location of the moiré potential minima, the moiré bands can show d-wave or g-wave altermagnetic splitting. The former can be enhanced by a finite twist angle; the latter is sensitive to strains that drive a transition to d-wave.
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Wednesday 2 September

Strongly Correlated Electrons

Antiferromagnetism-altered plasmon dynamics

Yifan Su, Suheng Xu, Rocco A. Vitalone et al. (+17 more)

The interaction between plasmons and magnons is a long-sought phenomenon with implications for fundamental physics and spintronics applications. In three-dimensional systems, this coupling is suppressed by the large mismatch in energy scales, but two-dimensional (2D) plasmons with gapless dispersion can overlap with magnons over a broad spectral range. Despite numerous theoretical predictions, experimental observation of magnon-plasmon interaction has remained elusive. In this work, we study a first-of-its-kind hybrid plasmon-magnon platform based on 2D materials. By deploying scattering-type scanning near-field optical microscopy (s-SNOM) with terahertz radiation, we image propagating plasmon wavepackets at a graphene/NiPS$_3$ interface and track their dynamics across the antiferromagnetic transition of NiPS$_3$. We observe a clear renormalization of the plasmon-polariton dispersion concurrent with the onset of antiferromagnetic order. With complementary Raman scattering and nano-terahertz spectroscopy, we unveil spectral weight redistribution and dielectric screening changes, potentially associated with the multi-magnon continuum, as the underlying mechanism. These results provide solid evidence of coupling between plasmon and antiferromagnetic order, marking a cornerstone for a potential platform for hybrid magnon-plasmon interactions in 2D materials, opening avenues for coherent spin-plasmon devices and tunable terahertz spintronic components.
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All authors Yifan Su, Suheng Xu, Rocco A. Vitalone, Ziyu Liu, Emil Viñas Boström, Na Wu, Chun-Ying Huang, Zhuquan Zhang, Jaehoon Jung, Daniel G. Chica, Vinícius da Silveira Lan Avelar, Yiping Wang, Takashi Taniguchi, Kenji Watanabe, James C. Hone, Xiaoyang Zhu, Cory R. Dean, Xavier Roy, Angel Rubio, D. N. Basov

Strongly Correlated Electrons

Orbital current rectifier and linear magnon Edelstein effect in $p$-wave antialtermagnets

Kristian Mæland and Björn Trauzettel

We show that $p$-wave magnets efficiently generate magnetization via a linear thermal Edelstein effect arising from the orbital magnetic moments of their magnons. Furthermore, they can generate perfectly nonreciprocal orbital currents through a purely even-order nonlinear response. This makes them promising candidates for orbital-current rectification. Because these transport phenomena originate from the orbital magnetic moment of magnons, they connect magnonics and orbitronics. More generally, we find that odd-parity-wave magnets with coplanar ground states host magnons with zero spin magnetic moment. Instead the magnons carry a collinear, out-of-plane orbital magnetic moment, realizing an orbital version of antialtermagnetism. We establish these general results using symmetry arguments and demonstrate them explicitly for a coplanar ground state with the minimal number of sublattices, inspired by the ground state of CeNiAsO. Our conclusions hold both in absence and presence of spin-orbit coupling.
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Strongly Correlated Electrons · Cross-listed from hep-th

Holographic zeros and poles from apparent singularities

Edwan Préau

We study general holographic fluctuation equations with apparent singularities, and the associated spectral functions. We consider the regime where an apparent singularity approaches the boundary, for which the near-boundary behavior of solutions can be determined via an inner/outer matching type of calculation. Depending on properties of the equation, we find that the merging of an apparent singularity with the boundary may generate either a zero or a pole for the corresponding spectral function. Our results also indicate that the recently introduced holographic product formula still applies in the presence of apparent singularities.
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Strongly Correlated Electrons · Cross-listed from quant-ph

Computing with qLDPC Codes by Climbing the Chain Map Hierarchy

Rahul Sahay, David M. Long and Vedika Khemani

We develop a framework for logical computation with qLDPC codes that places logical Pauli, Clifford, and non-Clifford operations on the same footing. This brings the simple homological description of Pauli logicals to the patchwork landscape of logical Clifford and non-Clifford operations, providing a tool for the discovery of new logical gates. In particular, we define the chain map hierarchy: a family of chain complexes whose homology classes encode logical unitary and code surgery operations at any level of the Clifford hierarchy, precisely analogous to the chain complex description of Pauli logicals. Consequently, intuition for Pauli logicals can be leveraged to discover new logical operations on qLDPC codes. For instance, the familiar ability to deform Pauli logicals with stabilizers—i.e. boundaries of the chain complex—becomes a way to search for constant-depth unitary implementations of (non-)Clifford logical gates. Using this strategy, we discover constant-depth unitary implementations of the full logical Clifford group on any number of blocks of the 2D toric code, including within a single block, and addressable logical CCZ gates on any triple of logical qubits on any number of blocks of the 3D toric code. Beyond manifold codes, we find addressable non-Clifford gates on codes with many encoded qubits. The chain map hierarchy naturally encompasses and extends other constructions of logical gadgets, for instance providing a universal parameterization of cup products. As such, our work provides a unified, useful, and intuitive language for computing with qLDPC codes.
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Strongly Correlated Electrons

Valley polarization dependence of candidate even-denominator fractional quantum Hall states in monolayer graphene

Saswata Sahu, Amarendra Pr. Indra and Moumita Indra

We investigate even-denominator fractional quantum Hall (EDFQH) states in monolayer graphene, focusing on the experimentally observed filling fractions $\nu = 1/2$ and $\nu = 1/4$. Owing to the approximate SU(4) symmetry arising from spin and valley degrees of freedom, graphene hosts a rich variety of multi-component fractional quantum Hall states with different polarization configurations. Using the Chern-Simons composite fermion framework for SU(4) systems, we construct candidate variational wave functions corresponding to distinct spin, valley, and mixed polarization states. The ground-state energies of these states are evaluated numerically using the Coulomb interaction in spherical geometry. By comparing energies across different polarization sectors, we identify the energetically favored configurations at $\nu = 1/2$ and $\nu = 1/4$. Our variational energy comparison suggests that the energetically favored candidate states correspond to multi-component correlated quantum Hall liquids. Within the restricted family of trial states considered here, these states differ from a simple composite-fermion Fermi-sea description. These findings provide insight into the role of valley polarization and SU(4) symmetry in stabilizing even-denominator fractional quantum Hall states in graphene.
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Strongly Correlated Electrons · Cross-listed from quant-ph

Effective quasiparticle conserving Lindbladians in the thermodynamic limit

Lea Lenke and Kai Phillip Schmidt

Open quantum many-body systems are commonly described by Lindblad master equations, yet the treatment of Lindbladian operators in the thermodynamic limit remains a major challenge. We develop a framework for constructing effective quasiparticle-conserving Lindbladian operators directly in the thermodynamic limit. Our approach extends continuous similarity transformations to non-Hermitian open quantum systems and enables the systematic block diagonalization of Lindbladians with respect to the quasiparticle (qp) number. We formulate two complementary methods. The first, projective continuous similarity transformations (pcst++), generalizes perturbative continuous unitary transformations to Lindblad operators so that a linked-cluster expansion allows us to obtain high-order series expansions of the infinite system. The second, deepCST, extends directly evaluated enhanced perturbative continuous unitary transformations by combining the same qp-conserving generator with a perturbative truncation scheme that yields non-perturbative effective Lindbladian operators directly in thermodynamic limit. We apply pcst++ and deepCST to the dissipative transverse-field Ising chain with local dissipation. We focus on the low-Ising regime. We purify the Lindbladian by splitting each spin into two sites. A spin flip then corresponds to two qps. We derive and analyze the effective, qp-conserving Lindbladian in the sectors with zero to two qps. We show how the Ising interaction renormalizes the decay rates of elementary spin-flip excitations and provides the microscopic mechanism for a competition between coherent interactions and dissipation. Our work establishes perturbative and non-perturbative continuous similarity transformations as a versatile tool for deriving effective qp pictures of open quantum many-body systems in the thermodynamic limit.
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Strongly Correlated Electrons · Cross-listed from cond-mat.supr-con

Electronic Structure and Superconductivity in La$_{1.55}$Sr$_{0.45}$CuO$_4$/La$_2$CuO$_4$ Artificial High-$T_c$ Superlattices Probed by Hard and Soft X-ray Spectroscopy

U. M. Jayathilake, S. Sheikh, T.-L. Lee et al. (+5 more)

In La$_{1.55}$Sr$_{0.45}$CuO$_4$/La$_2$CuO$_4$ (LSCO/LCO) artificial high-$T_c$ superlattices (AHTS), grown by quantum material design, the superconducting dome can be tuned by varying the geometric ratio $L/d$, where $L$ is the LCO layer thickness and $d$ is the superlattice nanoscale period. Here, we combine hard X-ray photoelectron spectroscopy (HAXPES) and polarization-dependent soft X-ray absorption spectroscopy (XAS) to probe how the electronic structure evolves across the $L/d$-tuned superconducting dome. We observe systematic chemical-potential evolution across the series, together with enhanced spectral weight near the Fermi level, enhanced local and non-local screening signatures, and increased in-plane orbital polarization near the top of the dome. Together, these spectroscopic signatures provide insight into the emergence of Fano-Feshbach resonant superconductivity in the LSCO/LCO AHTS series.
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All authors U. M. Jayathilake, S. Sheikh, T.-L. Lee, C. Klewe, G. Logvenov, G. Campi, A. Bianconi, A. X. Gray

Strongly Correlated Electrons

Disorder-driven competing magnetic interactions and glassy magnetic behavior in quaternary Heusler alloy FeRuMnGe

Manikantha Panda, Prabuddha Kant Mishra, Sonali S. Pradhan et al. (+6 more)

In this combined experimental and theoretical study, we investigate the role of disorder in governing the magnetic ground state of the quaternary Heusler alloy FeRuMnGe. In the FeRuMnZ (Z = Ga, Si) series, chemical substitution modifies atomic ordering and electronic structure, resulting in distinct magnetic ground states. Motivated by this, we extend the series to FeRuMnGe. X-ray diffraction reveals B2-type antisite disorder, where Fe–Ru and Mn–Ge intermix. Theoretical calculations show that such disorder modifies exchange interactions, leading to competing ferromagnetic and antiferromagnetic couplings, and drives the system from half-metallic to metallic. Magnetic measurements reveal competing interactions, giving rise to a cluster-glass state coexisting with long-range magnetic order. The absence of a thermodynamic signature at $T_f$, together with ac susceptibility and relaxation measurements, supports the presence of short-range magnetic interactions and a glassy magnetic state. The compound exhibits an enhanced magnetic response below $\sim 161$ K and a maximum magnetization of $\sim 1.64~\mu_B$/f.u. at 5 T. Overall, this work establishes a direct correlation between antisite disorder, competing exchange interactions, and glassy magnetism in quaternary Heusler alloys. Combined experimental results and theoretical calculations reveal that disorder drives the system from an AFM-dominated state in FeRuMnSi to an FM-dominated state in FeRuMnGe, providing deeper insight into the role of the extent of disorder in governing the magnetic properties of QHAs.
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All authors Manikantha Panda, Prabuddha Kant Mishra, Sonali S. Pradhan, Aarti Gautam, Bhagyashree Pol, P D Babu, Ashok Kumar Ganguli, V. Kanchana, Tapas Paramanik

Strongly Correlated Electrons

Prominent Dimensional Effects on High-order Harmonic Generation in Strongly Correlated Electron Systems

Kento Uchida, Shintaro Takayoshi, Yuta Murakami et al. (+6 more)

Dimensionality strongly affects elementary excitations in correlated quantum materials, yet its impact on extreme nonlinear optical responses remains largely unexplored. Here, we combine high-harmonic generation (HHG) experiments on quasi-one-dimensional SrCuO$_2$ and quasi-two-dimensional Pr$_2$CuO$_4$ with nonequilibrium simulations of Hubbard models. We find a pronounced dimensional contrast: SrCuO$_2$ exhibits a robust plateau-like high-harmonic spectrum with weak temperature dependence, whereas Pr$_2$CuO$_4$ shows a monotonic decrease in harmonic yield and strong thermal suppression, especially at higher harmonics. The simulations qualitatively reproduce these trends and identify dimensionality-dependent doublon–holon dephasing, governed by spin–charge coupling, as their microscopic origin. These results establish dimensionality and spin–charge coupling as key control parameters for extreme nonlinear optical responses in correlated insulators.
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All authors Kento Uchida, Shintaro Takayoshi, Yuta Murakami, Dongjoon Song, Alannah M. Hallas, Masayuki Watanabe, Takashi Konishi, Aiko Nakano, Koichiro Tanaka

Strongly Correlated Electrons

Thermodynamic Electric Toroidal Dipole and Intrinsic Longitudinal Spin Transport

Taisei Yamanaka, Takumi Sato and Satoru Hayami

Electric toroidal dipoles (ETDs) characterize ferroaxial order, yet their bulk definition in periodic crystals has remained elusive because conventional multipole operators involve the ill-defined position operator. Here we formulate a thermodynamic ETD by coupling a spatially varying electric field to the relativistic spin-induced electric polarization. The resulting expression is gauge invariant and provides a bulk order parameter for ferroaxial phases. We further establish a direct relation between the chemical-potential derivative of the ETD and the intrinsic longitudinal spin conductivity in insulating systems. To demonstrate the formulation, we construct a minimal ferroaxial extension of the Kane–Mele model. The ETD becomes finite exclusively in the ferroaxial phase and is strongly enhanced near a small band gap, accompanied by a sizable longitudinal spin current. Our results establish a thermodynamic theory of ETDs in crystalline solids and identify the longitudinal spin conductivity as a direct transport manifestation of ferroaxial order.
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Strongly Correlated Electrons · Cross-listed from cond-mat.mtrl-sci

Charge-Transfer Electronic Structure of NiX$_2$ (X = S, Se)

Atsushi Hariki, Takaki Okauchi, Daisuke Takegami et al. (+8 more)

We investigate the electronic structures of NiS$_2$ and NiSe$_2$ using density functional theory combined with dynamical mean-field theory (DFT+DMFT). A realistic electronic structure within DFT+DMFT was determined by optimizing the double-counting correction to reproduce experimental valence-band photoemission spectra. The validity of the present model is further confirmed by its successful description of the Ni 2$p$ core-level photoemission and Ni $L$-edge x-ray absorption spectra of NiS$_2$. Our results reveal a smaller charge-transfer energy than previously assumed, resulting in substantial ligand-to-Ni charge transfer and a reduced Ni local moment. We clarify how the relative position and interaction between the Ni upper Hubbard band and the antibonding chalcogen-dimer states shape the evolution of the low-energy electronic structure across the NiS$_{2-x}$Se$_x$ series.
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All authors Atsushi Hariki, Takaki Okauchi, Daisuke Takegami, Naoki Ito, Mizuki Furo, Ayako Yamamoto, Tomoya Higo, Satoru Nakatsuji, Masato Yoshimura, Takashi Mizokawa, Jan Kunes

Strongly Correlated Electrons · Cross-listed from quant-ph

Renormalization group and long-range conditional mutual information in hierarchical models

Yu-Hsueh Chen

A departure of a mixed quantum state from a local Gibbs description is generally invisible to local observables but can be detected by the conditional mutual information (CMI). Here we study the relationship between the renormalization group (RG) and CMI, and in particular, how RG constrains CMI. We first show that the CMI between nonadjacent regions $A$ and $C$, conditioned on the buffer region $B$, is UV-finite whenever the state admits a locally reversible RG with a fixed on-site Hilbert space dimension. We then study two hierarchical models that have long-range CMI and yet admit a simple RG description. The first model has a divergent Markov length at every temperature $0<T<\infty$ but nevertheless flows to an infinite-temperature product state under RG. The second model satisfies the local Markov condition while violating the global one and is stable against weak noise. At the critical noise strength, the two-point CMI decays only polynomially as a function of the system size, while the two-point mutual information vanishes.
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Strongly Correlated Electrons · Cross-listed from cond-mat.mtrl-sci

EuIn2Sb2: epitaxially stabilized axion insulator candidate with strong spin-orbit coupling

Hsiang Lee, Shinichi Nishihaya, Markus Kriener et al. (+5 more)

Eu triangular lattice layer compounds described by the general formula EuA2X2 have attracted growing attention due to a wide range of layered crystal structures and corresponding magnetic topological phases, arising from the coexistence of large Eu magnetic moments and energetically inverted A and X bands. Among the EuA2X2 family, EuIn2Sb2 has been predicted to host robust axion insulator and higher-order topological insulator states by stronger spin-orbit coupling but has not been experimentally realized, including the structural identification. Here we report the epitaxial stabilization of EuIn2Sb2 by adopting the molecular beam epitaxy technique. Structural characterization reveals that EuIn2Sb2 films are based on a unique In-on-In stacking, forming a new trilayer structure distinct from previously predicted structures in the EuA2X2 family. In addition to the in-plane antiferromagnetic ordering at TN=10.5K, the stronger spin-orbit coupling and enhanced In-In hybridization in EuIn2Sb2 make it an ideal candidate of axion insulator and higher-order topological insulator phases. These results establish EuIn2Sb2 as a new member of the EuA2X2 family and offer an advanced platform for exploring these magnetic topological phases.
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All authors Hsiang Lee, Shinichi Nishihaya, Markus Kriener, Ayano Nakamura, Tadashi Yoneda, Yuki Deguchi, Makuro Goto, Masaki Uchida

Strongly Correlated Electrons

Observation of g-wave altermagnetic multipole

Ryo Misawa, Rikuto Oiwa, Shunsuke Kitou et al. (+16 more)

Over the past few years, altermagnets have emerged as a new class of collinear magnets with broken time-reversal symmetry, offering novel opportunities for spintronics beyond conventional magnets. Rather than from net magnetization, as in ferromagnets, the unconventional time-reversal symmetry breaking of altermagnets originates from antiferroic magnetic dipoles locked to higher-order multipoles. Here we report the direct visualization of a $g$-wave altermagnetic multipole in the canonical altermagnet CrSb. Combining high-energy synchrotron X-ray diffraction with valence electron density (VED) analysis, we uncover a pronounced directional anisotropy of the VED distribution alternating between Cr sublattices. This evidences the antiferroic order of electric hexadecapoles predicted in $g$-wave altermagnets. Its coexistence with antiferroic magnetic dipoles induces ferroic magnetic multipoles, as probed by polarized neutron diffraction. We further identify a microscopic model of altermagnetism that directly relates the $g$-wave multipole and the $g$-wave spin splitting. Through direct observation and quantification of multipoles, this study provides a real-space fingerprint of altermagnetism and establishes a general probe of hidden multipole order in quantum materials.
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All authors Ryo Misawa, Rikuto Oiwa, Shunsuke Kitou, Tatsuya Miki, Motohiko Ezawa, Weiyi Yun, Rinsuke Yamada, Chihaya Koyama, J. Alberto Rodríguez Velamazán, Kamil K. Kolincio, Navid Qureshi, Elina Zhakina, Yuiga Nakamura, Jan Masell, Ilya Belopolski, Taka-hisa Arima, Yusuke Nomura, Satoru Hayami, Max Hirschberger

Tuesday 1 September

Strongly Correlated Electrons

Evolution from Kitaev to XXZ spin chains via distortion: Application to BaCo$_2$V$_2$O$_8$

Philip Richard, Mandev Bhullar and Hae-Young Kee

BaCo$_2$V$_2$O$_8$ is a prototypical spin-orbit-coupled Ising-chain antiferromagnet that provides a unique platform for studying field-induced quantum magnetism. Under a transverse magnetic field, it exhibits unusual magnetic properties, including an anomalous staggered magnetization and a pronounced anisotropy of the critical field with respect to the in-plane field direction. While these phenomena have been attributed phenomenologically to a site-dependent anisotropic $g$-tensor, a recent microscopic theory has shown that spin-orbit coupling naturally generates bond-dependent Heisenberg, Kitaev, and $\Gamma$ exchange interactions. Here, we unify these two pictures by extending the microscopic theory to incorporate distortions of the CoO$_6$ octahedra. We show that the distortions not only generate the site-dependent anisotropic $g$-tensor but also renormalize the staggered exchange interactions through a distortion-induced contribution that partially compensates the Kitaev-derived staggered term. Using point-charge calculations to estimate the $g$-tensor of BaCo$_2$V$_2$O$_8$, we demonstrate that the strong anisotropy of the critical field originates from the combined effects of the modified exchange interactions and the anisotropic $g$-tensor. Our work provides a unified microscopic framework for understanding the magnetic anisotropy of spin-orbit-coupled Ising-chain materials.
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Strongly Correlated Electrons · Cross-listed from cond-mat.mtrl-sci

A Spatial Localizer for Constituent-Resolved Exciton Wannier Functions

Haylen Gerhard and Wladimir A. Benalcazar

Excitons are composite quasiparticles: beyond a center-of-mass position, each carries an internal electron-hole dipole governing its coupling to electric fields and other excitons. Exciton Wannier functions locally represent exciton bands, but resolving this dipole requires localizing the electron and hole simultaneously. We show that, in one dimension, the projected electron and hole position operators fail to commute when the covariant derivative of the quantum geometric dipole (QGD) matrix (the difference between the hole and electron non-Abelian Berry connections) is nonzero. This precludes a common eigenbasis and bounds the joint electron-hole spread from below. For one band, the internal dipole is gauge invariant and center-of-mass methods suffice; for multiple bands, no existing construction yields a gauge minimizing both position uncertainties. We introduce an “exciton spatial localizer,” a Hermitian operator embedding both projected positions in a Clifford-algebra structure. Its spectral minima locate the exciton's center-of-mass and dipole coordinates, while its eigenvectors yield exciton Wannier functions jointly localized in electron and hole coordinates without gauge fixing, an ansatz, or iterative optimization. In an interacting bilayer model, combined reflection–time-reversal symmetry or a nonsymmorphic particle–hole symmetry forces the QGD matrix to be traceless at every momentum while allowing it to remain nonzero. A two-band exciton subspace with zero net internal dipole then decomposes into a symmetry-related pair of exciton Wannier functions with opposite center-of-mass positions and internal dipoles. Adding the interlayer dipole as a Clifford component further separates intralayer and interlayer exciton Wannier functions in a six-band subspace.
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Strongly Correlated Electrons · Cross-listed from quant-ph

Bilinear correlations in Fluctuating Gaussian States with anti-unitary symmetries

Xu Zhang

We study bilinear order-parameter correlations in a general class of sign problem-free systems that are described by what we call Fluctuating Gaussian States (FGS) with anti-unitary symmetries, where the weight of each Gaussian measurement in the space-time path integral is positive-definite. Supported by Monte Carlo simulations on fluctuating Gaussian fermionic and bosonic examples, we argue that such systems generally exhibit two types of broken-symmetry phases: a saddle point phase and a non-local contraction phase, with the competition between the two determined by the stability of the fluctuation saddle point of FGS. Due to the Fermi liquid instability in fermionic FGS with anti-unitary symmetries, we also discuss the possibility of representing fermionic FGS with a particular Projected Entangled-Pair State (PEPS) ansatz, which we argue a certain construction provides an efficient description for the saddle point phase, but not for the non-local contraction phase. Finally, we discuss the potential implication of our result in non-equilibrium systems and stabilizing a target bilinear order.
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Strongly Correlated Electrons · Cross-listed from quant-ph

Depth-1 expanders on the unitary group and applications

Anurag Anshu, Shankar Balasubramanian, Jonas Haferkamp et al. (+2 more)

We construct a constant-degree and constant-gap quantum expander on $n$ qubits where each unitary can be implemented by a depth-$1$ and 1D circuit of Pauli or CNOT gates. We provide two applications of this expander. First, we use it to construct a family of frustration-free 1D Hamiltonians whose ground states obey the entanglement-gap relation $S = \Theta(\Delta^{-1/2})$; this is believed to be optimal, but achieving it had been open. Second, we use it to provide a streaming protocol that tests for closeness to a class of 1D volume-law entangled states. Moreover, we extend our quantum expander to a constant-degree and constant-gap expander on the unitary group where each unitary is a single $T$ gate, a single $T^{\dagger}$ gate, or a depth-$1$ Clifford circuit. This implies that a random sequence of unitaries from the expander yields a gapped walk on a dense subgroup of the unitary group. This improves upon previous work by Bourgain and Gamburd which did not control the dependence of the gap on the dimension.
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All authors Anurag Anshu, Shankar Balasubramanian, Jonas Haferkamp, Aram W. Harrow, Xinyu Tan

Strongly Correlated Electrons

Self-Healing Diffusion Monte Carlo applied to a simple fermionic model: A critical assessment of the method

Michel Caffarel, Manon Pinar and Anthony Scemama

We investigate the Self-Healing Diffusion Monte Carlo (SHDMC) method using a one-dimensional model with periodic boundary conditions. An inversion symmetry is introduced to mimic the antisymmetry property of fermionic wave functions, with the bosonic and fermionic sectors being modeled by the even and odd eigenstates, respectively. As in realistic fermionic systems, the nodal structure is only partially constrained by symmetry, making this model a non-trivial testbed for nodal optimization algorithms such as SHDMC. We show that the nodal evolution under SHDMC iterations can be cast into a dynamical system exhibiting both attractive and repulsive fixed points. When applying SHDMC to this model in the absence of statistical noise, the fixed-node energy is found to increase during iterations and the node converges to an incorrect value. This occurs for any finite basis, even when the exact node is representable in it; the attractive fixed point approaches the exact node only as the basis becomes complete, while the convergence toward it becomes increasingly slow. We further show that this problem can be largely cured by modifying the nodal update criterion to give more importance to the nodal region. Our results suggest that, in the general case, an efficient nodal optimization may require both an improved nodal update criterion and a localized basis set allowing local distortions of the trial wave function, as is the case for this model.
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Strongly Correlated Electrons · Cross-listed from quant-ph

A nonabelian anyon violates Haag duality

Daniel Wallick and Henrik Wilming

We show that a superselction sector describing a single nonabelian anyon violates Haag duality, which is equivalent to the fundamental quantum-information theoretic principle of uniqueness of purifications. We provide a simple physics argument as well as a rigorous proof using sector theory, and instantiate our result concretely in Levin-Wen models. We also show that the associated ground state does not allow for quantum steering despite being a pure state. Finally, we show that this ground state violates approximate Haag duality, disproving the conjecture that all gapped ground states satisfy this condition. Our result implies that there are gapped phases of matter where Haag duality fails at every point in the phase.
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Strongly Correlated Electrons · Cross-listed from cond-mat.supr-con

Resolving unconventional gap structure in kagome superconductors with hybrid microwave circuits

Yejin Lee, Haolin Jin, Sushmita Chandra et al. (+12 more)

Unconventional superconductivity is a hallmark of exotic quantum matter, where determining the pairing symmetry is essential for uncovering its microscopic origin. Kagome superconductors provide a fertile landscape for emergent phenomena arising from strong electronic correlations and nontrivial band topology, yet their superconducting pairing symmetry remains elusive. The superconducting gap structure is commonly probed via electrodynamic response, but such measurements are inapplicable to thin flakes due to their small volume and delicate nature. Superconducting microwave resonators offer a coherent and highly sensitive platform for probing electrodynamic responses, with versatile designs that enable incorporation of diverse materials and geometries. Here, we integrate flakes into microwave circuits, enabling noninvasive access to the superfluid response through contactless coupling that preserves structural integrity. By engineering the device geometry to suppress parasitic two-level-system losses that dominate dissipation in microwave circuits, we isolate the intrinsic material response. Remarkably, the temperature-dependent superfluid density exhibits linear behavior at low temperatures, consistent with a nodal gap structure. Our approach establishes a noninvasive platform for probing electrodynamic response of fragile superconducting flakes while advancing hybrid microwave architectures for quantum technologies.
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All authors Yejin Lee, Haolin Jin, Sushmita Chandra, Berit H. Goodge, Edouard Lesne, Tommaso Confalone, Francesco Tafuri, Davide Massarotti, Golam Haider, Kornelius Nielsch, Bernd Büchner, Claudia Felser, Debanjan Chowdhury, Nicola Poccia, Uri Vool

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

Magnetic quantum defects in a uniaxial antiferromagnetic insulator

Shangfei Wu, Laur Peedu, Zhihao Wang et al. (+12 more)

Point defects have been successfully utilized in various quantum technologies, serving as quantum qubits for quantum computation, single-photon emitters for quantum communication, and nanoscale sensors for quantum metrology. However, their further development faces key challenges, particularly in discovering and exploring suitable defect-host systems that meet the necessary criteria for quantum applications. Here, using polarization-resolved Raman spectroscopy and terahertz absorption spectroscopy, we discover three distinct chromium-vacancy-induced excitations in the uniaxial antiferromagnetic insulator, Cr$_2$O$_3$. These vacancy-induced excitations have an energy scale of a few tens of millielectronvolts and are twofold degenerate, and the lowest one at 64 $cm^{-1}$ is sharp and sensitive to the external magnetic field along the easy-axis direction, particularly close to the spin-flop regime around 6T, where the mode softens from 64 to 27$cm^{-1}$. Based on the defect supercell first-principles calculations, we interpret the mode at 64 $cm^{-1}$ as a local magnetic excitation of the local moment within the chromium vacancy state. Our results establish that the magnetic defect states in Cr$_2$O$_3$ have potential for quantum applications.
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All authors Shangfei Wu, Laur Peedu, Zhihao Wang, Xuecong Wang, Xianghan Xu, Kai Du, Sang-Wook Cheong, Aleksei Boldin, Joosep Link, Ivo Heinmaa, Raivo Stern, Sai Mu, Urmas Nagel, Toomas Rõõm, Girsh Blumberg

Strongly Correlated Electrons

Correlation-Driven Nonlinear Magnetoelectric Response in an Altermagnet: A Dynamical Mean-Field Study

Robert Peters and Jun Ōiké

We investigate the optical nonlinear magnetoelectric effect (NMEE) in a strongly correlated altermagnet using dynamical mean-field theory. Unlike effective band descriptions with an imposed spin splitting, our approach determines the altermagnetic order, electronic spectrum, and optical nonlinear response self-consistently. We find that the NMEE is finite in the altermagnetic phase and vanishes in the paramagnetic phase. Its frequency dependence reflects the spin-resolved spectral structure and provides an estimate of the characteristic altermagnetic spin-splitting scale. Interaction and temperature tuning produce qualitatively different behavior: at low temperature, reducing the interaction strength toward the interaction-driven magnetic phase boundary enhances the response, whereas increasing the temperature suppresses it and drives it to zero above the critical temperature. These results establish the optical NMEE as a probe of correlated altermagnetic order and suggest that tuning parameters such as pressure, strain, or chemical substitution toward an interaction-driven phase boundary may provide a promising route to maximizing the response.
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Strongly Correlated Electrons · Cross-listed from cond-mat.mtrl-sci

GW and Bethe-Salpeter Theory for Molecular Polaritons, Quasiparticles, and Excitons

Soohaeng Yoo Willow, Gi Beom Sim, Tae Hyeon Park et al. (+6 more)

The electron self-energy is central to quasiparticle theory, yet how an optical cavity enters it remains unclear. We address this question for a molecule in a single-mode cavity using the dipole-gauge Pauli-Fierz Hamiltonian and a coherent-state QED Hartree-Fock reference. The cavity enters through three channels: the static dipole self-energy (DSE) shift of reference orbital energies, direct DSE augmentation of the screened interaction, and the polariton pole carrying the bilinear electron-photon coupling. We benchmark QED-$GW$ ionization potentials (IPs) and electron affinities (EAs) against a cavity $\Delta$-method ladder from QED-HF to correlated wave-function methods, whose cavity-induced shifts agree within 1 meV where directly comparable. For closed-shell molecules with unbound anions, $GW$ systematically overestimates cavity-induced IP redshifts, whereas EA shifts are reproduced nearly quantitatively, although this does not imply comparable accuracy for absolute EAs. For ionic molecules with bound anions, this ordering reverses, consistent with published QED coupled-cluster results. Coupling and detuning scans show that the error is predominantly quadratic in $\lambda$ and DSE-driven rather than resonant. The spectral function develops a polariton-replica photoemission sideband with weight scaling as $\lambda^2$. In the static screened interaction used in the Bethe-Salpeter equation, bare-photon exchange cancels the matching DSE contribution to the direct interaction, while exchange and polariton-screening corrections remain. Their net effect on the lowest excitation is appreciable only for ammonia in the molecules studied. Exciton-binding energies involving unbound anions are strongly basis-dependent and should therefore be viewed as diagnostics of electron-hole interactions rather than basis-converged molecular quantities.
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All authors Soohaeng Yoo Willow, Gi Beom Sim, Tae Hyeon Park, Tae In Kim, D. ChangMo Yang, Mikuláš Matoušek, Jiří Brabec, Libor Veis, Chang Woo Myung

Monday 31 August

Strongly Correlated Electrons

Bound states, resonances, and their thermodynamic properties in pseudospin-1 systems with short-range impurities

E. V. Gorbar and Pavlo Sukhachov

Bound states and resonances induced by short-range impurities modeled by circular potential wells are analyzed in the vicinity of flat and dispersive bands in gapped and gapless pseudospin-1 systems. We find that the bound and resonant states derived from the flat band show unusual characteristics originating from the multicomponent structure of pseudospin-1 fermions, which are distinct from those for pseudospin-$\tfrac{1}{2}$ fermions. Contrary to gapped Dirac systems and unlike bound states in the vicinity of the upper dispersive band, the bound states derived from the flat band occur for any value of the total angular momentum. The energies of these bound states with higher angular momentum $j$ tend to decrease with $|j|$. In addition, it is found that their wave functions are localized at the potential well edge and the localization increases with $|j|$. The signatures of the impurity states in the local density of states are determined. Using the Anderson model for independent electrons in the disorder potential, the thermodynamic potential, entropy density, and heat capacity are obtained. In the regime dominated by bound states derived from the flat band, the entropy density monotonically increases with temperature and saturates, whereas the heat capacity exhibits a single maximum.
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Strongly Correlated Electrons

Engineering tunable $p$-wave magnetism in antiferromagnetic bilayers

Yu-Han Lin, Jin-Wei Dong, Ziqiang Wang et al. (+1 more)

We propose a symmetry-guided route to engineer tunable $p$-wave magnetism in a bilayer system composed of two AB-stacked antiferromagnetic square lattices with their collinear moments nonparallel to each other. We show that an in-plane relative shift between the two layers selectively breaks the symmetries protecting spin degeneracy while preserving time-reversal-related constraints, thereby generating odd-parity spin splitting in a fully compensated magnetic state without spin-orbit coupling. The direction and the magnitude of the resulting $p$-wave spin-splitting, together with the associated spin responses, can be continuously tuned by the in-plane displacement, providing a potential knob for control. We further map the bilayer system onto an effective bond-modulated square lattice and investigate the corresponding Hubbard model within a mean-field framework. The calculated phase diagram reveals extended regions where $p$-wave magnetism with coplanar or chiral spin textures emerges spontaneously. Our results establish a minimal equilibrium platform for realizing and manipulating $p$-wave magnetism in two-dimensional systems.
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All authors Yu-Han Lin, Jin-Wei Dong, Ziqiang Wang, Sen Zhou

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

Entanglement from particle number fluctuations in a second-order topological insulator

Mahla Moridi Farimani and Kim Pöyhönen

Higher-order topological insulators are materials with a bulk energy gap featuring gapless modes at $(d-n)$-dimensional edges, where $n > 1$. Despite this modified bulk-boundary correspondence, their nontrivial topology can nevertheless be observed through the bipartite entanglement spectrum. Inspired by works suggesting fluctuations in conserved quantities could be used to study topology in one-dimensional systems, which likewise feature zero-dimensional edge modes, we extend this treatment to a two-dimensional second-order topological insulator where corner states play an analogous role. We show that the standard bipartite entanglement entropy and particle number fluctuation lack striking signals of topological phase transitions, both in the static case and in the time evolution after a quench, obscured by an area-law term from the subsystem edges. By introducing a quadripartite construction of the subsystem, we show that it is possible to isolate the topological contributions, producing sharp transition peaks in both quantities, observable by studying subsystems much smaller than the full system. As particle number fluctuations are accessible in existing experiments, these results establish a concrete and scalable path to experimental detection of the entanglement in higher-order topological insulators.
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