arXiv subjects

Matthias Vojta

Publications and source records attributed to Matthias Vojta.

At least 19 recordsLinked to original sources

arXiv2026

Quantum spin-glass criticality in disordered frustrated dimer magnets

We study quantum phase transitions of Mott insulators between spin-glass and featureless paramagnetic phases. Specifically, we consider a triangular-lattice bilayer Heisenberg model with bond disorder. The clean system has two phases, a dimer quantum paramagnet and a non-collinear antiferromagnet, separated by a quantum critical point. Bond disorder destroys the antiferromagnetic phase via the interference of dipolar textures and results in spin-glass order, such that the system features a quantum phase transition between spin-glass and dimer phases. We study the vicinity of this transition using a variant of bond-operator theory and calculate thermodynamic observables as well as excitation spectra. Bond disorder leads to strong inhomogeneities near the transition which suppresses non-collinearities and leads to anomalously weak glassiness in the near-critical quantum spin glass. We characterize the low-energy excitations which have a strong tendency towards spatial localization, and we track the behavior of the amplitude (i.e. Higgs) mode across the glass phase whose spectral weight we find to be strongly suppressed due to interference effects.

cond-mat.str-el↗

arXiv2026

Unveiling Magnetic Frustration via the Elastocaloric Effect

Motivated by experimental progress in pressure and strain tuning of quantum materials, we examine the thermodynamic response of frustrated magnets to uniaxial strain. Specifically, we study Ising and Heisenberg models on spatially anisotropic triangular (and, for the Ising model, also kagome) lattices. We determine the entropy as a function of temperature and strain, and use it to compute the elastic Gr\"uneisen ratio $\eta$. The Ising models can be strain-tuned into and out of classical spin-liquid phases, and we show that $\eta$ can become arbitrarily large at low temperature $T$ near the point of maximal frustration, a universal hallmark of an extensive ground-state entropy. In contrast, the spin-$1/2$ Heisenberg model is moderately frustrated and displays multiple $T=0$ phase transitions. These transitions dominate $\eta$ at low $T$ while the intermediate-$T$ behavior is similar to that of the Ising model. We discuss the extent to which the elastic Gr\"uneisen ratio can be used to deduce the phase diagram, and we connect our results to recent experiments on triangular-lattice magnets.

cond-mat.str-el↗

arXiv2026

Field-driven phases in a three-dimensional twisted Kitaev model for CoNb$_2$O$_6$: Interplay of frustration and spin-orbit coupling

The Ising chain in a transverse field stands out as a paradigmatic example for a quantum phase transition. CoNb$_2$O$_6$ has been discussed as a material realization of this physics, but it was later realized that its magnetic exchange couplings are more complicated, taking the form of twisted Kitaev chains. Here we study a three-dimensional model for CoNb$_2$O$_6$, taking into account both Kitaev physics and frustrated inter-chain coupling, in applied magnetic field. Using semiclassical techniques at zero temperature, we map out the sequence of field-driven phases for arbitrary field direction; these include phases with commensurate and incommensurate inter-chain order. As a result of spin-orbit coupling, the phase diagram is extremely sensitive to small changes in the field angle. We compute static observables as well as magnetic excitation spectra in the various phases and connect our results to existing experimental data.

cond-mat.str-el↗

arXiv2026

Quantum Coulomb Liquids of Different Rank in the Breathing Pyrochlore Antiferromagnet

Emergent gauge fields and Coulomb liquids have long been central to the physics of frustrated pyrochlore magnets, yet their realization beyond conventional, i.e. rank-1 $U(1)$, spin ice and into fully quantum higher-rank regimes has remained elusive. Here we provide a controlled demonstration of this physics in the spin-$\tfrac{1}{2}$ quantum Heisenberg antiferromagnet on the breathing pyrochlore lattice with symmetry-allowed Dzyaloshinskii--Moriya interactions, using the pseudofermion functional renormalization group. We show that tuning the breathing asymmetry stabilizes extended quantum analogues of both rank-1 and rank-2 $U(1)$ Coulomb liquids within a single microscopic model, directly distinguished by their characteristic pinch-point morphologies in momentum space. This provides the first controlled quantum realization in three dimensions where gauge theories of different rank emerge within a single microscopic spin Hamiltonian. In addition, quantum fluctuations qualitatively reshape the classical nearest-neighbor atlas of phases, causing an incommensurate spiral instability and an extended quantum-disordered regime without dipolar order, both absent from the classical model. Our results establish the breathing pyrochlore as a timely and experimentally relevant platform where higher-rank gauge constraints, conventional magnetic order, and fluctuation-driven quantum phases compete on equal footing, opening a direct route to diagnosing emergent gauge structure in three-dimensional quantum magnets.

cond-mat.str-el↗

arXiv2025

Quantum effects on pyrochlore higher-rank U(1) spin liquids: Pinch-line singularities, spin nematics, and connections to oxide materials

Motivated by the magnetism of pyrochlore oxides, we consider the effect of quantum fluctuations in the most general symmetry-allowed nearest-neighbor Kramers exchange Hamiltonian on the pyrochlore lattice. At the classical level, this Hamiltonian exhibits a rich landscape of classical spin liquids and a variety of nonconventional magnetic phases. In contrast, much remains unclear for the quantum model, where quantum fluctuations have the potential to alter the classical landscape and stabilize novel magnetic phases. Employing state-of-the-art pseudo-fermion functional renormalization group calculations for the spin-$1/2$ model, we determine the quantum phase diagram at relevant cross-sections, where the classical model hosts an algebraic nodal rank-2 spin liquid and a spin nematic order. We find large regions in parameter space on which dipolar magnetic order is absent and, based on known fingerprints in the correlation functions, we suggest that this nonconventional region is composed of an ensemble of distinct phases stabilized by quantum fluctuations. Our results hint at the existence of a spin nematic phase, and we identify the quantum analog of the classical rank-2 spin liquid. Furthermore, we highlight the importance of assessing the subtle interplay of quantum and thermal fluctuations in reconciling the experimental findings on the nature of magnetic order in Yb$_2$Ti$_2$O$_7$.

cond-mat.str-el↗

arXiv2025

Classical spin liquids from frustrated Ising models in hyperbolic space

Antiferromagnetic Ising models on frustrated lattices can realize classical spin liquids, with highly degenerate ground states and, possibly, fractionalized excitations and emergent gauge fields. Motivated by the recent interest in many-body system in negatively curved space, we study hyperbolic frustrated Ising models. Specifically, we consider nearest-neighbor Ising models on tesselations with odd-length loops in two-dimensional hyperbolic space. For finite systems with open boundaries we determine the ground-state degeneracy exactly, and we perform extensive finite-temperature Monte-Carlo simulations to obtain thermodynamic data as well as correlation functions. We show that the shape of the boundary, constituting an extensive part of the system, can be used to control low-energy states: Depending on the boundary, we find ordered or disordered ground states. Our results demonstrate how geometric frustration acts in curved space to produce classical spin liquids.

cond-mat.str-el↗

arXiv2025

Fractionalized Superconductivity Mediated by Majorana Fermions in the Kitaev-Kondo Lattice

Superconductivity usually emerges from a metallic normal state which follows the Fermi-liquid paradigm. If, in contrast, the normal state is a fractionalized non-Fermi liquid, then pairing may either eliminate fractionalization via a Higgs-type mechanism leading to a conventional superconducting state, or pairing can occur in the presence of fractionalization. Here we discuss a simple model for the latter case: Using a combination of perturbation theory and functional renormalization group, we show that the Kitaev--Kondo lattice model displays a fractionalized superconducting phase at weak Kondo coupling. This phase is characterized by Cooper pairing of conventional electronic quasiparticles, coexisting with a spin-liquid background and topological order. Depending on the sign of the Kitaev coupling, we find the pairing to be either of chiral $d$-wave or $p$-wave type for extended doping regions around the van-Hove filling. We discuss applications and extensions.

cond-mat.str-el↗

arXiv2025

Fragility of local moments against hybridization with flat bands

The Kondo screening of a localized magnetic moment crucially depends on the spectral properties of the electronic bath to which it is coupled. Unlike textbook examples, realistic systems as well as dynamical mean-field theory of correlated lattice models force us to consider sharp features in the hybridization function near the Fermi energy. Divergencies of this kind can play a relevant role in twisted bilayer graphene, for which local-moment formation and isospin entropy at finite temperature are currently under the spotlight. We clarify how a low-frequency singularity impacts the screening mechanisms by means of a toy model with a tunable $\delta$-peak in the hybridization function, superimposed to a regular part. Our analysis unveils an unexpectedly big impact on the local-moment physics already for a parametrically small weight of the flat band in the bath.

cond-mat.str-el↗

arXiv2025

Emergent Chiral Metal near a Kondo Breakdown Quantum Phase Transition

The destruction of the Kondo effect in a local-moment metal can lead to a topological non-Fermi-liquid phase, dubbed fractionalized Fermi liquid, with spinon-type excitations and an emergent gauge field. We demonstrate that, if the latter displays an internal $π$-flux structure, a chiral heavy-fermion metal emerges near the Kondo-breakdown transition. Utilizing a parton mean-field theory describing the transition between a conventional heavy Fermi liquid and a U(1) fractionalized Fermi liquid, we find a novel intermediate phase near the transition whose emergent flux pattern spontaneously breaks both translation and time-reversal symmetries. This phase is an orbital antiferromagnet, and we derive a Landau-type theory which shows that such a phase generically emerges from a $π$-flux spin liquid. We discuss the relevance to pertinent experiments.

cond-mat.str-el↗

arXiv2025

SU($N$) altermagnetism: Lattice models, magnon modes, and flavor-split bands

Altermagnetism, a type of magnetic order that combines properties of ferro- and antiferromagnets, has stirred great interest lately not only as a promising source of spintronics applications, but also as a potential gateway to exotic phases of matter. Here, we demonstrate how to generalize collinear altermagnetism to SU($N$) magnets with $N>2$. Guided by symmetry principles, we present a recipe to construct Heisenberg models for such generalized altermagnets and apply it explicitly for $N=3,4$. Using flavor-wave theory, we compute the excitation spectrum of a two-dimensional SU(3) model and show that it exhibits magnon bands with altermagnetic splitting according to magnetic quantum numbers; we connect this quantum-number splitting to the frequently used concept of magnon chirality. We also compute the electronic band structure for a metallic system of the same symmetry and map out the polarization of the resulting flavor-split bands.

cond-mat.str-el↗

arXiv2024

Bond disorder in extended Heisenberg-Kitaev models: Spin textures and in-gap states in the high-field regime

We study the effect of bond disorder in extended Heisenberg-Kitaev models on the honeycomb lattice, relevant for materials such as $α$-RuCl$_3$, in the semiclassical limit using a combination of T-matrix and real-space spin-wave approaches. Focusing on the regime of large applied magnetic field, we discuss two distinct but related disorder-induced phenomena, namely spin textures in the vicinity of isolated impurities and magnetic excitations below the bulk gap. A finite impurity concentration smears the field-tuned phase transition and turns the isolated in-gap states into impurity bands. As a result, there is a large field regime above the bulk transition into the high-field phase where impurity-induced states fill the bulk spin gap. We illustrate the field dependence of these in-gap states for parameters relevant for $α$-RuCl$_3$, and we connect our results to heat-transport and NMR data which indicated their presence.

cond-mat.str-el↗

arXiv2024

Hydride superconductivity: here to stay

The field of hydride superconductivity has recently been mired in a controversy that might divert attention from the question of central importance: do hydrides support genuine superconductivity or not? We examine some key papers from the field, and conclude that hydride superconductivity is real.

cond-mat.supr-con↗

arXiv2024

Critical properties of metallic and deconfined quantum phase transitions in Dirac systems

We characterize, by means of large-scale fermion quantum Monte Carlo simulations, metallic and deconfined quantum phase transitions in a bilayer honeycomb model in terms of their quantum critical and finite-temperature properties.The model features three different phases at zero temperature as function of interaction strength. At weak interaction, a fully symmetric Dirac semimetal state is realized. At intermediate and strong interaction, respectively, two long-range-ordered phases that break different symmetries are stabilized. The ordered phases feature partial and full, respectively, gap openings in the fermion spectrum. The first transition between the disordered and long-range-ordered semimetallic phases has previously been argued to be described by the $(2+1)$-dimensional Gross-Neveu-SO(3) field theory. By performing simulations with an improved symmetric Trotter decomposition, we further substantiate this claim by computing the critical exponents $1/ν$, $η_ϕ$, and $η_ψ$, which turn out to be consistent with the field-theoretical expectation within numerical and analytical uncertainties. The second transition between the two long-range-ordered phases has previously been proposed as a possible instance of a metallic deconfined quantum critical point. We further develop this scenario by analyzing the spectral functions in the single-particle, particle-hole, and particle-particle channels. Our results indicate gapless excitations with a unique velocity, supporting the emergence of Lorentz symmetry at criticality. We also compute the finite-temperature phase boundaries of the ordered states above the fully gapped state at large interaction. The phase boundaries vanishes smoothly in the vicinity of the putative metallic deconfined quantum critical point, in agreement with the expectation for a continuous or weakly-first-order transition.

cond-mat.str-el↗

arXiv2024

Discrete JT gravity as an Ising model

Inspired by the program of discrete holography, we show that Jackiw-Teitelboim (JT) gravity on a hyperbolic tiling of Euclidean AdS$_2$ gives rise to an Ising model on the dual lattice, subject to a topological constraint. The Ising model involves an asymptotic boundary condition with spins pointing opposite to the magnetic field. The topological constraint enforces a single domain wall between the spins of opposite direction, with the topology of a circle. The resolvent of JT gravity is related to the free energy of this Ising model, and the classical limit of JT gravity corresponds to the Ising low-temperature limit. We study this Ising model through a Monte Carlo approach and a mean-field approximation. For finite truncations of the infinite hyperbolic lattice, the map between both theories is only valid in a regime in which the domain wall has a finite size. For the extremal cases of large positive or negative coupling, the domain wall either shrinks to zero or touches the boundary of the lattice. This behavior is confirmed by the mean-field analysis. We expect that our results may be used as a starting point for establishing a holographic matrix model duality for discretized gravity.

hep-th↗

arXiv2024

Field-driven transition from quantum spin liquid to magnetic order in triangular-lattice antiferromagnets

Recently several triangular-lattice magnets with delafossite structure have been found to display spin-liquid behavior down to the lowest temperatures. Remarkably, applying a magnetic field destroys the spin liquid which then gives way to symmetry-breaking states, identified as semiclassical coplanar states including a magnetization plateau at 1/3 total magnetization. Here we provide a theoretical approach rationalizing this dichotomy, utilizing a Schwinger-boson theory that captures both ordered and disordered magnetic phases. We show that a zero-field spin liquid, driven by strong frustration, is naturally destabilized in a magnetic field via spinon condensation. Symmetry-breaking order akin to the standard triangular-lattice Heisenberg model then arises via an order-by-disorder mechanism. We discuss implications for pertinent experiments.

cond-mat.str-el↗

arXiv2024

Partial magnetic order in kagome spin ice

Motivated by the observation of partial magnetic order in kagome-based magnets, we study the classical kagome Ising antiferromagnet, known as kagome spin ice, including further-neighbor interactions at zero and finite temperature. While the nearest-neighbor model displays an extensive ground-state degeneracy, various symmetry-breaking states can appear upon including additional couplings. Among these, peculiar partially ordered states have been proposed. We present results from large-scale Monte-Carlo simulations, establishing that such partial order is stabilized by third-neighbor couplings along the hexagon diagonals. We show that these states arise due to the emergence of one-dimensional chains in an entirely frustrated environment, and that they are stable with respect to further couplings. We discuss their finite-temperature properties in detail, highlight the magnetic states' peculiarities depending on the diagonal couplings' sign, and suggest observing these states using thermodynamic probes.

cond-mat.str-el↗

arXiv2024

A Classical Chiral Spin-Liquid from Chiral Interactions on the Pyrochlore Lattice

Classical spin-liquids are paramagnetic phases which feature nontrivial patterns of spin correlations within their ground-state manifold whose degeneracy scales with system size. Often they harbor fractionalized excitations, and their low-energy fluctuations are described by emergent gauge theories. In this work, we discuss a model composed of chiral three-body spin interactions on the pyrochlore lattice that realizes a novel classical chiral spin-liquid. Employing both analytical and numerical techniques, we show that the ground-state manifold of this spin-liquid is given by a subset of the so-called color-ice states. We demonstrate that the ground states are captured by an effective gauge theory which possesses a divergence-free condition and an additional chiral term that constrains the total flux of the fields through a single tetrahedron. The divergence-free condition results in two-fold pinch points in the spin structure factor and the identification of bionic charges as the elementary excitations of the system. We discuss how the mobility of these elementary charges is restricted by the additional chiral term, which in turn suggests that these charges are fractons.

cond-mat.str-el↗

arXiv2024

Disorder effects in spiral spin liquids: Long-range spin textures, Friedel-like oscillations, and spiral spin glasses

Spiral spin liquids are correlated states of matter in which a frustrated magnetic system evades order by fluctuating between a set of (nearly) degenerate spin spirals. Here, we investigate the response of spiral spin liquids to quenched disorder in a $J_1$-$J_2$ honeycomb-lattice Heisenberg model. At the single-impurity level, we identify different order-by-quenched-disorder phenomena and analyze the ensuing spin textures. In particular, we show that the latter generally display Friedel-like oscillations, which encode direct information about the spiral contour, i.e., the classical ground-state manifold. At finite defect concentrations, we perform extensive numerical simulations and characterize the resulting phases at zero temperature. As a result, we find that the competition between incompatible order-by-quenched-disorder mechanisms can lead to spiral spin glass states already at low to moderate disorder. Finally, we discuss extensions of our conclusions to nonzero temperatures and higher-dimensional systems, as well as their applications to experiments.

cond-mat.str-el↗