arXiv subjects

Johannes Knolle

Publications and source records attributed to Johannes Knolle.

At least 37 records · Page 2Linked to original sources

arXiv2026

Odd-parity magnons in the Haldane-Hubbard model from topological exciton condensation

Odd-wave magnets are the counterparts to even-wave altermagnets realizing odd-parity spin splitting. Normally discussed for noncollinear systems, they have recently been shown to appear in collinear magnetic states in the presence of loop currents. Here we study collective excitations of the paramagnetic and magnetic phase of the seminal Haldane-Hubbard model. We identify the existence topological excitons in the paramagnetic phase, and their condensation as the driving mechanism into the collinear Néel state. The latter realizes an odd-wave magnet with odd-parity magnons displaying a characteristic $f$-wave splitting. We further uncover that an electron bandgap closing ensures magnon bandgap closing causing a change in odd-parity magnon topology, as well as a drastically enlarged spin splitting. Our results establish the presence of topological excitons and odd-parity magnons in the Haldane-Hubbard, with potential realizations in Floquet-driven materials and cold atomic gases.

cond-mat.str-el↗

arXiv2026

Raman Circular Dichroism and Quantum Geometry of Chiral Quantum Spin Liquids

We show that the quantum geometry of fractionalized spin excitations in Mott-insulating quantum spin liquids (QSL) gives rise to a finite Raman circular dichroism (RCD) signal. We demonstrate the equivalence between the Loudon-Fleury framework and the light-matter coupling approach for effective spinon bands. Using the latter, we derive an analytical decomposition of the RCD into different contributions of the quantum geometry. This reveals the sensitivity of the RCD to the underlying structure of the wave functions and the handedness of the excitations, rather than a nonzero Chern number of spinon excitations. To illustrate this, we apply our approach to two examples, the Kitaev honeycomb model in a magnetic field and a chiral $U(1)$ QSL on the triangular lattice, and discuss its experimental relevance for candidate materials.

cond-mat.str-el↗

arXiv2026

Fate of Topological Dirac Magnons in van der Waals Ferromagnets at Finite Temperature

Dirac magnons, the bosonic counterparts of Dirac fermions in graphene, provide a unique platform to explore symmetry-protected band crossings and quantum geometry in magnetic insulators, while promising high-velocity, low-dissipation spin transport for next-generation magnonic technologies. However, their stability under realistic, finite-temperature conditions remains an open question. Here, we develop a comprehensive microscopic theory of thermal magnon-magnon interactions in van der Waals honeycomb ferromagnets, focusing on both gapless and gapped Dirac magnons. Using nonlinear spin-wave theory with magnon self-energy corrections and a T-matrix resummation that captures two-magnon bound states, we quantitatively reproduce temperature- and momentum-dependent energy shifts and linewidths observed experimentally in the gapless Dirac magnon material CrBr$_3$, even near the Curie temperature. Our approach resolves discrepancies between prior theoretical predictions and experiment and highlight the significant role of bound states in enhancing magnon damping at low temperatures. For gapped Dirac magnon materials such as CrI$_3$, CrSiTe$_3$, and CrGeTe$_3$, we find a thermally induced reduction of the topological magnon gap but no evidence of thermally driven topological transitions. Classical atomistic spin dynamics simulations corroborate the gap' s robustness up to the Curie temperature. Furthermore, we establish a practical criterion for observing topological gaps by determining the minimum ratio of Dzyaloshinskii-Moriya interaction to Heisenberg exchange required to overcome thermal broadening throughout the ordered phase, typically around 5%. Our results clarify the interplay of thermal many-body effects and topology in low-dimensional magnets and provide a reliable framework for interpreting spectroscopic experiments.

cond-mat.str-el↗

arXiv2026

Meson spectroscopy of exotic symmetries of Ising criticality in Rydberg atom arrays

The Ising model serves as a canonical platform for exploring emergent symmetry in quantum critical systems. The critical point of the 1D Ising chain is described by a conformal Ising field theory, which remains integrable in the presence of a magnetic perturbation, leading to massive particles associated with the exceptional Lie algebra $E_8$. Weakly coupling two Ising chains into a ladder breaks this integrability and is predicted to confine the elementary excitations of each chain into a richer spectrum of bound states organized by a $\mathcal{D}^{(1)}_8$ symmetry. Experimental signatures of $E_8$ excitations have arguably been observed in scattering studies of the spin chain material CoNb$_2$O$_6$, but direct evidence of confinement driven by inter-chain coupling has remained elusive. Here, we probe these emergent symmetries in a Rydberg atom quantum processing unit, leveraging its tunable geometry to realize both chain and ladder configurations. We identify mass spectra consistent with $E_8$ at the single-chain critical point and, in the weakly coupled ladder, report the first signatures of confinement of Ising excitations into the bound-state spectrum predicted by $\mathcal{D}^{(1)}_8$ symmetry. Our results demonstrate the power of Rydberg platforms for investigating symmetry emergence in quantum many-body systems and provide a direct window into the interplay of confinement, geometry, and criticality.

quant-ph↗

arXiv2026

Dichroic Raman probes for chiral edge modes

The identification and manipulation of charge-neutral fractionalized quasi-particles, in particular chiral edge modes (CEM), is a long-standing quest in physics. Remarkably, the microscopically mediated interaction between light and charge-neutral excitations in Mott-Hubbard insulators can take an identical form to the Raman coupling between light and particles with electric charge. However, since CEMs are Raman-inactive due to conservation of lattice momentum, Raman probes have been deemed unsuitable for their identification. Here, using the Kitaev quantum spin liquid (KSL) as an illustrative example, we demonstrate that the long-range correlated disorder inherent to a closed edge can lead to a Raman circular dichroism (RCD) signal that avoids suppression by linear and angular momentum selection rules, and exhibits a dependence on experimentally tunable length and energy scales that are characteristic of CEM. Having calculated the low-frequency RCD response of generic KSL, we argue that the interaction of the chiral matter fermion with the $\Ztwo$ boundary charge leaves a unique fingerprint of the KSL via the anisotropic Zeeman field dependence.

cond-mat.str-el↗

arXiv2026

Dynamical dimer structure factor of the triangular $S=1/2$ Heisenberg antiferromagnet

The dynamical dimer structure factor is an observable probing spin-singlet excitations of quantum magnets distinct from those commonly studied by the spin structure factor. We report the dimer response for the extended spin-$1/2$ antiferromagnetic Heisenberg model on the triangular lattice using large-scale GPU-accelerated matrix-product-state simulations. We investigate the ordered phases with $120^\circ$ coplanar, collinear stripe, and tetrahedral spin order, as well as candidate quantum spin-liquid (QSL) regimes, comprising an expected gapless $U(1)$ Dirac QSL and a chiral QSL at finite spin-scalar-chirality coupling. In the ordered phases, we find low-energy modes below the onset of the two-magnon continuum illustrating avoided quasiparticle decay. Within the candidate gapless QSL, we observe absolute dispersion minima at momenta of half the Brillouin zone corners, $X\equiv K/2$, in agreement with field-theory predictions that singlet monopole excitations of the $U(1)$ Dirac spin liquid become gapless at these points. Thus, the high-resolution dynamical dimer response provides support for a $U(1)$ Dirac QSL with singlet monopole excitations.

cond-mat.str-el↗

arXiv2026

Uncovering Exotic Paired States in the 2D Spin-Imbalanced Fermi Gas with Neural Wave Functions

We study the zero-temperature phase diagram of the 2D spin-imbalanced Fermi gas with short-ranged attractive interactions using the recently developed neural network variational Monte Carlo method with the AGPs FermiNet Ansatz. The Fulde-Ferrell-Larkin-Ovchinnikov phase is observed in the weakly interacting BCS limit and a polarised superfluid is seen in the strongly interacting BEC limit. When the interactions are strong, the minority-spin momentum density is reduced almost to zero in the momentum-space region occupied by the unpaired majority-spin electrons. When the interactions are very strong, phase separation occurs, with regions containing bosonic pairs and unpaired regions occupied by the remaining majority-spin particles. In addition, we observe translational symmetry breaking at intermediate interaction strengths, where the system forms an exotic crystal of Cooper pairs in a Fermi fluid of unpaired majority-spin particles. We provide a possible explanation for the formation of the crystalline phase, explain the origins of the k-space momentum-density hole when the pairs are tightly bound, and discuss how our approach opens new directions for future work.

cond-mat.quant-gas↗

arXiv2026

Genuine quantum scars in Floquet chaotic many-body systems

Unstable periodic orbits act as organizing structures for classical chaotic systems and underpin quantum scarring. Long known in single-particle systems, genuine quantum scars based on unstable periodic orbits have been recently extended to isolated many-body systems for time-independent Hamiltonians. Their fate under periodic driving, however, remains largely uncharted, challenged by the expectation that these systems should in general heat to infinite temperature. Here, we investigate how genuine scarring competes with the drive in a Floquet many-body system. Using chaotic spin chains, we demonstrate that Floquet states remain scarred in the high-frequency limit. Beyond this static correspondence, we uncover additional, driving-induced Floquet scars with no static analog. We construct a rich dynamical stability diagram with intermediate-frequency regimes of enhanced and quenched scarring, which we understand with a classical analysis of the Lyapunov exponent. Our results position Floquet systems as a natural platform for tuning the scarring behavior of quantum many-body systems.

cond-mat.stat-mech↗

arXiv2026

Interaction-Mediated Non-Reciprocal Dynamics in Open Quantum Systems: From an Exactly Solvable Model to Generic Behavior

Reservoir engineering has emerged as a powerful paradigm to realize non-reciprocal dynamics in open quantum many-body systems. Here, we show that density-density interactions can transfer bath-induced non-reciprocity between different degrees of freedom. Specifically, we investigate a one-dimensional lattice of spin-$\frac{1}{2}$ fermions with all-to-all Hatsugai-Kohmoto interactions in the presence of an engineered reservoir. We establish the exact solvability of the Lindbladian dynamics and show that the interplay between non-reciprocity and interactions qualitatively reshapes the dynamics of excitations. Remarkably, interactions induce directional drift even in spin sectors that are not directly coupled to the reservoir. By analyzing a driven-dissipative Fermi-Hubbard chain, we show that the same mechanism persists for local interactions. The Hatsugai-Kohmoto model thus emerges as a minimal, exactly solvable platform for interaction-mediated non-reciprocal many-body dynamics.

quant-ph↗

arXiv2026

Proximate quantum spin liquids and Majorana continua in magnetically ordered Kitaev magnets

We study the spin excitation spectra in magnetically ordered phases proximate to the Kitaev quantum spin liquid (KQSL). Although the low-energy universal features should be governed by the magnetic orders, the $\textit{non-universal}$ high-energy features of the KQSL and adjacent phases can be remarkably similar. Therefore, we study the extended Kitaev model within a Stoner-like theory using Majorana partons, and compute the inelastic neutron scattering (INS) intensities in the random phase approximation. First, we benchmark against the antiferromagnetic (AFM) Heisenberg model and recover the AFM order with linear Goldstone modes. We then explore the phase diagram which agrees qualitatively with previous numerical results. In particular, the Majorana parton theory accurately captures Order-by-Disorder effects in the Kitaev-Heisenberg limit. We also find large INS intensities near the associated high-symmetry Brillouin zone (BZ) points of the magnetic orders. At intermediate and high energies, broad multi-spinon continua emerge across the BZ, providing a distinct mechanism for magnon decay and spectral broadening beyond the conventional multi-magnon decay scenario. Finally, we study the model Hamiltonian of candidate Kitaev material $α$-RuCl$_3$. The zigzag ground state agrees qualitatively with experiments, its stability under external magnetic field also exhibits strong anisotropy in the field directions, and broad scattering continua are recovered similar to those observed experimentally.

cond-mat.str-el↗

arXiv2026

Current-tunable room temperature ferromagnetism and current-driven phase transitions

It is generally assumed that the application of a charge-current in ferromagnetic metals suppresses their ferromagnetic order through trivial Joule heating. Here, we demonstrate that a charge current can instead enhance magnetic ordering. Using a WTe2/Fe3Ge2Te (FGT) stack as a model system, we show that a charge current flowing in WTe2 controls the ferromagnetic properties and magnetic phase transition of the adjacent FGT via a current-induced effective magnetic-field arising from orbital magnetization. Remarkably, the charge current drives a substantial enhancement of the Curie temperature, boosting it well above room temperature. Furthermore, we show that the charge-current enables controlled tuning of the phase transitions in FGT, which confirms the scaling behaviour of a ferromagnet-paramagnet phase transition. This work provides a pathway for integrating two-dimensional ferromagnets into spintronic functionalities at technologically relevant temperatures and for exploring novel current-driven phenomena in ferromagnetic systems.

cond-mat.mtrl-sci↗

arXiv2026

Tuning the magnetic properties of Kitaev materials via the antiferromagnetic proximity effect: Novel phases and application to an $α$-RuCl$_3$/MnPS$_3$ bilayer

In recent years, the increasing level of control over van der Waals (vdW) heterostructures has opened new routes to tune the properties of quantum materials. Motivated by these developments, we examine the potential consequences of interfacing a Kitaev honeycomb magnet, such as $α$-RuCl$_3$, with a nearly lattice-matched vdW antiferromagnet. By combining perturbation theory, exact diagonalization, and a classical energy-minimization method, we show that an effective staggered magnetic field originating from the vdW antiferromagnet can drive a monolayer of a Kitaev material into various novel phases, including an antichiral Kitaev spin liquid, a nonmagnetic nematic phase, and different types of skyrmion crystals. We then apply first-principle simulations to assess the prospect of concretely realizing this setup in a heterobilayer of $α$-RuCl$_3$ and the easy-axis antiferromagnet MnPS$_3$.

cond-mat.str-el↗

arXiv2026

Thermal Hall conductivity from semiclassical spin dynamics simulations: implementation and applications to chiral ferromagnets and Kitaev magnets

We investigate thermal Hall transport in magnetic systems, using semiclassical spin dynamics simulations. Building on a linear response framework, we discuss the intricacies of computing the thermal Hall conductivity from real-time energy current correlations and the energy magnetization. We then apply this methodology to two models: a square-lattice chiral magnet with in-plane Dzyaloshinskii-Moriya interaction, and the antiferromagnetic Kitaev model in a field. Our results demonstrate the efficiency of semiclassical spin dynamics to study thermal Hall transport capturing quantitative effects beyond the simple intrinsic non-interacting approximation. They can serve as a benchmark for comparison with experiments in regimes where non-linearities from magnon-magnon interactions and strong thermal fluctuations play a crucial role.

cond-mat.str-el↗

arXiv2026

Exciton Berryology

In translationally invariant semiconductors that host exciton bound states, one can define an infinite number of possible exciton Berry connections. These correspond to the different ways in which a many-body exciton state, at fixed total momentum, can be decomposed into free electron and hole Bloch states that are entangled by an exciton envelope wave function. Inspired by the modern theory of polarization, we define an exciton projected position operator whose eigenvalues single out two unique choices of exciton Berry phase and associated Berry connection - one for electrons, and one for holes. We clarify the physical meaning of these exciton Berry phases and provide a discrete Wilson loop formulation that allows for their numerical calculation without a smooth gauge. As a corollary, we obtain a gauge-invariant expression for the exciton polarisation at a given total momentum, i.e. the mean separation of the electron and hole within the exciton wave function. In the presence of crystalline inversion symmetry, the electron and hole exciton Berry phases are quantized to the same value and we derive how this value can be expressed in terms of inversion eigenvalues of the many-body exciton state. We then consider $C_2 \mathcal{T}$ symmetry, for which no symmetry eigenvalues are available as it is anti-unitary, and confirm that the exciton Berry phase remains quantized and still diagnoses topologically distinct exciton bands. The notion of shift excitons, whose exciton Wannier states are displaced from those of the non-interacting bands by a quantized amount, can therefore be generalised beyond symmetry indicators.

cond-mat.mes-hall↗

arXiv2026

Berry Curvature of Low-Energy Excitons in Rhombohedral Graphene

We investigate low energy excitons in rhombohedral pentalayer graphene encapsulated by hexagonal boron nitride (hBN/R5G/hBN), focusing on the regime at the experimental twist angle $θ= 0.77^\circ$ and with an applied electric field. We introduce a new low-energy two-band model of rhombohedral graphene that captures the band structure more accurately than previous models while keeping the number of parameters low. Using this model, we show that the centres of the exciton Wannier functions are displaced from the moiré unit cell origin by a quantised amount - they are instead localised at $C_3$-symmetric points on the boundary. We also find that the exciton shift is electrically tunable: by varying the electric field strength, the exciton Wannier centre can be exchanged between inequivalent corners of the moiré unit cell. Our results suggest the possibility of detecting excitonic corner or edge modes, as well as novel excitonic crystal defect responses in hBN/R5G/hBN. Lastly, we find that the excitons in hBN/R5G/hBN inherit excitonic Berry curvature from the underlying electronic bands, enriching their semiclassical transport properties. Our results position rhombohedral graphene as a compelling tunable platform for probing exciton topology in moiré materials.

cond-mat.mes-hall↗

arXiv2026

Collinear $p$-wave magnetism and hidden orbital ferrimagnetism

In the absence of spin-orbit coupling, collinear magnets are classified as even-wave magnets, i.e., either ferro-, antiferro-, or altermagnets. It is based on the belief that collinear magnets always feature an inversion-symmetric band structure, which forbids odd-wave magnetism. Here, we show that collinear magnets, which break time reversal symmetry in the non-magnetic sector, can have an inversion symmetry broken band structure and lead to unconventional types of collinear magnets. Hence, collinear odd-wave magnets do exist, and we explain that a magnetic field-induced Edelstein effect is their unique signature. We propose minimal models based on the coexistence of AFM order with compensated loop current orders for all types of collinear magnets. Our work provides a new perspective on collinear magnets and the spin-space group classification.

cond-mat.str-el↗

arXiv2026

Scattering Theory of Chiral Edge Modes in Topological Magnon Insulators

Topological magnon insulators exhibit robust edge modes with chiral properties similar to quantum Hall edge states. However, due to their strong localization at the edges, interactions between these chiral edge magnons can be significant, as we show in a model of coupled magnon-conserving spin chains in an electric field gradient. The chiral edge modes remain edge-localized and do not scatter into the bulk, and we characterize their scattering phase: for strongly-localized edge modes we observe significant deviation from the bare scattering phase. This renormalization of edge scattering can be attributed to bound bulk modes resonating with the chiral edge magnons, in the spirit of Feshbach resonances in atomic physics. We argue that the scattering dynamics can be probed experimentally with a real-time measurement protocol using inelastic scanning tunneling spectroscopy. Our results show that interaction among magnons can be encoded in an effective edge model of reduced dimensionality, where the interactions with the bulk renormalize the effective couplings. Our work introduces a systematic way to determine the many-body effective theory for edge states in topological magnon insulators.

cond-mat.str-el↗

arXiv2025

Altermagnetism Without Crystal Symmetry

Altermagnetism is a collinear magnetic order in which opposite spin species are exchanged under a real-space rotation. Hence, the search for physical realizations has focussed on crystalline solids with specific rotational symmetry. Here, we show that altermagnetism can also emerge in non-crystalline systems, such as amorphous solids, despite the lack of global rotational symmetries. We construct a Hamiltonian with two directional orbitals per site on an amorphous lattice with interactions that are invariant under spin rotation. Altermagnetism then arises due to spontaneous symmetry breaking in the spin and orbital degrees of freedom around each atom, displaying a common point group symmetry. This form of altermagnetism exhibits anisotropic spin transport and spin spectral functions, both experimentally measurable. Our mechanism generalizes to any lattice and any altermagnetic order, opening the search for altermagnetic phenomena to non-crystalline systems.

cond-mat.str-el↗