Data files for "Charge Order in the half-filled bond-Holstein Model"

Zenodo (CERN European Organization for Nuclear Research) · 2026-05-08

Title: Charge order in the half-filled bond-Holstein model Abstract: We use determinant quantum Monte Carlo to study the half-filled `bond-Holstein' model on a square lattice. We find that the model exhibits a \gls*{CDW} phase transition with a critical temperature $T_\mathrm{cdw}$ considerably higher than that of the canonical `site-Holstein' model. Using a finite-size scaling analysis of the charge structure factor $S_{\rm cdw}$, we obtain $T_\mathrm{cdw}$ to greater than one percent accuracy. At the same time, local observables also show clear signatures consistent with the transition temperatures inferred from our scaling analysis. We attribute the enhanced \gls*{CDW} tendencies to a phonon-mediated nearest-neighbor electron repulsion that is directly proportional to the dimensionless electron-phonon coupling $\lambda$ in the atomic ($t\rightarrow 0$) limit. This behavior contrasts with the site-Holstein case, where the same limit yields only an on-site attraction. We supplement our analysis with results from several unsupervised machine learning methods, which not only confirm our estimates of $T_\mathrm{cdw}$ but also provide insight into the high-temperature crossover between a metallic and bipolaron liquid regime. Ref: Physical Review B, in press. Preprint: https://doi.org/10.48550/arXiv.2601.13121

Data files for "Charge Order in the half-filled bond-Holstein Model"

Zenodo (CERN European Organization for Nuclear Research) · 2026-05-08

Title: Charge order in the half-filled bond-Holstein model Abstract: We use determinant quantum Monte Carlo to study the half-filled `bond-Holstein' model on a square lattice. We find that the model exhibits a \gls*{CDW} phase transition with a critical temperature $T_\mathrm{cdw}$ considerably higher than that of the canonical `site-Holstein' model. Using a finite-size scaling analysis of the charge structure factor $S_{\rm cdw}$, we obtain $T_\mathrm{cdw}$ to greater than one percent accuracy. At the same time, local observables also show clear signatures consistent with the transition temperatures inferred from our scaling analysis. We attribute the enhanced \gls*{CDW} tendencies to a phonon-mediated nearest-neighbor electron repulsion that is directly proportional to the dimensionless electron-phonon coupling $\lambda$ in the atomic ($t\rightarrow 0$) limit. This behavior contrasts with the site-Holstein case, where the same limit yields only an on-site attraction. We supplement our analysis with results from several unsupervised machine learning methods, which not only confirm our estimates of $T_\mathrm{cdw}$ but also provide insight into the high-temperature crossover between a metallic and bipolaron liquid regime. Ref: Physical Review B, in press. Preprint: https://doi.org/10.48550/arXiv.2601.13121

Antiferromagnetism and Kekulé valence bond order in the honeycomb optical Su-Schrieffer-Heeger-Hubbard model

Physical review. B./Physical review. B · 2026-02-26

The precise role of e-ph coupling in graphene and related materials on a honeycomb lattice is not yet fully understood, despite extensive research on these systems. Here, we perform sign-problem-free determinant quantum Monte Carlo (DQMC) simulations of the optical Su-Schrieffer-Heeger (oSSH)-Hubbard model on the honeycomb lattice, focusing on the parameters relevant to graphene. Performing finite-size scaling analyzes, we obtain the model's ground state phase diagram, which includes the semi-metal (SM), Kekulé Valence Bond Solid (KVBS), and anti-ferromagnetic (AFM) phases, as well as indications of a small KVBS/AFM coexistence region. We find that a weak to moderate Hubbard repulsion, tuned toward the SM-AFM critical value in the pure honeycomb Hubbard model, enhances KVBS correlations and can even stabilize the KVBS phase. Estimating the effective parameters for graphene places it in the SM region of the phase diagram, but near the SM-KVBS phase boundary. Notably, we predict that increasing either the on-site Hubbard repulsion or the e-ph coupling strength drives graphene toward the KVBS phase rather than the AFM phase, highlighting a synergistic effect that can be exploited to further control the remarkable properties of graphene and related materials.

Dispersive dark excitons in van der Waals ferromagnet CrI3

ArXiv.org · 2025-01-16 · 1 citations

Spin-flip dark excitons are optical-dipole-forbidden quasiparticles with remarkable potential in optoelectronics, especially when they are realized within cleavable van der Waals materials. Despite this potential, dark excitons have not yet been definitively identified in ferromagnetic van der Waals materials. Here, we report two dark excitons in a model ferromagnetic material CrI3 using high-resolution resonant inelastic x-ray scattering (RIXS) and show that they feature narrower linewidths compared to the bright excitons previously reported in this material. These excitons are shown to have spin-flip character, to disperse as a function of momentum, and to change through the ferromagnetic transition temperature. Given the versatility of van der Waals materials, these excitons hold promise for new types of magneto-optical functionality.

The ubiquity of variable radio emission and spin-down rates in pulsars

arXiv (Cornell University) · 2025-01-07

Pulsars are often lauded for their (relative) rotational and radio emission stability over long time scales. However, long-term observing programmes are identifying an increasing number of pulsars that deviate from this preconceived notion. Using Gaussian process regression and Bayesian inference techniques, we investigated the emission and rotational stability of 259 isolated radio pulsars that have been monitored using Murriyang, the Parkes 64 m radio telescope, over the past three decades. We found that 238 pulsars display significant variability in their spin-down rates, 52 of which also exhibit changes in profile shape. Including 23 known state-switching pulsars, this represents the largest catalogue of variable pulsars identified to date and indicates that these behaviours are ubiquitous among the wider population. The intensity of spin-down fluctuations positively scales with increasing pulsar spin-down rate, with only a marginal dependence on spin-frequency. This may have substantial implications for ongoing searches for gravitational waves in the ensemble timing of millisecond pulsars. We also discuss challenges in explaining the physical origins of quasi-periodic and transient profile/spin-down variations detected among a subset of our pulsars.

Observation of Anisotropic Dispersive Dark-Exciton Dynamics in CrSBr

Physical Review Letters · 2025-10-01 · 1 citations

Many-body excitons in CrSBr are attracting intense interest in view of their highly anisotropic magneto-optical coupling and their potential for novel optical interfaces within spintronic and magnonic devices. Characterizing the orbital character and propagation of these electronic excitations is crucial for understanding and controlling their behavior; however, this information is challenging to access. High resolution resonant inelastic x-ray scattering is a momentum-resolved technique that can address these crucial questions. We present measurements collected at the Cr L_{3}-edge which show a rich spectrum of excitations with a variety of spin-orbital characters. While most of these excitations appear to be localized, the dispersion of the lowest energy dark exciton indicates that it is able to propagate along both the a and b directions within the planes of the crystal. This two-dimensional character is surprising as it contrasts with electrical conductivity and the behavior of the bright exciton, both of which are strongly one dimensional. The discovery of this propagating dark exciton highlights an unusual coexistence of one- and two-dimensional electronic behaviors in CrSBr.

Beyond-Hubbard pairing in a cuprate ladder

ArXiv.org · 2025-01-17

The Hubbard model is believed to capture the essential physics of cuprate superconductors. However, recent theoretical studies suggest that it fails to reproduce a robust and homogeneous superconducting ground state. Here, using resonant inelastic x-ray scattering and density matrix renormalization group calculations, we show that magnetic excitations in the prototypical cuprate ladder Sr$_{14}$Cu$_{24}$O$_{41}$ are inconsistent with those of a simple Hubbard model. The magnetic response of hole carriers, contributing to an emergent branch of spin excitations, is strongly suppressed. This effect is the consequence of d-wave-like pairing, enhanced by nearly an order of magnitude through a large nearest-neighbor attractive interaction. The similarity between cuprate ladders and the two-dimensional compounds suggests that such an enhanced hole pairing may be a universal feature of superconducting cuprates.

The Thousand-Pulsar-Array programme on MeerKAT -- XVI. Mapping the Galactic magnetic field with pulsar observations

ArXiv.org · 2025-04-13

Measuring the magnetic field of the Milky Way reveals the structure and evolution of the galaxy. Pulsar rotation measures (RMs) provide a means to probe this Galactic magnetic field (GMF) in three dimensions. We use the largest single-origin data set of pulsar measurements, from the MeerKAT Thousand-Pulsar-Array, to map out GMF components parallel to pulsar lines of sight. We also present these measurements for easy integration into the consolidated RM catalogue, RMTable. Focusing on the Galactic disk, we investigate competing theories of how the GMF relates to the spiral arms, comparing our observational map with five analytic models of magnetic field structure. We also analyse RMs to extragalactic radio sources, to help build up a three-dimensional picture of the magnetic structure of the galaxy. In particular, our large number of measurements allows us to investigate differing magnetic field behaviour in the upper and lower halves of the Galactic plane. We find that the GMF is best explained as following the spiral arms in a roughly bisymmetric structure, with antisymmetric parity with respect to the Galactic plane. This picture is complicated by variations in parity on different spiral arms, and the parity change location appears to be shifted by a distance of 0.15 kpc perpendicular to the Galactic plane. This indicates a complex relationship between the large-scale distributions of matter and magnetic fields in our galaxy. Future pulsar discoveries will help reveal the origins of this relationship with greater precision, as well as probing the locations of local magnetic field inhomogenities.

Frequency evolution of pulsar emission

Astronomy and Astrophysics · 2025-10-28

Aims. We explore frequency-dependent changes in pulsar radio emission by analyzing their profile widths and emission heights, assessing whether the simple radius-to-frequency mapping (RFM) or the fan beam model can describe the data. Methods. Using wideband (704–4032 MHz) Murriyang (Parkes) observations of over 100 pulsars, we measured profile widths at multiple intensity levels, fit Gaussian components, and used aberration–retardation effects to estimate emission altitudes. We compared trends in width evolution and emission height with a fan beam model. Results. Similar to other recent studies, we find that while many pulsars show profiles narrowing with increasing frequency, a substantial fraction show the reverse. The Gaussian decomposition of the profiles reveals that the peak locations of the components vary little with frequency. However, the component widths do, in general, narrow with increasing frequency. This argues that propagation effects are responsible for the width evolution of the profiles rather than emission height. Overall, the evolution of the emission height with frequency is unclear and clouded by the assumptions in the model. Spin-down luminosity correlates weakly with profile narrowing but not with emission height. Conclusions. The classic picture where pulsars emit at a single emission height that decreases with increasing observing frequency cannot explain the diversity in behavior observed here. Instead, pulsar beams likely originate from extended regions at multiple altitudes, with fan beam or patchy structures dominating their frequency evolution. Future models must incorporate realistic plasma physics and multi-altitude emission to capture the range of pulsar behaviors.

Beyond-Hubbard Pairing in a Cuprate Ladder

Physical Review X · 2025-05-12 · 6 citations

The Hubbard model is believed to capture the essential physics of cuprate superconductors. However, recent theoretical studies suggest that it fails to reproduce a robust and homogeneous superconducting ground state. Here, using resonant inelastic x-ray scattering and density matrix renormalization group calculations, we show that magnetic excitations in the prototypical cuprate ladder <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:msub><a:mrow><a:mi>Sr</a:mi></a:mrow><a:mrow><a:mn>14</a:mn></a:mrow></a:msub><a:msub><a:mrow><a:mi>Cu</a:mi></a:mrow><a:mrow><a:mn>24</a:mn></a:mrow></a:msub><a:msub><a:mrow><a:mi mathvariant="normal">O</a:mi></a:mrow><a:mrow><a:mn>41</a:mn></a:mrow></a:msub></a:mrow></a:math> are inconsistent with those of a simple Hubbard model. The magnetic response of hole carriers, contributing to an emergent branch of spin-flip excitations, is strongly suppressed. This effect is the consequence of strong <d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline"><d:mi>d</d:mi></d:math>-wavelike pairing, enhanced by nearly an order of magnitude through a large nearest-neighbor attractive interaction and persisting up to at least 260 K. The close connection between the physics of cuprate ladders and that of the two-dimensional compounds suggests that such an enhanced hole pairing may be a universal feature of superconducting cuprates.