arXiv subjects

Marton Lajer

Publications and source records attributed to Marton Lajer.

5 recordsLinked to original sources

arXiv2025

Breakdown of Quantum Chaos in the Staggered-Field XXZ Chain: Confinement and Meson Formation

Confinement of fractionalized excitations can strongly restructure many-body spectra. We investigate this phenomenon in the gapped spin-$\frac{1}{2}$ XXZ chain subject to a staggered field, where spinons bind into domain-wall ``mesons'' deep in the antiferromagnetic phase. We present evidence that this non-integrable model exhibits both Hilbert space fractionalization and quantum scar formation as controlled by the anisotropy parameter $\Delta$. Exact diagonalization across symmetry-resolved sectors reveals a crossover from Gaussian-orthogonal (chaotic) level statistics at weak anisotropy $\Delta \sim 1$ to non-ergodic behavior deep in the antiferromagnetic regime $\Delta\gg 1$ through scrutinizing the adjacent gap ratios, accompanied by a striking banding of eigenstates by domain-wall number in correlation and entanglement measures. The Page-like entanglement dome characteristic of chaotic spectra gives way to suppressed, band-resolved entanglement consistent with emergent quasi-conservation of domain walls. To investigate further the formation mechanism of mesonic scar states, we carry out meson spectroscopy near the two-spinon threshold and compare with the analytic ladder predicted by Rutkevich [Phys. Rev. B 106, 134405 (2022)]. We test the theory through continuum-relative bindings, an offset-removed Airy scaling collapse, and explicit two-meson thresholds that determine the number of stable meson levels. The low-lying spectrum shows close quantitative agreement, while deviations at higher energies are consistent with finite-size and subleading corrections. These results establish a unified account of confinement-induced nonergodicity and provide a template for quantitative meson spectroscopy in quantum spin chains.

cond-mat.str-el↗

arXiv2024

Witnessing Quantum Entanglement Using Resonant Inelastic X-ray Scattering

Although entanglement is both a central ingredient in our understanding of quantum many-body systems and an essential resource for quantum technologies, we only have a limited ability to quantify entanglement in real quantum materials. Thus far, entanglement metrology in quantum materials has been limited to measurements involving Hermitian operators, such as the detection of spin entanglement using inelastic neutron scattering. Here, we devise a method to extract the quantum Fisher information (QFI) from non-Hermitian operators and formulate an entanglement witness for resonant inelastic x-ray scattering (RIXS). Our approach is then applied to the model iridate dimer system Ba$_3$CeIr$_2$O$_9$ and used to directly test for entanglement of the electronic orbitals between neighboring Ir sites. We find the entanglement can be detected if we account for the expected symmetries, parity, and electron number conservation, of the dimer system. We also consider the roles that the incident and outgoing x-ray polarizations and the incident photon energy play in entanglement detection. Our protocol provides a new handle for entanglement detection in quantum materials.

cond-mat.str-el↗

arXiv2022

Nuclear matter in 1+1 dimensions

We review the solution of QCD in two spacetime dimensions. Following the analysis of Baluni, for a single flavor the model can be analyzed using Abelian bosonization. The theory can be analyzed in strong coupling, when the quarks are much lighter than the gauge coupling. In this limit, the theory is given by a Luttinger liquid.

hep-ph↗

arXiv2021

When cold, dense quarks in 1+1 and 3+1 dimensions are not a Fermi liquid

We analyze the behavior of quarks coupled to a $SU(N_c)$ gauge theory in 1+1 dimensions. In the limit of strong coupling, the model reduces to a Wess-Zumino-Novikov-Witten (WZNW) model. At nonzero density, excitations near the Fermi surface form a non-Fermi liquid. With $N_f$ flavors, the finite density of quarks reduce to a free $U(1)$ field, which governs fluctuations in baryon number, together with a WZNW $SU(N_f)$ nonlinear sigma model at level $N_c$, from the pion/kaon modes. We compute the singularity in the charge susceptibility at the Fermi surface and the attendant power law correlations. We suggest that this is relevant to the quarkyonic regime of cold, dense QCD in 3+1 dimensions, in the limit that the Fermi surface is covered by many small patches, and the theory is effectively one dimensional. In this regime the dominant excitations near the Fermi surface are not baryons, but gapless bosonic modes.

hep-th↗

arXiv2019

Leading exponential finite size corrections for non-diagonal form factors

We derive the leading exponential finite volume corrections in two dimensional integrable models for non-diagonal form factors in diagonally scattering theories. These formulas are expressed in terms of the infinite volume form factors and scattering matrices. If the particles are bound states then the leading exponential finite-size corrections ($μ$-terms) are related to virtual processes in which the particles disintegrate into their constituents. For non-bound state particles the leading exponential finite-size corrections (F-terms) come from virtual particles traveling around the finite world. In these F-terms a specifically regulated infinite volume form factor is integrated for the momenta of the virtual particles. The F-term is also present for bound states and the $μ$-term can be obtained by taking an appropriate residue of the F-term integral. We check our results numerically in the Lee-Yang and sinh-Gordon models based on newly developed Hamiltonian truncations.

hep-th↗