arXiv subjects

Cesar A. Gallegos

Publications and source records attributed to Cesar A. Gallegos.

5 recordsLinked to original sources

arXiv2026

Revisiting magnon bound states: ferro-antiferromagnetic $J_1\!-\!J_2$ square-lattice model

A comprehensive analysis of pairing and bound states of magnons in the Heisenberg $S\!=\!1/2$ square-lattice $J_1\!-\!J_2$ ferro-antiferromagnetic model is presented. We highlight the similarities and differences between the bound states of spin flips on a lattice and those of particles in the continuum. Magnon bound states at finite pair momentum are studied throughout the Brillouin zone and a convenient lattice partial-wave nomenclature is advocated. The mechanisms of enhanced stability or fragility of these bound states for the high-symmetry pair momenta are identified and quantified as relating to the effective dimensional reduction or enhancement, respectively. These effects are also shown to control the evolution of the bound states with the model parameters, providing a transparent framework for understanding magnon pairing in a more general setting. A method to determine the bound state phase boundaries is presented.

cond-mat.str-el↗

arXiv2026

Spinless charged excitation at the interface between a conventional topological insulator and a topological Mott insulator

We investigate the interface separating two topologically distinct insulating phases of matter using extensive density-matrix renormalization group calculations to study the triangular-lattice Hofstadter-Hubbard model with a spatially varying interaction strength, chosen to realize both integer quantum Hall and chiral spin liquid states in different spatial regions. We find that the integer quantum Hall-chiral spin liquid interface hosts a spinless charged excitation that is bound to the interface. This mode at the interface is identified through charge and spin pumping, and by direct calculations of low-lying excited states. We also characterize bulk excitations in both phases, finding evidence for fractionalization in the chiral spin liquid and for spin-triplet exciton formation in the integer quantum Hall phase.

cond-mat.str-el↗

arXiv2026

Quantum Hall to Chiral Spin Liquid transition in a Triangular Lattice Hofstadter-Hubbard Model

We investigate the weak interaction integer quantum Hall (IQH) phase, the intermediate interaction phase identified as a chiral spin liquid (CSL) and the transition between them in the triangular lattice Hofstadter-Hubbard model at a density of one electron per site in an orbital magnetic field corresponding to one-quarter flux per plaquette. Our primary tool is the finite system density matrix renormalization group (DMRG) method with both interaction-strength scan and fixed interaction techniques for cylinders of circumference 3, 5, and 7 and lengths up to 240. For the IQH phase, we use single particle exact diagonalization to clarify finite size effects, including an excess charge on the edges of our cylinders, and the limitations of entanglement spectra degeneracies on small circumference cylinders. For both phases, we use DMRG to study the entanglement spectra, the entanglement entropy, and the effect of flux insertion on charge and spin pumping, all of which show key differences between the two phases. To study the transition, we use interaction-strength scans extending between the two phases, and apply a scaling data collapse of a bond-dimerization order parameter to extract critical exponents. We also extract critical behavior from the divergence of correlation lengths on the IQH side, measuring decay away from edges of both the dimerization order parameter and transverse edge currents. The critical behavior and exponents are consistent with an Ising transition in 1+1 dimensions. Finally, we obtain excited states in various quantum number sectors finding that the gap to a charge neutral momentum $π$ excitation corresponding to fluctuations of the dimerization order parameter closes in the vicinity of the critical point but gaps to other excitations remain large.

cond-mat.str-el↗

arXiv2025

Phase Diagram of the Easy-Axis Triangular-Lattice $J_1\!-\!J_2$ Model

The phase diagram of the $S\!=\!1/2$ easy-axis triangular-lattice $J_1\!-\!J_2$ model is investigated using the density-matrix renormalization group and analytical insights. We find a significant spin-liquid region extending from the Heisenberg limit and residing between the Y phase-known as the magnetic analogue of the "supersolid"-and collinear stripe phase. The order parameters of the supersolid are analyzed and an understanding of its lack of ferromagnetic moment is suggested.

cond-mat.str-el↗

arXiv2024

Magnon interactions in the quantum paramagnetic phase of CoNb$_2$O$_6$

In this work, we study effects of magnon interactions in the excitation spectrum of CoNb$_2$O$_6$ in the quantum paramagnetic phase in transverse field, where the $1/S$ spin-wave theory exhibits unphysical divergences at the critical field. We propose a self-consistent Hartree-Fock approach that eliminates such unphysical singularities while preserving the integrity of the singular threshold phenomena of magnon decay and spectrum renormalization that are present in both theory and experiment. With the microscopic parameters adopted from previous studies, this method yields a close quantitative agreement with the available experimental data for CoNb$_2$O$_6$ in the relevant regime. Insights into the general structure of the spin-anisotropic model of CoNb$_2$O$_6$ and related zigzag chain materials are also provided and a discussion of the effects of additional longitudinal field on the spectrum is given.

cond-mat.str-el↗