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arXiv:1805.09287 · Physical review. B./Physical review. B

Spectral signatures of fractionalization in the frustrated Heisenberg model on the square lattice

Francesco Ferrari, Federico Becca

Published Sep 17, 2018• 82 citations
arXivHTMLPDF

Abstract

We employ a variational Monte Carlo approach to efficiently obtain the dynamical structure factor for the spin-$1/2\phantom{\rule{0.16em}{0ex}}{J}_{1}\text{\ensuremath{-}}{J}_{2}$ Heisenberg model on the square lattice. Upon increasing the frustrating ratio ${J}_{2}/{J}_{1}$, the ground state undergoes a continuous transition from a N\'eel antiferromagnet to a ${\mathbb{Z}}_{2}$ gapless spin liquid. We identify the characteristic spectral features in both phases and highlight the existence of a broad continuum of excitations in the proximity of the spin-liquid phase. The magnon branch, which dominates the spectrum of the unfrustrated Heisenberg model, gradually loses its spectral weight, thus releasing nearly deconfined spinons, whose signatures are visible even in the magnetically ordered state. Our results provide an important example on how magnons fractionalize into deconfined spinons across a quantum critical point.

DOI
10.1103/physrevb.98.100405

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Quantum dynamics in a spin-1/2 square lattice J1J_{1}J1​-J2J_{2}J2​-δδδ altermagnet
A key feature of the newly discovered altermagnet is that its spin degeneracy is lifted, although it has an antiferromagnetic order and zero net magnetization. In this work, we investigate a frustrated spin-1/2 J1J_1J1​-J2J_2J2​-δδδ Heisenberg model on the square lattice by the tensor network methodin combination with the linear spin-wave theory, with our focus on both the magnon excitations and longitudinal excitations.For a small J2J_2J2​ and a finite range of δδδ we demonstrate that such a model hosts an altermagnetic ground state. Its magnon spectrum is split into two branches and the largest splitting occurs at (±π/2,±π/2)\left(\pmπ/2, \pmπ/2\right)(±π/2,±π/2) in the Brillouin zone. The magnitudes of splitting in the two magnon modes are equal with respect to the case of δ=0δ=0δ=0. Dynamical spin structure factors show that the low-energy peak in the longitudinal spectral weight around (π/2,π/2)(π/2, π/2)(π/2,π/2) is also split, and thus the relative positions of the magnon modes and longitudinal modes in energy may change in the presence of a finite δδδ. These findings demonstrate that the altermagnets harbor more complex quantum dynamics than the conventional collinear antiferromagnets.