Multi-triplet bound states and finite-temperature dynamics in highly frustrated quantum spin ladders

TL;DR

This study explores the finite-temperature dynamics of highly frustrated quantum spin ladders, revealing rapid spectral weight transfer and persistent multi-triplet bound states, providing insights relevant to materials like SrCu$_2$(BO$_3$)$_2$.

Contribution

It presents the first numerical analysis of finite-temperature dynamical structure factors in frustrated spin ladders, highlighting the behavior of multi-triplet bound states near quantum phase transitions.

Findings

Rapid transfer of spectral weight from one-triplet to multi-triplet states

Persistence of multi-triplet bound states up to infinite temperature

Strong spectral features near quantum phase transition

Abstract

Low-dimensional quantum magnets at finite temperatures present a complex interplay of quantum and thermal fluctuation effects in a restricted phase space. While some information about dynamical response functions is available from theoretical studies of the one-triplet dispersion in unfrustrated chains and ladders, little is known about the finite-temperature dynamics of frustrated systems. Experimentally, inelastic neutron scattering studies of the highly frustrated two-dimensional material SrCu$_2$(BO$_3$)$_2$ show an almost complete destruction of the one-triplet excitation band at a temperature only 1/3 of its gap energy, accompanied by strong scattering intensities for apparent multi-triplet excitations. We investigate these questions in the frustrated spin ladder and present numerical results from exact diagonalization for the dynamical structure factor as a function of temperature. We find anomalously rapid transfer of spectral weight out of the one-triplet band and into both broad and sharp spectral features at a wide range of energies, including below the zero-temperature gap of this excitation. These features are multi-triplet bound states, which develop particularly strongly near the quantum phase transition, fall to particularly low energies there, and persist to all the way to infinite temperature. Our results offer valuable insight into the physics of finite-temperature spectral functions in SrCu$_2$(BO$_3$)$_2$ and many other highly frustrated spin systems.