Excitation Spectra of Structurally Dimerized and Spin-Peierls Chains in a Magnetic Field

TL;DR

This paper investigates the excitation spectra of dimerized and spin-Peierls chains under magnetic fields, revealing distinct phases, spectral features, and the persistence of spin gaps in different regimes using exact diagonalization.

Contribution

It provides a detailed analysis of the dynamical spin structure factor and Raman response in these chains, highlighting differences in spin gap behavior across phases.

Findings

Spin liquid phase with interacting singlet dimers at low fields

Incommensurate regime with field-adapted triplet spectra modulation

Spin-Peierls chains remain gapped in all regimes

Abstract

The dynamical spin structure factor and the Raman response are calculated for structurally dimerized and spin-Peierls chains in a magnetic field, using exact diagonalization techniques. In both cases there is a spin liquid phase composed of interacting singlet dimers at small fields h < h_c1, an incommensurate regime (h_c1 < h < h_c2) in which the modulation of the triplet excitation spectra adapts to the applied field, and a fully spin polarized phase above an upper critical field h_c2. For structurally dimerized chains, the spin gap closes in the incommensurate phase, whereas spin-Peierls chains remain gapped. In the spin liquid regimes, the dominant feature of the triplet spectra is a one-magnon bound state, separated from a continuum of states at higher energies. There are also indications of a singlet bound state above the one-magnon triplet.