Field-Induced Gap in a Quantum Spin-1/2 Chain in a Strong Magnetic Field

TL;DR

This study investigates the magnetic excitations in a quantum spin-1/2 chain with a field-induced gap, using electron spin resonance spectroscopy and DMRG simulations to understand the transition from soliton-breather to magnon excitations near saturation.

Contribution

It provides the first detailed experimental and theoretical analysis of the field-induced spin gap and the transition between different excitation regimes in a real quantum spin chain.

Findings

Identification of a minimum in the spin gap near saturation field

Confirmation of the transition from sine-Gordon to spin-polarized state

Quantitative agreement between experimental data and DMRG simulations

Abstract

Magnetic excitations in copper pyrimidine dinitrate, a spin-1/2 antiferromagnetic chain with alternating $g$-tensor and Dzyaloshinskii-Moriya interactions that exhibits a field-induced spin gap, are probed by means of pulsed-field electron spin resonance spectroscopy. In particular, we report on a minimum of the gap in the vicinity of the saturation field $H_{sat}=48.5$ T associated with a transition from the sine-Gordon region (with soliton-breather elementary excitations) to a spin-polarized state (with magnon excitations). This interpretation is fully confirmed by the quantitative agreement over the entire field range of the experimental data with the DMRG investigation of the spin-1/2 Heisenberg chain with a staggered transverse field.