Spinon signatures in the critical phase of the (1,1/2) ferrimagnet in a magnetic field

TL;DR

This paper develops an effective theory for the critical phase of a quantum ferrimagnetic chain with alternating spins, successfully describing spinon excitations and specific heat features near critical magnetic fields.

Contribution

It introduces a mapping of the ferrimagnetic chain to an effective spin-1/2 XXZ model, providing new insights into spinon signatures and thermodynamic behavior.

Findings

Effective mapping to spin-1/2 XXZ chain explains spinon bands.

Accurate description of low-temperature specific heat peaks.

Identification of spinon contributions in experimental observables.

Abstract

We propose an effective theory for the critical phase of a quantum ferrimagnetic chain with alternating spins 1 and 1/2 in an external magnetic field. With the help of the matrix product variational approach, the system is mapped to a spin-1/2 XXZ chain in an (effective) magnetic field; as a byproduct, we obtain an excellent description of the optical magnon branch in the gapped phase. Recent finite-temperature DMRG results for the low-temperature part of the specific heat are well described by the present approach, and the ``pop-up'' peaks, developing near the critical field values and in the middle of the critical phase, are identified with the contributions from two different spinon bands of the effective spin-1/2 chain. The effect should be as well observable in other spin-gap systems in an external field, particularly in spin ladders.