Finite low-temperature entropy of some strongly frustrated quantum spin lattices in the vicinity of the saturation field

TL;DR

This paper rigorously calculates the finite residual entropy at the saturation field for certain highly frustrated quantum spin lattices, revealing a maximum in low-temperature entropy and robustness against small deviations, especially in spin-1/2 systems.

Contribution

It provides exact ground-state entropy calculations at saturation for specific frustrated spin lattices, highlighting the persistent residual entropy and its implications.

Findings

Residual ground-state entropy per site remains finite at saturation.

Maximum in the field dependence of low-temperature entropy occurs at saturation.

Enhancement of isothermal entropy is robust against small exchange deviations, especially for spin 1/2.

Abstract

For a class of highly frustrated antiferromagnetic quantum spin lattices the ground state exhibits a huge degeneracy in high magnetic fields due to the existence of localized magnon states. For some of these spin lattices (in particular, the 1D dimer-plaquette, sawtooth and kagom\'{e}-like chains as well as the 2D kagom\'{e} lattice) we calculate rigorously the ground-state entropy at the saturation field. We find that the ground-state entropy per site remains finite at saturation. This residual ground-state entropy produces a maximum in the field dependence of the isothermal entropy at low temperatures. By numerical calculation of the field dependence of the low-temperature entropy for the sawtooth chain we find that the enhancement of isothermal entropy is robust against small deviations in exchange constants. Moreover, the effect is most pronounced in the extreme quantum case of spin 1/2.