Specific heat and susceptibility of S=1/2 antiferromagnets on square, triangular, and kagome lattices

TL;DR

This study investigates the temperature-dependent thermodynamic properties of spin-1/2 antiferromagnets on various 2D lattices using the sine-square deformation method to approximate bulk behavior in finite systems.

Contribution

It introduces the application of sine-square deformation to compute thermodynamic properties of frustrated antiferromagnets, revealing distinct low-temperature behaviors.

Findings

Triangular and kagome lattices show double-peak structures in specific heat.

Kagome lattice exhibits strong susceptibility enhancement at low temperatures.

Low-energy excitations in kagome are mainly magnetic, unlike in triangular lattices.

Abstract

We study the temperature dependence of the thermodynamic properties of spin-1/2 antiferromagnets on two-dimensional lattices. Our analysis employs the sine-square deformation (SSD), in which a real-space envelope function is applied to the Hamiltonian so that the local energy scale is smoothly reduced to zero at the system boundaries. The quantum eigenstates of the SSD Hamiltonian exhibit bulk-like behavior near the system center, effectively mimicking the thermodynamic limit even in small finite-size calculations. Using these fictitious bulk states, we compute the energy density, specific heat, and magnetic susceptibility as functions of temperature. We find that both the triangular- and kagome-lattice antiferromagnets show either a shoulder or a pronounced double-peak structure in the low-temperature specific heat, whereas the kagome case particularly shows a strong enhancement of magnetic susceptibility down to the lowest temperature range. These direct comparisons, together with the square-lattice and one-dimensional cases, reveal that although both frustrated systems retain a substantial amount of entropy, the low-energy excitations below ~ 0.5J of the kagome lattice are predominantly governed by the magnetic excited states, whereas not much for the triangular lattice.