Thermodynamic properties of the macroscopically degenerate tetramer-dimer phase of the spin-1/2 Heisenberg model on the diamond-decorated square lattice

TL;DR

This study investigates the thermodynamic behavior of the spin-1/2 Heisenberg model on a diamond-decorated square lattice, revealing macroscopic degeneracy and anomalous properties near the dimer-tetramer phase, with implications for magnetic cooling.

Contribution

It provides a detailed analysis of the thermodynamics of the dimer-tetramer phase using multiple methods, highlighting the role of classical models in understanding quantum degeneracy.

Findings

Macroscopic degeneracy persists under magnetic field.

Enhanced magnetocaloric effect near the dimer-tetramer phase.

Thermodynamics near phase boundary described by extended monomer-dimer models.

Abstract

The spin-1/2 Heisenberg antiferromagnet on the diamond-decorated square lattice in the presence of a magnetic field displays various quantum phases including the Lieb-Mattis ferrimagnetic, dimer-tetramer, monomer-dimer, and spin-canted phases, in addition to the trivial fully saturated state. Thermodynamic properties of this model are investigated using several complementary analytical and numerical methods such as exact diagonalization up to the systems of 40 spins, an effective monomer-dimer description, sign-problem-free quantum Monte Carlo simulations for up to 180 spins, and a decoupling approximation. Our particular attention is focused on the parameter region favoring the dimer-tetramer phase. This ground state can be represented by a classical hard-dimer model on the square lattice and retains a macroscopic degeneracy even under a magnetic field. However, the description of the low-temperature thermodynamics close to the boundary between the macroscopically degenerate dimer-tetramer and the non-degenerate monomer-dimer phases requires an extended classical monomer-dimer lattice-gas model. Anomalous thermodynamic properties emerging in the vicinity of the dimer-tetramer phase are studied in detail. Under the adiabatic demagnetization we detect an enhanced magnetocaloric effect promoting an efficient cooling to absolute zero temperature, provided that the system reaches the dimer-tetramer ground state at zero field.