Unconventional quantum ordered and disordered states in the highly frustrated spin-1/2 Ising-Heisenberg model on triangles-in-triangles lattices

TL;DR

This paper provides exact analytical results for a highly frustrated spin-1/2 Ising-Heisenberg model on triangles-in-triangles lattices, revealing unconventional quantum phases and complex thermal behaviors due to geometric frustration.

Contribution

It introduces a rigorous mapping method to derive exact thermodynamic properties and uncovers novel quantum ordered and disordered ground states in the model.

Findings

Discovery of unconventional quantum ordered phases.

Identification of disordered quantum paramagnetic phases with residual entropy.

Observation of diverse specific heat behaviors including Schottky maxima.

Abstract

Ground state and thermodynamics of geometrically frustrated spin-1/2 Ising-Heisenberg model on two different but topologically related triangles-in-triangles lattices is investigated in particular. A rigorous mapping based on generalized star-triangle transformations enables one to derive exact analytical results for the free and internal energy, spontaneous magnetization, entropy and specific heat from the known exact results for the corresponding spin-1/2 Ising model on a triangular lattice. It is demonstrated that several unconventional quantum ordered and disordered phases may become the ground state in addition to the classical ferromagnetic phase with a perfect alignment of all constituent spins. A mutual competition between the ferromagnetic Ising and XXZ Heisenberg interactions may lead to an appearance of a peculiar quantum ferromagnetic phase, in which the magnetic moment of the Heisenberg spins undergoes a quantum reduction to one third of the saturation value in contrast with the fully saturated magnetic moment of the classical Ising spins. In the highly frustrated regime, we have found either the disordered quantum paramagnetic phase with the substantial residual entropy or a similar but strikingly spontaneously long-range ordered phase, in which a quantum reduction of the magnetic moment of the Heisenberg spins causes a small quantum reduction of the magnetic moment of otherwise purely classical Ising spins. It is evidenced that the specific heat exhibits diverse thermal dependences with or without a logarithmic divergence superimposed either on one or two round Schottky-type maxima.