Ground-state phase diagram and magnetic properties of a tetramerized spin-1/2 J_1-J_2 model: BEC of bound magnons and absence of the transverse magnetization

TL;DR

This paper investigates the ground state and magnetic phases of a frustrated spin-1/2 J1-J2 model with a plaquette structure, revealing a spin-nematic phase, a BEC phase without transverse magnetization, and other complex magnetic states.

Contribution

It introduces a detailed analysis of the phase diagram, including the discovery of a spin-nematic phase and a BEC phase lacking transverse magnetization, using various theoretical methods.

Findings

Identification of a spin-nematic phase near J1/J2 ~ -2

Discovery of a BEC phase without transverse magnetization

Observation of multiple magnetic phases including supersolid and plateau states

Abstract

We study the ground state and the magnetization process of a spin-1/2 $J_1$-$J_2$ model with a plaquette structure by using various methods. For small inter-plaquette interaction, this model is expected to have a spin-gap and we computed the first- and the second excitation energies. If the gap of the lowest excitation closes, the corresponding particle condenses to form magnetic orders. By analyzing the quintet gap and magnetic interactions among the quintet excitations, we find a spin-nematic phase around $J_1/J_2\sim -2$ due to the strong frustration and the quantum effect. When high magnetic moment is applied, not the spin-1 excitations but the spin-2 ones soften and dictate the magnetization process. We apply a mean-field approximation to the effective Hamiltonian to find three different types of phases (a conventional BEC phase, ``striped'' supersolid phases and a 1/2-plateau). Unlike the BEC in spin-dimer systems, this BEC phase is not accompanied by transverse magnetization. Possible connection to the recently discovered spin-gap compound (CuCl)LaNb2O7 is discussed.