Phase transitions in spin-orbital coupled model for pyroxene titanium oxides

TL;DR

This paper investigates phase transitions in a spin-orbital model for pyroxene titanium oxides, revealing competing ordered states and the influence of Hund's coupling and magnetic fields on phase behavior.

Contribution

It introduces a detailed analysis of phase transitions in a spin-orbital coupled model specific to pyroxene titanium oxides, highlighting the competition between spin and orbital orders.

Findings

Identification of two ordered states: spin-dimer/orbital-ferro and spin-ferro/orbital-antiferro.

Phase transition driven by Hund's-rule coupling strength and magnetic field.

Large fluctuations near multicritical points affect finite-temperature properties.

Abstract

We study the competing phases and the phase transition phenomena in an effective spin-orbital coupled model derived for pyroxene titanium oxides ATiSi2O6 (A=Na, Li). Using the mean-field-type analysis and the numerical quantum transfer matrix method, we show that the model exhibits two different ordered states, the spin-dimer and orbital-ferro state and the spin-ferro and orbital-antiferro state. The transition between two phases is driven by the relative strength of the Hund's-rule coupling to the onsite Coulomb repulsion and/or by the external magnetic field. The ground-state phase diagram is determined. There is a keen competition between orbital and spin degrees of freedom in the multicritical regime, which causes large fluctuations and significantly affects finite-temperature properties in the paramagnetic phase.