Orbital- and spin-driven lattice instabilities in quasi-one-dimensional CaV$_2$O$_4$

TL;DR

This study investigates the complex interplay of orbital and spin-driven lattice instabilities in CaV$_2$O$_4$, revealing temperature-dependent elastic softening linked to orbital fluctuations and a spin-state crossover in a quasi-one-dimensional zigzag chain system.

Contribution

It provides the first detailed ultrasound velocity analysis of CaV$_2$O$_4$, demonstrating the coexistence of orbital and spin-driven lattice instabilities and their temperature evolution.

Findings

Huge Curie-type softening of shear elastic moduli observed.

Orbital fluctuations present above the structural transition temperature.

Spin-state crossover occurs below the structural transition, driven by spin-lattice coupling.

Abstract

Calcium vanadate CaV$_2$O$_4$ has a crystal structure of quasi-one-dimensional zigzag chains composed of orbital-active V$^{3+}$ ions and undergoes successive structural and antiferromagnetic phase transitions at $T_s\sim 140$ K and $T_N \sim 70$ K, respectively. We perform ultrasound velocity measurements on a single crystal of CaV$_2$O$_4$. The temperature dependence of its shear elastic moduli exhibits huge Curie-type softening upon cooling that emerges above and below $T_s$ depending on the elastic mode. The softening above $T_s$ suggests the presence of either onsite Jahn-Teller-type or intersite ferro-type orbital fluctuations in the two inequivalent V$^{3+}$ zigzag chains. The softening below $T_s$ suggests the occurrence of a dimensional spin-state crossover, from quasi-one to three, that is driven by the spin-lattice coupling along the inter-zigzag-chain orthogonal direction. The successive emergence of the orbital- and spin-driven lattice instabilities above and below $T_s$, respectively, is unique to the orbital-spin zigzag chain system of CaV$_2$O$_4$.