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

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${}_4$.

7 pages, 6 figures