Charge and Orbital Orderings, and Frustration in Quasi-one-dimensional Ferrimagnetic Insulator $\beta$-V${}_2$O(PO${}_4$)

Abstract

Using ab initio calculations based on the correlated band theory, we have investigated the quasi-one-dimensional chain system $\beta$-V${}_2$O(PO${}_4$), showing both charge and spin orderings. Even in the uncorrelated region, the pure transition from the tetragonal to the monoclinic structure leads to a sizable charge difference between the two types of V ions, regardless of magnetic orders. In the ferrimagnetic phase, inclusion of the on-site Coulomb repulsion $U$ leads to a full orbital-polarization of V1 ($t_{2g}^{3\uparrow }$, $S=\frac{3}{2}$) and V2 ($a_{1g}^{1\downarrow }{e}_g^{\prime 1\downarrow }$, $S=1$) above $U_{eff}^c\approx 3.5$ eV, leading to local spin moments of 2.30 and --1.54 ${\mu }_B$, respectively, with small orbital moments of several hundredth ${\mu }_B$. So, the net moment is nearly 1 ${\mu }_B$ per formula unit, which is about 2--3 times larger than the experimental value. Our results show significant variations, strongly depending on the strength of $U_{eff}$, in energy differences between various magnetic states as well as a small magnetic anisotropy. These results suggest that the substantial difference between the calculated and experimental moments is attributed to quantum fluctuation of the pyrochlore-like weakly linked V${}_4$ tetrahedral structure. Our findings are expected to provide a good platform to investigate the interplay among the charge-, spin-, and lattice-degrees of freedom, and geometrical frustration.

8 pages