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

## TL;DR

This study uses ab initio calculations to explore charge and spin orderings, orbital polarization, and frustration effects in the quasi-one-dimensional ferrimagnetic insulator $eta$-V$_2$O(PO$_4$), revealing complex magnetic behaviors and the influence of Coulomb interactions.

## Contribution

The paper provides a detailed ab initio analysis of charge, spin, and orbital orderings in $eta$-V$_2$O(PO$_4$), highlighting the role of Coulomb repulsion and geometrical frustration in its magnetic properties.

## Key findings

- Charge difference between V ions persists even without correlations.
- Orbital polarization occurs above a critical Coulomb value, affecting magnetic moments.
- Calculated magnetic moments are larger than experimental values, indicating quantum fluctuations.

## 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 $U^c_{eff}\approx3.5$ 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.