Charged Magnons in NaV2O5

TL;DR

This study explores the optical properties of NaV2O5, revealing a non-centrosymmetric high-temperature phase, evidence of a broken-parity ground state, and a second-order phase transition at 34 K involving a larger unit cell.

Contribution

It provides the first direct evidence of a broken-parity ground state and a non-centrosymmetric structure in NaV2O5, and proposes a two-magnon optical absorption process.

Findings

Broken-parity electronic ground state in high-temperature phase

Non-centrosymmetric crystal structure confirmed

Phase transition at 34 K involving unit cell enlargement

Abstract

We investigated the temperature-dependent optical conductivity of NaV2O5 in the energy range 4 meV-4 eV. The intensities and the polarization dependence of the detected electronic excitations give a direct indication for a broken-parity electronic ground-state and for a non-centrosymmetric crystal structure of the system in the high-temperature phase. A direct two-magnon optical absorption process, proposed in this Letter, is in quantitative agreement with the optical data. By analyzing the optically allowed phonons at various temperatures above and below the phase transition, we conclude that a second-order change to a larger unit cell takes place below 34 K.