Optical spectroscopic study of the interplay of spin and charge in NaV2O5

TL;DR

This study explores the temperature-dependent optical properties of NaV2O5, revealing a phase transition, charge disproportionation, and spin-charge interplay, with implications for understanding strongly correlated electron systems.

Contribution

It provides detailed analysis of the structural and electronic changes in NaV2O5, introducing a charged-magnon model and evidence for local charge disproportionation without long-range order.

Findings

Phase transition occurs below 34 K with a larger unit cell.

Evidence of charge disproportionation on a local scale.

Detection of two-magnon optical absorption due to broken symmetry.

Abstract

We investigate the temperature dependent optical properties of NaV2O5, in the energy range 4meV-4eV. The symmetry of the system is discussed on the basis of infrared phonon spectra. By analyzing the optically allowed phonons at temperatures below and above the phase transition, we conclude that a second-order change to a larger unit cell takes place below 34 K, with a fluctuation regime extending over a broad temperature range. In the high temperature undistorted phase, we find good agreement with the recently proposed centrosymmetric space group Pmmn. On the other hand, the detailed analysis of the electronic excitations detected in the optical conductivity, provides direct evidence for a charge disproportionated electronic ground-state, at least on a locale scale: A consistent interpretation of both structural and optical conductivity data requires an asymmetrical charge distribution on each rung, without any long range order. We show that, because of the locally broken symmetry, spin-flip excitations carry a finite electric dipole moment, which is responsible for the detection of direct two-magnon optical absorption processes for E parallel to the a axis. The charged-magnon model, developed to interpret the optical conductivity of NaV2O5, is described in detail, and its relevance to other strongly correlated electron systems, where the interplay of spin and charge plays a crucial role in determining the low energy electrodynamics, is discussed.