Effects of thermal and spin fluctuations on the band structure of purple bronze Li2Mo12O34

Abstract

The band structures of ordered and thermally disordered Li 2 Mo 12 O 34 are calculated by use of the ab initio density-functional-theory-linear-muffin-tin (DFT-LMTO) method. The unusual, very one-dimensional band dispersion obtained in previous band calculations is confirmed for the ordered structure, and the overall band structure agrees reasonably well with existing photoemission data. Dispersion and band structure perpendicular to the main dispersive direction are obtained. A temperature-dependent band broadening is calculated from configurations with thermal disorder of the atomic positions within the unit cell. This leads to band broadening of the two bands at the Fermi energy which can become comparable to their energy separation. The bands are particularly sensitive to in-plane movements of Mo sites far from the Li sites, where the density of states (DOS) is highest. The latter fact makes the effect of Li vacancies on the two bands relatively small. Spin-polarized band results for the ordered structure show a surprisingly large exchange enhancement on the high DOS Mo sites. Consequences for spin fluctuations associated with a cell doubling along the conducting direction are discussed.

Effective Hamiltonian for the electronic properties of the quasi-one-dimensional material Li0.9Mo6O17Merino1, McKenzie2 2012Phys. Rev. B252490View full textAdd to dashboardCite

“…We keep the most relevant hopping amplitudes needed for the description of the band structure. The simple tight-binding model for the d xy orbitals is compared to previous 44 and more recent 55 LDA-DFT bandstructure calculations from which the hopping amplitudes can be extracted.…”

Calculated Effects of Disorder on the Mo Core Levels in Purple Bronze Li2Mo12O34

“…The force constant K, (where K = M A ω 2 , where M A is an atomic mass; here the mass of Mo is used because of its dominant role in the electronic DOS) can be calculated as K = d 2 E/du 2 (E is the total energy), or it can be determined experimentally. We use the measurements of the phonon DOS of the related blue bronze K 0.3 MoO 3 [30] to estimate K and the average displacements of Mo atoms, as is explained in [12].…”

“…The electronic structure of Li 2 Mo 12 O 34 (two formula units of stoichiometric purple bronze) has been calculated using the Linear Muffin-Tin Method (LMTO) [28,31,32], in the local density approximation (LDA) [33], with or without disorder [12]. The lattice dimension and atomic positions of the structure have been taken from Onoda et al [3].…”

“…The band structure shows that two bands are very close together near the crossing of the Fermi energy, E F , with a moderately large density-of-states (DOS). Thermal disorder and possible spin fluctuations on the valence bands lead to band broadening and partial gaps at E F [12].…”

“…However, structural disorder due to thermal vibrations are detectable through spectroscopic methods [12,[15][16][17][18], and it is recognized as being important for T-dependent physical properties in materials with particular fine structures in the DOS near E F [12,[19][20][21][22][23][24][25][26]. Here, for purple bronze, effects of thermal fluctuations at large T and zero-point motion (ZPM) at T = 0 might be pertinent for the degree of dimensionality and the band overlap between the two bands at E F [12]. The present work shows that core level broadening from thermal disorder can be large, and therefore it is important to recognize core level spectroscopy as a useful tool for the determination of the effects of disorder on valence states and material properties.…”