Probing localization effects inLi0.9Mo6O17

Abstract

We report the polarized reflectance and optical conductivity of the quasi-one-dimensional conductor Li 0.9 Mo 6 O 17 as a function of temperature. The compound displays an unusual ͑non-Drude-type͒ mobile carrier response at low-energy, with partially screened vibrational features along the highly conducting b axis. In addition, we observe Mo d→d transitions near 0.42, 0.57, and 1.3 eV, and an O p→Mo d charge-transfer band near 4 eV. Perpendicular to the b axis, Li 0.9 Mo 6 O 17 exhibits semiconducting behavior with an optical gap of 0.4 eV and electronic structure similar to that of the b axis at higher energies. The substantial temperature dependence of the vibrational modes in this direction reveals that the lattice of Li 0.9 Mo 6 O 17 is not rigid. However, no noticeable change in the lattice through the 25 K metal-insulator transition is observed. Comparing x-ray and infrared data for several model materials, we establish an upper bound on the size of any lattice distortion in Li 0.9 Mo 6 O 17. Based upon these combined results, we argue that localization effects dominate the bulk and microscopic properties of this material.

Luttinger-liquid theory of purple bronze Li0.9Mo6O17in the charge regime

“…The only visible discrepancies are the relative vertical shifts of the bands of order 0.1eV. The computed ratio between the Fermi velocities along b-and c-crystal axis, is about 40, compatible with the reported anisotropic 1D-like resistivity [4,5,7,21]. The velocity along a is even smaller.…”

“…Then the two bands seems to merge. This is also the energy scale revealed by optics [7]: it shows a formation of a first plasmon edge for electrons along b-axis (Γ − Y direction) and a gap value in perpendicular direction. Thus it sounds reasonable to take it as a point where the 1D physics forms.…”

“…2 in Ref. [7] and gives ω p = 0.65eV (ω p extracted from the LDA [17] is even larger ≈ 1.1eV ). The sum rule integral I σ was in fact already evaluated in Fig.…”

“…3 of Ref. [7]. Determining the value of the first saturation from the plot has of course some error attached to it.…”

“…Several very different experimental probes have been used: angle resolved photo-emission (ARPES) [2], scanning tunnelling microscopy (STM) [3], DC [4] and magneto-resistivity [5], thermal conductivity [6], optical conductivity [7], Nernst signal [8], muons spectroscopy [9], X-rays [10], thermal expansion [11], neutron scattering [12]. Although the main effort of those investigations was focused on the nature of a mysterious phase transition at around 25K, interesting knowledge about higher energy phase was also gathered.…”