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.

Sr 4 Ru 3 O 10 is characterized by a sharp metamagnetic transition and ferromagnetic behavior occurring within the basal plane and along the c-axis, respectively. Resistivity at magnetic field, B, exhibits low-frequency quantum oscillations when B||c-axis and large magnetoresistivity accompanied by critical fluctuations driven by the metamagnetism when B⊥c-axis. The complex behavior evidenced in resistivity, magnetization and specific heat presented is not characteristic of any obvious ground states, and points to an exotic state that shows a delicate balance between fluctuations and order.

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ABSTRACT:We have measured the specific heat of single crystals of the triple-layer RuddlesdenPopper material, Sr 4 Ru 3 O 10 , grown both in an image furnace and by flux-growth. The flux grown sample has a sharp mean-field-like anomaly at the onset of magnetic order, T C = 102 K, but a much broader anomaly, indicative of residual heterogeneity, is observed for the image furnace sample. Even for the flux grown sample, however, the anomaly is at least an order of magnitude smaller than one would expect for complete ordering of the spins. Neither sample exhibits an anomaly at T M ~ 50 K, where magnetic measurements suggest that basal plane antiferromagnetism sets in. Anomalous behavior (e.g. consistent with a term in the specific heat ∝ T 3/2 as would be observed for a threedimensional ferromagnet with weak exchange) is observed at low temperatures for both samples, indicative of the unusual magnetic order in this material.

Formation of an insulating ground state in the salt of -(BEDT-TTF) 2 Cu͓N(CN) 2 ͔I was studied by measuring the temperature dependence of the electronic transport properties ͑the anisotropy of the resistivity, thermopower along the principal in-plane directions, and the Hall effect͒ and the x-ray diffraction patterns. Qualitative agreement of all the transport properties with the metallic state predicted by the band structure calculation was found above 230 K. Transformation to the insulating state proceeds via an intermediate stage between 230 and ϳ100 K, in which the transport properties are affected by localization phenomena in the disordered metallic state. The state is induced by the formation of a commensurate superstructure in the anion layer, leading to a unit cell doubling in the c-direction. The electronic spectrum starts to form a real gap below 100 K, as indicated by the sharp thermopower change, which is mainly related to the destruction of the hole pocket. The gap formation is accompanied by an in-plane resistivity anisotropy increase. This transformation is due to a short-range ordering, with a wave vector close to c*/3, strongly interacting with the electronic system.