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.

We report results of a magnetic and transport study of SrRu 1-x Mn x O 3 (0≤ x<0.60), i.e., Mn doped SrRuO 3 . The Mn doping drives the system from the itinerant ferromagnetic state (T C =165 K for x=0) through a quantum critical point at x c =0.39 to an insulating antiferromagnetic state. The onset of antiferromagnetism is abrupt with a Néel temperature increasing from 205 K for x=0.44 to 250 K for x=0.59. Accompanying this quantum phase transition is a drastic change in resistivity by as much as 8 orders of magnitude as a function of x at low temperatures. The critical composition x c =0.39 sharply separates the two distinct ground states, namely the ferromagnetic metal from the antiferromagnetic insulator.

The 5d-electron based BaIrO 3 is a nonmetallic weak ferromagnet with a Curie temperature at Tc=175 K. Its largely extended orbitals generate strong electron-lattice coupling, and magnetism and electronic structure are thus critically linked to the lattice degree of freedom. Here we report results of our transport and magnetic study on slightly Sr doped BaIrO 3 . It is found that dilute Sr-doping drastically suppresses Tc, and instantaneously leads to a nonmetal-metal transition at high temperatures. All results highlight the instability of the ground state and the subtle relation between magnetic ordering and electron mobility. It is clear that BaIrO 3 along with very few other systems represents a class of materials where the magnetic and transport properties can effectively be tuned by slight alterations in lattice parameters.The layered iridates BaIrO 3 [1][2][3][4][5] and Sr n+1 Ir n O 3n+1 with n=1 and 2 [6-11] possess two unique features not found in other materials, namely, high-temperature weak ferromagnetism and an insulating ground state with phase proximity of a metallic state.These features defy common wisdom that 4d and 5d transition metal oxides should be much more conducting than their 3d counterparts because of the more extended d-orbitals that significantly reduce the Coulomb interaction. In fact, this extended characteristic significantly enhances crystalline field splittings and the d-p hybridization between the transition metal and the oxygen octahedron surrounding it. This, in turn, leads to strong electron-lattice coupling, which very often alters and distorts the metal-oxygen bonding lengths and angles, lifting degeneracies of t 2g orbitals and thus precipitating possible orbital ordering. It is this feature that defines a high sensitivity of the ground state to the atomic stacking sequence and structural distortions. This characteristic is illustrated in recent studies on 4d-electron based ruthenates where magnetism and electric transport change drastically and systematically with structural dimensionality. Although the layered iridates distinguish themselves from the ruthenates by showing conspicuously low magnetic moments and yet high Curie temperatures, magnetic and transport behavior of these 5d-electron systems is also critically linked to the lattice degree of freedom, requiring only dilute impurity doping to tip the balance across the metal-insulator borderline accompanying drastic changes in magnetic ordering.The Ir 4+ (5d 5 ) ions in these layered iridates are presumed to be in a low spin state with S=1/2 since the large extension of the 5d orbitals leads to large crystal field splittings and a reduced Coulomb repulsion [1][2][3][4][5][6][7][8][9][10][11]. BaIrO 3 is a nonmetallic weak ferromagnet with an exotic ground state. Recent experimental studies reveal evidence for 2 a simultaneous onset of weak ferromagnetism and charge density wave (CDW) formation at Tc=175 K [4], an unexpected occurrence that is also supported by results of tightbinding band structure calculations, whi...

Transport and magnetic studies of Ca3Ru2O7 for temperatures ranging from 0.4 to 56 K and magnetic fields B up to 45 T lead to strikingly different behavior when the field is applied along the different crystal axes. A ferromagnetic (FM) state with full spin polarization is achieved for the B//a axis, but colossal magnetoresistance is realized only for the B//b axis. For the B//c axis, Shubnikov-de Haas oscillations are observed and followed by a less resistive state than that for B//a. Hence, in contrast with standard colossal magnetoresistive materials, the FM phase is the least favorable for electron hopping. These properties together with highly unusual spin-charge-lattice coupling near the Mott transition (48 K) are driven by the orbital degrees of freedom.