NMR Study on La and Tl-based High-T_c Cuprates (High Field Superconductors)

NMR study on three types of high-T_C cuprates TlBa_2CaCu_2O_ (Tl1212), La_Ba_xCuO_4 (LBCO) and (La_Y_y)_Ce_xCuO_4 (LYCCO) is reported. First the Knight shift in the superconducting state was investigated for the Zn-substituted TlBa_2Ca(Cu_Zn_z)_2O_, which belongs to the over-doped region. The temperature dependence of the Knight shift was successfully explained in terms of the partially closed d-wave model proposed by Kitaoka et al. The reduction in T_C by Zn-substitution was also consistent with Miyake\u27s theoretical calculation on the potential scattering of the unitarity limit in the d-wave superconductors. Next, the impurity effect on the anomalous suppression of the superconductivity in La_Ba_xCuO_4 (LBCO) around x≅1/8 was investigated by La-NMR and ultrasonic measurements. The transition temperatures of the magnetic order and of the structural phase transformation in Zn^ and Ce^-doped LBCO have shown that the main and direct force to the suppression in the superconductivity is the magnetic ordering, and that the role of the structural phase transformation is the enhancement of the suppression. Lastly, the new electron doped cuprate free from 4f-spins has been synthesized and studied by NMR. Observed spectra of ^Cu without quadrupolar splitting similar to other conventional electron-doped cuprates indicate that the doped carrier in this system is electron like

Mott Transition of MnO under Pressure: Comparison of Correlated Band Theories

The electronic structure, magnetic moment, and volume collapse of MnO under pressure are obtained from four different correlated band theory methods; local density approximation + Hubbard U (LDA+U), pseudopotential self-interaction correction (pseudo-SIC), the hybrid functional (combined local exchange plus Hartree-Fock exchange), and the local spin density SIC (SIC-LSD) method. Each method treats correlation among the five Mn 3d orbitals (per spin), including their hybridization with three O $2p$ orbitals in the valence bands and their changes with pressure. The focus is on comparison of the methods for rocksalt MnO (neglecting the observed transition to the NiAs structure in the 90-100 GPa range). Each method predicts a first-order volume collapse, but with variation in the predicted volume and critical pressure. Accompanying the volume collapse is a moment collapse, which for all methods is from high-spin to low-spin (5/2 to 1/2), not to nonmagnetic as the simplest scenario would have. The specific manner in which the transition occurs varies considerably among the methods: pseudo-SIC and SIC-LSD give insulator-to-metal, while LDA+U gives insulator-to-insulator and the hybrid method gives an insulator-to-semimetal transition. Projected densities of states above and below the transition are presented for each of the methods and used to analyze the character of each transition. In some cases the rhombohedral symmetry of the antiferromagnetically ordered phase clearly influences the character of the transition.Comment: 14 pages, 9 figures. A 7 institute collaboration, Updated versio

Transition to a Virtually Incompressible Oxide Phase at a Shock Pressure of 120 GPa (1.2 Mbar): Gd3Ga5O12

Cubic, single-crystal, transparent Gd{sub 3}Ga{sub 5}O{sub 12} has a density of 7.10 g/cm{sup 3}, a Hugoniot elastic limit (HEL) of 30 GPa, and undergoes a continuous phase transition from 65 GPa to a quasi-incompressible (QI) phase at 120 GPa. Only diamond has a larger HEL. The QI phase of Gd{sub 3}Ga{sub 5}O{sub 12} is more incompressible than diamond from 170 to 260 GPa. Electrical conductivity measurements indicate the QI phase has a bandgap of 3.1 eV. Gd{sub 3}Ga{sub 5}O{sub 12} can be used to obtain substantially higher pressures and lower temperatures in metallic fluid hydrogen than was achieved previously by shock reverberation between Al{sub 2}O{sub 3} disks. Dynamic compression achieves pressures, densities, and temperatures that enable investigation of ultracondensed matter at conditions yet to be achieved by any other technique. The prototypical example is observation of minimum metallic conductivity (MMC) of dense fluid hydrogen at 140 GPa, nine-fold compression of liquid density, and {approx}3000 K [1-3]. The high pressure and density and relatively low temperature are achieved by multiple-shock compression [2]. Temperature T is relatively low in the sense that T/TP{sub F} {approx} 0.01, where T{sub F} is the Fermi temperature. The time scale of compression is sufficiently long to achieve thermal equilibrium and sufficiently short so the process is adiabatic. Similar results are obtained for oxygen [4] and nitrogen [5]. Fluid Cs and Rb undergo the same transition at 2000 K near their liquid-vapor critical points [6]. All five elemental fluids have essentially the same value of MMC and the density dependences of their semiconductivities scale with the quantum-mechanical charge-density distributions of the respective atoms [5]. Liquid H{sub 2} is one of the most compressible of all materials. In this paper, we report that the dielectric crystal Gd{sub 3}Ga{sub 5}O{sub 12} (GGG) transitions to a virtually incompressible phase at 120 GPa shock pressure

First-principles calculations of the phase stability of TiO2

Published versio

Thermal conductivity of high- T c superconductors

This paper reviews existing data on the thermal conductivity of high- T c superconductors. Included are discussions of pristine polycrystalline high- T c ceramics, single crystal specimens, and high- T c materials structurally modified by substitution or by radiation damage. The thermal conductivity of high- T c superconductors is compared with that of conventional superconductors, and dramatic differences are found between the two families. Mechanisms of thermal conductivity applicable to high- T c perovskites are discussed and implications for theories of high- T c superconductivity are noted.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/45121/1/10948_2004_Article_BF00617463.pd