L. C. Bourne

Elastic anomalies in the charge density wave conductor K0.3MoO3

Abstract We have measured Youngs modulus Y of the charge density wave (CDW) conductor K0.3MoO3, both parallel (Y|) and perpendicular (Y⊥) to the highly conducting (b) axis. The elastic constants are anisotropic, and both Y| and Y⊥ show sharp anomalies at the Peierls transition temperature at T=180K. The anomaly in Y⊥ is dramatic and similar to that oberved in TaS3. No change in Y⊥ or in internal friction is onserved upon CDW depinning. We compare the detailed form of Y⊥ near the transition temperature to predictions of a recent theoretical model which considers coupling between electrons and the soft phonon.

On the 17.5K transition in URu2Si2: thermopower and elasticity

Abstract We report thermoelectric power (TEP) and elasticity measurements on the heavy Fermion compound URu2Si2. The TEP is positive and roughly independent of temperature between 300K and 120K, becomes increasingly negative below 70K, and displays a sharp change in slope at Tp=17.5K. At 15K there is a dramatic reversal in the TEP. The Youngs modulus shows a strong dip near 18K, accompanied by an increase in the internal friction. The TEP and elastic anomalies are consistent with a Fermi surface instability at Tp, and we interpret our results in terms of charge or spin density wave formation.

Compositionally dependent superconducting transition temperature of YBaCu oxides

Abstract The superconducting transition temperatures of a family of YBaCu oxides have been explored by dc resistivity measurements. Substitutions of magnetic and non-magnetic ions on Y, Ba and Cu sites are reported. For some compositions, achieving high superconducting transition temperatures is strongly dependent on growth conditions for the material.

Pressure dependence of superconductivity in single-crystal Bi2(Sr, Ca)3Cu2O8

Abstract The pressure dependence of the superconducting transition temperature (T c ) of a single-crystal sample of Bi 2 (Sr, Ca) 3 Cu 2 O 8 was measured up to 42 Kbar. T c first increased slightly with pressure, reaching a maximum at around 10 Kbar, and then decreased with increasing pressure at a rate of about 0.05 K/Kbar. This non-linear pressure dependence of T c is discussed within a two-dimensional BCS model, and within two of the resonating-valence-bond models of high-T c superconductivity.

Magnetic and resistive determination of the oxygen isotope effect in La1.85Sr0.15CuO4

Abstract In polycrystalline samples of the superconducting oxide La1.85Sr0.15CuO4, the substantial substitution of 18O for 16O causes a small downward shift in the superconducting transition temperature Tc. Although Tc(onset) measured resistively differs by as much as two Kelvin from that measured magnetically, the two measurement methods yield comparable Tc shifts upon isotope substitution. Samples with different superconducting phases (as evidenced by resistive fine structure in the transition region) show similar isotope shifts for different phases. We determine an overall oxygen isotope shift in La1.85Sr0.15CuO4 corresponding to α = 0.140±.008, where Tc∼M−α and M is the oxygen mass.

Elastic properties of polycrystalline La2−xSrxCuO4

Abstract We report on the temperature dependence of the shear (G) and Youngs (E) moduli of polycrystalline La 1.85 Sr 0.15 CuO 4 and La 2 CuO 4 . We find that E∼3G, implying that the single crystal shear moduli are small, so that the polycrystalline E and G are determined by the single crystal shear moduli only. Contrary to expectations, there are large, but sample dependent, drops in moduli at T c for the superconducting Sr-alloy. The large shear softening associated with the tetragonal-orthorhombic transition at 180K is also very sample dependent, and does not appear simply related to the superconducting transition. The polycrystalline elastic moduli are very hysteretic below the structural transition, presumably due to the distortion driven variation in crystalline dimensions with temperature.

The isotope and superconducting oxides

The authors explored standard isotropic three-dimensional theory for the isotope effect with a purely electron-phonon attractive interaction. They find that additional features are necessary to account for the observed values of the isotope effect in the high-Tc superconducting oxides. Some recent proposals for possible explanations of the observed reduced isotope effects are discussed.

Electronic interference, elasticity, and scanning tunneling microscopy studies of the charge density wave conductor K0.3MoO3

Abstract Charge density wave (CDW) dynamics and statics are explored in the blue bronze K 0.3 MoO 3 . For very thin (optically transparent, d≈0.2 μ m) platelets highly coherent response obtains, and in the presence of dc electric fields the ac conductivity shows high frequency interference structure. This interference is discussed in terms of a resonant circuit analog. At low temperatures, the zero differential resistance state is associated with dramatic changes in the elastic constants of the crystal. However, the effects of sample core heating may be important. Both above and below the CDW transition temperature, the surface of K 0.3 MoO 3 has been examined by atomic resolution scanning tunneling microscopy. Although the underlying lattice is clearly resolved, no CDW superstructure has been imaged on the crystal surface.

Electron-phonon interactions in high-temperature oxide superconductors: Isotope effects and elasticity studies

Abstract The substitution of different oxygen isotopes into the high-T c oxide superconductors La 1.85 Sr 0.15 CuO 4 and YBa 2 Cu 3 O 7 is investigated by transport and magnetic measurements. For both materials, replacement of 16 O with 18 O depresses T c slightly. The observed shifts are much smaller than those expected from conventional electron-phonon pairing superconductivity. We also explore the elastic properties of La-, Y-, and Bi-based high-T c superconductors, including single crystals. Only La 1.85 Sr 0.15 CuO 4 shows a dramatic soft phonon mode above T c .

Experimental electronic structure of Bi 2 Ca Sr 2 Cu 2 O 8 + δ

Highly oriented polycrystalline Bi/sub 2/CaSr/sub 2/Cu/sub 2/O/sub 8+//sub delta/ has been studied by x-ray photoemission spectroscopy (XPS) to determine the oxidation states of its constituents and the valence electronic structure. Core-level shifts indicate 3/sup +/ and 2/sup +/ as main oxidation states for Bi and Cu, respectively. The Ca and Sr core levels exhibit pronounced shifts to lower binding energy compared to the metals. The Ca lines show two main components, indicating either inequivalent sites or the presence of a different phase. The XPS and ultraviolet photoemission spectroscopy valence-band spectra show weak but reproducible metallic emission at the Fermi level (E/sub F/) which we ascribe to states originating in the Bi-O planes. The main Cu d--O p band is between 2 and 8 eV below E/sub F/; this is about 1 eV lower than predicted by band-structure calculations. We argue that this is due to electronic correlations tending to increase the anisotropy in the ground-state charge distribution compared to that obtained in local-density-approximation calculations.