W.N. Creager

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.

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.

Pressure and magnetic field effects on transport in NbSe3

Abstract We have performed dc resistance measurements on the charge density wave (CDW) conductor NbSe 3 , at pressures up to 38kbar, and magnetic fields up to 8T. We show that a magnetic field generates no anomalous magnetoresistance in the lower CDW state if the CDW transition is suppressed by pressure. We also show that the upper CDW state is unaffected by the application of a magnetic field. Furthermore, we show evidence of a possible H-induced phase transition at ∼30K.

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 .

Anisotropic Transport in Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O

The anisotropic normal state transport properties of the superconducting oxides Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O are investigated by dc resistivity, thermoelectric power, high frequency conductivity, and uniaxial stress effects in single crystals. We also explore the superconducting state by measurements of Tc under c-axis stress, and oxygen isotope substitution. Energy gap structure is investigated by break junction single-crystal tunneling.

Model of dynamic charge density wave break-up in an applied temperature gradient

Abstract We examine theoretically the high-velocity dc dynamics of a charge density wave (CDW) condensate subjected to a uniform longitudinal temperature gradient. The condensate is treated within an elastic medium model where internal strain and carrier conversion are treated explicitly. For sufficiently large temperature gradients, the CDW breaks up into a series of N coexisting subdomains with independent phase velocities. N scales directly with the magnitude of the temperature gradient and with the length of the crystal. Unusual dynamical asymetries are also predicted depending on the relative directions of heat and electrical current flow through the CDW crystal.

Far infrared reflectance and conductivity of TaS3: Completion of the AC response spectrum

Abstract The far infrared reflectance of the charge density wave (CDW) conductor TaS 3 has been measured from 3 to 700cm −1 with incident radiation polarized parallel and perpendicular to the long axis of the crystals. For parallel polarization, a strong reflection edge correlated with the formation of the CDW appears near 70cm −1 . The associated conductivity shows no dramatic conduction mode in the far infrared. These results are in share contrast to the observed behavior in the related CDW materials (TaSe 4 ) 2 I and K 0.3 MoO 3 , and they rule out models of a “generic far infrared mode” in CDW excitations.

Asymmetry in the static and dynamic charge density wave states of NbSe3 : heat and electrical current flow

Abstract The dynamic and static states of the charge density wave (CDW) in NbSe 3 have been measured by narrow band noise and differential resistance measurements, in the presence of a symmetry-breaking temperature gradient. In the dynamic state, the CDW velocity is sensitive to the relative direction of heat and electrical current flow. In the pinned state, metastable state structure with dramatic hysteresis is observed. Our results are consistent with predictions of a phase strain model of CDW motion in the presence of a temperature gradient.

Magneto-elastic properties of NbSe3

Abstract The Youngs Modulus (Y) and internal friction (δ) have been measured in the charge density wave (CDW) conductor NbSe 3 . Anomalies are clearly resolved in Y and δ at the upper CDW transition, T 1 (144K) and in Y at the lower CDW transition, T 2 (59K). Below 100K, Y has been measured in transverse magnetic fields up to 8T. Small field-induced stiffening effects are observed.

Temperature-gradient-induced subdomain scaling in NbSe3

Abstract In a temperature gradient, the charge density were (CDW) in NbSe 3 breaks up into a series of N coexisting “subdomains” with independent CDW phase velocities. With ΔT the temperature difference applied across the ends of the sample, we find N ∼(Δ T 2 3 . This scaling relation has been predicted by a phase-strain model of CDW dynamics.