P J Ford

The effects of quenching and lead substitution on the ultrasonic wave velocity and attenuation in bismuth cuprate high Tc superconductors

The velocity and attenuation of longitudinal and shear ultrasonic waves propagated in ceramic specimens of single phase Bi(Pb)2223, Bi(Pb)2212 and lead-free Bi2212 have been measured between 10 and 295 K. The anomalous elastic effects and hysteresis in the range about 190-240 K reported by other workers have been found in samples of Bi(Pb)2223 and lead-free Bi2212. However, for the Bi(Pb)2212 phase, in which some Bi is substituted by Pb, these anomalies and hysteresis are suppressed. Quenching a sample of the Bi2212 phase from 800 degrees C into liquid nitrogen removes these anomalies, which have therefore been shown to be strongly dependent on chemical composition: either oxygen content or lead substitution for bismuth. The elastic stiffnesses of these bismuth cuprates are unusually small due largely to (i) their open microstructure on the macroscopic level but also in part as a result of (ii) their rather open layer-like structure with weak interlayer binding forces and (iii) the intermediate valence state of the copper ions. It is known that quenching the Bi2212 phase from just below the melting temperature (about 880 degrees C) into liquid nitrogen results in the reduction in oxygen content that is necessary to optimize the superconducting properties of this phase. The authors have now established that quenching also results in substantial longitudinal acoustic mode softening whilst enhancing Tc.

Anisotropic elastic and nonlinear acoustic properties of very dense textured Bi2Sr2CaCu2O8+y

The velocities of longitudinal and shear ultrasonic waves propagated in very dense (95% of theoretical), highly-textured, ceramic Bi2Sr2CaCu2O8+y (Bi2212) have been measured as functions of temperature (between 10 and 290 K) and hydrostatic pressure at 290 K. The main finding is a marked anisotropy of the elastic stiffness constants and their dependences on pressure. The authors have observed an elastic stiffness constant in the a-b plane that is substantially greater than that in the c direction. This observation is consistent with the crystallographic structure of Bi2212. The anomalous elastic effects and thermal hysteresis, observed previously by several groups in various Bi-based superconductors in the range of 19-240 K, have not been found in the present material. These observations suggest that these anomalies are not an inherent property of Bi-based cuprates, but instead may depend on the oxygen content and microstructure of the ceramic material. An analysis of sound velocity in a polycrystalline array of elastically anisotropic crystallites is presented.

Contrasting effects of lanthanum and samarium modifiers on the elastic and non-linear acoustic properties of phosphate glasses

While samarium phosphate (Sm2O3)x(P2O5)(1−x) glasses are known to show pronounced long-wavelength acoustic-mode softening, their lanthanum (La2O3)x(P2O5)(1−x) analogues do not. To investigate experimentally this different influence of lanthanide modifiers, a comparative study has been made of the effects of temperature and pressure on the velocities of ultrasonic waves propagated in binary ((La2O3)x(P2O5)(1−x) and (Sm2O3)x(P2O5)(1−x)) and ternary ((La2O3)x(Sm2O3)y(P2O5)(1−x−y)) phosphate glasses with compositions near to that corresponding to the metaphosphate (R2O3)0.25(P2O5)0.75. For each glass the second-order elastic stiffness tensor componentsCIJScontinue to increase down to 10 K in a manner consistent with ultrasonic interactions with two-level systems. Measurements of the effects of hydrostatic pressure on the ultrasonic wave velocities have been used to determine the hydrostatic pressure derivatives (∂CIJS/∂P)T,P=0 of the second-order elastic stiffness tensor components and (∂B0S/∂P)T,P=0 of the bulk modulusB0S at room temperature (293 K). For the ternary glasses (∂C11S/∂P)T,P=0, (∂C44S/∂P)T,P=0 and (∂B0S/∂P)T,P=0 are small but positive; these glasses stiffen under pressure. Replacement of samarium by lanthanum in the ternary glasses negates the acoustic-mode softening. Possible sources of the different effects of lanthanum and samarium modifiers on the non-linear acoustic properties of metaphosphate glasses are discussed.

A comparative study of the high-Tc electron superconductor Nd1.85Ce0.15CuO4-y and its parent compound Nd2CuO4-y

A study has been made of the electron-doped high-Tc superconducting oxide Nd1.85Ce0.15CuO4-y using ultrasonics, calorimetry, electrical property measurements and X-ray powder techniques. The behaviour is compared with that observed in the non-superconducting parent compound Nd2CuO4-y. Above about 50 K the electrical resistance of Nd1.85Ce0.15CuO4-y can be fitted by including a Kondo logarithmic term. In the superconducting specimen, there are no indications of the pronounced acoustic mode softening below room temperature of the kind that has been observed in La2-xSrxCuO4-y. However, at about 220 K there is a change in the temperature dependence of the longitudinal mode velocity and a much less pronounced effect for the shear mode. It is argued that this is consistent with the coupling of the longitudinal mode to the ionic volume change induced by enhancement of the 4+ contribution to the intermediate valence cerium ion. No such behaviour is observed in the parent compound Nd2CuO4-y. The intermediate valence state of the cerium ions plays an important role in the properties of this electron superconductor. Calorimetric and X-ray studies show no evidence for any structural transition in either the superconductor or the parent compound at around 200 K.

164 K resistive transition in yttrium barium copper oxide-a cautionary tale?

A halide-flux grown large grain size sample of YBa2Cu3O7-x has been found to develop a sharp drop in resistance at 164 K after thermal cycling. Susceptibility measurements show no evidence of a Meissner effect at this temperature. Ultrasonic measurements are presented that indicate a structural phase transition occurring at about 170 K in low oxygen content samples. The sharp drop in resistance is attributed to the structural phase transition and not to superconductivity. Attention is drawn to the similarity of these effects and those advanced as evidence of superconductivity in some new materials.

High-pressure ultrasonic determination of the phase diagram of antiferromagnetic Cr-0.3 at.% Ru alloy single-crystal

Abstract The temperature and hydrostatic pressure dependences of anomalies observed in the ultrasonic wave velocity have been used to determine (i) the boundary corresponding to the paramagnetic (P) to commensurate transition and (ii) the first-order phase boundary between the commensurate (C) and incommensurate (I) spin-density wave (SDW) phases on the pressure-temperature phase diagram for monocrystalline Cr-0.3 at.% Ru alloy. The paramagnetic-commensurate phase boundary is linear with a negative gradient d T PC /d P = -358±4 K / GPa . Both T CI and T IC increase approximately linearly with pressure, the mean values of dT CI / dP and dT IC / dP being 333±3 K/GPa and 277±5 K/GPa respectively. The position of the triple point is estimated as (315±5 K, 0.22±0.02 GPa). In the commensurate SDW state the alloy shows the extraordinary property that when pressure is applied it becomes easier to squeeze.

Magnetic origins of the phase transitional behaviour at 200 K in Ca0.5Zn0.5Fe2O4

Abstract Interest in the material Ca0.5Zn0.5Fe2O4 has been stimulated by the suggestion that it is superconducting up to 200K. We have examined this material using a variety of experimental techniques and have concluded that this is not the case. Instead, it is a mixture of two phases, one of which, having the orthorhombic CaFe2O4 structure, undergoes a rather unusual transition at around 200K whose physical properties and possible magnetic nature are discussed.

Elastic behaviour of Nd1.85Ce0.15CuO4-y and Nd2CuO4-Y

Ultrasonic measurements on the electron doped high Tc superconductor Nd1.85Ce0.15CuO4-y and Nd2CuO4-y its nonsuperconducting parent compound are presented. Although the superconductor shows no evidence of acoustic mode softening below room temperature, there is a clear change in the temperature dependence of the longitudinal mode velocity around 220 K. This effect is far less pronounced for the shear mode and is not observed in the parent compound, indicating that the intermediate valence state of the Ce ions plays an important role in the elastic behaviour of the superconductor. The first results for the pressure dependences of the elastic constants are reported.

Ultrasonic evidence of low bulk modulus and large vibrational anharmonicity in the bismuth cuprate high Tc superconductors

The velocities of longitudinal and shear ultrasonic waves propagated in ceramic specimens of single phase Bi(Pb)2223, Bi(Pb)2212 and lead-free Bi2212 have been measured as a function of hydrostatic pressure. The elastic stiffnesses of these bismuth cuprates are unusually small and their hydrostatic pressure derivatives ( delta CL/ delta P)P=0 and ( delta B/ delta P)P=0 at room temperature are large. The ceramic microstructure comprises loosely stacked, thin platelets; the substantial effects of porosity on the elastic properties have been approximated using wave scattering theory in a porous medium. The bismuth cuprates investigated show the common characteristic of high Tc mixed oxides based on copper that the bulk modulus B0 measured ultrasonically is much smaller than the BT(P) determined at very high pressures from X-ray measurements of lattice parameters in a diamond cell. This can be resolved by taking the contribution to the high pressure bulk modulus BT(P) of the pressure derivative ( delta B/ delta P)P=0 into account. In these materials the anharmonicity of the longitudinal acoustic modes in the long wavelength limit is large, probably as a result of the combination of (i) weak interlayer binding forces due to the lone pairs sited between the pairs of widely spaced BiO layers and (ii) intermediate valence effects.