R. L. Meng

Superconductivity at 52.5 k in the lanthanum-barium-copper-oxide system.

A superconducting transition with an onset temperature of 52.5 K has been observed under hydrostatic pressure in compounds with nominal compositions given by (La0.9Ba0.1)2 CuO4-Y. Possible causes for the high-temperature superconductivity are discussed.

Enhancement of flux pinning and critical currents in YBa2Cu3O7−δ films by nanoscale iridium pretreatment of substrate surfaces

We have acquired positive results in a controlled study to investigate the effects of substrate surface modification on the growth-induced flux-pinning nanostructures in YBa2Cu3O7−δ (YBCO) films. Nanoscale iridium (Ir) particles were applied to single-crystal SrTiO3 substrate surfaces using dc-magnetron sputtering. Superconducting properties of YBCO films grown on the Ir-modified substrates, measured by transport and magneto-optical imaging, have shown substantial improvement in the critical current densities (Jc) at 77 K over those on untreated, control substrates. Results also show a nearly uniform enhancement of Jc over all orientations of magnetic field. Present results are found to be consistent with cross-sectional transmission electron microscopy investigations. Ultimately, the objective of this approach is to produce enhancements in the properties of coated conductors by a simple pretreatment of the substrate surface.

Calorimetric evidence of antiferromagnetism in GdBa2Cu3O6+δ

Abstract Heat capacity measurements between 1.8 and 10K have been made on the superconductor GdBa 2 Cu 3 O 6+δ with δ ⋍ 0.7 and T c ⋍ 94K. A well defined magnetic transition prevails at 2.22K. The entropy involved agrees well with the complete antiferromagnetic ordering of Gd 3+ ions.

Local chemistry and vibrational behavior of the rare-earth in the high TC superconductor EuBa2Cu3O7

Temperature dependent 151Eu Mossbauer spectroscopy experiments on the high Tc≧90K superconductor EuBa2Cu3O7 in the range of 4.2K < T < 300K display evidence of Eu3+ site only. No evidence of Eu2+ sites is observed. Variation of isomer-shift σ(T) and recoil-free-fraction f(T) shows that the mean square displacement (lnf) and mean square velocity (σ) of the Eu3+ site are both characterized by a Debye temperature θD=280(5)K. No discernible evidence of a phonon mediated structural transition as a precursor to Tc is observed in the temperature range examined.

HIGH CRITICAL CURRENT DENSITY IN NEUTRON-IRRADIATED BULK YBa2Cu3O7

Bulk melt-textured YBa2Cu3O7 irradiated by fast neutrons has been investigated. We observed: (1) a critical current density (Jc) determined magnetically at 77 K exceeds 5×105A/cm2 in 0.1 T, 2×105A/cm2 in 1 T, and 1.2×103A/cm2 in 5.0 T; (2) at 5 K, Jc is >2.1×107A/cm2 in 0.5 T, ~1.8×107A/cm2 at 1 T and >8×106A/cm2 in 5.0 T and (3) the Jc-anisotropy decreases rapidly with the external field (H), e.g. at 77 K, it becomes 1 at ~4.8 T with Jc ~3×103A/cm2. We also found that (1) such a Jc-enhancement is most effective in low H but less so in high H, (2) the enhancement is temperature independent at low H, (3) the pinning potential remains unchanged to within ~10% as Jc varies by a factor of 15, and (4) thermal annealing up to 600°C recovers the un-irradiated state. These results and their implications are presented and discussed.

Thermodynamic properties in the normal and superconducting states of NaxCoO2.yH2O powder measured by heat capacity experiments

The heat capacity of superconducting Na(x)CoO(2)*yH(2)O was measured and the data are discussed based on three different models: The thermodynamic Ginzburg-Landau model, the BCS theory, and a model including the effects of line nodes in the superconducting gap function. The electronic heat capacity is separated from the lattice contribution in a thermodynamically consistent way maintaining the entropy balance of superconducting and normal states at the critical temperature. It is shown that for a fully gapped superconductor the data can only be explained by a reduced (about 55 %) superconducting volume fraction. The data are compatible with 100 % superconductivity in the case where line nodes are present in the superconducting gap function.

Granularity and intragrain penetration depth in RuSr2GdCu2O8

Abstract The multi-stage transition of RuSr 2 GdCu 2 O 8 (Ru1212) ceramic samples was investigated. An intergrain phase-lock temperature of T 2 is identified around 10–30 K. Slightly above T 2 , however, an isolated Ru1212 grain still behaves as a Josephson-junction array of superconductive nano-domains, which are phase-locked below an interdomain phase-lock temperature (intragrain transition-temperature) of T 1 ≈33–40 K. The intragrain penetration depth λ 1 was subsequently deduced from the ac susceptibility χ ′ of powders with different particle-size d (0.3⩽ d ⩽10 μm in this work) between T 1 and T 2 . The sample-dependent λ 1 ≈2.5–6 μm so obtained is extremely long, and bears a systematic correlation with the T 1 observed. A phase-separation model was proposed to interpret the data.

Far-infrared transmission spectra of high Tc (Y0.6Ba0.4)2CuO4−y superconductors

Abstract We have measured the far-infrared transmission spectra of two samples of the YBaCuO superconductor which have transition temperatures of 98K and 86K respectively. Both samples showed absorption peaks at 110 cm -1 , 141 cm -1 , 173 cm -1 , 209 cm -1 , 234 cm -1 , 273 cm -1 , and 328 cm -1 , in the frequency range of 50 – 340 cm -1 , in both the normal and the superconducting states. The peak frequencies did not vary with temperature. The linewidths were smaller for the sample with T c = 98 K. For one sample it was estimated from the temperature dependence of the transmission spectra that the superconducting energy gap was 2δ ≅ 3.3 kT c .

Study of intermetallic compounds isostructural to MgB/sub 2/

The small intergrain effect of MgB/sub 2/ on supercurrent makes it one of the most promising candidates for superconducting conductors due to its easier processing and the associated lower manufacturing cost. Unfortunately, the superconducting transition temperature T/sub c/ is only 40 K. However, band-structure calculations predict that a higher T/sub c/ than that of MgB/sub 2/ is possible for isostructural and isovalent intermetallic compounds with greater lattice parameters or greater unit cell volumes. The prediction appears to be consistent with the negative pressure effect on T/sub c/ observed. The substitution of the larger Ca-ions for the smaller Mg-ions has thus been suggested to raise T/sub c/, but not yet realized. Alternatively, we have synthesized and studied a series of binary and pseudobinary intermetallic compounds, AGa/sub 2/, AGa/sub 2-x/Si/sub x/, and AAl/sub 2-x/Si/sub x/, where A=Ca, Sr, or Ba, which are isostructural to MgB/sub 2/ and have greater lattice parameters than MgB/sub 2/. In spite of the greater lattice parameters, AGa/sub 2/ are not superconducting. However, all pseudobinary compounds AGa/sub 2-x/Si/sub x/ and AAl/sub 2-x/Si/sub x/ for 0.6<x<1.5 are superconducting. Superconductivity is reported for the first time in SrAlSi, with T/sub c/ of 5.1 K. Although T/sub c/ varies with x in a similar fashion for all members of the series, no specific correlation between T/sub c/ and lattice parameters or ionic mass is observed. The maximum T/sub c/ of these compound series with different As varies between 5.5 and 7.8 K, much lower than that of MgB/sub 2/. The results strongly suggest the unique role of B in the superconductivity of C32 intermetallic compounds. They also demonstrate that factors additional to the lattice parameters and densities of states must play an important role, and that the rigid-band model is not sufficient to account for the observations.

High Pressure Study on MgB2

The hydrostatic pressure effect on the newly discovered superconductor MgB2 has been determined. The transition temperature Tc was found to decrease linearly at a large rate of -1.6 K/GPa, in good quantitative agreement with the ensuing calculated value of -1.4 K/GPa within the BCS framework by Loa and Syassen, using the full-potential linearlized augmented plane-wave method. The relative pressure coefficient, dlnTc/dp, for MgB2 also falls between the known values for conventional sp- and d-superconductors. The observation, therefore, suggests that electron-phonon interaction plays a significant role in the superconductivity of the compound.