B. F. Woodfield

Specific heat of YBa2Cu3O7

Specific heat measurements, including measurements in magnetic fields and at both low temperatures and nearTc, on a number of YBa2Cu3O7 samples have revealed several correlations among strongly sample-dependent parameters. These correlations suggest that the sample dependence of the parameters reflects a sample dependence of the volume fraction of superconductivity, which is in turn correlated with a low concentration of Cu2+ moments. The correlations give a criterion for recognizing the values of the parameters characteristic of the fully superconducting material. Preliminary results on the effects of sample heat treatment are reported. New data on the “linear term” is presented and discussed.

Dependence of the low-temperature specific heat of YBa{sub 2}Cu{sub 3}O{sub 7-{delta}} on {delta} and magnetic field: The H{sup {1/2}}T term for H{ne}0; absence of evidence for a T{sup 2} term for H=0

Specific-heat measurements with improved precision on two new samples of YBa2Cu3O7−δ with relatively low concentrations of paramagnetic centers are reported. For one sample the measurements were made for different values of δ. The data are analyzed, and the data reported earlier for another sample reanalyzed, with an improved fitting expression. In each case, the data give a well defined H1/2T term in the mixed state. No persuasive evidence for a T2 term in the superconducting state is found.

Dependence of the low-temperature specific heat of YBa2Cu3O7−δ on δ and magnetic field: Concentrations of spin-2 and spin-1/2 paramagnetic centers

Specific heat data on two samples of YBa2Cu3O7−δ with relatively low concentrations of paramagnetic centers show the presence of both spin-2 and spin-1/2 moments. Measurements on one sample following incremental increases in δ show that for δ≤0.2 approximately one spin-1/2 moment is produced for each 100 O vacancies, with no substantial increase in the linear term of the specific heat. For several reasons it is concluded that these O-vacancy related moments are different from those that have figured in previously reported correlations with parameters characteristic of the superconductivity.

SPECIFIC HEAT OF YBA2CU3O7-DELTA AND (LA2-XSRX)CUO4 : EVIDENCE FOR D-WAVEPAIRING

New low-temperature specific-heat data for YBa2Cu3O7−δ, 0≤δ≤0.2, and (La1.85Sr0.15)CuO4 in magnetic fields (H) to 9 T are reported. They show features predicted for the lines of nodes in the energy gap that are expected for d-wave pairing; for H=0, a T2 term; for H≠0 an H1/2T term with a crossover to a more complicated T dependence at higher T and lower H. The results are compared with recent measurements in other laboratories.