Several investigators reported a Kosterlitz-Thouless (KT) transition in Yba2Cu3O7−δ films. Since such films are often in the 3D limit, such a finding is surprising. We performed sensitive measurements on the thinnest possible specimens, single-unit-cell films. Low sensitivity data are roughly consistent with a KT transition. However, IVs at the lowest currents are ohmic at temperatures far below the nominal KT temperature, contrary to the theory. An ohmic response at low currents and temperatures implies the existence of unbound vortices, most likely due to finite size effects.

We have performed nanovolt-level dc measurements on single-unit-cell films of YBa2Cu3O7-delta. When only the low-sensitivity data are analyzed, they are roughly consistent with the occurrence of a Kosterlitz-Thouless transition. At the lowest currents and voltages, however, the samples are ohmic at temperatures quite far below the nominal Kosterlitz-Thouless transition temperature, contrary to the theory. The ohmic response at low currents implies that there are unbound vortices present well below the nominal Kosterlitz-Thouless transition temperature.

Microwave absorption has been used to study the electromagnetic properties of the yttrium-barium-copper-oxide superconductor and other compounds in this phase system. The method can discriminate between superconductivity and metal-insulator and ferroelectric transitions in milligram size samples of powder. The transition that has been reported to occur near 240 K in several physical properties of samples of the high-temperature superconductor YBa2Cu3O7 is shown to be not an intrinsic property of this material but to be associated with impurity phases or interfaces.

We report a systematic study of the temperature dependences of microwave absorption and low field dc magnetization in powder samples of the superconductor Y1Ba2Cu3O6.9. Both measurements indicate a transition temperature near 90 K. The temperature dependence of the diamagnetic susceptibility, as well as its value at T⪡Tc are close to that of a conventional BCS superconductor.

The far-infrared absorption has been measured directly for a superconducting YBa2Cu3O7−x pellet for frequencies ≤ 25 cm−1 using a calorimetric technique at 6 K. Finite absorption is observed at all frequencies down to our lower limit of 5 cm−1.

Summary Viruses recovered from outbreaks of influenza in pigs in the British Isles have been established in the ferret and in the mouse. They produce a febrile catarrh in the ferret and pulmonary consolidation in mice resembling the changes induced by influenza A virus or the porcine virus of Shope. These viruses have been compared with standard strains (W. S., PR8, and Shope) by neutralization and vaccination tests in mice and the inhibition of fowl red-cell agglutination (Hirst technique). The neutralization and red-cell tests indicated that in general each strain was most effectively neutralized by its homologous antiserum. Vaccination experiments 'in which mice were tested for immunity either by the intranasal or by the intraperitoneal route, gave results which suggested that homologous vaccines were 14 to 100 times as effective as heterologous ones. It is still difficult to obtain a clear conception of the antigenic relations of these viruses. The impression has been gained that neither of the two strains examined is more closely related to the other or to the Shope pig virus than it is to a human influenza A (PR8) virus.