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A high critical-current density was observed in the material (Tl0.5Pb0.5)Sr2Ca2Cu3O9. Based on magnetic-hysteresis measurements, this single (insulating) Tl-O layer material has a Jc value of 1.24×105 A/cm2 when measured at a temperature of 77 K and a field of 1 T. This value of Jc is comparable to that measured here for YBa2Cu3O7 (1.04×105 A/cm2), having single-conducting Cu-O chains, but considerably greater than the value for the material Tl2Ba2Ca2Cu3O10 with double (insulating) Tl-O layers in which no hysteresis was observed under these same conditions. Our results from specific heat measurements indicate that the (Tl0.5Pb0.5)Sr2Ca2Cu3O9 compound has a three-dimensional character due to the enhancement of the superconducting coupling along the c axis between the Cu-O planes and thus results in an increase in Jc compared to the two-dimensional Bi- and Tl-2212 and 2223 [e.g., (Bi or Tl)2(Sr or Ba)2CaCu2O8 and (Bi or Tl)2(Sr or Ba)2Ca2Cu3O10, respectively] compounds. We believe that the high Jc is associated with an increase in the irreversibility field, rather than an increase in pinning centers.

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The ZnTe/CdSe superlattice is one of the wide-gap semiconductor superlattices grown successfully in recent years which have potential applications in blue-green diode lasers. There exists experimental evidence of interchange of entire atom layers across the interfaces of ZnTe/CdSe superlattices in spite of the resulting interface strain. First-principles pseudopotential calculations of the total energy of such systems confirmed the possibility of such interchanges. By using the combination of a first-principles pseudopotential method and an empirical method with parameters extracted from the available experimental data and pseudopotential calculations, we studied phonons in ZnTe/CdSe superlattices with interchange of atom layers across the interface. The results are compared with phonons in ZnTe/CdSe superlattices with ideal interfaces. Several unique features of phonon modes in superlattices with interchange of atomic layers across interfaces are identified and discussed. These results provide more information about interface structures in such heterostructures, and we hope that they will stimulate more experimental measurements in these materials. § Permanent address: