SUPERCONDUCTING PHASE IN NB1-XTAXSE3 (0.135-LESS-THAN-X-LESS-THAN-0.16)

We have found a superconducting phase in single-crystal Nb1-xTaxSe3 wi th 0.135 < x < 0.16 by use of resistivity and susceptibility measureme nts. Its crystal structure is identical to that of NbSe3, and the supe rconductivity occurs between x = 0.135 and 0.16, a much higher tantalu m concentration than that of the known phase with x = 0.02-0.08. The h ighest superconducting transition temperature T(c) is obtained as 5.8 K at x = 0.148, well beyond the T(c) = 1.5 K of Nb0.95Ta0.05Se3. Measu rements of the upper critical field H(c2) revealed its quasi-two-dimen sional nature and the angular dependence of H(c2) is described by an a nisotropic effective-mass model, which shows the intrinsic anisotropy of the electron band mass. The superconducting energy gap has been mea sured with an electron-tunneling technique and the observed gap values are discussed on the basis of the BCS theory.