Superconducting phase in Nb1-xTaxSe3 (0.135<x<0.16)

We have found a superconducting phase in single-crystal Nb1-xTaxSe3 with 0.135<x<0.16 by use of resistivity and susceptibility measurements. Its crystal structure is identical to that of NbSe3, and the superconductivity occurs between x=0.135 and 0.16, a much higher tantalum concentration than that of the known phase with x=0.02-0.08. The highest superconducting transition temperature Tc is obtained as 5.8 K at x=0.148, well beyond the Tc=1.5 K of Nb0.95Ta0.05Se3. Measurements of the upper critical field Hc2 revealed its quasi-two-dimensional nature and the angular dependence of Hc2 is described by an anisotropic effective-mass model, which shows the intrinsic anisotropy of the electron band mass. The superconducting energy gap has been measured with an electron-tunneling technique and the observed gap values are discussed on the basis of the BCS theory.