Superconductivity in rubidium substituted Ba1-xRbxTi2Sb2O

We report on the synthesis and the physical properties of Ba1-xRbxTi2Sb2O (x < 0.4) by x-ray diffraction, SQUID magnetometery, resistivity and specific heat measurements. Upon hole doping by substituting Ba with Rb, we find superconductivity with a maximum Tc = 5.4 K. Simultaneously, the charge-density-wave (CDW) transition temperature is strongly reduced from T_CDW 55 K in the parent compound BaTi2Sb2O and seems to be suppressed for x > 0.2. The bulk character of the superconducting state for the optimally doped sample (x = 0.2) is confirmed by the occurrence of a well developed discontinuity in the specific heat at Tc, with \DeltaC/Tc = 22 mJ/mol K2, as well as a large Meissner-shielding fraction of approximately 40 %. The lower and the upper critical fields of the optimally doped sample (x = 0.2) are estimated to \mu0Hc1(0) = 3.8 mT and \mu0Hc2(0) = 2.3 T, respectively, indicating that these compounds are strongly type-II superconductors.