Comprehensive low-temperature specific heat data C(T,H) of Na

To resolve the discrepancies of the superconducting order parameter in quasi-two-dimensional NbSe 2 , comprehensive specific-heat measurements have been carried out. By analyzing both the zero-field and mixed-state data with magnetic fields perpendicular ͑H Ќ c͒ to and parallel ͑H ʈ c͒ to the c axis of the crystal and using the two-gap model, we conclude that ͑1͒ more than one energy scale of the order parameter is required for superconducting NbSe 2 due to the thermodynamic consistency; ͑2͒ ⌬ L = 1.26 meV and ⌬ S = 0.73 meV are obtained; ͑3͒ N S ͑0͒ / N͑0͒ = 11% -20%; ͑4͒ the observation of the kink in ␥͑H͒ curve suggests that the two-gap scenario is more favorable than the anisotropic s-wave model to describe the gap structure of NbSe 2 ; and ͑5͒ ⌬ S is more isotropic and has a three-dimensional-like feature and is located either on the Se or the bonding Nb Fermi sheets.

Magnetic field dependence of low temperature specific heat of spinel oxide superconductor LiTi 2 O 4 has been elaborately investigated. In the normal state, the obtained electronic coefficient of specific heat ã n = 19.15 mJ/mol K 2 , the Debye temperature È D = 657 K and some other parameters are compared with those reported earlier. The superconducting transition at T c~1 1.4 K is very sharp (∆T c ~ 0.3 K) and the estimated äC/ã n T c is ~1.78. In the superconducting state, the best fit of data leads to the electronic specific heat C es /ã n T c = 9.87 exp (-1.58 T c /T) without field and ã(H) ∝ H 0.95 with fields. In addition, H c2 (0) ~ 11.7 T, H c (0)~0.32 T, ξ GL (0) ~ 55 Å , λ GL (0) ~ 1600 Å , and H c1 (0) ~ 26 mT are estimated from Werthamer-Helfand-Hohenberg (WHH) theory or other relevant relations. All results from the present study indicate that LiTi 2 O 4 can be well described by a typical type-II, BCS-like, moderate coupling, and fully gapped superconductor in the dirty limit. It is further suggested that LiTi 2 O 4 is a moderately electron-electron correlated system. 74.25.Bt, 74.25.Ha PACS number(s):

Colossal magnetoresistance (CMR) and electroresistance (CER) induced by the electric field in spinel multiferroic CdCr 2 S 4 are reported. It is found that a metal-insulator transition (MIT) in CdCr 2 S 4 is triggered by the electrical field. In magnetic fields, the resistivity  of CdCr 2 S 4 responds similarly to that of CMR manganites. Combing previous reports, these findings make CdCr 2 S 4 the unique compound to possess all four properties of the colossal magnetocapacitive (CMC), colossal electrocapacitive (CEC), CER, and CMR. The present results open a new venue for searching new materials to show CMR by tuning electric and magnetic fields.