We study the superconducting properties of the non-centrosymmetric compound LaNiC$_2$ by measuring the London penetration depth $Delta lambda (T)$, the specific heat $C(T,B)$ and the electrical resistivity $rho (T,B)$. Both $Deltalambda (T)$ and the electronic specific heat $C_e(T)$ exhibit exponential behavior at low temperatures and can be described in terms of a phenomenological two-gap BCS model. The residual Sommerfeld coefficient in the superconducting state, $gamma_0(B)$, shows a fast increase at low fields and then an eventual saturation with increasing magnetic field. A pronounced upturn curvature is observed in the upper critical field $B_{c2}(T)$ near $T_{c}$. All the experimental observations support the existence of two-gap superconductivity in LaNiC$_2$.
The upper critical field $mu_0H_{c2}(T_c)$ of LiFeAs single crystals has been determined by measuring the electrical resistivity using the facilities of pulsed magnetic field at Los Alamos. We found that $mu_0H_{c2}(T_c)$ of LiFeAs shows a moderate anisotropy among the layered iron-based superconductors; its anisotropic parameter $gamma$ monotonically decreases with decreasing temperature and approaches $gammasimeq 1.5$ as $Trightarrow 0$. The upper critical field reaches 15T ($Hparallel c$) and 24.2T ($Hparallel ab$) at $T=$1.4K, which value is much smaller than other iron-based high $T_c$ superconductors. The temperature dependence of $mu_0H_{c2}(T_c)$ can be described by the Werthamer-Helfand-Hohenberg (WHH) method, showing orbitally and (likely) spin-paramagnetically limited upper critical field for $Hparallel c$ and $Hparallel ab$, respectively.
The upper critical fields ($H_{c2}$) of the single crystals $rm(Sr,Na)Fe_2As_2$ and $rm Ba_{0.55}K_{0.45}Fe_2As_2$ were determined by means of measuring the electrical resistivity, $ rho_{xx}(mu_0H)$, using the facilities of pulsed magnetic field at Los Alamos. In general, these compounds possess a very large upper critical field ($H_{c2}(0)$) with a weak anisotropic effect. The detailed curvature of $H_{c2}(T_c)$ may depend on the magnetic field orientation and the sample compositions. We argue that such a difference mainly results from the multi-band effect, which might be modified via doping.
We presented that the relaxation of n coupling molecules in a molecular string exhibits n individual relaxation modes (RMs), each mode being characterized by a definite relaxation time and amplitude according to the string model. The n RMs behaving a single relaxation at high temperature, evolves to two relaxation species, at low temperature, with different temperature dependences for the respective relaxation times and amplitudes. Since the characteristics of the two relaxation species are in agreement with those exhibited by the universal alpha- and Johari-Goldstein (JG) beta-relaxations in glass dynamics, we provided a unified molecular level mechanism for these two processes.
