We study the slopes of the upper critical field $\partial_{ T}H_{ c2}|_{ T_{ c}}\equiv\partial H_{ _{ c2}}/\partial T$ at $T_{ c}$ in anisotropic superconductors with transport (non-magnetic) scattering employing the Ginzburg-Landau theory, developed for this situation by S. Pokrovsky and V. Pokrovsky, Phys. Rev. B 54, 13275 (1996). We found unexpected behavior of the slopes for a $d-$wave superconductor and in a more general case of materials with line nodes in the order parameter. Specifically, the presence of line nodes causes $\partial_{ T}H_{ c2}|_{ T_{ c}}$ to decrease with increasing non-magnetic scattering parameter $P$, unlike the nodeless case where the slope increases. In a pure $d-$wave case, the slope $\partial H_{ c2}|_{ T_{ c}}$ changes from decreasing to increasing when scattering parameter approaches $P\approx0.91\,P_{ { \rm crit}}$, where $P_{ { \rm crit}}\approx0.2807$ at which $T_{ c}\to0$ that implies the the existence of a gapless state in $d-$wave superconductors with transport scattering in the interval, $0.91\,P_{ { \rm { crit}}}<P<P_{ { \rm crit}}$. Furthermore, we have considered the mixed $s+d$ order parameter that has 4 nodes on a cylindrical Fermi surface when a $d-$part is dominant, or no nodes at all when an $s-$phase is the major one. We find that presence of nodes causes the slope $\partial_{ T}H_{ c2}|_{ T_{ c}},$ to decrease initially with increasing $P$, whereas in the nodeless state, $\partial_{ T}H_{ c2}|_{ T_{ c}}$ monotonically increases. Therefore, fairly straightforward experiments make it possible to decide whether or not the order parameter of a superconductor has nodes by measuring the disorder-dependence of the slope of $H_{ c2}$ at $T_{ c}$.
Categories: cond-mat.supr-con