RESUMO
We report on a study of epitaxially grown ultrathin Pb films that are only a few atoms thick and have parallel critical magnetic fields much higher than the expected limit set by the interaction of electron spins with a magnetic field, that is, the Clogston-Chandrasekhar limit. The epitaxial thin films are classified as dirty-limit superconductors because their mean-free paths, which are limited by surface scattering, are smaller than their superconducting coherence lengths. The uniformity of superconductivity in these thin films is established by comparing scanning tunneling spectroscopy, scanning superconducting quantum interference device (SQUID) magnetometry, double-coil mutual inductance, and magneto-transport, data that provide average superfluid rigidity on length scales covering the range from microscopic to macroscopic. We argue that the survival of superconductivity at Zeeman energies much larger than the superconducting gap can be understood only as the consequence of strong spin-orbit coupling that, together with substrate-induced inversion-symmetry breaking, produces spin splitting in the normal-state energy bands that is much larger than the superconductor's energy gap.
RESUMO
We report measurements of the ab-plane superfluid density n(s) (magnetic penetration depth lambda) of heavily underdoped films of YBa2Cu3O6+x, with T(C)'s from 6 to 50 K. We find the characteristic length for vortex unbinding transition equal to the film thickness, suggesting strongly coupled CuO2 layers. At the lowest dopings, T(C) is as much as 5 times larger than the upper limit set by the 2D Kosterlitz-Thouless-Berezinskii transition temperature calculated for individual CuO2 bilayers. Our main finding is that T(C) is not proportional to n(s)(0); instead, we find T(C) proportional to ns(1/2.3+/-0.4). This conflicts with a popular point of view that quasi-2D thermal phase fluctuations determine the transition temperature.
RESUMO
We report measurements of the inverse squared magnetic penetration depth, lambda(-2)(T), in Pr(2-x)Ce(x)CuO(4-delta) (0.115< or =x < or =0.152) superconducting films grown on SrTiO3 (001) substrates coated with a buffer layer of insulating Pr2CuO4. lambda(-2)(0), T(c), and normal-state resistivities of these films indicate that they are clean and homogeneous. Over a wide range of Ce doping, 0.124< or =x < or =0.144, lambda(-2)(T) at low T is flat: it changes by less than 0.15% over a factor of 3 change in T, indicating a gap in the superconducting density of states. Fits to the first 5% decrease in lambda(-2)(T) produce values of the minimum superconducting gap in the range of 0.29< or =Delta(min)/k(B)T(c)< or =1.01.
RESUMO
We present measurements of the ab-plane magnetic penetration depth, lambda(T), in five optimally doped Pr(1.855)Ce(0.145)CuO(4-y) films for 1.6 K< or =T < or =T(c) approximately 24 K. Low resistivities, high superfluid densities n(s)(T) proportional, variant lambda(-2)(T), high T(c)'s, and small transition widths are reproducible and indicative of excellent film quality. For all five films, lambda(-2)(T)/lambda(-2)(0) at low T is well fitted by an exponential temperature dependence with a gap, Delta(min), of 0.85k(B)T(c). This behavior is consistent with a nodeless gap and is incompatible with d-wave superconductivity.
RESUMO
We present measurements of the magnetic penetration depth, lambda(-2)(T), in Pr(2-x)Ce(x)CuO(4-y) and La(2-x)Ce(x)CuO(4-y) films at three Ce doping levels, x, near optimal. Optimal and overdoped films are qualitatively and quantitatively different from underdoped films. For example, lambda(-2)(0) decreases rapidly with underdoping but is roughly constant above optimal doping. Also, lambda(-2)(T) at low T is exponential at optimal and overdoping but is quadratic at underdoping. In light of other studies that suggest both d- and s-wave pairing symmetry in nominal optimally doped samples, our results are evidence for a transition from d- to s-wave pairing near optimal doping.
