Using simple four-terminal resistance measurements and two-coil superfluid stiffness measurements, we observe an extended temperature range wherein both the superfluid density and the resistance are substantially suppressed, in some cases below experimental resolution, in severely underdoped ultrathin films of Ca0.3Y0.7Ba2Cu3O7-delta (CaYBCO). This temperature range delta T-c, deemed the "offset," is in some films more than 1/2 of the resistive transition temperature. dTc scales linearly with the characteristic two-dimensional (2D) vortex unbinding temperature T-2D, growing larger with underdoping upon approach to the superconductor-insulator transition. Absent in three-dimensional samples of CaYBCO, we discuss the offset in the context of a previously observed 2D quantum critical point in the vicinity of the superconductor-insulator transition, as well as in the context of intrinsic and extrinsic inhomogeneities in the superconducting state of the film.

This paper models the dynamics of vortices that are generated in the middle of a thin, large-area, superconducting film by a low-frequency magnetic field from a small coil, motivated by a desire to better understand measurements of the superconducting coherence length made with a two-coil apparatus. When the applied field exceeds a critical value, vortices and antivortices originate near the middle of the film at the radius where the Lorentz force of the screening supercurrent is largest. The Lorentz force from the screening supercurrent pushes vortices toward the center of the film and antivortices outward. In an experiment, vortices are detected as an increase in mutual inductance between the drive coil and a coaxial “pickup” coil on the opposite side of the film. The model shows that the essential features of measurements are well described when vortex pinning and the attendant hysteresis are included.

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.

We report measurements of the superfluid density, lambda(-2)(T), in ferromagnet-on-superconductor (F/S) bilayers and S/F/S' trilayers comprising Nb with Ni, Py, CoFe, and NiV ferromagnets. Bilayers provide information about F/S interface transparency and the T dependence of lambda(-2) that inform interpretation of trilayer data. The Houzet-Meyer theory accounts well for the measured dependence of lambda(-2)(0) and T-c of F/S bilayers on thickness of F layer, d(F), except that lambda(-2)(0) is slightly under expectations for CoFe/Nb bilayers. For Nb/F/Nb' trilayers, we are able to extract T-c and and lambda(-2) for both Nb layers when F is thick enough to weaken interlayer coupling. The lower "T-c " is actually a crossover identified by onset of superfluid in the lower-T-c Nb layer. For Nb/NiV/Nb' trilayers, lambda(-2)(0) versus d(F) for both Nb layers has a minimum followed by a recovery, suggestive of a pi junction.

We apply a low-field (<100 G) technique to measure the critical pair momentum pc in thin underdoped films of Y0.7Ca0.3Ba2Cu3O7-delta and Bi2Sr2CaCu2O8+delta reflecting a wide range of hole doping. We observe that p(c) proportional to <(h)over bar>/xi scales with T-c and therefore superfluid density n(s) (T -> 0) in our two-dimensional cuprate films. This relationship was famously predicted by a universal model of the cuprates with a doping-independent superconducting gap, but has not been observed by high field (>10(4) G) measurements of the coherence length xi because high-field cuprate phenomena obscure the simple scaling we observe at much lower fields.

Sr2FeMoO6 (SFMO) films were grown on SrTiO3 (100)- and (111)-oriented substrates via pulsed laser deposition (PLD). In order to study the fundamental characteristics of deposition, films were grown in two different PLD chambers. In chamber I, the best films were grown with a relatively long substrate-to-target distance (89 mm), high substrate temperature (850 °C), and low pressure (50 mTorr) in a 95% Ar/5% H2 atmosphere. Although X-ray diffraction (XRD) measurements indicate these films are single phase, Rutherford Backscattering (RBS) measurements reveal considerable non-stoichiometry, corresponding to a Sr2Fe1−xMo1+xO6 composition with x ≅ 0.2–0.3. This level of non-stoichiometry results in inferior magnetic properties. In chamber II, the best films were grown with a much shorter substrate-to-target distance (38 mm), lower temperature (680 °C), and higher pressure (225 mTorr). XRD measurements show that the films are single phase, and RBS measurements indicate that they are nearly stoichiometric. The degree of ordering between Fe and Mo was dependent on both the temperature and pressure used during deposition, reaching a maximum order parameter of 85%. The saturation magnetization increases as the Fe/Mo ordering increases, reaching a maximum of 2.4 μB/f.u. Based on prior studies of bulk samples, one would expect a higher saturation magnetization for this degree of Fe/Mo order. The presence of extra strontium oxide layers in the form of Ruddlesden-Popper intergrowths appears to be responsible for the lower than expected saturation magnetization of these films.

We report the superconducting transition temperature T c versus thickness d F of Ferromagnet/Superconductor (F/S) bilayers, where F is a strong 3d ferromagnet (Ni, Ni 0.81 Fe 0.19 (Permalloy), Co 0.5 Fe 0.5 ) and S = Nb, taken from superfluid density measurements rather than resistivity. By regrouping the many physical parameters that appear in theory, we show that the effective exchange energy is determined from the F film thickness d F where T c versus d F begins to flatten out. Using this rearranged theory, we conclude 1) the effective exchange energy, E e x , is about 15 times smaller than measured by ARPES and five times smaller than deduced in previous studies similar to ours; 2) the dirty-limit coherence length, ξ F , for Cooper pairs in F is larger than the electron mean free path, ℓ F ; and 3) the 3d-F/Nb interface is enough of a barrier that Cooper pairs typically must hit it several times before getting through. The Py/Nb and CoFe/Nb interfaces are more transparent than the Ni/Nb interface.

This paper addresses two issues. Underdoped Bi-2212 has been shown to exhibit two-dimensional quantum fluctuations, with T-c vs. N-s (0) scaling qualitatively similar to ultrathin YBCO films that are 2-D by construction. The present paper shows that the quantitative difference in scaling is resolved by growing Bi-2212 films that are much smoother than previously used. Also, the absence of 2-D thermal critical behavior from the superfluid density of Bi-2212 films that exhibit 2-D quantum scaling is not likely due to an inhomogeneous T-c smearing out the 2-D thermal transition. Finally, we emphasize that there is as yet no consensus on the physics that drives the T -dependence of N-s at low to intermediate temperatures.

The penetration depth of MgB2 was measured using two methods of different mechanisms. The first method used MgB2 Josephson junctions and the magnetic field dependence of the junction critical current. The second method deduced the penetration depth from the inductance of a MgB2 microstrip used to modulate the voltage of a MgB2 DC SQUID. The two methods showed a consistent value of the low-temperature penetration depth for MgB2 to be about 40 nm. Both the small penetration depth value and its temperature dependence are in agreement with a microscopic theory for MgB2 in the clean limit.