A direct measurement of the London penetration depth in the high-T c superconductor YBa 2 Cu 3 O 6.92 has been made using low-energy SR with an effective background suppression method. The average magnetic field versus mean depth was measured in the Meissner state of high-purity detwinned crystals of YBa 2 Cu 3 O 6.92 . The resulting magnetic field profiles along the a and b axes are consistent with a local London model beyond 10 nm but there are deviations close to the surface. The absolute values of a and b extrapolated to zero temperature are 126.1Ϯ 1.2Ϯ 3 and 105.5Ϯ 1.0Ϯ 3 nm, respectively. These results are compared with other less direct methods and extend the use of low-energy SR to small crystals.One of the fundamental quantities of a superconductor is the London penetration depth, , which is the characteristic length scale that a magnetic field penetrates into the surface of a superconductor while in the Meissner state. 1 In the clean limit the absolute value of is directly related to the superfluid density n s via 1 / 2 = 0 e 2 n s / m and consequently its variation as a function of temperature, doping and orientation are of central importance in testing microscopic theories of exotic superconductors. For example, the linear variation in 1 / 2 with respect to temperature was a key finding confirming the d-wave nature of the pairing in YBa 2 Cu 3 O 6+x . 2,3 Also, early SR studies of the vortex phase in polycrystalline samples found a linear correlation between 1 / 2 and T c in the underdoped region. 4-6 More recent measurements on crystals both in the Meissner 7 and vortex states 8 indicate this systematic variation with doping is sublinear and may be connected with an approach to a quantum-critical point. 7,9 The large in-plane anisotropy of in YBa 2 Cu 3 O 6.95 measured with IR reflectivity, 10 was taken as evidence for a multiband effect, whereby a one-dimensional Fermi sheet associated with the CuO chains in YBa 2 Cu 3 O 6.95 contributes to the superfluid flowing in the chain direction. 11 This is a unique feature of YBa 2 Cu 3 O 6+x which distinguishes it from other cuprates. Accurate measurements of the absolute value of and its anisotropy are required to clarify central issues in YBa 2 Cu 3 O 6+x and more generally in the area of exotic superconductors.Unfortunately, accurate measurements of are difficult due to many possible systematic uncertainties. For example, in any bulk measurement the assumption of an exponential decay of the field in the Meissner state is only valid in the local London limit of a perfect surface. 1 This adds uncertainty to all bulk measurements where the field profile is assumed and not measured. Alternatively, one can determine from SR studies in the vortex state where the muon acts as a sensitive probe of the magnetic field distribution. 8,12,13 However, in this case the nonlocal and nonlinear effects complicate the theory 14 leading to an effective-fielddependent penetration depth. Until now there has been no way to verify to what extent this effective penetration de...

The electrical and thermal Hall conductivities of the cuprate superconductor YBa2Cu3Oy, σxy and κxy, were measured in a magnetic field up to 35 T, at a hole concentration (doping) p = 0.11. In the T = 0 limit, we find that the Wiedemann-Franz law, κxy/T = (π 2 /3)(kB/e) 2 σxy, is satisfied for fields immediately above the vortex-melting field Hvs. This rules out the existence of a vortex liquid at T = 0 and it puts a clear constraint on the nature of the normal state in underdoped cuprates, in a region of the doping phase diagram where charge-density-wave order is known to exist. As the temperature is raised, the Lorenz ratio, Lxy = κxy/(σxyT ), decreases rapidly, indicating that strong small-q scattering processes are involved.

We have studied the microwave electrodynamics of single crystal iron-based superconductors Ba0.72K0.28Fe2As2 (hole-doped, Tc ≈ 30 K) and Ba(Fe0.95Co0.05)2As2 (electron-doped, Tc ≈20 K), by cavity perturbation and broadband spectroscopy. SQUID magnetometry was used to confirm the quality and homogeneity of the samples under study. Through cavity perturbation techniques, the temperature dependence of the in-plane London penetration depth ∆λ(T ), and therefore the superfluid phase stiffness λ 2 (0)/λ 2 (T ) was measured. Down to 0.4 K, the data do not show the exponential saturation at low temperatures expected from a singly-, fully-gapped superconductor. Rather, both the electron-and the hole-doped systems seem to be best described by a power law behavior, with λ 2 (0)/λ 2 (T ) ∼ T n and n ≈ 2.5. In the three samples we studied, a weak feature near the sensitivity limit of our measurements appears near T /Tc = 0.04, hinting at a corresponding low energy feature in the superconducting density of states. The data can also be relatively well-described by a simple two-gap s-wave model of the order parameter, but this yields parameters which seem unrealistic and dependent on the fit range. Broadband surface resistance measurements reveal a sample dependent residual loss whose origin is unclear. The data from the Ba0.72K0.28Fe2As2 samples can be made to scale as ω 2 if the extrinsic loss is treated as an additive component, indicating large scattering rates. Finally, the temperature dependence of the surface resistance at 13 GHz obeys a power law very similar to those observed for ∆λ(T ).

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