Has the nonlinear Meissner effect been observed?

We examine recent high-precision experimental data on the magnetic field, ${\bf H}$, dependence of the penetration depth $\lambda(H)$ in $\rm{YBa_2Cu_3O_{7-\delta}}$ (YBCO) for several field directions in the $a-b$ plane. In a new theoretical analysis that incorporates the effects of orthorhombic symmetry, we show that the data at sufficiently high magnetic fields and low temperatures are in quantitative agreement with the theoretical predictions of the nonlinear Meissner effect.Comment: 4 text pages plus 3 postscript figure

Phenomenology of a-axis and b-axis charge dynamics from microwave spectroscopy of highly ordered YBa2Cu3O6.50 and YBa2Cu3O6.993

Extensive measurements of the microwave conductivity of highly pure and oxygen-ordered \YBCO single crystals have been performed as a means of exploring the intrinsic charge dynamics of a d-wave superconductor. Broadband and fixed-frequency microwave apparatus together provide a very clear picture of the electrodynamics of the superconducting condensate and its thermally excited nodal quasiparticles. The measurements reveal the existence of very long-lived excitations deep in the superconducting state, as evidenced by sharp cusp-like conductivity spectra with widths that fall well within our experimental bandwidth. We present a phenomenological model of the microwave conductivity that captures the physics of energy-dependent quasiparticle dynamics in a d-wave superconductor which, in turn, allows us to examine the scattering rate and oscillator strength of the thermally excited quasiparticles as functions of temperature. Our results are in close agreement with the Ferrell-Glover-Tinkham sum rule, giving confidence in both our experiments and the phenomenological model. Separate experiments for currents along the $\hat a$ and $\hat b$ directions of detwinned crystals allow us to isolate the role of the CuO chain layers in \YBCO, and a model is presented that incorporates both one-dimensional conduction from the chain electrons and two-dimensional transport associated with the \cuplane plane layers.Comment: 17 pages, 13 figure

CeCoIn5 - a quantum critical superfluid

We have made the first complete measurements of the London penetration depth $\lambda(T)$ of CeCoIn5, a quantum-critical metal where superconductivity arises from a non-Fermi-liquid normal state. Using a novel tunnel diode oscillator designed to avoid spurious contributions to $\lambda(T)$, we have established the existence of intrinsic and anomalous power-law behaviour at low temperature. A systematic analysis raises the possibility that the unusual observations are due to an extension of quantum criticality into the superconducting state.Comment: 5 pages, 3 figure

Effect of Impurity Scattering on the Nonlinear Microwave Response in High-Tc Superconductors

We theoretically investigate intermodulation distortion in high-Tc superconductors. We study the effect of nonmagnetic impurities on the real and imaginary parts of nonlinear conductivity. The nonlinear conductivity is proportional to the inverse of temperature owing to the dependence of the damping effect on energy, which arises from the phase shift deviating from the unitary limit. It is shown that the final-states interaction makes the real part predominant over the imaginary part. These effects have not been included in previous theories based on the two-fluid model, enabling a consistent explanation for the experiments with the rf and dc fields

Theory of Nonlinear Meissner Effect in High-Tc Superconductors

We investigate the nonlinear Meissner effect microscopically. Previous studies did not consider a certain type of interaction effect on the nonlinear phenomena. The scattering amplitude barely appears without being renormalized into the Fermi-liquid parameter. With this effect we can solve the outstanding issues (the quantitative problem, the temperature and angle dependences). The quantitative calculation is performed with use of the fluctuation-exchange approximation on the Hubbard model. It is also shown that the perturbation expansion on the supercurrent by the vector potential converges owing to the nonlocal effect

Field Induced Reduction of the Low Temperature Superfluid Density in YBa2Cu3O6.95

A novel high magnetic field (8 T) spectrometer for muon spin rotation has been used to measure the temperature dependence of the in-plane magnetic penetration depth in YBa2Cu3O6.95. At low H and low T, the penetration depth exhibits the characteristic linear T dependence associated with the energy gap of a d_x^2-y^2-wave superconductor. However, at higher fields the penetration depth is essentially temperature independent at low T. We discuss possible interpretations of this surprising new feature in the low-energy excitation spectrum.Comment: 8 pages, 4 figures, 1 RevTex file and 4 postscript figure

Meissner-London currents in superconductors with rectangular cross section

Exact analytic solutions are presented for the magnetic moment and screening currents in the Meissner state of superconductor strips with rectangular cross section in a perpendicular magnetic field and/or with transport current. The extension to finite London penetration is achieved by an elegant numerical method which works also for disks. The surface current in the specimen corners diverges as l^(-1/3) where l is the distance from the corner. This enhancement reduces the barrier for vortex penetration and should increase the nonlinear Meissner effect in d-wave superconductors

Free Energy and Magnetic Penetration Depth of a dd-Wave Superconductor in the Meissner State

We investigate the free energy and the penetration depth of a quasi-two-dimensional d-wave superconductor in the presence of a weak magnetic field by taking account of thermal, nonlocal and nonlinear effects. In an approximation in which the superfluid velocity $v_s$ is assumed to be slowly varying, the free energy is calculated and compared with available results in several limiting cases. It is shown that either nonlocal or nonlinear effects may cut off the linear-$T$ dependence of both the free energy and the penetration depth in all the experimental geometries. At extremely low $T$, the nonlocal effects will also generically modify the linear $H$ dependence of the penetration depth ("nonlinear Meissner effect") in most experimental geometries, but for supercurrents oriented along the nodal directions, the effect may be recovered. We compare our predictions with existing experiments on the cuprate superconductors.Comment: 18 revtex pages with 4 eps figures, final versio

Nonlinear electrodynamics of p-wave superconductors

We consider the Maxwell-London electrodynamics of three dimensional superconductors in p-wave pairing states with nodal points or lines in the energy gap. The current-velocity relation is then nonlinear in the applied field, cubic for point nodes and quadratic for lines. We obtain explicit angular and depth dependent expressions for measurable quantities such as the transverse magnetic moment, and associated torque. These dependences are different for point and line nodes and can be used to distinguish between different order parameters. We discuss the experimental feasibility of this method, and bring forth its advantages, as well as limitations that might be present.Comment: Fourteen pages RevTex plus four postscript figure