Shot noise in diffusive ferromagnetic metals

We show that shot noise in a diffusive ferromagnetic wire connected by tunnel contacts to two ferromagnetic electrodes can probe the intrinsic density of states and the extrinsic impurity scattering spin-polarization contributions in the polarization of the wire conductivity. The effect is more pronounced when the electrodes are perfectly polarized in opposite directions. While in this case the shot noise has a weak dependence on the impurity scattering polarization, it is strongly affected by the polarization of the density of states. For a finite spin-flip scattering rate the shot noise increases well above the normal state value and can reach the full Poissonian value when the density of states tends to be perfectly polarized. For the parallel configuration we find that the shot noise depends on the relative sign of the intrinsic and the extrinsic polarizations.Comment: 4 pages, 3 figure

Superconductor-Ferromagnet Bi-Layers: a Comparison of s-Wave and d-Wave Order Parameters

We study superconductor-ferromagnet bi-layers, not only for s-wave but also for d-wave superconductors. We observe oscillations of the critical temperature when varying the thickness of the ferromagnetic layer for both s-wave and d-wave superconductors. However, for a rotated d-wave order parameter the critical temperature differs considerably from that for the unrotated case. In addition we calculate the density of states for different thicknesses of the ferromagnetic layer; the results reflect the oscillatory behaviour of the superconducting correlations.Comment: 11 pages, 5 figures, accepted for publication in J. Phys.: Condens. Matte

Order parameter oscillations in Fe/Ag/Bi2Sr2CaCu2O{8+delta} tunnel junctions

We have performed temperature dependent tunneling conductance spectroscopy on Fe/Ag/Bi2Sr2CaCu2O8 (BSCCO) planar junctions. The multilayered Fe counterelectrode was designed to probe the proximity region of the ab-plane of BSCCO. The spectra manifested a coherent oscillatory behavior with magnitude and sign dependent on the energy, decaying with increasing distance from the junction barrier, in conjunction with the theoretical predictions involving d-wave superconductors coupled with ferromagnets. The conductance oscillates in antiphase at E = 0 and E = +/-Delta. Spectral features characteristic to a broken time-reversal pairing symmetry are detected and they do not depend on the geometrical characteristics of the ferromagnetic film.Comment: 4 pages and 4 figures Submitted to Physical Review Letter

Magnetic interference pattern in planar SNS Josephson junctions

We study the Josephson current through a ballistic normal metal layer of thickness $D$ on which two superconducting electrodes are deposited within a distance $L$ of each other. In the presence of an ({\it in-layer}) magnetic field we find that the oscillations of the critical current $I_c(\Phi)$ with the magnetic flux $\Phi$ are significantly different from an ordinary magnetic interference pattern. Depending on the ratio $L/D$ and temperature, $I_c(\Phi)$-oscillations can have a period smaller than flux quantum $\Phi_0$, nonzero minima and damping rate much smaller than $1/\Phi$. Similar anomalous magnetic interference pattern was recently observed experimentally.Comment: 6 pages, 4 figures, Accepted by Phys. Rev.

Nonsinusoidal current-phase relations and the 0−π0-\pi transition in diffusive ferromagnetic Josephson junctions

We study the effect of the interfacial transparency on the Josephson current in a diffusive ferromagnetic contact between two superconductors. In contrast to the cases of the fully transparent and the low-transparency interfaces, the current-phase relation is shown to be nonsinusoidal for a finite transparency. It is demonstrated that even for the nearly fully transparent interfaces the small corrections due to weak interfacial disorders contribute a small second-harmonic component in the current-phase relation. For a certain thicknesses of the ferromagnetic contact and the exchange field this can lead to a tiny minimum supercurrent at the crossover between 0 and $\pi$ states of the junction. Our theory has a satisfactory agreement with the recent experiments in which a finite supercurrent was observed at the transition temperature. We further explain the possibility for observation of a large residual supercurrent if the interfaces have an intermediate transparency.Comment: 7 pages, 4 figure

Enhanced superconducting proximity effect in clean ferromagnetic domain structures

We investigate the superconducting proximity effect in a clean magnetic structure consisting of two ferromagnetic layered domains with antiparallel magnetizations in contact with a superconductor. Within the quasiclassical Green's function approach we find that the penetration of the superconducting correlations into the magnetic domains can be enhanced as compared to the corresponding single domain structure. This enhancement depends on an effective exchange field which is determined by the thicknesses and the exchange fields of the two domains. The pair amplitude function oscillates spatially inside each domain with a period inversely proportional to the local exchange field. While the oscillations have a decreasing amplitude with distance inside the domain which is attached to the superconductor, they are enhancing in the other domain and can reach the corresponding normal metal value for a zero effective exchange field. We also find that the corresponding oscillations in the Fermi level proximity density of states as a function of the second domain's thickness has an growing amplitude over a range which depends on the effective exchange field. Our findings can be explained as the result of cancellation of the exchange fields induced phases gained by an electron inside the two domains with antiparallel magnetizations.Comment: 7 pages, 4 figure

On the Selfconsistent Theory of Josephson Effect in Ballistic Superconducting Microconstrictions

The microscopic theory of current carrying states in the ballistic superconducting microchannel is presented. The effects of the contact length L on the Josephson current are investigated. For the temperatures T close to the critical temperature T_c the problem is treated selfconsistently, with taking into account the distribution of the order parameter $\Delta (r)$ inside the contact. The closed integral equation for $\Delta$ in strongly inhomogeneous microcontact geometry ($L\lesssim \xi_{0}, \xi_{0}$ is the coherence length at T=0) replaces the differential Ginzburg-Landau equation. The critical current $I_{c}(L)$ is expressed in terms of solution of this integral equation. The limiting cases of $L\ll \xi_{0}$ and $L\gg \xi_{0}$ are considered. With increasing length L the critical current decreases, although the ballistic Sharvin resistance of the contact remains the same as at L=0. For ultra short channels with $L\lesssim a_{D}$ ($a_{D}\sim v_{F}/\omega_{D}, \omega_{D}$ is the Debye frequency) the corrections to the value of critical current I_c(L=0) are sensitive to the strong coupling effects.Comment: 15 pages LaTex, 3 jpg figure

Semiclassical theory of spin-polarized shot noise in mesoscopic diffusive conductors

We study fluctuations of spin-polarized currents in a three-terminal spin-valve system consisting of a diffusive normal metal wire connected by tunnel junctions to three ferromagnetic terminals. Based on a spin-dependent Boltzmann-Langevin equation, we develop a semiclassical theory of charge and spin currents and the correlations of the currents fluctuations. In the three terminal system, we show that current fluctuations are strongly affected by the spin-flip scattering in the normal metal and the spin polarizations of the terminals, which may point in different directions. We analyze the dependence of the shot noise and the cross-correlations on the spin-flip scattering rate in the full range of the spin polarizations and for different magnetic configurations. Our result demonstrate that noise measurements in multi-terminal devices allow to determine the spin-flip scattering rate by changing the polarizations of ferromagnetic terminals.Comment: 12 pages, 5 figure