Oriol T. Valls

Spin-polarized tunneling in ferromagnet/unconventional superconductor junctions

We study tunneling in ferromagnet/unconventional superconductor (F/S) junctions. We include the effects of spin polarization, interfacial resistance, and Fermi wave-vector mismatch between the F and S regions. Andreev reflection at the F/S interface, governing tunneling at low bias voltage, is strongly modified by these parameters. The conductance exhibits a wide variety of novel features as a function of applied voltage.

Tunneling spectroscopy for ferromagnet/superconductor junctions

In tunneling spectroscopy studies of ferromagnet/superconductor (F/S) junctions, the effects of spin polarization, Fermi wave-vector mismatch (FWM) between the F and S regions, and interfacial resistance play a crucial role. We study the low bias conductance spectrum of these junctions, governed by Andreev reflection at the F/S interface. We present results for a range of values of the relevant parameters and find that a rich variety of features appears, depending on pairing state and other conditions. We show that in the presence of FWM, spin polarization can enhance Andreev reflection and give rise to a zero-bias conductance peak for an s-wave superconductor. This implies that the extraction of spin polarization from measurements in F/S structures requires careful analysis. We consider both d- and s-wave superconductors as well as mixed states of the

Proximity effects and characteristic lengths in ferromagnet-superconductor structures

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Proximity effects at ferromagnet-superconductor interfaces

form and find that mixed states can have a different signature.

Odd triplet pairing in clean superconductor/ferromagnet heterostructures

We present an extensive theoretical investigation of the proximity effects that occur in clean ferromagnet/ superconductor (F/S) systems. We use a numerical method to solve self-consistently the Bogoliubov-de Gennes equations in the continuum. We obtain the pair amplitude and the local density of states, and use these results to extract the relevant lengths characterizing the leakage of superconductivity into the magnet and to study spin splitting into the superconductor. These phenomena are investigated as a function of parameters such as temperature, magnet polarization, interfacial scattering, sample size and Fermi wave vector mismatch, all of which turn out to have important influence on the results. These results should help characterize and analyze future data and are shown to be in agreement with existing experiments.

Angular Dependence of the Nonlinear Transverse Magnetic Moment of YBa2Cu3O6.95 in the Meissner State.

We study proximity effects at ferromagnet-superconductor interfaces by self-consistent numerical solution of the Bogoliubov-de Gennes equations for the continuum, without any approximations. Our procedures allow us to study systems with long superconducting coherence lengths. We obtain results for the pair potential, the pair amplitude, and the local density of states. We use these results to extract the relevant proximity lengths. We find that the superconducting correlations in the ferromagnet exhibit a damped oscillatory behavior that is reflected in both the pair amplitude and the local density of states. The characteristic length scale of these oscillations is approximately inversely proportional to the exchange field, and is independent of the superconducting coherence length in the range studied. We find the superconducting coherence length to be nearly independent of the ferromagnetic polarization.

Angular dependence of the superconducting transition temperature in ferromagnet-superconductor-ferromagnet trilayers

We study triplet pairing correlations in clean ferromagnet (F)/superconductor (S) nanojunctions, via fully self-consistent solution of the Bogoliubov-de Gennes equations. We consider FSF trilayers, with S being an s-wave superconductor, and an arbitrary angle alpha between the magnetizations of the two F layers. We find that contrary to some previous expectations, triplet correlations, odd in time, are induced in both the S and F layers in the clean limit. We investigate their behavior as a function of time, position, and alpha. The triplet amplitudes are largest at times on the order of the inverse Debye frequency, and at that time scale they are long-ranged in both S and F. The zero temperature condensation energy is found to be lowest when the magnetizations are antiparallel.

Induced triplet pairing in clean s -wave superconductor/ferromagnet layered structures

The angular dependence of the nonlinear transverse magnetic moment of untwinned high-quality single crystals of YBa{sub 2}Cu {sub 3}O{sub 6.95} has been studied at a temperature of 2.5thinspthinspK using a low frequency ac technique. The absence of any signature at angular period 2{pi}/4 is analyzed in light of the numerical predictions of such a signal for a pure d{sub x{sup 2}{minus}y{sup 2}} order parameter with line nodes. Implications of this null result for the existence of a nonzero gap at all angles on the Fermi surface are discussed. {copyright} {ital 1999} {ital The American Physical Society}

Angular dependence of superconductivity in superconductor/spin-valve heterostructures

The superconducting transition temperature T(c) of a ferromagnet (F)-superconductor (S)-ferromagnet trilayer depends on the mutual orientation of the magnetic moments of the F layers. This effect has been previously observed in F/S/F systems as a T(c) difference between parallel and antiparallel configurations of the F layers. Here we report measurements of T(c) in CuNi/Nb/CuNi trilayers as a function of the angle between the magnetic moments of the CuNi ferromagnets. The observed angular dependence of T(c) is in qualitative agreement with a F/S proximity theory that accounts for the odd triplet component of the condensate predicted to arise for noncollinear orientation of the magnetic moments of the F layers.

Diffusive motion in a model for particle hopping

We study induced triplet pairing correlations in clean ferromagnet/superconductor/ferromagnet heterostructures. The pairing state in the superconductor is the conventional singlet