ORAL SESSION 4D6: Novel Junctions

ORAL SESSION 4D6: Novel Junctions

Thursday, Feb. 24, 3:45 p.m. – 6:15 p.m., Room 301F (GRB)

Chairs: J. Clarke (U California-Berkeley), A. Braginski (Tohoku U)

4D6.1 Spin Injection and Andreev Reflection in Ferromagnet-Superconductor Oxide Heterostructures*

A.M. Goldman, P.A. Kraus, K. Nikolaev, A. Bhattacharya, and W. Cooley, School of Physics and Astronomy, University of Minnesota, 116 Church St. SE, Minneapolis, MN 55455, USA

Presenting Author: A.M. Goldman

Abstract not available.

4D6.2 Spin Injection in Ferromagnet/Superconductor/Ferromagnet Tunnel Junctions

Saburo Takahashi, Hiroshi Imamura, and Sadamichi Maekawa, Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan

Presenting Author: S. Takahashi

Spin-dependent transport in a ferromagnet/superconductor/ferromagnet (FM/SC/FM) tunnel junction is theoretically studied. The spin-polarized tunneling current driven from FM into SC gives rise to a nonequilibrium spin polarization in SC. When the thickness of SC is much smaller than the spin diffusion length, a strong interference appears between nonequilibrium spin densities injected from the left and right FM electrodes. In the parallel alignment of magnetization, the injected spin densities destructively interfere with each other and disappear in SC. In the antiparallel alignment, however, the spin densities interfere to enhance the spin accumulation in SC, so that the superconducting gap is strongly reduced by the Cooper pair breaking with increase of bias voltage and vanishes at a critical voltage V_c. Consequently, the tunnel magnetoresistance (TMR) exhibits a strong oscillation around V_c in the bias dependence. We also study the interplay between spin and charge imbalance in asymmetric tunnel junctions, where the height of the tunnel barriers and/or the strength of FMs are different, and discuss how the superconductivity is suppressed in the asymmetric junctions. Our results are consistent with recent experimental results on the critical current suppression in high-T_c SCs by spin injection.

4D6.3 Probing spin-charge separation using spin transport

Qimiao Si Dept. of Physics, Rice Univ., Houston, TX 77005-1892

Presenting Author: Q. Si

After some general comments on what constitutes a signature of spin-charge separation, I will present a specific proposal to test spin-charge separation in the normal state of the cuprates. The proposal is to compare the temperature dependences of the charge transport and spin transport properties. I will also show that it is experimentally feasible to measure the spin transport properties in the cuprates using the spin injection technique; included here is an estimate of the spin transport length and time scales. Finally, I will briefly discuss some theoretical work on spin injection into a d-wave superconductor and the implications of this theory for spin injection experiments in the superconducting cuprates.

4D6.6 Tunnel Junction Roughness Effects on Surface Bound States in d-Wave Superconductors

M.B. Walker and P. Pairor, Department of Physics, University of Toronto, Toronto, Ont. M5S 1A7, Canada

Presenting Author: M.B. Walker

We review recent theoretical results showing that the detailed properties of the zero bias conductance peak (ZBCP) observed in NIS (normal-insulator-superconductor) tunneling into high T_c superconductors are significantly affected by the tunnel junction roughness. The reason for this is that only grazing incidence quasiparticles can be reflected specularly at a rough tunnel junction, and hence only these quasiparticles can form the surface bound states believed to be the origin of the ZBCP. This result leads to the theoretical prediction that the ZBCP has a universal width (in agreement with experiment) independent of junction transparency, surface roughness, and impurity scattering rate, and is controlled by the magnitude of the superconducting gap. This is in spite of the fact that the height of the ZBCP varies dramatically with the above mentioned properties. Other results on the theory of tunneling at rough interfaces, such as the lack of validity of the conventional tunneling cone ideas for almost all real tunnel junctions, will also be discussed.

4D6.7 Measurement of the transport spin-polarization of oxides using Point Contact Andreev Reflection (PCAR)

Mike Osofsky, Boris Nadgorny, Igor Mazin, Jeff Byers, and Robert. J. Soulen, Jr., Naval Research Laboratory, Washington, DC 20375

Presenting Author: M.S. Osofsky

Results of high resolution (<1meV) transport measurements of the spin polarization of various ferromagnetic oxides at low temperatures (< 4.2K) using a new technique are presented. This technique utilizes point contact tunneling from a superconducting tip into a ferromagnet (FM) as a probe of the spin-polarization of the FM. Quantitative information can be extracted from the conductance data through a modified Blonder, Tinkham, Klapwijk (BTK) model of supercurrent conversion at a superconductor-metal interface (Andreev reflection) which includes the spin-polarization of the normal metal. Using this technique, the spin polarization of various thin film and crystals has been determined. Several of the samples show large spin polarizations that approach 90%. These results will be compared to previous measurements of spin polarization and with band structure predictions.