Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session U39: Superconducting Proximity Effect-S/N and S/F |
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Sponsoring Units: DCMP Chair: James Eckstein, University of Illinois, Urbana-Champaign Room: Baltimore Convention Center 342 |
Thursday, March 16, 2006 8:00AM - 8:12AM |
U39.00001: Proximity effect and Josephson current in clean strong/weak/strong superconducting tri-layers Frank Marsiglio, Lucian Covaci Recent measurements of the Josephson critical current through LSCO/under-doped LSCO/LSCO thin films showed an unusually large proximity effect. Using the Bogoliubov-de Gennes (BdG) equations for a tight binding Hamiltonian we describe the proximity effect in weak links between a superconductor with critical temperature $T_c$ and one with critical temperature $T_c'$, where $T_c>T_c'$. The weak link (N') is considered to be a superconductor above it's critical temperature and the superconducting regions can have either s-wave or d-wave symmetry. We observe that the proximity effect is enhanced due to the presence of superconducting correlations in the weak link. The dc Josephson current is also calculated, and we observe a non-zero value for temperatures greater than $T_c'$ for sizes of the weak links that are greater than the conventional coherence length. This effect alone is unable to explain the experimental results, instead, we also consider pockets of superconductivity in the weak link. [Preview Abstract] |
Thursday, March 16, 2006 8:12AM - 8:24AM |
U39.00002: Superconducting Proximity Effect in Semiconductor Films - Experiment Soren Flexner, Michael Vissers, Paul Welander, Kevin Inderhees, Jim Eckstein Interface transparency and device topology together determine the information regarding the superconducting proximity effect that can be obtained from transport measurements. We have introduced a new three terminal device design and use junctions formed entirely in-situ between niobium(S) and a thin heavily doped InGaAs epitaxial layer(N). The junction design allows us to separately extract the junction conductance and the sheet resistance of the InGaAs from the two terminal and three terminal voltage readings at low bias currents. We see evidence for both fluctuating and phase-stiff superconductivity (SC) in the normal material. At temperatures below, but close to Tc of the niobium, SC fluctuations cause the spreading resistance, Rs, on the normal side of the junction to drop. At lower temperatures, phase-stiff SC emerges in the InGaAs, effectively stealing volume from the normal region. This makes Rs appear to increase as the SC order sets in. The specific junction conductance, Gc, rises to values much greater than the normal value. We propose this is caused by the N-S boundary moving into the semiconductor. [Preview Abstract] |
Thursday, March 16, 2006 8:24AM - 8:36AM |
U39.00003: Superconducting Proximity Effect in Semiconductor Films - Device Theory Michael Vissers, Soren Flexner, Paul Welander, Kevin Inderhees, Jim Eckstein A new three terminal device architecture is introduced and analyzed for studying the superconducting proximity effect. It consists of a narrow superconducting injector line that injects current into a thin normal film. The current is extracted from one side of the injector line by a superconducting drain electrode that is many normal state coherence lengths ``downstream'' of the injector. A third voltage tap is provided on the other or ``upstream'' side of the injector. We present a theory showing how measurements made in various voltage sensing configurations can be combined to provide enough information to extract the two dimensional sheet resistance of the normal metal under the superconductor, as well as the specific contact conductance between the superconducting and normal parts of the device. This theory has been used to characterize the proximity effect in thin heavily doped InGaAs layers. A transition from fluctuating to phase stiff pair correlations in the normal layer has been observed at temperatures below Tc of the superconductor. [Preview Abstract] |
Thursday, March 16, 2006 8:36AM - 9:12AM |
U39.00004: Coexistence of, and Competition between, Magnetism and Superconductivity Invited Speaker: Magnetism and superconductivity are competitive types of order in correlated electron systems. However, when a magnetic material is brought in contact with a superconducting material to build a junction, a coexistence region exists near the boundary that can modify the properties of the heterostructure in a qualitative way. In the case of a ferromagnet in contact with a singlet superconductor, the importance of triplet pairing correlations in the interface region recently became the focus of research. Such triplet correlations have unusual properties. They are typically odd in frequency for the case of a diffusive material. For clean materials in addition a triplet component even in frequency but odd in momentum is present. We have predicted that such triplet correlations can lead to an unusual indirect Josephson effect in a superconductor/half-metal/superconductor structure. In the case of a long half-metal such a Josephson effect is solely due to equal-pair triplet superconducting correlations. The triplet supercurrent is converted into a singlet current in the interface regions of the structure. Although theoretically predicted, a direct experimental verification of the presence of triplet correlations in ferromagnet/superconductor hybrid structures is difficult. In addition to the above effect we propose to use the torque on a ferromagnet/superconductor/ferromagnet trilayer in an external magnetic field as a probe of the presence of triplet correlations in the superconducting phase. [Preview Abstract] |
Thursday, March 16, 2006 9:12AM - 9:24AM |
U39.00005: Magnetization-dependent T$_{c }$ shift in F/S/F trilayers with strong ferromagnets Ion C. Moraru, William P. Pratt, Norman O. Birge Hybrid systems combining ferromagnetic (F) and superconducting (S) metals in contact exhibit a wide range of fascinating behaviors. Several experimental groups have used weak ferromagnetic alloys in F/S experiments to enhance the penetration of Cooper pairs into the ferromagnet. In an F/S/F trilayer structure, a difference in the critical temperature T$_{c}$, based on the mutual orientation of the outer ferromagnets, has been reported [1] in CuNi/Nb/CuNi. Systems with strong ferromagnets, on the other hand, present new challenges, due to the very different density of states and Fermi velocity for the two different spin bands. Using the strong ferromagnets Ni and NiFe (Permalloy) in F/S/F exchange-biased spin valves [2], we observe that the T$_{c}$ for the parallel (P) orientation is lower than that of the anti-parallel (AP) case, i.e. T$_{c}$ (P) $<$ T$_{c}$ (AP). These results are consistent with theoretical expectations, but opposite to recent experimental work from another group where an inverse spin switch has been reported in a NiFe/Nb/NiFe structure [3]. [1] J. Y. Gu et al, Phys. Rev. Lett. 89, 267001 (2002). [2] I. C. Moraru et al., submitted for publication (2005). [3] A. Yu. Rusanov et al., cond-mat/0509156 (2005). [Preview Abstract] |
Thursday, March 16, 2006 9:24AM - 9:36AM |
U39.00006: Tunneling Studies of Superconductor/Strong Ferromagnet Bilayers Paul SanGiorgio, Serge Reymond, Malcolm Beasley, Tesu Kim, Junhyung Kwon, Kookrin Char Thin-film heterostructures composed of superconductors and ferromagnets have recently received much interest, as they provide a unique opportunity to study the proximity effect between superconductivity and magnetism. We report systematic tunneling density of states (DOS) measurements on superconductor (Nb) /strong ferromagnet (CoFe, Ni) bilayers made with high quality aluminum-oxide planar tunnel junctions as a function of ferromagnetic thickness, $d_F$. In CoFe, we find that as $d_F$ increases, the superconducting DOS exhibits a scaling behavior in which the deviations from the normal-state conductance have a universal shape, which decreases exponentially in amplitude. The decay length, $d_1$, is approximately $0.4$~nm. We do not see oscillations in the DOS as a function of $d_F$, as one would expect from predictions based on the Usadel equations using reasonable parameters, although an oscillation in $T_c(d_F)$ has been seen in the same materials. Measurements on Nb/Ni bilayers will also be presented. This work is supported by AFOSR, DOE, and KOSEF through CSCMR. [Preview Abstract] |
Thursday, March 16, 2006 9:36AM - 9:48AM |
U39.00007: Interplay between superconductivity and ferromagnetism in tunneling Martin Gronsleth, Jacob Linder, Asle Sudbo We study tunneling currents in a model consisting of ferromagnetic spin-triplet superconductors with magnetization in an arbitrary direction separated by a thin insulating layer. A novel effect is found with both ferromagnetic and superconducting phases entering in the expressions for the single- and two-particle tunneling currents in both spin and charge sector. This interplay between ferromagnetism and superconductivity is present when unconventional Cooper pairs with parallell spin pairing are allowed to form. [Preview Abstract] |
Thursday, March 16, 2006 9:48AM - 10:00AM |
U39.00008: Tunneling in Dilute Al-Mn Alloys: Observation of Resonant Scattering and Implications for High-Temperature Superconductors Steven Ruggiero, Gerald Arnold, Joseph Bychowski, Anthony Williams, Nan Sun, Anna Clark, Nathan Miller, Joel Ullom We report on the observation of superconducting energy gap suppression by resonant scattering. Tunneling measurements of dilute Al-Mn alloys demonstrate the absence of density-of-states smearing that accompanies pair breaking and verify the detailed predictions of the Kaiser resonant scattering theory. These materials represent model systems for quasi-particle scattering and interference phenomena in the high- temperature superconductors. [Preview Abstract] |
Thursday, March 16, 2006 10:00AM - 10:12AM |
U39.00009: Andreev reflection spectroscopy of the heavy-fermion superconductor CeCoIn$_{5}$ along different crystallographic orientations Wan Kyu Park, Laura Greene, John Sarrao, Joe Thompson We have performed Andreev reflection spectroscopy on single crystals of the heavy-fermion superconductor CeCoIn$_{5}$. Conductance spectra obtained along both (001) and (110) crystallographic orientations exhibit similar features including asymmetry in the background conductance, the magnitude of zero-bias conductance enhancement (12 - 13{\%}) and the energy scale for the conductance enhancement ($\sim $1 meV). Analysis of the (001) junction data taken at the lowest temperature (400 mK) using an extended Blonder-Tinkham-Klapwijk model gives 2\textit{$\Delta $/k}$_{B}T_{c}$ = 4.64 [1,2]. The failure to account for the full temperature dependence of the data sets requires further theoretical investigations to account for the magnitude of the Andreev signal, including the possibility of two-fluid behavior. Features in the (110) data may provide the first spectroscopic evidence for $d_{x2-y2}$ superconducting order parameter symmetry [2,3]. [1] W. K. Park \textit{et al}., Phys. Rev. B \textbf{72}, 052509 (2005). [2] W. K. Park \textit{et al}., cond-mat/0507353. [3] W. K. Park and L. H. Greene, cond-mat/0507489. This work was supported by the U.S. DoE Award No. DEFG02-91ER45439 through the FSMRL and the Center for Microanalysis of Materials at UIUC. [Preview Abstract] |
Thursday, March 16, 2006 10:12AM - 10:24AM |
U39.00010: Elastic Cotunnelling and Crossed Andreev Reflection in Normal-Superconductor Nanostructures Paul Cadden-Zimansky, Zhigang Jiang, Venkat Chandrasekhar Transport experiments were made on normal-superconductor-normal systems where the separation of the normal elements is less than a superconducting coherence length. For this geometry two coherent, nonlocal effects have been predicted. In elastic cotunnelling electrons from one normal element can tunnel to the other through a virtual state in the superconducting gap. In crossed Andreev reflection one electron from each spatially separated normal element join to enter the superconductor as a Cooper pair. We present evidence of these nonlocal effects and show that their spatial dependence agrees with theory. [Preview Abstract] |
Thursday, March 16, 2006 10:24AM - 10:36AM |
U39.00011: Anomalous Proximity Effect in Nb/Al/CoFe Trilayers Kookrin Char, Jun Hyung Kwon, Jinho Kim, Kyungmoon Kim, Hyeonjin Doh, Han-Yong Choi We have fabricated Nb/Al/CoFe(Ni, Cu$_{40}$Ni$_{60})$ trilayers to study the interaction between superconductivity and ferromagnetism. Increasing the thickness of Al in S/N/F trilayer, we observed that Tc values of S/N/F trilayers increase sharply close to the Tc of S/N bilayer until the Al thickness reaches 3 nm. As Al thickness increases from 3 nm to 180 nm, Tc value of S/N/F decreases again, following those of the S/N data. In order to fit the Tc data of Nb/Al/CoFe trilayers as a function of Al thickness in a conventional Usadel formalism, we had to use a large $\gamma _b ^{F}$(= R$_{b}$A/$\rho _{f}\xi _{f})$ value of about 4, which seems unphysically large. In order to examine the role of Al/CoFe interface, we fabricated Nb/Cu(2 nm)/Al(2 nm)/CoFe and Nb/Au(2 nm)/Al(2 nm)/CoFe and compared them with Nb/Al(4 nm)/CoFe. The Tc of the double N layer system showed lower value than the Tc of the single Al layer system, although the three systems shared the same Al/CoFe interfaces. Our data suggests the large $\gamma _b ^{F}$ value in a conventional Usadel picture is not sufficient and rather indicates towards the unique role of Al instead of the Al/CoFe interface. We will discuss other possibilities such as the triplet superconductivity in order to explain our experimental findings. [Preview Abstract] |
Thursday, March 16, 2006 10:36AM - 10:48AM |
U39.00012: The Role of Inelastic Scattering in Intermediate Spin Polarized Normal Metal/Superconductor Point Contacts Charles W. Smith, Paul J. Dolan, Jr. Charge transport in ferromagnetic normal metal/superconductor point contacts is constrained by both the limited minority spin population, which reduces the probability of the Andreev reflection process, and by quasiparticle finite-lifetime effects, i.e., inelastic scattering, which influences the probability of ordinary electron transport. For the case of intermediate polarization 0.30 $\le $ P $\le $ 0.60, where 0 $\le $ P $\le $ 1.0, these processes can play equally important roles. We present results for normalized conductance at zero bias, as a function of temperature, and for conductance as a function of voltage, at P = 0.40, parametrically, for the entire range of inelastic scattering. Experimental results for point contacts will be presented. [Preview Abstract] |
Thursday, March 16, 2006 10:48AM - 11:00AM |
U39.00013: Magnetically Induced Superconductor-Metal-Insulator Transition in Thin Tantalum Films Carlos Vicente, Yongguang Qin, Jongsoo Yoon Homogeneously disordered superconducting thin tantalum films are found to exhibit a metallic behavior in the limit of zero temperature when the superconductivity is suppressed by weak magnetic fields. The metallic behavior is characterized by an apparent saturation of sample resistance to a finite value, which can be order of magnitude smaller than the normal state resistance. This implies that the metallic state exists as a separate phase rather than a point in phase diagram. Such a metallic behavior is in strong contrast to the traditional belief that the electronic state of a 2D superconducting film can either be superconducting or insulating. We present details of transport characteristics in the magnetically induced metallic and insulating phase. We also discuss the influence of disorder, represented by normal conducting sheet resistance, on the metallic behavior and the superconductor-metal-insulator phase transition. [Preview Abstract] |
Thursday, March 16, 2006 11:00AM - 11:12AM |
U39.00014: Magnetic Field-Induced Metallic Behavior in Superconducting Tantalum Films Yongguang Qin, Carlos Vicente, Jongsoo Yoon We present the results of electronic transport measurements on superconducting thin tantalum films. The films are grown by dc sputtering on Si substrates. No sign of crystalline ordering is found from X- ray diffraction studies, particularly for films with thickness less than $\sim $ 5nm, indicating that the structure of the films is amorphous. The superconducting transition temperatures are found to continuously decrease with decreasing film thickness, which is characteristic of homogeneously disordered superconducting films. At zero magnetic field, a direct superconductor-insulator transition is observed at a critical thickness $\sim $ 3 nm. At this thickness the normal conducting sheet resistance is close to the quantum resistance, $h$/4$e^{2}$. When the superconductivity is suppressed by applying magnetic fields, however, the system exhibits an unexpected metallic behavior in the limit of zero temperature. The metallic behavior is characterized by a drop in resistance followed by an apparent saturation to a finite value as the temperature is reduced. We observe qualitatively different nonlinear voltage-current characteristics across the ``superconductor- metal'' boundary, and also the ``metal-insulator'' boundary. [Preview Abstract] |
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