Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session H9: Superconductivity: Josephson Junctions, Proximity Effect & Squids I |
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Sponsoring Units: DMP Chair: Laura Greene, University of Illinois Room: Colorado Convention Center Korbel 1D |
Tuesday, March 6, 2007 8:00AM - 8:12AM |
H9.00001: Search for second-order Josephson tunneling in Superconductor-Ferromagnet-Superconductor junctions M.J.A. Stoutimore, D.J. Van Harlingen, S.M. Frolov, V.V. Bolginov, V.A. Oboznov, V.V. Ryazanov We have fabricated Nb-CuNi-Nb SFS (Superconductor-Ferromagnet-Superconductor) $\pi $-Josephson junctions in a geometry that allows us to perform transport measurements to determine the uniformity of the sample and subsequent current-phase measurements to look for period doubling on the same junction. Junctions of this type exhibit transitions between the 0-junction and the $\pi $-junction state as a function of the ferromagnetic barrier thickness and temperature. Near the 0-$\pi $ transition points, it is predicted that first-order tunneling will be suppressed and the second-order term, proportional to sin(2$\phi )$, may dominate. The interpretation of past experiments to measure this term have been ambiguous due to concerns that non-uniformity in the ferromagnetic layer could mimic second-order Josephson behavior by producing half-integer Shapiro steps. [Preview Abstract] |
Tuesday, March 6, 2007 8:12AM - 8:24AM |
H9.00002: Superconducting Proximity Effects in Epitaxial Cr/Nb Bi-Layers: A Novel Approach Using a Three-Terminal Device Architecture Paul B. Welander, Michael R. Vissers, James N. Eckstein We have studied the proximity effects between a superconductor (Nb) and a SDW antiferromagnet (Cr) using epitaxial thin films and a novel three-terminal device structure. Single-crystal Cr/Nb bi-layers are grown on sapphire by means of molecular beam epitaxy. Interface quality is crucial -- oxidation of the Cr surface before Nb deposition can render the proximity effects unseen. Our three-terminal device architecture allows independent measurement of the bi-layer sheet-resistance and the Nb-Cr contact resistance. We find that the contact resistance goes to zero a few tenths of a degree below the Nb critical temperature. Over this temperature range the junction conductance increases at a rate well beyond that predicted by Andreev reflection alone. The bi-layer sheet resistance also shows a slight increase with decreasing temperature. As the devices are cooled further we find both linear and non-linear current-voltage regimes. [Preview Abstract] |
Tuesday, March 6, 2007 8:24AM - 8:36AM |
H9.00003: Proximity Effect in Nb/Mg/CoFe Trilayers. Seong Kook Choi, Jun Hyung Kwon, Kookrin Char We have fabricated the Nb/Mg bilayer and Nb/Mg/CoFe trilayer samples by varying Mg layer thickness and measured their superconducting transition temperature $T_c $ electrically using the 4-prove method. Mg normal layer was used to investigate the effect of its small atomic number, since we have observed previously the largely different behavior when Au, Cu and Al were used. When the Al layer was used, a very unusual behavior was found. Because of chemical interaction between Nb and Mg, the $T_c $ transition curves did not show sharp $T_c $ transition. In order to prevent this interaction, we inserted 2 nm thick Al layer between Nb and Mg. In the case of Nb/Al(2nm)/Mg, we observed $T_c $ behavior consistent with a conventional SN theory. In the case of Nb/Al(2nm)/Mg/CoFe with fixed thicknesses of Nb and CoFe layer, the $T_c $ values exhibited two distinct behavior as the thickness of Mg increased. The $T_c $ value of S/N/F trialyer increased rapidly until the Mg thickness reached a few nm. As Mg thickness increased further to 200 nm, the $T_c $ value of S/N/F decreased again, following closely those of the S/N data. Overall, the Mg data basically followed those of Al data, suggesting that the low atomic number of the normal layer is important in observing the unusual proximity effect in SNF trilayers. Our analysis of the interface effect using an Usadel picture will be presented. [Preview Abstract] |
Tuesday, March 6, 2007 8:36AM - 8:48AM |
H9.00004: Proximity effect in superconducting/magnetic nanostructures. Jose L. Vicent, Elena Navarro, Nuria O. Nunez, Alfonso Cebollada, Alejandro Perez-Junquera, Jose I. Martin Fe nanoparticles have been grown by sputtering technique on sapphire substrates. The fabrication parameters (substrate temperature, rate, {\ldots}) allow us to change the nanoparticle sizes at will. Different elements have been deposit on top of these Fe nanoparticles. The magnetic behaviour of the Fe nanoislands strongly depends of the capping material, for instance capping with thin Al film does not change the magnetic behaviour of the nanoislands, but capping with thin Pt film induces an enhancement of the magnetic nanoparticles coupling, and finally exchange bias develops in the nanostructured system Cr layer on top of Fe nanoislands. Nb films have been deposited on top of these nanostructures. The superconducting/magnetic proximity effect has been studied for different capping materials (Al, Pt, and Cr) and different Fe nanoparticle sizes. We will show that the superconducting proximity effect strongly depends on the capping materials, with interplay between exchange bias and superconductivity leading to enhancement of the superconducting critical temperature. [Preview Abstract] |
Tuesday, March 6, 2007 8:48AM - 9:00AM |
H9.00005: Effects of magnetic fluctuation on 0-$\pi$ transition in a superconductor-ferromagnet-superconductor junction Michiyasu Mori, Shin'ichi Hikino, Saburo Takahashi, Sadamichi Maekawa There has been growing interest in a superconductor- ferromagnetic metal-superconductor (SFS) junction, in which the Josephson critical current, $I_{\rm c}$, shows a cusp as a function of thickness of ferromagnetic-layer, $d$, and/or temperature, $T$. Such a non-monotonous behavior, which is in marked contrast to $I_{\rm c}$ in a conventional Josephson junction, originates from the fact that the current-phase relation is shifted by $\pi$. This is called $\pi$-state. We study the influence of magnetic fluctuation on $I_{\rm c}$ in the SFS junction by a tunneling Hamiltonian approach. An analytical formula of $I_{\rm c}$ is given in the fourth order perturbation theory as regards the tunneling matrix element. Electrons propagate diffusively in the FM due to non-magnetic- and magnetic scatterings. The $I_{\rm c}$ exhibits the damped oscillatory dependence on $d$, and shows the transition between ${\it 0}$- and $\pi$-{\it states}. When the superconducting transition temperature is comparable to the ferromagnetic Curie temperature, the period of oscillation is elongated by increasing $T$ due to the magnetic fluctuation, which plays an important role in the $0$-$\pi$ transition, in particular, with $T$. Our results present an appropriate combination of a superconductor and a ferromagnetic metal to control the ${\it 0} $- and the $\pi$-{\it states}. [Preview Abstract] |
Tuesday, March 6, 2007 9:00AM - 9:12AM |
H9.00006: Theory of long range superconducting proximity effect in half-metallic ferromagnets: the role of disorder Matthias Eschrig, Tomas Lofwander A Josephson supercurrent between two singlet superconducting electrodes separated by half-metallic CrO$_2$ has been recently reported. Only conduction electrons with equal spins can be paired in a half metal. Both the mechanism involved in the conversion process from singlet to equal spin Cooper pairs at the interface, as well as whether the symmetry of the pairing correlations in ferromagnets is odd-frequency s-wave or even frequency p-wave for different amounts of disorder, are subjects currently under debate. We report an extensive theoretical investigation of the Josephson effect through half-metals with arbitrary impurity concentration. We discuss a model for the conversion mechanism between the singlet and triplet supercurrents at the interfaces and we analyze the symmetry of the pairing components that contribute to the supercurrent in the entire range from ballistic to diffusive transport. [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:24AM |
H9.00007: Electrical Characterization of Superconducting Microbridge Josephson Junctions with Ferromagnetic Strip Luis Gomez, Shinichi Kitamura, Takahiro Kubo, Haruhisa Kitano, Atsutaka Maeda We have fabricated high temperature superconductor microbridges where thin ferromagnetic strips, with nanometer size widths, have been deposited across the center of the microbridge. We characterized these devices as a function of temperature, magnetic field, and microwave power and frequency in order to evaluate their potential as Josephson junctions. The devices fabricated until now present promising Josephson-like characteristics and could be regarded as $S-S'-S$ weak links, although their $I-V's$ are flux flow type for all temperatures below $T_{c}$. We are currently modifying the bridge as well as the ferromagnetic strip dimensions (width and thickness) in order to improve the shape of the $I-V's$. In this talk, we will present our results to date. [Preview Abstract] |
Tuesday, March 6, 2007 9:24AM - 9:36AM |
H9.00008: Long range odd frequency triplet components in F/S/F trilayers Han-Yong Choi, Nayoung Lee We calculate the singlet and triplet pairing amplitudes of the ferromagnet/superconductor/ferromagnet (F/S/F) trilayers by solving the Usadel equation linearized near the transition temperature $T_c$ in the diffusive limit. Here, S is a conventional singlet s-wave supercondcutor like Nb and F is a ferromagnet like CoFe. A particular focus is on the odd frequency triplet pairing components which have a long length scale in F region unlike the singlet pairing component. The triplet components in F/S/F are induced by the proximity effects when the two F's have non-colinear orientations. We will identify the fingerprints of the odd frequency triplet pairing components which will help to directly observe them in the F/S/F trilayers. [Preview Abstract] |
Tuesday, March 6, 2007 9:36AM - 9:48AM |
H9.00009: A Strategy for Fitting a Three Parameter Model of Charge Transport in Ferromagnetic/Superconductor Point Contacts Charles W. Smith, Paul J. Dolan, Jr. We study charge transport in ferromagnet/superconductor point contacts that exhibit effects of spin polarization (P), interface elastic scattering (Z), and bulk inelastic scattering ($\Gamma$). We demonstrate a strategy to extract values for P, Z and $\Gamma$ from conductance data, as a function of temperature. The resulting parameter set can then be used to fit a model that describes charge transport in this type of point contact. Experimental examples will be presented. [Preview Abstract] |
Tuesday, March 6, 2007 9:48AM - 10:00AM |
H9.00010: Tunneling through the metal-insulator transition with Nb/Nb$_{x}$Si$_{1-x}$/Nb Josephson junctions Burm Baek, Alan Kleinsasser, George Kerber, Paul Dresselhaus, Samuel Benz We have successfully fabricated Josephson junctions with Nb$_{x}$Si$_{1-x}$ barriers to study the metal-insulator transition. By tuning the relative concentration of the barrier we can explore the electrical properties as different junctions are controllably tuned to concentrations in either the metallic or insulating states.. By cosputtering Nb and Si, reliable control of barrier properties has been achieved. In the metallic barrier regime that has higher Nb content, the uniformity, reproducibility, and tunability have already proven useful for voltage standard applications. In the insulating regime that has low Nb concentration, we observe hysteretic junction behavior. Details of the crossover from metallic to insulating behavior of the Josephson barrier will be discussed along with the ramifications for superconductive devices using this technology. [Preview Abstract] |
Tuesday, March 6, 2007 10:00AM - 10:12AM |
H9.00011: Superconducting Proximity Effect in Thin Semiconducting Films Michael Vissers, Soren Flexner, Paul Welander, Kevin Inderhees, James Eckstein The superconducting proximity effect changes both the transport properties of the N-layer as well as the conductance between the N and S layers. We use a novel 3 terminal device structure to probe this which provides two resistance measurements allowing us to measure both the N-layer sheet resistance, Rs, as well as the junction conductance, Gc. When the N-layer is a degenerate semiconductor, the changes in these quantities are large. Gc increases much more than the factor of 2 that Andreev reflection theory predicts, and both Rs as well as Gc exhibit reentrance as a function of temperature. We interpret these changes as the N-S boundary moving into the semiconductor increasing Gc while simultaneously removing volume in the N-layer that had been used in normal transport. Magnetic fields applied both parallel and perpendicular to the junction cause the maximum conductance to increase while the sheet resistance rises. The magnitude of the necessary field implies a local proximity effect. This work was supported by the DOE BES at the F. Seitz Materials Research Laboratory at the University of Illinois, Urbana. [Preview Abstract] |
Tuesday, March 6, 2007 10:12AM - 10:24AM |
H9.00012: Effect of heavy electron mass $m^{\star}$ on Andreev reflection (AR) in heavy-fermion/superconductor (HF/S) point-contacts H. Stalzer, W.K. Park, L.H. Greene, J.L. Sarrao, J.D. Thompson, J. Frederick, P. Canfield, L.D. Pham, Z. Fisk We investigate the effect of $m^{\star}$ on Andreev reflection in HF/S point-contacts (PC) by measuring the differential electrical conductance at temperatures between 1.5 and 10\,K. An electrochemically etched Nb tip ($T_{\rm c}$\,=\,9.2\,K) is brought into contact with HF single crystals (CeCoIn$_5$, CeRhIn$_5$, YbAl$_3$) of varying $m^{\star}$. Our conductance signals show a clear superconducting gap structure expected for PC in the Sharvin regime which can be fitted by the Blonder- Tinkham-Klapwijk model. Preliminary results on CeCoIn$_5$ at temperatures above its $T_{\rm c}$\,=\,2.3\,K indicate an enhanced AR signal of similar magnitude as in Au/Nb PC which is in contrast to CeCoIn$_5$/Au PC as reported earlier\,[1]. We discuss this in the context of a two fluid model which considers heaviness and bandstructure of the HF quasiparticles and which may also explain the asymmetric conductance background observed in many heavy-fermions below a characteristic temperature $T^{\star}$. [1] W.~K. Park {\it et al.}, PRB {\bf 72}, 052509 (2005). $-$ This work was supported by the Deutsche Forschungsgemeinschaft, and U.S. DoE Award No. DEFG02-91ER45439 through the FSMRL and the CMM at UIUC and the NSF-DMR-0503360 at UC. [Preview Abstract] |
Tuesday, March 6, 2007 10:24AM - 10:36AM |
H9.00013: Density of States measurements of AlMn alloys with tunable superconducting gaps Galen O'Neil, Dan Schmidt, Nathan Miller, Joel Ullom, Anthony Williams, Gerald Arnold, Steven Ruggiero Superconductors with tunable transition temperatures and energy gaps are useful for a variety of device applications. For instance, transition-edge $\mu$-calorimeter sensors and electron-tunneling $\mu$-refrigerators have been made based on Al with various levels of Mn doping. The transition temperature of AlMn can be continuously tuned from about $\sim 1.35~K$ to below 10~mK by Mn concentrations up to several thousand ppm. Here, we present detailed measurements of the superconducting density of states of AlMn made with both normal metal-insulator-superconductor and superconductor-insulator-superconductor tunnel junctions. We compare the data to theoretical expectations for magnetic and non-magnetic impurities in superconductors. In particular we show that the density of states of AlMn is not gapless as we would expect from magnetic impurities. Rather, present evidence indicates that the density of states is essentially BCS-like with an increased Dynes parameter. The increased Dynes parameter corresponds to a broadened peak at the gap and an increase in subgap states. We discuss the implications of this behavior for tunnel junction devices with AlMn electrodes. [Preview Abstract] |
Tuesday, March 6, 2007 10:36AM - 10:48AM |
H9.00014: Making an Analogy between Forming a Josephson Junction and the Use of Wave Functionals to Form Soliton- Anti Soliton Pairs in Both Biological and Condensed Matter Physics Andrew Beckwith Our paper generalizes techniques initially explicitly developed for CDW applications only with respect to what is needed for multi dimensional instantons forming in complex condensed matter and/or bio physics applications. This involves necessary conditions for formulation of a soliton- anti soliton pair, assuming a minimum distance between charge centers, and discusses the prior density wave physics example as to why a Pierels gap term is added to the tilted washboard potential for insuring the formation of scalar potential fields. We state that much the same methodology is needed for higher dimensional condensed matter systems and bio physics, with strict conditions stated as to necessary potential terms needed to form a Josephson junction interpretation as to how to form wave functionals with necessary Gaussian character which can model instanton physics via a process analogous to Pierels gap and Brillouin zone boundary physics. [Preview Abstract] |
Tuesday, March 6, 2007 10:48AM - 11:00AM |
H9.00015: Dynamics of a current-biased Bi$_2$Sr$_2$CaCu$_2$O$_{\8+d}$ surface intrinsic Shaoxiong Li, Wei Qiu, Siyuan Han, Y. F. Wei, X. B. Zhu, C. Z. Gu, S. P. Zhao, H. B. Wang The dynamics of switching from superconducting to voltage state of a current-biased Bi$_2$Sr$_2$CaCu$_2$O$_{\8+d}$ surface intrinsic Josephson junction is studied by measuring the temperature-dependent switching current distributions at temperatures ranging from 15 mK to 4.8 K. Crossover from phase diffusion to Kramers switching and thermal activation to quantum tunneling have been observed. Quantitative analysis indicates that the dynamics of a single high T$_c}$ Josephson junction can be described very well by theories whose predictions have been confirmed by experiments performed on low T$_c}$ Josephson junctions. Our result aslo indicate that the d-wave order parameter symmetry has no observable effect on the dynamics of Bi$_2$Sr$_2$CaCu$_2$O$_{\8+d}$ surface intrinsic Josephson junctions where current is along c-axis of the crystal. [Preview Abstract] |
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