Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session B47: Superconductors/Ferromagnet Multilayer Structures |
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Sponsoring Units: DCMP Chair: Chia-Ling Chien, Johns Hopkins University Room: Mile High Ballroom 4F |
Monday, March 3, 2014 11:15AM - 11:27AM |
B47.00001: Proximity effect in superconductor/ferromagnet hetero-structures as a function of interface properties Julio Sarmiento, Edgar J. Patino Superconductor/ferromagnet heterostructures are currently a subject of strong research due to novel phenomena resulting from the proximity effect. Among the most investigated ones are the oscillations of the critical temperature as function of the ferromagnet thickness. The oscillatory behavior of Tc is theoretically explained as to be result of the generation of the FFLO (Fulde-Ferrel-Larkin-Ovchinnikov) state of Cooper pairs under the presence of the exchange field of the ferromagnet [1,2]. With the advancement of experimental techniques for S/F bilayers growth new questions regarding the effect of the interface transparency can to be addressed. For instance the influence of the interface roughness on the proximity effect [3]. For studying this phenomenon Nb/Co and Nb/Cu/Co samples were sputtered on SiO2 substrates with different roughness. Characterization of these samples show a significant variation of Tc with the interface roughness. This results point towards a possible relationship between transparency and roughness of the interface. References [1] J. S. Jiang, et al., Phys. Rev Lett. 74 (1995) 314. [2] C. Cirillo, et al . Phys. Rev. 72, 144511 (2005) [3] E. J. Pati\~{n}o, Study of The Influence of Domain Walls in The S/F Proximity Effect. Cambridge (2005). [Preview Abstract] |
Monday, March 3, 2014 11:27AM - 11:39AM |
B47.00002: Signatures of triplet supercurrents in hybrid S/F structures Caroline Richard, Manuel Houzet, Julia Meyer While ferromagnetism and conventional superconductivity appear as antagonist phases, the proximity effect in hybrid S/F structures offers a unique opportunity to study their interplay. In particular, spin-triplet odd-frequency superconducting correlations may be induced in a diffusive ferromagnet. We study the Josephson effect through a long ferromagnetic bilayer in the diffusive regime [1]. For a non-collinear magnetization of the bilayer, we find that the current phase relation is dominated by its second harmonic, and corresponds to the long-range coherent propagation of two triplet pairs. Here, the superharmonicity is a signature of the Josephson coupling between a singlet superconductor and an effective triplet superconductor induced at the end of the ferromagnetic bilayer attached to the other lead. Then, we further study the critical current flowing between such two effective triplet reservoirs through a conventional superconducting layer. As a result of the competition between triplet/triplet and triplet/singlet couplings and under quite general conditions, we find that the critical current exhibits a maximum in the vicinity of the superconducting transition of the central layer. [1] C. Richard, M. Houzet, and J.S. Meyer, PRL. 110, 217004 (2013) [Preview Abstract] |
Monday, March 3, 2014 11:39AM - 11:51AM |
B47.00003: Tunneling and Transport in Clean Ferromagnet-Superconductor Heterostructures Chien-Te Wu, Oriol Valls, Klaus Halterman We study charge and spin transport in clean Ferromagnet (F)-Superconductor (S) layered structures. By combining a transfer matrix method with a numerical self-consistent solution of the Bogoliubov-de Gennes (BdG) equations, we compute the spin dependent tunneling conductance in F-F-S trilayers in a range of exchange fields and layer thicknesses. In particular, we investigate the dependence of the tunneling conductance on the angle $\alpha$ between the magnetizations in two F layers. We find a variety of non-monotonic and switching behaviors in these heterostructures. We also present results for charge and spin transport in S-F-F-S Josephson junctions. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:03PM |
B47.00004: Electron transport in p-wave superconductor-normal metal junctions Ahmet Keles, Anton Andreev, Boris Spivak We study low temperature electron transport in p-wave superconductor-insulator-normal metal junctions. In diffusive metals the p-wave component of the order parameter decays exponentially at distances larger than the mean free path l. At the superconductor-normal metal boundary, due to spin-orbit interaction, there is a triplet to singlet conversion of the superconducting order parameter. The singlet component survives at distances much larger than l from the boundary. It is this component that controls the low temperature resistance of the junctions. As a result, the resistance of the system strongly depends on the angle between the insulating boundary and the d-vector characterizing the spin structure of the triplet superconducting order parameter. We also analyze the spatial dependence of the electric potential in the presence of the current, and show that the electric field is suppressed in the insulating boundary as well as in the normal metal at distances of order of the coherence length away from the boundary. This is very different from the case of the normal metal-insulator-normal metal junctions, where the voltage drop takes place predominantly at the insulator. [Preview Abstract] |
Monday, March 3, 2014 12:03PM - 12:15PM |
B47.00005: Theory of proximity induced triplet superconductivity in spin-orbit-coupled systems Xin Liu, Jainendra Jain, Chao-Xing Liu We study proximity induced triplet superconductivity in a spin-orbit-coupled system, and show that the $\vec{d}$-vector of the induced triplet superconductivity undergoes precession that can be controlled by varying the Rashba and Dresselhaus spin-orbit couplings. In particular, a long range triplet-helix mode is predicted when the two spin-orbit couplings have equal strengths. We also study the Josephson junction geometry and show that a transition between 0- and $\pi$- junctions can be induced by controlling the spin-orbit coupling strength. An experimental setup is proposed to verify these effects. Conversely, the observation of these effects can serve as a direct confirmation of the triplet nature of the superconductivity. [Preview Abstract] |
Monday, March 3, 2014 12:15PM - 12:27PM |
B47.00006: Control of Spin-Triplet S/F/S Josephson junctions E.C. Gingrich, B.M. Niedzielski, A.M. Cramer, W.P. Pratt, Jr., Norman O. Birge We present recent work on S/F'/F/F''/S Josephson junctions, where F' is a hard ferromagnet, F is a synthetic antiferromagnet (SAF), and F'' is a soft ferromagnet. With the hard and soft ferromagnets magnetizations lying in plane, and the central ferromagnet possessing a non-colinear magnetization with respect to the F' and F'' layers, the Josephson junctions experience an enhancement in their critical current due to the generation of Spin-Triplet pair correlations. The direction of the F'' layer is predicted to control the state of the junction. We are fabricating such junctions with the goal of controllably switching the state of the junction between the $0$ and $\pi$ states. By integrating the junctions into a SQUID device, the state of the junctions can be measured. We will report on our progress. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B47.00007: Exploration of long-range spin-triplet correlations in superconductor/ferromagnetic hybrid systems William Martinez, W.P. Pratt, Jr., Norman O. Birge Since the prediction of long-range spin-triplet correlations (LRTCs) in superconductor/ferromagnet (S/F) systems,[1] their realization has been investigated by many groups. From F-N bilayers to intrinsic generation of spin-triplet through domain walls, there is wide interest in observing a signal at ranges beyond the tens of $nm$ observed in earlier work.[2] In this work, we examine the propagation of LRTCs extrinsically generated through noncollinear magnetization, at long $(100nm)$ length scales. We will report on our recent progress. \\ $[1]$ A.F. Volkov, F.S. Bergeret and K.B. Efetov, Phys. Rev. Lett., \textbf{90}, 117006 (2003).\\ $[2]$ T.S. Khaire, M.A. Khasawneh, W.P.Pratt, Jr, and N.O. Birge, Phys. Rev. Lett. \textbf{104}, 137002 (2010). [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B47.00008: Dimensionality crossover in ferromagnetic/superconducting films: Role of magnetic history Luis Ruiz-Valdepe\~nas, Fernando Valdes-Bango, Luis Alvarez-Prado, Jose Martin, Elena Navarro, Maria Velez, Jose Alameda, Jose Vicent Amorphous NdCo$_{5}$ films are ferromagnetic samples with a weak perpendicular magnetic anisotropy which can show small magnetic domain sizes (less than 100 nm) with labyrinthine structures. Sputtering technique is used to fabricate Nb/Al(5nm)/ NdCo$_{5}$ superconducting films on Si substrates. The temperature dependence of the upper critical field shows features which could be related to an ``imprinting'' of the domain structure of NdCo5 layers in the superconducting Nb film. This peculiar proximity effect governs the superconductivity dimensionality crossover from 1D to a regime between 1D and 2D typical of superconducting wire network. This superconducting crossover can be connected to the NdCo$_{5}$ magnetic history. [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:03PM |
B47.00009: Angular Dependence of Superconductivity in Superconductor / Spin Valve Heterostructures Alejandro Jara, Christopher Safranski, Ilya Krivorotov, Chien-Te Wu, Oriol Valls The superconducting condensate in superconductor / ferromagnet (S/F) multilayers consists of singlet and triplet components. For a non-collinear state of magnetization of the multilayer, all three spin components $S_{z}=(0,\pm 1)$ of the triplet condensate are generally non-zero, which can result in a long range proximity effect in S/F multilayers. Indeed, the $S_z= \pm 1$ triplet components of the condensate are immune to pair breaking by the exchange field and, unlike the singlet and the $S_z =0$ triplet components, they can penetrate deep into the ferromagnetic layers. Here we report measurements demonstrating magnetic control of the triplet component amplitude in Nb/Co/Cu/Co/CoOx superconducting spin valves. We find that for all values of the layer thicknesses employed in the experiment, Tc shows non-monotonic angular dependence with a minimum near perpendicular orientation of the Co layers. This drop in Tc is evidence of the enhanced long-range triplet amplitude in the maximally non-collinear configuration of the spin valve. We will present detailed measurements of the magnitude of this effect as a function of thicknesses of both Co and Cu layers of the spin valve. We will also compare our data to theoretical predictions of the angular dependence of Tc for this system. [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B47.00010: Superconducting proximity effects in YBa$_{2}$Cu$_{3}$O$_{7}$/[Co/Pt] multilayers J.E. Villegas, C. Visani, A. Verso, F. Cuellar, C. Deranlot, R. Bernard, A.F. Volkov, F.S. Bergeret We have studied the penetration of superconducting correlations into Co/Pt multilayers and single Pt thin films deposited on top of c-axis YBa$_{2}$Cu$_{3}$O$_{7}$. We used tunneling conductance measurements across an AlO$_{\mathrm{x}}$ barrier in order to track changes in the electron density of states of the Co/Pt (or Pt) films. A large number of junctions were studied, varying several parameters. We found more pronounced, longer-ranged signatures of proximity-induced superconductivity (energy-gap below T$_{\mathrm{C}})$ in Co/Pt than in single Pt films. This is interpreted by considering a series of magnetic effects specific to the Co/Pt interface, which induce inhomogeneities in the exchange field felt by the conduction electrons and explain the presence of long-range proximity effects. [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B47.00011: Proximity effect in superconductor/conical magnet/ferromagnet heterostructures James Annett, Daniel Fritsch At the interface between a superconductor and a ferromagnetic metal spin-singlet Cooper pairs can penetrate into the ferromagnetic part of the heterostructure with an oscillating and decaying spin-singlet Cooper pair density. However, if the interface allows for a spin-mixing effect, equal-spin spin-triplet Cooper pairs can be generated that can penetrate much further into the ferromagnetic part of the heterostructure, known as the long-range proximity effect. Here, we present results of spin-mixing based on self-consistent solutions of the microscopic Bogoliubov-de Gennes equations incorporating a tight-binding model. In particular, we include a conical magnet into our model heterostructure to generate the spin-triplet Cooper pairs and analyze the influence of conical and ferromagnetic layer thickness on the unequal-spin and equal-spin spin-triplet pairing correlations. It will be shown that, in agreement with experimental observations [1], a minimum thickness of the conical magnet is necessary to generate a sufficient amount of equal-spin spin-triplet Cooper pairs allowing for the long-range proximity effect [2]. \newline [1] J. W. A. Robinson, J. D. S. Witt, and M. G. Blamire. Science {\bf 329}, 59 (2010). \newline [2] D. Fritsch and J. F. Annett, arXiv:1311.3278 (2013). [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B47.00012: Giant thermoelectric effects in a proximity-coupled superconductor-ferromagnet device Peter Machon, Matthias Eschrig, Wolfgang Belzig The usually negligibly small thermoelectric effects in superconducting heterostructures can be boosted dramatically due to the simultaneous effect of spin-dependent scattering and spin-filtering. Build on our idea [Phys. Rev. Lett. \textbf{110}, 047002 (2013)], we propose realistic setups to measure local thermoelectric effects in superconductor heterostructures. We focus on the Seebeck-effect which is a direct measure of the local thermoelectric response and find that a thermopower $\sim100\mu V/K$ can be achieved if a third terminal allows to drain the thermal current. A measurement of the thermopower can furthermore be used to determine quantitatively the spin-dependent interface parameters that induce the spin splitting. For applications in nano-cooling we discuss the figure of merit that we found to exceed one for realistic parameters. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B47.00013: Direct Detection of Cooper Pair Momentum in Fulde-Ferrell Superconductors Wei Chen, Rui Shen, Dingyu Xing, Ming Gong Finite momentum pairing for Cooper pairs in inhomogeneous superconductors is an important conceptual extension of the celebrated Bardeen-Cooper-Schrieffer theory of superconductivity. In the past five decades, great efforts have been paid and only indirect evidences related to the possible finite momentum pairing have been reported. In this work, we propose an Andreev interferometer based on branched Y-junction to directly detect the Cooper pair momentum in the Fulde-Ferrell (FF) superconductor, which can provide the most convincing evidences for finite momentum pairing. The subgap conductance of the interferometer is a unique function of the phase difference induced by the FF superconductor, providing full information of the Cooper pair momentums. We demonstrate that the function of the device will not be affected by other uncontrollable phases during the Andreev scattering processes. The interferometer has important applications in non-centrosymmetric superconductors, where FF type pairing is generally expected under proper in-plane magnetic field. [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B47.00014: Switching Josephson coupling through a pseudo-spin-valve barrier with an exchange-field effect Burm Baek, William Rippard, Samuel Benz, Stephen Russek, Paul Dresselhaus, Matthew Pufall, Horst Rogalla With respect to information technology applications, Josephson junctions can be used in circuits that perform logic operations in picoseconds, which may result in energy-efficient, high-performance cryogenic computers, provided that memory elements can be developed that can be switched between two stable states by integrated superconducting logic circuits. We show that Josephson junctions based on pseudo-spin-valve barriers could enable such memory elements. We tuned the magnetic materials and carefully analyzed the full magnetic field dependence of the critical current in order to differentiate the controlled changes in Josephson coupling from the spatial superconducting phase modulation mediated by the remanent fields. We observed clear changes in Josephson critical current that can be either in magnitude or phase, which are explained well by the direct exchange-field effect on the spin-singlet superconducting pairs. These devices are the first that demonstrate nonvolatile, size-independent switching of the Josephson coupling in magnitude or phase, and they may allow for the first scalable superconducting memory devices. [Preview Abstract] |
Monday, March 3, 2014 2:03PM - 2:15PM |
B47.00015: Anomalous Resistance Behavior in Ferromagnet/Superconductor Heterostructure Lu-Kuei Lin, Jin-Hua Huang, Shang-Fan Lee We report on anomalous resistance peaks larger than the normal-state values in ferromagnetic NiFe wires with superconductor Nb electrodes. The resistance peaks occurs just below the onset of superconducting transition. The peak resistance temperature was suppressed and the peak height diminished by magnetic field together with the reduction of onset transition temperature. It is distinct from the charge imbalance behavior which is sensitive to magnetic field. The spin accumulation is one possible mechanism but the resistance peak is three-order larger than the predicted value. Contrary to the typical interfacial resistance contribution from band structure mismatch and electron spin imbalance, which should increase as the size of superimposed area was reduced, when we reduced the size of the ferromagnet/superconductor overlapping area, the resistance peaks vanished. Possible mechanisms of the electronic transport in the ferromagnet/superconductor interface will be discussed. [Preview Abstract] |
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