Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session Q11: Focus Session: Hybrid Magnetic-Superconducting Systems II |
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Sponsoring Units: DMP Chair: Anand Bhattacharya, Argonne National Laboratory Room: Morial Convention Center RO9 |
Wednesday, March 12, 2008 11:15AM - 11:51AM |
Q11.00001: Proximity Effects and crossed Andreev reflections in Perovskite Superconductor-Ferromagnet Bilayers Invited Speaker: Since the ferromagnetic side of a superconductor-ferromagnet (S-F) junction is spin polarized, Andreev reflections are suppressed. Consequently, the induced S order parameter in the F is expected to decay rapidly, on a nm scale. Our scanning tunneling spectroscopy measurements of thin epitaxial (100)YBa$_{2}$Cu$_{3}$O$_{7-\delta }$-SrRuO$_{3}$ (YBCO-SRO, S-F) bilayers indeed show that on most of the junction area the S order parameter vanishes in the SRO over a distance less than 8 nm. However, we find localized regions, arranged along narrow strips, where a superconductor-like gap penetrates the SRO more than 25 nm. This is attributed to ``crossed Andreev reflections" taking place at domain boundaries, where an electron from one magnetic domain is retro reflected as a hole with opposite spin in an adjacent domain. Another intriguing finding was the reduction in the F ordering temperature of the SRO layer when T$_{Curie}$ was in the pseudogap temperature regime of the (underdoped) YBCO film. This can be due to injection of correlated electron pairs into the SRO, thus supporting the preformed pairs scenario for the pseudogap. With that regard, I will discuss T$_{c}$ enhancement in cuprate bilayers. We have also studied (110)YBCO-SRO bilayers, where, surprisingly, a clear penetration of the Andreev bound states (residing on the (110) YBCO surface) into the F layer was revealed. The penetration is manifested in the density of states as a split zero bias conductance peak with an imbalance between peak heights. Our data indicate that the splitting occurs at the S side, possibly as a consequence of induced magnetization, consistent with recent theoretical predictions. The imbalance is attributed to the spin polarization in SRO. [Preview Abstract] |
Wednesday, March 12, 2008 11:51AM - 12:03PM |
Q11.00002: Scanning Tunneling Spectroscopy Study of Bi-Layer La$_{0.7}$Ca$_{0.3}$MnO$_3$/YBa$_2$Cu$_3$O$_{7-\delta}$ Thin Films I. Fridman, P. Morales, J.Y.T. Wei Recent experiments have reported long-range proximity effect in ferromagnet/superconductor (F/S) heterostructures comprised of transition-metal perovskites. To look for direct evidence of this effect, we have performed scanning tunneling spectroscopy (STS) on La$_{0.7}$Ca$_{0.3}$MnO$_3$/YBa$_2$Cu$_3$O$_{7-\delta}$ (LCMO/YBCO) bi-layer thin films. The bi-layer films were epitaxially grown on $<$001$>$ SrTiO$_{3}$ substrates using pulsed laser deposition. STS measurements were made at 4.2 K on bi-layer films with varying LCMO thickness ($\sim$20 to 60 nm) and in a magnetic field applied parallel to the film. The STS data were analyzed for spectral signatures of a pairing gap on the LCMO layer induced by the YBCO layer, to determine the length scale of the F/S proximity effect and the role played by magnetic domain walls. [Preview Abstract] |
Wednesday, March 12, 2008 12:03PM - 12:15PM |
Q11.00003: Anomalous Sub-Gap features in Nb/Ni Tunneling DOS Paul SanGiorgio, Malcolm Beasley, Junhyung Kwon, Kookrin Char We report the existence of anomalous sub-gap features in tunneling DOS measurements of Superconductor/Strong Ferromagnet bilayers. By measuring the DOS as a function of ferromagnet thickness, $d_F$, we are able to document the spatial evolution of the Cooper pairs diffusing into the ferromagnet. This technique proves to be much less sensitive to variations in boundary conditions and provides more detailed information per sample than traditional $T_c$ or $J_c$ measurements. In addition to the sub-gap features, we find that the DOS shows the usual peaks at $\pm\Delta_0$, which ``invert'' at $d_F = 3.5$~nm. We analyze the results with the fully non-linear Usadel equation, including the effects of various scattering mechanisms and spin-dependent boundary conditions. We are able to quantitatively account for the features at $\pm\Delta_0$ through the addition of spin-orbit scattering in the F-layer, but we find that no known parameter recreates the sub-gap features. By examining the behavior of the sub-gap in magnetic field, we propose that it is a signature of a long-range triplet condensate. [Preview Abstract] |
Wednesday, March 12, 2008 12:15PM - 12:27PM |
Q11.00004: Spin Polarization And Inelastic Scattering In High And Low Transparency Ferromagnet/Superconductor Point Contacts Paul J. Dolan, Jr., Charles W. Smith For point contact spectroscopy studies of ferromagnet/superconductor charge transport, the effects of spin polarization and inelastic scattering can dominate conductance data. Model calculations show that accurate values of the spin polarization parameter can most easily be determined in high transparency contacts (the ballistic limit) when inelastic scattering is low, and in low transparency contacts (the tunneling limit) when inelastic scattering is high. Comparison with data for both cases will be shown. [Preview Abstract] |
Wednesday, March 12, 2008 12:27PM - 12:39PM |
Q11.00005: Electrical properties of interfaces in high-T$_{c}$ superconducting heterostructures Maarten van Zalk, Jos Boschker, Menno Veldhorst, Alexander Brinkman, Hans Hilgenkamp Electrical contacts between materials of different functionality are often required, both for technological applications as well as for fundamental research. However, contacts between different complex oxides frequently do not behave like expected. For example, it is well known that tunnel junctions are difficult to fabricate from high-T$_{c}$ superconductors, due to the presence of a degraded, non-superconducting layer underneath the tunnel barrier. We investigated the interface properties of a number of different oxide heterostructures, prepared by pulsed laser deposition. Specifically, the causes of non-ideal interface behavior, such as changes in the stoichiometry, oxygen deficiency, structural changes and electronic reconstruction, were investigated. [Preview Abstract] |
Wednesday, March 12, 2008 12:39PM - 12:51PM |
Q11.00006: All-oxide inverse superconducting spin switch Jacobo Santamaria, Norbert Nemes, Cristina Visani, Javier Garcia-Barriocanal, Zouhair Sefrioui, Diego Arias, Carlos Leon, Mar Garcia-Hernandez, Suzanne te Velthuis, Axel Hoffmann In proximity coupled ferromagnet/superconductor/ferromagnet (F/S/F) structures the critical temperature is modulated by the relative orientation of the magnetization in the F layers (superconducting spin switch- SSS-). A larger T$_{c}$ with antiparallel (AP) compared to parallel (P) magnetizations results from the averaging of the exchange field over the coherent volume. Recent reports have shown an SSS behavior in F/S/F structures with strong ferromagnets, where superconductivity is favored for P orientation of the adjacent magnetizations. Its origin is a subject of debate. While some reports suggest enhanced pair breaking by spin-polarized quasiparticles in the AP configurations, others emphasize the effect of stray fields in depressing the superconductivity. Comparing data of bilayers and trilayers, we show that the SSS effect of our epitaxial LCMO/YBCO/LCMO structures is governed by spin transport with limited influence of stray fields. [Preview Abstract] |
Wednesday, March 12, 2008 12:51PM - 1:03PM |
Q11.00007: Covalent Bonding and Orbital Reconstruction at an Oxide Interface. Jak Chakhalian, John Freeland, Hans-Ulrich Habermeier, Georg Cristiani, G. Khaliullin, Michel van Veenendaal, Bernhard Keimer Atomically controlled interfaces between two materials can give rise to novel physical phenomena and functionalities. Modern synthesis methods have yielded high-quality heterostructures of oxide materials with competing order parameters. Orbital reconstructions and covalent bonding must be considered as important factors in the rational design of oxide heterostructures. Here we examine the interface between high-temperature superconducting (Y,Ca)Ba2Cu3O7 and metallic La2/3Ca1/3MnO3 by resonant x-ray spectroscopy. The resulting data show that electrons are transferred from Mn to Cu ions across the interface. This phenomenon is accompanied by the major reconstruction of the orbital occupation and symmetry in the CuO2 plane. Specifically, we report the experimental finding that unlike bulk at the interface Cu d3z2-r2 orbital is partially occupied and electronically active. This observation opens a path to orbital engineering of interface-controlled materials. J. Chakhalian et al, ``Covalent Bonding and Orbital Reconstruction at an Oxide Interface'', Science, v. 318, 1155 (2007). [Preview Abstract] |
Wednesday, March 12, 2008 1:03PM - 1:15PM |
Q11.00008: Spin-triplet correlations in clean superconductor-ferromagnet multilayers Zoran Radovic, Zorica Pajovic, Milos Bozovic, Jerome Cayssol, Alexander Buzdin We study transport phenomena in clean superconductor-ferromagnet (S-F) multilayers for a general case of arbitrary relative orientation of in-plane magnetizations and interface transparencies. The scattering problem based on the Bogoliubov-de Gennes equation is solved, taking into account both spin-singlet and spin-triplet superconducting correlations in two geometries, FSF and SFFS. We find a monotonic dependence of conductance spectra on the angle of misorientation of magnetizations $\alpha $ as their alignment is changed from parallel to antiparallel in FSF. Moreover, the critical Josephson current in SFFS multilayers is also a monotonic function of $\alpha $ when the junction is far enough from 0-$\pi $ transitions. In contrast to the diffusive case, no substantial impact of long--range spin--triplet superconducting correlations neither on conductance nor on the Josephson current has been found in the clean limit. [Preview Abstract] |
Wednesday, March 12, 2008 1:15PM - 1:27PM |
Q11.00009: Properties of Hybrid Superconductor/Half-metal Systems Matthias Eschrig, Tomas Lofwander Interfaces between materials that exhibit different types of order present unique opportunities to study fundamental problems in physics. One example is the interface between a singlet superconductor and a half-metallic ferromagnet, where Cooper pairing occurs between electrons with opposite spin on one side, while the other displays 100\% spin polarisation. New effects, like spin mixing and spin rotation are characteristic for such interfaces [1], providing a mechanism for conversion between unpolarized and completely spin polarized supercurrents. Our predictions have been confirmed in a recent experiment observing a supercurrent through half-metallic CrO$_2$ [2]. The interface region exhibits a superconducting state of mixed-spin pairs with highly unusual symmetry properties that opens up new perspectives for exotic Josephson devices [3]. We present results for Andreev bound state spectra and for point contact Andreev conductance spectra. The role of disorder and symmetry of the pairing amplitudes on the spectra and the temperature dependence of the Josephson current is elucidated.\\ {\small [1] M. Eschrig et al., Phys. Rev. Lett. {\bf 90}, 137003 (2003).}\\ {\small [2] R.S. Keizer et al., Nature, {\bf 439}, 825 (2006).}\\ {\small [3] M. Eschrig, T. Lofwander, submitted to Nature Physics (2007).} [Preview Abstract] |
Wednesday, March 12, 2008 1:27PM - 1:39PM |
Q11.00010: Novel Josephson junctions of high-$T_c$ cuprate using magnetic islands: Fabrication and Properties A. Maeda, L. Gomez, S. Kitamura In spite of the potential high performance, no one has succeeded in fabricating good Josephson junctions of high-$T_c$ cuprate superconductors with a large $I_cR_N$ product and textbook like $I-V$ characteristics. This is probably because the fabrications of the oxide barrier layer even on the almost perfect surface of the cuprate causes many unexpected troubles, suggesting that there are still many issues to be solved in the surface science of the cuprate superconductors. We have tried the high-$T_c$ Junction fabrication with a novel method, where we do not need to fabricate oxide barrier layer; A small island of Fe was merely placed on a strip of a cuprate superconductor, LSCO. By measuring $I-V$ characteristics, microwave radiation effect, and magnetic field effect, it turned out that this simple novel structure becomes a typical Josephson junction. What is remarkable is that the structure often exhibits a large $I_cR_N$ value ($\sim$ 10-20 meV), suggesting that our method is promising. Together with various kinds of comparative experiments, we will discuss the mechanism of the weak coupling. [Preview Abstract] |
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