Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session V8: Superconductivity: Proximity Effects and SN Junctions II |
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Sponsoring Units: DCMP Room: 304 |
Thursday, March 17, 2016 2:30PM - 2:42PM |
V8.00001: Evidence for a $\pi $-junction in Nb/F/Nb' trilayers from superfluid density measurements Thomas Lemberger, Michael Hinton, Stanley Steers, Bryan Peters, Fengyuan Yang Two-coil measurements of the sheet superfluid density of Nb/NiV/Nb' trilayers reveal the transition temperatures and volume superfluid densities of both Nb layers, as functions of the thickness, $d_{F}$, of the intervening ferromagnetic (F) Ni$_{\mathrm{0.96}}$V$_{\mathrm{0.04}}$ layer. The upper transition occurs when the thicker Nb layer goes superconducting and superfluid first appears. Fitting the high-temperature superfluid density to an appropriate functional form reveals the presence of a lower ``transition'' where additional superfluid appears. This event is really a crossover, but the difference is irrelevant here. There is a surprising minimum in superfluid densities of both Nb layers at $d_{F} \quad \approx $ 30 {\AA}, followed by a slow rise. This behavior suggests that a $\pi $ phase difference between the Nb layers develops at $d_{F} \quad \approx $ 30 {\AA} and continues to larger F thickness. [Preview Abstract] |
Thursday, March 17, 2016 2:42PM - 2:54PM |
V8.00002: Checking for odd-triplet pairing using novel superconducting spin valves Pavel N. Lapa, Trupti Khaire, Junjia Ding, John E. Pearson, Valentyn Novosad, Axel Hoffmann, J.S. Jiang An excitation of odd-triplet pairing in a superconducting spin valve can be revealed by measuring the dependence of the superconducting critical temperature Tc with increasing non-collinearity of the magnetizations in adjacent ferromagnetic layers. A standard approach to create such a non-collinear magnetization configuration is to pin one ferromagnetic layers and control the magnetization in another layer by rotating the multilayer in a small magnetic field. Unfortunately, the rotation can modify the vortex current which also strongly affects the critical temperature. To exclude such spurious effects, we designed and fabricated a novel superconducting spin valve which allows us to create non-collinear magnetization configurations without using a sample rotator. The valve's operational principle is based on pinning of a synthetic antiferromagnet (SAF) by exchange coupling it to FeMn layer. The ability to imprint non-collinear magnetization configurations in the spin valve was confirmed using giant magneto resistance (GMR) measurements. The response of the magnetizations on an external magnetic field was simulated based on a coherent rotation model. The dependence of the Nb layer Tc on imprinted magnetization configuration will be presented. [Preview Abstract] |
Thursday, March 17, 2016 2:54PM - 3:06PM |
V8.00003: Colossal Triplet Spin-Valve Effect in Heterostructures containing 100\% Spin Polarised Fe0.8Co0.2Si Gavin Burnell, Nathan Satchell, Benjamin Steele, Priyasmita Sinha, Christopher Marrows, Sean Langridge At the interface between a superconductor (S) and ferromagnet (F), an inhomogeneity can convert singlet Cooper pairs into the (spin aligned) long ranged triplet component (LRTC). Manipulation of the LRTC forms the basis of the emerging field of super-spintronics. The prototypical device in this field is the superconducting spin valve (SSV), where LRTC generation can be controlled by the relative orientation of two F layers in a heterostructure. This generation is accompanied by an observed suppression in the superconductors critical temperature (T$_{c}$). Motivated by a recent report of ‘colossal proximity effects’ in a F$_{1}$/F$_{2}$/S SSV containing 100\% spin polarized CrO$_{2}$ as the bottom drainage layer\footnote{A. Singh, \textit{et al.}, \textbf{Phy. Rev. X} 5, 021019}, we explore the possibility of using highly spin polarized, Fe$_{0.8}$Co$_{0.2}$Si as F$_{1}$. The observed T$_{c}$ suppression of 830 mK is nearly an order of magnitude larger than previous studies using standard F layers with Nb, and is consistent with that seen in CrO$_{2}$. Our results confirm the special importance of high spin polarization in the formation of the LRTC, and we offer the field a new material as a fundamental building block for incorporation into future super-spintronic devices. [Preview Abstract] |
Thursday, March 17, 2016 3:06PM - 3:18PM |
V8.00004: Remote Induced Magnetism in a Normal Metal coupled to a Superconductor/Ferromagnet Heterostructure Nathan Satchell, Machiel G. Flokstra, Jangyong Kim, Gavin Burnell, Peter J. Curran, Simon J. Bending, Joshaniel F. K. Cooper, Christy J. Kinane, Sean Langridge, Aldo Isidori, Matthias Eschrig, Natalia Pugach, Hubertus Luetkens, Andreas Suter, Thomas Prokscha, Stephen L. Lee Integrating superconductors (S) into ferromagnetic (F) heterostructures has revealed a rich area of novel physics and led to the development of superconducting spintronics. Of particular interest is the prototypical device, the S spin valve. In this work we use neutron and muon techniques to study the local magnetic profile in such a device, looking for an induced magnetism expected at the S/F interface. Instead we observe an additional unexpected moment arising neither in the S nor F layers, but in the normal metal cap\footnote{M. G. Flokstra, \textit{et al.}, \textbf{Nat. Phys.} doi:10.1038/nphys3486 (2015)}. The magnetisation is always antiparallel to the direction of an applied field (to align the F layers) and appears at the onset of superconductivity, increasing in strength with decreasing temperature. The profile of this induced moment is inconsistent with any known or predicted phenomena. What is particularly remarkable is that there is no applied current or temperature gradients meaning the effect manifests in equilibrium. [Preview Abstract] |
Thursday, March 17, 2016 3:18PM - 3:30PM |
V8.00005: Controlling superconducting spin flow with a single homogeneous ferromagnet: interference, torque and spin-flip immunity Sol Jacobsen, Iryna Kulagina, Jacob Linder Superconducting spintronics has the potential to overcome the Joule heating and short decay lengths of electron transport by harnessing the dissipationless spin currents of superconductors in thin-film devices. Using conventional singlet superconductive sources, such dissipationless currents have only been demonstrated experimentally using intricate magnetically inhomogeneous multilayers, which can be difficult to construct, control and measure. Here we present analytic and numerical results proving the possibility of both generating and controlling a long-ranged spin supercurrent using only one single homogeneous magnetic element (arXiv:1510.02488). The spin supercurrent generated in this way does not decay spatially, in stark contrast to normal spin currents that remain polarized only up to the spin relaxation length. Through a novel interference term between long-ranged and short-ranged Cooper pairs, we expose the existence of a superconductivity-mediated torque even without magnetic inhomogeneities, showing that the different components of the spin supercurrent polarization respond fundamentally differently to a change in the superconducting phase difference. This establishes a mechanism for tuning dissipationless spin and charge flow separately via superconductors. [Preview Abstract] |
Thursday, March 17, 2016 3:30PM - 3:42PM |
V8.00006: Field-History Dependence of the Superconducting Transition Temperature in Erbium/Niobium Bilayers James Witt, Nathan Satchell, Sean Langridge, Gavin Burnell Recently, there has been much interest in a new class of superconducting (S) spintronic devices based upon hybrid S/F (ferromagnet) heterostructures. The prototypical super-spintronic device is the superconducting spin valve (SSV), within which the critical temperature (Tc) of an S layer can be controlled by the relative orientation of two or more F layers. Such manipulation of the F layers requires careful engineering of the heterostructure and the rotation of the structure with respect to an applied magnetic field. Here, we show that such control over Tc is also possible in a simple S/F bilayer. By manipulating the remenant magnetic state of a thin Er layer -- which is proximity coupled to a Nb S layer -- we are able to demonstrate a high level of control over the Tc of the Nb (which is measured in zero field). The shifts in Tc are comparable in size to the largest seen in the SSV and are manipulated using solely the field history. The system can be reset by warming the sample through the Er Curie temperature (approximately 20 K). Our results are of particular interest due to the simplicity of both the bilayer and the measurement geometry in comparison to the SSV. [Preview Abstract] |
Thursday, March 17, 2016 3:42PM - 3:54PM |
V8.00007: Conductance spectroscopy of topological superconductor wire junctions F. Setiawan, Philip Brydon, Jay Sau We study the zero-temperature transport properties of one-dimensional normal metal-superconductor (NS) junctions with topological superconductors across their topological transitions. Working within the Blonder-Tinkham-Klapwijk (BTK) formalism generalized for topological NS junctions, we analytically calculate the differential conductance for tunneling into two models of a topological superconductor: a spinless intrinsic $p$-wave superconductor and a spin-orbit-coupled $s$-wave superconductor in a Zeeman field. The zero-bias conductance takes nonuniversal values in the nontopological phase while it is robustly quantized at $2e^2/h$ in the topological regime. Despite this quantization at zero voltage, the zero-bias conductance only develops a peak (or a local maximum) as a function of voltage for sufficiently large interfacial barrier strength, or certain parameter regimes of spin-orbit coupling strength. Our calculated BTK conductance also shows that the conductance is finite inside the superconducting gap region because of the finite barrier transparency, providing a possible mechanism for the observed “soft gap” feature in the experimental studies. [Preview Abstract] |
Thursday, March 17, 2016 3:54PM - 4:06PM |
V8.00008: Detecting a quantum critical point in topological SN junctions Yashar Komijani, Ian Affleck A spin-orbit coupled quantum wire, with one end proximate to an s-wave superconductor, can become a topological superconductor, with a Majorana mode localized at each end of the superconducting region. It was recently shown that coupling one end of such a topological superconductor to two normal channels of interacting electrons leads to a novel type of frustration and a quantum critical point when both channels couple with equal strength. We propose an experimental method to access this critical point in a single quantum wire and show its resilience to disorder. [Preview Abstract] |
Thursday, March 17, 2016 4:06PM - 4:18PM |
V8.00009: On correlation between zero bias conductance peaks and topological invariants in semiconductor Rashba nanowires Amit Nag, Jay Sau The observed zero bias peak in tunneling conductance experiments on semiconductor Rashba nanowire is a signature of presence of Majorana zero modes. Characteristics of zero bias conductance peak (ZBCP) namely, height, width and peak splitting, are a function of microscopic parameters. Zero modes have finite splitting as a result of finiteness of the nanowire rendering the ground state only approximately topological i.e. zero modes are only approximately Majoranas. We calculate the scattering matrix topological invariant to quantify the quality of approximate Majorana modes and study its relation to observed characteristics of ZBCP. Furthermore we study the effect of dephasing on the topological invariant. Finally, we draw connection between the characteristics of the ZBCP and probability of observing non-Abelian statistics in proposed future experiments involving braiding of Majorana modes. [Preview Abstract] |
Thursday, March 17, 2016 4:18PM - 4:30PM |
V8.00010: ABSTRACT WITHDRAWN |
Thursday, March 17, 2016 4:30PM - 4:42PM |
V8.00011: SPT 2-Channel Kondo Model in the Structure of Normal Metal/Quantum Dot/$\backslash $emph\textbraceleft DIII\textbraceright -class Topological Superconductor Wei-Jiang Gong, Zhen Gao We investigate the Kondo effect in a structure which is constructed by embedding one quantum dot between a normal metal and a $\backslash $emph\textbraceleft DIII\textbraceright -class topological superconductor supporting Majorana doublets at its ends. It is observed that Kondo correlation occurs between the localized state in the dot and two continuum states simultaneously, i.e., the continuum state in the metal and the continuum Andreev reflection state between the metal and topological superconductor. As a result, the Kondo model Hamiltonian is topologically protected by the {\$}SU(2)\textunderscore s$\backslash $rtimes Z\textunderscore 2\textasciicircum T{\$} symmetry. More interestingly, two new Kondo temperatures appear in this system, in comparison with the normal Kondo model. This phenomenon exactly reflects the special role of Majorana doublet in tuning the Kondo effect. [Preview Abstract] |
Thursday, March 17, 2016 4:42PM - 4:54PM |
V8.00012: Controlling the critical temperature of superconducting hybrid structures with spin-orbit coupling Jabir Ali Ouassou, Sol Jacobsen, Jacob Linder Based on our recent publication Phys.~Rev.~B~\textbf{92}~024510~(2015), we present theoretical predictions for the effect of spin-orbit coupling on the critical temperature of superconductor/ferromagnet bilayers. More specifically, we consider mesoscopic diffusive bilayers where the ferromagnet has (i)~pure Rashba coupling and~(ii) Rashba--Dresselhaus coupling, and show that one can achieve a superconducting spin-valve effect in both of these structures. Furthermore, it is shown that if the Rashba and Dresselhaus coupling have similar magnitudes, the critical temperature of the bilayer can change with over 35 percent as the in-plane magnetization is rotated by 90 degrees. In contrast to existing designs for superconducting spin-valves which require inhomogeneous magnetization, such as having multiple layers with noncollinear magnetizations, the critical temperature in our proposed setup is tunable with one single homogeneous ferromagnet. Thus, these results highlight a new way to exert control over superconductivity in proximity structures, which may prove easier to manufacture and control than the existing designs. [Preview Abstract] |
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