Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session M11: Superconductivity: Mostly Meso-Nano Scale |
Hide Abstracts |
Sponsoring Units: DCMP Chair: Timir Datta, University of South Carolina Room: 007B |
Wednesday, March 4, 2015 11:15AM - 11:27AM |
M11.00001: Size dependent suppression of superconductivity in granular mesoscopic Nb islands Malcolm Durkin, Sarang Gopalakrishnan, Rita Garrido Menacho, Nadya Mason While the suppression of superconductivity has been studied in bulk, 2D, and 1D systems, it remains largely unstudied in systems of mesoscopic granular islands. We investigate critical temperature (T$_{\mathrm{c}})$ suppression in mesoscopic granular Nb islands as a function of island diameter. By performing transport measurements, we find that superconductivity is suppressed at diameters considerably larger than the coherence length of Nb. This behavior cannot be explained by competition between charging and Josephson energies. Instead, variations in island T$_{\mathrm{c}}$ for fixed diameters suggest that the onset ---and suppression--- of superconductivity may be determined by rare region effects. [Preview Abstract] |
Wednesday, March 4, 2015 11:27AM - 11:39AM |
M11.00002: Origin of superconductivity and ferromagnetism in Bi/Ni/Bi sandwich trilayers Weiwei Zhao, Cui-zu Chang, Duk-Y Kim, Jagadeesh Moodera, Moses Chan Coexistence of superconductivity and ferromagnetism was experimentally observed in Bi/Ni bilayers [PRL 94, 037006 (2005)]. To reveal the origin of the superconductivity in this system, we here systematically studied the superconductivity and the ferromagnetism in Bi/Ni/Bi trilayers, Bi/Ni bilayers and Ni/Bi bilayers. We found the superconducting transition temperature to be independent of the thickness of Ni layers from 1nm to 45nm in Bi/Ni/Bi trilayers. The superconducting critical magnetic fields of trilayers were higher than those of bilayers. In addition, we observed conventional ferromagnetism in both trilayers and bilayers. These observations can be explained by a model that the superconductivity originates from the Bi/Ni interfaces with spin triplet pairing. [Preview Abstract] |
Wednesday, March 4, 2015 11:39AM - 11:51AM |
M11.00003: Electron Pairing Without Superconductivity Jeremy Levy, G. Cheng, M. Tomczyk, S. Lu, J.P. Veazey, M. Huang, P. Irvin, S. Ryu, H. Lee, C.-B. Eom, C.S. Hellberg Strontium titanate (SrTiO$_3$) exhibits an extremely low carrier density threshold for superconductivity, and possesses a phase diagram similar to high-temperature superconductorsâ€”two factors that suggest an unconventional pairing mechanism. We describe transport experiments with nanowire-based quantum dots localized at the interface between SrTiO$_3$ and LaAlO$_3$. Electrostatic gating of the quantum dot reveals a series of two-electron conductance resonances--paired electron states--that bifurcate above a critical magnetic field B$_p$~1-4 Tesla, an order of magnitude larger than the superconducting critical magnetic field. For B$<$B$_p$, these resonances are insensitive to applied magnetic fields; for B$>$B$_p$, the resonances exhibit a linear Zeeman-like energy splitting. Electron pairing is stable at temperatures as high as $T=900$ mK, far above the superconducting transition temperature (T$_c$~300 mK). These experiments demonstrate the existence of a robust electronic phase in which electrons pair without forming a superconducting state. Key experimental signatures are captured by an attractive-U Hubbard model that describes real-space electron pairing as a precursor to superconductivity. [Preview Abstract] |
Wednesday, March 4, 2015 11:51AM - 12:03PM |
M11.00004: Andreev Bound States in Confined Superconducting Semiconductor Nanowires Guanglei Cheng, Michelle Tomczyk, Shicheng Lu, Josh Veazey, Mengchen Huang, Patrick Irvin, Jeremy Levy, Hyungwoo Lee, Sangwoo Ryu, Chang-Beom Eom The hybridization of superconductors (SC) and semiconducting nanowires leads to a variety of interesting phenomena including nanoscale superconductivity and Majorana fermion physics. Andreev bound states (ABS), which are a discrete energy spectrum as a result of Andreev reflections of electrons and holes between two normal-SC interfaces, are predicted to support Majorana bound states under certain conditions. The LaAlO$_3$/SrTiO$_3$ interface possesses native superconductivity and strong spin-orbit coupling and is thus a promising platform for observing signatures of Majorana fermions. Here we investigate a superconducting nanowire quantum dot created by reversible ``write'' and ``erase'' processes using a conductive atomic force microscope tip. Transport studies show that electrons can travel in different regimes dominated by resonant pair tunneling and Andreev reflection in a single device. [Preview Abstract] |
Wednesday, March 4, 2015 12:03PM - 12:15PM |
M11.00005: Detection of vortex trapping in mesoscopic single-crystal loops of NbSe$_2$ by magnetoresistance oscillations Shaun Mills, Chenyi Shen, Zhaun Xu, Ying Liu Vortex crossing of a doubly-connected mesoscopic loop of a type II superconductor will lead to magnetoresistance oscillations because the free energy of the loop is modulated by the enclosed flux. The amplitude and temperature dependence of the oscillations in these mesoscopic loops of type II superconductors differ from the conventional Little-Parks effect. In addition to Abrikosov vortex crossing, vortex trapping within the arms of a mesoscopic loop should be possible. London calculations predict a phase shift in the free energy modulation when a single vortex is trapped within the arm of a superconducting loop. We present magnetoresistance measurements on mesoscopic, single-crystal NbSe$_2$ loops exhibiting the anticipated free energy modulation phase shift at a critical vortex trapping field that can be tuned by sample geometry, temperature, and external current in agreement with theoretical expectations. [Preview Abstract] |
Wednesday, March 4, 2015 12:15PM - 12:27PM |
M11.00006: Andreev states in the spin-symmetric solution of a quantum dot attached to superconducting leads Vaclav Janis, Vladislav Pokorny A quantum dot with Coulomb repulsion attached to left and right superconducting leads is studied via the perturbation expansion in the interaction strength. We use the Nambu formalism and the standard many-body diagrammatic representation of the impurity Green functions. We formulate the perturbation expansion in the spectral representation so that to be able to separate contributions from the isolated gap states (Andreev bound states) and from the continuous band states. We resolve exactly the spin-symmetric state in the asymptotic limit with Andreev states approaching the Fermi energy. We demonstrate that a spin-symmetric state reaches saturation at zero temperature at a critical interaction at which the Andreev states merge and freeze at the Fermi energy. There is no solution above this critical interaction with Andreev states split from the Fermi energy unless external magnetic field breaking the spin symmetry is applied and degeneracy of the ground state lifted. [Preview Abstract] |
Wednesday, March 4, 2015 12:27PM - 12:39PM |
M11.00007: Splitting individual Cooper pairs and single photon detection in superconducting aluminium Megan Edwards, Nicholas Lambert, Adam Esmail, Andrew Ferguson, Brendon Lovett, Felix Pollock Using superconducting aluminum quantum dots, we demonstrate the splitting of individual Cooper pairs using microwave light [1]. Within our nanoscale devices, the competition at low temperatures between the charging energy, superconducting gap and Josephson energy enables single Cooper pairs to be split and reformed. The delivery of constituent quasi-particles to separate quantum dots has facilitated measurements of both splitting and recombination. The devices are probed by radio-frequency reflectometry, a technique sensitive to the `quantum capacitance' of the device band structure [2]. We intentionally induce splitting via the application of a microwave field, indicating a system able to detect individual photons of microwave light [1]. To controllably separate a Cooper pair into two quasi-particles may have important implications for quantum information processing; future experiments will investigate the coupling of a double quantum dot to a microwave resonator for single photon detection [3], with applications to circuit quantum electrodynamics. \\[4pt] [1] N. J. Lambert \textit{et al}., Phys. Rev. B., 90, 140503(R), 2014\\[0pt] [2] T. Duty \textit{et al.}, Phys. Rev. Lett., 95, 206807, 2005\\[0pt] [3] M. G\"{o}ppl \textit{et al.}, J. Appl. Phys., 104, 113904, 2008 [Preview Abstract] |
Wednesday, March 4, 2015 12:39PM - 12:51PM |
M11.00008: Stripe-like nanoscale structural phase separation and optimal inhomogeneity in superconducting BaPb$_{1-x}$Bi$_x$O$_3$ Paula Giraldo-Gallo, Ying Zhang, Carolina Parra, Hari Manoharan, Malcolm Beasley, Theodore Geballe, Matthew Kramer, Ian Fisher Structural phase separation in the form of partially disordered stripes, with characteristic length scales in the nanometer range, is observed for superconducting BaPb$_{1-x}$Bi$_x$O$_3$. The evolution of the superconducting coherence length with composition relative to the size of these stripes suggests an important role of the nanostructure in determining the shape of the superconducting dome. It is proposed that the maximum T$_c$ is determined by a kind of ``optimal inhomogeneity,'' characterized by a crossover from an inhomogeneous macroscopic superconductor to a granular superconductor for which phase fluctuations suppress T$_c$. [Preview Abstract] |
Wednesday, March 4, 2015 12:51PM - 1:03PM |
M11.00009: ABSTRACT WITHDRAWN |
Wednesday, March 4, 2015 1:03PM - 1:15PM |
M11.00010: Interaction Determined Electron Energy Levels in One-Dimension Michael Pepper, Sanjeev Kumar, Kalarikad Thomas, Luke Smith, Graham Creeth, Ian Farrer, David Ritchie, Geraint Jones, Griffiths Jonathan We have investigated electron transport in a quasi-one dimensional electron gas in the GaAs-AlGaAs heterostructure designed so that the confinement potential can be progressively weakened. This causes the energy levels to decrease in energy relative to each other, however this decrease occurs at different rates, a feature attributed to the energy being determined by both confinement and the electron-electron repulsion which varies with the shape of the wavefunction. It is found that the initial ground state crosses the higher levels so resulting in missing plateaux of quantised conductance. A change in the nature of the ground state to a more extended form causes an increase in the capacitance between the confining gates and the electrons. Both crossings and anti-crossings of the levels are found and these will be discussed along with other consequences of the form of the level interactions. The effects of level crossing on the spin dependent 0.7 structure will be presented. [Preview Abstract] |
Wednesday, March 4, 2015 1:15PM - 1:27PM |
M11.00011: Strong effects of weak ac driving in short superconducting junctions Roman-Pascal Riwar, Manuel Houzet, Julia S. Meyer, Yuli V. Nazarov We study a short superconducting junction subject to a dc and ac phase bias. The ac modulation changes the occupation of the Andreev bound states formed at the constriction by transitions between bound states and the continuum. In a short junction, the non-equilibrium Andreev bound state population may relax through processes that conserve parity of the occupation number on the same bound state and processes that do not conserve it. We argue that the parity conserving processes occur on a much faster time scale. In this case, even a weak driving may lead to a large deviation of the supercurrent from its equilibrium value. We show that this effect is accompanied by a quasiparticle current which may lead to a measurable charge imbalance in the vicinity of the junction. Furthermore, we study the time evolution of the supercurrent after switching off the ac drive. On a time scale where parity relaxation is negligible, the supercurrent relaxes to a stationary non-equilibrium state. Finally, we briefly outline the regime of ultraweak driving where the ac-induced processes occur on a time scale comparable to parity relaxation. [Preview Abstract] |
Wednesday, March 4, 2015 1:27PM - 1:39PM |
M11.00012: Weber blockade in superconducting nanowires Tyler Morgan-Wall, Benjamin Leith, Nikolaus Hartman, Atikur Rahman, Nina Markovic Vortices in superconductors are topological excitations that carry quantized magnetic flux and can be viewed as basic degrees of freedom that describe the low-energy states of the system. Here we show that a short superconducting nanowire can behave as a quantum dot for vortices. In the range of magnetic fields in which vortices can enter the nanowire in a single row, we find regular oscillations of the critical current as a function of magnetic field, with each oscillation corresponding to the addition of a single vortex to the nanowire. A charge-vortex dual of the Coulomb-blockaded quantum dot for electrons, the nanowire shows diamond-shaped regions of zero resistance as a function of current and magnetic field, in which the number of vortices is fixed. Besides demonstrating that macroscopic objects such as vortices can behave as fundamental particles, the fine control over critical currents and vortex configurations may prove useful for quantum devices that employ superconducting circuits. [Preview Abstract] |
Wednesday, March 4, 2015 1:39PM - 1:51PM |
M11.00013: Generation of photon pairs at different frequencies: route toward quantum microwave source Daniel Esteve, Olivier Parlavecchio, Carles Altimiras, Philippe Joyez, Denis Vion, Patrice Roche, Fabien Portier The dynamical Coulomb blockade (DCB) is a quantum phenomenon where the tunneling of charge through a tunnel junction is modified by its electromagnetic environment. The sudden charge transfer generates photons in the electromagnetic modes. We coupled a Josephson junction to two resonators at frequencies $\nu_1\neq\nu_2$; when voltage-biased at $2eV=h\nu_1+h\nu_2$, Cooper pairs can tunnel only if two photons, one at each frequency, are simultaneously emitted. We measured the cross-correlations between the emission rates and showed that a Cauchy-Schwarz inequality is violated. This result, in agreement with theoretical prediction made by Leppakangas and coworkers\footnote{J. Lepp\"{a}kangas, G. Johansson, M. Marthaler, and M. Fogelstrom, ``Nonclassical Photon Pair production in a Voltage-Biased Josephson Junction,'' Phys. Rev. Lett. \textbf{110}, 267004-5 (2013).}, reveals the amplitude two-mode squeezing. Our setup is a easy way to produce non-classical microwave radiation from a battery. We believe that this source is a good candidate for producing pairs of entangled photons with high rate (few hundreds of MHz). [Preview Abstract] |
Wednesday, March 4, 2015 1:51PM - 2:03PM |
M11.00014: Point-contact spectroscopy on the 3-Dimensional Dirac Semi-metal Cd$_3$As$_2$ Goutam Sheet, Abhishek Gaurav, Gohil Singh Thakur, Zeba Haque, Ashok Kumar Ganguli, Leena Aggarwal The three dimensional (3D) Dirac semi-metals exist close to topological phase boundaries. Therefore, in principle, it should be possible to drive them into exotic new phases by breaking certain symmetries. We will discuss point-contact spectroscopic measurements on the 3D Dirac semi-metal Cd$_3$As$_2$ using several normal metallic tips. We have found that the mesoscopic point-contacts between elemental normal metals (like silver (Ag), platinum (Pt) and gold (Au)) and Cd$_3$As$_2$ exhibit signatures of certain exotic new phases. The possible origin of such phases in the confined region on Cd$_3$As$_2$ will also be discussed. [Preview Abstract] |
Wednesday, March 4, 2015 2:03PM - 2:15PM |
M11.00015: Tilting of the magnetic field in Majorana nanowires Stefan Rex, Asle Sudb{\O} Semiconductor nanowires with strong spin-orbit coupling and proximity-induced $s$-wave superconductivity in an external magnetic field have been the most promising settings for approaches towards experimental evidence of topological Majorana zero modes. We investigate the effect of tilting the magnetic field relative to the spin-orbit coupling direction in a simple continuum model and provide an analytical derivation of the critical angle, at which the topological states disappear. We also obtain the differential conductance characteristic of a junction with a normal wire for different tilting angles and propose a qualitative change of the dependence of the zero-energy differential conductance on the tunnel barrier strength at the critical angle as a criterion for establishing the topological nature of the observed signal. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700