Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session F50: Superconductivity: Josephson and Proximity Effects |
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Sponsoring Units: DCMP Chair: Jian-Xin Zhu, Los Alamos National Laboratory Room: Mile High Ballroom 1D |
Tuesday, March 4, 2014 8:00AM - 8:12AM |
F50.00001: Gate-tuned Fraunhofer-type Conductance Modulation in Graphene-based Andreev Interferometers Minsoo Kim, Dongchan Jeong, Gil-Ho Lee, Yun-Sok Shin, Hyun-Woo Lee, Hu-Jong Lee The interplay between superconductivity and the Dirac-fermionic nature of electronic states of graphene leads to unique phase-coherent transport, when graphene is in proximity contact with superconducting electrodes. In this study, we report gate-tuned phase-coherent nonlocal magnetoconductance oscillations in Andreev interferometers consisting of a superconducting Al loop in contact with two ends of a T-shaped mono-layer graphene bar. The conductance oscillations arise from the flux change through the superconducting Al loop, with a gate-dependent Fraunhofer-type modulation of the envelope, which is independent of the sample-specific impurity configuration in the graphene sheet. We confirm that the modulation of envelope is caused by the gate-dependent nonlocal pair coherence along with the change of flux threading the phase-coherent region of graphene between the Al electrodes. The finite-bias effect on the conductance oscillations is also examined in terms of the Onsager-B\"uttiker relation and the BTK-type Andreev reflection probability. [Preview Abstract] |
Tuesday, March 4, 2014 8:12AM - 8:24AM |
F50.00002: ABSTRACT WITHDRAWN |
Tuesday, March 4, 2014 8:24AM - 8:36AM |
F50.00003: Effect of a spin-active interface on proximity-induced superconductivity in topological insulators Christopher Triola, Enrico Rossi, Alexander Balatsky We examine the effect of a spin-active interface on the symmetry of proximity-induced superconducting pairing amplitudes in topological insulators. We employ diagramatic techniques to investigate the leading order contribution to the pairing amplitude considering 3 different kinds of spin-active interfaces: 1) those for which the interface leads to the wavefunctions of transmitted electrons picking up spin-dependent phases in addition to flipping the spin of transmitted electrons, 2) those with only spin-dependent phases and no spin-flipping, and 3) those with only spin-flipping and no spin-dependent phases. We find that in cases (1) and (2) a considerable odd-frequency spinful-triplet pairing is induced in the TI while for case (3) no spin triplet pairing is induced to leading order. We compare our results to those for a normal metal and ferromagnetic materials finding that the nontrivial spin structure of the TI leads to qualitatively different behavior. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F50.00004: Josephson Junctions based Suspended Bi2Se3 nanoribbons Yanmeng Shi, Zhiyong Wang, Jing Shi, Chun Ning Lau As an important member of topological insulator family, Bi2Se3 has Dirac surface states and a 300meV bulk energy gap. Hybrid Bi2Se3/superconductor junctions have the promise of realizing Majorana fermions, and have attracted much interest recently. In our work, we fabricate suspended Bi2Se3 nanoribbon devices with superconducting Al electrodes, and study their transport transport properties. We will present our latest transport data at the meeting. [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F50.00005: Observation of short ballistic Josephson effect in vertical graphene junctions Gil-Ho Lee, Hu-Jong Lee The current-phase relation (CPR) of vertical graphene Josephson junctions (vGJJs) was measured using phase-sensitive dc-SQUID interferometry. A vGJJ, realized by vertically sandwiching a monolayer graphene between two Al electrodes, had an atomically short channel with transparent contacts for the highly coherent junction nature. The measured CPR was almost perfectly skewed, which rigorously confirmed the short ballisticity of the vGJJs. The short ballistic character of a Josephson junction has been predicted since 1970's but has never been realized in scalable hybrid systems. The CPR also provided energy spectrum of Andreev levels formed inside the junction, which offered a promising prospect for scalable quantum information devices such as Andreev-level qubits. This vertical-junction scheme is also readily applicable to the other cleavable materials such as three-dimensional topological insulators or transition metal dichalcogenides, opening a new pathway for uncovering exotic coherence phenomena arising in an atomic scale. [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F50.00006: Majorana Fermion Signatures in Flux Quantum Tunneling Pedro Lopes, Vasudha Shivamoggi, Amir Caldeira We propose Majorana fermions signatures in quantum tunneling experiments on SQUIDs composed of topological superconductors. Majorana fermions in a single Josephon junction have well-studied signatures which rely on single electron transfers. We investigate the effect of Majorana fermions on the flux, rather than charge, degree of freedom of a superconducting loop. Tuning of the applied magnetic flux through the loop can cause the system to tunnel between states with different enclosed flux quanta. We study how this tunneling picture may be modified in the presence of Majorana fermions. We use a 1+1D spinless p-wave model which hosts Majorana fermions in the topological phase to introduce and argue in favor of a phenomenological model, and demonstrate that novel phase slipping and quantum tunneling physics arise as a consequence. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F50.00007: Current filamentation in large Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$ mesas observed by luminescent and scanning laser thermal microscopy Timothy Benseman, Yang Hao, Alex Koshelev, Vitalii Vlasko-Vlasov, Ulrich Welp, Wai-Kwong Kwok, Boris Gross, Matthias Lange, Dieter Koelle, Reinhold Kleiner, Kazuo Kadowaki Self-heating is a critical issue in stacked intrinsic Josephson junction devices designed for terahertz emission. Some theoretical models, as well as experimental evidence, suggest that self-heating may indeed be helpful for maximizing THz power output. Here we study the self-heating of a Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$ mesa terahertz source via two techniques. We show that low-temperature scanning-laser microscopy measurements - a sensitive, but indirect probe of device temperature - agree well with direct temperature measurements obtained via a thermoluminescent imaging technique. Due to the semiconductor-like c-axis resistivity of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$, we find that at low temperatures device self-heating is highly non-uniform, displaying hysteretic nucleation of narrow hotspots with elevated current density. Also, the hotspot radius grows with increasing device temperature. These behaviors are consistent with theoretical predictions for a current filament forming in a material whose resistance falls with increasing temperature. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F50.00008: Spectral investigation of hot-spot and cavity resonance effects on the terahertz radiation emitted from high-$T_{\mathrm{c}}$ superconducting Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ single crystal mesa structures Kazuo Kadowaki, Chiharu Watanabe, Hidetoshi Minami, Takashi Yamamoto, Takanari Kashiwagi, Richard Klemm Terahertz (THz) electromagnetic radiation emitted from high-$T_{\mathrm{c}}$ superconducting Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ mesa structures in the case of single mesa and series-connected mesas is investigated by the FTIR spectroscopic technique while observing its temperature distribution simultaneously by a SiC photoluminescence technique. Changing the bias level, sudden jumps of the hot-spot position were clearly observed. Although the radiation intensity changes drastically associated with the jump of the hot spot position, the frequency is unaffected as long as the voltage per junction is kept constant. Since the frequency of the intense radiation satisfies the cavity resonance condition, we confirmed that the cavity resonance is of primarily importance for the synchronization of whole intrinsic Josephson junctions in the mesa for high power radiation. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F50.00009: Supercurrents in InSb nanowire Josephson junctions Jun Chen, Peng Yu, S\'ebastien Plissard, Diana Car, Vincent Mourik, Kun Zuo, David van Woerkom, Daniel Szombati, Leo Kouwenhoven, Erik Bakkers, Sergey Frolov Majorana fermions have been predicted in one-dimensional semiconductor nanowires with strong spin-orbit interactions coupled to superconductors. Effects such as odd number Shapiro steps disappearing and critical currents oscillating in magnetic field have been proposed as signatures of Majorana fermions in Josephson junctions. Here we investigate supercurrents in NbTiN-InSb nanowire-NbTiN Josephson junctions as a function of back gate and magnetic field. When an external magnetic field was applied along the nanowire, we observe gate-tunable oscillations in the critical current. To clarify the origin of this oscillating critical current, we are studying the spectra of Shapiro steps, which may give us a better understanding of such Josephson junctions and guide the search for additional signatures of Majorana fermions. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F50.00010: Anomalous oscillations of the Josephson supercurrent in InSb nanowires Attila Geresdi, D\'{a}niel B. Szombati, Ludo J. Cornelissen, Diana Car, S\'{e}bastien R. Plissard, Erik P.A.M. Bakkers, Leo P. Kouwenhoven Semiconductor nanowires proximity coupled to superconducting leads provide an ideal experimental platform to investigate the Josephson effect in tunable ballistic channels in the presence of strong spin-orbit coupling and large Land\'{e} g-factor. The interplay of an external magnetic field perpendicular to the intrinsic spin-orbit field may lead to an anomalous supercurrent which is a proposed signature of the coupling between two Majorana modes through the channel. Here we present our experimental studies of the Josephson supercurrent in InSb nanowires. Ohmic contacts to bulk superconductor NbTiN leads enable us to trace supercurrents up to $B=3\,$T magnetic field. The gate control over the channel allows us to investigate the amplitude of the critical current from the tunneling regime to a few transparent modes, where nonsinusoidal current-phase relationship (CPR) is expected, verified by the presence of fractional Shapiro steps under microwave irradiation. The evolution of the critical current with the external magnetic field is shown to exhibit non-monotonic behavior depending on the gate configuration, consistently with the theory of Josephson junctions hosting Majorana modes. [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F50.00011: Nonlinear Superconducting Metamaterials in Free-Space at mm-wave Frequencies Steven Anlage, Daimeng Zhang, Melissa Trepanier, Oleg Mukhanov, K. Delfanazari, V. Savinov, N. Zheludev Superconducting metamaterials show the promise of low loss, compact size and extreme tunability and nonlinearity, allowing for new applications. Most demonstrations of these metamaterials have been conducted in waveguide geometries, either in co-planar form or three-dimensional single-conductor structures. Here we demonstrate for the first time a widely tunable superconducting metamaterial operating under the free-space illumination of a quasi-optical beam in the 100 GHz regime. The meta-atoms are Radio Frequency Superconducting QUantum Interference Devices (RF SQUIDs) that form compact self-resonant objects endowed with the nonlinearity of the Josephson effect. The metamaterial is tuned with dc magnetic flux, temperature and mm-wave power, and holds promise for a new generation of mm-wave agile devices. [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F50.00012: Multi-photon Transitions in a near-millimeter-sized superconducting device Roberto Ramos, Steven Carabello, Joseph Lambert, Daniel Cunnane, Wenqing Dai, Ke Chen, Qi Li, Xiaoxing Xi The washboard potential well of a current-biased Josephson junction is a natural testbed for studying the quantum dynamics of trapped particles. At sufficiently low temperatures, the dynamics of the device is similar to that of a quantum ``phase particle'' that can access quantized states within the well. When photons are strongly coupled to such a quantum system such as by weak microwave irradiation, multi-photon transitions can be observed between two energy levels. This occurs when the quantum energy of the radiation, multiplied by an integer, matches the spacing between levels. These quantum-behaving devices are typically tens of microns across and are current-biased so that the well is shallow enough to accommodate few energy levels. In contrast, we have observed single- and multi-photon transitions in junctions with areas 600 times bigger than conventional junctions previously shown to display multi-photon transitions. These relatively large devices are MgB2 junctions, measuring up to 0.3 mm across. The data fits consistently with theoretical models of junctions behaving in the quantum limit. On the other hand, the data and the use of large capacitance junctions suggests observation of such transitions even in a deep well with many quantum levels. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F50.00013: A quantum accurate waveform synthesizer as a voltage reference for an electronic primary thermometer Alessio Pollarolo, Samuel Benz, Horst Rogalla, Paul Dresselhaus We are using a quantum voltage noise source (QVNS) for use as an intrinsically accurate voltage reference for a new type of electronic temperature standard. In Johnson Noise Thermometry (JNT) the noise of a resistor is used to measure temperature or Boltzmann's constant $k$, because the Nyquist equation \textless $V^{\mathrm{2}}$\textgreater $=$4\textit{kTR}$\Delta f$ shows that the power spectral density \textless $V^{\mathrm{2}}$\textgreater is proportional to $k$, temperature $T$, resistance $R$ and measurement bandwidth $\Delta f$. The QVNS is a digital to analog converter used to synthesize a voltage waveform that resembles pseudo-random noise comparable in amplitude to the resistor noise. The signal generated is a frequency comb of harmonics tones that are equally spaced in frequency, all having identical amplitudes but random phases. The QVNS is an array superconducting Josephson junctions that are biased with a pulsed waveform clocked at 10 GHz. The accuracy of the voltage waveform derives from the identical voltage pulses produced by each junction that are perfectly quantized because their time-integrals are always equal to flux quantum $h$/2$e$. The time-dependent output voltage waveform is determined by the number of pulses and their density in time. The measurement electronics exploits cross-correlation techniques to reduce the uncorrelated measurement noise so as to reveal the resistor noise, both of which are on the order of 2 nV/$\surd $Hz. With this technique we have measured $k$ with an uncertainty of about one part in 10$^{\mathrm{5}}$, which we hope to improve by another order of magnitude with further research. [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F50.00014: Proximity induced superconductivity in Au(111) surface state with strong Rashba spin-orbit coupling Peng Wei, Andrew Potter, Ferhat Katmis, Patrick Lee, Jagadeesh Moodera The surface state with Rashba type spin-orbit coupling (SOC) and induced $s$-wave superconductivity (SC) has been predicted as an excellent platform for topological SC.$^{\mathrm{1,2}}$ Large SOC energy splitting is essential in protecting the topological SC from disorders. Metallic surface states, i.e. Au(111), has been known to possess SOC splitting as large as 50 meV - a candidate superior to many semiconductor materials. We present our experimental demonstrations on the proximity induced SC in Au(111), for the first time, by tunneling studies. We have successfully achieved epitaxial growth of gold/SC bilayers with clean interface. Structural analyses show excellent sample quality with minimal surface roughness. Planar tunnel junctions are made, where clear Shockley surface state of Au(111) was observed at about 450 meV below the Fermi energy. SC tunneling spectroscopy reveals unconventional two gap features on Au(111) that is attributed to the induced surface SC. Further results about ongoing tunneling studies in lithographically patterned Au(111) nanowires will also be discussed. [1] A. C. Potter {\&} P. A. Lee. \textit{Phys Rev B} \textbf{83}, 094525 (2011); [2] A. C. Potter {\&} P. A. Lee. \textit{Phys Rev B} \textbf{85}, 094516 (2012) [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F50.00015: Supercurrent Spectroscopy of Andreev States \c{C}a\u{g}lar Girit, Landry Bretheau, Cristian Urbina, Daniel Esteve, Hugues Pothier We measure the excitation spectrum of a superconducting atomic contact. In addition to the usual continuum above the superconducting gap, the single particle excitation spectrum contains discrete, spin-degenerate Andreev levels inside the gap. Quasiparticle excitations are induced by a broadband on-chip microwave source and detected by measuring changes in the supercurrent flowing through the atomic contact. Since microwave photons excite quasiparticles in pairs, two types of transitions are observed: Andreev transitions, which consists of putting two quasiparticles in an Andreev level, and transitions to odd states with a single quasiparticle in an Andreev level and the other one in the continuum. [Preview Abstract] |
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