Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session F24: Superconducting Quantum Devices and Josephson Junctions |
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Sponsoring Units: DMP Chair: Laura Adams, Harvard University Room: 203AB |
Tuesday, March 3, 2015 8:00AM - 8:12AM |
F24.00001: Direct measurements of the current-phase relation in long-range spin-triplet SFS Josephson junctions David Hamilton, Dale Van Harlingen, Yixing Wang, Norman Birge We present direct measurements of the current-phase relation (CPR) of Josephson junctions which use multiple ferromagnetic layers to generate long-range spin-triplet pair correlations. Using a phase-sensitive Josephson interferometry technique, we obtain the phase and temperature dependence of this spin-triplet supercurrent. We also demonstrate the use of an inductive shunt to enhance this technique at higher critical currents. Our data suggest that the current-phase relation of these junctions is harmonic in character. Further measurements are planned in order to determine the ground state phase shift of these junctions. [Preview Abstract] |
Tuesday, March 3, 2015 8:12AM - 8:24AM |
F24.00002: Parity conservation in a Cooper-pair transistor David Van Woerkom, Attila Geresdi, Sebastian Rubbert, Anton Akhmerov, Leo Kouwenhoven In a small superconducting island, hosting an even number of electrons, all charge carriers form Cooper pairs, defining the ground state of the Cooper-pair transistor (CPT). An additional, unpaired electron can only occupy a higher energy level, determined by the superconducting order parameter. This even-odd (parity) energy difference makes the CPT a very sensitive charge detector as well as a prototype superconducting qubit, whose coherence relies on the conservation of the parity of the island. Here we report parity conservation in a niobium-based superconductor, NbTiN, for the first time. NbTiN is a popular superconductor since it can sustain high parallel and perpendicular magnetic fields which is often a requirement for hybrid devices. The parity conversation resulted in the first 2e-periodicity measurements in a non-Aluminium CPT. The highest reported parity lifetime ever, which was longer than one minute, was measured. The parity lifetime didn't saturate down to a base temperature of 12mK, showing state-of-the-art device shielding of thermal photons. We show that our CPT is magnetic field compatible, opening new possibilities for coupling spin degrees of freedom to superconducting circuits and qubits and for topological superconductivity, enabling qubits based on Majorana fermions. [Preview Abstract] |
Tuesday, March 3, 2015 8:24AM - 8:36AM |
F24.00003: Ferromagnetic Josephson Junctions for Cryogenic Memory Bethany M. Niedzielski, Eric C. Gingrich, Mazin A. Khasawneh, Reza Loloee, William P. Pratt Jr., Norman O. Birge Josephson junctions containing ferromagnetic materials are of interest for both scientific and technological purposes. In principle, either the amplitude of the critical current or superconducting phase shift across the junction can be controlled by the relative magnetization directions of the ferromagnetic layers in the junction. Our approach concentrates on phase control utilizing two junctions in a SQUID geometry. We will report on efforts to control the phase of junctions carrying either spin-singlet or spin-triplet supercurrent for cryogenic memory applications. [Preview Abstract] |
Tuesday, March 3, 2015 8:36AM - 8:48AM |
F24.00004: Control of triplet supercurrent in superconductor/ferromagnet hybrid systems William Martinez, W.P. Pratt, Jr., Norman O. Birge A lot of excitement has been generated in superconductor/ferromagnet (S/F) systems since long-range spin-triplet correlations (LRTCs) were predicted. \footnote{A.F. Volkov, F.S. Bergeret and K.B. Efetov, Phys. Rev. Lett., \textbf{90}, 117006 (2003)} Despite the many breakthroughs so far in this field, the ability to control the triplet generation reliably still needs to be realized before these devices can be used in technological applications. One possible direction to control the state in such structures is to manipulate the magnetizations of the various F layers within, specifically to switch between colinear and non-colinear directions between the layers. In this work, we report on the progress made to control LRTC generation in such a way. \\ Supported by the DOE-BES under grant DE-FG-02-06ER46341. [Preview Abstract] |
Tuesday, March 3, 2015 8:48AM - 9:00AM |
F24.00005: Fabrication and characterization of single domain magnetic Josephson Mazin Khasawneh, Bethany Niedzielski, Erich Gingrich, Reza Loloee, William Pratt, Jr, Norman Birge A nice effect that can be observed in Ferromagnetic (F) Josephson junctions is the crossover from a standard Josephson junction (0-junction) to a ``$\pi $-junction'' as a function of the thickness of the F layer, d$_{\mathrm{F}}$. This observation is interesting not only from the scientific point of view but also from a practical point of view, as it could be used in cryogenic memory, for example. In this work we are fabricating and measuring micron-scale Josephson junctions containing a soft magnetic material such as NiFe. Such junctions exhibit clear switching of the single-domain magnetic element as a function of applied field. We will report on our recent progress. [Preview Abstract] |
Tuesday, March 3, 2015 9:00AM - 9:12AM |
F24.00006: Wireless Josephson Junction Arrays Laura Adams We report low temperature, microwave transmission measurements on a wireless two- dimensional network of Josephson junction arrays composed of superconductor-insulator -superconductor tunnel junctions. Unlike their biased counterparts, by removing all electrical contacts to the arrays and superfluous microwave components and interconnects in the transmission line, we observe new collective behavior in the transmission spectra. In particular we will show emergent behavior that systematically responds to changes in microwave power at fixed temperature. Likewise we will show the dynamic and collective response of the arrays while tuning the temperature at fixed microwave power. We discuss these spectra in terms of the Berezinskii-Kosterlitz-Thouless phase transition and Shapiro steps. We gratefully acknowledge the support Prof. Steven Anlage at the University of Maryland and Prof. Allen Goldman at the University of Minnesota. [Preview Abstract] |
Tuesday, March 3, 2015 9:12AM - 9:24AM |
F24.00007: ABSTRACT WITHDRAWN |
Tuesday, March 3, 2015 9:24AM - 9:36AM |
F24.00008: $8\pi$-periodic Josephson effects in a quantum dot / quantum spin-Hall josephson junction system Hoi-Yin Hui, Jay Sau Josephson junctions made of conventional $s$-wave superconductors display $2\pi$ periodicity. On the other hand, $4\pi$-periodic fractional Josephson effect is known to be a characteristic signature of topological superconductors and Majorana fermions [1]. Zhang and Kane have shown that Josephson junctions made of topological superconductors are $8\pi$-periodic if interaction is used to avoid dissipation [2]. Here we present a general argument for how time-reversal symmetry and $Z_2$ non-trivial topology constrains the Josephson periodicity to be $8\pi$. We then illustrate this through a microscopic model of a quantum dot in a quantum spin-hall Josephson junction. [Preview Abstract] |
Tuesday, March 3, 2015 9:36AM - 9:48AM |
F24.00009: Superconducting-semiconducting nanowire hybrid microwave circuits G. De Lange, B. van Heck, A. Bruno, D. van Woerkom, A. Geresdi, S. R. Plissard, E. P. A. M. Bakkers, A. R. Akhmerov, L. DiCarlo Hybrid superconducting-semiconducting circuits offer a versatile platform for studying quantum effects in mesoscopic solid-state systems. We report the realization of hybrid artificial atoms based on Indium-Arsenide nanowire Josephson elements in a circuit quantum electrodynamics architecture. Transmon-like single-junction devices have gate-tunable transition frequencies. Split-junction devices behave as transmons near zero applied flux and as flux qubits near half flux quantum, wherein states with oppositely flowing persistent current can be driven by microwaves. This flux-qubit like behaviour results from non-sinusoidal current-phase relations in the nanowire Josephson elements. These hybrid microwave circuits are made entirely of magnetic-field compatible materials, offering new opportunities for hybrid experiments combining microwave circuits with polarized spin ensembles and Majorana bound states. [Preview Abstract] |
Tuesday, March 3, 2015 9:48AM - 10:00AM |
F24.00010: An Ultra-Sensitive Electrometer based on the Cavity-Embedded Cooper-Pair Transistor Juliang Li, Marco Miller, Alex Rimberg We discuss use of a cavity-embedded Cooper-pair transistor (cCPT) as a potentially quantum-limited electrometer. The cCPT consists of a Cooper pair transistor placed at the voltage antinode of a 5.7 GHz shorted quarter-wave resonator so that the CPT provides a galvanic connection between the cavity's central conductor and ground plane. The quantum inductance of the CPT, which appears in parallel with the effective inductance of the cavity resonance, can be modulated by application of either a gate voltage to the CPT island or a flux bias to the CPT/cavity loop. Changes in the CPT inductance shift the cavity resonant frequency, and therefore the phase of a microwave signal reflected from the cavity. The reflected wave is amplified by both SLUG and HEMT amplifiers before its phase is measured. Results of recent measurements on the cCPT electrometer will be compared with theoretical predictions. [Preview Abstract] |
Tuesday, March 3, 2015 10:00AM - 10:12AM |
F24.00011: Microwave Resonant Activation Results in Hybrid MgB2/I/Pb and MgB2/I/Sn Josephson Junctions with Low to Moderate Damping Steven Carabello, Joseph Lambert, Wenqing Dai, Qi Li, Ke Chen, Daniel Cunnane, X.X. Xi, Roberto Ramos Superconducting-to-normal switching in ``hybrid' single-gap/2-gap junctions is a subject of recent theoretical study. Most analysis is in the absence of thermal or microwave fluctuations. We have performed experiments on such hybrid junctions, incorporating microwave excitations, on multiple MgB$_2$/I/Sn and MgB$_2$/I/Pb junctions with small to moderate damping. At small damping, the primary and resonant peaks in the histogram of superconducting to normal switching events are easily distinguishable, when the appropriate microwave power is applied. At moderate damping, the resonant behavior is still apparent, although separate primary and resonant peaks do not appear together in the same switching histogram, at any power. In both cases, additional features appear beyond those in single-gap/single-gap junctions, which may reflect the presence of additional resonant and tunneling modes. [Preview Abstract] |
Tuesday, March 3, 2015 10:12AM - 10:24AM |
F24.00012: Improved tunable microstrip SQUID amplifier for the Axion Dark Matter eXperiment (ADMX) Sean O'Kelley, Jorn Hansen, Jan-Michael Mol, Gene Hilton, John Clarke We present a series of tunable microstrip SQUID amplifiers (MSAs) for use in ADMX. The axion dark matter candidate is detected via Primakoff conversion to a microwave photon in a high-Q ($\approx $ 10$^{\mathrm{5}})$ tunable microwave cavity, cooled to 1.6 K or lower, in the presence of a 7-tesla magnetic field. The microwave photon frequency is a function of the unknown axion mass, so that the cavity and amplifier must be scanned over a broad frequency range. An MSA is constructed by flux-coupling a resonant microstrip to a resistively-shunted SQUID biased into the voltage state. We demonstrate gains exceeding 20 dB, at frequencies above 900 MHz. Tunability is achieved by terminating the microstrip with a low inductance GaAs varactor diode that operates at cryogenic temperatures. By varying the voltage bias of the varactor we vary its capacitance, allowing a reflected phase varying from nearly 0 to $\pi $ at the end of the microstrip, and thus a standing wave tunable from nearly $\lambda $/2 to $\lambda $/4. With proper design of the microwave environment, a noise temperature of 1/2 to 1/4 of the physical temperature is demonstrated. [Preview Abstract] |
Tuesday, March 3, 2015 10:24AM - 10:36AM |
F24.00013: Magnetic Field Dependence of the Critical Current of Planar Geometry Josephson Junctions Meng Ma, Ethan Cho, Chuong Huynh, Shane Cybart, Robert Dynes We report a study on the magnetic field dependence of the critical current of planar geometry Josephson junctions. We have fabricated Josephson junctions by using a focused helium ion beam to irradiate a narrow barrier in the plane of a 25~nm thick Y--Ba--Cu--O film. The London penetration depth $\lambda_L$ is large ($\sim$1~$\mu$m) because of the ultra--thin thickness of the film. As a result, calculations of the Josephson penetration depth $\lambda_J$ are not realistic nor physical. Therefore in this work, we measure $\lambda_J$ experimentally. We tested devices with bridge widths ranging from 4 to 50~$\mu$m, and present measurements of the Fraunhofer quantum diffraction pattern ($I_C(B)$). We observe a crossover from short to long junction behavior, which gives an experimentally measured $\lambda_J$ that ranges between 3~$\mu$m to 5~$\mu$m. The shape of the $I_C(B)$ pattern is strongly affected by the width of the bridge because of self-field effects. As the bridge width increases, Josephson vortices enter the junction and skew the patterns. This work shows that the electronic properties of the planar junctions are very different than those classical ``sandwich'' junctions due to the differences in geometry. [Preview Abstract] |
Tuesday, March 3, 2015 10:36AM - 10:48AM |
F24.00014: Transition from superconducting-normal metal to superconducting-insulating barrier in focused helium beam YBCO Josephson junctions Ethan Cho, Shane Cybart, Chuong Huynh, Robert Dynes We report measurements of the metal--insulator transition (MIT) in high--transition--temperature--superconductor, planar Y--Ba--Cu--O Josephson junctions. The junctions were made by irradiation of a $\sim$1~nm barrier with a focused helium ion beam. To describe the MIT we use a theoretical model by Blonder, Tinkham and Klapwijk (BTK). This model explains transition of the transport at a superconducting normal metal/insulator interface using a single parameter, the barrier strength (Z). In our experiment, the dose of the He$^+$ ions serves the role of Z. We tested 20 junctions created using 20 different doses ranging from $10^{16}$~He$^+$/cm$^2$ to $10^{17}$~He$^+$/cm$^2$. We measured current--voltage characteristics of the junctions for several temperatures, and extract critical current ($I_C$), normal state resistance ($R_N$) and $I_CR_N$ from fits to the data. We show that the $R_N$ of the junctions show metallic behavior for lower doses and change continuously to insulating at higher doses. We have demonstrated precise control over the junction parameters with He$^+$ ion dose, which opens up an avenue for studying materials sensitive to disorder as well as reproducible production of Josephson junctions for application. [Preview Abstract] |
Tuesday, March 3, 2015 10:48AM - 11:00AM |
F24.00015: Modeling Excess Current in Josephson I-V Characteristics Bjorn Wehlin, Ethan Cho, Meng Ma, Shane Cybart, Robert Dynes The current-voltage characteristics ($I$-$V$) of some metal-barrier Josephson junctions include a current component that does not exhibit the DC, or AC Josephson effects. This current is referred to as the excess current, $I_\chi$. Moreover, while $I_\chi$ is negligible around the transition temperature $T_C$, it increases and can exceed 50\% of the total current as temperature is decreased. Normally, $I_\chi$ is omitted from Josephson $I$-$V$ models and this leads to a large overestimate of the product of Josephson critical current, $I_C$, and normal-state resistance, $R_N$ (i.e., $I_CR_N$). We have developed an extended $I$-$V$ model based on the Stewart-McCumber and Ambegaokar-Halperin models that includes $I_\chi$. Using our model, we fit experimental $I$-$V$ data for planar Y-Ba-Cu-O junctions over a range of temperatures. From these fits we obtain values for $I_\chi$, $I_C$ and $R_N$, as well as noise temperature. Our values agree well with experimental measurements. $I_C$ is suppressed using a magnetic field. Further, our $I_C$ exhibits an asymptotic temperature dependence like that expected for a superconducting energy gap well below $T_C$. Our model is an improvement over existing models as it reliably estimates $I_C R_N$. [Preview Abstract] |
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