Virtual Journal of Applications of Superconductivity

Note: A sample holder design for sensitive magnetic measurements at high temperatures in a magnetic properties measurement system

A. Arauzo, E. Guerrero, A. Urtizberea, J. Stankiewicz, and C. Rillo
A sample holder design for high temperature measurements in a commercial MPMS SQUID magnetometer from Quantum Design is presented. It fulfills the requirements for the simultaneous use of the oven and reciprocating sample option (RSO) options, thus allowing sensitive magnetic measurements up to 800 ... [Rev. Sci. Instrum. 83, 066106 (2012)] published Fri Jun 15, 2012.

Influence of artificial pinning on vortex lattice instability in superconducting films

A Silhanek, A Leo, G Grimaldi, G Berdiyorov, M Miloevi et al.
In superconducting films under an applied dc current, we analyze experimentally and theoretically the influence of engineered pinning on the vortex velocity at which the flux-flow dissipation undergoes an abrupt transition from low to high resistance. We argue, based on a nonuniform distribution of ... [New J. Phys. 14, 053006 (2012)] published Mon Jun 4, 2012.

Dynamics of flux-line bundles and voltage-current characteristics in a Ba(K)FeAs single crystal

K.-H. Muller, S. K. H. Lam, X. L. Wang, and S. X. Dou
Understanding the flux-line dynamics in layered superconductors is of utmost importance for many superconductor applications, yet extracting from measurements the relevant parameters remains a major challenge. Here we report on measurements and theoretical model calculations of voltage-current (V-I) ... [Phys. Rev. B 85, 224516 ] published .

Scanning tunneling microscopy/spectroscopy of vortices in LiFeAs

T. Hanaguri, K. Kitagawa, K. Matsubayashi, Y. Mazaki, Y. Uwatoko et al.
We investigate vortices in LiFeAs using scanning tunneling microscopy/spectroscopy. Zero-field tunneling spectra show two superconducting gaps without detectable spectral weight near the Fermi energy, evidencing fully gapped multiband superconductivity. We image vortices in a wide field range from 0 ... [Phys. Rev. B 85, 214505 ] published .

Fabrication of binary FeSe superconducting wires by diffusion process

Toshinori Ozaki, Keita Deguchi, Yoshikazu Mizuguchi, Yasuna Kawasaki, Takayoshi Tanaka et al.
We report successful fabrication of multi- and mono-core FeSe wires with high transport critical current density J using a simple in-situ Fe-diffusion process based on the powder-in-tube (Fe-diffusion PIT) method. The seven-core wire showed transport J of as high as 1027 A/cm at 4.2 K. The supercond ... [J. Appl. Phys. 111, 112620 (2012)] published Fri Jun 15, 2012.

Design and performance of a new induction furnace for heat treatment of superconducting radiofrequency niobium cavities

Pashupati Dhakal, Gianluigi Ciovati, Wayne Rigby, John Wallace, and Ganapati Rao Myneni
Superconducting radio frequency (SRF) cavities made of high purity niobium (Nb) are the building blocks of many modern particle accelerators. The fabrication process includes several cycles of chemical and heat treatment at low (~120 degrees C) and high (~800 degrees C) temperatures. In this contr ... [Rev. Sci. Instrum. 83, 065105 (2012)] published Mon Jun 11, 2012.

Mesoscopic variations of local density of states in disordered superconductors

A. E. Koshelev and A. A. Varlamov
We explore correlations of inhomogeneous local density of states (LDoS) for impure superconductors with different symmetries of the order parameter (s wave and d wave) and different types of scatterers (elastic and magnetic impurities). It turns out that the LDoS correlation function of superconduct ... [Phys. Rev. B 85, 214507 ] published .

The role of Cu-O bond length fluctuations in the high temperature superconductivity mechanism

Guy Deutscher
We review three different kinds of experiments that emphasize the non-BCS, inhomogeneous aspects of superconductivity in the high Tc cuprates. The first is the existence of two different energy scales in the superconducting state, initially identified by a comparison between tunneling and AndreevSai ... [J. Appl. Phys. 111, 112603 (2012)] published Fri Jun 15, 2012.

Strong field-matching effects in superconducting YBaCuO films with vortex energy landscapes engineered via masked ion irradiation

I. Swiecicki, C. Ulysse, T. Wolf, R. Bernard, N. Bergeal et al.
We have developed a masked ion irradiation technique to engineer the energy landscape for vortices in oxide superconductors. This approach associates the possibility to design the landscape geometry at the nanoscale with the unique capability to adjust the depth of the energy wells for vortices. Thi ... [Phys. Rev. B 85, 224502 ] published .

Extension of the measurement capabilities of the quadrupole resonator

T. Junginger, W. Weingarten, and C. Welsch
The quadrupole resonator, designed to measure the surface resistance of superconducting samples at 400 MHz has been refurbished. The accuracy of its RF-dc compensation measurement technique is tested by an independent method. It is shown that the device enables also measurements at 800 and 1200 MHz ... [Rev. Sci. Instrum. 83, 063902 (2012)] published Wed Jun 6, 2012.

Magnetoresistance and transistor-like behavior of a double quantum-dot via crossed Andreev reflections

E. C. Siqueira and G. G. Cabrera
The electric current and the magnetoresistance effect are studied in a double quantum-dot system, where one of the dots QD is coupled to two ferromagnetic electrodes (F,F), while the second QD is connected to a superconductor S. For energy scales within the superconductor gap, electric conduction is ... [J. Appl. Phys. 111, 113905 (2012)] published Mon Jun 4, 2012.

On-chip integration of HTS bandpass and lowpass filters with Josephson mixer

T. Zhang, J. Du, Y.J. Guo, and X.-W. Sun
A compact high-T superconducting (HTS) monolithic downconverter is presented. The HTS passive and active devices are integrated on one single chip to achieve compactness and high coupling efficiency. The downconverter consists of a HTS YBaCuO (YBCO) bandpass filter for RF input, a lowpass filter for ... [Electron. Lett. 48, 729 (2012)] published Thu Jun 7, 2012.

Dynamics of superconducting nanowires shunted with an external resistor

Matthew W. Brenner, Dibyendu Roy, Nayana Shah, and Alexey Bezryadin
We present a study of superconducting nanowires shunted with an external resistor, geared towards understanding and controlling coherence and dissipation in nanowires. The dynamics is probed by measuring the evolution of the V-I characteristics and the distributions of switching and retrapping curre ... [Phys. Rev. B 85, 224507 ] published .

Evanescent states and nonequilibrium in driven superconducting nanowires

N. Vercruyssen, T. G. A. Verhagen, M. G. Flokstra, J. P. Pekola, and T. M. Klapwijk
We study the nonlinear response of current transport in a superconducting diffusive nanowire between normal reservoirs. We demonstrate theoretically and experimentally the existence of two different superconducting states appearing when the wire is driven out of equilibrium by an applied bias, calle ... [Phys. Rev. B 85, 224503 ] published .

Effect of thermal inhomogeneity for terahertz radiation from intrinsic Josephson junction stacks of BiSrCaCuO

Itsuhiro Kakeya, Yuta Omukai, Takashi Yamamoto, Kazuo Kadowaki, and Minoru Suzuki
Terahertz radiation from the mesa structures of BiSrCaCuO is detected in samples with thin electrodes <100 nm. In samples with thick electrodes ~=400 nm, neither radiations nor voltage jumps in current-voltage characteristics are detected. This suggests that the thin electrode helps excite the Josep ... [Appl. Phys. Lett. 100, 242603 (2012)] published Tue Jun 12, 2012.

A hybrid superconductor-normal metal electron trap as a photon detector

S. V. Lotkhov and A. B. Zorin
A single-electron trap built with two superconductor-insulator-normal metal tunnel junctions and coupled to a readout SINIS-type single-electron transistor A (SET A) was studied in a photon detection regime. As a source of photon irradiation, we used an operating second SINIS-type SET B positioned i ... [Appl. Phys. Lett. 100, 242601 (2012)] published Mon Jun 11, 2012.

Photon condensation in circuit quantum electrodynamics by engineered dissipation

D Marcos, A Tomadin, S Diehl, and P Rabl
We study photon condensation phenomena in a driven and dissipative array of superconducting microwave resonators. Specifically, we show that by using an appropriately designed coupling of microwave photons to superconducting qubits, an effective dissipative mechanism can be engineered, which scatter ... [New J. Phys. 14, 055005 (2012)] published Mon Jun 4, 2012.

Black-Box Superconducting Circuit Quantization

Simon E. Nigg, Hanhee Paik, Brian Vlastakis, Gerhard Kirchmair, S. Shankar et al.
We present a semiclassical method for determining the effective low-energy quantum Hamiltonian of weakly anharmonic superconducting circuits containing mesoscopic Josephson junctions coupled to electromagnetic environments made of an arbitrary combination of distributed and lumped elements. A conven ... [Phys. Rev. Lett. 108, 240502 ] published .

Quantum superposition of three macroscopic states and superconducting qutrit detector

V. I. Shnyrkov, A. A. Soroka, and O. G. Turutanov
Superconducting quantum coherent circuits have opened up a novel area of fundamental low-temperature science since they could potentially be the element base for future quantum computers. Here we report a quasi-three-level coherent system, the so-called superconducting qutrit, which has some advanta ... [Phys. Rev. B 85, 224512 ] published .

Measurements of Quasiparticle Tunneling Dynamics in a Band-Gap-Engineered Transmon Qubit

L. Sun, L. DiCarlo, M. D. Reed, G. Catelani, Lev S. Bishop et al.
We have engineered the band gap profile of transmon qubits by combining oxygen-doped Al for tunnel junction electrodes and clean Al as quasiparticle traps to investigate energy relaxation due to quasiparticle tunneling. The relaxation time T of the qubits is shown to be insensitive to this band gap ... [Phys. Rev. Lett. 108, 230509 ] published .

Pure dephasing in flux qubits due to flux noise with spectral density scaling as 1/f

S. M. Anton, C. Muller, J. S. Birenbaum, S. R. O'Kelley, A. D. Fefferman et al.
For many types of superconducting qubits, magnetic flux noise is a source of pure dephasing. Measurements on a representative dc superconducting quantum interference device (SQUID) over a range of temperatures show that S(f)=A/(f/1 Hz), where S is the flux noise spectral density, A is of the order ... [Phys. Rev. B 85, 224505 ] published .

Geometric phases in superconducting qubits beyond the two-level approximation

S. Berger, M. Pechal, S. Pugnetti, A. A. Abdumalikov, Jr., L. Steffen et al.
Geometric phases, which accompany the evolution of a quantum system and depend only on its trajectory in state space, are commonly studied in two-level systems. Here, however, we study the adiabatic geometric phase in a weakly anharmonic and strongly driven multilevel system, realized as a supercond ... [Phys. Rev. B 85, 220502 ] published .

Optimal operating conditions of an entangling two-transmon gate

Antonio D'Arrigo and Elisabetta Paladino
We identify the optimal operating conditions of an entangling two-qubit gate realized by capacitive coupling of two superconducting charge qubits in a transmission line resonator (the so-called transmons). We demonstrate that the sensitivity of the optimized gate to 1/f flux and critical current noi ... [New J. Phys. 14, 053035 (2012)] published Mon Jun 4, 2012.

Robust Toffoli gate originating from Stark shifts

Xiao-Qiang Shao, Tai-Yu Zheng, and Shou Zhang
A method for synthesizing the Toffoli gate is proposed based solely on the Stark shifts of three superconducting quantum interference devices (SQUIDs). This scheme is robust against the effect of decoherence, since it operates with no excitation of SQUIDs and the coplanar waveguide cavity. The obtai ... [J. Opt. Soc. Am. B 29, 1203 (2012)] published Thu Jun 7, 2012.