Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session Y9: Superconductivity: Josephson Junctions, Proximity Effect & Squids II |
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Sponsoring Units: DMP Chair: Wan Kyu Park, University of Illinois at Urbana-Champaign Room: Colorado Convention Center Korbel 1D |
Friday, March 9, 2007 11:15AM - 11:27AM |
Y9.00001: Microwave characterization of Josephson junctions arrays for Coulomb blockade Vladimir Manucharyan, Michael Metcalfe, R. Vijay, Etienne Boaknin, Michel Devoret Coulomb Blockade of a single Josephson junction leads to oscillations in time of the voltage across the junction when a DC electrical current is applied. Because the frequency $f$ of these so-called Bloch oscillations is related to the current $I$ only through the charge $2e$ of a Cooper pair, $f = I/2e$, the phenomenon can be utilized to build a primary standard of electrical current. However, to reach the regime of Coulomb blockade, the current must be applied to the junction through the leads with electromagnetic impedance exceeding the quantum of resistance for Cooper pairs ($6.5$ k$\Omega$) at frequencies $\omega_c \approx \hbar/E_c$, $E_c$ being the charging energy of the junction. For typical parameters $\omega_c$ lies in the microwave domain, where electromagnetic impedances tend to be of the order of the free space impedance ($377$ $\Omega$). This fundamental mismatch in the impedance turns the realization of a current standard based on Bloch oscillations into a very challenging problem. Our proposal is to use kinetic impedance of a superconductor, namely, the Josephson inductance. We have fabricated arrays of large Josephson junctions which are superconducting at DC and characterized them at microwave frequencies. Our results indicate that these arrays are capable of beating the impedance quantum and we are setting up an experiment on electrical current metrology. [Preview Abstract] |
Friday, March 9, 2007 11:27AM - 11:39AM |
Y9.00002: Low-frequency Critical Current Fluctuation Measurements in Nb/AlOx/Nb Junctions Shawn Pottorf, Vijay Patel, J. E. Lukens We have measured the low frequency critical current noise in Nb/AlO$\mathrm{_{x}}$/Nb Josephson junctions used for qubits in quantum computation circuits. Low frequency current noise measurements were made using a bridge circuit with a SQUID null detector. The current noise spectra density showed a 1/$f$ component at low frequencies for both an unshunted junction biased near 6 mV and a shunted junction biased near $\sim 7$ $\mu$V. In both cases this corresponded to critical current fluctuations with a spectral density at 1 Hz of $2.2 \cdot 10^{-24}$ $\mathrm{A^{2}/Hz}$. Our measured value of critical current fluctuations is roughly two orders of magnitude less than the average of various technologies reported by Van Harlingen et al. (2004). [Preview Abstract] |
Friday, March 9, 2007 11:39AM - 11:51AM |
Y9.00003: Microscopic Model of 1/f Noise in Josephson Junctions Magdalena Constantin, Clare Yu, John Martinis We present a simple microscopic model to show how fluctuating two-level systems in the Josephson junction tunnel barrier can modify the potential energy of the barrier and produce critical current noise spectra as well as charge noise. We find $1/f$ critical current and charge noise at low frequencies. Our values are in good quantitative agreement with recent experimental measurements of noise in Al/AlO$_{x}$/Al Josephson junctions. We also investigate the sensitivity of the critical current noise on the nonuniformity of the tunnel barrier. [Preview Abstract] |
Friday, March 9, 2007 11:51AM - 12:03PM |
Y9.00004: High-frequency spectroscopy and emission properties of a single-Cooper pair transistor Pierre-Marie Billangeon, Fr\'ed\'eric Pierre, H\'el\`ene Bouchiat, Richard Deblock We have characterized the high-frequency properties of a single-Cooper pair transistor (SCPT), by a capacitive coupling with a Josephson junction. We have alternately used the Josephson junction (JJ) as a high-frequency generator by using the AC Josephson effect, and as a high-frequency detector by using the photo-assisted tunneling current. We have been able to induce transitions between the first energy levels of a SCPT, thanks to the high-energy photons emitted by the AC Josephson effect. This allowed us to perform a high-frequency spectroscopy of a SCPT(10-200 GHz). Moreover, as the emitted photons can have an frequency higher than the superconducting gap of the island, we can not only induce the transfer of Cooper pairs, but also quasiparticles, allowing us to control the poisoning of the SCPT. Then, we used the Squid geometry of the Josephson junction in order to tune its Josephson energy to zero, and use it as a high-frequency detector. We have been able to detect different kinds of high-frequency emission process associated with the coherent transfer of Cooper pairs through the SCPT (AC Josephson effect), and the resonant transfer of Cooper pairs, already characterized in transport measurements by Joyez et al. [Preview Abstract] |
Friday, March 9, 2007 12:03PM - 12:15PM |
Y9.00005: Using a High-Q Josephson Resonator as a Non-Dissipative RF-SQUID J.A. Strong, K.D. Osborn, A.J. Sirois, R.W. Simmonds Superconducting Quantum Interference Devices (SQUIDs) have been used for years to measure small magnetic fields. Such devices measure the DC voltage across a Josephson junction as a function of magnetic flux. It is well known, however, that a voltage-biased Josephson junction radiates energy. This is problematic for many superconducting quantum device applications including readout methods for superconducting quantum bits and SET's. Here, we examine a newly developed Josephson junction resonator as a new breed of SQUID, wherein the resonator's resonant frequency (instead of the junction's voltage) is measured as a function of magnetic flux. In this way, the Josephson junction is kept perpetually in the super- current state, with zero DC voltage and therefore no Josephson radiation. We examine issues of sensitivity, noise, and read- out speed. [Preview Abstract] |
Friday, March 9, 2007 12:15PM - 12:27PM |
Y9.00006: Cooper-pair Tunneling in a High Impedance Environment. M. A. Castellanos-Beltran, K. W. Lehnert Coulomb blockade of current and coherent oscillations in the voltage across a small tunnel junction can only be observed if the junction is embedded in the appropriate electromagnetic environment. The demanding condition is an environment with an impedance large compared to the quantum of resistance ($R_Q=6.5$~k$\Omega$) at all relevant frequencies (DC - 20~GHz). We will show results from two different experiments that characterize the impedance of one-dimensional Josephson junction arrays. First, by using the array to bias a SQUID made from small area junctions, we show the behavior of this system can only be explained if the array creates an environment with an impedance several times $R_Q$. Second, we measure the speed at which microwave signals propagate through coplanar waveguides whose inner conductors are formed from an array of Josephson junctions. We find that these waveguides behave as LC transmission lines with wave impedances of several k$\Omega$ and wave speeds less than 1\% of the speed of light in free space. [Preview Abstract] |
Friday, March 9, 2007 12:27PM - 12:39PM |
Y9.00007: Adiabatic Perturbing a Bloch Transistor by Microwave Irradiation: Inversion of Coulomb Oscillation Watson Kuo, Saxon Liou, Y.W. Suen, W.H. Hsieh, C.S. Wu, C.D. Chen We experimentally studied the switching current and DC current-voltage (IV) characteristics of a Bloch transistor irradiated with microwaves of frequency from several GHz up to 18GHz. The photon energy is well below the level spacing of the two-level quantum states so that the Bloch transistor is perturbed in the adiabatic regime. Phase-charge duality in a Josephson junction is clearly seen in the switching current distribution as a function of gate voltage. The reduction of switching current due to photon excitation is significant when the phase fluctuation is small. In particular, an inversion of Coulomb oscillation of switching current is observed at a higher microwave power level. When the microwave frequency is below 7GHz, the IV characteristics of the Bloch transistor evolve from being superconductor-like to being blockade-like as the microwave power level increases, and the zero-bias resistance R$_{0}$ shows a Coulomb oscillation accordingly: when R$_{0}$ is maximal, the switching current is also maximal, opposite to that without microwave irradiation. As the microwave power level increases further, Shapiro steps in IV characteristics are observed. The step height can be analyzed using a model for an ac voltage source applied to a single Josephson junction. [Preview Abstract] |
Friday, March 9, 2007 12:39PM - 12:51PM |
Y9.00008: Superconductor-normal metal contact conductance of a graph node Vladimir Lukic, Elisabeth Nicol We study the conductance of a superconductor-normal metal (SN) contact with the topology of a graph node. We derive the extension of the Blonder-Tinkham-Klapwijk (BTK) equations using the boundary conditions for a wavefunction at a graph node, and show that in the appropriate limit they reduce to the standard BTK formula for an SN contact. Qualitatively new conductance features arise from crossed Andreev reflection and interference of partially reflected waves from different graph legs, and we demonstrate their importance by using the Landauer method to rederive the formula for conductance. The relevance of these effects to experiment will be discussed. [Preview Abstract] |
Friday, March 9, 2007 12:51PM - 1:03PM |
Y9.00009: Singular length dependence of critical current in superconductor/normal-metal/superconductor bridges Alex Levchenko, Alex Kamenev, Leonid Glazman We examine the dependence of the critical Josephson current on the length $L$ of the normal bridge $N$ between two bulk superconductors. This dependence turns out to be nonanalytic at small $L$. The nonanalyticity originates from the contribution of extended quasiparticle states with energies well above the superconducting gap. This should be contrasted with the more familiar contribution to the Josephson current coming from Andreev bound states localized in the normal region at energies below the gap. We also have preliminary results on the ac Josephson effect above the critical temperature $T_{c}$ where we have studied the influence of the superconducting fluctuations on the current noise. It turns out that the current noise acquires singular in $T-T_c$ correction, which is peaked at the Josephson frequency. This correction originates from the fluctuating ac Josephson current. [Preview Abstract] |
Friday, March 9, 2007 1:03PM - 1:15PM |
Y9.00010: Interaction of breathers with moving vortices in a Josephson junction ladder Devin Edwards, Ken Segall, Juan Mazo Josephson junction arrays offer an important method to experimentally study nonlinear dynamics. We have studied Nb-AlOx-Nb Josephson junction ladder arrays with 24 periods which support two distinct nonlinear modes. The first is a discrete breather, which is a spatially localized excitation that does not propagate through the ladder. The second is a propagating two pi phase shift, or moving vortex. Both modes have been observed independently in our arrays. Predictions have been made regarding the dynamics of the interactions of these two modes, but these have not been verified experimentally. One such prediction is that under some conditions the breather will `pin' the propagating vortices and prevent them from passing beyond the breather. We will present theoretical simulations and recent experiments attempting to observe their interaction. [Preview Abstract] |
Friday, March 9, 2007 1:15PM - 1:27PM |
Y9.00011: Fluxon ratchet dynamics in a Josephson junction array Kenneth Segall, Adam Dioguardi, Nikhil Fernandes, Ushnish Ray, Juan Mazo, Fernando Naranjo We present theoretical and experimental work on the ratchet dynamics of fluxons in an array of Josephson junctions. Fluxons trapped in a parallel array of Josephson junctions upon cooldown experience a potential determined by the junction critical currents and the cell inductances. By varying these quantities in an asymmetric way, the potential can be made ratchet. We probe the dynamics of the fluxon with switching current measurements, which allow determination of the transition rate of the fluxon from its pinned state to a running state. We find two temperature regimes, both experimentally and in simulations. At low temperatures, the fluxon behaves like a single particle and undergoes thermal activation. At intermediate temperatures, the fluxon undergoes diffusion for several periods and then jumps to the running state. The dynamics in this region cannot be explained with a single particle picture. We have observed two temperature-dependent crossovers in the direction of transport in this temperature region. We present temperature-dependent measurements, comparisons with simulation, and possible interpretations of the crossovers. [Preview Abstract] |
Friday, March 9, 2007 1:27PM - 1:39PM |
Y9.00012: Manipulation of half-integer flux quanta C.J.M. Verwijs, Ariando, J.A. Boschker, H. Hilgenkamp Because of the different pairing symmetry in high-T$_{c}$ cuprates ($d$-wave) and low-T$_{c}$ superconductors ($s$-wave) it is possible to realize hybrid superconducting rings with a built-in $\pi $-phase shift. These rings have a twofold degenerate groundstate characterized by spontaneously generated circulating currents corresponding to a half flux quantum. We have realized such rings by connecting the cuprate YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ to the low-T$_{c}$ superconductor Nb via ramp-type Josephson junctions. We present the integration of $\pi $-rings in a superconducting quantum electronics device, a toggle flip-flop, in which the fractional flux polarity is toggled by applying single flux quantum pulses. We will also present experiments in which the half flux quanta at the discontinuities of 0-$\pi $-0 Josephson corner junctions are manipulated. [Preview Abstract] |
Friday, March 9, 2007 1:39PM - 1:51PM |
Y9.00013: Vortex flow characteristics of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ Long Intrinsic Josephson Junctions Kazuo Kadowaki, Kohei Kawamata, Yuimaru Kubo, Kazuki Fukui, Takashi Yamamoto, Itsuhiro Kakeya Intrinsic Josephson junction characters in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$, where the junction length $L$ is much longer than the Josephson length \textit{$\lambda $}$_{J}$=\textit{$\gamma $s}, have been studied from view point of Josephson vortex dynamics, since they are expected to be released from the strong geometrical constraint, which confines Josephson vortices into a square box, and as a result they are free to move in the two dimensional channels. We fabricated such long junctions with $L$=20-40 $\mu $m and have measured $c$-axis transport characteristics. We have found that Josephson flow resistance $R_{f}$ suddenly suppressed at the intermediate field region of 1-2 T, then begins to reappear gradually as field is increased. The sharp lock-in transition as a function of angle appearing below it where the periodic oscillation of $R_{f}$ is seen, becomes immediately broad and a round peak above it, where the periodic oscillation of $R_{f}$ is no longer observable. It seems that the transition field becomes lower as $L$ is increased so that in short junctions it cannot be observed in a field region up to 6 T. [Preview Abstract] |
Friday, March 9, 2007 1:51PM - 2:03PM |
Y9.00014: Optimization of HighTc Josephson nanojunctions by Monte Carlo simulations M. Sirena, N. Bergeal, J. Lesueur, G. Faini, R. Bernard, J. Briatico, D. Crete The fabrication of YBCO JJ by ion damage is the best method that allows closed packed JJ series within the nanoscale and that could operate at high temperature. However, the strong variation of the JJ's critical current with temperature (T) and the increase of dispersion for high irradiation dose are still important issues for several applications. Reproducible HTc JJ have been produced combining electron beam lithography and ion beam irradiation, whose characteristics can be adjusted on a wide range of T. To further improve the homogeneity of planar JJ and optimize their behaviour, we have studied its lateral ion damage distribution (LDD) for different ions and incident energies using Monte Carlo simulations. The LDD was used to calculate the transition temperature (Tc') of the irradiated zone and its resistance as a function of T. Dispersion in the irradiation mask's size was introduced as the source of the JJ's in-homogeneity. The simulations results reproduce quite well the observed dispersion of the irradiated JJ. A linear behaviour of the JJ's Tc' dispersion with basically the LDD width was found, independent of the incident ions mass, its energy, the films thickness, etc. By choosing the appropriate parameters is possible to increase the JJ homogeneity, reducing the LDD width. [Preview Abstract] |
Friday, March 9, 2007 2:03PM - 2:15PM |
Y9.00015: Fiske and size-independent resonances in $I-V$ characteristics of micron-sized Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ single crystals Itsuhiro Kakeya, Yuimaru Kubo, Masashi Kohri, Kazuki Fukui, Kohei Kawamata, Takashi Yamamoto, Kazuo Kadowaki We have investigated the $c$-axis transport properties of micron-size Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (Bi2212) single crystals fabricated by the focused ion beam method under magnetic field parallel to the $ab$-plane. It was found that periodic current steps in current-voltage ($I-V$) characteristics, whose features are similar to the Fiske step known in a single Josephson junction. We also found another current step with non- oscillating field dependence in low voltage region. Since the voltage of this step does not depend on the sample size unlikely to the Fiske step, it is considered that the step is attributed to an intrinsic phase excitation of Bi2212. [Preview Abstract] |
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