Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session B38: Superconducting Devices and AppIications |
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Sponsoring Units: DCMP Chair: Michael Bleiweiss, Naval Academy Prep School Room: Baltimore Convention Center 341 |
Monday, March 13, 2006 11:15AM - 11:27AM |
B38.00001: Transition-edge sensors based on superconducting nanowires. Matthew Bell, Andrei Sergeev, Gregory Goltsman, Jonathan Bird, Aleksandr Verevkin We present our experimental study of superconducting NbN nanowire-based sensor. The responsivity of the sensor is strongly affected by the superconducting transition width of the nanostructure, which, in turn, is determined by the phase slip centers (PCSs) dynamics. The fluctuations and noise properties of the sensor are also discussed, as well as the devices' behavior at high magnetic fields. The ultimate performance of the sensor and prospects of the devices will be discussed, as well. [Preview Abstract] |
Monday, March 13, 2006 11:27AM - 11:39AM |
B38.00002: Frequency Noise in Superconducting Thin-Film Resonators Shwetank Kumar, Peter Day, Henry LeDuc, Benjamin Mazin, Megan Eckart, Jiansong Gao, Jonas Zmuidzinas We present the results of low temperature (120 -- 1200 mK) noise measurements performed on thin-film superconducting niobium resonators fabricated on a silicon substrate. The devices studied use coplanar waveguide (CPW) transmission lines and have resonance frequencies of around 4 GHz and quality factors in the range of Q $\sim $ 10$^{4}$ to 10$^{6}$. These resonators are similar to those used to make novel photon detectors and read out charge qubits. These resonators show excess frequency noise which varies as approximately f$^{-1/2}$. This excess noise limits the sensitivity of our photon detectors and likely effects the qubit performance as well. Two level systems (TLS) in amorphous thin-film dielectrics and oxide tunnel barriers have been shown to cause dissipation and decoherence in phase qubits. We suggest that noise in our resonators is also caused by TLS most likely near the surfaces of the substrate and metal films. To test this idea, we have measured the frequency shift, the quality factor and the frequency noise as a function of the device temperature and the microwave readout power. The frequency shift data agrees well with existing weak field TLS theory. We also find that the frequency noise decreases with increasing readout power and temperature and that decreased noise at higher powers is not due to simply device heating. [Preview Abstract] |
Monday, March 13, 2006 11:39AM - 11:51AM |
B38.00003: Measurement of environmental impedance at plasma frequency of Josephson junctions with microwave induced escapes Bo Mao, Siyuan Han Impedance of electromagnetic environment of a Josephson junction is critical to its dynamics. We show that microwave induced escape rate of Josephson junctions as a function of microwave frequency and power provides an excellent quantitative measurement of the environmental impedance at plasma frequency of the junction. It is shown that in strong microwave fields the effect of anharmonicity of the junction's potential well must be taken into account. Direct comparison of experimental result with numerical simulation allows one to extract environmental impedance at junction's plasma frequency. [Preview Abstract] |
Monday, March 13, 2006 11:51AM - 12:03PM |
B38.00004: Probing superconducting samples using a two-superconducting-nanowire device David Pekker, Alexey Bezryadin, David S. Hopkins, Paul M. Goldbart An NQUID (nanowire SQUID) is a device featuring two thin-film superconducting leads connected by a pair of parallel narrow superconducting wires [1,2]. Thermal fluctuations of the order parameter in the superconducting wires endow the device with a nonzero lead-to-lead resistance ($R$). For short wires, $R$ also depends on the phase profile of the order parameter in the leads. Here, we consider the situation in which one of the leads carries a supercurrent perpendicular to the wires (a cross-current). We show that $R$ is a periodic function of this cross-current. Minima of $R$ occur whenever the phase-gain between the wire attachment-points along the lead carrying the cross-current is an integer multiple of $2\pi$. NQUID devices such as these may be useful for probing superconducting order in the leads or determining current-phase relations in various settings. The results of experiments on devices carrying cross-currents will be presented in a companion talk by Hopkins et al. \newline [1] Hopkins et al., Science 308, 1762 (2005).\newline [2] Pekker et al., Phys. Rev. B 72, 104517 (2005). [Preview Abstract] |
Monday, March 13, 2006 12:03PM - 12:15PM |
B38.00005: Measurement of superconducting phase gradients using a DNA-templated nanowire interference device David S. Hopkins, David Pekker, Paul M. Goldbart, Alexey Bezryadin We have fabricated an NQUID (i.e., a Nanowire SQUID) [1,2] in which two thin ($\sim$15 nm) superconducting nanowires templated by DNA molecules are used to directly measure phase gradients of the Ginzburg-Landau order parameter along a superconducting strip of width $\sim$1 $\mu$m. A supercurrent flowing along the strip creates a phase difference between the contact-points of the nanowires. This causes the resistance of the two-wire device to oscillate as a function of the current flowing along the strip with a period equal to the amount of current required to create a 2$\pi$ phase difference between the contact-points of the wires. We find that the period increases with decreasing temperature, due to the increase in superfluid density in the film. Using this device we are able to study the process of superconductivity suppression in the thin film strip caused by a high bias-current and/or a magnetic field. A theory of the operation of such NQUIDs will be presented in a companion talk by Pekker et al. [1] Hopkins et al., Science 308, 1762 (2005). [2] Pekker et al., Phys. Rev. B 72, 104517 (2005). [Preview Abstract] |
Monday, March 13, 2006 12:15PM - 12:27PM |
B38.00006: Impedance matching using superconducting planar spiral inductors Utku Kemiktarak, Keith Schwab, Kamil L. Ekinci We discuss the use of superconducting Nb inductors for impedance matching. The micro-fabricated inductors we used in our experiments had submicron line-widths with 50 to 200 turns. We first characterized these inductors by measuring their inductances, quality factors and self resonance frequencies. Then we compared these results with existing models. With the measured parameters, we showed that these inductors could be used for matching impedances on the order of megaohms to 50 $\Omega $. [Preview Abstract] |
Monday, March 13, 2006 12:27PM - 12:39PM |
B38.00007: Microstrip SQUID amplifiers with cooling fins Darin Kinion, John Clarke Amplifiers based on dc SQUIDs (superconducting quantum interference devices) with integrated input coils configured as a microstrip resonator operate at frequencies between 50 MHz and 2 GHz, and in principle are capable of reaching the Standard Quantum Limit (SQL) for linear amplifiers. The SQUIDs are fabricated with Nb-AlOx-Nb tunnel junctions with Pd shunt resistors to eliminate hysteresis. In practice, Johnson noise in the shunt resistors often limits the noise temperature before the SQL is reached. To reduce this noise contribution we have attached large area cooling fins to the shunts to minimize hot- electron effects when the amplifier is operated at miilikelvin temperatures. Previous measurements were performed at frequencies corresponding to peak gain, but theory predicts that the lowest noise temperature should be attained at a slightly lower frequency. We have measured the noise temperature of a number of devices as a function of frequency and bath temperature. We compare our results to the predicted frequency dependence of the noise temperature and to the predicted value of the SQL. [Preview Abstract] |
Monday, March 13, 2006 12:39PM - 12:51PM |
B38.00008: RF Amplifiers Based on DC SQUID for 3-4 GHz Band G.V. Prokopenko, K.D. Osborn, S.V. Shitov, M. Maezawa, A.J. Sirois, K. Cicak, R.W. Simmonds DC SQUID based RF Amplifiers (SRFAs) are known to dissipate very little power and can be integrated on-chip with existing micro-fabricated circuits for low-noise, low temperature measurements. The SRFA chip has 4 independent channels, which amplify at different signal frequencies and have integrated output filters, which prevent the leakage of high frequency resonances associated with the Josephson frequency. A compact two-layer input signal coil is integrated with the washer of the SQUID (L=20 pH) and planar capacitors for tuned frequency and impedance matching. The input reflected power is further reduced using proven balanced configuration on the SQUID. The shunt resistors are made from a multilayer film Ti(2 nm)/Pd(55 nm)/Ti(2 nm), which allow the SRFA~ to operate down to 0.4 K. Low-noise SRFAs have been tested with an operating frequency range of 3-4 GHz. [Preview Abstract] |
Monday, March 13, 2006 12:51PM - 1:03PM |
B38.00009: SQUID-detected magnetic resonance imaging in zero static magnetic field N. Kelso, S-K. Lee, L. Bouchard, V. Demas, A. Pines, J. Clarke Conventional magnetic resonance imaging (MRI) is performed in a static homogenous magnetic field B$_{0}$ in the presence of applied field gradients that generate a magnetic field change $\Delta $B $<<$ B$_{0}$ across the sample. In this case, the concomitant gradients can be ignored and the applied gradients are unidirectional. When $\Delta $B $\sim $ B$_{0}$, this approximation breaks down and the concomitant gradients distort the image. In the limit B$_{0}\to $ 0 these distortions can be eliminated by means of a pulse sequence consisting of a train of short, spatially uniform magnetic field pulses. Between the pulses, the spins evolve in a pure gradient field (with zero spatial average). The effect of the pulse train is to average out the concomitant terms to leave an effectively unidirectional gradient field (Meriles \textit{et al}., \textit{PNAS} 102, 1840 (2005)). We acquire magnetic signals with a superconducting gradiometer coupled to the input loop of a low-transition temperature superconducting quantum interference device. Using this pulse sequence we have acquired undistorted two-dimensional images of methanol phantoms in a residual static field $<$ 1 $\mu $T. Supported by USDOE.. [Preview Abstract] |
Monday, March 13, 2006 1:03PM - 1:15PM |
B38.00010: Fast Non-Destructive Evaluation of Superconducting Magnet Wires using a Flow-Through SQUID Microscope John Matthews, Frederick Wellstood, Harold Weinstock We have developed a cryocooled high-Tc SQUID microscope for fast non-destructive evaluation (NDE) of long wires, designed for detecting defects in superconducting magnet wire. A feedthrough mechanism pulls the wire at speeds of up to 20 cm/s through a thin mylar tube that separates the room temperature wire from the SQUID. We present results on test wires where we detect defects down to about 0.3 mm diameter. We discuss how we extract information from the data, such as defect size and location, and also outline a method for fast automated detection of defects in long wires. [Preview Abstract] |
Monday, March 13, 2006 1:15PM - 1:27PM |
B38.00011: Low-field SQUID MRI: To tune or not to tune? Michael Hatridge, Whittier Myers, Michael M\"{o}{\ss}le, Nathan Kelso, John Clarke, Ben Inglis, Andreas Trabesinger Our magnetic resonance imaging (MRI) system detects protons precessing at 5.6 kHz in a 132-$\mu $T field using a superconducting quantum interference device (SQUID) coupled to an untuned superconducting second-order gradiometer with 65-mm diameter coils. The magnetic flux noise of the SQUID corresponds to a magnetic field noise of 0.4 fT Hz$^{-1/2}$. We consider whether the use of a tuned input circuit could reduce the noise. The high intrinsic quality factor of the superconducting circuit must be damped to obtain the bandwidth required for MRI. The simplest approach is to insert a resistor in series with the gradiometer. However, the associated Nyquist noise causes this configuration to underperform the untuned version at frequencies below 100 kHz. In a more sophisticated approach Seton and coworkers damped the input circuit by coupling the SQUID output to the pickup coil. In principle, such feedback damping enables the tuned detector to achieve a field noise of 0.05 fT Hz$^{-1/2}$ at 5.6 kHz. The relative merits of these approaches over a wide frequency range are discussed. Work supported by USDOE. [Preview Abstract] |
Monday, March 13, 2006 1:27PM - 1:39PM |
B38.00012: SQUID-detected microtesla MRI: a new modality for tumor detection? Michael M\"o{\ss}le, Sarah Busch, Michael Hatridge, Whittier Myers, Alexander Pines, John Clarke, Lars Schmitt, Jeff Simko We are investigating the use of low-field magnetic resonance imaging with enhanced longitudinal-relaxation-time (T$_{1})$-weighted contrast to detect tumors. Our technique involves prepolarizing the sample in a magnetic field up to 0.3 T and detecting the nuclear magnetic resonance (NMR) signal at microtesla fields using a superconducting quantum interference device. This technique enables us to obtain T$_{1}$ dispersion curves and T$_{1}$-weighted contrast images in fields from 1.4 $\mu $T to 0.3 T. We have shown that for materials such as agarose gel the T$_{1}$-weighted contrast is greatly enhanced in microtesla fields. To investigate the use of this enhancement for tumor imaging we measured the T$_{1}$ relaxation times of healthy and cancerous tissue specimens, maintained at 4\r{ }C, shortly after their surgical removal. To minimize artifacts we measured normal and cancerous tissues simultaneously and separated their NMR signals by applying a magnetic field gradient. We present T$_{1}$ dispersion curves for several sets of samples, and discuss the applicability of this technique to \textit{in vivo} imaging. Work supported by USDOE. [Preview Abstract] |
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