SQUID Loop Research Articles

Depinning of single vortices in niobium thin film dc supercond interference devices by rf demagnetisation

To test the influence of an rf magnetic field on pinning and depinning of single vortices in thin niobium films we prepared dc SQUIDs with a square loop of 1 /spl mu/m line width and a hole size of 20 /spl mu/m/spl times/20 /spl mu/m. On one side the niobium strip widens to an area of 5 /spl mu/m/spl times/5 /spl mu/m, which is large enough for the pinning of at least one vortex. The motion of the trapped vortices from one pinning site to another generates flux changes in the SQUID loop and can thus be detected by the SQUID. We report on measurements of two level fluctuations caused by vortices hopping between different pinning sites. The investigations were made in a temperature range from 2.8 K to 4.5 K. By measuring the time, for which a vortex remains in one of the pinning sites, one can deduce the difference of the pinning energies and the activation energy for hopping between the pinning sites. By the use of a high frequency demagnetisation process the vortex can be forced to change the pinning site or to leave the superconductor, depending on the power and the frequency of the rf field.

The effect of thermal noise on the operation of DC SQUIDs at 77 K-a fundamental analytical approach

A rigorous analytical analysis of DC SQUIDs having reduced inductance smaller than 1//spl pi/ and operating in the presence of thermal fluctuations has been developed. On the basis of the Fokker-Planck equation the expressions for the output voltage across the Josephson junctions and the circulating current in the SQUID loop were found. This reduces the problem of finding all important SQUID characteristics (current-voltage curves, dynamic resistance, transfer function, and energy sensitivity) and their optimization to a simple exercise of elementary mathematics. As expected, the present analytical approach gives unique insights and significantly contributes to understand the physics related to the effects of large thermal fluctuations on system's operation.

YBCO SQUIDs fabricated by field-emission electron beam source

We report the applicability of YBa/sub 2/Cu/sub 3/O/sub 7-y/ (YBCO) SQUIDs fabricated by field-emission electron beam source. The junctions show hysteresis and self-resonance steps related to a high frequency resonance in the dc SQUID loop at a voltage V/sub res/=/spl Phi//sub 0//2/spl pi/(LC/2)/sup 1/2/ around 0.3 T/sub c/. The noise spectrum of the SQUIDs was measured with dc-bias schemes at different temperatures and showed values around 10/sup -4/ /spl Phi//sub 0//Hz/sup 1/2/ down to frequencies of 10 Hz at 40 K. The influence of magnetic flux, operating temperatures and flux-voltage transfer factor (|dV/d/spl Phi/|) were investigated. We also tested inverter operation of a dc-biased Josephson logic gate composed of an rf SQUID and a dc SQUID to realize logic circuit performance. Operation of the inverter has been successfully demonstrated although the operating temperature is low, around 10 K.

Environmentally decoupled sds -wave Josephson junctions for quantum computing

Unconventional superconductors exhibit an order parameter symmetry lower than the symmetry of the underlying crystal lattice. Recent phase sensitive experiments on YBCO single crystals have established the d-wave nature of the cuprate materials, thus identifying unambiguously the first unconventional superconductor. The sign change in the order parameter can be exploited to construct a new type of s-wave - d-wave - s-wave Josephson junction exhibiting a degenerate ground state and a double-periodic current-phase characteristic. Here we discuss how to make use of these special junction characteristics in the construction of a quantum computer. Combining such junctions together with a usual s-wave link into a SQUID loop we obtain what we call a `quiet' qubit --- a solid state implementation of a quantum bit which remains optimally isolated from its environment.

Determination of the Parameters of High- T C RF SQUID from Its Small Signal Voltage-Frequency Characteristics

The present study is the extension of a recent theory of high-TC RF SQUIDs, operating in the presence of large thermal fluctuations. First, the explicit analytical expressions for the amplitudes of higher harmonics of the current circulating in a SQUID loop which are necessary for the determination of the junction critical current IC and the inductive coupling coefficient of the SQUID loop to its tank circuit, k2 , are found. Second, in order to find the junction resistance R with acceptable accuracy, the solution of Fokker–Plank equation to first order in parameter q = ωL/R, where ω is a bias frequency, L is a loop inductance, is obtained. These results allow one to derive the simple expressions for the determination of the junction critical current IC, its normal resistance R, and the inductive coupling coefficient of the SQUID loop to its tank circuit, k2 , from the experimental measured small signal voltage-frequency characteristics.

Signal Enhancement in a Nonlinear Transfer Characteristic

We study nonlinear behavior in a model of a periodically modulated, overdamped rf SQUID loop operating in the dispersive (i.e., nonhysteretic) mode. In the presence of correlated noise we find an enhancement of the output signal-to-noise ratio (SNR) as a function of the nonlinearity parameter of the device. The calculation involves knowledge only of the input-output transfer characteristic of the device. These signal enhancement properties appear to be generic to devices characterized by nonlinear transfer characteristics. We also use our transfer characteristic approach to explain recent experimental results showing SNR enhancement in dc SQUIDs as a function of dc bias current and flux.

The response of high-Tc SQUID magnetometers to small changes in temperature

We have investigated the response of the flux-locked output of several high-Tc SQUID magnetometers to small changes in temperature and for magnetic fields 0–30 μT. The temperature response DT≡dΦS/dT is observed to be linear in the applied magnetic field Ba and can be as large as 500 mΦ0/K where ΦS is the flux through the SQUID loop and Φ0 is the flux quantum. Our measurements can be explained using a simple model that takes into account the geometry of a given device and is based on the idea that DT is due to the temperature dependence of the superconducting penetration depth. Our results can be used to optimize device performance in applications where the noise of a device is dominated by ambient temperature fluctuations.

Improved self-shielded d.c.-SQUID

We report on the design, fabrication and performance of a thin-film multiloop d.c.-SQUID, built in such a way to realize an efficient shielding against uniform ambient fields. The multiloop geometry is a design which allows one to reduce the SQUID inductance. This is an important item to improve the device sensitivity towards the quantum limit, but it has the drawback of a lower coupling coefficient with the input coil, compared to other geometries such as the washer one. In this new design, we improve the design of the input coil to overcome the problem of the coupling and arrange the SQUID loops in a gradiometric configuration to obtain self-shielding of the device.

High Tc SQUIDs with two or three junctions for application in disturbed environment

Abstract Different dc SQUIDs for galvanometer type high T c SQUID sensors are investigated. The aim is to achieve a high flux-to-voltage transfer function and good sensitivity to the current originated in a directly coupled pickup loop. Furthermore, the stability against external fields is investigated. For these purposes 2 and 3 junction SQUIDs with various layouts are compared. The 3 junction SQUID showed no advantage for the application in a galvanometer type SQUID sensor. For the layout slim and long SQUID loops give best results in all respects.

Low excess flux noise in YBa/sub 2/Cu/sub 3/O/sub 7-x/ dc SQUIDs cooled in static magnetic fields

We have investigated the effect of device geometry on the excess low frequency 1/f noise of thin-film YBCO dc SQUIDs cooled in static magnetic fields. The key factor in eliminating this noise is the reduction of the linewidth of the SQUID loop to a value below the average separation of the flux vortices. The spectral density of the flux noise in these devices was independent of cooling field up to 33 /spl mu/T in the best case. Estimates indicate that incorporating this device into a directly-coupled magnetometer would not increase the noise further.

Noise-Controlled Resonance Behavior in Nonlinear Dynamical Systems with Broken Symmetry.

We study noise-controlled resonant behavior in periodically modulated, overdamped bistable dynamic elements subject to a symmetry-breaking dc signal. The spectral amplitudes of the harmonics of the modulation frequency are found to exhibit multiple maxima whose occurrence depends on matchings of deterministic and stochastic time scales; in turn, these times depend on the noise statistics and the degree of asymmetry. We demonstrate the phenomenological results via analytical and numerical computations on an rf SQUID loop, and propose this technique to detect weak signals using a ``frequency hopping'' mechanism to circumvent detector noise limitations.

The phase-dependent dissipation effect in a d.c. SQUID

The magnetic field modulation of the voltage output of a d.c. SQUID, calculated numerically including the phase-dependent dissipation (PDD) term with a negative prefactor ε in Josephson junctions, exhibits maxima (minima) at integer (half-integer) values of the magnetic filling factor f in a larger bias current beyond I ∗ , the values of which depends on the inductance parameter β L and the prefactor ε. Asymmetry in both junction critical currents and inductances between the two branches of a d.c. SQUID loop does not change the essential feature of the magnetic-field modulation of the resistance due to the PDD effect.

An integrated planar gradiometer based on a double relaxation oscillation SQUID

The design and performance of an integrated planar gradiometer based on a double relaxation oscillation SQUID (DROS) are presented. The DROS was made from hysteretic junctions and the devices were fabricated by a simple four-level process. The signal SQUID loop is a gradiometric type with two square holes connected in parallel and a reference junction is used instead of the reference SQUID. The high flux-to-voltage transfer coefficient of typically enabled direct readout by a simple electronics with a modest voltage noise. The pickup coil integrated on the same wafer as the SQUID consists of two planar coils connected in series and has a baseline of 30 mm. The overall size of the device is . The field gradient noise is in the white region and at 1 Hz.

The ORPHEUS dark matter experiment

Abstract A progress report of the ORPHEUS dark matter experiment in the Bern Underground Laboratory is presented. A description of the ORPHEUS detector and its sensitivity to WIMPs is given. The detector will consist of 1 to 2 kg Sn granules operating in a magnetic field of approximately 320 G and at a temperature of 50 mK. In the first phase, the detector will be read out by conventional pickup coils, followed by a second phase with SQUID loops. Preliminary results on background and radioactivity measurements are shown.

Zener tunneling in small Josephson junctions with dissipation

We investigated charging effects in a small-area Josephson tunnel junction in a SQUID geometry, biased through high-resistance thin-film leads. The Josephson coupling energyEJ was varied by an external magnetic flux inside the SQUID loop. At a certain value of the current the Zener tunneling rate exceeds that of dissipative relaxation and the system “jumps” from the lowest to higher Brillouin zones. The shape of I–V curve and crossover currentIcr are in good agreement with a theory of Zener tunneling with dissipation.

Vertical c-axis microbridge junctions in YBa/sub 2/Cu/sub 3/O/sub 7//PrBa/sub 2/Cu/sub 3/O/sub 7/ thin films

The fabrication and measurement of microbridge junctions in the vertical c-axis direction of YBCO/PBCO/YBCO evaporated thin films is described. Single junctions show critical current modulation with applied magnetic field, the period scaling with junction size. The IV characteristics show Shapiro steps to high order when irradiated with microwaves. Initial measurements of linear arrays suggest that with further optimisation of the processing steps a reproducible junction technology could emerge. Very low inductance (L/spl ap/5-15 pH) SQUIDs have been demonstrated with dV/d/spl Phi/>1 mV//spl Phi//sub 0/ at 50 K. The vertical SQUID loop geometry of these SQUIDs has allowed a direct determination of the penetration depth as a function of temperature.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Effects of high energy proton bombardment (50-280 MeV) on dc SQUIDS

Three thin film dc SQUIDs of varied construction were bombarded with energetic protons in the energy range of 50 to 280 MeV. Measurements of the voltage output of the dc SQUIDs were taken in open loop, as well as flux locked mode, in an environment of proton flux that was varied from 10/sup 4/ to 10/sup 7/ protons/cm/sup 2//s. Discrete voltage jumps corresponding to 0.01 to 0.001 flux quanta were observed in two of the three SQUIDs in the flux locked mode; discrete changes in the open loop SQUID output voltage were also observed. Some data appear to be consistent with proton-induced flux motion in the body of the SQUID loop.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Inductance measurements in multilevel high T c step-edge grain boundary SQUIDS

Multilevel high Tc SQUIDs, suitable for digital circuit applications, have been fabricated and tested. The devices employ a YBa2Cu3O7−δ (YBCO) ground plane, an epitaxial SrTiO3 insulator, and a YBCO microstrip layer. Junctions are formed by the step-edge grain boundary process, with a ground plane contact for the ‘‘low’’ side of each junction, using only isotropic sputtering and milling techniques. Control current is directly injected in a microstrip segment of the SQUID loop, allowing us to measure the microstrip inductance, and thus to infer the magnetic penetration depth of the YBCO. The SQUIDs are operational above 77 K, at which temperature we infer a penetration depth of 350 nm. The temperature dependence of the penetration depth is found to be in reasonable agreement with the Gorter–Casimir form close to Tc.

YBa2Cu3O7 Josephson junctions and dc SQUIDs by mechanically induced growth disorder

A simple and reproducible method for the fabrication of high-Tc Josephson junctions and SQUIDs is presented. A very weak disturbance is produced by pressing a diamond needle onto the substrate surface before film deposition. The junction parameters such as the critical current Ic, the IcRn product, or β=2IcL/Φ0 (with L the self-inductance of the SQUID loop) are accessible by controlling the film thickness and can be varied by several orders of magnitude. The junctions usually have critical current densities jc in the range of 5×104–1×105 A/cm2 at 77 K and show Shapiro steps when exposed to rf power. The current-voltage characteristics are resistively shunted junctionlike and nonhysteretic. In dc SQUIDs a maximum peak-to-peak voltage response to dc flux of up to 12 μV at 77 K is observed.

Processing of DC SQUIDs for radiofrequency amplification

The goal of this studies is to use dc Squids as low noise radiofrequency amplifier (Tn≃1K) in the frequency range 1-150 Mhz and to integrate them in NMR probe. DC Squids have been built using planar technology and with two types of superconducting materials: low Tc(Nb) and High Tc(YBa 2 Cu 3 O 7 ). Contrary to the traditional set up used in magnetometry (flux locked loop) the dc Squid is here fluxed biased near Oz=(2n+1)O o /4 and no flux locked loop is connected. Small variations of flux (O s <<O o ) introduced from signal source via the input coil are amplified following the transfer characteristic V O =dV/dO. The low Tc devices are set up to now the best, concerning the reproducibility and the qualities of the Josephson Junctions obtained with a three layer process Nb/Al-Al 2 O 3 /Nb. Such junctions have been realized following the so called SNOP process, without anodization. The main characteristic obtained at 4.2K are R J /R N =10, 2Δ=2.8mev and a Josephson current density J≃10 3 A/cm 2 . The high Tc devices have been built with bi-epitaxial YBa 2 Cu 3 O 7 junctions and with a 30×30μm 2 area loop. I(V) and V(O) characteristics curves obtained are promizing for applications as RF detection in the temperature range: 4-77K. Modelization studies are currently in progress for the optimization of those devices (low Tc and high Tc) as RF amplifiers (minimization of parasitic capacitance between the input coil and the Squid loop) in order to increase their upper frequency limit