SQUID Voltmeter Research Articles

Vortex loops in the critical Casimir effect in superfluid and superconducting films

Vortex-loop renormalization techniques are used to calculate the magnitude of the critical Casimir forces in superfluid and superconducting thin films. The force is found to become appreciable when the size of the thermally excited vortex loops is comparable to the film thickness, and the results for T< T c are found to match very well with perturbative renormalization-group theories that can only be carried out for T> T c. In helium films the Casimir force leads to a change in the film thickness close to T c that has been observed experimentally. A similar effect is predicted to occur near the transition temperature of high- T c superconducting films, which is also a vortex-loop phase transition. In this case the Casimir force takes the form of a voltage difference that will appear at the junction between a thin film and a bulk sample. Estimates show that this voltage can be appreciable (tens of microvolts), and it may be possible to observe the effect by measuring the voltage across two Josephson tunnel junctions to the film and to the bulk, using a SQUID voltmeter.

Application of high performance DC SQUIDs in precision measurement technique

In this review the performance of a DC SQUID system with Nb-NbO/sub x/-Pb/In/Au window-type Josephson junctions is described. The SQUIDs and the SQUID control units were developed for universal applications in precision measurement technique. Under optimum conditions a flux noise level of 2/spl times/10/sup -6/ /spl Phi//sub 0///spl radic/Hz was achieved corresponding to an energy resolution of 3/spl times/10/sup -31/ J/Hz. The contribution gives the design of the DC SQUID device and summarizes recent results of its application in precision measurement technique. The applications include a cryogenic current comparator for non-destructive measurement of the beam intensity produced in a particle accelerator, a high-sensitive SQUID voltmeter for investigations of the low-field Hall coefficient of high purity metals, the upgrade of a multichannel RF SQUID neuromagnetometer for biomagnetic research, and a planned high-precision experiment in fundamental physics.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

SQUID picovoltometry of single crystal Bi2Sr2CaCu2O8+ delta : Observation of the crossover from high-temperature Arrhenius to low-temperature vortex-glass behavior.

Using a SQUID voltmeter we have measured current-voltage curves in untwinned crystals of ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ as a function of temperature and magnetic field. Our data show a clear crossover from high-temperature Arrhenius behavior to a critical region associated with the low-temperature three-dimensional vortex-glass phase transition. The critical exponents \ensuremath{\nu}(z-1)=7\ifmmode\pm\else\textpm\fi{}1 in this system are in accord with theoretical models and previous measurements in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$. The width of the critical region collapses below 2 T, reflecting the changing role of dimensionality with field.

Thermopower and resistance of superconducting-normal interfaces II. Experiment

This paper presents experimental data on the thermopower and resistance of superconducting-normal interfaces in SNS junctions made from pure copper foils to which rods of pure lead or PbBi alloys are attached, measured using a SQUID voltmeter. The data are analysed using the theory presented in the preceding paper, which is generally successful in its region of validity for the purer alloys near to Tc, and more successful than earlier theories, particularly as concerns the thermopower. The bulk thermopower for the superconducting excitations is found to be continuous with its normal value at Tc. The junctions made using dirty alloys in which the heat flow is phonon dominated show an unexplained large rise in boundary thermopower at low temperatures.

Measurement of the low-field Hall coefficient by d.c. SQUID technique

A d.c. SQUID voltmeter adapted for the measurement of the low-field Hall coefficient at 4.2 K is described. On the basis of this device Hall voltages of high purity metals of the magnitude of a few nanovolts produced by a current up to 1 A and magnetic induction up to 10 mT can be detected with a resolution of 1.5 pV Hz −1 2 . The reproducibility of the Hall voltage measured between different cooling cycles of the SQUID system is close to 0.1%. Results are presented for samples of high purity copper which contain dislocations.

Use of YBCO wires to extend the temperature range of a SQUID voltmeter

The authors describe modifications to a conventional SQUID voltmeter involving the use of YBCO wires in the superconducting potentiometer circuit. This modification allows measurements to be made of voltages across samples at temperatures well above the 8K limit set by the transition temperature of the original superconducting wires.

Electron spin injection and detection at a ferromagnetic-paramagnetic interface (invited)

A new technique for investigating conduction electron magnetization is presented. A coupling between electronic charge and spin transport in ferromagnetic metals follows from generalizing the linear transport equations. In practice, this means that ferromagnetic films act as conduction electron spin polarizers. At an interface between a ferromagnetic and a paramagnetic metal, one can inject and detect nonequilibrium magnetization in the paramagnet, and use sensitive electric measurements to probe spin transport. Application of a small external magnetic field dephases the spins. The result is a new technique for measuring conduction spin relaxation times, which is demonstrated on bulk aluminum samples. Three advantages of the technique make it applicable to a wide range of studies. (1) The measurement can be made in the limit of zero field so that systems with interesting properties which are altered or destroyed by magnetic fields can now be probed (spin glasses, superconductors, metals with g anisotropies). (2) A SQUID voltmeter with picovolt noise floor provides a sensitivity that allows detection of one nonequilibrium spin in a background of 1012 equilibrium spins. (3) The experimental geometry utilizes microfabrication techniques and is ideal for application to small-size systems that are inaccessible to conventional electron spin resonance methods. The geometry conveniently needs but one good surface; other sample surfaces are available for alteration in situ for the study of surface relaxation effects. Finally, there are obvious applications to the study of ferromagnets and interfaces.

The thermoelectric effect in SNS' sandwiches

A SQUID voltmeter with a noise of 8*10-15 V Hz-1/2 at 1K has been used to measure the resistance and thermoelectric properties of a number of superconductor-normal-metal-superconductor (SNS) sandwiches. Both the resistance and thermoelectric EMF diverge near the lower transition temperature in a fashion which is well explained by the results of a new theory for Pb/Cu/Pb sandwiches but sandwiches of Pb/Cu/eutectic PbBi show more puzzling features.

Investigation of electric field penetration into superconducting tantalum near a normal-superconducting interface

The temperature dependence of the excess resistance of Ta-Cu-Ta sandwiches was investigated. The samples were prepared by the mutual rolling of the bulk materials, with the thickness of the superconducting plates (⋍0.1 cm) much greater than the penetration length of the electric field into the superconductorlE up to temperatures very close toTc. The penetration of the electric field through the clean NS interface into a superconductor with a relatively long electronic mean path (1⋍ξ0) was studied. We used an rf SQUID voltmeter to measure the resistance, ∼10−9 Ω, at currents not exceeding 1 mA. The experimental data were compared with the Artemenko-Volkov and Hsiang-Clarke theories. The electron-phonon relaxation time τɛ for our tantalum sample was estimated to be ∼1×10−10 sec.

The squid picovoltmeter operating in magnetic fields up to 3.2 T

A method which enables the Squid dc voltmeter to be operated with a sample exposed to a significant external magnetic field is presented. A special superconducting shield with a finite time constant has been used to suppress time-dependent components of the field. With the shield, the Squid voltmeter has been able to work in magnetic fields up to 0.7 T in the flux-locked loop mode. The total voltage noise per unit bandwidth V rms (referred to input) does not exceed the value of 1 × 10 −12 V Hz − 1 2 . To extend the range of performance to higher magnetic fields, simple low pass filters have been built into the input circuit to increase its time constant. With the signal coil of the Squid shunted by a resistor R B = 3 × 10 −6 ω, the Squid voltmeter could be operated in fields up to 3.2 T with V rms ⩽ 1.2 × 10 −11 V Hz − 1 2

Percolation in inhomogeneous superconducting composite wires

We report the initial results of SQUID voltmeter measurements on Cu-alloy wires which contain elongated μm-scale inclusions of superconducting Nb. Below the transition temperature T c of the Nb inclusions the wires have a temperature-dependent resistance which can be fit to R ∝ (T - T cl) s where s = 1.06 ± 0.04. We develop a simple model employing percolation theory to explain this behavior.

Low-level measurements of DC voltages using a SQUID

The input circuitry of a SQUID voltmeter system has been modified by using a shorted signal coil which acts as a low-pass filter with a cut-off frequency of less than 1 Hz. This enables the SQUID to be used under adverse noise conditions in order to measure voltages generated in an unscreened sample. The incorporation of a superconducting switch is also described which enables sample voltages to be isolated without changes in the applied conditions. The frequency response of the low-pass filter is measured in the magnetometer mode.

Practical SQUID Technique for the Precise Measurements of Transport Coefficients

A practical system of SQUID voltmeter for precise measurements of thermoelectric power and other transport coefficients in metals and alloys at low temperatures is described. The cryostat is designed to allow temperature control of the sample continuously from 1.3 K to 8.5 K, while the SQUID and the standard resistors are kept at a fixed temperature of 4.2 K. The sensitivity was found to reach the Johnson noise level for a source resistance lower than 0.02 Ω; typically 0.9×10-13 V Hz-1/2 for 0.4×10-4 Ω at 4.2 K. A simple noise analysis indicates that the system has an ultimate sensitivity of at least 1.5×10-14 V with a time constant of 1 sec for a source resistance lower than 10-6 Ω at helium temperatures.

Low frequency noise in Josephson junctions

Using a high resolution SQUID voltmeter, we have measured the spectrum of low frequency voltage fluctuations across a thin-film Josephson tunnel junction biased at a constant current I greater than the junction critical current I <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> . We find that the frequency dependence of the voltage spectrum <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V^{2}(f)</tex> may be accurately represented by the power law <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V^{2}(f) \propto f^{-1}</tex> over the frequency range of our data: <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10^{-2} &lt; f &lt; 10</tex> Hz. The dependence of the magnitude of the spectra at any single frequency upon the value of the bias current I and upon the sample temperature T supports our hypothesis that the observed voltage fluctuations arise from a modulation of the junction critical current I <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> by equilibrium, thermodynamic temperature fluctuations in the active junction volume. We are able to interpret our measurements in terms of the semi-empirical theory of Clarke and Voss for the low frequency fluctuation spectrum of systems obeying a diffusion equation. This interpretation provides design criteria which may prove useful in reducing the level of long-term drifts in systems employing Josephson tunnel junctions.

Comparison between the constant-potential steps of two point-contact Josephson junctions

Microwave-induced steps in the current-potential characteristics of two niobium point-contact junctions have been compared using a SQUID voltmeter with a resolution of less than 10−12 V. It has been established that the uncertainty in step potential at the 1-mV level is less than 2 in 109.