Wolfgang Buck

Consistency of low temperature scales and the temperature of the HE-3 melting pressure minimum

Abstract Two independent low temperature scales, based on noise thermometry below 0.6 K and on extrapolation from 1.5 K using magnetic thermometry, have been realized and compared. Both scales coincide at the pressure minimum of the He-3 melting curve within ±0.3 mK and determine its temperature as 315.1 mK.

Combined time-resolved magnetooptical and electrical flux detection in thin-film superconductors

The dynamic behavior of the current-induced dissipative flux-flow state in a thin-film type I superconductor was studied by simultaneous stroboscopic magnetooptical flux detection and direct recording of the time-resolved flux-flow voltage. Employing high-resolution magnetooptical flux detection with a high-speed stroboscope, it was possible to visualize globally individual multiquantum flux tubes during their rapid motion across the superconducting Pb film, yielding spatial and temporal resolution of better than 1 µm and 0.1 µsec, respectively. Simultaneously, the temporal structure of the flux-flow voltage was recorded using a highly sensitive signal-averaging procedure, thereby yielding a voltage resolution of about 30 nV at a time resolution of 10 nsec (corresponding to a recording bandwidth of 25 MHz). The recorded temporal voltage structures agreed well with the voltages expected from the velocity profiles of all flux tubes existing simultaneously obtained from the magnetooptical data. The experiments are the first to demonstrate full agreement between both independent flux-detection measurements, clearly confirming the existing theory.

Temporal Structure of the Flux-Flow Voltage in Constricted Superconducting Indium Films

Time-resolved measurements of the flux-flow voltage generated in the current-induced resistive state of constricted superconducting indium films are reported. Using a computer-aided signal averaging technique, it has been possible to obtain a voltage resolution of better than 10 nV at a time resolution of 10 nsec. The results are in good agreement with theoretical model calculations.

A nuclear demagnetization cryostat for thermometry

Abstract A single-stage nuclear demagnetization cryostat is reported with 33 moles of copper in a magnetic induction of 8 T developed for the investigation of low temperature thermometry. The lowest temperature measured with a Pt NMR thermometer was 130 μK, and the time dependent heat leak approached 0.3 nW after 80 days.

High‐performance waveform‐sampling analysis based on digital multipoint signal averaging

This paper describes a versatile digital multipoint waveform‐recording system utilizing computer‐aided signal‐averaging procedures. Basically, it consists of a Biomation 8100 transient recorder coupled to a DEC PDP 11/04 computer via an electronic interface device and a CAMAC I/O‐register NE 9066 supported by an appropriate software system. Additional use of an extremely low‐noise wideband preamplifier system together with proper shielding, filtering, and differential measuring techniques minimize the disturbing influence of random and coherent electromagnetic noise. Employing our waveform‐recording system to sensitive time‐resolved flux‐flow measurements in superconductors yields both a voltage resolution approaching 1 nV and a time resolution of 10 ns at a recording bandwidth of 25 MHz. Such a novel scheme of high‐performance waveform‐sampling analysis yields an overall efficiency limited at the characteristic rise‐time sensitivity product of about 10−17 V s, corresponding to a magnetic flux sensitivity...

High-resolution recording of the time-dependent flux-flow voltage in thin-film superconductors

Abstract The temporal structure of the instantaneous voltage generated by multiquantum flux tubes moving through a constricted type-I superconducting microbridge (in films of 2 μm thickness) is measured. By employing a high-resolution signal-averaging technique, we are able to record the total time development of the flux-flow voltage thereby yielding a voltage resolution of better than 10 nV at a recording bandwidth of 25 MHz (corresponding to a time resolution of 10 ns). All experimentally obtained voltage profiles agree well with the behavior expected from theoretical model calculations.

The fixed points of the3He melting curve

Abstract After the temperature value of the3He melting pressure minimum of 315.1 mK has been confirmed by other authors, now the pressure values of the other fixed points of the melting curve are determined. To avoid the limitations due to the uncertainty of the absolut pressure scale it is more precise to rely on pressure differences related to the minimum pressure or the pressure of the superfluid A transition than on absolute pressures. The distance between both is determined to be (0.50264±0.00008) MPa and differs from Greywalls data by 0.00060 MPa. The difference between the magnetic transition in the solid3He and the A transition is (0.00521±0.00005) MPa. Based on the NIST minimum pressure of 2.93118 MPa the present absolut values are, for comparison, 3.43382 MPa, 3.43588 MPa, and 3.43903 MPa for the A, B and magnetic transition, respectively.

Temporal voltage profile of a single flux tube moving across a superconducting strip

Abstract The temporal voltage profile which is generated by a single multiquantum flux tube crossing a superconducting strip of type-I material is simulated by calculations based on the Gibbs free-energy barrier model. The simulation can reproduce the experimental data in every detail without any fitting procedure except a correction of the critical current due to the unknown sample shape around the nucleation site of the flux tube.

Transient Phenomena during the Flux-Flow Onset in Thin-Film Superconductors

From time-resolved measurements of the flux-flow voltage detailed information can be derived about the spatial current distribution and the migration of flux domains across superconducting thin-film samples. Calculations based on the Gibbs-free-energy-barrier model achieve excellent agreement with the experimental voltage signal generated by a single flux domain. For a series of consecutive flux tubes the monopole-interaction model has to be included. In this case, the computer simulation of experimental voltage patterns reveals a damped oscillation of the number of flux quanta per domain. This number approaches an equilibrium value much smaller than the domain size in the case of a singularly existing flux tube. Flux flow in superconducting films turns out to be a convenient phenomenon for studying general properties of transient behaviour during its onset.

Analysis of the current-induced breakdown of superconductivity

Recently, the breakdown of superconductivity has been investigated in thin-film samples with extreme temporal resolution. As a result, the voltage drop across the flux-flow path shows a temporal pattern of striking structure caused by migrating domains of magnetic flux. This reveals the dynamic character of the so-called current-induced resistive state. This voltage structure is analyzed on the basis of the extended Gibbs free-energy-barrier model, and quantitative agreement is achieved for the single-domain and the multidomain cases as well. In the single-domain case, the influence of fluctuations of the transport current on the time-resolved voltage is demonstrated. In the multidomain case, where more than one domain exists within the sample, a transient oscillation of the number of flux quanta per domain is revealed occurring immediately after the onset of flux flow. This size oscillation dies out after a small number of cycles and approaches the stationary value observed in previous experiments. Thus, a successful explanation of the temporal aspects of the breakdown of superconductivity is established, which holds from its very onset to the stationary regime of the current-induced resistive state.