Tom Coffey

Measuring radio frequency properties of materials in pulsed magnetic fields with a tunnel diode oscillator

Tunnel diode oscillators have been used in many types of experiments that measure the properties of materials. We present the details of an apparatus that extend these tunnel diode techniques to measure the properties of materials in pulsed magnetic fields. In the most common version of this method, a sample is placed in the inductor of a small rf tank circuit powered by a tunnel diode and the conductivity, magnetization, or penetration depth is measured. We explain in this article how the sample and configuration of the radio frequency fields determine which property is measured. Our major innovations are to stabilize the tunnel diode oscillator during a magnet pulse by using compensated coils in the tank circuit and the development of two methods, one digital and one analog, to measure the frequency and amplitude shifts in the oscillator during the short (10 s of ms) magnet pulse. We illustrate the power of this new measurement method by showing preliminary results of the superconducting transition and ...

Radio frequency measurements of the superconducting transition in κ-(ET)2Cu(NCS)2 using a tunnel diode oscillator in pulsed magnetic fields

We have measured the penetration depth in the superconductor κ-(ET) 2 Cu(NCS) 2 as a function of magnetic field at constant temperature. We consistently find a sharp transition as the experiment is repeated with the sample at different orientations with respect to the magnetic field. When the magnetic field is near parallel to the conducting planes we see an inflection in the data that may indicate a change of state. We have also shown that the tunnel diode oscillator can be used effectively in ms pulsed magnetic fields.

The anomalous superconducting phase diagram of (BEDO-TTF)2ReO4 · H2O

Abstract We have measured the critical magnetic fields in the quasi 2D superconductor (BEDO-TTF)ReO 4 ·H 2 O as a function of temperature with the magnetic field perpendicular and parallel to the conducting planes. A sharp positive change in the curvature, or kink, in the H-T phase line, similar to one in the phase diagram of the superconductor λ-(BETS) 2 GaCl 4 occurs in the low temperature part of the phase diagram near T/T c = 0.4. These kinks in the H-T phase line could be the signature of a change in the superconducting order parameter. We discuss other possibilities, such as magnetic transitions, and why they are less likely to exist in these salts. To support our claims we also present recent data showing part of the H-T phase diagram of λ-(BETS) 2 GaCl 4 with the applied magnetic field parallel to the conducting planes.

RESISTIVITY AND PENETRATION DEPTH MEASUREMENTS OF ORGANIC SUPERCONDUCTORS IN HIGH MAGNETIC FIELDS USING A TUNNEL DIODE OSCILLATOR

We have made measurements of resistivity and penetration depth in dc-and pulsed-magnetic fields of the organic superconductors α-(ET)2NH4Hg(SCN)4, and κ-(ET)2Cu(NCS)2 using a resonant rf circuit powered by a tunnel diode oscillator (TDO). We compare the critical fields as measured by resistivity, and the TDO. All the superconductors we have studied have anisotropic critical fields with a ratio Hc2// to Hc2⊥ of 5-17. In κ-(ET)2Cu(NCS)2 we observed an almost monotonic increase in the parallel critical fields up to 24 T at 390 mK, and a sharp cusp in the critical field as a function of angle when the sample is parallel to the magnetic field. This suggests that the superconducting layers are only Josephson-coupled and the superconductivity is two-dimensional(2D). Another salt, α-(ET)2NH4Hg(SCN)4, shows similar cusp behavior, but this material shows evidence of extreme Pauli limiting when the field is turned parallel to the conducting layers.

Studies of the organic superconductor (BEDO)2ReO4H2O at high pressures and high magnetic fields

Abstract We have measured magnetoresistance in the organic conductor (BEDO) 2 ReO 4 H 2 O up to 27 tesla and 15 kbar. From the Shubnikov de Haas oscillations we have determined the change in the size of closed orbits on the Fermi surface and the effective mass as a function of pressure. We have also measured the magnetoresistance in a pulsed magnetic field up to 50 tesla, which is in the extreme quantum limit for one of the Fermi surfaces.