Jean-Louis Bret

Design and applications of a very low losses superconducting LC resonator at 4.2 K

Abstract A superconducting electrical LC resonator has been developed with a niobium wire having a quality factor Q larger than 7 × 10 5 at 7 kHz and 4.2 K. This device is very easy to construct but very high Q values are obtained only if great care is taken to minimize the losses. To measure such Q values accurately at relatively low frequencies, an original experimental set-up has been adopted based on the use of a spectrum analyser. Applications to noise currents and contactless resistivity measurements are discussed.

High frequency thermal modulation of superconducting NbTi thin film by use of laser diode

Abstract Continuous modulation of the temperature of a fraction of a sputtered NbTi thin film has been performed around its critical temperature at frequencies up to 900 kHz using a pigtailed laser diode to provide the optical power. The high frequency chopping was achieved on a low thermal conductivity substrate. Possible applications of this type of switching in the areas of sensitive electronic and magnetic measurements are discussed.

An absolute cryogenic magnetometer

Abstract The high frequency thermal modulation of sputtered superconductors enhances the absolute measurement of very weak magnetic fields. We have used a dual device, made with neighbouring 1000 A thick NbTi disk and ring, whose present sensitivity is 10−12 T Hz− 1 2 . The thermal modulation of each of these elements is performed by two pigtailed laser diodes. The modulation of the temperature of the NbTi disk around its critical temperature gives the absolute value of the perpendicular applied magnetic field, because of the Meissner effect. The modulation of the temperature of a part of a superconducting ring just below Tc is sensitive to the variation of the magnetic induction, because of the divergence of the kinetic inductance. The detected voltage comes from a very high-Q LC resonator tuned to the frequency of the thermal modulation, and set at the input of a cryogenically cooled FET.