The observation of resistance oscillations in metals at high magnetic fields using a SQUID

Quantum oscillatory effects provide particularly unambiguous information about the properties of quasiparticles in metals, and have consequently been used for the purpose of understanding the low temperature behaviour of a wide variety of metallic elements and compounds, including the recently discovered heavy fermion materials. Unfortunately, the signals which are produced by these oscillations are often so weak as to be undetectable, because of the high effective masses of the quasiparticles, or the presence of imperfections in the samples, or otherwise. In order to improve our ability to measure these effects, the author has developed a system for the detection of resistance oscillations which uses a SQUID as the amplifying element and which fits inside a toploading dilution refrigerator probe. It has proven possible to use this instrument to investigate very elusive phenomena in samples which have been cooled to 0.02 K in magnetic fields of 13.7 tesla. For example, although no resistance oscillations have previously been seen in aluminum, except under conditions of magnetic breakdown, the new system has made it possible to detect resistance oscillations in this material under relatively arbitrary conditions. In the case of the heavy fermion compound CeRu₂Si₂, oscillations of 10^–9 have been detected against a background resistance of about 10^–5 , using excitation currents of around 10^–2 A.