Magnetic Susceptibility of Heavy Fermion ³He-Bilayers

Abstract

A ³He bilayer film adsorbed on graphite preplated with a bilayer of solid ⁴He has recently been shown to be a new heavy fermion system that can be tuned towards a quantum critical point (QCP) by varying the ³He coverage. We report new measurements of the temperature dependence of the magnetic susceptibility of this system over the temperature range 0.3 to 300 mK. The magnetisation of the ³He film is measured using pulsed NMR at a frequency of 60 kHz, on a SQUID detector setup with an untuned input circuit. The signal sensitivity attainable with this setup enables us to collect high-precision data with reasonable acquisition times, despite the low surface area of the sample (2 m²) and the small liquid susceptibility. The detailed NMR response provides an essential diagnostic tool to precisely adjust the ⁴He preplating. On cooling, the magnetisation evolves from Curie-law, through a distinct Kondo-like maximum at a temperature T ^*, to a weakly rising magnetisation far below the Curie law. This behaviour signifies the formation of the heavy fermion state of the coupled ³He layers. As the ³He coverage is increased towards the putative QCP, T ^* is tuned to lower temperature. These preliminary observations demonstrate the power of the technique, confirm previous results obtained in a different sample cell by a different method, and pave the way for a detailed study close to the QCP.