We describe here the design and performance of a current sensing noise thermometer using a low T c DC SQUID as the front end amplifier. The DC SQUID is used to measure the thermal noise current in a resistor and the temperature is then obtained from the Nyquist formula. The thermometer is fast, absolute and precise and is usable over a wide temperature range below 4.2 K, in principle down to well below 1 mK. The excellent energy sensitivity of the DC SQUID, operated at fixed temperature, enables the use of a relatively large noise resistor, in the m range. This requires relatively short averaging times when measuring the spectrum of noise fluctuations. We have shown that it is possible to determine absolute temperature with a precision of 1% in a measuring time of 10 seconds with an amplifier noise temperature, T N , of the order of 30 µK, and to an accuracy better than 0.3%. The percentage precision is independent of temperature for temperatures much greater than T N. Our method of heat sinking the noise resistor ensures proper cooling of the electrons. We incorporate a fixed point device for checking the gain calibration. We have cooled the thermometer successfully to below 1 mK, achieving a minimum electron temperature of 300 µK. We present the results of a preliminary comparison with a 3 He melting curve thermometer (MCT) above 4.5 mK, and with a platinum NMR thermometer down to the lowest temperatures.

The nuclear magnetic susceptibility of 3 He adsorbed on the surface of graphite, plated with a monolayer of solid 4 He, has been measured at surface densities less than 0.055 A ~2 and at temperatures down to 6 mK. A temperature-independent susceptibility is observed at low temperatures, which is strongly enhanced over the ideal-gas value, due to quasiparticle interactions. The inferred Landau-Fermi-liquid parameters allow a comparison with microscopic theories of interacting Fermi systems. PACS numbers: 67.50.-b, 67.70.+nThe fluid phase of adsorbed 3 He is potentially an important and well characterized example of a lowdimensional system of highly correlated fermions. In this Letter we show that 3 He adsorbed on graphite plated with a single atomic layer of 4 He behaves as a twodimensional Landau Fermi liquid, in which the interactions can be varied over a wide range and the crossover from degenerate to nondegenerate behavior can be clearly explored. We present the first measurements on twodimensional 3 He to show a constant low-temperature nuclear magnetic susceptibility which is strongly enhanced by quasiparticle interactions above ideal-gas values. The Fermi-liquid parameters inferred from these measurements provide a test of microscopic models of interacting Fermi systems, such as the almost localized fermion model.The two related systems that have received the most attention are 3 He adsorbed on exfoliated graphite [1] and 3 He-4 He mixture films adsorbed on heterogeneous substrates [2,3]. In the mixture films the 3 He atoms reside near the surface of a 4 He film which coats the substrate, smoothing out effects of substrate heterogeneity. The interaction of the 3 He atoms with the 4 He film gives rise to a mass enhancement of the single 3 He "impurity" due to hydrodynamic backflow. In addition the 3 He atom has available excited states, at relatively low energy, associated with motion perpendicular to the surface. These states become populated therefore at relatively low coverages -0.03 A -2 , and need to be taken into account at temperatures above a few hundred millikelvin [2,4].The system we have chosen to study is the second-layer fluid phase of 3 He on Grafoil [5l. Here the significantly higher fluid density of 0.055 A -2 is accessible before solidification commences [6,7] (the onset of promotion to the third layer occurs at 0.069 A -2 ). At these higher fluid densities strong correlation effects are observable. The binding energy of the second-layer atoms is relatively large, -25 K, and since the excitation of atoms out of this layer is negligible below 1 K the onset of degeneracy in the two-dimensional fluid can be investigated quantitatively. The more strongly bound first-layer submonolayer film also exhibits a fluid phase [1]. However, the range of

We describe the design and performance of a series of fast, precise current sensing noise thermometers. The thermometers have been fabricated with a range of resistances from 1.290 Ω down to 0.2 mΩ. This results in either a thermometer that has been optimised for speed, taking advantage of the improvements in superconducting quantum interference device (SQUID) noise and bandwidth, or a thermometer optimised for ultra-low temperature measurement, minimising the system noise temperature. With a single temperature calibration point, we show that noise thermometers can be used for accurate measurements over a wide range of temperatures below 4 K. Comparisons with a melting curve thermometer, a calibrated germanium thermometer and a pulsed platinum nuclear magnetic resonance thermometer are presented. For the 1.290 Ω resistance we measure a 1 % precision in just 100 ms, and have shown this to be independent of temperature.

Measurements of the nuclear magnetic susceptibility of multilayer films of 'He adsorbed on graphite plated with a monolayer of 4He are reported for 'He surface densities up to 0.27A-2 and temperatures in the range (6+-600)mK. The coverage dependence of the exchange constant has been measured; this is antiferromagnetic when the second layer first solimes and becomes ferromagnetic with the formation of a fluid overlayer. The second-layer solid density evolves in a stepwise fashion with increasing coverage, while the susceptibility of the fluid overlayer shows structure associated with layering in the film.

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