Magnetic susceptibility of liquid 3He

International audience3He is a model of Fermi liquid, isotropic, its Fermi temperature is attainable and the interaction between atoms can be controlled by changing the pressure on the liquid. In this paper we present accurate cw-NMR measurements of the nuclear magnetic susceptibility of liquid 3He as a function of temperature and pressure. The emphasis has been placed in reliable thermometry, 3He pressure measurements directly in the cell to increase the measuring range until solidification, and an accurate characterization of the NMR spectrometer. Our measurements give effective Fermi temperatures substantially lower than former results

Dynamique dans les fluides quantiques (Etude des excitations collectives dans un liquide de Fermi 2D)

L'4He et l'3He sont des systèmes modèles pour comprendre les propriétés quantiques de la matière fortement corrélée. C'est pour cette raison que plusieurs études ont été consacrées à la compréhension de leur dynamique. A basses températures où les effets quantiques jouent un rôle essentiel, les excitations élémentaires dans l'4He sont décrites par un mode collectif d'excitations: phonon-roton. Par contre pour un système d'3He la description est plus complexe, le spectre d'excitation a deux composantes: un mode collectif (zéro-son) et un continuum d'excitations incohérentes de type particule-trou. Les deux sont bien décrites par la théorie de Landau des liquides de Fermi qui trouve sa validité pour des petits vecteurs d'onde. Jusqu'à présent, on supposait que la dynamique dans les liquides de Fermi à vecteurs d'onde élevés était essentiellement incohérente. Cette thèse porte sur l'exploration, par diffusion inélastique de neutrons, des excitations collectives dans l'3He liquide 2D adsorbé sur un substrat de graphite. Un tel travail expérimental requiert trois ingrédients essentiels : un réfrigérateur à dilution afin de travailler à basses températures, un spectromètre temps de vol afin de mesurer le facteur de structure dynamique du système et un substrat solide (graphite exfolié ZYX) pour la préparation de films d'3He-2D par physisorption. Nos expériences sur ces films d'3He déposés en deuxième couche sur de l'4He solide adsorbé sur le graphite nous ont permis de faire les observations suivantes : à petit vecteur d'onde, le zéro-son est plus proche de la bande particule-trou que celui observé dans le cas de l'3He massif, tandis qu'à fort vecteur d'onde le mode collectif entre dans le continuum et réapparait de l'autre côté. Cette nouvelle branche, observée pour la première fois, est aujourd'hui décrite par la théorie dynamique à N-corps développée par nos collaborateurs de l'université Johannes Kepler de Linz, Autriche. Au cours de ce travail de thèse plusieurs techniques expérimentales ont été développées, en particulier, un réfrigérateur à dilution sans fluide cryogénique robuste adapté à des expériences de diffusion neutronique. Son optimisation a permis de réduire le temps de refroidissement de ce type de réfrigérateurs.4He and 3He are model systems for understanding quantum properties of strongly interacting matter. For this reason many studies have been devoted for the understanding of their dynamics. At low temperatures at which quantum effects play an essential role, the elementary excitations in 4He are described by a phonon-roton collective mode. For 3He, the physical description is more complicated, the spectrum has two components: collective excitations (zero-sound) and incoherent particle-hole excitations. Both are described by Landau's theory of Fermi liquids which is valid at low wave vectors. So far, it was thus believed that the dynamics at high wave vectors is essentially incoherent. This thesis is mainly concerned by exploring the collective excitations of a two dimensional 3He film adsorbed on graphite, using inelastic neutron scattering. Such an experiment has three main requirements: a dilution refrigerator in order to work at low temperatures, a time of flight spectrometer for measuring the dynamical structure factor of 3He and a solid substrate (exfoliated graphite ZYX) to obtain a two dimensional film by physical adsorption. Our investigations of the dynamics in two-dimensional 3He adsorbed on graphite preplated with 4He films have revealed important features: At low wave-vectors, the zero-sound mode is considerably depressed compared to bulk 3He. At higher wave vectors, the collective excitations branch enters the particle-hole continuum, and reappears at the lower energy branch of the continuum. This new branch, observed for the first time, is described by the dynamic many-body theory developed by our collaborators from Johannes Kepler University, Linz, Austria. During this work several low temperature techniques have been developed, in particular a robust, cryogen-free dilution refrigerator adapted to the demanding conditions of a neutron scattering experiments. Due to its efficient design, the cooling time has been considerably reduced compared to that of refrigerators of the same type developed in the past.SAVOIE-SCD - Bib.électronique (730659901) / SudocGRENOBLE1/INP-Bib.électronique (384210012) / SudocGRENOBLE2/3-Bib.électronique (384219901) / SudocSudocFranceF

Very low resistance Al/Cu joints for use at cryogenic temperatures

We present two different techniques for achieving low resistance ($<$20 n$\rm \Omega$) contacts between copper and aluminium at cryogenic temperatures. The best method is based on gold plating of the surfaces in an e-beam evaporator immediately after Ar plasma etching in the same apparatus, yielding resistances as low as 3 n$\rm \Omega$ that are stable over time. The second approach involves inserting indium in the Al/Cu joint. For both methods, we believe key elements are surface polishing, total removal of the aluminum oxide surface layer, and temporary application of large (typ. 11 kN) compression forces. We believe the values for gold plated contacts are the lowest ever reported for a Cu/Al joint of a few $\rm cm^{2}$. This technology could simplify the construction of thermal links for advanced cryogenics applications, in particular that of extremely low resistance heat switches for nuclear demagnetization refrigerators.Comment: Accepted by Journal of Low Temperature Physic

Superconducting aluminum heat switch with 3 nΩ\Omega equivalent resistance

Superconducting heat switches with extremely low normal state resistances are needed for constructing continuous nuclear demagnetization refrigerators with high cooling power. Aluminum is a suitable superconductor for the heat switch because of its high Debye temperature and its commercial availability in high purity. We have constructed a high quality Al heat switch whose design is significantly different than that of previous heat switches. In order to join the Al to Cu with low contact resistance, we plasma etched the Al to remove its oxide layer then immediately deposited Au without breaking the vacuum of the e-beam evaporator. In the normal state of the heat switch, we measured a thermal conductance of $8 T$ W/K$^2$ which is equivalent to an electrical resistance of 3 n$\Omega$ according to the Wiedemann-Franz law. In the superconducting state we measured a thermal conductance that is $2\times10^6$ times lower than that of the normal state at 50 mK

Niobium Nitride Thin Films for Very Low Temperature Resistive Thermometry

International audienceWe investigate thin film resistive thermometry based on metal-to-insulator-transition (niobium nitride) materials down to very low temperature. The variation of the NbN thermometer resistance have calibrated versus temperature and magnetic field. High sensitivity in tempertaure variation detection is demonstrated through efficient temperature coefficient of resistance. The nitrogen content of the niobium nitride thin films can be tuned to adjust the optimal working temperature range. In the present experiment, we show the versatility of the NbN thin film technology through applications in very different low temperature use-cases. We demonstrate that thin film re-sistive thermometry can be extended to temperatures below 30 mK with low electrical impedance

Nouvelles mesures de la suceptibilité nucléaire de l'3He liquide à très basse température

TOULOUSE3-BU Sciences (315552104) / SudocSudocFranceF

'Fast Exchange' model visualized with 3He confined in aerogel: a Fermi liquid in contact with a Ferromagnetic solid

International audience3He confined in aerogel in the millikelvin temperature domain exemplifies a Fermi liquid in the presence of disorder. In confined 3He systems, a solid layer of 3He atoms forms on the confining medium. This system can then be viewed as a model system for the study of the (strongly interacting) Fermi liquid in contact with a (ferromagnetic) "2D-like" adsorbed solid. This interaction, studied experimentally through NMR T2 experiments, is described in the framework of the "fast exchange" model. A complete analytical descripion of the model is given, explaining our measurements as well as related normal-state confined 3He NMR literature

A closed-cycle dilution refrigerator for space applications

International audienceWe discuss the development of a gravity-insensitive dilution refrigerator adapted from the open-cycle refrigerator used for the Planck mission. Since the 3He and 4He components are circulated (the 4He by a fountain-effect pump operating at about 2 K, the 3He by a compressor at room temperature) rather than ejected into space, the lifetime of a closed-cycle refrigerator is not limited by the quantity of 3He and 4He available. In this work, we concentrate on the design and performance of the cold end (counterflow heat exchanger and mixing chamber) of the refrigerator. We discuss the sizing of the heat exchanger and present cooling power measurements. We detail the working of the fountain pump. We also briefly touch upon some practical issues including the choice of a 3He compressor and the pre-cooling requirements for the dilution refrigerator