Micrometre-scale refrigerators

A superconductor with a gap in the density of states or a quantum dot with discrete energy levels is a central building block in realizing an electronic on-chip cooler. They can work as energy filters, allowing only hot quasiparticles to tunnel out from the electrode to be cooled. This principle has been employed experimentally since the early 1990s in investigations and demonstrations of micrometre-scale coolers at sub-kelvin temperatures. In this paper, we review the basic experimental conditions in realizing the coolers and the main practical issues that are known to limit their performance. We give an update of experiments performed on cryogenic micrometre-scale coolers in the past five years

Untersuchung der magnetischen Eigenschaften kubischer Antiferromagnete

Mit der vorliegenden Arbeit konnten durch Messungen in Feldern von bis zu 14.5 Tesla neue Erkenntnisse zu den magnetischen Eigenschaften der antiferromagnetischen Ordnung in der FCC Symmetrie gewonnen werden. Diese Symmetrie ist durch geometrische Frustration gekennzeichnet, welche zu einer Entartung des Grundzustandes der antiferromagnetischen Ordnung führt. Als Modellsystem wurde die Substanz K2IrCl6 ausgewählt, da hier die bekannte magnetische Struktur von einer besonders schwachen Kopplung stabilisiert wird, so dass die Induzierung eines magnetischen Phasenüberganges mit erreichbaren Feldstärken erwartet werden konnte. Zunächst wurde durch Magnetisierungsmessungen mit einem SQUID Magnetometer die makroskopische Magnetisierung in externen Feldern bis hin zu 9 Tesla untersucht. Diese Messungen deckten die Existenz eines magnetischen Phasenüberganges auf und gaben Aufschluss über die Besetzung der magnetischen Domänen. Die gewonnenen Erkenntnisse erlaubten einen gezielten Einsatz der Methode der Neutronenstreuung zur magnetischen Strukturbestimmung der entdeckten Hochfeldphase. Die experimentellen Rahmenbedingungen für die Neutronenstreuung waren bei der Substanz K2IrCl6 besonders anspruchsvoll, da die Iridiumatome Neutronen stark absorbieren, Chlor ein starkes Untergrundsignal erzeugt und aufgrund des kleinen Momentes (Spin = ½) nur geringe magnetische Streuintensitäten vorhanden sind. Beispielsweise konnte die Charakterisierung der Hochfeldphase nur mit polarisierten Neutronen erfolgreich abgeschlossen werden. Aufgrund des erstellten Strukturmodells wurde der Einfluss von Fluktuationen auf die Stabilisierung dieser magnetischen Phase diskutiert. Somit wurde erstmalig ein neuer experimenteller Zugang zur Charakterisierung der Fluktuationen beim Antiferromagnetismus der FCC-Symmetrie ermöglicht. Einen weiteren Schwerpunkt der experimentellen Untersuchungen bilden die Messungen der magnetischen und strukturellen Eigenschaften von K2IrCl6 am Phasenübergang ohne äußeres Feld. Neben Neutronenstreuexperimenten zur Eingrenzung des kritischen Verhaltens wurde die Kristallstruktur mit Synchrotronstrahlung untersucht. Neben der Charakterisierung der Magnetostriktion wurde eine geringe Symmetriebrechung entdeckt und deren Einfluss auf die magnetische Ordnung diskutiert

Primary tunnel junction thermometry

We describe the concept and experimental demonstration of primary thermometry based on a four probe measurement of a single tunnel junction embedded within four arrays of junctions. We show that in this configuration random sample specific and environment-related errors can be avoided. This method relates temperature directly to Boltzmann constant, which will form the basis of the definition of temperature and realization of official temperature scales in the future

Static structure factor of two-dimensional liquid 3He adsorbed on graphite

International audienceLiquid 3He is a model system for strongly correlated Fermi liquids. For this reason, many X-ray and neutron scattering experiments have been performed to understand the structure and dynamics of this quantum fluid. We have recently shown that two-dimensional liquid 3He sustains long-lived zero-sound excitations at large wave-vectors (Nature 483, 576, 2012). Here we show that its static structure factor can be obtained with reasonable accuracy by integrating the experimental S(Q,ω) over a suitable energy range. A good agreement is found between the static structure factor deduced from the experiment and theoretical models: Quantum Monte Carlo simulations and Dynamical Many Body Theory (DMBT). At high wave-vectors, the experimental values are underestimated because of the limited accessible phase space; nevertheless, even at atomic wave-vectors a semiquantitative agreement is observed with the theoretical predictions

Heat Transistor: Demonstration of Gate-Controlled Electron Refrigeration

We present experiments on a superconductor-normal metal electron refrigerator in a regime where single-electron charging effects are significant. The system functions as a heat transistor, i.e., the heat flux out from the normal metal island can be controlled with a gate voltage. A theoretical model developed within the framework of single-electron tunneling provides a full quantitative agreement with the experiment. This work serves as the first experimental observation of Coulombic control of heat transfer and, in particular, of refrigeration in a mesoscopic system.Comment: 4 pages, 3 color figure

Traceable Coulomb Blockade Thermometry

We present a measurement and analysis scheme for determining traceable thermodynamic temperature at cryogenic temperatures using Coulomb blockade thermometry. The uncertainty of the electrical measurement is improved by utilizing two sampling digital voltmeters instead of the traditional lock-in technique. The remaining uncertainty is dominated by that of the numerical analysis of the measurement data. Two analysis methods are demonstrated: numerical fitting of the full conductance curve and measuring the height of the conductance dip. The complete uncertainty analysis shows that using either analysis method the relative combined standard uncertainty (k = 1) in determining the thermodynamic temperature in the temperature range from 20 mK to 200 mK is below 0.5 %. In this temperature range, both analysis methods produced temperature estimates that deviated from 0.39 % to 0.67 % from the reference temperatures provided by a superconducting reference point device calibrated against the Provisional Low Temperature Scale of 2000.Comment: 11 page

Electronic Refrigeration at the Quantum Limit

We demonstrate quantum limited electronic refrigeration of a metallic island in a low temperature micro-circuit. We show that matching the impedance of the circuit enables refrigeration at a distance, of about 50 um in our case, through superconducting leads with a cooling power determined by the quantum of thermal conductance. In a reference sample with a mismatched circuit this effect is absent. Our results are consistent with the concept of electromagnetic heat transport. We observe and analyze the crossover between electromagnetic and quasiparticle heat flux in a superconductor.Comment: 5 pages, 3 figure

Quantized current of a hybrid single-electron transistor with superconducting leads and a normal-metal island

We discuss the operation of the superconductor - insulator - normal-metal - insulator - superconductor (SINIS) turnstile. This voltage-biased hybrid single-electron transistor (SET) provides current quantization even with only one radio-frequency (rf) control parameter, namely the gate voltage of the single island. We give an overview of the main error mechanisms of the turnstile and consider its feasibility as a quantum current standard. We also present experimental results of pumping with the SINIS structure which show decreased leakage current compared to earlier measurements with the opposite NISIN structure.Comment: 10 pages, 9 figures; Text edited, one new figure added, references update

Observation of zero-sound at atomic wave-vectors in a monolayer of liquid 3He

International audienceThe elementary excitations of a strongly interacting two-dimensional Fermi liquid have been investigated by inelastic neutron scattering in an experimental model system: a monolayer of liquid3He adsorbed on graphite preplated by a monolayer of solid 4He. We observed for the first time the particle-hole excitations characterizing the Fermi liquid state of two-dimensional liquid 3He, and we were also able to identify the highly interesting zero-sound collective mode above a particle-hole band. Contrarily to bulk 3He, at low wave-vectors this mode lies very close to the particle-hole band. At intermediate wave-vectors, the collective mode enters the particle-hole band, where it is strongly broadened by Landau damping. At high wave-vectors, where the Landau theory is not applicable, the zero-sound collective mode reappears beyond the particle hole band as a well defined excitation, with a dispersion relation quite similar to that of superfluid 4He. This spectacular effect is observed for the first time in a Fermi liquid (including plasmons excitations in electronic systems)