Tunneling spectroscopy in the magnetic superconductor TmNi2B2C

We present new measurements about the tunneling conductance in the borocarbide superconductor TmNi$_2$B$_2$C. The results show a very good agreement with weak coupling BCS theory, without any lifetime broadening parameter, over the whole sample surface. We detect no particular change of the tunneling spectroscopy below 1.5K, when both the antiferromagnetic (AF) phase and the superconducting order coexist.Comment: Submitted to Phys. Rev. B, Rapid Communication

Phonon-mediated anisotropic superconductivity in the Y and Lu nickel borocarbides

We present scanning tunneling spectroscopy and microscopy measurements at low temperatures in the borocarbide materials RNi2B2C (R=Y, Lu). The characteristic strong coupling structure due to the pairing interaction is unambiguously resolved in the superconducting density of states. It is located at the superconducting gap plus the energy corresponding to a phonon mode identified in previous neutron scattering experiments. These measurements also show that this mode is coupled to the electrons through a highly anisotropic electron-phonon interaction originated by a nesting feature of the Fermi surface. Our experiments, from which we can extract a large electron-phonon coupling parameter lambda (between 0.5 and 0.8), demonstrate that this anisotropic electron-phonon coupling has an essential contribution to the pairing interaction. The tunneling spectra show an anisotropic s-wave superconducting gap function.Comment: 5 pages, 3 figure

SAFARI engineering model 50 mK cooler

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Performances of the 50mK ADR/sorption cooler

International audienceCEA/SBT is currently testing a 50 mK cooler developed in the framework of a European Space Agency Technological Research Program targeted for the Advanced Telescope for High Energy Astrophysics space mission. This cooler is composed of a small demagnetization refrigerator pre cooled by a sorption cooler stage. This Engineering Model is able to produce 1 lW of net heat lift at 50 mK and an additional 10 lW at 300 mK provided by the sorption cooler stage. The autonomy of the cooler is 24 h, and once the low temperature phase at 50 mK is over, it can be recycled in about 8 h with 10 lW and 100 lW available at respectively the 2.5 and 15 K heat sinks. These performances are in agreement with the European Space Agency requirements. In this paper, we present the detailed thermal performances of the cooler in nominal conditions as well as sensitivity measurements of the variation of the heat sink and the cold end temperatures

300 mK CONTINUOUS COOLING, SORPTION-ADR SYSTEM

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SPICA/SAFARI Sub-Kelvin Cryogenic Chain

Presented at the 16th International Cryocooler Conference, held May 17-20, 2008 in Atlanta, Georgia.SPICA, a Japanese led mission, is part of the JAXA future science program and is planned for launch in 2018. SPICA will perform imaging and spectroscopic observations in the 5 to 210 mm waveband. The SPICA payload features three instruments, one of which, SAFARI, is developed by a European based consortium. SPICA’s distinctive feature is to use an actively cooled telescope down to 4 K. In addition SPICA is a cryogen-free satellite and all the cooling will be provided by radiative cooling (L2 orbit) down to 30 K and by mechanical coolers for lower temperatures. The satellite will be launched warm and slowly reach its operating temperatures once in orbit. This warm launch approach allows to suppress any large liquid cryogen tank and to use the mass saved to launch a large diameter telescope (3.5 meters). This 4 K cooled telescope allows significantly reduced thermal radiation, offering superior sensitivity in the infrared region. The cryogenic system that enables this warm launch/cooled telescope concept is a key issue of the mission. This cryogenic chain features a number of cooling stages comprising passive radiators, Stirling coolers and several Joule Thomson loops, offering cooling powers at typically 20, 4.5, 2.5 and 1.7 K. The SAFARI detectors require cooling to temperatures as low as 50 mK, and thus the SAFARI instrument cooler will be operated from these heat sinks. It is composed of a small adiabatic demagnetization refrigerator (ADR) pre-cooled by a sorption cooler. This hybrid architecture allows a lower weight cooler able to reach 50 mK. Because the sorption cooler/ADR combination is probably the lightest solution to produce sub-Kelvin temperatures, it allows the stringent SAFARI mass budget to be met

Progress in the Development of the IXO 50 mK Sorption-ADR stage

Presented at the 16th International Cryocooler Conference, held May 17-20, 2008 in Atlanta, Georgia.The nominal temperature of the new generations of detectors for the next space mission International X-ray Observatory (IXO) is expected to be around 50 mK. The coupling of a 3He cooler with an ADR provides an elegant cooler in this temperature range with low mass and few interfaces. As part of an ESA contract to develop such a solution, we designed an efficient assembly based on low thermal interfaces at 15 K and at 2.5 K. The cooler is sized to provide simultaneously net heat lifts of 1 μW at 50 mK and 10 μW at 300 mK for an autonomy exceeding 24 hours. The design of an engineering model is presented, as well as mechanical analysis and simulated results. The influence of the interface parameters are discussed together with different cycling scenario possibility

Evaluation of the vapor thermodynamic state in php

International audiencePulsating Heat Pipe (PHP) is a recently invented kind of thermal link of high thermal performance. The heat is transferred due to self-sustained oscillations of vapor bubbles and liquid plugs in a unique capillary tube which links the hot and cold sources in several turns or branches. Increasingly strong efforts are devoted to PHP simulation models which need to be experimentally assessed. Recent models show the importance of the thermodynamic state of the vapor bulk. Indeed it may deviate from the saturation conditions due to vapor transient compression/expansion. This study aims at measuring and calculating the vapor state in a horizontal single branch PHP which allows generating oscillations of a liquid meniscus in a capillary tube with one open end, at which the pressure is imposed. The experimental setup uses oxygen at cryogenic temperatures which reduces as much as possible the radiation losses of both sensors (microscopic thermocouple) used for the direct vapor temperature measurement and of the whole PHP. We present here the simultaneous experimental measurements of the vapor pressure and temperature. The data are compared to the theoretical modeling obtained within the film evaporation/condensation model of PHP [Das et al., 2010]. The model is extended to account for the thermal gradient along and across the tube walls formed because of the poor heat conductivity of its material. The vapor is found superheated, which demonstrates the relevance of the model. The vapor temperature oscillates around the evaporator temperature. The heat transfer between the vapor and the dry evaporator wall explains this result. The corresponding heat transfer coefficient is calculated using DNS. The comparison shows an agreement between the experiment and the model

SPICA sub-Kelvin cryogenic chains

SPICA, a Japanese led mission, is part of the JAXA future science program and is planned for launch in 2018. SPICA will perform imaging and spectroscopic observations in the mid- and far-IR waveband, and is developing instrumentation spanning the 5–400 μm range. The SPICA payload features several candidate instruments, some of them requiring temperature down to 50 mK. This is currently the case for SAFARI, a core instrument developed by a European-based consortium, and BLISS proposed by CALTECH/JPL in the US. SPICA’s distinctive feature is to actively cool its telescope to below 6 K. In addition, SPICA is a liquid cryogen free satellite and all the cooling will be provided by radiative cooling (L2 orbit) down to 30 K and by mechanical coolers for lower temperatures. The satellite will launch warm and slowly equilibrate to its operating temperatures once in orbit. This warm launch approach makes it possible to eliminate a large liquid cryogen tank and to use the mass saved to launch a large diameter telescope (3.2 m). This 4 K cooled telescope significantly reduces its own thermal radiation, offering superior sensitivity in the infrared region. The cryogenic system that enables this warm launch/cooled telescope concept is a key issue of the mission. This cryogenic chain features a number of cooling stages comprising passive radiators, Stirling coolers and several Joule Thomson loops, offering cooling powers at typically 20, 4.5, 2.5 and 1.7 K. The SAFARI and BLISS detectors require cooling to temperatures as low as 50 mK. The instrument coolers will be operated from these heat sinks. They are composed of a small demagnetization refrigerator (ADR) pre cooled by either a single or a double sorption cooler, respectively for SAFARI and BLISS. The BLISS cooler maintains continuous cooling at 300 mK and thus suppresses the thermal equilibrium time constant of the large focal plane. These hybrid architectures allow designing low weight coolers able to reach 50 mK. Because the sorption cooler has extremely low mass for a sub-Kelvin cooler, it allows the stringent mass budget to be met. These concepts are discussed in this paper

Experimental results on MgB 2 used as ADR magnetic shields, and comparison to NbTi

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