Design of a novel on-chip electronic refrigerator based on a normal-insulator-superconductor tunnel junction

We present a design for a novel electronic refrigerator having a base temperature of about 18 mK when operating from a bath temperature as high as 1.5 K. This all-electronic refrigerator is a factor of 10⁴ smaller and lighter than dilution and adiabatic demagnetization refrigerators, and is compatible with conventional photolithographic fabrication. The refrigerator, based on the unique thermal transport properties of a normal-insulator-superconductor (NIS) tunnel junction, preferentially removes electrons whose energy is higher than the Fermi energy from a normal metal. Electrons with an average energy equal to the Fermi energy are returned to the metal by a superconductor contact. Consequently, high energy thermal excitations are removed from the normal metal, thus cooling the electrons. In our configuration, the junction is deposited on a Si₃N₄ membrane of submicron thickness that thermally isolates the normal electrode from the bath. As a result, both electrons and phonons in the metal are cooled below the bath temperature. We calculate a cooling power of 2 nW at 100 mK, and a base temperature of 18 mK for a refrigerator area of about 100X100 m². Using 10⁵ such refrigerator circuits, the cooling power can be increased to 200 W.     

An improved He3-He4 mixing chamber for single-cycle experiments.

A new type of mixing chamber is described with which any dilution refrigerator, capable of reaching 30 mK continuously, can now attain temperatures below 3 mK when operating in the single-cycle mode.In a preliminary experiment this mixing chamber used in a dilution refrigerator with an additional superfluid He⁴ circulation of 2 × 10^?3 moles s^?1 reached a temperature of 4 mK continuously.     

Dielectric Constant Measurements of Solid 4He

Careful measurements of the dielectric properties of solid ⁴He have been carried out down to 35 mK, considerably lower than the temperature range of previous studies. The sample was prepared from high purity gas with ³He concentrations of the order of 200 ppb and were formed by the blocked capillary method. The molar volume of the sample was 20.30 cm³. The dielectric constant of the samples was found to be independent of temperature down to 120 mK before showing a continuous increase with decreasing temperature and saturating below 50 mK. The total increase in ?? is 2 parts in 10^?5. The temperature dependence of ?? mimics the increase in the resonant frequency found in the torsional oscillator studies and also the increase found in the shear modulus measurements.     

Compact dilution refrigerator with a cryogenic circulation cycle of He3

A He³-He⁴ dilution refrigerator has been constructed which circulates He³ by using two alternately operating adsorption pumps which are in the same cryostat as the dilution refrigerator. The scheme described here organised the low temperature circulation cycle such that the construction and cryostat communications were essentially simplified, the dimensions of the arrangement were decreased which resulted in a circulation velocity of 10^?4 mole s^?1, which was sufficient to obtain a temperature in the order of 10 mK.     

Practical design of heat exchangers for dilution refrigerators: part 2

A dilution refrigerator with a He³ circulation rate 80–730 μmols^?1 and a minimum temperature of 2.85 mK which is continuously maintained has been built. Discrete heat exchangers with pressed metal (Cu and Ag) powder are used. The heat exchangers are newly developed, based on the analysis of Radebaugh and Siegwarth. The machine is designed for many purposes such as the base of nuclear cooling and studies of general solid state physics. The design, construction and performance of this refrigerator are described.     

A Cryogen-Free Miniature Dilution Refrigerator for Low-Temperature Detector Applications

We describe a miniature dilution refrigerator, suitable for many detector applications at temperatures down to 50 mK. Unlike conventional systems, the ³He gas is recycled internally which eliminates the need for any room-temperature pumps and gas handling system. No fine capillaries, moving parts or cooled O-rings are required which makes the system mechanically very reliable and minimizes the risk of developing blockages or cryogenic leaks during thermal cycles. We have integrated our dilution refrigerator with a pulse-tube cooler to create a cryogen-free, push button system which is now being used for TES detector developments. Temperatures below 50 mK and a cooling power of several micro-Watt at 100 mK is available for more than 15 hours at the time, with a re-cycle period of about 4 hours.     

Development of Adiabatic Demagnetization Refrigerator for X-ray Microcalorimeter Operation

We are developing of adiabatic demagnetization refrigerator for X-ray microcalorimeter operation. By improving thermal environment in the cryostat, it achieved the lowest temperature of 80 mK, temperature stability of 9?μK rms at 100?mK, and a hold time of 7 hours at 100?mK. The excess heat load to the saltpill decreased from 10?μW to 3?μW. Using this system, we demonstrated a measurement of R–T curve of a TES microcalorimeter and ?–V curve of a SQUID.     

On Mössbauer thermometry in the Millikelvin region

Resonant absorption of γ-rays by⁵⁷Fe and¹¹⁹Sn in iron was investigated to study the possibility of ultralow-temperature measurements by the Mössbauer effect. A dilution refrigerator which could provide temperatures down to 6 mK was used. The Mössbauer temperature of¹¹⁹Sn coincided with the temperature in the mixing chamber down to 6 mK if the heat input to the absorber was low enough (?10^?5 erg/sec). But the Mössbauer temperatures of⁵⁷Fe stayed almost invariable below 15 mK. In the same iron sample the nuclei of⁵⁷Fe and¹¹⁹Sn had different Mössbauer temperatures below 15 mK. The reasons for this phenomenon are not yet clear.     

A Pomeranchuk cell which is precooled and pressurized by a He4-cycling He3 - He4 dilution refrigerator

This paper deals with a special Pomeranchuk cooling device which is used in combination with a He⁴-cycling He³-He⁴ dilution refrigerator at high pressures. The lowest temperature obtained was 1.1 mK.     

Mass Coupling and Q ?1 of Impurity-Limited Normal 3He in?a?Torsion Pendulum

We present results of the Q ^?1 and period shift, ??P, for ³He confined in a 98% nominal open aerogel on a torsion pendulum. The aerogel is compressed uniaxially by 10% along a direction aligned to the torsion pendulum axis and was grown within a 400???m tall pancake (after compression) similar to an Andronikashvili geometry. The result is a high Q pendulum able to resolve Q ^?1 and mass coupling of the impurity-limited ³He over the whole temperature range. After measuring the empty cell background, we filled the cell above the critical point and observe a temperature dependent period shift, ??P, between 100?mK and 3?mK that is 2.9% of the period shift (after filling) at 100?mK. The Q ^?1 due to the ³He decreases by an order of magnitude between 100?mK and 3?mK at a pressure of 0.14??0.03?bar. We compare the observable quantities to the corresponding calculated Q ^?1 and period shift for bulk ³He.     

Development of a 3He Refrigerator for Possible Experiments of Solid 4He on a Small Jet Plane

The gravity on the Earth (g _E) has not been taken seriously to mask the fundamental phenomena on quantum solids, though there are some important studies on the critical phenomena of superfluid ⁴He under microgravity. We are planning to investigate the effect of gravity on the equilibrium shape of solid ⁴He. Since we had a chance to do such an experiment on a small jet plane through the ground based program by JAXA, we got started to construct a cryostat which could cool down as low as 500?mK and meet severe restrictions of experiments on a jet plane. The main part of the refrigerator was a usual ³He-evaporator pumped by a scroll pump. A?small GM refrigerator was installed to provide 4?K stage. 1?K pot was also put in which was also pumped by another scroll pump to condense ³He gas and sample ⁴He. The cryostat was designed to have two optical windows to be able to observe solid ⁴He under microgravity. In the test flight for the refrigerator, the minimum temperature of 690?mK was kept during the entire flight of two hours in which 7 to 8 times parabolic flight was performed. Each parabolic flight includes about 20?seconds microgravity and 20?seconds 1.5 to 2.0?g _E period before and after the microgravity. We did some preliminary experiments with bcc solid ⁴He under microgravity. The crystal remained stuck to the bottom of the sample cell even in the 20 seconds microgravity condition.     

Experimental Evaluation of a Single Stage Superfluid Stirling Refrigerator Using a Large Plastic Recuperator

This paper describes the low and high temperature experimental performance of the second single stage superfluid Stirling refrigerator (SSR) to use a plastic recuperator. This SSR has a total internal volume of 96.6 cm³ and uses a Kapton recuperator which has 12.10 cm³ devoted to recuperative heat transfer. Operating from a high temperature of 1.0 K and with 1.5% and 3.0% ³He–⁴He mixtures, this SSR achieves a low temperature of 291 mK and delivers net cooling powers of 3705 W at 750 mK, 977 W at 500 mK, and 409 W at 400 mK. Cooling power versus cold piston temperature for various frequencies of operation and for two piston stroke configurations are also provided. These results are non-dimensionalized and compared to the Schmidt model of a regenerative Stirling refrigerator and the adiabatic model of a recuperative Stirling refrigerator. The SSR was also operated from high temperatures between 1.0 K and 2.0 K. This SSR achieves low temperatures of 412 mK, 620 mK, 1.069 K, and 1.459 K operating from high temperatures of 1.2 K, 1.4 K, 1.6 K, and 1.8 K respectively. This high temperature performance is compared to the theoretical performance of the SSR using a phonon–roton gas model.     

NMR Studies of 3He Impurities in 4He in the Proposed Supersolid Phase

Preliminary results are reported for measurements of the NMR relaxation times of very dilute ³He in samples of solid ⁴He at low temperature, 0.25 K <T< 1.3 K. The results were obtained for carefully prepared samples with different ³He concentrations. The measurements of the spin-spin relaxation time, T ₂, show several interesting features. A temperature independent plateau attributed to the exchange motional narrowing is observed down to the lowest temperature studied, and the observed variation of T ₂ with ³He concentration favors the nonlinear theory suggested by Landesman. The best fit to the data is given by T ₂ ∝ x ₃ ^?1.89±0.1 rather than x ₃ ^?1 . No evidence of an exchange-phonon bottleneck for the spin-lattice relaxation is seen down to 25 mK. The vacancy activation energy is determined to be 13.5±0.3 K for a sample with x ₃=5×10^?4 and molar volume 20.9 cm³.     

Powerful dilution refrigerator for use in the study of polarized liquid 3He and nuclear cooling

A ³He⁴He dilution refrigerator with a lowest temperature of 1.90 ± 0.05 mK and a maximum circulation rate of 10 mmol s⁻¹ has been constructed. The apparatus is at present used for the study of ³He in high magnetic fields and as a precooling stage for nuclear demagnetization. The design and construction of the dilution refrigerator are described.     

NMR Study of Superfluid Phases of 3He Confined in 97.5% Porous Silica Aerogel

We have studied the scattering effect from aerogel strands on superfluid phases of ³He by a cw NMR method at 920 kHz. Liquid ³He at a pressure of 13 bar was confined in 97.5% porous aerogel from the same batch as that of a recent 4th sound study. The NMR experiment was performed in a magnetic field of 28.4 mT down to 0.3 mK. As temperature decreased, the NMR resonant frequency increased below 0.76 mK. The temperature of 0.76 mK agrees with the superfluid transition temperature T ^aerogel _c observed in the 4th sound study at the same pressure. Below T ^aerogel _c the behavior of thefrequency shift as a function of temperature indicates that there is no phasetransition to the other superfluid phase down to about 0.4 T ^aerogel _c . Owing to a very large surface solid ³He magnetization, we could not determine the superfluid phase of ³He in the aerogel in the magnetization measurement.     

Improved Optical Setup to Measure Growth and Megting of Solid 3He Below 1 mK in Magnetic Fields Up to 9 Tesla

Recent experiments on crystal growth and crystal shape in ³He at zero magnetic field at 0.55 mK by Tsepelin et al. ^{1, 2} in Helsinki have revealed many new facets and strong anisotropy in the growth rates. Since ³He presents the unique possibility of investigating the influence of a magnetic field on crystal growth properties, we intend to extend our experimental studies of the shape and growth rate of solid ³He to magnetic fields up to 9 T at temperatures as low as possible.For that purpose our optical access cryostat with low temperature CCD has been upgraded and modified. The low temperature LED is replaced by a room temperature source coupled to the cell with an optical fiber to obtain clear images of the growth process. Our Pomeranchuk cell has been fitted with a new optical section with pure quartz windows. It has a larger diameter, a larger distance between the optical windows, and an improved thermal link. Also the heat switches between the nuclear stage and the dilution refrigerator were replaced according to a more sturdy design. Initial resugts show clear crystals with sharp facet edges down to T=0.4 mK.     

Operation of a TES microcalorimeter cooled by a compact liquid-helium-free He-He dilution refrigerator directly coupled to a Gifford-McMahon cooler

A superconducting transition edge thermosensor (TES) microcalorimeter was irradiated with LX-ray photons emitted by an ²⁴¹Am source maintained at an operating temperature of 120 mK using a compact liquid-helium-free ³He-⁴He dilution refrigerator directly coupled to a Gifford-McMahon (GM) cooler. The first and second stages of the GM cooler were directly coupled to the first and second pre-cool heat exchangers of a stick shaped dilution unit through copper plates in a vacuum chamber. The helium-free dilution refrigerator provided a cooling power of 20 μW at 100 mK. Detection signals of LX-ray photons emitted by the ²⁴¹Am source were observed by operating the TES microcalorimeter in a severe noise environment induced by the mechanical vibrations of the GM cooler.     

A tiltable single-shot miniature dilution refrigerator for astrophysical applications

We present a ³He/⁴He dilution refrigerator designed for cooling astronomical mm-wave telescope receivers to around 100 mK. Used in combination with a Gifford–McMahon closed-cycle refrigerator, ⁴He and ³He sorption-pumped refrigerators, our cryogen-free system is capable of achieving 2 μW cooling power at 87 mK. A receiver attached directly to the telescope optics is required to rotate with respect to the downward direction. This scenario, of variable tilt, has proved difficult for typical dilution refrigerators, but our design has a geometry chosen to allow tilt to 45° and beyond.     

GaAs Quantum Dot Thermometry Using Direct Transport and Charge Sensing

We present measurements of the electron temperature using gate-defined quantum dots formed in a GaAs 2D electron gas in both direct transport and charge sensing mode. Decent agreement with the refrigerator temperature was observed over a broad range of temperatures down to 10 mK. Upon cooling nuclear demagnetization stages integrated into the sample wires below 1 mK, the device electron temperature saturates, remaining close to 10 mK. The extreme sensitivity of the thermometer to its environment as well as electronic noise complicates temperature measurements but could potentially provide further insight into the device characteristics. We discuss thermal coupling mechanisms, address possible reasons for the temperature saturation and delineate the prospects of further reducing the device electron temperature.