Table of contents

Usually liquid nitrogen (LN₂) transfer from a container to a laboratory equipment takes place by applying pressure to the container to push out liquid or pouring liquid into the cryostat directly by lifting the container. In order to overcome inconvenience of pressuring or lifting containers, we have been developing the Liquid Nitrogen Centrifugal Pump of a small electric turbine pump. Significant advantages that both reducing time to fill LN₂and controlling the flow rate of liquid into the cryostat are obtained by introducing this pump. We have achieved the lift of about 800mm with the vessel's opening diameter of 28mm.

The low temperature acoustic and thermal properties of amorphous, glassy materials are remarkably similar. All these properties are described theoretically with reasonable quantitative accuracy by assuming that the amorphous solid contains dynamical defects that can be described at low temperatures as an ensemble of two-level systems (TLS), but the deep nature of these TLSs is not clarified yet. Moreover, glassy properties were found also in disordered crystals, quasicrystals, and even perfect crystals with a large number of atoms in the unit cell. In crystals, the glassy properties are not universal, like in amorphous materials, and also exhibit anisotropy. Recently it was proposed a model for the interaction of two-level systems with arbitrary strain fields (Phys. Rev. B 75, 64202, 2007), which was used to calculate the thermal properties of nanoscopic membranes at low temperatures. The model is also suitable for the description of anisotropic crystals. We describe here the results of the calculation of anisotropic glass-like properties in crystals of various lattice symmetries.

We report on the mutual relationship between the current-voltage (I-V) and power-voltage (P-V) characteristics in metal-vacuum-metal tunnel junctions (TJs). Performed studies of power redistribution between individual electrodes of TJ have shown good correlation of measured P-V curves with ones numerically derived from I-V characteristics. This supports our earlier proposal that careful studies of power dissipation in individual electrodes of TJ by tunneling calorimetry could be a source of similar spectroscopic information as that derived from I-V characteristics. Moreover, we advert to the possibility to extend the tunneling calorimetry for detection of energy emission processes due to charge tunneling, what represents a base for development of qualitatively new scanning probe microscopy techniques.

For studies of the superfmid phases of ³He the technique low of frequency (500 kHz) NMR is widely used. One way to read out the NMR signal is with the continuous wave experiment. In this experiment the NMR signal is proportional with the quality factor of a tank circuit. However direct connection with a coax cable will, because of its resistivity and parasitic capacitance load the tank circuit and by that lower the quality factor In this paper two passive methods, which minimize the loading to read out the NMR signal are described and simulated. The first method reads the NMR signal over the parasitic capacitance of the coax cable, which is put in series with the tank circuit. The second method makes use of a pick up coil, which is weakly coupled to the coil of the tank circuit Both methods can preserve a high quality factor, and are optimized for best SNR

We describe the design and fabrication of a cryogenic probe for measurements of magnetic penetration depth (λ) down to 1.8 K. Penetration depth provides fundamental information about the nature of superconductivity and sets the length scale for vortex dynamics. Our probe employs the single coil self inductance technique at radio frequencies, in which the coil configuration along with the temperature stabilized tunnel diode oscillator enable the measurement of λ for thin film samples. There is also a provision for studying modulation of λ in the presence of small magnetic fields generated from an inbuilt coaxial superconducting coil. We present the performance of this probe in controlled sustenance of temperatures below 4.2 K and attainment of a signal-to-noise ratio of ∼10⁵,while measuring superconducting transition of a foil of indium.

Weakly Interacting Massive Particles (WIMPs) are a strong candidate for the Cold Dark Matter of the Universe. CDMS-II is a direct-search WIMP search experiment, operating at 50 mK and housed at the Soudan mine, Minnesota. The 250 gram Ge detectors utilize athermal phonon sensors where tungsten transition edge sensors are operated in negative electrothermal feedback. The search at Soudan is ongoing with an expected final reach of CDMS-II by the end of 2008 of a WIMP-nucleon cross-section sensitivity of 2.1 x10^-44 cm², at a WIMP mass of 60 GeV/c². To proceed further, we have proposed the SuperCDMS program.

We describe the design and fabrication of a compact Low Temperature Scanning Tunneling Microscope (LT-STM) together with a dipper cryostat for cooling the STM down to liquid helium temperatures. The STM, based on the piezo-tube walker as coarse approach mechanism, is suspended inside a cryostat vacuum can using three soft helical springs. The can is dipped into a liquid helium storage container for cooling the STM. Its compact size makes it less susceptible to mechanical vibrations and so the STM works with atomic resolution with a simple spring suspension. We demonstrate the performance of this STM for atomic resolution imaging and tunneling spectroscopy by observing the 3 ×3 charge modulation and the energy gap in the Charge Density Wave (CDW) phase of 2H-NbSe₂ at liquid helium temperatures.

Transport phenomena in superfluid helium can be described using the two-fluid Landau-Khalatnikov model and the Gorter-Mellink mutual friction. Here we discuss a mathematical formulation of the two-fluid model that uses macroscopic conservation balances of mass, momentum and energy of each species, and assumes local thermodynamic equilibrium. A particularity of this model is that it describes the state of He II as well as that of each of the two-fluid components in terms of pressure p and temperature T, which is convenient for stable numerical solution. The equations of the model form a system of partial differential equations (PDE) that can be written in matrix form for convenience. On this base, a three-dimensional numerical model using a complete and consistent, while still practical, system of PDEs was developed. In the form described, the PDE can be solved using three-dimensional Lagrangian finite element in space supplemented by a Beam-Warming time-marching algorithm. Once validated, this solver will allow to simulate He II thermal counter-flow applied to arbitrary geometry.

In many applications, one needs to measure elastic moduli of materials at low frequencies and small strains. With most techniques, such as ultrasonic pulse transmission or acoustic resonance, measurements are limited to a few discrete frequencies. These sound speed measurements are also indirect, since they involve density as well as elastic constants. We have developed a direct, quasi-static method to measure the shear modulus of solid ⁴He. A ⁴He crystal is grown in a narrow gap (0.2 to 0.5 mm) between two rigidly mounted lead zirconium titanate (PZT) piezoelectric transducers. A voltage applied to one transducer generates a shear displacement at its surface and a corresponding shear strain in the sample. The resulting shear stress at the surface of the second transducer generates a piezoelectric charge which is measured using a low noise current amplifier. The ratio between the measured stress and the applied strain gives the sample's shear modulus. Measurements can be made at any frequency between the mechanical resonances of the sample holder and cell (around 8 kHz in our arrangement) and a few Hz (a limit set by our current amplifier). At 2000 Hz, the minimum detectable stress is about 10^-5 Pa, corresponding to a strain of order 10^-12.

In many thermal systems spontaneous mechanical oscillations are generated under the influence of large temperature gradients. Well-known examples are Taconis oscillations in liquid-helium cryostats and oscillations in thermoacoustic systems. In split Stirling refrigerators the compressor and the cold finger are connected by a flexible tube. The displacer in the cold head is suspended by a spring. Its motion is pneumatically driven by the pressure oscillations generated by the compressor. In this paper we give the basic dynamic equations of split Stirling refrigerators and investigate the possibility of spontaneous mechanical oscillations if a large temperature gradient develops in the cold finger, e.g. during or after cool down. These oscillations would be superimposed on the pressure oscillations of the compressor and could ruin the cooler performance.

In the dilution refrigerator³He circulates due to its condensation in a vessel with the temperature of 0.35–0.4K. The latter is cooled by the pumping of ³He from another bath by means of a sorption pump. A temperature <0.1 K is maintained for >12 hours. The sample holder is placed in an upper part of the refrigerator and is connected with a mixer by a copper heat conductor. They are surrounded by screens at temperatures 0.4, 4.2 and ≈100K. The low temperature part is tied to the 0.4 K screen and centered by a polymer threads. A heat flux from the 0.4 K screen to the mixer is less than 0.1 μW. Inner volumes are filled with 0.2 mol of ⁴He, 0.1 mol of ³He and 0.05 mol of a mixture 40%³He+60%⁴He respectively. These gases remain all time inside the apparatus. The refrigerator is working when inserted in a 35 l transport cryostat with a liquid helium and operate during 5–6 days. The refrigerator is designed to cool down low temperature detectors or samples in experiments that do not require a high refrigerating capacity.

Thermometry over broader temperature ranges below 4 K usually requires the application of different thermometers to have sufficient precision and accuracy. We have developed dc SQUID-based noise thermometers which cover the temperature range from 4 K to below 10 mK. The working principle is based on the detection of thermal magnetic flux noise generated by noise currents in a metallic temperature sensor. The noise thermometers exhibit a linear characteristic via the Nyquist relation, are fast, compact, and easy to use. The current sensing noise thermometer and the magnetic field fluctuation thermometer we present can be designed to meet different requirements concerning speed and accuracy of the temperature measurement. In both variants the temperature sensor and the SQUID are located at the same temperature level. Experimental results will be given for the characteristics of the thermometers as well as for comparison measurements with the PLTS-2000. A commercial noise thermometer version now is available through the company MAGNICON.

An increasing number of experiments and applications employ low temperature particle detectors. Following the calorimetric detection principles, the energy released in the detector leads to a temperature increase which is measured by a very sensitive sensor. Metallic magnetic calorimeters are composed by an energy absorber, optimized for the particles to be detected, in good thermal contact with a metallic paramagnetic sensor positioned in a weak magnetic field. A change in the sensor magnetisation follows the change of the detector temperature. High energy resolution can be obtained by using a low-noise, high-bandwidth DC-SQUID to measure the corresponding change of flux. We discuss the thermodynamic properties, the energy resolution, the microfabrication and general design considerations of magnetic calorimeters as well as their application in high resolution x-ray spectroscopy, beta spectroscopy and absolute activity measurements.

Studies of quantum turbulence (QT) require a means of generating well-characterised QT. For systems containing negligible normal fluid density this must be achieved without simultaneously introducing extraneous heating due, for example, to friction between mechanical components. Methods relying on oscillating objects produce negligible extraneous heating, but suffer from the disadvantages of both spatially and temporally nonuniform velocity. In addition, the vibrating grid goes back and forth through the QT it has itself created. Hence the characteristic length scale is subject to uncertainty. We are therefore developing a one-dimensional linear motor to draw a grid steadily, once, through He-II at mK temperatures. Frictionless magnetic bearings are used to levitate the moving elements in contrast to other designs, precluding mechanical dissipation. Details of the design and of the initial tests, and a quantitative comparison of theoretical and actual cryogenic performance, will be presented and discussed.

An ultra-low temperature multi-frequency electron spin resonance (ESR) apparatus has been developed for use at a temperature down to about 200 mK by utilizing a ³He-⁴He compact dilution refrigerator and a vector network analyzer. The microwave frequency can be varied between 30 GHz and 700 GHz almost continuously. Either of the two kinds of copper blocks with a sample holder for typical two configurations, namely Faraday and Voigt configurations, is attached to a stainless steel block neighboring to a mixing chamber which provides ultra-low temperarures down to 160 mK. A magnetic field up to 16 T is produced with a superconducting magnet (14 T at 4.2 K and 16 T at 2.2 K). Thus, we have a potential to perform ESR measurements in a very wide frequency-field window at ultra-low temperatures.

We have constructed a versatile current to voltage (I-V) converter operating at liquid helium temperature, using a commercially available all-CMOS OPamp. It is valuable for cryogenic measurements of electrical current of nano-pico amperes, for example, in scanning probe microscopy. The I-V converter is thermally linked to liquid helium bath and self-heated up to 10.7 K. We have confirmed its capability of a transimpedance gain of 10⁶ V/A and a bandwidth from DC to 200 kHz. In order to test the practical use for a frequency-modulation atomic force microscope, we have measured the resonance frequency shift of a quartz tuning fork at 32 kHz. In the operation of the I-V converter close to the sensor at liquid helium temperature, the signal-to-noise ratio has been improved to a factor of 13.6 compared to the operation at room temperature.

The high pressure nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) are conventionally performed up to 3 GPa using piston cylinder cell. However, the NMR/NQR measurements beyond this pressure range are scarcely performed owing to the technical difficulty. Recently, we developed new high pressure NMR/NQR technique using cubic anvil apparatus with which highly hydrostatic pressure was obtained. Using the new method, the ⁶³Cu-NQR signal of Cu²O was observed up to 7.2GPa in nearly hydrostatic condition with high sensitivity. It was noted that the appropriate choice of pressure-transmitting medium is important for accurate experiments.

Arrays of nanoscale apertures have been found to exhibit fascinating quantum phenomena such as the Josephson effect and collective quantized phase-slippage. To be in the Josephson regime, the aperture size must be comparable to the healing length of the 4He order parameter. For 50nm apertures this regime is attained ∼mK below T_λ. Collective phase slippage occurs at considerably lower temperatures.

Fabricating aperture arrays with appropriate properties (strength, temporal stability and reproducibility) at these length scales has been a long-standing goal for our group. Here, we present the techniques used thus far, based on recent work performed at the Cornell Nanoscale Facility. We discuss some issues that arise and their possible solutions.

We have studied experimentally the nature of phonon transport in 750 nm and 200 nm thick free-standing silicon nitride (SiN_x) membranes over a temperature range of 0.1–1K. Measurements were performed by using a radially symmetric DC heater and several normal metal-insulator-superconductor (NIS) tunnel junction thermometers to detect the phonon temperature T_p at several different distances from the heater. Our data indicates that phonon transport is three dimensional in both membranes, and mostly ballistic at the low temperature range of our experiment. At temperatures above 0.5 K there are indications of a transition to a more diffusive regime.

Progress in fields such as astronomy and fundamental physics can require increasingly complex instrumentation operating at millikelvin temperatures. Such instruments often have demands on materials and components which have not been seen previously, particularly for space based instrumentation. The large scale of these projects and tight timescales forces as conservative design as possible. However, building these instruments with conventional techniques and materials is often impractical and sometimes impossible. It is therefore common for the design stage of such instruments to include test and measurement programmes. This adds risk to the development schedule, and such programmes also suffer from the problem that they are tightly focused on the exact needs of one particular instrument. We are setting up a laboratory to measure material properties and develop cryogenic components for general use in future large millikelvin instruments. By decoupling these measurements from a particular instrument programme, we have the freedom to make more speculative measurements, such as measuring new polymers whose cryogenic properties are completely unknown. We describe our set-up and the results of initial work.

The design and performance of a novel scanning Hall probe microscope for milliKelvin magnetic imaging with submicron lateral resolution is presented. The microscope head is housed in the vacuum chamber of a commercial ³He-refrigerator and operates between room temperature and 300 mK in magnetic fields up to 10 T. Mapping of the local magnetic induction at the sample surface is performed by a micro-fabricated 2DEG Hall probe equipped with an integrated STM tip. The latter provides a reliable mechanism of surface tracking by sensing and controlling the tunnel currents. We discuss the results of tests of the system and illustrate its potential with images of suitable reference samples captured in different modes of operation.

Recent increase in liquid helium cost caused by global helium supply problems rose significant concern about affordability of conventional cryogenic equipment. Luckily the progress in cryo-cooler technology offers a new generation of cryogenic systems with significantly reduced consumption and in some cases nearly complete elimination of cryogens. These cryogen-free systems also offer the advantage of operational simplicity and require less space than conventional cryogen-cooled systems. The ISIS facility carries on an internal development program intended to substitute gradually all conventional cryogenic systems with cryogen free systems preferably based on pulse tube refrigerators. A unique feature of this cryo-cooler is the absence of cold moving parts. This considerably reduces vibrations and increases the reliability of the cold head. The program includes few development projects which are aiming to deliver range of cryogen free equipment including top-loading cryostat, superconducting magnets and dilution refrigerators. Here we are going to describe the design of these systems and discuss the results of prototypes testing.

The COMPASS experiment in the CERN M2 beam line is using 1508 cm³ granular solid ¹⁴NH₃ as polarized proton target material to study the nuclear spin with deep inelastic scattering of polarized muons. The target consists of 2.5 T solenoid and 0.63 T dipole magnets, microwave cavity and large dilution cryostat. Continuous wave NMR is used to determine the polarization of the nuclei that are polarized with dynamic nuclear polarization method using 4 mm microwaves. Determination of the target proton polarization with thermal equilibrium NMR signals at temperatures 1.0 - 1.6 K is discussed.

Two variants are described of ³He-⁴He dilution refrigerators for experiments with polarized nuclei in particle beams, which often have to be performed under adverse experimental conditions. The first is used for experiments in a beam of cold neutrons that do not allow any ³He on their beam path. The sample is therefore indirectly cooled by liquid ⁴He.

The second variant is used for experiments in a beam of heavy nuclei that only allow a very thin film of material, in this case ⁴He, to cool a thin polarized polystyrene target. Such a polarized target is a powerful spectroscopic tool to investigate nuclear structure and reaction mechanisms, which is of interest for the study of nuclei far from stability.

We have investigated electronic cooling of suspended nanowires with SINIS tunnel junction coolers. The suspended samples consist of a free standing nanowire suspended by four narrow (∼ 200 nm) bridges. We have compared two different cooler designs for cooling the suspended nanowire. We demonstrate that cooling of the nanowire is possible with a proper SINIS cooler design.

At low temperatures helium crystals can grow and melt so fast that a melting-freezing wave may propagate along the liquid-solid interface. In ⁴He these crystallization waves have been observed at temperatures below 0.5K. The required temperature for the observation of the crystallization waves in ³He is predicted to be about 0.2 mK. In order to reach such low temperature at the melting pressure of ³He, special care has to be taken in the design of the experimental cell. Here we present the draft of the experimental cell which is designed for observation of the crystallization waves in ³He.

In order to measure the Cosmological Microwave Background (CMB), high performance "bolometric cameras" similar to CCDs are currently developed. They are made out of thousands of pixels, each of which is a bolometer on its own. In order to meet the requirements for future CMB experiments - notably the measurement of the CMB B-mode polarization - the sensitivity of each pixel should be improved by one or two orders of magnitude compared to what now exists. Taking advantage of the solid-state properties of amorphous Nb_xSi_1-x thin films, we here present a proposal for a new bolometer structure that would increase the pixels' sensitivity, its response time and allow a simplification of the fabrication process. In this resistive detector (that can be either high impedance or TES) the three functions of a classical bolometer (wave absorption, temperature measurement and thermal decoupling) are achieved in a single Nb_xSi_1-x film. The frequency properties of this material allow the merger of the two first functions. The natural thermal decoupling between electrons and phonons at low temperature then makes it possible to use this single object as bolometer. This new type of detector solely uses the electronic properties of the Nb_xSi_1-x thin films and is free of any phononic mediation of the energy.

Magnetic refrigeration which is based on the magnetocaloric effect of solids has the potential to achieve high thermal efficiency for hydrogen liquefaction. We have been developing a magnetic refrigerator for hydrogen liquefaction which cools down hydrogen gas from liquid natural gas temperature and liquefies at 20 K. The magnetic liquefaction system consists of two magnetic refrigerators: Carnot magnetic refrigerator (CMR) and active magnetic regenerator (AMR) device. CMR with Carnot cycle succeeded in liquefying hydrogen at 20K. Above liquefaction temperature, a regenerative refrigeration cycle should be necessary to precool hydrogen gas, because adiabatic temperature change of magnetic material is reduced due to a large lattice specific heat of magnetic materials. We have tested an AMR device as the precooling stage. It was confirmed for the first time that AMR cycle worked around 20 K.

A high-speed and a high-sensitive laser scanning magneto-optical (MO) imaging system have been developed. In the high-speed imaging mode, we have succeeded in almost the real time observation of MO images with the sensitivity better than 100 μT. On the other hand, in the high-sensitive mode using an acoustic-optic modulator and a RF lock-in amplifier, it was found that the system noise reduced into the magnetic field strength was less than 6 μT at the data acquisition time of 10 μs. In this mode we have also succeeded in the fast MO imaging of the magnetic flux penetration into a square-shaped YBa₂Cu₃O_7-δ n film at ∼0.3 s/frame for 128x128 and ∼5 s/frame for 500x500 pixels' image.

We have succeeded to construct a novel ³He cryostat by improving a low cost commercial two stage Gifford-McMahon (GM) refrigerator whose cooling power is 0.3 W at 4.2 K. This main portion of this system consists of 4 K pot, 1 K pot, heat exchanger and ³He pot, which are connected to the second cold stage. The main portion is covered with the 1st radiation shield attached to the 1st cold stage and the ³He pot is covered with the 2nd radiation shield attached to the heat exchanger, where the 1st shield must be further covered with more than five layers of super-insulation films in order to realize ³He temperature. The achieved temperature is 0.4 K and persists over one day. The temperature oscillation is less than 1 mK below 4 K. The required time to attain the lowest temperature is typically about 8 hours. This system allows us to deal with almost all the low temperature experiments of condensed matter physics without requiring low temperature technique.

To study the nanoscale electronic order in strongly correlated electron systems and vortex states in high-T_c superconductors in high magnetic fields, we have developed scanning tunneling microscopy (STM) for the 18 T cryocooled superconducting magnet (18T-CSM). The test results of the STM operation in the 18T-CSM at room temperature indicate that our STM has a good atomic resolution up to 18 T when we use the nonmagnetic vibration-isolation table which reduce the vibration noise from the cryocoolers of the 18T-CSM. In this paper, we report on the design of the high-field STM system for large-scale magnets and its performance.

Adiabatic Demagnetization Refrigeration (ADR) does not use working fluids contrary to conventional refrigerators that make use of the fluid density difference, which leads to superiority of the ADR under the weak gravity condition. In this study, we developed a continuous ADR system to provide constant cooling temperatures ∼0.1 K. The system consists of four stages of magnetic materials and magnets cascaded with heat switches. The magnetic materials CPA and GdLiF₄ are used for 3 stages between 0.1K and 1.4 K, and single stage between 1.4 K and 4 K, respectively. Passive heat switches are used for the stages > 0.3 K and a superconducting heat switch is used for the continuous stage at ∼0.1 K. A G-M cycle cooler with a 100 V compressor unit is used to cool the ADR and cryostat shieldings. Total mass of flight model is less than 60 kg. Cooling tests with Transition Edge Sensor on the ground showed that the ADR provided continuous cooling temperatures between 105 mK and 120 mK and it successfully operated the TES. Airborne flight experiments confirmed the ability of the cooling system under the mili-gravity condition. The experimental results showed that the ADR could provide stable temperature under the weak gravity, however, strong vibrations coming from turbulence or takeoff affected to the stability of ADR cycle.

Basic characteristics of micro-SQUIDs (superconducting quantum interference devices) with very small tunnel junctions are presented. Through electron-beam lithography, angled deposition and lift-off, we have made aluminum SQUIDs, the loop size of which was from 2μm to 20μm. The Al-AlOx-Al tunnel junctions were 0.15 μm× 0.15 μm in size and the parallel resistance was of the order of 10 kΩ Transport measurement at low temperatures down to 30 mK reveals the following features of the device: (1) It does not show dissipationless supercurrent, i.e. the zero-bias resistance remains finite even at the lowest temperature. (2) The resistance varies periodically as a function of the magnetic fields. When the field is perpendicular to the SQUID loop plane, the period corresponds to one flux quantum through the loop. (3) In almost-parallel magnetic fields, the resistance oscillations continue up to about 0.9T, though the amplitude diminishes with the magnetic field. Thus, the device is applicable for the magnetic sensing in moderate magnetic fields. Noteworthy feature is that the power dissipation necessary for operation is extremely small, i.e., of the order of 1 fW.

A new probe has been developed that allows for both optical irradiation and uniaxial rotation, all in the low temperature environment of a commercial superconducting quantum interference device (SQUID) magnetometer. As part of the design process, various materials were investigated and characterized for their low temperature structural and magnetic properties, including nylon, Vespel, Delrin, Spiderwire monofilament, and PowerPro braided microfilament. Using this information, a prototype was built and operated. Characteristics of the probe will be presented along with a summary of the low temperature (T ≥ 2 K) and high magnetic field (H ≤ 7 T) properties of the construction materials.

A commercially available SQUID (Superconducting QUantum Interference Device) DC magnetometer is often limited by its relatively high temperature (≥ 1.9 K) and low magnetic field (≤ 7 T) operating environment. The need for the lower temperature and higher field DC magnetization measurements keeps growing as more materials show interesting physical phenomena with relevant energy scales that require millikelvin temperatures. To meet these needs we have developed a SQUID DC magnetometer which operates in the top loading dilution refrigerator of a 16 T superconducting magnet. An essential part of this low temperature and high field SQUID magnetometer is a specialized probe which can adapt the SQUID electronics and low friction mechanical sample shaft. The details of magnetometer probe and preliminary testing results are described in this paper.

The basis for worldwide uniformity of low and ultra-low temperature measurements is provided by two international temperature scales, the International Temperature Scale of 1990 (ITS-90) for temperatures above 0.65 K and the Provisional Low Temperature Scale of 2000 (PLTS-2000) for temperatures in the range 0.9 mK to 1 K. Over the past 10 years, the thermometry research in the Netherlands provided substantial contributions to the definition, realization and dissemination of these scales. We first give an overview of the Dutch contributions to the ITS-90 realization: a) ³He and ⁴He vapour pressure thermometer range of the ITS-90, 0.65 K to 4 K (1997), b) ⁴He interpolating constant volume gas thermometry for the ITS-90 range 3 K to 24.5 K (2007) and c) cryogenic fixed points for the ITS-90 range 13.8 K to 273.16 K (2005). Then we highlight our work on ³He melting pressure thermometry from 10 mK to 1 K (2003) to support the dissemination of the PLTS-2000. Finally we present the current status of the Dutch calibration facilities and dissemination devices providing for traceable low and ultra-low temperatures for use in science and industry: a) the NMi-VSL cryogenic calibration facility for the range 0.65 K to 273.16 K and b) the SRD1000 superconductive reference devices for the range 10 mK to 1 K.

We have developed a novel procedure for constructing high surface area silver sinter heat exchangers. Our recipe incorporates nylon fibers having a diameter of ∼50 μm and thin wires of bulk silver in the heat exchanger. In order to increase the thermal conductance of liquid helium within the heat exchanger, prior to sintering, the nylon fibers are dissolved with an organic acid leaving a network of channels. In addition, the silver wires reinforce the structural integrity, and reduce the resistance, of the silver sinter. We have constructed a ³He melting curve thermometer (MCT) with this type of heat exchanger and measured the thermal time response of the liquid ³He inside the MCT in the temperature range T ≈ 2 — 150 mK. We find a thermal relaxation time of ∼490 s at T ≈1 mK. We have used scanning electron microscopy (SEM) to characterize the heat exchanger and BET absorption for determination of the specific surface area.

We report experimental results on the free cooling power available at the level of the second stage regenerator of a 4K pulse tube cooler. By using two localised heat exchangers we obtained additional cooling power, in the range 400 and 600 mW at 4.8 K or between 500 and 700 mW at 18 K. We have investigated in detail the thermal behavior of the system. In this manuscript we report on the evolution of the temperature of the heat exchangers and the pulse tube stages under different distributions of the total heat load.

A frequency modulation-atomic force microscope (FM-AFM) designed to measure atomic scale topography at low temperatures is developed. Piezoelectric quartz tuning forks are employed as a force sensor for low temperatures use. In order to perform high-resolution measurements, detection of attractive forces between a tip and a sample is important. Measurements of attractive van der Waals forces on a SrTiO₃ substrate is successfully performed down to 4.2 K by minimizing an amplitude of the tuning fork. Topographic imaging of atomic steps of the SrTiO₃ substrate is also achieved at room temperatures in vacuum (10^-3 Pa).

Below 2 K the speed of second sound in mixtures of liquid ³He and ⁴He first increases to a maximum of 30–40 m/s at about 1 K and then decreases again at lower temperatures to values below 15 m/s. The exact values depend on the concentration and pressure of the mixture. This can be exploited to provide fixed points in temperature by utilizing a resonator with appropriate dimensions and frequency to excite standing waves in the resonator cavity filled with helium mixture. We demonstrate that commercially mass produced quartz tuning forks can be used for this purpose. They are meant for frequency standards operating at 32 kHz. Their dimensions are typically of order 1 mm matching the wavelength of the second sound in helium mixtures at certain values of temperature. Due to the complicated geometry, we observe some 20 sharp acoustic resonances in the range 0.1ℓ 2 K having temperature resolution of order 1 μK. The quartz resonators are cheap, compact, simple to implement, easy to measure with great accuracy, and, above all, they are not sensitive to magnetic field, which is a great advantage compared to fixed point devices based on superconductivity transitions. The reproducibility of the resonance pattern upon thermal cycling remains to be verified.

We developed a new method to design linear field gradient coils wound inside of a superconducting shield, which is attached to the outside of a superconducting solenoid. A compact palm-sized MRI magnet is constructed under these design and is tested to have enough performance.

CUORE is a new generation of 1-ton scale cryogenic detector for rare-events physics. CUORE, a detector to search Neutrinoless Double Beta Decay of ¹³⁰Te, is an array of 988 TeO₂ crystals of a mass of 750 g each. To build the cryogenic system, where the CUORE detector will be installed in the Gran Sasso Underground Laboratory, is really a challenge. It is a large cryogen-free cryostat cooled by pulse tubes and by a high power dilution refrigerator. To avoid radioactive background, about 10000 kg of lead will be cooled to below 1 K and only few construction materials are acceptable. the detector will have a total mass of about 1500 kg and must be cooled to less than 10 mK in a vibration-free environment.

Sensitivity of the capacitive method for determining the melting pressure of helium can be enhanced by loading the empty side of the capacitor with helium at a pressure nearly equal to that desired to be measured and by using a relatively thin and flexible membrane in between. This way one can achieve a nanobar resolution at the level of 30 bar, which is two orders of magnitude better than that of the best gauges with vacuum reference. This extends the applicability of melting curve thermometry to lower temperatures and would allow detecting tiny anomalies in the melting pressure, which must be associated with any phenomena contributing to the entropy of the liquid or solid phases. We demonstrated this principle in measurements of the crystallization pressure of isotopic helium mixtures at millikelvin temperatures by using partly solid pure ⁴He as the reference substance providing the best possible universal reference pressure. The achieved sensitivity was good enough for melting curve thermometry on mixtures down to 100 μK. Similar system can be used on pure isotopes by virtue of a blocked capillary giving a stable reference condition with liquid slightly below the melting pressure in the reference volume. This was tested with pure ⁴He at temperatures 0.08–0.3 K. To avoid spurious heating effects, one must carefully choose and arrange any dielectric materials close to the active capacitor. We observed some 100 pW loading at moderate excitation voltages.

We discuss the performance of the automated heat conductivity measurement system manufactured by the Quantum Design, Inc. The Thermal Transport Option implemented into the Physical Properties Measurement System (PPMS) measures the thermal transport properties of materials (thermal conductivity, Seebeck coefficient and electrical resistivity simultaneously) in the temperature range 1.8 - 395 K and in magnetic fields generated by the installed superconducting solenoid. Recently, discrepancies up to 30% in measured quantities at 390 K have been reported. We critically analyze the experimental method used to measure the above mentioned quantities and show possible sources of problems.

In order to enable AC magnetometry with the ³He insert that has been designed for Quantum Design's MPMS SQUID magnetometer, we have calibrated the phase shift caused by the insert. Using gadolinium gallium garnet (GGG) as a zero-phase reference, we have succeeded in determining the phase contribution from the ³He insert down to about 0.5K, below which GGG develops a spin-glass-like transition. This means that AC magnetometry is now feasible down to 0.5 K for frequencies at least up to 100Hz on the common MPMS platform. At lower frequencies (≤ 1 Hz) virtually no phase shift is added by the ³He insert to the existing shift due to the MPMS sample tube. The sensitivity of the AC magnetization amplitude is estimated to be about 2 x 10^-7 emu, although the AC susceptibility of the order of 1 x 10^-4 emu shifted by ca. -2 x 10^-6 emu when it was measured with the ³He insert, as compared to the normal measurement. This development is expected to facilitate studies of time-dependent magnetic phenomena, such as spin-glass transitions and quantum magnetization tunnelings of single-molecule magnets.

Two new Janis dilution refrigerators for scanning tunnelling microscopy (STM) in ultra-high vacuum (UHV) and magnetic field up to 15 Tesla are presented. Their design details, including differences between top and bottom sample loading features are described. Base temperature, cooling power, internal vacuum and other experimental results are discussed.

We have developed two custom designed bottom loading He-3 cryostats with sample in UHV space for basic research. The first cryostat is a single-shot, bottom loading system for Scanning Tunnelling Microscopy (STM) applications with a number of design enhancements. The second cryostat is a continuous flow system with relatively high cooling power that is developed for sub-K Angular Resolution Photoemission Spectroscopy (ARPES). Design and performance details will be discussed.

Quartz tuning forks mass-produced as frequency standards for watches proved to serve as very useful tools for generating and probing flows of gaseous and liquid helium. Their cryogenic use as thermometers, pressure- and viscometers as well as generators and detectors of cavitation and turbulence has been recently widely discussed in the literature [JLTP 136, 1 (2004); 146, 537 (2007); 150, 525 (2008)]. Here we report our preliminary experiments where the vibrating fork is used to detect the externally applied flow in superfluid ⁴He.

We report on the thermoelectric properties of FeSb₂ in comparison with the iso-structural RuSb₂, both of which are narrow-gap semiconductors. Significant difference in the thermoelectric properties is found between the two systems. Despite similar charge carrier concentrations, the thermoelectric power factor differs as much as two orders of magnitude whereas the thermal conductivity is of the same order. The thermopower of strongly correlated FeSb₂ seems to possess a huge contribution due to correlated electrons that are absent in RuSb₂.

We report a Helium Circulation System (HCS) that re-liquefies all the evaporating helium gas, consumes far less power and has extremely lower magnetic noise compared with conventional systems. It collects warm helium gas about 300 K, cools it to about 40K and returns it to the neck tube of the Dewar to keep it cold. It also collects helium gas just above the liquid helium surface while it is still cold, re-liquefies and returns it to the Dewar. A special transfer tube (TT) about 2 m length with 7 multi-concentric pipes was developed to allow the dual helium streams. It separates the HCS with a MEG to reduce magnetic noise. A refiner to collect the contaminating gases such as oxygen and nitrogen effectively by freezing the gases is developed. It has an electric heater to remove the frozen contamination in the form of gases into the air. A gas flow controller is also developed, which automatically control the heater to cleanup the contamination. The developed TT has very low heat inflow less than 0.1W/m to the liquid helium ensuring the efficient operation. The HCS can re-liquefy up to 35.5 1/D of liquid helium from the evaporated helium gas using two 1.5W@4.2K GM cryocoolers (SRDK-415D, Sumitomo Heavy Industries, Ltd.). It has been confirmed that the HCS could be used with the real MEG system without any noise problem for over one year. The maintenance cost (electricity charges and cryocoolers maintenance fee) of the MEG has reduced to be less than 1/10 of the previous cost.

A superconducting transition edge thermosensor (TES) microcalorimeter was cooled by a compact liquid-helium-free ³He-⁴He dilution refrigerator with loading a Gifford-McMahon (GM) cooler for detection of LX-ray photons emitted from an ²⁴¹Am source. The first and second stages of the GM cooler are directly coupled with the first and the second precool heat exchangers of a stick shaped dilution unit through copper plates in the vacuum chamber, respectively. The circulating ³He-⁴He gas through the precooled heat exchangers is condensed into a liquid of condense mixture by the isoenthalpic expansion through the Joule-Thomson impedance. A cascade of two mixing chambers are employed for achieving sufficient cooling power. The helium-free dilution refrigerator performs the cooling power of 20 μW at 100 mK. The TES and SQUID chips suffered from mechanical vibrations induced by a reciprocating motion of the displacer of the GM cooler. Detection signals of LX-ray photons emitted from ²⁴¹Am source were observed by operating the TES microcalorimeter in severe noise environment induced by mechanical vibrations.

We have constructed a novel compact cryostat for optical measurements at temperatures below 2 K. The desktop cryostat, small enough to be placed under the objective of a standard commercial polarized light microscope, functions in a single shot mode, with a five hour autonomy at 1.5 K. Central to its conception are four charcoal pumps for adsorption and desorption of He contained in a closed circuit, and novel thermal switches allowing for thermalization of the pumps and of the two 1 K pots. The latter are connected to the 1" diameter sample holder through braids. Sample access is immediate, through the simple removal of the optical windows. In this contribution, we shall present first results on magneto-optical imaging of flux penetration in the heavy-fermion superconductor CeCoIn₅.

We have developed two small-scale helium liquefaction systems that provide solutions for liquid helium usage in laboratories. These helium liquefaction systems use two-stage pulse tube cryocoolers to provide cooling at 4 K. The cold head/liquefier resides inside of the neck of a dewar. The room temperature helium gas to be liquefied enters the neck of the dewar and is efficiently pre-cooled down to 5–6 K by means of the regenerators and pulse tubes of the cold head before being liquefied. Two models of liquefaction system, LHeP12 and LHeP18, produce liquid helium from room temperature gas with the rates of >12 L/day and 18 L/day.

A sensitive Straty-Adams type strain gauge has been developed for precise pressure measurements at ultra-low temperatures and high magnetic fields. The new design, without using any plastic material for the insulation spacer, can avoid the remarkable pressure dependence at low temperatures observed in previous designs. The sensitivity can be easily modified by changing the thickness between the two diaphragms without reconstruction of the sensor after it has been made. The sensitivity, accuracy and reproducibility have been tested at low temperatures and in high magnetic fields. The general design can be applied to a variety of precise pressure measurements.

We developed the new liquid-helium-free dilution refrigerator system, in which the Gifford-McMahon (GM) cycle cryocooler and dilution refrigerator (DR) unit are separated. We obtained the base temperature below 50 mK in this DR system. In usual liquid-helium-free DR systems, the DR unit directly couples with GM-cryocooler in the same vacuum chamber. Therefore the mechanical vibration of GM-cryocooler is hardly removed from DR unit. In order to eliminate the vibration problem, the separated vacuum chamber contacting the GM-cryocooler is connected with the DR unit chamber by the flexible hose with length of about 1 meter. Thin flexible tubes used for circulation of the refrigerant gas and radiation shield are installed in the connection hose. The ⁴He gas, cooled in the GM-cryocooler unit, transfers to the DR unit throw the thin flexible tubes. After cooling the DR unit, the gas returns to GM-cryocooler unit with cooling of the radiation shield. We expect that our separate-type dilution refrigerator becomes a useful piece of apparatus for the low temperature experiments.

A cryogen-free dilution refrigerator equipped with an 8 T superconducting magnet is developed. The dilution unit and the magnet are cooled together by a common Gifford-McMahon refrigerator in the same vacuum chamber. A heat flow from room temperature terminals through a pair of copper current leads is well absorbed by thermal anchors connected to a 1st stage of the GM refrigerator. Between the 1st stage and the 2nd stage, a pair of flexible high-T_C DyBCO coated conductors is used as a material for the current leads to reduce the heat leakage to the magnet. The dilution cycle is not affected by coexistence of the magnet, because the heat load imposed by the magnet is much less than that by the dilution unit. When a quench event happens, the temperatures of the magnet and a 4 K plate of the dilution unit rise rapidly up to 20 K, but safely recover in 18 min without causing serious trouble in the dilution process.

In order to study the vortex dynamics in Josephson junction arrays under temperature perturbations we have build Pb/Cu structures on top of Si₃N₄ membranes. Membranes with an area of 1 mm² and with a thickness of 650 nm were fabricated by anisotropic etching of a silicon substrate previously covered with a low stress nitride film. By e-beam lithography and thermal evaporation we defined two similar SNS Josephson arrays one on top of the membrane and another on the bulk substrate. The heat source was provided illuminating the sample space with a conventional LED placed a few millimeters away from the substrate. Using the superconducting transition of both arrays the thermal response of the system has been characterized obtaining a cut-off frequency of 150 Hz and the power efficiency of our setup.