mK Temperatures Research Articles

Custom-built research refrigerators for ultra-low temperature

The Janis line of ultra-low-temperature research refrigerators is presented. Cryogen-containing and cryogen-free 3He, 3He–4He dilution and adiabatic demagnetization refrigerators for the mK temperature range are described in detail and their possible use for cooling low-temperature sensors and detectors is discussed. A special emphasis is made on the Janis temperature scale down to 10mK, maintained with a newly designed compact CMN thermometer and superconducting fixed point device.

Massive triplet excitations in a magnetized anisotropic Haldane spin chain

Inelastic neutron scattering experiments on the Haldane-gap quantum antiferromagnet $\mathrm{Ni}({\mathrm{C}}_{5}{\mathrm{D}}_{14}{\mathrm{N}}_{2}{)}_{2}{\mathrm{N}}_{3}({\mathrm{PF}}_{6})$ are performed at mK temperatures in magnetic fields of almost twice the critical field ${H}_{c}$ applied perpendicular to the spin chains. Above ${H}_{c}$ a reopening of the spin gap is clearly observed. In the high-field N\'eel-ordered state the spectrum is dominated by three distinct excitation branches. A theoretical model consistently describing the experimental data is proposed.

Solar active regions: The footpoints of 1 MK loops

On-disc SOHO and TRACE observations of the footpoints of 1 MK quiescent loops in a solar active region are presented. These types of loops are long-lived and high-lying features that are best seen in the TRACE 173 A passband, sensitive to 1 MK temperatures. SOHO/CDS data are used to show the clear association between the high-lying 1 MK coronal emission and emission formed at lower heights and temperatures, in the transition region. The 1 MK loops are rooted in strong unipolar regions located at the supergranular network boundaries. These loops have near-isothermal distributions at each location along their length, with peak emission measures at T = 0.7 MK, where electron densities are ∼2 x 10 9 cm - 3 . The loops showed photospheric abundances, at odds with previous results based on Skylab data.

Sound propagation in coexistent Bose and Fermi superfluids in aerogel.

We report the first observation of longitudinal sound propagation in three dimensionally distributed Bose and Fermi superfluids in an acoustic investigation of phase separated 3He-4He mixtures confined to aerogel. At mK temperatures, this inhomogeneous system exhibits simultaneous 3He and 4He superfluidity leading to two "slow modes" along with the conventional sound mode. We also infer the superfluidity of isolated bubbles of pure 3He in a large 4He concentration sample.

He 4 nanoclusters around vacancies in solid solutions of He4 in He3

Precise measurements of the pressure in solid solutions of 4He in 3He are performed with temperature cycling in the phase-separation range. It is found that as a result of such cycling the difference ΔP of the pressures between the minimum (≈100 mK) and maximum (≈200 mK) temperatures decreases by a factor of approximately 3 and then remains unchanged for a long time. The initial values of ΔP and the pressure P0 in a uniform solid solution 4He-3He are restored only after the sample is heated to a temperature substantially above the phase-separation point. The data obtained are explained on the basis of the hypothesis that under these conditions clusters of pure 4He form around quasiequilibrium vacancies. A thermodynamic calculation of such clusters is performed. The computational results made it possible to give a quantitative explanation of the distinctive features observed. Comparing the calculations with experiment made it possible to determine the cluster concentration, 8×10−5, and the cluster radius, which is 3–4 interatomic distances at low temperatures. The characteristic equilibration time under these conditions is determined, and it is suggested that processes occurring at cluster boundaries determine its value.

Superfluidity and quantized vortex studies under rotation up to [formula omitted] at mK and [formula omitted] at sub-mK temperatures

Two new high-speed rotating cryostats at the ISSP, University of Tokyo, capable of operating at mK temperatures, are currently employed for studies of superfluidity and quantized vortex states in 4 He and 3 He . A stable high- Q torsional oscillator is used for the study of superfluidity in low-density 4 He films adsorbed on 3-D connected porous substrates. At T c, the critical phenomena of 3-D Bose superfluids is seen. Under rotation a new type of vortex state is observed. High-resolution NMR studies of superfluid 3 He , contained in capillary arrays of two different diameters, are made as a function of rotational speed. In these experiments, single vortex intrusion events as well as texural modification under rotation are clearly resolved.

Superfluid He in Rotation: Single-Vortex Resolution and Requirements on Rotation

The present generation of rotating refrigerators, which are used for the study of quantized vorticity in helium superfluids, are often capable of cooling to mK temperatures and rotate up to ∼ 1 rev/s. To achieve single-vortex resolution at all rotation velocities, smooth and stable rotation is required. This calls for low and stable rotational friction, sufficient axial alignment and lateral balancing of weights, to promote mechanical stability and precession-free rotation. We comment on general design principles, discuss the rotation noise spectrum of our installation, and describe interference problems from the rotation in low-noise measurement.

Spin accumulation in ferromagnetic–superconductor–ferromagnetic double-barrier junctions

We have investigated experimentally the spin transport properties in ferromagnetic–superconductor–ferromagnetic double-barrier tunnel junctions. The transport measurements were conducted at mK temperatures with magnetic fields applied along the leads. When the magnetic moments of the two ferromagnetic leads were in opposite orientations (i.e. antiferromagnetic alignment), majority of the spins accumulated in the superconducting island. This was evidenced by a suppression of the superconducting gap.

Intermittent Switching between Potential Flow and Turbulence in Superfluid Helium at mK Temperatures

Superfluid flow around an oscillating microsphere is investigated at temperatures down to 25 mK. Stable laminar flow below a critical velocity and turbulence at large drives are found to be separated below 0.5 K by an intermediate range of driving forces where the flow is unstable, intermittently switching between laminar and turbulent phases. We have recorded time series of this switching phenomenon and have made a statistical analysis of the switching probability. The mean lifetime of the turbulent phases grows with increasing drive and becomes infinite at a critical value. Stability of the laminar phases above the critical velocity is limited by natural background radioactivity or cosmic rays.

Optical dephasing in terrylene-doped polymer microspheres

We present a progress report on investigations of dye-doped microstructures. Motivated by the effect that a confined sample volume may have upon the optical linewidths of dye doped into a small sample, we studied polystyrene microspheres (300 nm in diameter) doped with terrylene. Terrylene doped into polystyrene microspheres has a quantum efficiency of hole burning of ∼10 −6. The hole burning also has a very efficient light-induced back reaction. The temperature dependence of the hole width in the microsphere sample has a greater slope as compared to the bulk sample indicating a higher concentration of two level systems associated with the high surface area to volume ratio. The microsphere sample also shows a plateau in the hole widths at low (<200 mK) temperature. The narrowest hole which could be burnt at these temperatures was ∼350 MHz wide. This width is considerably broader than expected from the lifetime-limited value.

Magnetization ofSn1−xGdxTe

The magnetization of ${\mathrm{Sn}}_{1\ensuremath{-}x}{\mathrm{Gd}}_{x}\mathrm{Te}$ for different concentrations of holes and x values up to 0.1 has been measured by means of both the vibrating sample magnetometer and cantilever techniques. The cantilever measurements were carried out at mK temperatures. The strength of the pair exchange interaction, $\ensuremath{-}{J}_{P}{/k}_{B},$ was found to depend on the concentration of holes with a maximum value of about 1 K in the ``resonant'' concentration range between $2.6\ifmmode\times\else\texttimes\fi{}{10}^{20} {\mathrm{cm}}^{\ensuremath{-}3}$ and $4.2\ifmmode\times\else\texttimes\fi{}{10}^{20} {\mathrm{cm}}^{\ensuremath{-}3}.$ No magnetization steps were observed even at temperatures of 30 mK. We suggest that this absence of steps is a result of the long range of the exchange interaction in this system.

Hard correlation gap observed in quench-condensed ultrathin beryllium.

We report on the tunneling density of states (DOS) in strongly disordered ultrathin Be films quench condensed at 20 K. Above 5 K, the DOS shows the well-known logarithmic anomaly at the Fermi level. Only in a narrow temperature range near 2 K is the DOS linearly dependent on energy, as predicted by Efros and Shklovskii. However, both the zero-bias conductance and the slope of the linear DOS are found to decrease drastically with decreasing temperature. Tunneling measurements at mK temperatures have revealed conclusively that a hard correlation gap opens up in the DOS.

The Physical Basis of Luminosity Classification in the Late A-, F-, and Early G-Type Stars. I. Precise Spectral Types for 372 Stars

This is the first in a series of two papers that address the problem of the physical nature of luminosity classification in the late A-, F-, and early G-type stars. In this paper, we present precise spectral classifications of 372 stars on the MK system. For those stars in the set with Stromgren uvbyβ photometry, we derive reddenings and present a calibration of MK temperature types in terms of the intrinsic Stromgren (b-y)0 index. We also examine the relationship between the luminosity class and the Stromgren c1 index, which measures the Balmer jump. The second paper will address the derivation of the physical parameters of these stars, and the relationships between these physical parameters and the luminosity class. Stars classified in this paper include one new λ Bootis star and 10 of the F- and G-type dwarfs with recently discovered planets.

Hydrated Cr(V) Peroxychromates M3CrO8·XH2O (M = Li, Na, Cs): Model 3d1Systems Exhibiting Linear Chain Behavior and Antiferromagnetic Interactions

A new class of Cr(V) peroxychromates, those containing waters of hydration (Li 3- CrO 8 .10H 2 O, Na 3 CrO 8 .14H 2 O and Cs 3 CrO 8 .3H 2 O), has been synthesized and characterized by single-crystal X-ray diffraction, magnetic susceptibility, specific heat, and EPR techniques. The Li salt crystallizes into the orthorhombic (Cmcm) group, whereas the Na and Cs salts exhibit the triclinic (P1) and monoclinic P2(1)/n space groups, respectively. Crystal structure analysis indicated the availability of low-dimensional spin-exchange pathways. The orientation dependence of the EPR line width exhibited the (3 cos 2 θ - 1) 4/3 behavior characteristic of linear chain compounds, with θ = 0 coinciding with the chain axis. Correlation of the EPR and X-ray data allowed for the explicit determination of the magnetic chain axes in the crystal and molecular frameworks. Magnetic susceptibility, X , measurements on oriented single crystals yielded negative values for the Curie-Weiss temperatures, indicating dominant antiferromagnetic interactions. The X data yielded the Curie constants that appear to be inversely related to the number of waters of hydration in the crystal structure; this observation is not yet understood. Correlation of the EPR line width, nearest neighbor distances, and Curie constants attested to the essential correctness of the linear chain spin exchange and dipolar fields in the Li and Cs salts, and to a lesser extent also for the Na salt. The specific heat data indicated that all these compounds have the potential to be useful for devices requiring strong heat sinks as well as magnetic refrigerants in the 250 mK temperature regime. They appear to be good as model systems for theoretical investigations on low-dimensional magnetic lattices and for understanding aspects of Cr(VI)-based catalysis and carcinogenesis.

Measurements of discrete electronic states in a gold nanoparticle using tunnel junctions formed from self-assembled monolayers

We present results from nanometer-scale tunnel junctions fabricated using organic self-assembled monolayers (SAMs) as tunnel barriers. Single junctions have resistances consistent with tunneling through a single layer of molecules. In several devices, a gold nanoparticle nucleated within the SAM barrier. Such samples allow a sensitive test of mechanical stability—some are sufficiently stable to permit the observation of electron tunneling through individual electron-in-a-box states at low voltages and mK temperatures. We also observe anomalous transport characteristics at larger voltages, which may be due to the motion of the gold nanoparticle within the monolayer.

Dynamics of vortex tangle without mutual friction in superfluid4He

A recent experiment has shown that a tangle of quantized vortices in superfluid $^4$He decayed even at mK temperatures where the normal fluid was negligible and no mutual friction worked. Motivated by this experiment, this work studies numerically the dynamics of the vortex tangle without the mutual friction, thus showing that a self-similar cascade process, whereby large vortex loops break up to smaller ones, proceeds in the vortex tangle and is closely related with its free decay. This cascade process which may be covered with the mutual friction at higher temperatures is just the one at zero temperature Feynman proposed long ago. The full Biot-Savart calculation is made for dilute vortices, while the localized induction approximation is used for a dense tangle. The former finds the elementary scenario: the reconnection of the vortices excites vortex waves along them and makes them kinked, which could be suppressed if the mutual friction worked. The kinked parts reconnect with the vortex they belong to, dividing into small loops. The latter simulation under the localized induction approximation shows that such cascade process actually proceeds self-similarly in a dense tangle and continues to make small vortices. Considering that the vortices of the interatomic size no longer keep the picture of vortex, the cascade process leads to the decay of the vortex line density. The presence of the cascade process is supported also by investigating the classification of the reconnection type and the size distribution of vortices. The decay of the vortex line density is consistent with the solution of the Vinen's equation which was originally derived on the basis of the idea of homogeneous turbulence with the cascade process. The obtained result is compared with the recent Vinen's theory.

A liquid-He-free dilution refrigerator for neutron scattering

A liquid-He-free dilution refrigerator, ‘mK cryocooler’, for neutron scattering research was developed. The mK cryocooler is composed of a pulse tube cryocooler as 4 K stage and a Joule–Thomson type dilution refrigerator as mK stage. The lowest sample temperature obtained in neutron scattering experiments was 41 mK. An automatic cooling operation was developed. We expect that any researcher is able to carry out neutron scattering experiments at mK temperatures with good movability, easy handling, low running cost, stable and continuous operation, and no neutron beam loss.

Decay of vortex tangle in superfluid 4He at very low temperatures

Recently, the Lancaster group observed the free decay of vortex tangle in superfluid 4He at mK temperatures. Since the system at such low temperatures is free from the normal fluid and the usual mutual friction, the mechanism of the free decay is unknown. In order to understand these phenomena, this work studies numerically the vortex dynamics without the mutual friction. The absence of mutual friction prevents the vortex from smoothing. The resulting kinked structure promotes vortex reconnection, thus making lots of vortex loops with small size. Such cascade process as breakup of vortices to smaller ones and the nonlocal interaction can decay the length of vortices.

Hyperfine enhanced nuclear antiferromagnetism in PrS

Compared to unenhanced nuclear magnets, e.g. Ag, Cu, AuIn 2, which undergo magnetic order at pK, nK, and μK temperatures, hyperfine enhanced nuclear magnets offer the possibility to study spontaneous nuclear magnetic ordering effects up to mK temperatures. However, the archetypal systems, PrNi 5 (T c =0.4 mK) and PrCu 6 (T c =2.7 mK) are restricted in their magnetic variety due to their clear tendency towards ferromagnetism. Here we present measurements of the dynamic susceptibility of the cubic Van Vleck paramagnet PrS at ultralow temperatures, 60 μK⩽T⩽100 mK , which give clear hints for an antiferromagnetic ordering transition of the hyperfine enhanced Pr nuclear moments at T N =90 μK . The nuclear fcc spin- 5 2 antiferromagnet PrS is a promising candidate for interesting spin structures.

Electromagnetically induced transparency in a thin vapor film

We examine the effect of electromagnetically induced transparency in a thin cell filled with a vapor of three-level $\ensuremath{\Lambda}$ atoms. We show that the absorption spectrum of the probe field and the fluorescence signal from the upper atomic level contain two sub-Doppler dips that have considerably different widths. The narrow dip corresponds to the effect of electromagnetically induced transparency, whereas the broader one results from the optical pumping of the external atomic states. Under the condition of exact resonance of the coupling field with the corresponding atomic transition, these two dips are superimposed on the frequency scale, while in the case of nonzero detuning of the coupling field, the two dips are shifted with respect to each other, which makes it possible to observe a competition between the two effects. In the frequency region where the absorption is low, large optical nonlinearities conditioned merely by the slow atoms having an effective 1 mK temperature are also found.