Rotating nuclear demagnetization refrigerator for experiments on superfluid he3

A nuclear demagnetization cryostat, capable of producing submillikelvin temperatures during rotational motion, was constructed. The apparatus has been used for NMR experiments on superfluid He³ at angular velocities up to 1.5 rad s^?1. The essential technical details and the operation of the cryostat are described.     

Thermalization of a manganin band-cable at 1.4 K

The thermalization of wires for electrical connections was measured at temperatures of ∼1.4 K in a pumped ⁴He cryostat, using a simple but efficient device composed of a band-cable of 50 manganin wires clamped into a heat sink. We applied a varied input power from 1 up to to the wires and we observed that the power dissipation can be reduced down to very low levels.     

An apparatus for infrared transmittance and reflectance measurements at cryogenic temperatures

A facility for measuring the optical properties of solid materials at cryogenic temperatures is being developed at the National Institute of Standards and Technology. A cryostat that houses four ur bolometric detectors and a six-position sample holder was designed and built. The bolometers operate near 5 K, and the sample temperature can he varied from 6 to 100 K. The beam from a Fourier transform spectrometer is directed to the cryostat by reflective optical components. The measurable wavelengths extend from 1m to 1 mm, with appropriate sources and beamsplitters in the spectrometer as well as windows and detectors in the cryostat. The angle of incidence on the sample ranges from 7.5 to 60. The mechanical electrical, and optical designs are described in this paper. Initial measurement results at wavelengths from 2 to 30m and a sample temperature of 10 K are presented.     

A Versatile Nuclear Demagnetization Cryostat for Ultralow Temperature Research

A new cascade nuclear demagnetization cryostat has been designed and constructed. Our aim was to build a versatile, modular cryostat, with a large experimental space providing an excellent platform for various types of ultralow temperature measurements. A powerful dilution refrigerator, combined with a massive copper nuclear cooling stage, enables us to reach lattice temperatures below 100 K continuously for more than two months. The cryostat is equipped with a second magnet for operating a double-stage nuclear demagnetization setup. Details of the design and performance are presented.     

Optical flow cryostat with rotatable sample stage

No Heading Matrix isolation of atoms and molecules for energy studies is very popular. In such work, an atom, molecule, adduct, or cluster is isolated in a solid gas matrix, which tends to act as a rigid vacuum. For example, a single atom may be frozen in argon, so that its properties as an isolated atom may be measured, usually by spectroscopic means. The many variations on this approach include studying clusters of atoms or molecules at low temperature and in magnetic fields. For this type of work, and other work as well, we designed and constructed a liquid helium flow cryostat that has the following advantages: it is small and light, so that it may be mounted on almost any spectrometer; it has a short (8 minute) cool down and warm up time; the sample stage may be rotated while cold, allowing it to be exposed to a number of gas nozzles for sample growth and optical access windows for measurements; and the cryostat is very simple and inexpensive to construct.PACS numbers: 07.20.Mc, 7.57Tg, 32.30Bv, 33.20Ea, 39.90.+d     

Optical interferometry in superfluid3He-B

We report interferometric measurements on 0.1 ... 1 mm thick films of superfluid ³He-B. The menisci of three different rotational states of the superfluid were observed and analyzed theoretically using two-fluid hydrodynamics: These are (i) the equilibrium vortex state in which the superfluid and the normal components corotate (solid body rotation), (ii) the vortex-free state (the Landau state), in which only the normal component rotates, and (iii) the quasistationary vortex state in which only the superfluid fraction rotates (pure superfluid rotation). The Landau state manifested itself by a reduced parabolic meniscus at rotation speeds below the critical angular velocity 0.2 rad/s for vortex formation. Transition from the Landau state to the equilibrium vortex state yielded a sudden deepening of the meniscus when _c was exceeded. After a rapid halt of the cryostat, we observed a novel meniscus which was produced by the superfluid rotation while the normal component was at rest. The enhanced depth of this meniscus is governed by the reactive mutual friction parameter B'.By employing laser light, both for imaging and for thermomechanical excitation, we measured the response of a thin superfluid layer to a heat pulse and analyzed it within the theory of two-fluid hydrodynamics. The data were employed, using the dispersion relation for thin film oscillations, to deduce the second viscosity coefficient ₃ close to T_c.     

A temperature-variable sample rotating cryostat in high magnetic fields

A temperature-variable sample rotating cryostat has been developed in order to measure the angular and temperature dependence of the upper critical field in several single crystal Chevrel phase superconductors. In the cryostat, the temperature of the sample can be varied from 2 to 15 K within an accuracy of ±10 mK. The sample can be rotated in an adiabatic vacuum can around a horizontal axis at the centre of our superconducting magnets up to 16.5 T within an accuracy of ±1°.     

Method for limiting the losses of liquid helium in transitions of superconducting magnets to the normal state

A simple method for limiting the losses of liquid helium and the pressure rise in a cryostat in transitions of superconducting magnets to the normal state is suggested. It involves the application of a layer of porous fiber insulation to the magnet surface. We investigated the influence of such a layer on the amount of liquid helium evaporated and on the pressure rise in a cryostat in the transition of a superconducting laboratory-scale magnet to the normal state.Russian Scientific Center Kurchatov Institute, Moscow. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 68, No. 6, pp. 883–886, November–December, 1996.     

NMR Line Shape of Rotating 3He-B at Large Counterflow Velocity

We have investigated the NMR line shapes of superfluid ³He-B in a rotating cylinder. In the vortex-free state at sufficiently large angular velocity of rotation, 1 rad/s, the main feature of the absorption spectrum is a large frequency-shifted peak above the Larmor frequency. The shape of this counterflow peak is both measured and calculated as a function of external magnetic field strength, angular velocity, temperature, and number of vortex lines. The NMR spectrum is derived from the calculated order-parameter texture. Reasonable agreement with the measured line shape is obtained by including line-broadening effects due to the external field inhomogeneity and Leggett–Takagi relaxation.     

Embedding Impurities into Liquid Helium

We have introduced guest particles into superfluid helium using a directed helium jet containing traces of species under study. The distinguishing peculiarity of the method consists in that the whole system is sealed from the cryostat main helium bath. This allows: (i) on the account of the absence of evaporating helium upflow to realize a complete capture of the impurities from the jet into liquid helium; (ii) to eliminate the dependence of the process conditions on liquid He level position in the main bath as well as on the amount of liquid He condensed inside a cell; and (iii) this method can be used to introduce impurities into liquid ³He. Two modifications of the technique have been designed—one for an optical cryostat and another for a cryostat with narrow 1 tail typical for use in a very high magnetic field. Optical and X-ray diffraction studies have confirmed the possibility of embedding in superfluid helium samples consisting of submicron D₂ particles with a rate of 10mmoles per hour. Such samples are necessary for the achievement of strong D₂ nuclear spin polarization by the brute force method.     

Optical Measurement of Wetting by 3He-4He Mixtures Near Their Tri-Critical Point

We have measured the contact angle of the interface of phase-separated ³ He- ⁴ He mixtures against a sapphire window using an interferometric method in an optical cryostat. Our measurement has been performed at saturated vapor pressure, along the phase separation line and near T_t, the temperature of the tri-critical point. We have found that the contact angle is finite, contrary to what was generally accepted up to now, i.e., complete wetting by the ⁴ He-rich phase. Furthermore, this angle does not tend to zero when the temperature approaches T_t. This is a remarkable exception to critical point wetting, a phenomenon which is usually observed near ordinary critical points.     

Heat Inputs to Sub-mK Temperature Cryostats and Experiments from γ-Radiation and Cosmic Ray Muons

We measured the spectrum of energies deposited by -radiation, emanating from radioactive materials in the laboratory that houses our mK cryostat, and by cosmic ray muons. This allows us to quantify the heat input that adversely affects the lowest temperature accessible in sub-mK experiments. We use our nuclear stage, stage plate and experimental cell as a prototype model system, and calculate the power deposited due to low energy (below 2.65 MeV) background radiation quanta (~20 pW). This is significantly less than the power (~120 pW) deposited in the nuclear and experimental stages by muons. Installation of a 5 cm thick lead wall around the cryostat reduced the energy due to the flux of quanta by a factor of ~10 to ~2 pW, and the number of quanta by a factor of ~20. The lower energy, soft cosmic ray component was also affected by introducing the same thickness of lead, reducing the overall count of cosmic ray derived particles by ~15% and the heat leak to ~100 pW.     

Persistent currents in superfluid3He

Measurements are reported of persistent currents in superfluid³He-B and³He-A. An ac gyroscope filled with 20 µm powder and mounted into a rotating nuclear refrigerator was employed. In³He-B, undiminished circulation was observed for 48 h; this implies an effective viscosity at least 12 orders of magnitude lower than in the normal fluid at the same temperature. AtP<15 bar, the observed critical velocity is independent of temperature but it is a weak function of pressure;v _c varies between 4 and 6 mm/sec. The response to rotation is hysteretic, with elastic potential flow at slow rotation and irreversible vortex flow at higher angular velocities. The persistent angular momentumL is reversible when thermally cycled in the B phase, and proportional to the superfluid fraction _s /. Above 15 bar the B phase splits into separate regions with different critical velocities. The measuredv _c in the phase existing only at high pressures is dependent on magnetic field; for example, at 23.0 bar,v _c (H=0) =5 mm/sec, butv _c (H=40 G) =15 mm/sec. In the low pressure phase,v _c is insensitive to a change in the magnetic field. The phase transition is of first order; the latent heatQ _G (1 µJ/mole) depends on the maximum angular velocity at which the cryostat was rotated. The transition is proposed to occur in the core structure of pinned quantized vortices sustaining persistent currents. In³He-A, currents could not be found to persist on an observable level. Direct measurements ofL atH=0 and atH=40 G, and repeated thermal cycling, showed that either the current decays rapidly orv _c <0.5 mm/sec.     

NMR experiments on rotating superfluid3He-A and3He-B and their theoretical interpretation

We have constructed a rotating nuclear demagnetization cryostat and used it for continuous-wave NMR experiments on superfluid³He-A and³He-B. The measurements were performed in a long cylindrical geometry of 5 mm diameter, with the cylinder axis parallel to the axis of rotation and with the external magnetic field H₀=284 or 142 Oe in the same direction. The angular velocity of rotation was varied between 0.2 and 1.5 rad/sec, and the experiments were done under 29.3 bar pressure at temperatures between T_c=2.72 and about 1.4 mK. As a guide to the new and esoteric field of superfluid³He in rotation, we first review the general theory at some length in relatively simple terms. Pictorial explanations are often given.In³He-A, a rotation-dependent NMR satellite was found; its intensity a rotation-dependent NMR satellite peak was discovered; its relative intensity increases linearly with . The position of the satellite is independent of and H, and does not depend on whether the sample was cooled from the Fermi-liquid region to the A phase while rotating or at rest. At temperatures 0.1<1–T/T_c<0.3, the frequency shift of the satellite can be described by the parameter R_t=0.86–1.1(1–T/T_c). Cooldown under rotation produced systematically larger satellite intensities than cooldown at rest. A second, metastable satellite, best seen at rest and disappearing in less than 30 min, was also discovered. Furthermore, the main NMR peak broadens during rotation, while the total NMR absorption remains the same. The behavior of the rotation-dependent satellite strongly supports the existence of vortices in³He-A, their number being proportional to ; the satellite is caused by localized spin wave modes trapped by vortex cores. Theoretical calculations agree quite well with our experimental data if continuous vortices, without a singularity in the order parameter, are assumed. Their presence is also responsible for the additional broadening of the main peak, due either to increased spin diffusion or to scattering of spin waves. The metastable satellite is caused by textural boundaries, probably by twist solitons in the superfluid, created by the rapid cooldown of the sample.In³He-B, a series of nearly equally spaced NMR satellites was found on the high-frequency side of the main peak with the cryostat at rest. Under rotation the separation between the satellites increases linearly with . The spacing displays a jump, proportional to , at 1–T/T_c=0.40. The discontinuity occurred only during start/stop experiments, not if the cryostat was continuously rotated while warming over the transition region. Immediately after rotation had been started the whole NMR spectrum shifted toward higher frequencies for about 30 sec; these transients were seen only at >0.25 rad/sec. In³He-B, the order parameter is strongly influenced by the wall of the container, producing the so-called flareout texture, with the angle between the vector andH equal to 63° at the walls. The satellites can be explained as spin wave modes arising from an almost harmonic potential well formed by the texture. The creation of vortices changes the texture and increases the steepness of the potential and therefore increases the satellite spacing during rotation. The vortices themselves perturb the texture due to the long-range orientating effect of their cores on the order parameter. The discontinuity in the satellite splitting at 1–T/T_c=0.40 is explained as being due to a first-order phase change in the vortex core at this temperature. The transient shift in the NMR spectrum, immediately after the start of rotation when vortices are not yet present, is caused by the large superfluid vs. normal liquid counterflow; this phenomenon thus gives an estimate for the time needed to create vortices in³He-B.     

Thin and thick YBCO films grown by MOCVD processes. Study of the angular magnetic field dependence of current transport properties

Thin and thick YBCO Films have been grown by Aerosol Assisted as well as by thermal MOCVD. The Aerosol Assisted MOCVD technique allows the growth of YBCO films from a single liquid source at deposition rates of up to 10m/h.Transport measurements (I-V) on etched microbridges using a single pulse technique have been performed. The angular magnetic field dependence J_c() of the critical current density from T_c down to 50 K has been measured. Transport properties are reported and discussed with respect to the microstructural features as determined by X-ray diffraction and electron microscopy.     

Deformation created by the impact of small angular and spherical particles on individual grains

Damage mechanism induced by 25 m, average size, angular and spherical particles impacting the surface of Cu-30% Zn (-brass) was investigated. Particles were impacted to the surface at normal incidence, with a velocity of 12 m/s. The primary characterization tools used were scanning electron microscopy (SEM) and atomic force microscopy (AFM). AFM allowed for the measurement of impact profiles, those of the cut surface and material pile-ups. Deformed volume by the particles was limited to a single grain. The impacts produced were asymmetrical and the chip formation was also highly directional. These asymmetries were the same in a single grain but they varied from grain to grain. Regardless of particle geometry, similar deformation features were observed on the target surface impacted by angular and spherical particles. The direction of the deformation appeared to be imposed by the mechanical response of the deforming grain. Since the deformation induced by a single impact is limited to a single grain, anisotropic mechanical properties of the individual grains observed were attributed to their impact damage morphology.     

Lamellae deformation and structural evolution in an Al-33%Cu eutectic alloy during equal-channel angular pressing

Equal channel angular pressing was successfully applied on a lamellae eutectic alloy of Al-33%Cu at 400°C, up to an equivalent strain of 8. A homogeneous fine equiaxed duplex microstructure with an average grain size of 1.1 ± 0.3 m was obtained. Deformation accommodation is realized in various form of periodical shear banding: periodical bending, periodical shear banding, shear switching, and periodical shear cutting in the eutectic. The shear essence of the strain mode involved in ECAP determines the very different behavior, from that in drawing, of lamellae structural evolution.     

The spin-up problem in Helium II

The laminar spin-up of Helium II is studied by solving the linearized equations of motion for the normal and superfluid components and the quantized vortex lines in a simple case. The fluid is taken to be confined between two parallel planes whose angular velocity increases at a small, steady rate. The vortex lines are treated as a continuum. No direct interactions between the vortex lines and the walls are included. Two mechanisms are identified for the transfer of angular momentum from the container to the interior fluid. In the first place, classical Ekman pumping occurs in the normal fluid component. Secondly, mutual friction between the normal Ekman layer and the vortex lines produces an (Ekman-like) secondary flow in the superfluid component. In both mechanisms, mutual friction in the interior couples the normal and superfluid components together, so that both components spin up. Normal-fluid Ekman pumping is found to dominate at temperatures close to the -point (T=2.17 K), while the second mechanism becomes progressively more important at lower temperatures. In the small-Ekman-number limit, when the vertical container dimension 2a is much larger than the Ekman layer thickness, the spin-up time (i.e., the time lag between the container and the interior fluid) for both components ist _spin-upf(T)a ₀ ^–1/2 , where ₀ is the angular velocity andf(T) is a decreasing function of temperature. Although some experimental spin-up times in He II have been reported in the literature, their analysis involves many uncertainties. Thus, new experiments to test this model should be highly desirable.     

Angular variation of coercivity of interacting fine acicular skeleton particles

For fine, acicular skeleton particles of -Fe prepared from -FeOOH, the effect of interparticle interaction on the angular variation ofH _c was numerically investigated using a direct expansion scheme of the chain-of-spheres fanning model to an interacting chain system forming an orthorhombic type of regular space lattice. The model theory reveals that only magnetostatic lateral interchain interaction can affect the angular variation ofH _c. The results have been used to explain the experimentally observed effects of particle morphology and packing fraction on the angular variation ofH _c in systems made from fine acicular particles prepared for audio/video magnetic recording media. It is suggested that a local aggregate of the so-called multiple type, is unavoidably generated in the system.