Search

Experiments with helium in thin films and small pores show that the bulk properties are greatly modified. With 4He, the onset of superfluidity is depressed and the specific heat anomaly is rounded and also depressed to lower temperatures. With 3He new features are found such as surface magnetism and free path effects. Bulk superfluid 4He is shown to have a sharp transition to turbulence, and bulk 3He the characteristics of a Fermi fluid.

We report measurements of the low-temperature specific heat of superconducting and non-superconducting Pr1.85Ce0.15CuO4 and non-superconducting Pr1.875Ce0.125CuO4 between 0.2 K and 4 K in magnetic fields up to 4 tesla. For all three samples, the zero-field specific heat C(T) consists of three contributions, a nuclear (T−2) term, a phononic (T3) term and a residual low-power term proportional to Tn where n

We report measurements of the specific heat of a large single crystal of Ba0.6K0.4BiO3 taken between 0.4K and 4.2K. Upon subtracting the phonon and nuclear quadrupole contributions, the residual specific heat is found to have a Tn temperature dependence with n ≈ 0.8. A similar low-power temperature dependence has been observed in the low temperature specific heat of the cuprate high-Tc superconductors and suggests a common origin for this term in both systems, possibly arising from localization of the quasiparticles at low temperatures.

We have measured the heat capacity of molecular hydrogen in Vycor porous glass from 6K to 15K using two different methods. The first gives results similar to previous work, with broad peaks corresponding to freezing and melting and at lower temperatures a smooth decrease. The second, which are much closer to thermodynamic equilibrium, resolve three distinct peaks at freezing and reveal a small reproducible anomaly at a lower temperature, not previously observed. Below the anomaly the results fit a roton-type function, and we discuss the possibility that this is consistent with a superfluid state, although our experiments cannot detect it.

We have used a plastic diaphragm to generate flow in superfluid3HeB, through a cylindrical geometry, whilst monitoring the transverse NMR frequency. It was found that the critical flow velocity needed to induce NMR frequency shifts was dependent on the magnetic field. The results are explained by a numerical calculation of the texture in the3HeB, under the influences of magnetic field, fluid flow and surface energy terms. Also presented are measurements of the B phase NMR longitudinal frequency, derived from the magnitude of the flow-induced change in the transverse frequency. A flow-induced texture transition, which caused a dramatic narrowing of the NMR lineshape, is described.

The Multi-Object Double Spectrographs (MODS) are two identical high-throughput optical low- to medium-resolution CCD spectrometers being deployed at the Large Binocular Telescope (LBT). Operating in the 340-1000nm range, they use a large dichroic to split light into separately-optimized red and blue channels that feature reflective collimators and decentered Maksutov-Schmidt cameras with monolithic 8×3K CCD detectors. A parallel infrared laser closed-loop image motion compensation system nulls spectrograph flexure giving it high calibration stability. The two MODS instruments may be operated together with digital data combination as a single instrument giving the LBT an effective aperture of 11.8-meter, or separately configured to flexibly use the twin 8.4-meter apertures. This paper describes the properties and performance of the completed MODS1 instrument. MODS1 was delivered to LBT in May 2010 and is being prepared for first-light in September 2010.

We present measurements of the thermal conductivity of polycrystalline Pr 2 Cu 1− x Zn x O 4 (0⩽ x ⩽0.02) in the temperature range 6–300 K. The thermal conductivity is found to increase with Zn substitution up to x =0.005 but to decrease with Zn concentration when x >0.005. This behaviour is attributed to the strong interaction of phonons with spin excitations.

Specific heat in applied magnetic fields were measured to a low temperature 0.08 K and resistance to 0.5 K. The specific heat shows a wide peak at 1 K, and a high value of the electronic coefficient gamma(T --> 0) approximate to 1200 mJ/mol.K-2 is obtained. The temperature independence of the wide peak in specific heat at 1 K in magnetic fields further suggests that the high value gamma of CeCuGa is due to Kondo effect.

The recently commissioned system for aluminizing the 8.408 meter diameter Large Binocular Telescope mirrors has a variety of unusual features. Among them are aluminizing the mirror in the telescope, the mirror is horizon pointing when aluminized, boron nitride crucibles are used for the sources, only 28 sources are used, the sources are powered with 280 Volts at 20 kHz, high vacuum is produced with a LN2 cooled charcoal cryo-panel, an inflatable edge seal is used to isolate the rough vacuum behind the mirror from the high vacuum space, and a burst disk is mounted in the center hole to protect the mirror from overpressure. We present a description of these features. Results from aluminizing both primary mirrors are presented.

We describe an instrument that is capable of taking simultaneous images at one optical (UBVRI) and one near-infrared (JHK) wavelength. The instrument uses relatively simple optics and a dichroic to image the same field on to an optical CCD and an HgCdTe array. The mechanical and thermal design is similar to previous instruments built by our group and the array controllers are based on the same architecture. The instrument has been in use for the past four years on the CTIO/Yale 1m telescope in Chile and has an excellent operational/reliability record. A number of notable science results have been obtained with the instrument; especially interesting are several photometric monitoring projects that have been possible, since the instrument is available every night on the telescope.

The low temperature heat capacity measurements on LSCO samples containing Zn impurities have been performed by means of pulsed differential calorimetry technique. The interpretation of the Zn- concentration dependence of the residual g-coefficient in the linear temperature part of Cel(T) at T<

A single crystal of La2CuO4 was made superconducting after treatment in a high pressure oxygen furnace. Measurements are presented of the low temperature heat capacity of the insulating crystal in zero field between 1K and 3K, and of the superconducting crystal in fields of 0, 2 and 4 Tesla applied parallel to the ab plane between 0.2K and 3K. A linear heat capacity contribution is observed for the crystal in both the insulating and superconducting states. This linear heat capacity contribution is larger in the superconducting state and is insensitive to the applied magnetic field.

Measurements are presented of the low temperature specific heat of three polycrystalline samples of Pr2−xCexCuO4 in magnetic fields of 0, 2 and 4 Tesla. The residual specific heat contribution in zero field seems to have a form αTn (n

We report on the specific heat of a Bi 2 Sr 2 CuO y single crystal measured between 0.1K and 5K. The measurements were carried out with the single crystal first in the semiconducting state and then, after oxygenation in a superconducting state. The specific heat of the latter showed a well defined peak centred at 4.1K associated with the superconducting transition. At low temperatures both states exhibited a 1/T 2 term associated with the nuclear spins of the copper ions. In addition both could be fitted well to a function which includes a residual term of the form αT n where n is less than unity. The exponent n is smaller for the superconducting state than for the semiconducting state, as is found in some other systems.

We describe measurements of the specific heat of parahydrogen in Vycor porous glass, using two different methods, between 6K and 15K. Results from the first method are very similar to those from other laboratories, showing broad peaks at solidification and melting at temperatures well below the bulk, and decreasing smoothly at lower temperatures. The second set were carried out very slowly and we believe are much closer to thermodynamic equilibrium. They show new phenomena not resolved in the previous measurements, namely three distinct peaks in the freezing anomaly, and at lower temperatures a small cusp-like feature which is reproducible in four separate experiments over a period of several weeks. Below the anomaly the specific heat follows a roton-type function, but these measurements cannot determine whether superfluid is present.

We present measurements of the low temperature specific heat (0.09K≤ T ≤ 1.5K) of seven samples of the La2− x Sr x CuO4 system (0.05≤ x ≤0.27). All the data were found to fit the expression, C = A( x )T −2 +α( x )T n( x ) +β( x )T 3 . The coefficient of the nuclear specific heat contribution A( x ) was found to have a minimum value for 0.15 ≤ x ≤ 0.2, i.e. near optimum doping. The exponent of the second term n( x ) was always less than unity and showed a very systematic variation with Sr-doping, having a minimum value of n ≈ 0.33 ± 0.07 for x = 0.15 .

Transient heat pulse techniques allow specific heat ( C ) and thermal conductivity (κ) to be determined from a single measurement. We have developed a new numerical fitting technique to analyse such data, which improves the accuracy of results obtained from heat pulse calorimetry. In this paper we present two further improvements to this technique which compensate for the finite duration of the heat pulse injected into the sample and any distortion of the temperature response caused by measurement time constants.

We present simultaneous measurements of the low temperature (0.04K to 1.2K) specific heat and thermal conductivity for a series of four sintered samples from the La 2−x Sr x CuO 4 system. By systematically varying the strontium (hole) concentration from x =0.05 (near-insulator) through to x =0.27 (near-metallic), we were able to study the thermal properties of the different superconducting regimes in this cuprate system. The thermal conductivity data are characterised by a weakening of the temperature dependence from T =3 to T =1 , observed in all the samples below 1K. A linear temperature variation in the specific heat data for the two most inhomogeneous superconductors ( x =0.05 and 0.27) was replaced by a T 0.5 dependence for the samples with the higher transition temperatures ( x =0.1 and 0.15). The specific heats of all the samples showed a large upturn (proportional to T −2 ) at the lowest temperatures. The coefficients of this T −2 term were found to have a minimum for x =0.15. We discuss these effects in terms of possible transport mechanisms.

We report on the magnetoresistance of La 2−x Sr x CuO 4−δ as a function of temperature, in applied magnetic fields up to 5 Tesla. These measurements were carried out on five samples ranging in strontium concentration from x=0.07 to x=0.20. In an earlier work on similar samples, the results of differential specific heat experiments were reported. In this present work we derive average values for the upper critical field H c2 (O) and the electronic specific heat term γ is then estimated. Analysing the results in terms of fluctuation theory we obtain estimates for the coherence length ξ(O) and finally showthat the fluctuations appear to have a two dimensional character.

NMR and compressional cooling experiments have been carried out in Vycor porous glass immersed in liquid 3He in a Pomeranchuk cell at various temperatures and pressures. The expected nucleation of a solid phase near the substrate at a few bar does not take place, but compressional cooling occurs at about 2/3 bar below the melting curve. Modification of a simple statistical layer model to take into account the two-dimensional nature of the second layer gives better agreement with the observations, but other considerations show that the phase is probably fluid. These experiments therefore show the existence of a previously undiscovered high-density (>solid) disordered phase which is probably mobile. The NMR measurements show that the ferromagnetic interactions in the adsorbed 3He are larger than in bulk liquid, and increase with increasing pressure and decreasing temperature.

We have measured the flow-induced shift in the NMR spectrum of /sup 3/He-B. Above a temperature-independent critical velocity, the absorption line is observed to shift to higher frequencies as the flow is increased, in qualitative agreement with the theory of textural deformation in the presence of flow due to Fetter.

We present the first measurements at millikelvin temperatures of both the real and the imaginary parts of the transverse-acoustic impedance of an oscillating surface in contact with dilute solutions of $^{3}\mathrm{He}$ in $^{4}\mathrm{He}$, as well as with pure $^{3}\mathrm{He}$ with and without a surface boundary layer of $^{4}\mathrm{He}$. Except for pure $^{3}\mathrm{He}$, large departures from hydrodynamic behavior are found which are inconsistent with slip theories incorporating specular scattering at the boundary. The effects of the $^{4}\mathrm{rich}$ surface boundary layer and of surface roughness are discussed.

Measurements have been made of the viscosity of pure liquid 3 He at the saturated vapor pressure over the temperature range 20 mK to 1 K, by observation of the decay of torsional oscillations of a crystalline quartz cylinder. The results generally confirm previous measurements by other methods, but are more accurate and extend over a wider temperature range. Careful data analysis has been carried out to determine the best fit, which is given by the equation (ηT 2 )−1 =A 0 +A 1 T n , withA 0 =0.5450,A 1 =−1.594, andn=1 (with η in micropoise) and which indicates that higher-order terms inT (n>1) are inappropriate. The coefficientA 1 is four times larger than that given by theory which calculates it with use of only theZ 0 Landau parameter, indicating that other interaction parameters should be included.

In this paper we present reliable measurements of the pressure variation of the spin diffusion coefficient of normal liquid3He in the limiting low-temperature Fermi fluid regime. Deviations from the bulk liquid behavior are observed for a sample confined to a 5-µm glass capillary array in which the mean free path of the3He quasiparticles is of the order of the capillary diameter. These deviations are believed to indicate that the spin momentum specular reflection coefficient is less than unity but greater than zero in this system.

Measurements of the thermal conduction of liquid 3 He in the collisionless regime, where the mean free path of the thermal carriers is much greater than the channel diameter, allow a direct measurement of the carrier velocity. These experiments have been done using Vycor porous glass as the channel (average diameter ∼70 A) at temperatures down to 16 mK. They show that the carrier velocity (∼43 m/sec) is that of quasiparticles rather than of bare 3 He atoms, whose velocity would be a factor of m*/m (∼3) greater, and thus confirm quite independently of any other measurement the validity of Landau quasiparticle theory. The data also strongly suggest a zero quasiparticle specular reflection coefficient, which allows an estimate to be made of the quasiparticle lifetime. This shows that the energy levels of quasiparticle states have a width more than ten times greater thankT at 20 mK. As an incidental result, a value for the low-temperature thermal conductivity of Vycor glass has been calculated.

With a proper choice of coordinates and for certain substances over a limited temperature range, the information contained in a whole family of adsorption isotherms coalesces into a single universal curve. Such behaviour can be related to a particularly simple thermodynamic model. This model and some of its implications are explored in detail. In particular, thermodynamic internal self-consistency demands that there be a unique (generally non-linear) relationship between the mass adsorbed and the film thickness which is completely deducible from measured adsorption isotherms plus bulk thermodynamic data. It is independent of assumptions regarding the form of the van der Waals attraction binding the adsorbate to the adsorbent substrate. The integral equation embodying this relationship is derived and exhibited. As an example of its use the equation is applied to the case of thin films of helium II.

Measurements are reported of the specific heat of a number of PtMn alloys. Interaction between the manganese impurities gives rise to antiferromagnetic or ferromagnetic ordering depending on the impurity concentration, the critical concentration for ferromagnetism being 0.8 at % Mn. Using the effect on the specific heat anomaly of the statistical fluctuations in the number of the impurities in a given volume, the interaction range of the impurities is estimated and found to agree with the band structure of platinum.

Superflow of 3He–B in a channel can be observed by nuclear magnetic resonance (nmr) techniques, the simplest arrangement being one in which the magnetic field is perpendicular to the flow. Above a certain critical velocity, the transverse resonance frequency shifts away from the Larmor frequency, and the velocity gives a value of the ratio of the flow coupling constant to the dipole coupling constant, which is found to differ substantially from theoretical prediction. The nmr shifts can be used to detect persistent currents in a doubly connected geometry, and some preliminary observations are described as well as an analysis of the way in which currents with multiply quantized circulation may be set up.

We report measurements of supercritical heat conduction in liquid helium II at pressures above the vapor pressure. The measurements were made in glass capillary tubes of diameter 50.5 and 366.8 Μm in the temperature range from 1.2 to 2.1 K. As in our previous results at the vapor pressure, a clear distinction can be made between two regimes in which the average separation of vortex lines is either comparable with or larger than the size of the flow channel. In the wider tube, at high relative velocities v between the superfluid and normal component, where the vortex spacing is small, a modified form of Gorter-Mellink mutual friction Fsn is observed which varies as Fsn=Aρsρnv(v−v0)2. The coefficient A increases with pressure, and its variation with T/Tλ(p) is in fairly good agreement with Schwarz ' microscopic theory of superfluid turbulence. However, the absolute magnitude of A is in general larger than in Schwarz ' theory and also larger than most other experimental values. In the narrower tube the measurements are all in the regime where the vortex spacing is comparable with the tube size, and they do not agree with the predictions of the Gorter-Mellink law. At present, the data cannot be analyzed quantitatively in the terms of the vortex line density, because B, the vortex line scattering coefficient, is not known at high pressures.

When a solid surface is in contact with liquid helium, the strong van der Waals forces between helium and the solid substrate result in a layer of high-density helium localized on the substrate. It was expected that, upon raising the pressure, further solid layers would grow out from the wall. Many experiments have now shown that this usually does not occur, and even at solidification pressures $^{4}\mathrm{He}$ liquid, rather than solid, wets a disordered surface. This implies that, in the neighborhood of some solid surfaces, liquid helium can exist at pressures above bulk solidification pressure.

Measurements are reported which relate to the dissipation and superfluid density of /sup 3/He-A and /sup 3/He-B flowing through a long rectangular channel of cross section 48 ..mu..m x 2.86 mm at currents between 5 x 10/sup -2/ g cm/sup -2/ sec/sup -1/ and 5 x 10/sup -5/ g cm/sup -2/ sec/sup -1/ and at temperatures down to T/T/sub c/=0.52. The results show a complex velocity dependence in both phases.

dc flow of superfluid /sup 3/He-B through a rectangular superleak exhibits two dissipative regimes and two critical currents with temperature dependence of the form J/sub c/ = b(1-T/T/sub c/)/sup a/. At low pressures aapprox. =(3/2) and b increases with pressure. Around 21.5 bars a crossover occurs to a new dissipation regime with aapprox. =2 and the prefactor b then decreases with pressure.

The critical transfer rate of the unsaturated helium II film has been measured on surfaces of glass and german silver by a heat conduction method. It is found that a considerable reduction of the transfer rate occurs when the vapour pressure over the film is decreased only slightly below the saturation value. At a given percentage of the saturation pressure, there exists a critical temperature above which film flow will not take place. This critical temperature is shown to be sharply defined, and to decrease with decreasing percentage satura­tion. At the full saturation pressure the transfer rate is markedly different on the various surfaces used, but as the vapour pressure over the film decreases the flow rate tends to become the same for all surfaces. The critical temperature for onset of superfluidity is also independent of the substrate. The temperature dependence of the transfer rate is different from that for the saturated film, but very similar to the variation found in the flow of liquid through channels of width less than that of the saturated film.

Specific heat results are presented for near-monolayer films of3He,4He, and isotopic He mixtures adsorbed on Vycor porous glass in the temperature range between 1 and 4°K. In the case of the pure He monolayers and submonolayers, the specific heats depend only onT 2 , like a two-dimensional Debye elastic medium. A film of slightly greater than monolayer coverage needs an additional temperature-independent term. Monolayer coverages of mixtures are difficult to define, but all coverages used, some of which are submonolayer, need bothT 2 andT-independent terms. The constant term may arise from atoms in the second layer forming a classical noninteracting system, which in the mixture case would indicate selective adsorption of4He in preference to3He at the substrate wall, in agreement with previous adsorption experiments on the same substrate.

The pressure and temperature gradients accompanying heat flow in liquid helium II have been measured simultaneously. The experiments were made in a glass tube of 107·6 μ diameter and 10·2 cm length between 1·3°k and 1·8°k. Significant deviations from Allen and Reekie's rule were observed end are provisionally attributed to an eddy viscosity, ηe, due to turbulence in the superfluid component. An equation, baaed on dimensional arguments, is proposed for ηe which satisfactorily represents the experimental date available at present.

The anomalously small thermal boundary resistance between liquid He3 and powdered cerous magnesium nitrate (CMN) crystals must be due either to an unexpectedly large surface area, to a porous surface layer, or to a new coupling mechanism between the thermal excitations on either side of the interface. Measurement of the surface area of powdered CMN by the BET adsorption method shows that the first two possibilities can be ruled out.

Previous measurements of adsorption isotherms of He4 on an argon-coated copper substrate have suggested that a phase transition may take place in adsorbed He4 films at temperatures above the normal bulk liquid lambda point. We have made high-precision adsorption measurements on a substrate of porous Vycor glass (mean pore diameter 62.4 A, surface area 580 m2) and failed to find any evidence of anomalous adsorption. Reasons for the conflict in results are discussed.

Measurements have been made of the heat conduction by liquid helium II in capillary tubes of diameter 52.0 microns, 107.6 microns and 366 microns. The results on subcritical flow have been described previously, and the present paper is concerned with observations in the transition region to supercritical conduction. Outstanding features of the transition are (i) spontaneous fluctuations in the thermal resistance; (ii) very large hysteresis effects in the resistance at low temperatures in the wide tubes, which decrease as the lambda point is approached, and which depend on the previous history of the helium; and (iii) sensitivity of the thermal resistance to mechanical shock or vibration, including permanent changes from one branch of the hysteresis loop to the other. The observations, which include measurements of the critical velocity as a function of tube size and temperature', are discussed in terms of the vortex line theory of Feynman and its development by Vinen, which are found to give good...

The heat of transport of helium II has been measured accurately from 1.15°k to 2.1°k in order to compare it with the calorimetric entropy data. The measurements were made with a thermally isolated glass vessel, which was connected to an isothermal liquid helium bath by a tube packed with fine powder. The heat of transport was determined (i) from the temperature and hydrostatic pressure changes in the vessel during thermomechanical flow, and (ii) from the heat input necessary to produce a measured flow rate. The results agree well with the indirect measurements of the fountain effect by van den Meydenberg, Taconis, Beenakker and Wansink and with the calorimetric data of Kramers, Wasscher and Gorter. The agreement with Kramers et al. is within the experimental error up to 1.8°k but at higher temperatures the present measurements are slightly higher. Oscillations in the temperature of the vessel were observed even during thermomechanical flow, provided that the rate was subcritical. As expected, the oscillations were damped out above the critical rate. When the flow was just critical, a pronounced peak in the temperature-time graph was observed at the beginning of the flow, which appears to be due to an unexplained irreversible process.

The specific heat of liquid He/sup 3/ was measured near the saturation vapor pressure in the range 0.085 to 0.75 deg K, and at pressures up to the melting pressure between 0.12 and 0.6 deg K. There is no evidence of a specific heat transition in the range of measurements. Near 0.1 deg K the specific heat near the saturated vapor pressure appears to tend linearly to zero at O'K with a slope of 4.0 plus or minus 0.1 cal/ mole deg/sup 2/, providing evidence for regarding He/sup 3/ as a Fermi liquid near this temperature with an effective excitation mass of (2.00 plus or minus 0.05) x the mass of a He/sup 3/ atom. Entropies calculated with this linear extrapolation and with the assumption that no transition occurs below 0.085 deg K agree well with previous evaluations. Measurements at higher pressures show a negative dependence of specific heat on pressure above 0.16 deg K, but a positive dependence below this temperature, in contradistinction to the case of an ideal Femni gas. (auth)

Measurements have been made in glass capillary tubes of 52 and 108 u diameter, in the temperature range from 1.15 to 2.15 °K. For sufficiently small temperature gradients the thermal resistance is independent of the heat flow and is caused solely by the viscosity of the normal fluid, n n . This viscosity has been calculated from the measured thermal resistance data with the use of the existing entropy values, and the results from the two tubes are in very good agreement except for a small deviation at low temperatures which is attributed to slip. The viscosity at the saturated vapour pressure, when corrected for slip, agrees closely with the results obtained by the rotating cylinder viscometer but does not agree with the oscillating disk experiments. The variation of n n at low temperatures follows the theory of Landau & Khalatnikov if A/ k is taken as 8.9 °K, but the measured values of the slip correction are slightly larger than would be expected from the theoretical phonon and roton mean free paths. In the 52 u tube the measurements have been extended up to the solidification pressure so as to give n n as a function of density. Near the lambda line n n increases rapidly with density, but at low temperatures the variation is quite small and it is found that the contribution from the rotons is almost independent of density.

Measurements are described of the stable supercritical thermal resistance of liquid helium II between 1.2°k and 2.0°K in capillary tubes of diameter 52.0 microns, 107.6 microns and 366 microns. The results are presented in terms of the quantized vortex line theory, and values of L 0, the effective length of vortex line per unit volume, are calculated. Graphs of L 0 1/2 against relative velocity v are straight lines at high velocities, corresponding to a cube law of mutual friction of the form F sn = A ρsρn v(v−v 0)2, as expected from the theory of Vinen. Values of A are independent of tube diameter and length, and increase with temperature in the same way as found by Vinen, but are somewhat larger. At low velocities, where the average spacing of vortex lines becomes comparable with the channel width, the effective value of A drops rather abruptly by about one-half. In this region, Vinen's theory can no longer be expected to apply, but a plot of the dimensionless variables (DL 0 1/2) against (mvD/...

A brief survey is given of papers read at the Ninth International Conference on Low Temperature Physics (LT9). The main topics dealt with are superfluidity, quantization and vortices in superconductors and liquid helium, quantum liquids, Fermi surfaces, low temperature transitions, and dilute alloys with transition metals. Some remarks are made about the value of the `LT' conferences.