Helium Sample Research Articles

Observation of Classical Rotational Inertia and Nonclassical Supersolid Signals in SolidHe4below 250 mK

We have confirmed the existence, as first reported by Kim and Chan, of a supersolid state in solid 4He at temperatures below 250 mK. We have employed a torsional oscillator cell with a square cross section to insure a locking of the solid to the oscillating cell. We find that the nonclassical rotational inertia signal is not a universal property of solid 4He but can be eliminated through an annealing of the solid helium sample. This result has important implications for our understanding of the supersolid state.

Improved helium exchange gas cryostat and sample tube designs for automated gas sampling and cryopumping

In order to eliminate the use of liquid helium for the extraction of atmospheric gases from polar ice cores, two units of a redesigned top load helium exchange gas cryostat were built and tested. The cryostats feature the shortest and largest diameter sample wells built to date, a base temperature below 7 Kelvin, and a sample well without baffles. The cryostats allowed shortening the length and thus increasing the gas pressure inside our sample tubes by 58% and increasing the amount of sample ending up in the mass spectrometer by 4.4%. The cryostats can either be used as mobile stand‐alone units for manual gas processing lines or integrated into a fully automated vacuum extraction and gas analysis line. For the latter application the cryostat was equipped with a custom‐designed automated changeover system.

An x-ray topographic study of diamond anvils: Correlation between defects and helium diffusion

X-ray topographic images of several dozens of diamonds have been recorded before and after their use as anvils in a diamond anvil high pressure cell. The aim of this study is to better understand and prevent the breakage of diamond anvils when they are used in contact with a helium sample. Indeed, helium is a good hydrostatic medium for very high pressure studies but many anvils are subjected to helium embrittlement. X-ray topographic observations suggest that helium atoms enter the diamond anvil by a diffusion process enhanced by subsurface strains and/or extended preexisting defects and nonhydrostatic stress. Then these atoms open cracks during nonhydrostatic load of the diamond anvil. The diamond anvils can be classified into three types depending on the nature of the defects. An associated criterion for the selection of stones resistant to helium is proposed.

3He and4He in the local interstellar gas as observed with the COLLISA foil experiment on the Mir space station

With the COLLISA foil experiment onboard the Mir space station we have collected samples of interstellar helium, which have been returned to the Earth and investigated by mass spectrometric analysis. Recently, we have been able to reduce the experimental uncertainties as given earlier (Salerno et al. 2003). Our improved estimate of the helium isotopic ratio in the local interstellar medium is now ( (Busemann et al. 2005).

Cosorption effect in gas chromatography: flow fluctuations caused by adsorbing carrier gases

Adsorbing carrier gases have a number of advantages in analytical and preparative gas chromatography, such as clearer detector signals and higher column efficiencies. This work shows that adsorbing carrier gases also may be useful because they cause the mobile phase flow rate to become unsteady after injecting a small amount of sample. This work shows that a 100 microL sample of helium can liberate enough carbon dioxide carrier gas from a zeolite 5A packed column at 373 K, that the departure from the steady-state flow rate had an upper lobe area of 586 microL of carrier gas. This was confirmed by coupling a modified Langmuir kinetic model with the Ergun equation.

‘Heat from Above’ Heat Capacity Measurements in Liquid4He

We have made heat capacity measurements of superfluid He-4 at temperatures very close to the lambda point, T(sub lambda) , in a constant heat flux, Q, when the helium sample is heated from above. In this configuration the helium enters a self-organized (SOC) heat transport state at a temperature T(sub SOC)(Q), which for Q greater than or = 100 nW/sq cm lies below T(sub lambda). At low Q we observe little or no deviation from the bulk Q = 0 heat capacity up to T(sub SOC)(Q); beyond this temperature the heat capacity appears to be sharply depressed, deviating dramatically from its bulk behaviour. This marks the formation and propagation of a SOC/superfluid two phase state, which we confirm with a simple model. The excellent agreement between data and model serves as an independent confirmation of the existence of the SOC state. As Q is increased (up to 6 micron W/sq cm) we observe a Q dependant depression in the heat capacity that occurs just below T(sub SOC)(Q), when the entire sample is still superfluid. This is due to the emergence of a large thermal resistance in the sample, which we have measured and used to model the observed heat capacity depression. Our measurements of the superfluid thermal resistivity are a factor of ten larger than previous measurements by Baddar et al.

Experiments with4He Heated from Above, Very Near the Lambda Point

We report on new transport phenomena observed within a column of 4He heated from above and close to its transition to superfluidity. In this configuration the helium sample will self-organize to a state where the temperature gradient across the column remains equal to the pressure-induced gradient of Tλ (1.273 µK/cm) even as the heat flux is varied by a factor of about a thousand. On this self-organized state we have observed a new temperature-entropy wave that propagates opposite to the direction of a steady heat flux Q. This propagating mode is due to a non-linear temperature dependence of the thermoconductance of the helium near criticality. Such a mode had been predicted to exist in this state for Q less than about 100 nW/cm2. We confirm that this mode exists in this regime, however we also observe that it propagates at higher values of Q, even when the helium is pushed away from the self-organized state into the normal state.

Corrective correlation method for the analysis of gases by plasma/laser-excited fluorescence spectrometry

Traces of gaseous impurities in permanent gases can be quantitatively determined by recording their fluorescence produced in a dual process of excitation in a plasma discharge combined with excitation in a resonant field of laser radiation. However, variations in the composition of the gas samples can cause systematic errors of determination. Such errors can be corrected using a correlation relationship established between the fluorescence intensity and concentrations of components of the sample. As an example, the possibility of accurate determination of low neon concentrations in a sample of helium containing an undetermined amount of argon is presented. The correlation relationship between the level of the saturated fluorescence of neon and the saturation parameter (spectral density of laser power required to reach the 50% level of neon fluorescence intensity relative to the level of the completely saturated fluorescence) over a broad argon content range in helium was established and used. The objective was to neutralize possible obscuring impacts of unknown uncontrolled impurities that may or may not be present in a gas sample on the analytical results.

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.

Second sound measurements very near the lambda point

High-resolution measurements of the second sound velocity near T λ in superfluid 4He at the vapor pressure are reported. Experiments were performed using the resonant technique in cavities 13, 4 and 2 mm in height. A disc of paramagnetic salt, copper ammonium bromide, in direct contact with the helium sample and coupled to a SQUID magnetometer, was used as the sensor. The sound was generated by wire-wound heaters embedded in the end opposite the sensor in each cavity. The superfluid density was determined from second sound measurements and the critical exponent ν was obtained from fits to the data. The results for the exponent were found to be very sensitive to the treatment of systematic effects in the data.

The fast alternative cryogenic experiment testbed

Subsystems for a “proof of concept” cryogenic payload have been developed to demonstrate the ability to accommodate low temperature science investigations within the constraints of the Hitchhiker siderail (HH-S) carrier on the Space Shuttle. These subsystems include: a hybrid solid neon – superfluid helium cryostat, a multi-channel Versa Modular European (VME) architecture Germanium Resistance Thermometer (GRT) readout and heater control servo system, and a multiple thermal isolation stage “probe” for thermal control of helium samples. The analysis and tests of these subsystems have proven the feasibility of a cryogenic HH-S carrier payload.

Analysis of decomposition of impurity–helium solid phase

The elemental composition of the impurity–helium solid phase (IHSP) grown by injecting of a gas jet containing Ne, Ar, Kr, and Xe atoms and N2 molecules into superfluid HeII is studied. The measured stoichiometric ratios S=NHe/NIm are much larger than the values predicted by the model of frozen together monolayer helium clusters. The theoretical possibility of freezing together of two-layered clusters is justified in the continual model of the helium subsystem of IHSP which fills the space between rigid impurity centers. Regularities of decomposition of “dry” samples (extracted from liquid helium) are analyzed in the temperature range 1.5–12 K under pressures from 10 to 500 torr. Two stages of sample decomposition are discovered: a slow stage accompanied by cooling and a rapid stage accompanied by heat release. These results suggest the presence of two types of helium in IHSP, viz., weakly bound and strongly bound helium which can be attributed respectively to the second and first coordination spheres of helium formed around heavy impurity particles. A tendency to elevation of the thermal stability of impurity–helium (IH) samples upon an increase in the mass of impurity center has been observed. An increase in the helium vapor pressure above the samples also increases their stability. It is found that the decomposition of IH samples containing nitrogen atoms in the temperature range 3–4.5 K is accompanied with luminescence induced by recombination of atoms. This indicates the possibility of a wide range of chemical reactions in solidified helium.

Heat capacity experiment for very high resolution tests of the theory of confined materials

We report the current status of an experiment to measure the heat capacity of helium confined within a stack of evenly spaced silicon plates at temperatures very close to the superfluid transition. Newly developed high-resolution thermometry has substantially improved our ability to look into regions where three-dimensional crosses over to two-dimensional behaviour and where two-dimensional behaviour dominates. These regions have been of interest to theorists and experimentalists for decades. The main part of the apparatus consists of a high-purity copper calorimeter containing a stack of 408 silicon plates spaced 57 μm apart and a pair of high-resolution, fast response, paramagnetic salt thermometers. The thermometers have been shown to have a noise level of 10 −10K with 1 Hz bandwidth. The expected resolution of the heat capacity measurements is 10 −9 K. To avoid the smearing effects of gravity in the 4.5-cm high helium sample, the measurements will be performed on the Space Shuttle. The results from the experiment can be combined with supplementary ground measurements on smaller length scales to perform additional tests of the renormalization group predictions for crossover to lower-dimensional behaviour.

Reducing gravity at the superfluid transition in helium-4

There are two intrinsic experimental limitations in measurements near Tλ for traditional ground based experiments: the gravitationally induced pressure variations present in any macroscopic helium sample limit how closely the transition can be approached, and the onset of convection in a sample with T>Tλ limits the range of heat values that can be used. To overcome these limitations, we have built a low gravity simulator consisting of a superconducting magnet with a magnetic field profile shaped to provide a magnetic force opposite that of gravity. Preliminary measurements have shown a decrease in the range of reduced temperatures across a sample but not the expected suppression of the onset of convection.

NMR investigation of the magnetic susceptibility anisotropy in3He-A

We have investigated transverse continuous wave NMR in a geometry where the modes are influenced by susceptibility anisotropy. The helium sample is separated by mylar foils into layers 12 μm thick perpendicular to the external dc magnetic field. In this geometryl is constrained to be parallel to the field andd switches from parallel to perpendicular to the field over a small range of field around the dipole unlocking field. The mode frequencies below this critical field yield a value for the susceptibility anisotropy. Above the critical field the expected negative frequency shift is observed, as well as a positive frequency shift due to bulk material and an unexpected signal close to the Larmor frequency.

The amplitude dependence of the velocity of nonlinear second sound in helium II nearT ?

The amplitude dependence of the velocity of nonlinear second sound in Helium II, as given by the Khalatnikov nonlinear coefficient, was measured nearTλ using a single-pulse travel-time technique. Using nonplanar pulses which were amplified and focused by a spherical reflector, the confirmation of Khalanikov's second order two fluid model prediction was extended to within 7 mK of Tλ much closer to the critical temperature than previously reported. This study was restricted to single pulses of second sound in order to ensure that the measurement pertained only to an undisturbed, vortextangle-free sample of Helium II.

First observation of self-focusing of nonlinear second sound in superfluid helium near T lambda.

Nonlinear second sound was studied in a large undisturbed sample of superfluid helium at temperatures near ${T}_{\ensuremath{\lambda}}$. Since the source of second sound was finite sized, the second-sound pulses were expected to show the rarefaction and amplitude attenuation characteristic of diffracting three-dimensional pulses. It was found instead that moderately high amplitude pulses propagating far from reflecting walls self-focused into stable plane wave pulses as the temperature approached the critical temperature ${T}_{\ensuremath{\lambda}}$. Self-focusing is a nonlinear phenomenon previously unreported for a sound wave.

Acoustic viscometer for measurements in gases

Martin Greenspan and Francis N. Wimenitz [‘‘An Acoustic Viscometer for Gases,’’ NBS Rep. 2658 (1953)] proposed the use of double Helmholtz resonators for measurement of the viscous diffusivity of gases. Such resonators, consisting of two cavities coupled by a duct, have a low-frequency mode where the gas oscillates between the cavities. A theoretical model that includes the viscous and thermal losses in the duct, thermal losses in the cavities, duct-end corrections, and fill ducts, have been developed. The resonator dimensions can be chosen so that the damping of the lowest mode is primarily due to viscous losses at the duct walls, and thermal losses at the cavity walls are very small. Several resonators have been constructed, including all-metal and mainly glass designs. Measurements on argon, helium, and propane samples are in very good agreement with the theoretical model.

Dynamic measurements near the lambda-point in a low-gravity simulator on the ground

The properties of liquid helium very near the lambda-transition in the presence of a heat current has received recent theoretical and experimental attention. In this regime, gravity induced pressure effects place severe constraints on the types of experiments that can be performed. A new experiment is described which largely overcomes these difficulties by magnetostrictively cancelling gravity influences in the helium sample with a suitable magnetic coil. Design limitations of the technique and a discussion of proposed experiments is presented.

Cryostat for neutron investigations on a large volume of liquid ho between 5.2 and 1.2K at pressures up to 30 bars

Investigation of liquid helium-4 by neutrons scattering methods demands special cryogenic equipment. The experimental conditions require a prolonged exhibition of the large liquid helium sample. Besides, helium-4 is one of the difficult subjects for cooling, as it possesses high heat content, the thermal capacity peak in λ-point, and the thermal conductivity in the superfluid state. A cryostat for the inelastic neutron scattering research is described. Cryostat permit to operate with the large liquid helium sample (2600cm 3) at the temperature range 5.2 − 1.2K and pressure up to 30 bars.