Evidence for Superfluidity in Para-Hydrogen Clusters Inside Helium-4 Droplets at 0.15 Kelvin

A linear carbonyl sulfide (OCS) molecule surrounded by 14 to 16 para-hydrogen (pH(2)) molecules, or similar numbers of ortho-deuterium (oD(2)) molecules, within large helium-4 ((4)He) droplets and inside mixed (4)He/(3)He droplets was investigated by infrared spectroscopy. In the pure (4)He droplets (0.38 kelvin), both systems exhibited spectral features that indicate the excitation of angular momentum around the OCS axis. In the colder (4)He/(3)He droplets (0.15 kelvin), these features remained in the oD(2) cluster spectra but disappeared in the pH(2) spectra, indicating that the angular momentum is no longer excited. These results are consistent with the onset of superfluidity, thereby providing the first evidence for superfluidity in a liquid other than helium.     

Thermodynamics of an incommensurate quantum crystal

We present a theory of the thermodynamics of an incommensurate quantum solid. The ground state of the solid is assumed to be an incommensurate crystal, with quantum zero-point vacancies and interstitials and thus a non-integer number of atoms per unit cell. We show that at low temperature T, the variation of the net vacancy concentration should be as T4 and that the first correction to the specific heat due to this varies as T7; these are quite consistent with experiments on solid helium-4. We also make some observations about the recent experimental reports of "supersolidity" in solid helium-4 that motivate a renewed interest in quantum crystals.     

Observation of superflow in solid helium

We report on the observation of nonclassical rotational inertia in solid helium-4 confined to an annular channel in a sample cell under torsional motion, demonstrating superfluid behavior. The effect shows up as a drop in the resonant oscillation period as the sample cell is cooled below 230 millikelvin. Measurement of 17 solid samples allows us to map out the boundary of this superfluid-like solid or supersolid phase from the melting line up to 66 bars. This experiment indicates that superfluid behavior is found in all three phases of matter.     

A Major Helium-3 Source at 15{degrees}S on the East Pacific Rise

An extensive plume of water enriched with helium-3 has been discovered in the deep Pacific Ocean at latitude 15 degrees S on the East Pacific Rise. In the core of the plume, at a depth of 2500 meters over the ridge crest, the helium-3/helium-4 ratio is 50 percent higher than the ratio in atmospheric helium, indicating a strong injection of mantle or primordial helium at the spreading center axis through local hydrothermal systems. The helium-3 plume is completely absent east of the rise, but it can be traced over 2000 kilometers to the west above a newly observed physical feature: a density discontinuity here caled the "ridge-crest front." The injected plume provides a unique deep-sea tracer with an asymmetric distribution which shows that the deep circulation across the rise is from east to west. The striking intensity and lateral extent of this helium-3 anomaly, compared to observations at known oceanic hydrohrmal sites, suggest that the largest hydrothermal fields in the ocean are yet to be discovered and that they will be found near 15 degrees S on the East Pacific Rise.     

Helium-3 from the mantle: primordial signal or cosmic dust?

Helium-3 in hotspot magmas has been used as unambiguous evidence for the existence of a primordial, undegassed reservoir deep in the Earth's mantle. However, a large amount of helium-3 is delivered to the Earth's surface by interplanetary dust particles (IDPs). Recycling of deep-sea sediments containing these particles to the mantle, and eventual incorporation in magma, can explain the high helium-3/helium-4 ratios of hotspot magmas. Basafts with high helium-3/helium-4 ratios may represent degassing of helium introduced by ancient (probably 1.5 to 2.0 billion years old) pelagic sediments rather than degassing of primordial lower mantle material brought to the surface in plumes. Influx of IDPs can also explain the neon and siderophile compositions of mantle samples.     

Helium isotope effect in solution in water and seawater

The isotope effect in the solution of helium in water from 0 degrees to 40 degrees C has been determined by microgasometric measurements of the solubilities of pure helium-3 and helium4. At 0 degrees C helium-3 is less soluble than helium-4 in both distilled water and sea-water by 1.2 percent. The observed fractionation factor is 0.988+/-0.002 at 0 degrees C and appears to decrease with increasing temperature at the rate of 0.0001 per degree Centigrade, although the existence of this trend is of limited statistical certainty. The measured isotope effect is in agreement with the ratio of helium-3 to helium-4 in surface ocean water reported by Clarke, Beg, and Craig.     

Primitive helium in diamonds

Thirteen diamond stones from various unspecified mines in South Africa were analyzed for the isotopic ratio of helium-3 to helium-4. Values of the ratio ranged from less than 10(-7) to (3.2 +/- 0.25) x 10(-4). The latter value is higher than the primordial helium-3/helium-4 ratio in meteorites and close to the ratio for solar-type helium. Such extremely high values may represent primitive helium that evolved very little (that is, showed very little increase in radiogenic helium-4) since the formation of the earth.     

Interplay of rotational, relaxational, and shear dynamics in solid ⁴He

Using a high-sensitivity torsional oscillator (TO) technique, we mapped the rotational and relaxational dynamics of solid helium-4 ((4)He) throughout the parameter range of the proposed supersolidity. We found evidence that the same microscopic excitations controlling the torsional oscillator motions are generated independently by thermal and mechanical stimulation. Moreover, a measure for the relaxation times of these excitations diverges smoothly without any indication for a critical temperature or critical velocity of a supersolid transition. Finally, we demonstrated that the combined temperature-velocity dependence of the TO response is indistinguishable from the combined temperature-strain dependence of the solid's shear modulus. This implies that the rotational responses of solid (4)He attributed to supersolidity are associated with generation of the same microscopic excitations as those produced by direct shear strain.     

Bilayer 3He: a simple two-dimensional heavy-fermion system with quantum criticality

Two-dimensional helium-3 (3He) provides a simple model for the experimental investigation of the emergence of quantum complexity in a strongly correlated Fermi system. We have observed two-dimensional, two-band heavy-fermion behavior in bilayer films of 3He atoms when adsorbed on the surface of graphite preplated by a solid bilayer of 4He. Thermodynamic measurements on this system showed that the relevant control parameter is the total density of the 3He film. The 3He bilayer system can be driven toward a quantum critical point at which the effective mass appears to diverge, interband coupling vanishes, and a local-moment state appears. It opens a new testing ground for theories of quantum criticality in heavy-fermion materials.     

Mantle plume helium in submarine basalts from the galapagos platform

Helium-3/helium-4 ratios in submarine basalt glasses from the Galapagos Archipelago range up to 23 times the atmospheric ratio in the west and southwest. These results indicate the presence of a relatively undegassed mantle plume at the Galápagos hot spot and place Galápagos alongside Hawaii, Iceland, and Samoa as the only localities known to have such high helium-3/helium-4 ratios. Lower ratios across the rest of the Galápagos Archipelago reflect systematic variations in the degree of dilution of the plume by entrainment of depleted material from the asthenosphere. These spatial variations reveal the dynamics of the underlying mantle plume and its interaction with the nearby Galápagos Spreading Center.     

A short duration of the Cretaceous-Tertiary boundary event: evidence from extraterrestrial helium-3

Analyses of marine carbonates through the interval 63.9 to 65.4 million years ago indicate a near-constant flux of extraterrestrial helium-3, a tracer of the accretion rate of interplanetary dust to Earth. This observation indicates that the bolide associated with the Cretaceous-Tertiary (K-T) extinction event was not accompanied by enhanced solar system dustiness and so could not have been a member of a comet shower. The use of helium-3 as a constant-flux proxy of sedimentation rate implies deposition of the K-T boundary clay in (10 +/- 2) x 10(3) years, precluding the possibility of a long hiatus at the boundary and requiring extremely rapid faunal turnover.     

Helium Isotopic Evidence for a Lower Mantle Component in Depleted Archean Komatiite

Archean magnesium-rich komatiites require hot and presumably deep mantle sources, but their trace-element composition and radiogenic isotope composition are similar to those of modern mid-ocean ridge basalts, which originate in the upper mantle. The isotopic composition of helium extracted by sequential crushing of fresh olivines separated from two Archean and one mid-Proterozoic komatiites varies over three orders of magnitude, between a radiogenic end-member rich in helium-4 and a component rich in helium-3. Such helium-3 enrichment suggests the presence of a lower mantle component in Archean komatiites.     

Advection of pore fluids through sediments in the equatorial East pacific

Measurements of the ratio of helium-4 to helium-3 and of calcium ion in the pore waters of sediments at two locations in the eastern equatorial Pacific indicate that solution advection is occurring through the sediments. Both the helium ratio and the calcium ion profile yield velocity values for advective flow of about 20 centimeters per year. Mass balance constraints are also consistent with the interpretation presented. Flow appears to be occurring thorugh relatively thick sediments, on the order of 300 meters.     

Rare gas isotopes in hawaiian ultramafic nodules and volcanic rocks: constraint on genetic relationships

Differences in the rare gas isotopic ratios, especially the ratios of helium-3 to helium-4 and of argon-40 to argon-36, in Hawaiian ultramafic nodules and phenocrysts in volcanic rocks indicate that the nodules and phenocrysts were derived from different sources. The isotopic ratios in ultramafic nodules are similar to those in oceanic tholeiites. The phenocrysts seem to have formed in equilibrium with source materials richer in primordial components than those of the oceanic tholeiites. Mixing between the sources is quite likely.     

Superfluidity of grain boundaries and supersolid behavior

When two communicating vessels are filled to a different height with liquid, the two levels equilibrate because the liquid can flow. We have looked for such equilibration with solid (4)He. For crystals with no grain boundaries, we see no flow of mass, whereas for crystals containing several grain boundaries, we detect a mass flow. Our results suggest that the transport of mass is due to the superfluidity of grain boundaries.     

Fermionic superfluidity with imbalanced spin populations

We established superfluidity in a two-state mixture of ultracold fermionic atoms with imbalanced state populations. This study relates to the long-standing debate about the nature of the superfluid state in Fermi systems. Indicators for superfluidity were condensates of fermion pairs and vortices in rotating clouds. For strong interactions, near a Feshbach resonance, superfluidity was observed for a broad range of population imbalances. We mapped out the superfluid regime as a function of interaction strength and population imbalance and characterized the quantum phase transition to the normal state, known as the Pauli limit of superfluidity.     

Evidence of supersolidity in rotating solid helium

Supersolidity, the appearance of zero-viscosity flow in solids, was first indicated in helium-4 torsional oscillator (TO) experiments. In this apparatus, the irrotationality of the superfluid component causes it to decouple from the underlying normal solid, leading to a reduction in the resonant period of the TO. However, the resonant period may be altered for reasons other than supersolidity, such as the temperature dependence of the elastic modulus of solid helium. Superimposing rotation onto oscillatory measurements may distinguish between supersolidity and classical effects. We performed such simultaneous measurements of the TO and the shear modulus, and observed substantial change in the resonant period with rotational speed where the modulus remained unchanged. This contrasting behavior suggests that the decrease in the TO period is a result of supersolidity.     

Tritium and argon radioactivties in lunar material

Tritium and argon radioactivities, attributable to galactic and solar cosmic-ray interactions, were measured in lunar soil and in three lunar rocks. The tritium in the soil, 325 +/- 17 disintegrations per minute per kilogram, is slightly higher than that in the rocks, 212 to 250 dpm/kg. For two rocks, the tritium was combined with the helium-3 in order to calculate exposure ages of 375 +/- 40 and 205 +/- 25 million years. The argon-37 radioactivities, 21.0 to 27.2 dpm/kg, and the argon-39 radioactivities, 12.1 to 16.4 dpm/kg, are slightly higher than those in stony meteorites. Higher exposure ages were obtained from the argon isotopes than from tritium and helium-3. On the basis of the known galactic cosmic-ray flux and the known cross section, at least half of the observed radioactivities are produced by solar cosmic rays.     

Apollo 11 solar wind composition experiment: first results

The helium-4 solar wind flux during the Apollo 11 lunar surface excursion was (6.3 +/- 1.2) x 10(6) atoms per square centimeter per second. The solar wind direction and energy are essentially not perturbed by the moon. Evidence for a lunar solar wind albedo was found.     

Rare gases in lunar samples: study of distribution and variafions by a microprobe technique

The rare gas distribution in lunar soil, breccias, and rocks was studied with a micro-helium-probe. Gases are concentrated in grain surfaces and originate from solar wind. Helium-4 concentrations of different mineral components vary by more than a factor of 10 apart from individual fluctuations for each type. Also grains with no detectable helium-4 exist. Titanium-rich components have the highest, calcium-rich minerals the lowest concentrations. The solar wind was redistributed by diffusion. Mean gas layer thicknesses are 10, 6, and 5 microm for helium, neon, and argon respectively. Lithic fragments in breccias contain no solar gases. Glass pitted surfaces of crystalline rocks contain about 10(-2) cubic centimeter of helium-4 per square centimeter. Etched dust grains clearly show spallogenic and radiogenic components. The apparent mean exposure age of dust is approximately 500 x 10(6) years, its potassium-argon age is approximately 3.5 x 10(9) yerars. Cavities of crystalline rocks contain helium-4, radiogenic argon, H(2), and N(2).