Trace detection of metastable helium molecules in superfluid helium by laser-induced fluorescence

We describe an approach to detecting ionizing radiation that combines the special properties of superfluid helium with the sensitivity of quantum optics techniques. Ionization in liquid helium results in the copious production of metastable He2 molecules, which can be detected by laser-induced fluorescence. Each molecule can be probed many times using a cycling transition, resulting in the detection of individual molecules with high signal to noise. This technique could be used to detect neutrinos, weakly interacting massive particles, and ultracold neutrons, and to image superfluid flow in liquid 4He.     

Onset of superfluidity in small CO2(4He)N clusters

We provide definitive theoretical evidence for the onset of superfluidity in small helium clusters doped with molecules at less than one solvation shell, with quantitative analysis of spectroscopic constants for CO2 in (4)He(N) in terms of nonclassical rotational inertia and helium superfluidity calculated by path integral methods. We find a significant superfluid response for N>/=5, with essentially unit response to rotations around the CO2 axis and partial response to rotations about an axis perpendicular to the CO2 axis for N>/=6. This anisotropic superfluid response is shown to be responsible for the N dependence of measured CO2 rotational spectra in (4)He(N).     

Superfluidity in a doped helium droplet

Path-integral Monte Carlo calculations of the superfluid density throughout 4He droplets doped with linear impurities are presented. After deriving a local estimator for the superfluid density distribution, we find a decreased superfluid response in the cylindrically symmetric region of the first solvation layer. The helium in this region has a superfluid transition temperature similar to that of a two-dimensional helium system and may be responsible for previously unexplained experimental Q-branch measurements.     

High-resolution spectroscopy of NO in helium droplets: a prototype for open shell molecular interactions in a quantum solvent

We have measured the high-resolution infrared spectrum of the radical NO in the (2)Pi(1/2) state in superfluid helium nanodroplets. The features are attributed to the -doubling splitting and the hyperfine structure. The hyperfine interaction is found to be unaffected by the He solvation. For the Lambda-doubling splitting, we find a considerable increase by 55% compared to the gas phase. This is explained by a confinement of the electronically excited NO states by the surrounding He. The rotational level spacing is decreased to 76% of the gas phase value. The IR transition to the J=1.5 state is found to be homogeneously broadened. We attribute both observations to the coupling between the molecular rotation and phonon/roton excitations in superfluid (4)He droplets.     

The two-fluid model with superfluid entropy

Summary The two-fluid model of liquid helium is generalized to the case that the superfluid fraction has a small entropy content. We present theoretical arguments in favour of such a small superfluid entropy. In the generalized two-fluid model various sound modes of He II are investigated. In a superleak carrying a persistent current the superfluid entropy leads to a new sound mode which we call sixth sound. The relation between the sixth sound and the superfluid entropy is discussed in detail.     

Superfluid helium interferometry: an introduction

This article describes, at an introductory level, how superfluids can be used to measure absolute rotations. To make it self-contained to some degree, I first introduce briefly the two-fluid model for superfluid helium and the concept of superfluid order parameter. These ideas, which were put forward for the superfluid heliums, are now widely used, in particular for the BEC gases which are the main topic of this volume. They are presented in the somewhat different perspective of helium physics. The second part will deal with the Josephson effects, the real engine behind superfluid interferometry. These effects were predicted in the early sixties for superconductors and were promptly observed in the laboratory. It was quickly realised that they would also exist in superfluids but the search took longer and conclusive experiments were performed in the eighties only in the B-phase of superfluid ³He. How these experiments are done, and how they can be used to measure the rotation of the Earth by superfluid interferometry is surveyed in the last two sections.     

Hydrodynamics of superfluid helium in a single nanohole

The flow of liquid helium through a single nanohole with radius smaller than 25 nm was studied. Mass flow was induced by applying a pressure difference of up to 1.4 bar across a 50 nm thick Si(3)N(4) membrane and was measured directly by means of mass spectrometry. In liquid He I, we experimentally show that the fluid is not clamped by the short pipe with diameter-to-length ratio D/L?1, despite the small diameter of the nanohole. This viscous flow is quantitatively understood by making use of a model of flow in short pipes. In liquid He II, a two-fluid model for mass flow is used to extract the superfluid velocity in the nanohole for different pressure heads at temperatures close to the superfluid transition. These velocities compare well to existing data for the critical superflow of liquid helium in other confined systems.     

Heat conduction through narrow channels and phase separation of He II at zero gravity

We explain the heat flow properties of a system which works as a phase separator for superfluid helium at zero gravity. Differently from most previous work the flow of He II in a narrow slit between a helium bath and a vapour space (instead of a second He bath) is studied. Due to the Fountain effect most of the He II stays contained in the bath, and a small amount is evaporated to carry away the heat transported through the slit. The role of evaporation kinetics is discussed and its contribution, raising the heat flow resistance, is calculated. At higher heat flow there is a sharp transition to a different flow state, which we identify with the Gorter-Mellink regime where the supercomponent flows dissipatively and liquid enters into the vent line so that an additional heat exchanger is necessary for its evaporation. In this regime the mass and heat flow are still controlled by the applied pressure gradients or-important for practical use:-by varying the length of the slit.     

Surface fluctuations of normal and superfluid 3He probed by Wigner solid dynamics

Electron scattering from surface fluctuations on normal and superfluid 3He has been measured by its effect on the linewidth of the low-wave-vector transverse magnetophonon mode of the electron crystal (the Wigner solid) floating on the helium surface. The relaxation rate becomes anomalously low below 70 mK, and reaches a plateau at about 3 times less than its expected value before dropping further at the superfluid transition. The absence of such anomalous behavior on 4He suggests that the effect is specific to liquid 3He.     

Neutron diffraction study of polycrystalline 4He in a porous medium

The elastic (diffraction) component of the neutron scattering cross section, which carries information on the atomic structure of solid helium confined in silica aerogel, has been studied. Analysis of the crystalline structure of solid helium in a porous medium, which is determined from the existing neutron diffraction data, indicates that the superfluid phase is localized inside a hexagonal close-packed phase and is not present in a body-centered cubic crystal. It has also been revealed that the addition of the ³He isotope changes the structure of solid helium and hardly affects the formation of a superfluid phase.     

Study of the electric field and wall voltage in a high pressure ac-PDP cell by laser induced fluorescence spectroscopy

The electric field in a surface discharge type ac-PDP cell with He or He/Xe(0.1%) mixture has been measured over a wide range of pressure (5－50 kPa) using laser induced fluorescence detection. The wall voltage was estimated from the measured electric field. The Stark manifolds of triplet atomic helium Rydberg state (2s^3S) with principal quantum numbers (n=8 and 9) have been used to measure the electric field, as the lifetime of 2s^3S is longer than the single atomic helium Rydberg state (2s^1S) in high pressure discharge. Comparison of the Stark manifolds between the n=9 and n=8 shows that the measurement accuracy of electric field can be increased by 10%. The maximum electric field strength during discharge and the wall voltage at the end of pulse decreases with the increase of pressure. The comparison of He and He/Xe(0.1%) discharge at 13 kPa showed that He/Xe gas mixture discharge can accumulate more wall charge on MgO surface and the electric field was somewhat higher than those of pure helium discharge during pulse off period under the same discharge conditions.     

Kerr effect in liquid helium at temperatures below the superfluid transition

The electro-optical Kerr effect induced by a slowly varying electric field in liquid helium at temperatures below the lambda point is investigated. The Kerr constant of liquid helium is measured to be (1.43+/-0.02(stat)+/-0.04(sys)) x 10(-20) (cm/V)(2) at T=1.5 K. Within experimental uncertainty, the Kerr constant is independent of temperature in the range T=1.5 K to 2.17 K, which implies that the Kerr constant of the superfluid component of liquid helium is the same as that of normal liquid helium. Pair and higher correlations of He atoms in the liquid phase account for about 23% of the measured Kerr constant. Liquid nitrogen was used to test the experimental setup; the result for the liquid nitrogen Kerr constant is (4.38+/-0.15) x 10(-18) (cm/V)(2). Kerr effect can be used as a noncontact technique for measuring the magnitude and mapping out the distribution of electric fields inside these cryogenic insulants.     

Nonclassical rotational inertia in helium crystals

It has been proposed that the observed nonclassical rotational inertia (NCRI) in solid helium results from the superflow of thin liquid films along interconnected grain boundaries within the sample. We have observed NCRI in large (4)He crystals grown at constant temperature and pressure, demonstrating that the superfluid grain boundary model cannot explain the phenomenon.     

Measurement of a long electronic spin relaxation time of cesium atoms in superfluid helium

The longitudinal electronic spin relaxation time of Cs atoms optically polarized in superfluid helium (He II, 1.5 K) has been measured with special care to cope with a serious decrease in the number of Cs atoms in the observation region. This decrease, mainly caused by helium convection in introducing the atoms into He II by laser sputtering, was significantly reduced using a new atom implantation method. Combined with a careful correction for the number of atoms, we have determined the relaxation time to be 2.24(19) s or longer, roughly twice as long as that in solid He.     

Magnetic dichroism of potassium atoms on the surface of helium nanodroplets

The population ratio of Zeeman sublevels of atoms on the surface of superfluid helium droplets (T=0.37 K) has been measured. Laser induced fluorescence spectra of K atoms are measured in the presence of a moderately strong magnetic field (2.9 kG). The relative difference between the two states of circular polarization of the exciting laser is used to determine the electron spin polarization of the ensemble. Equal fluorescence levels indicate that the two spin sublevels of the ground-state K atom are equipopulated, within 1%. Thermalization to 0.37 K would give a population ratio of 0.35. We deduce that the rate of spin relaxation induced by the droplet must be <520/s. For the K2 triplet dimer we find instead full thermalization of the spin.     

Condensed water in superfluid He-II

It was found that when ⁴He gas containing water vapor as an impurity condenses on the surface of superfluid He-II cooled to ∼ 1.4 K, semitransparent clouds (icebergs) form in the volume of a glass cell filled with He-II below the He-II surface. The form of the icebergs extracted from the superfluid liquid remains virtually unchanged on heating up to ∼ 1.8 K. In the temperature range 1.8–2.2 K the thermometers register sharp temperature jumps, which are accompanied by jumps in the gas pressure in the cell and a repeated decrease, by more than two orders of magnitude, in the total volume of the condensate, i.e., the water content in the volume of an iceberg does not exceed 10²⁰H₂O molecules per 1 cm³. It can be inferred that porous icebergs, permeated with superfluid liquid and containing cores consisting of small clusters surrounded by a layer of solidified helium, form in the volume of He-II as the gas mixture condenses. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 11, 744–748 (10 December 1999)     

Spectroscopic exploration of atomic scale superfluidity in doped helium nanoclusters

We present high resolution spectra of He(N)-OCS clusters with N up to 39 in the microwave and 72 in the infrared regions, observed with apparatus-limited line widths of about 15 kHz and 0.001 cm(-1), respectively. The derived rotational constant, B (proportional to the inverse moment of inertia), passes through a minimum at N=9, then rises due to onset of superfluid effects, and exhibits broad oscillations with maxima at N=24, 47 and minima at 36, 62. We interpret these unexpected oscillations as a manifestation of the aufbau of a nonclassical helium solvation shell structure. These results bridge an important part of the gap between individual molecules and bulk matter with atom by atom resolution, providing new insight into microscopic superfluidity and a critical challenge for theory.     

Observation of a superfluid phase in solid helium

Evidence of a superfluid liquid phase present in polycrystalline helium at a temperature of 0.2 K and a pressure of 51 bar has been obtained by means of inelastic neutron scattering. The superfluid component is absent at a temperature of 0.6 K and the same pressure. Thus, a “solid helium-superfluid helium” phase transition has been discovered. The sample of solid helium in a porous medium (silica aerogel) has been prepared with the use of a capillary blocking technique. The shape of the structure factor of the superfluid phase indicates the presence of clusters or the effects of a restricted geometry. The results may be used to explain the nonclassical rotational inertia phenomenon in solid helium (often referred to as supersolidity, Nature, 2004).     

Dynamical effects in superfluid helium 4

The close structural similarity between the commutation relations of harmonic oscillator operators and the operators for Bose fields is exploited to study the excitation spectrum in superfluid helium 4. By applying ‘broken symmetry’ condition it is shown how the creation of phonon gives rise to superfluid behaviour of liquid He 4. The energy gap needed for roton excitation is derived.     

A1 and A2 transitions in superfluid 3He in 98% porosity aerogel

Superfluid 3He in high porosity aerogel is the system in which the effects of static impurities on a p-wave superfluid can be investigated in a systematic manner. We performed shear acoustic impedance measurements on this system (98% porosity aerogel) in the presence of magnetic fields up to 15 T at the sample pressures of 28.4 and 33.5 bars. We observed the splitting of the superfluid transition into two transitions in high fields in both bulk and liquid in aerogel. The field dependence of the splitting in aerogel resembles that of the bulk superfluid 3He caused by the presence and growth of the A1 phase. Our results provide the first evidence of the A1 phase in superfluid (3)He/aerogel.