3He Atoms Research Articles

Variational Calculation of 4He Tetramer Ground and Excited States Using Realistic 4He–4He Potentials

We calculated binding energies and wave functions of the 4He tetramer ground and excited states employing various realistic 4He−4He potentials which includes the currently most accurate one with the adiabatic, relativistic, QED and residual retardation corrections. We used our Gaussian expansion method (GEM) for ab initio variational calculations of few-body systems. We found that precisely the same shape of the short-range correlation (r ij < 4A) in the dimer appear in the ground and excited states of trimer and tetramer. The four kinds of the binding energies of the trimer and tetramer ground and excited states, $${B_3^{(0)}, B_3^{(1)}, B_4^{(0)}}$$ and $${B_4^{(1)}}$$ , for the different potentials exhibit perfect linear correlations over the range of binding energies relevant for 4He atoms; namely, six types of the generalized atomic Tjon lines were observed.

Final report on P1-APMP.EM-S9: VNIIM/KRISS bilateral comparison of DC magnetic flux densityby means of a transfer standard coil

The purpose of this bilateral comparison is to check the conformance of the base quantities of magnetic measurements, DC magnetic flux density and its ratio to a current, as reproduced at VNIIM and KRISS. In these institutes adequate conditions for precise measurements in low magnetic fields are provided and the appropriate equipment for attaining a high level of accuracy is available.The results in this report cover the comparisons of two units, T/A and T, reproduced by the two institutes. The experimental comparison data show good agreement within the estimated uncertainty components of the standards. The coordinated values of the unit of DC magnetic flux density and its ratios to DC current show a standard uncertainty at the level of 1 × 10−6 to 1.2 × 10−6 (k = 1) using the value of the gyromagnetic ratio of the shielded protons γp that was recommended by CODATA in 2010, the experimental determination of the ratio (γ4He/γp) of 4He atoms to protons, and the standards of the two institutes.The results give a basis for carrying out multilateral comparisons of standard quantum magnetometers of metrological institutes in the framework of APMP with participation of geomagnetic observatories, which require the establishment of a unified standard of the unit of DC magnetic flux density. They also show the possibility of decreasing the uncertainty of the determination of the unit of DC magnetic flux density from direct comparisons of standard quantum magnetometers.Main text.To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.The final report has been peer-reviewed and approved for publication by APMP, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

The Quantum Potential: The Missing Interaction in the Density Maximum of He4 at the Lambda Point

The lambda point in liquid He4 is a well established phenomenon acknowledged as an example of Bose-Einstain condensation. This is generally accepted, but there are serious discrepancies between the theory and experimental results, namely the lower value of the transition temperature T and the negative value of dT /dP. These discrepancies can be explained in term of the quantum stochastic hydrodynamic analogy (SQHA). The SQHA shows that at the He4IHe4II superfluid transition the quantum coherence length c becomes of order of the distance up to which the wave function of a couple of He4 atoms extends itself. In this case, the He42 state is quantum and the quantum pseudo-potential brings a repulsive interaction that leads to the negative dT /dP behavior. This fact overcomes the difficulty to explain the phenomenon by introducing a Hamiltonian inter-atomic repulsive potential that would obstacle the gas-liquid transition.

Neutron reflection from the surface of a liquid 4He-3He mixture

We have used neutron reflection from the liquid surface at ultra-low temperatures to study the surface properties of liquid helium. We measured neutron scattering from the free surfaces of commercially pure 4He (0.3 ppm of 3He impurities) and of a 3He-4He mixture with a 3He concentration of 0.5% for temperatures in the range from 340mK to 2.2K We compare the reflected neutron intensity for different temperatures and we fit a model that describes the collected data. The data are described well by a diffusive 3He layer of a few hundred Angstrom thickness on the bulk 4He liquid surface. Even at high temperatures (∼2K) there is an increased concentration of 3He atoms near the liquid surface. The distribution does not change very much with temperature. At low temperatures the neutron absorption increases significantly, which might be an indication of the formation of Andreev states. However, the shapes of the curves do not change very much which seems to suggest that the layer formed by the 3He atoms in Andreev states is very thin ∼ 10Å. The experimental method, based on neutron reflectometry, opens up new opportunities for the study of the surfaces and interfaces of quantum fluids and solids.

Emission profiles of K-He exciplexes in cold helium gas

Emission spectra of exciplexes composed of a light alkali atom in the first excited state and 4He atoms have been observed in cryogenic gas in the spectral range from the atomic D lines to 6300 cm−1. A unified semi-classical theory of line broadening has been used to determine the total profile from the center to the far wings of emission profiles of potassium perturbed by helium at low temperatures and high He density. The agreement of the theoretical peak positions of K*Hen exciplexes compared to the experimental determinations is fairly good. Such comparisons provide a critical test of the calculated molecular potentials and the relevance of the theoretical approach which has been used.

Spatial effects in the 800 keV 3He implantation in W followed by isochronal annealing at 900 K

The effect of fluence on the depth distribution of vacancy clusters in the track region of room temperature 800keV 3He atom implanted tungsten further annealed up to 900K has been investigated using an Object Kinetic Monte Carlo (OKMC) approach. The results show that the width and the position of the maximum size of the depth distribution strongly depend on the implantation fluence. For the high implantation fluence, the vacancy clusters are the largest in the first 100nm close to the surface, whereas, for the low implantation fluence, the distribution is more uniform in size between 100 and 300nm from the surface. The vacancy clusters are always smaller very close to the surface, and this whatever the fluence. The trends observed are in good agreement with the experimental Positron Annihilation Spectroscopy (PAS) results.

Density Functional Theory and Bose Statistics for the Freezing of Superfluid 4He

Density functional theory so far has experienced pathological results in the description of the freezing of superfluid 4He, even with the use of accurate inputs, namely the density response function, χ(q), and the equation of state of the superfluid. In fact, while the second order approximation of Ramakrishnan-Yussouff type gives a too much stable solid phase (Likos et al. in Phys. Rev. B 55:8867, 1997; Minoguchi and Galli in J. Law Temp. Phys. 162:160, 2011), a non-perturbative treatment called modified weighted density approximation (MWDA) gives, on the other hand, a too stable superfluid phase (Minoguchi et al. in J. Phys. Conf. Ser. 2012). We have preliminarily performed a MWDA calculation for a specific Gaussian distortion α=0.387 A−2 of liquid 4He. By using χ(q) and the equation of state of a system of liquid 4He in which Bose statistics has been suppressed, we have found that the calculation provides a lower energy. This indicates that the solid may be better described by a distorted normal liquid. The input data has been obtained via Path Integral Monte Carlo simulations of a system of distinguishable 4He atoms. The χ(q) has been obtained from the dynamic structure factors extracted, via the Genetic Inversion via Falsification of Theories (GIFT) method (Vitali et al. in Phys. Rev. B 82:174510, 2010) from imaginary time density–density correlation functions.

Effects of a 3He impurity on the Elastic Anomalies of 4He at T=0

Effects on the elastic constants of a system formed from 4He atoms with a 3He impurity concentration of 0.14 % are investigated in a range of densities varying from 0.029 to 0.035 A−3. A model wave function of the shadow class is used to compute elastic constants and quantities defined through relations satisfied by them. The linear compressibility assumes the isotropic character expected for an hcp crystal with a constant value of the structure axis ratio c/a. This is in contrast with bulk 4He where an anisotropic behavior is observed.

Structural and Superfluid Properties of the 4He Monolayer on a C28 Molecule

We have performed path-integral Monte Carlo calculations to study the adsorption of N4He atoms on a single C28 fullerene molecule. Radial density distributions show a layer-by-layer growth of 4He with the first adlayer being located at a distance of 5.3 A from the center of C28. The monolayer is found to show a commensurate structure at N=16 with each of the 16 adsorption sites on the molecular surface being occupied by one 4He atom. As more helium atoms are adsorbed beyond N=16, the adlayer is in a mixed state consisting of 4 localized atoms at the hexagonal faces and the other atoms delocalized over the pentagonal faces. Another structurally-ordered state is observed at N=32, where the helium layer shows the same crystalline structure with an icosahedral symmetry as observed for 32 4He atoms on a C60 molecule. It is found that more 4He atoms can be squeezed into the first layer to disrupt this icosahedral structure when enough 4He atoms are added in the second layer. Finally we observe the reentrant superfluid response of the monolayer with superfluidity being quenched completely at the ordered states of N=16 and 32.

Gaussian expansion method for few-body systems and its applications to atomic and nuclear physics

The Gaussian expansion method and its application to various three-, four-, and five-body problems are reviewed. As examples for the application, we review i) the application to three- and four-body 4He-atom clusters and ii) benchmark testing for four-nucleon bound states using realistic force and calculation of the second 0+ state of 4He, iii) the four-body calculation of |${}^4_{\Lambda }$|H and |${}^4_{\Lambda }$|He taking ΛN–ΣN coupling, and iv) the five-body calculation of a double Λ hypernucleus, |${}^{11}_{\Lambda }$|Be. In addition, we discuss the understanding of the structure and the mechanisms of those systems together with some useful techniques for the calculations. We obtain the first numerically reliable solution to the binding energies and wave functions of the four-body system of 4He atoms interacting with an extremely strong short-range repulsion and a weak van der Waals attraction. By applying the method to the calculations of the four-nucleon bound state, we find that the drastic change in the spatial structure between the 0+1 to 0+2 states is well understood in terms of the GEM four-body calculation. The four-body calculations are performed for |${}^4_{\Lambda }$|H and |${}^4_{\Lambda }$|He and the role of Λ–Σ conversion in these hypernuclei is discussed. Energy levels of the double Λ hypernucleus, |${}^{11}_{\Lambda \Lambda }$|Be, are calculated within the framework of an ααnΛΛ five-body model. The Hida event, recently observed in the KEK-E373 experiment, is interpreted as an observation of the ground state of |${}^{11}_{\Lambda \Lambda }$|Be.

Leftover Superfluid 4He in Aerogel and Its Crystallization by Cooling

Using a variable volume cell, we were able to crystallize 4He in aerogels at a constant temperature. The entire crystallization process was monitored visually owing to the transparency of the aerogel. Two different crystallization processes of 4He in aerogels were observed: creep at high temperatures and avalanche at low temperatures. In a 96 % porosity aerogel, we noticed that 4He remained liquid in some parts of the cell even though other parts of the aerogel were completely crystallized. Once such a situation was formed, the application of additional pressure did not further crystallize the liquid. This is presumably because a supply path of 4He atoms from the bulk crystal was blocked by the crystals in the aerogel. This leftover liquid, however, was found to begin to crystallize via avalanches when cooled below a particular temperature. If the crystallization pressure in aerogel is temperature independent at low temperatures as the bulk crystallization pressure, the crystallization by cooling is rather unusual. Possible explanations would be a decrease of the crystallization pressure in aerogel in the low temperature region, or the supersolidity of crystals in aerogel playing some role in mass transport.

A Summary of Mass Flux Measurements in Solid 4He

Here we provide a summary and brief review of some of the work done with solid 4He at the University of Massachusetts Amherst below a sample pressure of 28 bar. The motivation for the work has been to attempt to pass 4He atoms through solid 4He without directly applying mechanical pressure to the solid itself. The specific technique chosen is limited to pressures near the melting curve and was initially designed to provide a yes/no answer to the question of whether or not it might be possible to observe such a mass flux. The thermo-mechanical effect and direct mass injection have been separately used to create chemical potential differences between two reservoirs of superfluid 4He connected to each other through superfluid-filled Vycor rods in series with solid 4He, which is in the hcp region of the phase diagram. The thermo-mechanical effect is a more versatile approach. And, in a particular symmetric application it is designed to provide a mass flux with little or no net increase in the density of the solid. Our observations, off but near the melting curve, have included: (1) the presence of an increasing DC flux of atoms through the solid-filled cell with decreasing temperature below ≈650 mK and no flux above this temperature; (2) the presence of a flux minimum and flux instability in the vicinity of 75–80 mK, with a flux increase at lower temperatures; (3) the temperature dependence of the flux above 100 mK and the dependence of the flux on the net driving chemical potential difference provide interesting insights on the possible mechanism that leads to the flux above 100 mK. The most recent data suggest that whatever is responsible for the flux in solid 4He, at least for T>100 mK, may be an example of a Bosonic Luttinger liquid.

On the mechanism of electromagnetic microwave absorption in superfluid helium

In experiments on electromagnetic (EM) wave absorption in the microwave range in superfluid (SF) helium [1–3], a narrow EM field absorption line with a width on the order of (20–200) kHz was observed against the background of a wide absorption band with a width of 30–40 GHz at frequencies f 0 ≈ 110–180 GHz corresponding to the roton gap energy Δ r (T) in the temperature range 1.4–2.2 K. Using the so-called flexoelectric mechanism of polarization of helium atoms (4He) in the presence of density gradients in SF helium (HeII), we show that nonresonance microwave absorption in the frequency range 170–200 GHz can be due to the existence of time-varying local density gradients produced by roton excitations in the bulk HeII. The absorption bandwidth is determined by the roton-roton scattering time in an equilibrium Boltzmann gas of rotons, which is t r-r ≈ 3.4 × 10−11 s at T = 1.4 K and decreases upon heating. We propose that the anomalously narrow microwave resonance absorption line in HeII at the roton frequency f 0(T) = Δr(T)/2πħ appears due to the following two factors: (i) the discrete structure of the spectrum of the surface EM resonator modes in the form of a periodic sequence of narrow peaks and (ii) the presence of a stationary dipole layer in HeII near the resonator surface, which forms due to polarization of 4He atoms under the action of the density gradient associated with the vanishing of the density of the SF component at the solid wall. For this reason, the relaxation of nonequilibrium rotons generated in such a surface dipole layer is strongly suppressed, and the shape and width of the microwave resonance absorption line are determined by the roton density of states, which has a sharp peak at the edge of the roton gap in the case of weak dissipation. The effective dipole moments of rotons in the dipole layer can be directed either along or across the normal to the resonator surface, which explains the experimentally observed symmetric doublet splitting of the resonance absorption line in an external dc electric field perpendicular to the resonator surface. We show that negative absorption (induced emission) of EM field quanta observed after triggering a Kapitza “heat gun” occurs when the occupation numbers for roton states due to “pumping” of rotons exceed the occupation numbers of EM field photons in the resonator.

Thermal Casimir-Polder interaction of different atoms with graphene

The thermal correction to the energy of Casimir-Polder interaction of atoms with a suspended graphene membrane described by the Dirac model is investigated. We show that a major impact on the thermal correction is made by the size of the gap in the energy spectrum of graphene quasiparticles. Specifically, if the temperature is much smaller than the gap parameter (alternatively, larger or of the order of the gap parameter), the thermal correction is shown to be relatively small (alternatively, large). We have calculated the free energy of the thermal Casimir-Polder interaction of atoms of He, Na, Rb, and Cs with graphene described by both the hydrodynamic and Dirac models. It is shown that in exact computations using the Dirac model, one should use the polarization operator at nonzero temperature. The computational results for the Casimir-Polder free energy obtained in the framework of hydrodynamic model of graphene are several times larger than in the Dirac model within the separation region below 2$\mu$m. We conclude that the theoretical predictions following from the two models can be reliably discriminated in experiments on quantum reflection of different atoms on graphene.

Magnetization of 3He layers in ferromagnetic regime: Cluster size effects

Abstract Magnetization of pure 3He monolayers on graphite and solid 3He on 4He-preplated graphite in the ferromagnetic regime is investigated theoretically within a two-dimensional spin-1/2 Heisenberg model in an external magnetic field. We develop an analytical approach based on a second-order two-time Green function formalism which describes the thermodynamic functions for finite as well as for infinite spin systems in the whole temperature range at arbitrary fields h. In particular, the present theory provides a proper description of the magnetization in the intermediate temperature region h T J at h well below the exchange J, that is in the region where the measurements for solid 3He monolayers are usually made. It is known from the experiment that only a part of 3He atoms is involved into ferromagnetic exchange. The ferromagnetic spins form clusters whose size is a function of coverage. The coverage dependences of the exchange constant, saturation magnetization, and average cluster size are found and analyzed. Nonmonotonic variation of the exchange constant with coverage is discussed. The effect of the cluster size on the temperature dependence of the magnetization is studied. To clarify how the cluster shape and boundary conditions affect magnetic properties of nanoclusters we calculate numerically the magnetization of small magnetic systems using the exact diagonalization method.

Modelling of the implantation and the annealing stages of 800 keV 3He implanted tungsten: Formation of nanovoids in the near surface region

The formation of voids in tungsten implanted at room temperature with 800keV 3He atoms and subsequently annealed from 300K to 900K is modelled using an Object Kinetic Monte Carlo code. Different fluences are investigated ranging from 1017 to 5×1020ionsm−2 and comparisons are made with Positron Annihilation Spectroscopy results. Good agreements with the experimental results are obtained regarding the temperature range at which the vacancy clustering occurs and the dependency of the nanovoid size with fluence. Despite the small amount of He atoms in the investigated region named “track region”, their role is underlined and it is shown that they act as nuclei for the nanovoid formation. The non trivial consequence is that the higher the fluence, the smaller the nanovoids in the track region.

Calculation of cascade processes rates and simulation of the transitions in Kaonic 4He atom

Interactions of K − in a 4He target and the cascade processes rates in K4He atoms are investigated by a full-quantum mechanical approach. Then the cascade processes and x-ray yields of kaonic helium atoms in liquid helium are simulated by Monte-Carlo method. For this purpose the rates of scattering, capturing and internal Auger effect are calculated, after that the initial distribution of n, l states in Kα + ion are determined, then the cascade processes rates in Kα + ion are calculated by quantum mechanical methods. The relative x-ray L series are compared with the experimental results.

Benchmark helium dimer and trimer calculations with a public few-body code

We present detailed calculations of bound and scattering states of dimers and trimers of He to produce highly accurate data and to test a non-relativistic three-body code currently in development for public distribution. For these systems, uncertainties and inaccuracies in the fundamental constants frequently used in published works can substantially exceed numerical errors. Our benchmark calculations include specific estimates of the numerical accuracy of the calculations and also explore sensitivity to fundamental constants and their uncertainties. The use of an inexact coupling constant in the previous calculations leads to 0.08% error for the ground state energy, 0.3% error for the excited state energy and up to 0.15% error for the atom–dimer scattering length in the system of three 4He atoms. The corresponding errors for the asymmetric 4He23He system are 0.3% for the bound state energy and 0.03% for the atom–dimer scattering length.

Modelling radiation damage and He production in tungsten

Tungsten is a candidate material for the divertor and for first wall armor of future thermonuclear fusion reactors (ITER and DEMO). In such irradiation conditions, it is well known that the microstructure, and as a result the properties of the materials, will evolve. In this perspective, the fate of irradiation-induced defects (helium atoms, vacancies, self-interstitials and the complexes they can form) has to be understood. In particular, He migration properties are of fundamental as well as practical interest, as they can affect the microstructure evolution and eventually influence physical and mechanical properties, the most significant example being high-temperature helium embrittlement. In this work, we show how our ab initio parameterized object kinetic Monte Carlo code LAKIMOCA can model the evolution, during implantation, of the point defect population in the track region of 800 keV 3He atoms implanted in W samples. We also discuss possible improvements of the model.

Cold and ultracold collisions of MgH (2Σ+) with helium

The cross sections for rotational, fine-structure and projection-change transitions in collision of MgH (2Σ+) molecule with 3He and 4He atoms were computed at cold and ultracold temperatures using the ab initio potential energy surface. The significant suppression feature and resonance pattern caused by spin-rotation interaction were found and discussed. The collisions exhibit a strong isotope effect in the ultracold regime. The rotational relaxations of the fine-structure excited energy levels tendentiously preserve the F index. The ratio of the cross sections for elastic and spin-flipping collision with 3He partner is always greater than seven orders of magnitude for the energy range 10-6−1cm-1 and suggests the MgH molecule is a good candidate for 3He buffer gas cooling.