Dynamics of confined quantum fluids

We examine the static and dynamic properties of liquid ⁴He in confined geometries. Confinement is modeled by placing the liquid between two rigid, attractive walls with strengths corresponding to Geltech, Vycor, or glass. The liquid arranges itself in a series of layers, with increasing areal density it undergoes a sequence of layering transitions familiar from classical fluids. We identify “bulk” excitations that propagate throughout the film, and “layer” excitations that propagate only close to the substrate. Both have the typical phonon-roton dispersion relation, but the energy of the layer-roton minimum depends sensitively on the substrate strength, thus providing a mechanism for a direct measurement of this quantity. Bulk-like roton excitations are largely independent of the interaction between the matrix and the helium atoms. While the bulk-like rotons are very similar to their true bulk counterparts, the layer modes are not in close relation to two-dimensional rotons and should be regarded as a completely independent kind of excitation.     

Dynamics of atom scattering from 4He nanoclusters

We present a microscopic theory and results of atom scattering calculations to determine the dispersion of surface modes (ripplons) of superfluid helium-4 nanodroplets, expanding previous work [J. Chem. Phys. 115, 10161 (2001)]. A quantum transport formalism is adapted to the many-body scattering problem, yielding both elastic and inelastic fluxes. We demonstrate that, in analogy to the dynamic structure function S(k,ω) obtained from neutron scattering, a dynamic structure function σ(k,ω) can be obtained from ³He scattering. The ³He dynamic structure function σ(k,ω) is sensitive to surface dynamics, whereas the neutron dynamic structure function S(k,ω) is dominated by bulk-like excitations, in particular by rotons. Unlike for neutron-scattering, the total inelastic cross section for atom-scattering on ⁴He nanodroplets is large which we believe makes experimental detection feasible. We also show that scattering identical particles, i.e. ⁴He atoms, does not provide information about the dispersion of surface modes. Instead, inelastically scattered ⁴He atoms preferably lose roughly half their energy.     

Theory of quantum evaporation from superfluid helium

The scattering of atoms and elementary excitations at the free surface of superfluid⁴He is studied in the framework of linearized time dependent mean field theory. The probability associated with the evaporation and reflection processes involving phonons, rotons and free atoms is evaluated. The analysis is carried out for different incidence angles. The consistency of the results with general properties of the scattering matrix, such as unitarity and time reversal, is discussed.     

Thermodynamic evidence for nanoscale bose-einstein condensation in (4)He confined in nanoporous media

We report the measurements of the heat capacity of (4)He confined in nanoporous Gelsil glass that has nanopores of 2.5-nm diameter at pressures up to 5.3 MPa. The heat capacity has a broad peak at a temperature much higher than the superfluid transition temperature obtained using the torsional oscillator technique. The peak provides definite thermodynamic evidence for the formation of localized Bose-Einstein condensates on nanometer length scales. The temperature dependence of the heat capacity is described well by the excitations of phonons and rotons, supporting the existence of localized Bose-Einstein condensates.     

Excitations of metastable superfluid 4He at pressures up to 40 bars

Neutron scattering measurements of the fundamental excitations of liquid 4He confined in 44 A pore diameter gelsil glass at pressures up to 40 bars in the wave vector range 0.41.6 A(-1), especially the rotons, are observed up to complete solidification of all the liquid at a pressure of approximately 40 bars where the roton vanishes. At and above a pressure of 35.1 bars, Bragg peaks are observed, indicating coexistence of liquid and solid in the pores at pressures 35 less than or approximately equal P less than or approximately equal 40 bars.     

Roton-roton crossover in strongly correlated dipolar Bose-Einstein condensates

We study the pair correlations and excitations of a dipolar Bose gas layer. The anisotropy of the dipole-dipole interaction allows us to tune the strength of pair correlations from strong to weak perpendicular and weak to strong parallel to the layer by increasing the perpendicular trap frequency. This change is accompanied by a roton-roton crossover in the spectrum of collective excitations, from a roton caused by the head-to-tail attraction of dipoles to a roton caused by the side-by-side repulsion, while there is no roton excitation for intermediate trap frequencies. We discuss the nature of these two kinds of rotons and the relation to instabilities of dipolar Bose gases. In both regimes of trap frequencies where rotons occur, we observe strong damping of collective excitations by decay into two rotons.     

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?C3], a narrow EM field absorption line with a width on the order of (20?C200) kHz was observed against the background of a wide absorption band with a width of 30?C40 GHz at frequencies f ₀ ?? 110?C180 GHz corresponding to the roton gap energy ?? _r (T) in the temperature range 1.4?C2.2 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?C200 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) = ??r(T)/2??? 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.     

A model of Wigner liquid

A two-dimensional low-density system of charge carriers with strong Coulomb interaction, which can lead to the appearance of a short-wavelength soft mode (precursor of crystallization) is examined. This system provides elementary excitations of two types: Fermi excitations and Bose excitations with a gap in the spectrum. The latter excitations are similar to rotons in superfluid helium. A model involving the Fermi liquid and the soft mode is proposed, and interaction of different excitations with each other is described phenomenologically as in the Landau theory of Fermi liquid. By solving the derived equations, it was found that, as the temperature increases, the effective mass of Fermi excitations decreases and the gap in the Bose excitation spectrum increases.     

Semiclassical mechanics of rotons

The roton elementary excitations in superfluid liquid ⁴He have an unusual dispersion curve. The energy is an approximately quadratic function of (p - p₀), the difference between the magnitude of the momentum p and a characteristic value p₀. As a result, while for p > p₀ a roton has its (group) velocity parallel to its momentum, when p < p₀ the velocity and momentum are antiparallel. When p = p₀, the roton has non-zero momentum but zero velocity. These kinematic properties lead to unusual trajectories when rotons scatter or experience external forces. This paper examines this behaviour in the classical (ray optics) limit, where the roton wavelength is small compared with all other dimensions. Several experiments illustrate these effects. The examples are interesting in themselves, and also offer unconventional pedagogical possibilities.     

Variational theory for the ground state and collective excitations of an elongated dipolar condensate

We develop a variational theory for a dipolar condensate in an elongated(cigar shaped)confinement potential. Our formulation provides an effective one-dimensional extended meanfield theory for the ground state and its collective excitations. We apply our theory to investigate the properties of rotons in the system comparing the variational treatment to a full numerical solution. We consider the effect of quantum fluctuations on the scattering length at which the roton excitation softens to zero energy.     

HeⅡ中元激发和色散关系的计算机模拟研究

液体HeⅡ的色散关系涉及到元激发的类型以及它们之间的相互作用,对液体4He物理性质的解释和实际应用都是至关重要的,因此从理论和实验两个方面展开对它的研究一直是液体4He研究的热点之一.本文采用计算机模拟的方法对液体HeⅡ的元激发和色散关系进行了比较深入的研究,给出了在一定温度范围内色散关系的统一解析表达,并在此基础上计算出自由能以及熵随温度的变化关系,模拟结果与实验数据吻合得很好.     

Exciton dispersion and electronic excitations in hcp 4He

We present first measurements of the dispersion of excitons in solid helium, taken on a single hcp 4He crystal along the c axis. In agreement with studies on helium clusters, the major energy-loss peak can be interpreted as an intermediate molecular-type exciton, as we do not observe Wannier-like excitations. The measurements are in the (0 0 2) periodic zone, with the exciton energy dispersing along the c axis with a minimum at the gamma point. A calculated conduction band minimum at 31.0 eV above the valence band at gamma is supported by our data at energies above the exciton energy, leading to an exciton binding energy of 8.4 eV.     

Coherence during scattering of fast H atoms from a LiF(001) surface

The coherence for diffraction effects during grazing scattering of fast hydrogen and helium atoms from a LiF(001) surface with energies up to some keV is investigated via the coincident detection of two-dimensional angular distributions for scattered projectiles with their energy loss. For keV H atoms, we identify electronic excitations of the target surface as the dominant mechanism for decoherence, whereas for He atoms this contribution is small. The suppression of electronic excitations owing to the band gap of insulators plays an essential role for preserving quantum coherence and thus for the application of fast atom diffraction as a surface analytical tool.     

Detection of 4He atoms separately evaporated by phonons and rotons at a free superfluid surface

Using heat pulses, we detect ⁴He atoms separately by phonons and rotons from a superfluid surface.     

Interaction of first and second sound in solid (4)He: properties of a possible Bose condensate

A previous experiment in this laboratory found that the addition of a few tens of parts per 10(6) of (3)He impurities to solid (4)He crystals resulted in a phase transition below 200 mK. In the experiment described here the interaction of acoustic waves with waves generated by heat pulses is measured and shows that there is an energy gap in the excitations of the high temperature phase and that coherent waves are carried by nonphonon excitations at the lowest temperatures.     

On the dispersion of the “roton” second sound in the superfluid helium

At sufficiently high frequencies of the sound when the energy equilibrium between the phonon and roton gases is not established in the superfluid helium there may propagate the second sound through the gas of rotons (“roton” second sound). In the systems of rotons the equilibrium with respect to the number of rotons can be not complete (the chemical potential is not exactly equal to zero). The dispersion of the roton second sound in these conditions is investigated.     

Structure and dynamics of water confined in silica hydrogels: X-ray scattering and dielectric spectroscopy studies

Liquid ⁴He immersed in porous media such as aerogel, Vycor, and Geltech silica are excellent examples of bosons in disorder and confinement. Of special interest is the impact of disorder on Bose-Einstein condensation (BEC), on the elementary excitations of the superfluid and on their connection to the superfluid properties. Indeed, the modifications induced by disorder can be used to reveal the interdependence of BEC, the excitations and superfluidity. To date, the superfluid properties in porous media are much more completely documented than BEC or the excitations. In this paper, we review measurements of the excitations by neutron scattering, focusing particularly on their temperature dependence and the existence of phonon-roton excitations at higher temperatures. The weight of single excitation response at higher temperatures suggests the existence of localized BEC above the superfluid-normal transition temperature in porous media. We sketch several recent predictions made for BEC, the excitations, and the superfluid properties in disorder. Connections with other Dirty Bose systems are made.Received: 1 January 2003, Published online: 21 October 2003PACS: 03.75.Kk Dynamic properties of condensates; collective and hydrodynamic excitations, superfluid flow - 61.12.Ex Neutron scattering (including small-angle scattering) - 67.40.Yv Boson degeneracy and superfluidity of ⁴He: Impurities and other defects     

Results from a search for light-mass dark matter with a p-type point contact germanium detector

We report on several features in the energy spectrum from an ultralow-noise germanium detector operated deep underground. By implementing a new technique able to reject surface events, a number of cosmogenic peaks can be observed for the first time. We discuss an irreducible excess of bulklike events below 3 keV in ionization energy. These could be caused by unknown backgrounds, but also dark matter interactions consistent with DAMA/LIBRA. It is not yet possible to determine their origin. Improved constraints are placed on a cosmological origin for the DAMA/LIBRA effect.     

Theory of resonant Raman scattering from low-energy collective excitations of interacting electrons in quantum wires

The Raman spectra of quantum wires in the region of electronic intra-band excitations are investigated using one- and two-band models based on the Luttinger approximation with spin. Structures related to charge and spin density modes are identified, and analyzed with respect to their behavior with photon energy and temperature. It is found that the low-energy peaks in the polarized spectra, close to resonance that are commonly assigned to “single particle excitations”, can be interpreted as the signature of spin density excitations. A broad structure in the resonant depolarized spectrum is predicted above the frequency of the spin density excitations. This is due to simultaneous but independent propagation of spin and charge density modes. The results, when compared with experiment, show, that the electronic collective excitations of quantum wires at low energies are characteristic for a non-Fermi liquid. Received: 25 March 1998 / Accepted: 3 June 1998