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We report on the first observation of 3He diffusion anisotropy in 3He–4He liquid mixture confined in ordered aerogels at 1.5–4.2 K temperatures. The used aerogels are arrays of long Al2O3 parallel 8 nm strands. The possible origins of diffusion anisotropy are considered and the changes of roton properties introduced by parallel aerogel strands are discussed. Among the responsible mechanisms we account for Knudsen diffusion, potential anisotropy of layer mode excitations or of bulklike excitations, and helium vortices. The observed reduced 3He diffusion in aerogels is discussed and suggested to appear due to helium excitations at strong confinement conditions. These observations pave the way for future experiments to gain insight into the crossover regime expected at lower temperatures (below 1 K) for which roton density is lower and 3He collisions with strands play significant role.

In this article, we discuss a method to control the long-range interactions between bosons in a three-dimensional Bose-Fermi mixture with the help of optical lattices on fermions. We find the range and the peaked momentum of the fermion-mediated interactions can be tuned by the optical lattice depth and the fermion density. If the fermion density is close to half-filling, roton excitations can be generated with weak Bose-Fermi interactions. Further, if the fermions are not exact at half-filling, multi-roton structure may emerge, implying competing density orders. Therefore, tuning the lattice depth and the fermion density in a Bose-Fermi mixture serves as an effective way to control the interaction range and resonant momentum between bosons.

We solve numerically exactly the many-body 1D model of bosons interacting via short-range and dipolar forces and moving in the box with periodic boundary conditions. We show that the lowest energy states with fixed total momentum can be smoothly transformed from the typical states of collective character to states resembling single particle excitations. In particular, we identify the celebrated roton state. The smooth transition is realized by simultaneous tuning short-range interactions and adjusting a trap geometry. With our methods we study the weakly interacting regime as well as the regime beyond the range of validity of the Bogoliubov approximation., 7 pages, 4 figures

We study a system of penetrable bosons on a line, focusing on the high-density/weak-interaction regime, where the ground state is, to a good approximation, a condensate. Under compression, the system clusterizes at zero temperature, i.e., particles gather together in separate, equally populated bunches. We compare predictions from the Gross-Pitaevskii (GP) equation with those of two distinct variational approximations of the single-particle state, written as either a sum of Gaussians or the square root of it. Not only the wave functions in the three theories are similar, but also the phase-transition density is the same for all. In particular, clusterization occurs together with the softening of roton excitations in GP theory. Compared to the latter theory, Gaussian variational theory has the advantage that the mean-field energy functional is written in (almost) closed form, which enables us to extract the phase-transition and high-density behaviors in fully analytic terms. We also compute the superfluid fraction of the clustered system, uncovering its exact behavior close, as well as very far away from, the transition., Comment: 27 pages, 4 figures

Quantum wires occupy a unique status among the semiconducting nanostructures with reduced dimensionality – no other system seems to have engaged researchers with as many appealing features to pursue. This letter aims at a core issue related with the magnetoplasmon excitations in the quantum wires characterized by the confining harmonic potential and subjected to a longitudinal electric field and a perpendicular magnetic field in the symmetric gauge. Despite the substantive complexity, we obtain the exact analytical expressions for the eigenfunction and eigenenergy, using the scheme of ladder operators, which fundamentally characterize the quantal system. Crucial to this inquiry is an intersubband collective excitation that evolves into a magnetoroton – above a threshold value of magnetic field – which observes a negative group velocity between the maxon and the roton. The evidence of negative group velocity implies anomalous dispersion in a gain medium with the population inversion that forms the basis for the lasing action of lasers. Thus, the technological pathway that unfolds is the route to devices exploiting the magnetoroton features for designing the novel optical amplifiers at nanoscale and hence paving the way to a new generation of lasers.

We consider the Gross-Pitaevskii(GP) model of a Bose-Einstein Condensate(BEC) for single-component and multi-component BEC. The pseudopotential for s-wave scattering between atoms is taken to be of width of the order of the s-wave scattering length. Such an interaction giving rise to a roton minimum in the spectrum of elementary excitations of a single component BEC is well known. However, softening roton modes takes us in the strongly interacting BEC regime where three body losses occur. We study the roton mode softening for a multi-component BEC. We show that by increasing the number of components of a multi-component BEC, the roton mode can be softened at a progressively lower value of the gas parameter ($a^{3}n$), thus reducing three body losses., 10 pages, 10 figures

We study the topological structure of the vortex system in a superfluid film. Explicit expressions for the vortex density and velocity field as functions of the superfluid order parameter are derived. The evolution of vortices is also studied from the topological properties of the superfluid order parameter field.

We present high-resolution data of the energy widths Γ of elementary excitations in superfluid 4He on the phonon branch 0.3A−1 < Q < 0.6 A−1 and both the energies and the widths on the roton branch 1.93A−1 < Q < 2.6A−1 at T = 0.98 K. In both Q-regions a broadening of Γ is clearly observed. The roton energies and group velocities observed in the present experiment are significantly smaller than those reported from previous experiments.

I develop here a semi-microscopic quantum theory for description of creation of phonons and rotons in superfluid helium by a solid heater. Starting with a correct transfer Hamiltonian describing a coupling between solid and liquid 4He the probabilities of transformation of a single phonon in the solid into (i) single helium phonon, (ii) two helium phonons, and (iii) single helium roton are found out. All the obtained expressions account for different polarizations of phonons in the solid. The heat transfer associated with single phonon and single roton channels are calculated. Particularly, the obtained expression for heat flux via the single phonon channel calculated in the framework of present microscopic theory coincides exactly with the well known Khalatnikov formula obtained initially in the framework of acoustic-mismatch theory. The impossibility of direct creation of R(-) rotons becomes clear in the used framework due to accurate account of the boundary conditions at the solid–liquid helium interface, which is in agreement with recent experimental results.

We present experimental results on the condensation of 4He atoms on the surface of liquid 4He. We show that there is quantum condensation with the creation of phonons and R+rotons in one-to-one processes as atoms go into the Bose–Einstein condensate. These phonon and R+roton signals can be recognized by their time of flight, their angular distribution and their dependence on the ambient temperature. Condensing atoms also create ripplons with a high probability. We derive rate equations for the growth of the ripplon density and the ripplon decay by the creation of bulk phonons. During the atom pulse a dynamic equilibrium is established. The low-energy phonons that are created by ripplon–ripplon scattering can easily be detected and distinguished from the phonons created by quantum condensation.From measured data, estimates are made of the probabilities of creating phonons, R+rotons and ripplons in condensation processes as well as estimates of the quantum evaporation probabilities of phonons and rotons.We clearly detect R+rotons directly with a bolometer in the liquid 4He and show why the signal of R+rotons is obscured by the low-energy phonon signal when both the source and detector are in the liquid helium. Values of the Kapitza conductance for the Zn bolometers are derived.

A theory of 3He diffusion coefficient, D, in liquid 4He for temperatures below 0.65 K is presented. The theory is consistent with the experiments of Lamoreux et al. only if the recoil of the 3He quasiparticle is included. The theory predicts that DT7 is not constant for temperatures T below 0.5 K.

The second-order phase transition of 4He from a normal fluid to a superfluid is ideally suited for studies of critical behaviour. In particular, effects of confinement have been studied recently to verify theoretical predictions of correlation-length scaling and calculations of specific scaling functions. These predictions are summarized for the specific heat and the superfluid density. The method of achieving confinement is discussed, as well as the measuring technique. The specific heat and the superfluid density in planar confinement are examined. It is found that the specific heat scales well on the normal side, and just as well on the superfluid side until the region of the specific heat maximum is reached. Here deviations from scaling are seen. It is possible that this behaviour is associated with the specific crossover in two dimensions. The superfluid fraction, which has been measured for the same type of confinement in two different ways, does not scale. Results of a calculation for the superfluid density to assess the role of the inhomogeneity induced by the van der Waals attraction at the confining walls are presented.

High-resolution neutron inelastic scattering studies of the wavevector and temperature dependence of the excitation spectrum of 4 He in the superfluid and normal-fluid phases have been made in the wavevector region ‘beyond the roton’, 2.0

High-energy resolution inelastic neutron scattering data on excitations in superfluid and normal 4He at SVP at wave vectors Q beyond the roton, 2.0 ≤ Q ≤ 4.0 A−1, are presented. The narrow energy resolution of these measurements reveals that the energy of the elementary excitation comes up to twice the roton energy, 2Δ, at Q = 2.8−1 and the energy remains constant at 2Δ between Q 3.0−1 and the end point of the dispersion curve at Q = 3.6−1. The width of the peak is also unobservably small from Q = 2.8−1 out to the end point, W = 2Γ < 20 μeV.

Systematic high-resolution neutron inelastic scattering measurements have been made of the collective phonon - roton excitations in normal and superfluid as a function of both pressure (density) and temperature. The IN6 time-of-flight spectrometer at the Institut Laue - Langevin has been used to obtain accurate and continuous data over a wavevector range with a resolution of 0.12 meV. We present the experimental data and discuss the energies and widths of the single excitations; changes concomitant with the transition are discussed. The structure of the multiphonon continuum at pressure is also discussed and is shown to be consistent with theory.

Liquid 4 He is a unique liquid because of its Bose nature but it can be used to study general problems. A number of such properties are reviewed including quantum evaporation, interactions between excitations and wetting studies with Cs.

A simple relation is demonstrated between the phonon dispersion curves of crystalline 4He in both the h.c.p. and the b.c.c. phase near melting and the dispersion law of elementary excitations in the superfluid phase near freezing. The collective dynamics as seen in neutron inelastic-scattering experiments from all these phases near coexistence reflects primarily the static density response function of the liquid and a suitably defined Debye-Waller factor. It is further pointed out that the curvature of the dispersion law at the roton minimum in the liquid is approximately related to the c44 shear elastic constant of the b.c.c. phase.

We sudy the impact of a weak random potential with a Gaussian correlation function on the thermodynamics of a two-dimensional (2D) dipolar bosonic gas. Analytical expressions for the quantum depletion, anomalous density, the ground state energy, the equation of state and the sound velocity are derived in the roton regime within the framework of the Bogoliubov theory. Surprisingly, we find that the condensate depletion and the anomalous density are comparable. The structure factor and the superfluid fraction are also obtained analytically and numerically. We show that these quantities acquire dramatically modified profiles when the roton is close to zero yielding the transition to an unusual quantum state., 7 pages, 4 figures. arXiv admin note: text overlap with arXiv:1505.07758

By employing both vortex-dynamical and self-consistent Navier-Stokes methods of analysis, an investigation of interactions between normal-fluid and superfluid vortex rings in helium-4 is performed. It is shown that, under the influence of normal-fluid vortices, superfluid vortices grow in size in order to match the size of the latter. In turbulence, this mechanism hints to a tendency towards scale-to-scale energy equipartition in the spectral regimes corresponding to the inertial-range in the normal-fluid. Yet, the analysis shows that the induced superfluid vorticity is not trapped by the cores of the normal-fluid vortices, and, consequently, there is no strong correlation between normal-fluid and induced superfluid vorticity patterns. Hence, there is no significant vortex locking effect.

In this paper, we describe recent neutron inelastic scattering studies of the elementary phonon-roton excitations of liquid 4He. The paper deals with a systematic investigation in both superfluid and normal phases of the temperature dependence of the excitation spectrum, covering a range in wavevector from 0.3 to 2.1 AA-1. These measurements are discussed in the light of theoretical models of the excitation spectrum of liquid 4He.

The shadow representation of the wave function of superfluid 4He is reviewed. Analogies with and differences from a classical fluid are discussed and the role of quantum delocalization is pointed out. Results for g(r), S(q) and the momentum distribution of superfluid 4He in the ground state and at finite T are presented.

The superfluid transition of 4 He in porous media is discussed. The eigenstates of the Hamiltonian may be localized whether the disorder is spatially correlated or not. Based on this argument we proposed a phenomenological model to describe the eects of localization on phase transitions. The system may be divided into many isolated blocks because of the localization. We calculated the superfluid density and the free energy density for the superfluid transition according to this model. These thermodynamical quantities are described by power laws in a certain temperature region but the hyperscaling relation is violated as one can expect. The comparison of the result of this model with the experiment on the superfluid transition of 4 He in gels is given.

A polaron-type model is proposed to describe the deformation in the superfluid 4He ground state, due to the presence of a 3He atom. The interaction matrix element of the model is expressed in terms of the structure factor of the superfluid on the basis of the hypernetted chain approximation. Both the use of an intermediate polaron coupling theory (rather than perturbation theory) and the determination of the precise interaction matrix elements are crucial ingredients to obtain an adequate theoretical description based on a polaron-type model. A generalization of Feynman's path-integral technique is used for the treatment of intermediate coupling. The present work solves i.a. an outstanding problem regarding the 3He effective mass in superfluid 4He. The theoretical result (an effective mass of 2.3123m3 as compared to the experimental value of 2.3m3 where m3 is the bare 3He mass) confirms that our formalism accurately describes the deformation cloud around the 3He atom. Also the difference between the specific heat of the mixture and the specific heat of pure superfluid 4He, and the diffusion of 3He in the superfluid, are accurately explained as the result of the interaction of 3He with the excitations of the superfluid.

The shadow wave function for the excited states of superfluid 4He is used to construct the density matrix of the system within the independent excitation picture. From the temperature dependence of the total and of the kinetic energy we infer that most of the energy of a roton is kinetic and that a roton lowers the potential energy. From the density matrix we compute the static structure factor S(k; T) and find that thermally excited rotons lead to an increased local order in good agreement with experiment.

The excitations in liquid 4He are phonons and rotons. They have been extensively studied both theoretically and experimentally over a long period and we now have a considerable understanding of them. This work is briefly reviewed here. There are still important outstanding questions such as how phonons and rotons are created by hot surfaces, the scattering of high energy excitations and the nature of the roton.

We study the effect of the superfluidity of 4He of a superradiant interaction among the 4He-atoms. We show that: i) a microscopic two-fluid picture emerges; ii) the T-dependence of the roton energy Δ(T) is correctly described; iii) the p = 0 fraction of the Bose condensate is ρc 0.15; iv) the observed values of the critical velocity vcrit can be adequately understood.

Excitations in many areas of condensed matter physics have dispersion relations which are very unlike those of simple newtonian objects. An unusual energy-momentum relation should lead to unconventional dynamics. Unfortunately most excitations are trapped in lattices or are otherwise inaccessible. One place where dynamic experiments are possible is in the superfluids, since here the excitations propagate freely in the superfluid condensate which we can regard as a mechanical vacuum. We can now cool superfluid 3He to a temperature where the excitation mean free paths are long enough for dynamic experiments to be feasible, which implies temperatures of the order of 100μK. Since the dispersion relation is tied to the rest frame of the condensate, it will be distorted by condensate motion, providing us with a way of influencing the dispersion curve. For example, a container of superfluid with a velocity gradient will have a range of excitation dispersion curves depending on the local velocity of the condensate. This means that the density of states available to the excitations will vary with position. We discuss the dynamical implications of this in terms of the behaviour of a vibrating wire resonator in the liquid, which shows unusual and often counter-intuitive behaviour. Among other things, for example, at a low enough temperature the force on a moving object in the excitation gas becomes independent of the velocity.

R+ rotons can be injected into liquid 4He and for ambient temperature T < 0.1 K the rotons have long lifetimes. This means that beams of ballistic rotons can be created. Such beams can be efficiently detected using quantum evaporation. The total time of flight of the R+ roton and atom is a function only of the roton wavevector and the path lengths, so by injecting short pulses of rotons it is possible to do roton spectroscopy. If two beams of ballistic rotons intersect there is scattering. Such an experiment is described and how the roton-roton cross-section may be obtained from it. It appears that the cross-section is strongly wavevector dependent. The scattering of high energy photons (ω/k ≥ 10 K) from slow rotons is compared to roton scattering from similar slow rotons and the ratio agrees with that found by neutron scattering.

Quantum evaporation of 4He atoms by phonons and rotons in liquid 4He is measured for a wide range of conditions. The results confirm that an atom is evaporated in a single-excitation to single-atom process and that the boundary conditions of conservation of energy and parallel momentum are obeyed. Excitations in the liquid are generated by pulse heating a thin metal film heater, and the evaporated atoms are detected by the energy yielded on condensation at the surface of a superconducting transition edge bolometer. The heater and bolometer can be rotated about a common axis in the liquid surface, and the collimation of the excitations and atoms into beams allows the angles of incidence and evaporation to be defined. It is first shown that the input power and pulse length must be carefully chosen to produce beams of phonons and rotons that are fully ballistic. The time of flight from the heater to the bolometer is measured, and the angular distribution of the evaporated atoms is determined for different angles of incidence. In particular, the wavevector dependence of the roton to atom pulse shape can be seen. The authors see no sign of Pitaevskii roton decay over the long liquid path lengths involved ( approximately 6 mm), and there is no indication that ripplons are created in the evaporation process.

We investigate the excitation spectrum and spin conductivity in square lattice antiferromagnets by nonlinear spin-wave theory. We have found that the excitation spectrum have a rotonlike minimum at about 3/4 of the saturation field. The "roton" appears and softens rapidly to zero as a function of the magnetic field as a result of the especially strong three-magnon interactions. For the spin conductivity, it becomes clear that a two-magnon sideband appears for sufficiently strong three-magnon interactions.

We present vortex solutions for the homogeneous two-dimensional Bose-Einstein condensate featuring dipolar atomic interactions, mapped out as a function of the dipolar interaction strength (relative to the contact interactions) and polarization direction. Stable vortex solutions arise in the regimes where the fully homogeneous system is stable to the phonon or roton instabilities. Close to these instabilities, the vortex profile differs significantly from that of a vortex in a nondipolar quantum gas, developing, for example, density ripples and an anisotropic core. Meanwhile, the vortex itself generates a mesoscopic dipolar potential which, at distance, scales as 1/r^2 and has an angular dependence which mimics the microscopic dipolar interaction., 11 pages, 5 figures. Proceedings of the 22nd International Laser Physics Workshop (Prague, July 2013

Bose-Einstein condensed gas of two-dimensional tilted dipoles in a thin layer is under consideration. The problem of stability is resolved. Phase diagram for system of two-dimensional dipole atoms in a thin layer is obtained. A formation of density waves and their orientation control possibility via polarizing field are considered. These effects can be experimentally observed for magnetically dipolar atoms at the Feshbach resonance or polar molecules in one-dimensional optical lattices.

We investigate the stability properties and the dynamics of Bose-Einstein condensates with axial symmetry, especially with dipolar long-range interaction, using both simulations on grids and variational calculations. We present an extended variational ansatz which is applicable for axial symmetry and show that this ansatz can reproduce the lowest eigenfrequencies of the Bogoliubov spectrum, and also the corresponding eigenfunctions. Our variational ansatz is capable of describing the roton instability of pancake-shaped dipolar condensates for arbitrary angular momenta. After investigating the linear regime we apply the ansatz to determine the dynamics and show how the angular collapse is correctly described within the variational framework., 27 pages, 11 figures

We present neutron scattering measurements of the phonon-roton (P-R) and layer modes of liquid 4He confined in MCM-41 under pressure up to 38 bar. The data shows unambiguously that the P-R mode exists at low temperature only. As temperature is increased, there is a gradual transfer of intensity from the P-R mode to the normal liquid response, which lies at a lower energy at higher pressure. The transfer takes place with no observable mode broadening. The loss of P-R modes is identified with the loss of Bose-Einstein condensation (BEC). The mode giving rise to the specific heat, cV, of liquid 4He in porous media (e.g., gelsil) at higher temperature is the layer mode since the energy of the mode extracted from cV and the layer mode energy are the same.

We show that, under achievable experimental conditions, a Bose-Einstein condensate (BEC) of polar molecules can exhibit dielectric character. In particular, we derive a set of self-consistent mean-field equations that couple the condensate density to its electric dipole field, leading to the emergence of polarization modes that are coupled to the rich quasiparticle spectrum of the condensate. While the usual roton instability is suppressed in this system, the coupling can give rise to a phonon-like instability that is characteristic of a dielectric material with a negative static dielectric function., Comment: Version published in New Journal of Physics, 11+ pages, 4 figures

Superfluid helium consists of two inter-penetrating fluids, a viscous normal fluid and an inviscid superfluid, coupled by a mutual friction. We develop a two-fluid shell model to study superfluid turbulence and investigate the energy spectra and the balance of fluxes between the two fluids in a steady state. At sufficiently low temperatures a 'bottle-neck' develops at high wavenumbers suggesting the need for a further dissipative effect, such as the Kelvin wave cascade.

The collective-excitation spectrum from a filled Landau level in a rotating dipolar Fermi gas is investigated through the equation of motion method. The predicted excitation spectrum exhibits a roton structure which is affected by the strength of dipolar interactions and trapping potential. The correlation effect on the roton minimum is observed by varying the dipolar interaction strength. An increase of the dipolar interaction strength leads to a deeper roton minimum. The roton minimum can even become zero, where the system exhibits instability, as the dipolar strength approaches a critical value. As the dynamical stability of the system is studied through the roton structure, we find that this zero-value roton minimum is a precursor of the occurrence of a bubble crystal.

We present a study of the excitation spectra and of the spectral functions for the elementary excitations in superfluid neutron star matter below the crust. We restrict the analysis to the proton and electron components within the Random Phase Approximation generalized to the superfluid case. The excitation spectrum shows the interplay of these two components and their relevance for the excitation modes. The results are relevant for neutrino physics and thermodynamical processes in neutron stars.

High-pressure experiments of hydrogen hydrate were performed using a diamond anvil cell under conditions of 0.1–44.2 GPa and at room temperature. Also, high pressure Raman studies of solid hydrogen were performed in the pressure range of 0.1–43.7 GPa. X-ray diffractometry (XRD) for hydrogen hydrate revealed that a known high-pressure structure, filled ice Ic structure, of hydrogen hydrate transformed to a new high-pressure structure at approximately 35–40 GPa. A comparison of the Raman spectroscopy of a vibron for hydrogen molecules between hydrogen hydrate and solid hydrogen revealed that the extraction of hydrogen molecules from hydrogen hydrate occurred above 20 GPa. Also, the Raman spectra of a roton revealed that the rotation of hydrogen molecules in hydrogen hydrate was suppressed at around 20 GPa and that the rotation recovered under higher pressure. These results indicated that remarkable intermolecular interactions in hydrogen hydrate between neighboring hydrogen molecules and between guest hydrogen molecules and host water molecules might occur. These intermolecular interactions could produce the stability of hydrogen hydrate.

We address the question of collective excitations in two-dimensional (2D) dipolar systems. The main issue is that such systems have no Hartree and no random-phase-approximation (RPA) limits and calculations have to include particle correlations from the outset. We focus on the longitudinal collective mode representing the density oscillations of the dipoles and on the transverse collective mode representing shear waves. Our theoretical approach is based on the quasi-localized charge approximation (QLCA) adapted to point-dipole systems interacting through a 1/r3 potential. Our analytical calculation is accompanied by classical molecular dynamics (MD) simulation. At long wavelengths, the longitudinal and transverse collective excitations exhibit acoustic behaviors with phase velocities that vary linearly with the dipole strength and are wholly maintained by particle correlations. At finite wavenumbers, the mode dispersion resulting from our classical MD simulations shows a roton-like behavior. Comparison with the quantum Monte Carlo dispersion generated through the Feynman relation (Astrakharchik et al 2007 Phys. Rev. Lett. 98 060405) shows a remarkably good quantitative agreement between the two.

It has been speculated earlier, that a superflow exceeding the roton Landau critical velocity at zero temperature may undergo a second order phase transition into a layered state. We investigate thermodynamic stability of this state and discuss possible phase diagrams at low temperature.

金沢大学理工研究域数物科学系, In the present work we propose the explanation of the recently observed at temperatures about 1.7 K anomalous ultrasound attenuation in high-porous aerogels filled in by liquid 4He by means of 2D-3D roton transformations. The existence of so-called 2D rotons in the system "aerogel-liquid 4He" has been found early in the experiments on inelastic neutron scattering T < 1.5 K where the spectrum of roton excitations is well-defined. The difference in energies of 3D-rotons and 2D-rotons at low temperatures exceeds the energy of acoustic phonons and the process of transformation of 2D-roton to 3D-roton is forbidden by energy conservation law. But with increasing temperature the 2D-roton and 3D-roton states begin to be broadened (the lifetime of excitations is decreasing) and possibility of the mutual transformation of rotons with the participation of a phonon appears. The further increasing of temperature leads to the essential decreasing of superfluid density and the efficiency of the proposed mechanism decreases too. Numerical simulations show the qualitative agreement of the proposed mechanism with the observed data. © 2009 IOP Publishing Ltd.

We obtain the dispersion curve for ripplons using a simple analytic model of He II. Asymptotes are derived for small frequencies and close to the roton threshold. At large wave vectors the curve is shown to be determined by the rotons' dispersion, and it approaches the roton threshold with zero derivative, at the peak of inverted parabola. The results are compared with experimental data.

We investigate two-magnon Raman scattering from the S = 1/2 Heisenberg antiferromagnet on the triangular lattice (THAF), considering both isotropic and anisotropic exchange interactions. We find that the Raman intensity for the isotropic THAF is insensitive to the scattering geometry, while both the line profile and the intensity of the Raman response for the anisotropic THAF shows a strong dependence on the scattering geometry. For the isotropic case we present an analytical and numerical study of the Raman intensity including both the efiect of renormalization of the one-magnon spectrum by 1 = S corrections and final-state magnonmagnon interactions. The bare Raman intensity displays two peaks related to one-magnon van-Hove singularities. We find that 1 = S self-energy corrections to the one-magnon spectrum strongly modify this intensity profile. The central Raman-peak is significantly enhanced due to plateaus in the magnon dispersion, the high frequency peak is suppressed due to magnon damping, and the overall spectral support narrows considerably. Additionally we investigate final-state interactions by solving the Bethe-Salpeter equation to O(1 = S). In contrast to collinear antiferromagnets, the non-collinear nature of the magnetic ground state leads to an irreducible magnon scattering which is retarded and non-separable already to lowest order. We show that final-state interactions lead to a rather broad Raman-continuum centered around approximately twice the 'roton'-energy.

We find the pressure, due to the thermal excitations of superfluid helium, at the interface with a solid. The separate contributions of phonons, $R^-$ rotons and $R^+$ rotons are derived. The pressure due to $R^-$ rotons is shown to be negative and partially compensates the positive contribution of $R^+$ rotons, so the total roton pressure is positive but several times less than the separate $R^-$ and $R^+$ roton contributions. The pressure of the quasiparticle gas is shown to account for the fountain effect in $HeI I$. An experiment is proposed to observe the negative pressure due to $R^-$ rotons., Comment: 14 pages, 4 figures

We have proposed a model of roton cluster excitations in liquid 4He based on a Schrodinger-type equation with a self-consistent confining potential. We have derived an equation for the number of atoms in the roton cluster, which can be treated as quantum 3D solitons depending on vibrational quantum numbers. It is shown that the smallest roton cluster is in the symmetric vibrational quantum state and consists of 13 helium atoms. We have also used a modified Born approximation to calculate the s-scattering length for helium atoms. This allows us to calculate all parameters of Landau's roton excitation spectrum, in agreement to high accuracy with experimental measurements from neutron scattering. It is also shown that the model leads to the saturation of the elementary excitation energy curve at twice the roton gap.

We have developed an apparatus which can be used to monitor the motion of individual electrons in liquid helium. A sound wave is used to explode an electron bubble for a fraction of a microsecond. While the bubble is expanded, it is illuminated by light from a flash lamp. We describe the details of the experiment and show the results we obtained. The phonon- roton in liquid helium drags the electron bubbles across the experiment cell. In some cases, an electron is seen to follow a snakelike path. Our tentative explanation is that the electron is sliding along a quantized vortex line.

By including an adequate four-particle interaction, correlations between the proton and neutron fluids in deformed nuclei are induced. As a result, superfluidities of the two nuclear systems may be generated by one another and the phase structure is enriched by a new superfluid phase dominated by alpha-type correlations. Among the consequences upon the nuclear structure and dynamics figure the prediction of new bands of elementary excitations and the increase of superfluid enhancement factors for alpha and pair transfer reactions.