[en] A model of e^+-, p, H atoms, D atoms, and an e⁺ and e^- in the liquid ⁴He is presented, here. This model is the extension of the theories of Woo, Tan and Massey (and Massey, Woo and Tan), and Yim and Massey. The detailed numerical results are not presented here. However, it is reserved in the later time. (author)

[en] In this thesis we develop a gapless theory of BEC which can be applied to both trapped and homogeneous gases at zero and finite temperature. The starting point for the theory is the second quantized, many-body Hamiltonian for a system of structureless bosons with pairwise interactions. A number conserving approach is used to rewrite this Hamiltonian in a form which is approximately quadratic with higher order cubic and quartic terms. The quadratic part of the Hamiltonian can be diagonalized exactly by transforming to a quasiparticle basis, while requiring that the condensate satisfy the Gross-Pitaevskii equation. The non-quadratic terms are assumed to have a small effect and are dealt with using first and second order perturbation theory. The conventional treatment of these terms, based on factorization approximations, is shown to be inconsistent. Infra-red divergences can appear in individual terms of the perturbation expansion, but we show analytically that the total contribution beyond quadratic order is finite. The resulting excitation spectrum is gapless and the energy shifts are small for a dilute gas away from the critical region, justifying the use of perturbation theory. Ultra-violet divergences can appear if a contact potential is used to describe particle interactions. We show that the use of this potential as an approximation to the two-body T-matrix leads naturally to a high-energy renormalization. The theory developed in this thesis is therefore well-defined at both low and high energy and provides a systematic description of Bose-Einstein condensation in dilute gases. It can be used to calculate the energies and decay rates of the excitations of the system at temperatures approaching the phase transition. We present analytical results for these quantities in the homogeneous limit and describe how they can be calculated numerically for trapped gases. (author)

[en] The authors consider stationary nonequilibrium states induced in the superfluid phases of ³He by phonon pumping at frequencies lower than the average pair-binding energy and far from the resonances of the collective triplet-condensate modes. The kinematic exclusion of the phonon single-particle absorption channel in He-3 interferes with the simulation mechanism known from superconductivity physics. Superfluidity can nonetheless be stimulated by phonon-absorption events in which several Fermi excitations participate. The calculations are performed for both the B and A phases of He-3. For the latter, account is taken of competing production of excess excitations by phonons via direct condensate-pair breaking. The stimulation is found to predominate at low frequencies, so that phonon emission can induce an A-B phase transition

[en] We report the results of recent experiments on magnetoplasma modes in circular sheets of ⁴He⁺ ions trapped below the surface of superfluid helium at a low temperature. The modes we observe include bulk modes, conventional edge modes, multipole edge modes, and extra satellites of unknown origin. The results are compared with earlier observations of bulk and conventional edge modes. Theories of the modes are reviewed and extended, and a detailed comparison with experiment is carried out. copyright 1997 The American Physical Society

[en] We present a new approach for studying the energy spectrum of superfluid Helium 4. It is based on the assumption that there exist localized modes in addition to the usual Feynman density fluctuations (phonons). They correspond to the short range behaviour in the liquid where effects of quantum statistics are important. We describe in a phenomenological way the hybridization of those two kinds of excitations and we compare the resulting energy spectrum with experimental results, e.g. the structure factor and the single excitation scattering intensity. We also predict the existence of another type of excitation interpreted as a vortex loop. The energy of this mode agrees both with the Raman scattering data and critical velocity experiments of Varoquaux et al.

[en] Full text: The theoretical solution of quantum mechanical electronic conduction systems is essential for the understanding of high-T_c superconductivity. We have made progress along this line, by solving the Hubbard and tJ models in d=3 with renormalization-group theory and obtaining their phase diagrams and thermodynamic properties for all temperatures, densities, and coupling strengths. In the Hubbard model, at low temperatures and around half filling, an antiferromagnetic phase, of purely quantum mechanical origin, is obtained. Furthermore: (1) At strong coupling and 30-35% electron or hole doping from half filling; (2) At weak and intermediate coupling and 10-18% electron or hole doping; two distinct novel phases, which we call τ phases, were found. The distinguishing characteristic of these phases is that the electron or hole hopping expectation value is non-zero at all length scales. The weak-intermediate coupling τ phase exhibits, as in BCS superconductivity, an excitation spectrum gap and, in the specific heat, a low-temperature exponential decay and a cusp phase transition singularity. The strong coupling phase exhibits, as in BEC superconducti-vity, in the specific heat, an α∼-1 phase transition singularity and a pair-formation peak above the phase transition temperature. In the limit of large Coulomb repulsion, the tJ model is obtained. In our calculations for this model, we find that the superfluid weight increases with doping, passes through a maximum within the τ phase at 32-37% doping, and decreases, and that the free carrier density also increases to a maximum value at 32-37% doping but remains at this value for further doping. These characteristics, seen experimentally in high-T_c superconductors, have not been obtained in previous theoretical studies. Results with spatially anisotropic systems will also be given. This research has been performed with my doctorate student Michael Hinczewski

[en] High-density composite solitons in superfluid ³He-A form a regular one-dimensional lattice. The magnetic-resonance frequencies and the intensities of these soliton lattices are determined theoretically. Furthermore, if the soliton lattice has open ends, the soliton lattice relaxes by uncoiling the l and d vectors. The characteristic time for the uncoiling is calculated. The present theory appears to account for unusual NMR behavior in magnetization-tipping experiments

[en] We have investigated the isospin dependence of the neutron and proton ³PF₂ superfluidity in isospin-asymmetric nuclear matter within the framework of the Brueckner-Hartree-Fock approach and the BCS theory. We show that the ³PF₂ neutron and proton pairing gaps depend sensitively on isospin asymmetry of asymmetric nuclear matter. As the isospin asymmetry increases, the neutron ³PF₂ superfluidity becomes stronger and the peak value of the neutron ³PF₂ pairing gap increases rapidly. The isospin dependence of the proton ³PF₂ superfluidity is shown to be opposite to the neutron one. The proton ³PF₂ superfluidity becomes weaker at a higher asymmetry and it even vanishes at high enough asymmetries. At high asymmetries, the neutron ³PF₂ superfluidity turns out to be much stronger than the proton one, implying that the neutron ³PF₂ superfluidity is dominated in the highly asymmetric dense interior of neutron stars. (authors)

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No abstract available