No description available.

Cette thèse s'intéresse à la turbulence dans l'hélium (4He) superfluide, à des températures comprises entre 1.15 K et la température de transition superfluide, Tlambda = 2.17 K, ce qui correspond à une fraction de superfluide rho_s/rho comprise entre 97.6% et 0%. Il s'agit d'un travail essentiellement expérimental dont le but est de comparer la turbulence classique et la turbulence quantique, à l'aide de mesures locales de fluctuations de vitesse et de vorticité. Ces mesures sont complétées par l'analyse de champs de vitesse issus de simulations numériques.Nous avons développé une instrumentation spécifique, adaptée aux écoulements cryogéniques: des tubes de Pitot miniatures, dont les dimensions effectives ont pu être rendues sub-millimétriques, et un capteur original, basé sur la déflection d'une micro-poutrelle (300 microns x 100 microns x 1 microns) mesurée à l'aide d'un micro-résonateur supraconducteur dans la gamme de fréquence du GHz. Un premier prototype de ce capteur, micro-fabriqué à partir d'une galette de silicium, a été réalisé en salle blanche puis validé dans une conduite cryogénique. La résolution spatiale obtenue est du même ordre que celle des meilleurs anémomètres en He II, et il devrait être possible de l'améliorer d'une décade.Les tubes de Pitot ont été placés dans les souffleries superfluides TSF et TOUPIE. La première, fruit d'une collaboration nationale, a fourni un écoulement stationnaire de grille (Rlambda= 250, 1.65 K < T < 2.6 K, rho_s/rho < 80%) et un sillage proche. La seconde soufflerie, refroidie pour la première fois dans le cadre de cette thèse, a fourni un écoulement de sillage lointain (Rlambda = 1100, 1.55 K < T < 4.2 K, rho_s/rho < 86%).Les mesures ont mis en évidence dans ces écoulements des similarités fortes avec les écoulements classiques, aux échelles inertielles: spectre de vitesse en k^{-5/3}, constante de Kolmogorov et taux de turbulence identiques aux écoulements classiques, loi des 4/5, exposants anormaux pour les fonctions de structures des incréments de vitesse (intermittence). À plus petite échelle, les simulations numériques (1.15 K < T < 2.1565 K) mettent en évidence un comportement exotique : l'énergie s'accumule et tend vers l'équipartition, ce qui se traduit par un spectre de vitesse simulé en k^2. Ce phénomène s'accentue à basse température. Enfin, des mesures locales de fluctuation de vorticité ont été réalisées à l'aide de pinces à second son sur une gamme de température comprise entre 1.69 K (rho_s/rho=77%) et 2.01 K (rho_s/rho = 42%). Nous avons observé un raidissement de la pente des spectres de vorticité lorsque la température diminue. Ce résultat peut être interprété comme une conséquence du phénomène d'équipartition mis en évidence dans les simulations numériques à petite échelle. / The focus of this thesis is the turbulence of superfluid 4He at temperatures between 1.15K and the superfluid transition temperature Tlambda = 2.17K, corresponding to a superfluid fraction rho_s/rho between 97.6% and 0%. This work is mostly experimental. We aim to compare classical and quantum turbulence, using local velocity and vorticity fluctuations measurements. These measurements are backed up by numerical simulations.We developed dedicated probes, designed for cryogenic flows: Pitot tubes with sub-millimeter effective size and a new cantilever-based probe (300 microns x 100 microns x 1 microns) whose deflection is measured with a superconducting micro-resonator in the GHz frequency range. A first prototype was micro-machined from a silicon wafer in cleanroom and validated in a superfluid wind tunnel. The resolution was found similar to the one of the best anemometers operating in He~II and will be further improved.The Pitot tubes have been inserted inside two superfluid wind tunnels, TSF and TOUPIE. The former, designed and operated within a national collaboration, provided a stationary grid flow (Rlambda = 250, 1.65 K < T < 2.6 K, rho_s/rho < 80%) and a near-wake flow. The latter (Rlambda = 1100, 1.55 K < T < 4.2 K, rho_s/rho < 86%), which was cooled down for the first time during this thesis, provided a far-wake flow.The measurements have highlighted strong similarities with classical flows at inertial scales: k^{-5/3} velocity spectra, Kolmogorov constant and turbulence intensity indistinguishable above and below the superfluid transition, 4/5-law, anomalous velocity structure functions exponents (intermittency). At smaller scales, the numerical simulations (1.15 K < T < 2.1565 K) exhibit exotic behavior: kinetic energy piles up and tends to equipartition, which makes the simulated velocity spectrum scale like k^2. This phenomenon is enhanced at low temperature. Finally, local vorticity fluctuations measurements have been achieved using second sound tweezers over a temperature range, between 1.69 K (rhos/rho=77%) and 2.01 K (rhos/rho = 42%). We observed that the spectrum scaling steepens as the temperature decreases. This can be interpreted as a consequence of the equipartitioned reservoir evidenced by numerical simulations at small scale.

The main theme of this thesis is the interplay between superuidity and localization, in a system of strongly correlated Bose particles. Driving this investigation is the search for yet unobserved phases of matter, such as the so-called supersolid. Using state-of-the- art, numerically exact computer simulations, we have carried out an extensive theoretical investigation of the effects of long-range interactions, inhomogeneity, disorder and frustration in a simple model of lattice Bosons. In particular, we explore the scenario of vacancy- and interstitial-based supersolid phases of hard core bosons on a square lattice, interacting repulsively via a nearest-neighbour and next-nearest neighbour potential. Secondly, in an attempt to model the physics of a layer of helium adsorbed on a corrugated substrate, an additional superlattice of the absorption sites is imposed to the system of hard core bosons, and the resulting low temperature phase diagram is studied. Finally, the possibility of actually inducing by disorder superuidity (superglass) in a system that does not display it in the absence of disorder is demonstrated. The quantitative and qual- itative predictions at which we have arrived appear to be at least in principle testable experimentally, for example by performing measurements on ultracold atoms in optical lattices.

Abstract Detailed in this manuscript is a methodology to model ground state properties of 4He droplets at zero pressure and zero Kelvin using a density functional theory of liquid helium. The density functional approach examined here consists of two noted functionals from the literature and corresponding mean field definitions. A mean field and trial density are defined for each system and optimized to self-consistency using a matrix diagonalization technique. Initial calculations of planar slabs are performed and demonstrate reasonable agreement with experiment and with prior studies using density functional theory. Quantum properties of droplets and droplets containing atomic dopants are calculated. Three different He-dopant potentials are examined to test the limits of the functional methods. For each impurity interaction, an average of 12 atoms were found to reside in the first solvation shell with an atomic dopant placed at the droplet center. Maximum densities in the first solvation shell reached those of solid helium as predicted by DF methods.

Thesis (Ph. D.)--University of Washington, 2001. / Vita. Includes bibliographical references (leaves 75-83).

No description available.

Thesis (Ph. D.)--Ohio State University, 2007. / Title from first page of PDF file. Includes bibliographical references (p. 187-192).

Thesis (Ph.D.); Submitted to Iowa State Univ., Ames, IA (US); 12 Dec 2003. / Published through the Information Bridge: DOE Scientific and Technical Information. "IS-T 1995" Wang-Chi Vincent Yeh. 12/12/2003. Report is also available in paper and microfiche from NTIS.

SANTOS, Victor Pereira do Nascimento. Dinâmica de solitons em superfluidos holográficos. 2010. 62 f. Dissertação (Mestrado em Física) - Programa de Pós-Graduação em Física, Departamento de Física, Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2010. / Submitted by Edvander Pires (edvanderpires@gmail.com) on 2015-06-23T20:57:29Z No. of bitstreams: 1 2010_dis_vpnsantos.pdf: 969083 bytes, checksum: 66187abe576db7775b0fcfd1ae693c87 (MD5) / Approved for entry into archive by Edvander Pires(edvanderpires@gmail.com) on 2015-06-23T21:04:21Z (GMT) No. of bitstreams: 1 2010_dis_vpnsantos.pdf: 969083 bytes, checksum: 66187abe576db7775b0fcfd1ae693c87 (MD5) / Made available in DSpace on 2015-06-23T21:04:21Z (GMT). No. of bitstreams: 1 2010_dis_vpnsantos.pdf: 969083 bytes, checksum: 66187abe576db7775b0fcfd1ae693c87 (MD5) Previous issue date: 2010 / We numerically study the formation and dynamics of domain-wall-like topological defects in superfluids, using a (3+1)-dimensional abelian Maxwell-Higgs model, under the AdS/CFT correspondence. We obtain the bulk-boundary propagators, as well as the correlation functions on the boundary for a scalar field, both in massive and non-massive cases. If we impose that the fields depends only on a boundary coordinate and the bulk radial coordinate, we obtain in the dual theory the domain wall solutions found in literature. From these solutions we found that the superfluid is characterized by two length scales, one for the order parameter and other for the charge density. We also study the change of charge density in the region near the interface of the defect, and finally we investigate the modifications needed in theory to study the dynamics of these solutions. / Neste trabalho, estudamos numericamente a formação e a dinâmica de defeitos topológicos do tipo parede de domínio em superfluidos em um modelo (3+1)-dimensional do tipo Maxwell-Higgs abeliano, no contexto da correspondência AdS/CFT. Obtemos os propagadores bulk-fronteira, assim como as funções de correlação na fronteira, para um campo escalar nos casos massivo e não-massivo. Se impusermos que os campos dependem apenas da coordenada radial no bulk e de uma coordenada na fronteira, conseguimos obter na teoria dual as soluções do tipo parede de domínio encontradas na literatura. A partir dessas soluções, estudamos suas propriedades, verificando que o superfluido é caracterizado por duas escalas de comprimento, uma para o parâmetro de ordem e outra para a densidade de carga. Estudamos também a variação da densidade de carga na região próxima à interface do defeito, e por fim, investigamos as modificações necessárias na teoria para se estudar a dinâmica dessas soluções.

M.Sc. / The content of this dissertation is directed at a clearer understanding of pulsar behaviour. Pulsars are neutron stars in relatively early stages of their evolution. The intervals between observed arrival times of pulsar beams have been observed to suddenly decrease implying an increase in the star’s angular velocity Ω. This phenomenon is called a glitch. The first observed glitch occurred in the Vela pulsar (PSR 0833 -45) in 1969. The largest observed glitch also occurred in the Vela pulsar and exhibited a relative spin-up, Ω∕Ω~10−6 [1]. These relative pulsar spin-ups generally fall within the range 10−9 to 10−6 [2]. Pulsar glitches occur as a result of the processes in the interior of a neutron star [3]. Thus, a rigorous study of neutron star interiors is justified. Neutron star interiors are composed of hadronic matter: mostly neutrons, with a few per cent of protons (and, of course, non-hadronic electrons to conserve electric charge). The density of a neutron star interior increases with increasing depth as illustrated in 1.1.