Página 11 dos resultados de 186982 itens digitais encontrados em 0.113 segundos

Non-Fermi liquid corrections to the neutrino mean free path in dense quark matter

Pal, Kausik; Dutt-Mazumder, Abhee K.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
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We calculate the neutrino mean free path with non-Fermi liquid (NFL) corrections in quark matter from scattering and absorption processes for both degenerate and nondegenerate neutrinos. We show that the mean free path decreases due to the non-Fermi liquid corrections leading to $l_{mean}^{-1}\sim[......+ .... C_F^2\alpha_s^2\ln(m_D/T)^2]$. These reduction results in higher rate of scattering.; Comment: 12 pages, 2 eps figures, published in PRD

Nonlinear quantum interferometry with Bose condensed atoms

Lee, Chaohong; Huang, Jiahao; Deng, Haiming; Dai, Hui; Xu, Jun
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
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In quantum interferometry, it is vital to control and utilize nonlinear interactions for achieving high-precision measurements. Attribute to their long coherent time and high controllability, ultracold atoms including Bose condensed atoms have been widely used for implementing quantum interferometry. Here, we review the recent progresses in theoretical studies of quantum interferometry with Bose condensed atoms. In particular, we focus on the nonlinear phenomena induced by the atom-atom interaction and how to control and utilize these nonlinear phenomena. Under the mean-field description, due to the atom-atom interaction, matter-wave solitons appear in the interference patterns, and macroscopic quantum self-trapping exists in the Bose-Josephson junctions. Under the many-body description, the atom-atom interaction can generate non-classical entanglement, which may be utilized to achieve high-precision measurements beyond the standard quantum limit.; Comment: 22 pages, 10 figures, resubmitted to Frontiers of Physics. The references have been updated

Electronic Pumping of Quasiequilibrium Bose-Einstein Condensed Magnons

Bender, Scott A.; Duine, Rembert A.; Tserkovnyak, Yaroslav
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
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We theoretically investigate spin transfer between a system of quasiequilibrated Bose-Einstein condensed magnons in an insulator in direct contact with a conductor. While charge transfer is prohibited across the interface, spin transport arises from the exchange coupling between insulator and conductor spins. In normal insulator phase, spin transport is governed solely by the presence of thermal and spin-diffusive gradients; the presence of Bose-Einstein condensation (BEC), meanwhile, gives rise to a temperature-independent condensate spin current. Depending on the thermodynamic bias of the system, spin may flow in either direction across the interface, engendering the possibility of a dynamical phase transition of magnons. We discuss experimental feasibility of observing a BEC steady state (fomented by a spin Seebeck effect), which is contrasted to the more familiar spin-transfer induced classical instabilities.; Comment: 7 pages, 4 figures

Free-fall expansion of finite-temperature Bose-Einstein condensed gas in the non Thomas-Fermi regime

Zawada, M.; Abdoul, R.; Chwedeńczuk, J.; Gartman, R.; Szczepkowski, J.; Tracewski, L.; Witkowski, M.; Gawlik, W.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 11/11/2008
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We report on our study of the free-fall expansion of a finite-temperature Bose-Einstein condensed cloud of 87Rb. The experiments are performed with a variable total number of atoms while keeping constant the number of atoms in the condensate. The results provide evidence that the BEC dynamics depends on the interaction with thermal fraction. In particular, they provide experimental evidence that thermal cloud compresses the condensate.; Comment: 8 pages, 4 figures

Manipulating Bose-Einstein condensed atoms in toroidal traps

Jackson, A. D.; Kavoulakis, G. M.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 24/07/2006
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We consider Bose-Einstein condensed atoms confined in a toroidal trap. We demonstrate that under conditions of one-dimensional behavior, the density distribution of the atoms may be exponentially localized/delocalized, even for very small variations in the trapping potential along the torus. This observation allows one to control the atom density externally via slight modifications of the trapping potential. For similar reasons, small irregularities of the trap may also have a very pronounced effect on the density of the cloud.; Comment: 4 pages, RevTex, 2 figures

Gap and screening in Raman scattering of a Bose condensed gas

Navez, Patrick; Bongs, Kai
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
448.55062%
We propose different spectroscopic methods to explore the nature of the thermal excitations of a trapped Bose condensed gas: 1) a four photon process to probe the uniform region in the trap center: 2) a stimulated Raman process in order to analyze the influence of a momentum transfer in the resulting scattered atom momentum distribution. We apply these methods to address specifically the energy spectrum and the scattering amplitude of these excitations in a transition between two hyperfine levels of the gas atoms. In particular, we exemplify the potential offered by these proposed techniques by contrasting the spectrum expected, from the {\it non conserving} Bogoliubov approximation valid for weak depletion, to the spectrum of the finite temperature extensions like the {\it conserving} generalized random phase approximation (GRPA). Both predict the existence of the Bogoliubov collective excitations but the GRPA approximation distinguishes them from the single atom excitations with a gapped and parabolic dispersion relation and accounts for the dynamical screening of any external perturbation applied to the gas. We propose two feasible experiments, one concerns the observation of the gap associated to this second branch of excitations and the other deals with this screening effect.; Comment: 6 pages...

Manifestation of superfluidity in an evolving Bose-condensed gas

Kagan, Yu.; Maksimov, L. A.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 14/08/2000
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We study the generation of excitations due to an ''impurity''(static perturbation) placed into an oscillating Bose-condensed gas in the time-dependent trapping field. It is shown that there are two regions for the position of the local perturbation. In the first region the condensate flows around the ''impurity'' without generation of excitations demonstrating superfluid properties. In the second region the creation of excitations occurs, at least within a limited time interval, revealing destruction of superfluidity. The phenomenon can be studied by measuring the damping of condensate oscillations at different positions of the ''impurity''.

Anisotropic Magnetoconductance in Quench-Condensed Ultrathin Beryllium Films

Bielejec, E.; Ruan, J.; Wu, Wenhao
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
448.55062%
Near the superconductor-insulator (S-I) transition, quench-condensed ultrathin Be films show a large magnetoconductance which is highly anisotropic in the direction of the applied field. Film conductance can drop as much as seven orders of magnitude in a weak perpendicular field (< 1 T), but is insensitive to a parallel field in the same field range. We believe that this negative magnetoconductance is due to the field de-phasing of the superconducting pair wavefunction. This idea enables us to extract the finite superconducting phase coherence length in nearly superconducting films. Our data indicate that this local phase coherence persists even in highly insulating films in the vicinity of the S-I transition.; Comment: 4 pages, 4 figure RevTex, Typos Corrected

Correlated bosons on a lattice: Dynamical mean-field theory for Bose-Einstein condensed and normal phases

Byczuk, Krzysztof; Vollhardt, Dieter
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
448.55062%
We formulate a bosonic dynamical mean-field theory (B-DMFT) which provides a comprehensive, thermodynamically consistent framework for the theoretical investigation of correlated lattice bosons. The B-DMFT is applicable for arbitrary values of the coupling parameters and temperature and becomes exact in the limit of high spatial dimensions d or coordination number Z of the lattice. In contrast to its fermionic counterpart the construction of the B-DMFT requires different scalings of the hopping amplitudes with Z depending on whether the bosons are in their normal state or in the Bose-Einstein condensate. A detailed discussion of how this conceptual problem can be overcome by performing the scaling in the action rather than in the Hamiltonian itself is presented. The B-DMFT treats normal and condensed bosons on equal footing and thus includes the effects caused by their dynamic coupling. It reproduces all previously investigated limits in parameter space such as the Beliaev-Popov and Hartree-Fock-Bogoliubov approximations and generalizes the existing mean-field theories of interacting bosons. The self-consistency equations of the B-DMFT are those of a bosonic single-impurity coupled to two reservoirs corresponding to bosons in the condensate and in the normal state...

Formation of an order in a system of exciton condensed phase islands in quantum wells

Sugakov, V. I.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 15/07/2004
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448.55062%
A theory of exciton condensed phase creation in two-dimensional system is presented. The theory is applied to explain the appearance of the periodical fragmentation which was observed last years in luminescence from the ring around laser spot in crystal with double quantum wells.; Comment: 10 pages, 4 figures

Quantum Kinetic Equations of Many-Particle Systems in Condensed States

Gerasimenko, V. I.; Tsvir, Zh. A.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
448.55062%
This paper is devoted to the description of the evolution of states of quantum many-particle systems within the framework of a one-particle density operator, which enables to construct the kinetic equations in scaling limits in the presence of correlations of particle states at initial time, for instance, correlations characterizing the condensed states.; Comment: 10 pages

Bose-Condensed Gases in a 1D Optical Lattice at Finite Temperatures

Arahata, E.; Nikuni, T.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
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We study equilibrium properties of Bose-Condensed gases in a one-dimensional (1D) optical lattice at finite temperatures. We assume that an additional harmonic confinement is highly anisotropic, in which the confinement in the radial directions is much tighter than in the axial direction. We derive a quasi-1D model of the Gross-Pitaeavkill equation and the Bogoliubov equations, and numerically solve these equations to obtain the condensate fraction as a function of the temperature.; Comment: Comments: 6 pages, 3 figures, submitted to Quantum Fluids and Solids Conference (QFS 2006)

Mixtures of correlated bosons and fermions: Dynamical mean-field theory for normal and condensed phases

Byczuk, Krzysztof; Vollhardt, Dieter
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
448.55062%
We derive a dynamical mean-field theory for mixtures of interacting bosons and fermions on a lattice (BF-DMFT). The BF-DMFT is a comprehensive, thermodynamically consistent framework for the theoretical investigation of Bose-Fermi mixtures and is applicable for arbitrary values of the coupling parameters and temperatures. It becomes exact in the limit of high spatial dimensions d or coordination number Z of the lattice. In particular, the BF-DMFT treats normal and condensed bosons on equal footing and thus includes the effects caused by their dynamic coupling. Using the BF-DMFT we investigate two different interaction models of correlated lattice bosons and fermions, one where all particles are spinless (model I) and one where fermions carry a spin one-half (model II). In model I the local, repulsive interaction between bosons and fermions can give rise to an attractive effective interaction between the bosons. In model II it can also lead to an attraction between the fermions.; Comment: 11 pages, removed style-files for Greek letters

Interference effects between two initially independent Bose-condensed gases

Xiong, Hongwei; Liu, Shujuan
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 21/07/2005
Relevância na Pesquisa
448.55062%
When two initially independent Bose-condensed gases are allowed to overlap, we investigate the density expectation value of the whole system by using the second quantization method. In the presence of interatomic interaction, based on the exact expression of the density expectation value, it is found that there is a nonzero interference term in the density expectation value of the whole system. The evolution of the density expectation value is shown for different coupling constants. The present work shows clearly that there is an interaction-induced coherence process between two initially independent condensates.; Comment: 6 pages, 2 figures

Drastic enhancement of magnon thermal conductivity in the Bose-Einstein condensed state of TlCuCl$_3$

Kudo, K.; Yamazaki, M.; Kawamata, T.; Noji, T.; Koike, Y.; Nishizaki, T.; Kobayashi, N.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
448.55062%
We have measured the thermal conductivity of a TlCuCl$_3$ single crystal in magnetic fields up to 14 T. It has been found that the temperature dependence of the thermal conductivity exhibits a sharp peak at 4 K in zero field, which is suppressed by the application of magnetic fields up to 7 T. The peak is concluded to be attributable to the enhancement of the thermal conductivity due to phonons because of the formation of a spin-gap state. In high magnetic fields above 7 T, on the other hand, another sharp peak appears around 4 K and this is enhanced with increasing magnetic field. This peak is regarded as being attributable to the enhancement of the thermal conductivity due to magnons and/or phonons because of the drastic extension of the mean free path of magnons and/or phonons in the Bose-Einstein condensed state.; Comment: 4 pages, 4 figures

Formation of a condensed state with macroscopic number of phonons in ultracold Bose gases

Kagan, Yu.; Manakova, L. A.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 29/05/2007
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A mechanism for the formation of a new type of stationary state with macroscopical number of phonons in condensed atomic gases is proposed. This mechanism is based on generating longitudinal phonons as a result of parametric resonance caused by a permanent modulation of the transverse trap frequency in an elongated trap. The phonon-phonon interaction predetermines the self-consistent evolution which is completed with macroscopic population of one from all levels within the energy interval of parametric amplification. This level proves to be shifted to the edge of this interval. All other levels end the evolution with zero population.; Comment: 9 pages, 8 figures

Bifurcations and bistability in cavity assisted photoassociation of Bose-Einstein condensed molecules

Jaaskelainen, Markku; Jeong, Jaeyoon; Search, Christopher P.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 31/05/2007
Relevância na Pesquisa
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We study the photo-association of Bose-Einstein condensed atoms into molecules using an optical cavity field. The driven cavity field introduces a new dynamical degree of freedom into the photoassociation process, whose role in determining the stationary behavior has not previously been considered. The semiclassical stationary solutions for the atom and molecules as well as the intracavity field are found and their stability and scaling properties are determined in terms of experimentally controllable parameters including driving amplitude of the cavity and the nonlinear interactions between atoms and molecules. For weak cavity driving, we find a bifurcation in the atom and molecule number occurs that signals a transition from a stable steady state to nonlinear Rabi oscillations. For a strongly driven cavity, there exists bistability in the atom and molecule number.

Vortices in Bose-Einstein Condensed Na Atoms

Shrivastava, Keshav N.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 20/11/2001
Relevância na Pesquisa
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There are surface modes on the Bose-Einstein condensed Na atoms so that the number of vortices diverges when the stirring frequency becomes equal to that of the surface waves. We introduce the finite life time of the surface modes so that the number of vortices becomes finite. Usually the number of vortices is a linear function of the stirring frequency. We find that this linearity is destroyed by the finite life time and a peaked function emerges with several peaks, one for each surface mode. The vortices become normal, as they should be, so that there occurs a phase transition from normal to the superfluid state.; Comment: 4 pages

Condensed phase of converting boson-fermion mixtures

Alexandrov, A. S.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 04/02/2004
Relevância na Pesquisa
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Theory of a condensed state of hybridised bosons and fermions is developed. Normal and anomalous Green's functions are obtained diagrammatically and analytically using the Hamiltonian of the boson-fermion model (BFM). A pairing of bosons analogous to the Cooper pairing of fermions is found. There are three coupled condensates in the model, described by the off-diagonal single-particle boson, pair-fermion and pair-boson fields. The Gor'kov expansion in the strength of the order parameter near the transition yields no linear homogeneous term in the Ginzburg-Landau equation, and the infinite Levanyuk-Ginzburg parameter, Gi, which indicates that previous mean-field discussions of BFM are flawed.; Comment: 4 pages, 2 figures, spelling corrected

Charge constrained density functional molecular dynamics for simulation of condensed phase electron transfer reactions

Oberhofer, H.; Blumberger, J.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
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We present a plane-wave basis set implementation of charge constrained density functional molecular dynamics (CDFT-MD) for simulation of electron transfer reactions in condensed phase systems. Following earlier work of Wu et al. Phys. Rev. A 72, 024502 (2005), the density functional is minimized under the constraint that the charge difference between donor and acceptor is equal to a given value. The classical ion dynamics is propagated on the Born-Oppenheimer surface of the charge constrained state. We investigate the dependence of the constrained energy and of the energy gap on the definition of the charge, and present expressions for the constraint forces. The method is applied to the Ru2+-Ru3+ electron self-exchange reaction in aqueous solution. Sampling the vertical energy gap along CDFT-MD trajectories, and correcting for finite size effects, a reorganization free energy of 1.6 eV is obtained. This is 0.1-0.2 eV lower than a previous estimate based on a continuum model for solvation. smaller value for reorganization free energy can be explained by fact that the Ru-O distances of the divalent and trivalent Ru-hexahydrates are predicted to be more similar in the electron transfer complex than for the separated aqua-ions.; Comment: 12 pages...