1,721,118 research outputs found
Calculation of the roton-roton contribution to the viscosity of liquid he II
No full textA new potential, which accounts for roton-roton interaction is proposed. Using this potential without any adjustable parameters, the calculated roton-roton contribution to the viscosity of He II in the temperature range between 1.35 °K and 1.9 °K is found to be in qualitative agreement with the experimental value. © 1969 Società Italiana di Fisica
Response Function of A Finite Electron-gas
No full textThe full response matrix of an idealized metal with two plane parallel surfaces is calculated in the RPA approximation using the equation of motion for the retarded density-density Green's function. The surfaces are approximated by infinite barriers and the induced charge density is related to the perturbing charge instead of the Hartree field. It is shown that the Hamiltonian of the system splits into two parts: one due to bulk interactions and the other to the presence of the surfaces. The response matrix is then formally calculated and surface plasmons are shown to be described by the poles of a function related to the response matrix
Radiation Enhanced Diffusion In Glasses
No full textWe discuss a possible new mechanism leading to stimulated ionic transport processes in electron-irradiated glasses. By invoking percolation through structurally relaxed units following defect recombination we can give a unified picture of various experimental results. Peculiarities in the transport properties are shown to give some insight into the nature of the irradiation-induced defects in glasses
PROPERTIES OF AN ELECTRON BUBBLE APPROACHING THE SURFACE OF LIQUID-HELIUM
No full textWe have studied the properties of an electron bubble close to the surface of liquid He-4, by using a Density Functional approach. We find that up to an electron-surface distance d(0) similar to 23 Angstrom the bubble is stable, while at smaller distances it becomes unstable and bursts. A potential energy barrier Phi/K-B similar to 38 degrees K for the thermal emission of electrons is obtained from our results, in agreement with experiments. Even when the electron-surface distance is larger than do, however, tunneling through the surface layer dominates the electron escape probability. Large deviations of the electron potential energy from its ideal value are found close to the surface. These deviations have a profound effect on the calculated decay rates of the tunneling curent, which are much smaller than those obtained previously and in semi-quantitative agreement with experiments
Shell effects in the first sound velocity of an ultracold Fermi gas
No full textWe investigate the first sound of a normal dilute and ultracold two-component Fermi gas in a cylindrical trap with harmonic radial confinement. We show that the velocity of the sound that propagates along the axial direction strongly varies in the dimensional crossover of the system. In particular, we predict that the first-sound velocity exhibits shell effects: by increasing the density, that is by inducing the crossover from one to three-dimensions, the first-sound velocity shows jumps in correspondence with the filling of harmonic modes. The experimental achievability of these effects is discussed by considering K-40 atoms
Dipolar Bose gas in a highly anharmonic trap
No full textBy means of mean-field theory, we have studied the structure and excitation spectrum of a purely dipolar
Bose gas in a pancake shaped trap where the confinement in the x-y plane is provided by a highly anharmonic
potential, resulting in an almost uniform confinement in the plane. We show that the stable condensates are
characterized by marked radially structured density profiles. The stability diagram is calculated by independently
varying the strength of the interaction and the trap geometry. By computing the Bogoliubov excitation spectrum
near the instability line we show that soft “angular” rotons are responsible for the collapse of the system. The
free expansion of the cloud after the trap is released is also studied by means of time-dependent calculations,
showing that a prolate cigar-shaped condensate is dynamically stabilized during the expansion, which would
otherwise collapse. Dipolar condensates rotating with sufficiently high angular velocity show the formation of
multiply quantized giant vortices, while the condensates acquire a ring-shaped form
COMPUTER-SIMULATIONS OF EXCESS ELECTRON-TRANSPORT IN NEON
No full textThe behavior of excess electrons in neon gas in a wide range of densities is investigated using molecular-dynamics simulations with a parameter-free interparticle potential. A realistic pseudopotential reproducing the measured electron-Ne low-energy scattering properties is used. A transition from quasi-free behavior to a localized regime where the electron is trapped in a bubblelike cavity is observed as the density is increased beyond a value that is close to the experimental one. The calculated electron mobilities in a wide range of densities are also found to be in reasonable agreement with the experimental data
Zero-point energy of ultracold atoms
No full textWe analyze the divergent zero-point energy of a dilute and ultracold gas of atoms in D spatial dimensions. For bosonic atoms we explicitly show how to regularize this divergent contribution, which appears in the Gaussian fluctuations of the functional integration, by using three different regularization approaches: dimensional regularization, momentum cutoff regularization and convergence-factor regularization. In the case of the ideal Bose gas the divergent zero-point fluctuations are completely removed, while in the case of the interacting Bose gas these zero-point fluctuations give rise to a finite correction to the equation of state. The final convergent equation of state is independent of the regularization procedure but depends on the dimensionality of the system and the two-dimensional case is highly nontrivial. We also discuss very recent theoretical results on the divergent zero-point energy of the D-dimensional superfluid Fermi gas in the BCS-BEC crossover. In this case the zero-point energy is due to both fermionic single-particle excitations and bosonic collective excitations, and its regularization gives remarkable analytical results in the BEC regime of composite bosons. We compare the beyond-mean field equations of state of both bosons and fermions with relevant experimental data on dilute and ultracold atoms quantitatively confirming the contribution of zero-point-energy quantum fluctuations to the thermodynamics of ultracold atoms at very low temperatures
Pair condensation in the BCS-BEC crossover of ultracold atoms loaded onto a two-dimensional square lattice
No full textWe investigate the crossover from the Bardeen-Cooper-Schrieffer (BCS) state of weakly bound Cooper pairs to the Bose-Einstein condensate (BEC) of strongly bound molecular dimers in a gas of ultracold atoms loaded on a two-dimensional optical lattice. By using the mean-field BCS equations of the emerging Hubbard model and the concept of off-diagonal long-range order for fermions we calculate analytically and numerically the pair binding energy, the energy gap, and the condensate fraction of Cooper pairs as a function of interaction strength and filling factor of atoms in the lattice at zero temperature
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