769 research outputs found
Experiments on Quantum and Thermal Desorption from ^4He Films
Desorption of He atoms from thin films may be resolved experimentally into quantum and thermal components. We show that quantum desorption becomes the dominant part of the signal in submonolayer films. We also show that, when all effects of collisions between desorbed atoms are eliminated, quantum desorption is not focused normal to the surface of optically polished sapphire crystals
Adsorption and Specific-Heat Studies of Monolayer and Submonolayer Films of He3 and He4
A study has been made of the adsorption of He3 and He4 at 4°K on a substrate consisting of a monolayer of argon adsorbed on a sintered copper sponge. The isotherms display distinct steps indicating the completion of first and second adsorbed layers. Comparisons among the adsorption isotherms of helium and of Ar and N2 at 77.4°K yield a self-consistent set of molecular areas. Measurements have been made of the specific heat of five submonolayer coverages of He3 and He4 on Ar-plated Cu sponge. The heat capacities of nearly complete monolayers vary as T2 from 0.3 to 4°K, yielding two-dimensional Debye temperatures Theta (He4)=28±1°K, and Theta (He3)=31±1°K. At lower coverages the molar heat capacities increase and develop contributions linear in T below 1°K. At an intermediate coverage, the heat capacity of He4 exhibits a broad and pronounced maximum near 3°K. Possible mechanisms for the linear terms and the maximum are discussed briefly. Evidence for considerable mobility of He atoms along the surface is adduced from the temperature and coverage dependence of the heat capacity. The T2 behavior for the complete monolayers yields an upper limit of ~10^-11 sec for the lifetime of a He atom in any individual adsorption site, consistent with a theoretical estimate
Spin Waves in Striped Phases
In many antiferromagnetic, quasi-two-dimensional materials, doping with holes
leads to "stripe" phases, in which the holes congregate along antiphase domain
walls in the otherwise antiferromagnetic texture. Using a suitably parametrized
two-dimensional Heisenberg model on a square lattice, we study the spin wave
spectra of well-ordered spin stripes, comparing bond-centered antiphase domain
walls to site-centered antiphase domain walls for a range of spacings between
the stripes and for stripes both aligned with the lattice ("vertical") and
oriented along the diagonals of the lattice ("diagonal"). Our results establish
that there are qualitative differences between the expected neutron scattering
responses for the bond-centered and site-centered cases. In particular,
bond-centered stripes of odd spacing generically exhibit more elastic peaks
than their site-centered counterparts. For inelastic scattering, we find that
bond-centered stripes produce more spin wave bands than site-centered stripes
of the same spacing and that bond-centered stripes produce rather isotropic low
energy spin wave cones for a large range of parameters, despite local
microscopic anisotropy. We find that extra scattering intensity due to the
crossing of spin wave modes (which may be linked to the "resonance peak" in the
cuprates) is more likely for diagonal stripes, whether site- or bond-centered,
whereas spin wave bands generically repel, rather than cross, when stripes are
vertical.Comment: 12 pages, 12 figures, for some high-res.pics, see
http://physics.bu.edu/~yaodx/spinwave/spinw.htm
Thickness of the flowing superfluid film
It has recently been shown experimentally that the thickness of a saturated helium film is independent of its state of superflow. We present a simple model in which the thickness of such a film is fixed by dynamical considerations at the gas-film interface, and the required balance of forces is then provided by relatively minor adjustments in the film surface tension
Socio-economic utility and chemical potential
In statistical physics, the conservation of particle number results in the
equalization of the chemical potential throughout a system at equilibrium. In
contrast, the homogeneity of utility in socio-economic models is usually
thought to rely on the competition between individuals, leading to Nash
equilibrium. We show that both views can be reconciled by introducing a notion
of chemical potential in a wide class of socio-economic models, and by relating
it in a direct way to the equilibrium value of the utility. This approach also
allows the dependence of utility across the system to be determined when agents
take decisions in a probabilistic way. Numerical simulations of a urban
economic model also suggest that our result is valid beyond the initially
considered class of solvable models.Comment: 6 pages, 3 figures, final versio
Phase change in adsorbed helium at low temperature
Several recent reports indicate a growing interest in the properties of adsorbed helium at low temperatures. This interest is based on the expected influence of restrictions of mobility and dimensions on thermodynamic properties such as heat capacity,(1,2) adsorption isotherms,(3) magnetic susceptibility,(4-6) and nuclear polarization.(7) This Letter presents specific heat data for adsorbed helium in the temperature range from 0.25 to 4°K, at several partial monolayer coverages. We interpret the results as indicating the existence of two surface phases.
See Also:
Erratum: D. L. Goodstein, J. G. Dash, and W. D. McCormick, Phase Change in Adsorbed Helium at Low Temperature., Phys. Rev. Lett. 15, 740 (1965)
The Relationship Between \u27\u27Achievement Imagery and Stuttering Behavior in College Males
The empirical work of Johnson (3), Darley (1), Moncur (9) and the yet unpublished research recently completed at the University of Iowa Speech Clinic under a grant from the Hill Family Foundation lends considerable support to these speculations. These studies have reported that the parents of stuttering children, as compared with the parents of non-stutterers, are generally more perfectionistic, have higher standards and expectations both for themselves and their children and are less well satisfied with the progress they and their children make in achieving these expectations
Planck Oscillators in the Background Dark Energy
We consider a model for an underpinning of the universe: there are
oscillators at the Planck scale in the background dark energy. Starting from a
coherent array of such oscillators it is possible to get a description from
elementary particles to Black Holes including the usual Hawking-Beckenstein
theory. There is also a description of Gravitation in the above model which
points to a unified description with electromagnetism.Comment: 18 pages latex; talk at the Max Born Symposium 2009, Wrocla
The effect of social media communication on consumer perceptions of brands
Researchers and brand managers have limited understanding of the effects social media communication has on how consumers perceive brands. We investigated 504 Facebook users in order to observe the impact of firm-created and user-generated social media communication on brand equity, brand attitude and purchase intention by using a standardized online survey throughout Poland. To test the conceptual model, we analyzed 60 brands across three different industries: non-alcoholic beverages, clothing and mobile network operators. When analyzing the data, we applied the structural equation modeling technique to both investigate the interplay of firm-created and user-generated social media communication and examine industry-specific differences. The results of the empirical studies showed that user-generated social media communication had a positive influence on both brand equity and brand attitude, whereas firm-created social media communication affected only brand attitude. Both brand equity and brand attitude were shown to have a positive influence on purchase intention. In addition, we assessed measurement invariance using a multi-group structural modeling equation. The findings revealed that the proposed measurement model was invariant across the researched industries. However, structural path differences were detected across the models
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