Spin critical opalescence in zero temperature Bose-Einstein Condensates

Cold atom developments suggest the prospect of measuring scaling properties and long-range fluctuations of continuous phase transitions at zero-temperature. We discuss the conditions for characterizing the phase separation of Bose-Einstein condensates of boson atoms in two distinct hyperfine spin states. The mean-field description breaks down as the system approaches the transition from the miscible side. An effective spin description clarifies the ferromagnetic nature of the transition. We show that a difference in the scattering lengths for the bosons in the same spin state leads to an effective internal magnetic field. The conditions at which the internal magnetic field vanishes (i.e., equal values of the like-boson scattering lengths) is a special point. We show that the long range density fluctuations are suppressed near that point while the effective spin exhibits the long-range fluctuations that characterize critical points. The zero-temperature system exhibits critical opalescence with respect to long wavelength waves of impurity atoms that interact with the bosons in a spin-dependent manner.Comment: 6 pages, 2 figure

Surface scattering analysis of phonon transport in the quantum limit using an elastic model

We have investigated the effect on phonon energy transport in mesoscopic systems and the reduction in the thermal conductance in the quantum limit due to phonon scattering by surface roughness using full 3-dimensional elasticity theory for an elastic beam with a rectangular cross-section. At low frequencies we find power laws for the scattering coefficients that are strongly mode dependent, and different from the $\omega^{2}$ dependence, deriving from Rayleigh scattering of scalar waves, that is often assumed. The scattering gives contributions to the reduction in thermal conductance with the same power laws. At higher frequencies the scattering coefficients becomes large at the onset frequency of each mode due to the flat dispersion here. We use our results to attempt a quantitative understanding of the suppression of the thermal conductance from the universal value observed in experiment.Comment: 27 pages, 13 figure

Acoustic phonon transport through a double-bend quantum waveguide

In this work, using the scattering matrix method, we have investigated the transmission coefficients and the thermal conductivity in a double-bend waveguide structure. The transmission coefficients show strong resonances due to the scattering in the midsection of a double-bend structure; the positions and the widths of the resonance peaks are determined by the dimensions of the midsection of the structure. And the scattering in the double-bend structure makes the thermal conductivity decreases with the increasing of the temperature first, then increases after reaches a minimum. Furthermore, the investigations of the multiple double-bend structures indicate that the first additional double-bend structure suppresses the transmission coefficient and the frequency gap formed; and the additional double-bend structures determine the numbers of the resonance peaks at the frequency just above the gap region. These results could be useful for the design of phonon devices.Comment: 13 pages, 6 figures, elsart.cls is use

Spatial and Electronic Manipulation of Silicon Nanocrystals by Atomic Force Microscopy

[As silicon-based devices shnnk, interest is increasing in fast, low-power devices sensitive to small numbers of electrons. Recent work suggests that MOS structures with large arrays of Si nanocrystals comprising a floating gate can be extremely fast, reliable and nonvolatile relative to conventional floating gate memories. In these structures approximately one electron is stored per nanocrystal. Despite promising initial results, current devices have a distribution of charge transit times during writing of nanocrystal ensembles, which limits speed. This behavior is not completely understood, but could be related to a dispersion in oxide thicknesses, nanocrystals interface states, or shifts in the electronic bound states due to size variations. To address these limitations, we have developed an aerosol vapor synthesis/deposition technique for silicon nanocrystals with active size classification, enabling narrow distributions of nanocrystal size (~10-15% of particle in the 2-10 nm size range). The first goal of these experiments has been to use scanning probe techniques to perform particle manipulation and to characterize particle electronic properties and charging on a single-particle basis. Si nanocrystal structures (lines, arrows and other objects) have been formed by contact-mode operation and subsequently imaged in noncontact mode without additional particle motion. Further, single nanocrystal charging by a conducting AFM tip has been observed, detected as an apparent height change due to electrostatic force, followed by a slow relaxation as the stored charge dissipates. Ongoing and future efforts will also be briefly discussed, including narrowing of nanocrystal size distributions, control of oxide thickness on the nanocrystals, and measurements of electron transport through individual particles and ensembles

The Effect of Surface Roughness on the Universal Thermal Conductance

We explain the reduction of the thermal conductance below the predicted universal value observed by Schwab et al. in terms of the scattering of thermal phonons off surface roughness using a scalar model for the elastic waves. Our analysis shows that the thermal conductance depends on two roughness parameters: the roughness amplitude $\delta$ and the correlation length $a$. At sufficiently low temperatures the conductance decrease from the universal value quadratically with temperature at a rate proportional to $\delta ^{2}a$. Values of $\delta$ equal to 0.22 and $a$ equal to about 0.75 of the width of the conduction pathway give a good fit to the data.Comment: 10 pages, 5 figures. Ref. added, typo correcte