893 research outputs found
Dynamics of tilt-based browsing on mobile devices
A tilt-controlled photo browsing method for small mobile devices is presented. The implementation uses continuous inputs from an accelerometer, and a multimodal (visual, audio and vibrotactile) display coupled with the states of this model. The model is based on a simple physical model, with its characteristics shaped to enhance usability. We show how the dynamics of the physical model can be shaped to make the handling qualities of the mobile device fit the browsing task. We implemented the proposed algorithm on Samsung MITs PDA with tri-axis accelerometer and a vibrotactile motor. The experiment used seven novice users browsing from 100 photos. We compare a tilt-based interaction method with a button-based browser and an iPod wheel. We discuss the usability performance and contrast this with subjective experience from the users. The iPod wheel has significantly poorer performance than button pushing or tilt interaction, despite its commercial popularity
Coulomb drag in high Landau levels
Recent experiments on Coulomb drag in the quantum Hall regime have yielded a
number of surprises. The most striking observations are that the Coulomb drag
can become negative in high Landau levels and that its temperature dependence
is non-monotonous. We develop a systematic diagrammatic theory of Coulomb drag
in strong magnetic fields explaining these puzzling experiments. The theory is
applicable both in the diffusive and the ballistic regimes; we focus on the
experimentally relevant ballistic regime (interlayer distance smaller than
the cyclotron radius ). It is shown that the drag at strong magnetic
fields is an interplay of two contributions arising from different sources of
particle-hole asymmetry, namely the curvature of the zero-field electron
dispersion and the particle-hole asymmetry associated with Landau quantization.
The former contribution is positive and governs the high-temperature increase
in the drag resistivity. On the other hand, the latter one, which is dominant
at low , has an oscillatory sign (depending on the difference in filling
factors of the two layers) and gives rise to a sharp peak in the temperature
dependence at of the order of the Landau level width.Comment: 26 pages, 13 figure
Flux Creep and Flux Jumping
We consider the flux jump instability of the Bean's critical state arising in
the flux creep regime in type-II superconductors. We find the flux jump field,
, that determines the superconducting state stability criterion. We
calculate the dependence of on the external magnetic field ramp rate,
. We demonstrate that under the conditions typical for most of the
magnetization experiments the slope of the current-voltage curve in the flux
creep regime determines the stability of the Bean's critical state, {\it i.e.},
the value of . We show that a flux jump can be preceded by the
magneto-thermal oscillations and find the frequency of these oscillations as a
function of .Comment: 7 pages, ReVTeX, 2 figures attached as postscript file
Angular 21 cm Power Spectrum of a Scaling Distribution of Cosmic String Wakes
Cosmic string wakes lead to a large signal in 21 cm redshift maps at
redshifts larger than that corresponding to reionization. Here, we compute the
angular power spectrum of 21 cm radiation as predicted by a scaling
distribution of cosmic strings whose wakes have undergone shock heating.Comment: 13 pages, 6 figures; v2: minor modifications, journal versio
Singularities in the Fermi liquid description of a partially filled Landau level and the energy gaps of fractional quantum Hall states
We consider a two dimensional electron system in an external magnetic field
at and near an even denominator Landau level filling fraction. Using a
fermionic Chern--Simons approach we study the description of the system's low
energy excitations within an extension of Landau's Fermi liquid theory. We
calculate perturbatively the effective mass and the quasi--particle interaction
function characterizing this description. We find that at an even denominator
filling fraction the fermion's effective mass diverges logarithmically at the
Fermi level, and argue that this divergence allows for an {\it exact}
calculation of the energy gaps of the fractional quantized Hall states
asymptotically approaching these filling fractions. We find that the
quasi--particle interaction function approaches a delta function. This singular
behavior leads to a cancelation of the diverging effective mass from the long
wavelength low frequency linear response functions at even denominator filling
fractions.Comment: 46 pages, RevTeX, 5 figures included in a uuencoded postscript file.
Minor revisions relative to the original version. The paper will be published
in the Physical Review B, and can be retrieved from the World Wide Web, in
http://cmtw.harvard.edu/~ster
Transport of Surface States in the Bulk Quantum Hall Effect
The two-dimensional surface of a coupled multilayer integer quantum Hall
system consists of an anisotropic chiral metal. This unusual metal is
characterized by ballistic motion transverse and diffusive motion parallel
(\hat{z}) to the magnetic field. Employing a network model, we calculate
numerically the phase coherent two-terminal z-axis conductance and its
mesoscopic fluctuations. Quasi-1d localization effects are evident in the limit
of many layers. We consider the role of inelastic de-phasing effects in
modifying the transport of the chiral surface sheath, discussing their
importance in the recent experiments of Druist et al.Comment: 9 pages LaTex, 9 postscript figures included using eps
"quasi-particles" in bosonization theory of interacting fermion liquids at arbitrary dimensions
Within bosonization theory we introduce in this paper a new definition of
"quasi-particles" for interacting fermions at arbitrary space dimenions. In
dimensions higher than one we show that the constructed quasi-particles are
consistent with quasi-particle descriptions in Landau Fermi liquid theory
whereas in one-dimension the quasi-particles" are non-perturbative objects
(spinons and holons) obeying fractional statistics. The more general situation
of Fermi liquids with singular Landau interaction is discussed.Comment: 10 page
Bounds from Primordial Black Holes with a Near Critical Collapse Initial Mass Function
Recent numerical evidence suggests that a mass spectrum of primordial black
holes (PBHs) is produced as a consequence of near critical gravitational
collapse. Assuming that these holes formed from the initial density
perturbations seeded by inflation, we calculate model independent upper bounds
on the mass variance at the reheating temperature by requiring the mass density
not exceed the critical density and the photon emission not exceed current
diffuse gamma-ray measurements. We then translate these results into bounds on
the spectral index n by utilizing the COBE data to normalize the mass variance
at large scales, assuming a constant power law, then scaling this result to the
reheating temperature. We find that our bounds on n differ substantially
(\delta n > 0.05) from those calculated using initial mass functions derived
under the assumption that the black hole mass is proportional to the horizon
mass at the collapse epoch. We also find a change in the shape of the diffuse
gamma-ray spectrum which results from the Hawking radiation. Finally, we study
the impact of a nonzero cosmological constant and find that the bounds on n are
strengthened considerably if the universe is indeed vacuum-energy dominated
today.Comment: 24 pages, REVTeX, 5 figures; minor typos fixed, two refs added,
version to be published in PR
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