14,400 research outputs found
A low-temperature dynamic mode scanning force microscope operating in high magnetic fields
A scanning force microscope was implemented operating at temperatures below
4.2K and in magnetic fields up to 8T. Piezoelectric quartz tuning forks were
employed for non optical tip-sample distance control in the dynamic operation
mode. Fast response was achieved by using a phase-locked loop for driving the
mechanical oscillator. Possible applications of this setup for various scanning
probe techniques are discussed.Comment: 5 pages, 5 figures, submitted to "Review of Scientific Instruments
Broad Histogram Monte Carlo
We propose a new Monte Carlo technique in which the degeneracy of energy
states is obtained with a Markovian process analogous to that of Metropolis
used currently in canonical simulations. The obtained histograms are much
broader than those of the canonical histogram technique studied by Ferrenberg
and Swendsen. Thus we can reliably reconstruct thermodynamic functions over a
much larger temperature scale also away from the critical point. We show for
the two-dimensional Ising model how our new method reproduces exact results
more accurately and using less computer time than the conventional histogram
method. We also show data in three dimensions for the Ising ferromagnet and the
Edwards Anderson spin glass.Comment: 6 pages of a TeX file with 4 PS figures. Related papers at
http://www.if.uff.br/~tjp
Segregation in a fluidized binary granular mixture: Competition between buoyancy and geometric forces
Starting from the hydrodynamic equations of binary granular mixtures, we
derive an evolution equation for the relative velocity of the intruders, which
is shown to be coupled to the inertia of the smaller particles. The onset of
Brazil-nut segregation is explained as a competition between the buoyancy and
geometric forces: the Archimedean buoyancy force, a buoyancy force due to the
difference between the energies of two granular species, and two geometric
forces, one compressive and the other-one tensile in nature, due to the
size-difference. We show that inelastic dissipation strongly affects the phase
diagram of the Brazil nut phenomenon and our model is able to explain the
experimental results of Breu et al. (PRL, 2003, vol. 90, p. 01402).Comment: 5 pages, 2 figure
Diffusion and spectral dimension on Eden tree
We calculate the eigenspectrum of random walks on the Eden tree in two and
three dimensions. From this, we calculate the spectral dimension and the
walk dimension and test the scaling relation (
for an Eden tree). Finite-size induced crossovers are observed, whereby the
system crosses over from a short-time regime where this relation is violated
(particularly in two dimensions) to a long-time regime where the behavior
appears to be complicated and dependent on dimension even qualitatively.Comment: 11 pages, Plain TeX with J-Phys.sty style, HLRZ 93/9
On the magnetic stability at the surface in strongly correlated electron systems
The stability of ferromagnetism at the surface at finite temperatures is
investigated within the strongly correlated Hubbard model on a semi-infinite
lattice. Due to the reduced surface coordination number the effective Coulomb
correlation is enhanced at the surface compared to the bulk. Therefore, within
the well-known Stoner-picture of band ferromagnetism one would expect the
magnetic stability at the surface to be enhanced as well. However, by taking
electron correlations into account well beyond the Hartree-Fock (Stoner) level
we find the opposite behavior: As a function of temperature the magnetization
of the surface layer decreases faster than in the bulk. By varying the hopping
integral within the surface layer this behavior becomes even more pronounced. A
reduced hopping integral at the surface tends to destabilize surface
ferromagnetism whereas the magnetic stability gets enhanced by an increased
hopping integral. This behavior represents a pure correlation effect and can be
understood in terms of general arguments which are based on exact results in
the limit of strong Coulomb interaction.Comment: 6 pages, RevTeX, 4 eps figures, accepted (Phys. Rev. B), for related
work and info see http://orion.physik.hu-berlin.d
Shot noise in carbon nanotube based Fabry-Perot interferometers
We report on shot noise measurements in carbon nanotube based Fabry-Perot
electronic interferometers. As a consequence of quantum interferences, the
noise power spectral density oscillates as a function of the voltage applied to
the gate electrode. The quantum shot noise theory accounts for the data
quantitatively. It allows to confirm the existence of two nearly degenerate
orbitals. At resonance, the transmission of the nanotube approaches unity, and
the nanotube becomes noiseless, as observed in quantum point contacts. In this
weak backscattering regime, the dependence of the noise on the backscattering
current is found weaker than expected, pointing either to electron-electron
interactions or to weak decoherence
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