5,128 research outputs found
Collective charge density wave motion through an ensemble of Aharonov-Bohm rings
We investigate theoretically the collective charge density wave motion
through an ensemble of small disordered Aharonov-Bohm rings. It is shown that
the magnetic flux modulates the threshold field and the magnetoresistance with
a half flux quantum periodicity , resulting from ensemble
averaging over random scattering phases of multiple rings. The magnitude of the
magnetoresistance oscillations decreases rapidly with increasing bias. This is
consistent with recent experiments on in presence of columnar defects
[Phys. Rev. Lett. 78, 919 (1997)].Comment: 4 pages Revtex, 2 figures. Submitted to Phys. Rev. Let
Comment on "Peierls Gap in Mesoscopic Ring Threated by a Magnetic Flux"
In a recent letter, Yi et al. PRL 78, 3523 (1997), have considered the
stability of a Charge Density Wave in a one-dimensional ring, in the presence
of an Aharonov-Bohm flux. This comment shows that, in one dimension, the
stability of the Charge Density Wave depends on the parity of the number of
electrons in the ring. This effect is similar to the parity effect known for
the persistent current in one-dimensional rings.Comment: Latex, 1 page, 2 figure
Spin-torque switching: Fokker-Planck rate calculation
We describe a new approach to understanding and calculating magnetization
switching rates and noise in the recently observed phenomenon of "spin-torque
switching". In this phenomenon, which has possible applications to information
storage, a large current passing from a pinned ferromagnetic (FM) layer to a
free FM layer switches the free layer. Our main result is that the spin-torque
effect increases the Arrhenius factor in the switching rate, not
by lowering the barrier , but by raising the effective spin temperature .
To calculate this effect quantitatively, we extend Kramers' 1940 treatment of
reaction rates, deriving and solving a Fokker-Planck equation for the energy
distribution including a current-induced spin torque of the Slonczewski type.
This method can be used to calculate slow switching rates without long-time
simulations; in this Letter we calculate rates for telegraph noise that are in
good qualitative agreement with recent experiments. The method also allows the
calculation of current-induced magnetic noise in CPP (current perpendicular to
plane) spin valve read heads.Comment: 11 pages, 8 figures, 1 appendix Original version in Nature format,
replaced by Phys. Rev. Letters format. No substantive change
Physiological requirements for carbonate precipitation during biofilm development of Bacillus subtilis etfA mutant
Although the implications of calcium carbonate (CaCO(3)) precipitation by microorganisms in natural environments are quite relevant, the physiology and genetics of this phenomenon are poorly understood. We have chosen Bacillus subtilis 168 as our model to study which physiological aspects are associated with CaCO(3) (calcite) formation during biofilm development when grown on precipitation medium. A B. subtilis eftA mutant named FBC5 impaired in calcite precipitation was used for comparative studies. Our results demonstrate that inactivation of etfA causes a decrease in the pH of the precipitation medium during biofilm development. Further analysis demonstrated that eftA extrudes an excess of 0.7 mol H(+) L(-1) with respect to B. subtilis 168 strain. Using media buffered at different pH values, we were able to control calcite formation. Because etfA encodes the alpha-subunit of a putative flavoprotein involved in fatty acid metabolism, we compared the intracellular levels of NADH among strains. Our physiological assay showed that FBC5 accumulated up to 32 times more NADH than the wild-type strain. We propose that the accumulation of NADH causes a deregulation in the generation of the proton motive force (DeltamicroH(+)) in FBC5 producing the acidification
Is Random Close Packing of Spheres Well Defined?
Despite its long history, there are many fundamental issues concerning random
packings of spheres that remain elusive, including a precise definition of
random close packing (RCP). We argue that the current picture of RCP cannot be
made mathematically precise and support this conclusion via a molecular
dynamics study of hard spheres using the Lubachevsky-Stillinger compression
algorithm. We suggest that this impasse can be broken by introducing the new
concept of a maximally random jammed state, which can be made precise.Comment: 6 pages total, 2 figure
Surface effects on nanowire transport: numerical investigation using the Boltzmann equation
A direct numerical solution of the steady-state Boltzmann equation in a
cylindrical geometry is reported. Finite-size effects are investigated in large
semiconducting nanowires using the relaxation-time approximation. A nanowire is
modelled as a combination of an interior with local transport parameters
identical to those in the bulk, and a finite surface region across whose width
the carrier density decays radially to zero. The roughness of the surface is
incorporated by using lower relaxation-times there than in the interior.
An argument supported by our numerical results challenges a commonly used
zero-width parametrization of the surface layer. In the non-degenerate limit,
appropriate for moderately doped semiconductors, a finite surface width model
does produce a positive longitudinal magneto-conductance, in agreement with
existing theory. However, the effect is seen to be quite small (a few per cent)
for realistic values of the wire parameters even at the highest practical
magnetic fields. Physical insights emerging from the results are discussed.Comment: 15 pages, 7 figure
On the derivation of the t-J model: electron spectrum and exchange interactions in narrow energy bands
A derivation of the t-J model of a highly-correlated solid is given starting
from the general many-electron Hamiltonian with account of the
non-orthogonality of atomic wave functions. Asymmetry of the Hubbard subbands
(i.e. of ``electron'' and ``hole''cases) for a nearly half-filled bare band is
demonstrated. The non-orthogonality corrections are shown to lead to occurrence
of indirect antiferromagnetic exchange interaction even in the limit of the
infinite on-site Coulomb repulsion. Consequences of this treatment for the
magnetism formation in narrow energy bands are discussed. Peculiarities of the
case of ``frustrated'' lattices, which contain triangles of nearest neighbors,
are considered.Comment: 4 pages, RevTe
Conductance Fluctuations in a Metallic Wire Interrupted by a Tunnel Junction
The conductance fluctuations of a metallic wire which is interrupted by a
small tunnel junction has been explored experimentally. In this system, the
bias voltage V, which drops almost completely inside the tunnel barrier, is
used to probe the energy dependence of conductance fluctuations due to disorder
in the wire. We find that the variance of the fluctuations is directly
proportional to V. The experimental data are consistently described by a
theoretical model with two phenomenological parameters: the phase breaking time
at low temperatures and the diffusion coefficient.Comment: 9 pages RevTeX and 4 PS figures (accepted for publication in Physical
Review Letters
Screening and interlayer coupling in multilayer graphene field-effect transistors
With the motivation of improving the performance and reliability of
aggressively scaled nano-patterned graphene field-effect transistors, we
present the first systematic experimental study on charge and current
distribution in multilayer graphene field-effect transistors. We find a very
particular thickness dependence for Ion, Ioff, and the Ion/Ioff ratio, and
propose a resistor network model including screening and interlayer coupling to
explain the experimental findings. In particular, our model does not invoke
modification of the linear energy-band structure of graphene for the multilayer
case. Noise reduction in nano-scale few-layer graphene transistors is
experimentally demonstrated and can be understood within this model as well.Comment: 13 pages, 4 figures, 20 reference
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