182,007 research outputs found
Electronic Structure of New AFFeAs Prototype of Iron Arsenide Superconductors
This work is provoked by recent discovery of new class prototype systems
AFFeAs (A=Sr,Ca) of novel layered ironpnictide High-Tc superconductors
(Tc=36K). Here we report ab initio LDA results for electronic structure of the
AFFeAs systems. We provide detailed comparison between electronic properties of
both new systems and reference LaOFeAs (La111) compound. In the vicinity of the
Fermi level all three systems have essentially the same band dispersions.
However for iron fluoride systems F(2p) states were found to be separated in
energy from As(4p) ones in contrast to La111, where O(2p) states strongly
overlaps with As(4p). Thus it should be more plausible to include only Fe(3d)
and As(4p) orbitals into a realistic noninteracting model than for La111.
Moreover Sr substitution with smaller ionic radius Ca in AFFeAs materials leads
to a lattice contruction and stronger Fe(3d)-As(4p) hybridization resulting in
smaller value of the density of states at the Fermi level in the case of Ca
compound. So to some extend Ca system reminds RE111 with later Rare Earths.
However Fermi surface of new fluorides is found to be nearly perfect
two-dimensional. Also we do not expect strong dependence of superconducting
properties with respect to different types of A substitutes.Comment: 5 pages, 4 figure
Control of laser wake field acceleration by plasma density profile
We show that both the maximum energy gain and the accelerated beam quality
can be efficiently controlled by the plasma density profile. Choosing a proper
density gradient one can uplift the dephasing limitation. When a periodic wake
field is exploited, the phase synchronism between the bunch of relativistic
particles and the plasma wave can be maintained over extended distances due to
the plasma density gradient. Putting electrons into the th wake period
behind the driving laser pulse, the maximum energy gain is increased by the
factor over that in the case of uniform plasma. The acceleration is
limited then by laser depletion rather than by dephasing. Further, we show that
the natural energy spread of the particle bunch acquired at the acceleration
stage can be effectively removed by a matched deceleration stage, where a
larger plasma density is used
Radio Frequency Spectroscopy of Trapped Fermi Gases with Population Imbalance
Motivated by recent experiments, we address, in a fully self consistent
fashion, the behavior and evolution of radio frequency (RF) spectra as
temperature and polarization are varied in population imbalanced Fermi gases.
We discuss a series of scenarios for the experimentally observed zero
temperature pseudogap phase and show how present and future RF experiments may
help in its elucidation. We conclude that the MIT experiments at the lowest
may well reflect ground state properties, but take issue with their claim that
the pairing gap survives up to temperatures of the order of the degeneracy
temperature at unitarity.Comment: 4 page, 3 figures, submitted to PRA Rapi
Aharonov-Bohm oscillations in the local density of states
The scattering of electrons with inhomogeneities produces modulations in the
local density of states of a metal. We show that electron interference
contributions to these modulations are affected by the magnetic field via the
Aharonov-Bohm effect. This can be exploited in a simple STM setup that serves
as an Aharonov-Bohm interferometer at the nanometer scale.Comment: 4 pages, 2 figures. v2 added reference
Why is the bulk resistivity of topological insulators so small?
As-grown topological insulators (TIs) are typically heavily-doped -type
crystals. Compensation by acceptors is used to move the Fermi level to the
middle of the band gap, but even then TIs have a frustratingly small bulk
resistivity. We show that this small resistivity is the result of band bending
by poorly screened fluctuations in the random Coulomb potential. Using
numerical simulations of a completely compensated TI, we find that the bulk
resistivity has an activation energy of just 0.15 times the band gap, in good
agreement with experimental data. At lower temperatures activated transport
crosses over to variable range hopping with a relatively large localization
length.Comment: 4+ pages, 3 figures; published versio
Spin torque ferromagnetic resonance with magnetic field modulation
We demonstrate a technique of broadband spin torque ferromagnetic resonance
(ST-FMR) with magnetic field modulation for measurements of spin wave
properties in magnetic nanostructures. This technique gives great improvement
in sensitivity over the conventional ST-FMR measurements, and application of
this technique to nanoscale magnetic tunnel junctions (MTJs) reveals a rich
spectrum of standing spin wave eigenmodes. Comparison of the ST-FMR
measurements with micromagnetic simulations of the spin wave spectrum allows us
to explain the character of low-frequency magnetic excitations in nanoscale
MTJs.Comment: Also see: http://faculty.sites.uci.edu/krivorotovgroup
Quantum criticality in a Mott pn-junction in an armchair carbon nanotube
In an armchair carbon nanotube pn junction the p- and n- regions are
separated by a region of a Mott insulator, which can backscatter electrons only
in pairs. We predict a quantum-critical behavior in such a pn junction.
Depending on the junction's built-in electric field E, its conductance G scales
either to zero or to the ideal value G=4e^2/h as the temperature T is lowered.
The two types of the G(T) dependence indicate the existence, at some special
value of E, of an intermediate quantum critical point with a finite conductance
G<4e^2/h. This makes the pn junction drastically different from a simple
barrier in a Luttinger liquid.Comment: 5 pages, 1 figur
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