8,097 research outputs found
Quantum transport through 3D Dirac materials
Bismuth and its alloys provide a paradigm to realize three dimensional
materials whose low-energy effective theory is given by Dirac equation in 3+1
dimensions. We study the quantum transport properties of three dimensional
Dirac materials within the framework of Landauer-B\"uttiker formalism. Charge
carriers in normal metal satisfying the Schr\"odinger equation, can be split
into four-component with appropriate matching conditions at the boundary with
the three dimensional Dirac material (3DDM). We calculate the conductance and
the Fano factor of an interface separating 3DDM from a normal metal, as well as
the conductance through a slab of 3DDM. Under certain circumstances the 3DDM
appears transparent to electrons hitting the 3DDM. We find that electrons
hitting the metal-3DDM interface from metallic side can enter 3DDM in a
reversed spin state as soon as their angle of incidence deviates from the the
direction perpendicular to interface. However the presence of a second
interface completely cancels this effect.Comment: compile with the .bbl file include
Stable local moments of vacancies and hollow-site impurities in graphene
Taking into account the possibility of a p-wave hybridization function
of ad-atom with Dirac electrons in graphene -- which arises for
vacancies and hollow-site impurities -- we study the nature of magnetic moment
formation within the single impurity Anderson model (SIAM). Compared to the
s-wave hybridization function, we find that the local moments formed within the
Hartree mean field are robust against the change in the parameters of the
model. Further we investigate the stability of the local moments with respect
to quantum fluctuations by going beyond the Hartree approximation. We find that
for parameter regimes where local moments formed by top-site ad-atoms are
completely washed out by quantum fluctuations, those formed by vacancies (or
hollow-site impurities) survive the quantum fluctuations captured by
post-Hartree approximation. Hence vacancies and hollow-site ad-atoms are
suitable candidates to produce stable local moments.Comment: 7 page, 6 figure
Dynamical Mean Field Theory equations on nearly real frequency axis
The Iterated Perturbation Theory (IPT) equations of the Dynamical Mean Field
Theory (DMFT) for the half-filled Hubbard model, are solved on nearly real
frequencies at various values of the Hubbard parameters , to investigate the
nature of metal-insulator transition (MIT) at finite temperatures. This method
avoids the instabilities associated with the infamous Pad\'e analytic
continuation and reveals fine structures across the MIT at finite temperatures,
which {\em can not be captured} by conventional methods for solving DMFT
equations on Matsubara frequencies. Our method suggests that at finite
temperatures, there is an abrupt decrease in the height of the quasi-particle
(Kondo) peak at a critical value of , to a non-zero but small bump which
gradually suppresses as one moves deeper into the {\em bad} insulator regime.
In contrast to Vollhardt and coworkers [J. Phys. Soc. Jpn. {\bf 74} (2005)
136], down to of the half-bandwidth we find no separating bad
insulator from a true Mott insulator.Comment: revisions corresponding to adding a new Fig.
Reply to "Comment on Anderson Transition in Disordered Graphene"
We show that the very small numeric effects discussed in the comment by
Schleede et al (arXiv:1005.0497) is not the source of mobility edge predicted
in graphene in our letter [Eur. Phys. Lett., 87 (2009) 37002].Comment: two pages, two figure
Neutral Triplet Collective Mode in Doped Graphene
Particle-hole continuum in Dirac sea of graphene has a unique window
underneath, which provides a unique opportunity for emergence of a pole in the
susceptibility of the {\em triplet} particle-hole channel in the entire
Brillouin zone (BZ). Here we use random phase approximation (RPA) to study such
collective mode at zero temperature, in a single layer of doped graphene. We
find that due to the chiral nature of one-particle states, in undoped graphene,
the wave function overlap factors do not lead to qualitative differences, while
in doped graphene they will kill small momentum part of the branch of magnetic
excitations by pushing it to touch the lower part of the continuum. The pole
corresponding to magnetic excitations survives for for larger momenta in the
BZ.Comment: major revision
Localized magnetic states in three dimensional Dirac solids
Formation of localized magnetic states in a metallic host is a classic
problem ofcondensed matter physics formalized by P. W. Anderson within the so
called single impurity Anderson model (SIAM). The general picture in a host of
a simple one-band metal is that a large Hubbard in the impurity orbital is
pre-requisite for the formation of localized magnetic states. In recent years
three dimensional (3D) Dirac solids have emerged the hallmark of which is
strong spin-orbit interaction. In this work we show that such a strong
spin-orbit interaction allows to form localized magnetic states even with small
values of Hubbard . This opens up the fascinating possibility of forming
magnetic states with or orbital impurities -- different from
traditional paradigms of or orbital based magnetic moments
Effects of coating rate on morphology of copper surfaces
We have used standard fractal analysis and Markov approach to obtain further
insights on roughness and multifractality of different surfaces. The effect of
coating rates on generating topographic rough surfaces in copper thin films
with same thickness has been studied using atomic force microscopy technique
(AFM). Our results show that by increasing the coating rates, correlation
length (grain sizes) and Markov length are decreased and roughness exponent is
decreased and our surfaces become more multifractal. Indeed, by decreasing the
coating rate, the relaxation time of embedding the particles is increased
Spin Hall effect originated from fractal surface
Spin hall effect (SHE) in thin films is inherited by surface roughness.
Although roughness effect on SHE has been studied in thin films, but roughness
is not only parameter in rough surfaces. Our results show that how other
statistical parameters of rough surface play important role in SHE. In this
paper we investigate theoretically the effects of correlated surface roughness
in the SHE with self affine fractal surface in non-heavy metallic thin films in
the frame work of the Born approximation. The surface roughness is described by
the k-correlation model and is characterized by the roughness exponent H (0 <=
H <= 1), the in plane correlation length kesi and the rms roughness amplitude
delta. We show that the spin Hall angle can increase by one order of magnitude
when H decreasing from H = 1 to H = 0. We also demonstrate the SHE for surface
roughness with distribution function of the Gaussian profile is mainly
contributed by the side jump scattering while for that with a non-Gaussian
profile, both side jump and skew scattering are present. our achievements
demonstrate the important role of roughness texture profile for SHE in
non-heavy metals.Comment: 15 pages,7 figure
Assessment of petrophysical quantities inspired by joint multifractal approach
In this paper joint multifractal random walk approach is carried out to
analyze some petrophysical quantities for characterizing the petroleum
reservoir. These quantities include Gamma emission (GR), sonic transient time
(DT) and Neutron porosity (NPHI) which are collected from four wells of a
reservoir. To quantify mutual interaction of petrophysical quantities, joint
multifractal random walk is implemented. This approach is based on the mutual
multiplicative cascade notion in the multifractal formalism and in this
approach represents a benchmark to describe the nature of
cross-correlation between two series. The analysis of the petrophysical
quantities revealed that GR for all wells has strongly multifractal nature due
to the considerable abundance of large fluctuations in various scales. The
variance of probability distribution function, , at scale
and its intercept determine the multifractal properties of the data sets
sourced by probability density function. The value of for NPHI
data set is less than GR's, however, DT shows a nearly monofractal behavior,
namely , so we find that . While, the value of
Hurst exponents can not discriminate between series GR, NPHI and DT. Joint
analysis of the petrophysical quantities for considered wells demonstrates that
has negative value for GR-NPHI confirming that finding shaly layers is in
competition with finding porous medium while it takes positive value for GR-DT
determining that continuum medium can be detectable by evaluating the
statistical properties of GR and its cross-correlation to DT signal.Comment: 13 pages, 5 figures and 3 table
Analytical expression for wave scattering from exponential height correlated rough surfaces
Wave scattering from rough surfaces in addition the inverse scattering is an
interesting approach to obtain the surface topography properties in various
fields. Analytical expression in wave scattering from some known rough
surfaces, not only help us to understand the scattering phenomena, but also
would prove adequate to be a criterion to measure the information for empirical
rough surfaces. For a rough surface with an exponential height correlation
function, we derive an analytical expression for the diffused part and expanded
it in two asymptotic regimes. We consider one surface as slightly rough and the
other as very rough based on the framework of the Kirchhoff theory. In the end,
we have measured the role of various Hurst exponents and correlation lengths on
scattering intensity in self-affine surfaces. We have shown that by increasing
the Hurst exponent from H=0 to H=1, the diffuse scattering decreases with the
scattering angle.Comment: 13 pages, 4 figure
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