1,313 research outputs found
Conductivity of epitaxial and CVD graphene with correlated line defects
Transport properties of single-layer graphene with correlated one-dimensional
defects are studied using the time-dependent real-space Kubo-Greenwood
formalism. Such defects are present in epitaxial graphene, comprising atomic
terraces and steps due to the substrate morphology, and in polycrystalline
chemically-vapor-deposited (CVD) graphene due to the grain boundaries, composed
of a periodic array of dislocations, or quasi-periodic nanoripples originated
from the metal substrate. The extended line defects are described by the
long-range Lorentzian-type scattering potential. The dc conductivity is
calculated numerically for different cases of distribution of line defects.
This includes a random (uncorrelated) and a correlated distribution with a
prevailing direction in the orientation of lines. The anisotropy of the
conductivity along and across the line defects is revealed, which agrees with
experimental measurements for epitaxial graphene grown on SiC. We performed a
detailed study of the conductivity for different defect correlations,
introducing the correlation angle alpha_max (i.e. the maximum possible angle
between any two lines). We find that for a given electron density, the relative
enhancement of the conductivity for the case of fully correlated line defects
in comparison to the case of uncorrelated ones is larger for a higher defect
density. Finally, we study the conductivity of realistic samples where both
extended line defects as well as point-like scatterers such as adatoms and
charged impurities are presented.Comment: 8 pages, 7 figure
Influence of correlated impurities on conductivity of graphene sheets: Time-dependent real-space Kubo approach
Exact numerical calculations of the conductivity of graphene sheets with
random and correlated distributions of disorders have been performed using the
time-dependent real-space Kubo formalism. The disorder was modeled by the
long-range Gaussian potential describing screened charged impurities and by the
short-range potential describing neutral adatoms both in the weak and strong
scattering regime. Our central result is that correlation in the spatial
distribution for the strong short-range scatterers and for the long-range
Gaussian potential do not lead to any enhancement of the conductivity in
comparison to the uncorrelated case. Our results strongly indicate that the
temperature enhancement of the conductivity reported in the recent study (Yan
and Fuhrer, Phys. Rev. Lett. 107, 206601 (2011)) and attributed to the effect
of dopant correlations was most likely caused by other factors not related to
the correlations in the scattering potential.Comment: 14 pages, 10 figure
Diffusivities and kinetics of short-range and long-range orderings in Ni-Fe permalloys
The microscopic model of atomic diffusion is considered to describe the
short-range order relaxation kinetics within the f.c.c.-Ni-Fe Permalloys. The
model takes into account both the discrete and anisotropic characters of atomic
jumps within the long-range field of concentration heterogeneities of the
interacting atoms. The diffusion coefficients and activation energies for the
disordered Ni-Fe permalloy are estimated with the evaluated probabilities of
atomic jumps. As shown, the increasing of a temperature with a fixed
composition influences on the 'potential' field of interatomic interaction
ambiguously: the field 'potential' increases for defined coordination shells
and decreases for some of other ones. Although the temperature increasing
promotes the increasing of any atomic-probabilities jumps generally, but
decreasing of the action of 'potential' field generated by the atoms of defined
element and caused by its concentration heterogeneities onto the distant sites
results in increasing of the atomic-jumps' probabilities of just this element,
first of all, into the sites, which are more distant from the 'source' of
heterogeneity. Within the framework of the static concentration waves' method
along with the self-consistent field approximation, the Onsager-type kinetics
equation is obtained to describe the long-range order relaxation by the
L12-type superstructure. To calculate diffusivities for the ordered Ni3Fe
permalloy, the independent, diffraction experimental data of the long-range
order parameter relaxation are used. Theoretical curves of the long-range order
time evolution for the non-stoichiometric f.c.c.-Ni-Fe permalloys are plotted.
Decreasing of the concentration of alloying element results in decelerating of
the long-range order parameter change and in increasing of its relaxation time
- …