2,619 research outputs found
Robust Transport Properties in Graphene
Two-dimensional Dirac fermions are used to discuss quasiparticles in graphene
in the presence of impurity scattering. Transport properties are completely
dominated by diffusion. This may explain why recent experiments did not find
weak localization in graphene. The diffusion coefficient of the quasiparticles
decreases strongly with increasing strength of disorder. Using the Kubo
formalism, however, we find a robust minimal conductivity that is independent
of disorder. This is a consequence of the fact that the change of the diffusion
coefficient is fully compensated by a change of the number of delocalized
quasiparticle states.Comment: 4 pages, 1 figur
First Principles Study of CaFe2As2 "Collapse" Under Pressure
We perform first principles calculations on CaFe2As2 under hydrostatic
pressure. Our total energy calculations show that though the striped
antiferromagnetic (AFM) orthorhombic (OR) phase is favored at P=0, a
non-magnetic collapsed tetragonal (cT) phase with diminished c-parameter is
favored for P > 0.36 GPa, in agreement with experiments. Rather than a
mechanical instability, this is an enthalpically driven transition from the
higher volume OR phase to the lower volume cT phase. Calculations of electronic
density of states reveal pseudogaps in both OR and cT phases, though As(p)
hybridization with Fe(d) is more pronounced in the OR phase. We provide an
estimate for the inter-planar magnetic coupling. Phonon entropy considerations
provide an interpretation of the finite temperature phase boundaries of the cT
phase.Comment: 4 pages, 4 figures, 1 Tabl
Resonant atom-field interaction in large-size coupled-cavity arrays
We consider an array of coupled cavities with staggered inter-cavity
couplings, where each cavity mode interacts with an atom. In contrast to
large-size arrays with uniform-hopping rates where the atomic dynamics is known
to be frozen in the strong-hopping regime, we show that resonant atom-field
dynamics with significant energy exchange can occur in the case of staggered
hopping rates even in the thermodynamic limit. This effect arises from the
joint emergence of an energy gap in the free photonic dispersion relation and a
discrete frequency at the gap's center. The latter corresponds to a bound
normal mode stemming solely from the finiteness of the array length. Depending
on which cavity is excited, either the atomic dynamics is frozen or a
Jaynes-Cummings-like energy exchange is triggered between the bound photonic
mode and its atomic analogue. As these phenomena are effective with any number
of cavities, they are prone to be experimentally observed even in small-size
arrays.Comment: 12 pages, 4 figures. Added 5 mathematical appendice
Comment on "Photon energy and carrier density dependence of spin dynamics in bulk CdTe crystal at room temperature"
We comment on the conclusion by Ma et al. [Appl. Phys. Lett. {\bf 94}, 241112
(2009)] that the Elliott-Yafet mechanism is more important than the
D'yakonov-Perel' mechanism at high carrier density in intrinsic bulk CdTe at
room temperature. We point out that the spin relaxation is solely from the
D'yakonov-Perel' mechanism. The observed peak in the density dependence of spin
relaxation time is exactly what we predicted in a recent work [Phys. Rev. B
{\bf 79}, 125206 (2009)].Comment: 2 page
Linear magnetoresistance on the topological surface
A positive, non-saturating and dominantly linear magnetoresistance is
demonstrated to occur in the surface state of a topological insulator having a
wavevector-linear energy dispersion together with a finite positive Zeeman
energy splitting. This linear magnetoresistance shows up within quite wide
magnetic-field range in a spatially homogenous system of high carrier density
and low mobility in which the conduction electrons are in extended states and
spread over many smeared Landau levels, and is robust against increasing
temperature, in agreement with recent experimental findings in BiSe
nanoribbons.Comment: 7 pages, 4 figure
8-band k·p modelling of mid-infrared intersubband absorption in Ge quantum wells
The 8-band k·p parameters which include the direct band coupling between the conduction and the valence bands are derived and used to model optical intersubband transitions in Ge quantum well heterostructure material grown on Si substrates. Whilst for Si rich quantum wells the coupling between the conduction bands and valence bands is not important for accurate modelling, the present work demonstrates that the inclusion of such coupling is essential to accurately determine intersubband transitions between hole states in Ge and Ge-rich Si1− x Ge x quantum wells. This is due to the direct bandgap being far smaller in energy in Ge compared to Si. Compositional bowing parameters for a range of the key modelling input parameters required for Ge/SiGe heterostructures, including the Kane matrix elements, the effective mass of the Γ2'Γ2′ conduction band, and the Dresselhaus parameters for both 6- and 8-band k·p modelling, have been determined. These have been used to understand valence band intersubband transitions in a range of Ge quantum well intersubband photodetector devices in the mid-infrared wavelength range
Theoretical prediction of topological insulator in ternary rare earth chalcogenides
A new class of three-dimensional topological insulator, ternary rare earth
chalcogenides, is theoretically investigated with ab initio calculations. Based
on both bulk band structure analysis and the direct calculation of topological
surface states, we demonstrate that LaBiTe3 is a topological insulator. La can
be substituted by other rare earth elements, which provide candidates for novel
topological states such as quantum anomalous Hall insulator, axionic insulator
and topological Kondo insulator. Moreover, YBiTe3 and YSbTe3 are found to be
normal insulators. They can be used as protecting barrier materials for both
LaBiTe3 and Bi2Te3 families of topological insulators for their well matched
lattice constants and chemical composition.Comment: 5 pages, 3 figures and 1 tabl
Ultraviolet light emission from Si in a scanning tunneling microscope
Ultraviolet and visible radiation is observed from the contacts of a scanning
tunneling microscope with Si(100) and (111) wafers. This luminescence relies on
the presence of hot electrons in silicon, which are supplied, at positive bias
on n- and p-type samples, through the injection from the tip, or, at negative
bias on p-samples, by Zener tunneling. Measured spectra reveal a contribution
of direct optical transitions in Si bulk. The necessary holes well below the
valence band edge are injected from the tip or generated by Auger processes
The most probable wave function of a single free moving particle
We develop the most probable wave functions for a single free quantum
particle given its momentum and energy by imposing its quantum probability
density to maximize Shannon information entropy. We show that there is a class
of solutions in which the quantum probability density is self-trapped with
finite-size spatial support, uniformly moving hence keeping its form unchanged.Comment: revtex, 4 page
Quantized Transport in Two-Dimensional Spin-Ordered Structures
We study in detail the transport properties of a model of conducting
electrons in the presence of double-exchange between localized spins arranged
on a 2D Kagome lattice, as introduced by Ohgushi, Murakami, and Nagaosa (2000).
The relationship between the canting angle of the spin texture and the
Berry phase field flux per triangular plaquette is derived explicitly
and we emphasize the similarities between this model and Haldane's honeycomb
lattice version of the quantum Hall effect (Haldane, 1988). The quantization of
the transverse (Hall) conductivity is derived explicitly from the
Kubo formula and a direct calculation of the longitudinal conductivity
shows the existence of a metal-insulator transition as a function
of the canting angle (or flux density ). This transition might
be linked to that observable in the manganite compounds or in the pyrochlore
ones, as the spin ordering changes from ferromagnetic to canted.Comment: 17 pages, 12 figure
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