1,045 research outputs found
Coherent wave-packet evolution in coupled bands
We develop a formalism for treating coherent wave-packet dynamics of charge
and spin carriers in degenerate and nearly degenerate bands. We consider the
two-band case carefully in view of spintronics applications, where transitions
between spin-split bands often occur even for relatively weak electromagnetic
fields. We demonstrate that much of the semiclassical formalism developed for
the single-band case can be generalized to multiple bands, and examine the
nontrivial non-Abelian corrections arising from the additional degree of
freedom. Along with the center of mass motion in crystal momentum and real
space, one must also include a pseudo-spin to characterize the dynamics between
the bands. We derive the wave packet energy up to the first order gradient
correction and obtain the equations of motion for the real- and -space
center of the wave-packet, as well as for the pseudo-spin. These equations
include the non-Abelian Berry curvature terms and a non-Abelian correction to
the group velocity. As an example, we apply our formalism to describe coherent
wave-packet evolution under the action of an electric field, demonstrating that
it leads to electrical separation of up and down spins. A sizable separation
will be observed, with a large degree of tunability, making this mechanism a
practical method of generating a spin polarization. We then turn our attention
to a magnetic field, where we recover Larmor precession, which cannot be
obtained from a single-band point of view. In this case, the gradient energy
correction can be regarded as due to a magnetic moment from the self-rotation
of the wave-packet, and we calculate its value for the light holes in the
spherical four-band Luttinger model.Comment: 8 pages, 1 figur
A multiband envelope function model for quantum transport in a tunneling diode
We present a simple model for electron transport in semiconductor devices
that exhibit tunneling between the conduction and valence bands. The model is
derived within the usual Bloch-Wannier formalism by a k-expansion, and is
formulated in terms of a set of coupled equations for the electron envelope
functions. Its connection with other models present in literature is discussed.
As an application we consider the case of a Resonant Interband Tunneling Diode,
demonstrating the ability of the model to reproduce the expected behaviour of
the current as a function of the applied voltageComment: 8 pages, 4 figure
A semi-quantitative scattering theory of amorphous materials
It is argued that topological disorder in amorphous solids can be described
by local strains related to local reference crystals and local rotations. An
intuitive localization criterion is formulated from this point of view. The
Inverse Participation Ratio and the location of mobility edges in band tails is
directly related to the character of the disorder potential in amorphous solid,
the coordination number, the transition integral and the nodes of wave
functions of the corresponding reference crystal. The dependence of the decay
rate of band tails on temperature and static disorder are derived. \textit{Ab
initio} simulations on a-Si and experiments on a-Si:H are compared to these
predictions.Comment: 4 pages, 2 figures, will be submitted to Phys. Rev. Let
A priori Wannier functions from modified Hartree-Fock and Kohn-Sham equations
The Hartree-Fock equations are modified to directly yield Wannier functions
following a proposal of Shukla et al. [Chem. Phys. Lett. 262, 213-218 (1996)].
This approach circumvents the a posteriori application of the Wannier
transformation to Bloch functions. I give a novel and rigorous derivation of
the relevant equations by introducing an orthogonalizing potential to ensure
the orthogonality among the resulting functions. The properties of these,
so-called a priori Wannier functions, are analyzed and the relation of the
modified Hartree-Fock equations to the conventional, Bloch-function-based
equations is elucidated. It is pointed out that the modified equations offer a
different route to maximally localized Wannier functions. Their computational
solution is found to involve an effort that is comparable to the effort for the
solution of the conventional equations. Above all, I show how a priori Wannier
functions can be obtained by a modification of the Kohn-Sham equations of
density-functional theory.Comment: 7 pages, RevTeX4, revise
Quasiparticle band structure of infinite hydrogen fluoride and hydrogen chloride chains
We study the quasiparticle band structure of isolated, infinite HF and HCl
bent (zigzag) chains and examine the effect of the crystal field on the energy
levels of the constituent monomers. The chains are one of the simplest but
realistic models of the corresponding three-dimensional crystalline solids. To
describe the isolated monomers and the chains, we set out from the Hartree-Fock
approximation, harnessing the advanced Green's function methods "local
molecular orbital algebraic diagrammatic construction" (ADC) scheme and "local
crystal orbital ADC" (CO-ADC) in a strict second order approximation, ADC(2,2)
and CO-ADC(2,2), respectively, to account for electron correlations. The
configuration space of the periodic correlation calculations is found to
converge rapidly only requiring nearest-neighbor contributions to be regarded.
Although electron correlations cause a pronounced shift of the quasiparticle
band structure of the chains with respect to the Hartree-Fock result, the
bandwidth essentially remains unaltered in contrast to, e.g., covalently bound
compounds.Comment: 11 pages, 6 figures, 6 tables, RevTeX4, corrected typoe
Generalized acceleration theorem for spatiotemporal Bloch waves
A representation is put forward for wave functions of quantum particles in
periodic lattice potentials subjected to homogeneous time-periodic forcing,
based on an expansion with respect to Bloch-like states which embody both the
spatial and the temporal periodicity. It is shown that there exists a
generalization of Bloch's famous acceleration theorem which grows out of this
representation and captures the effect of a weak probe force applied in
addition to a strong dressing force. Taken together, these elements point at a
"dressing and probing" strategy for coherent wave-packet manipulation, which
could be implemented in present experiments with optical lattices.Comment: 12 pages, 4 figure
Atomic correlations in itinerant ferromagnets: quasi-particle bands of nickel
We measure the band structure of nickel along various high-symmetry lines of
the bulk Brillouin zone with angle-resolved photoelectron spectroscopy. The
Gutzwiller theory for a nine-band Hubbard model whose tight-binding parameters
are obtained from non-magnetic density-functional theory resolves most of the
long-standing discrepancies between experiment and theory on nickel. Thereby we
support the view of itinerant ferromagnetism as induced by atomic correlations.Comment: 4 page REVTeX 4.0, one figure, one tabl
Calculation of nanowire thermal conductivity using complete phonon dispersion relations
The lattice thermal conductivity of crystalline Si nanowires is calculated.
The calculation uses complete phonon dispersions, and does not require any
externally imposed frequency cutoffs. No adjustment to nanowire thermal
conductivity measurements is required. Good agreement with experimental results
for nanowires wider than 35 nm is obtained. A formulation in terms of the
transmission function is given. Also, the use of a simpler, nondispersive
"Callaway formula", is discussed from the complete dispersions perspective.Comment: 4 pages, 3 figures. Accepted in Phys. Rev.
A Liquid Model Analogue for Black Hole Thermodynamics
We are able to characterize a 2--dimensional classical fluid sharing some of
the same thermodynamic state functions as the Schwarzschild black hole. This
phenomenological correspondence between black holes and fluids is established
by means of the model liquid's pair-correlation function and the two-body
atomic interaction potential. These latter two functions are calculated exactly
in terms of the black hole internal (quasilocal) energy and the isothermal
compressibility. We find the existence of a ``screening" like effect for the
components of the liquid.Comment: 20 pages and 6 Encapsulated PostScript figure
Resonant Impurity Scattering in a Strongly Correlated Electron Model
Scattering by a single impurity introduced in a strongly correlated
electronic system is studied by exact diagonalization of small clusters. It is
shown that an inert site which is spinless and unable to accomodate holes can
give rise to strong resonant scattering. A calculation of the local density of
state reveals that, for increasing antiferromagnetic exchange coupling, d, s
and p-wave symmetry bound states in which a mobile hole is trapped by the
impurity potential induced by a local distortion of the antiferromagnetic
background successively pull out from the continuum.Comment: 10 pages, 4 figures available on request, report LPQTH-93-2
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