6,076 research outputs found
Atomic Effective Pseudopotentials for Semiconductors
We derive an analytic connection between the screened self-consistent
effective potential from density functional theory (DFT) and atomic effective
pseudopotentials (AEPs). The motivation to derive AEPs is to address structures
with thousands to hundred thousand atoms, as given in most nanostructures. The
use of AEPs allows to bypass a self-consistent procedure and to address
eigenstates around a certain region of the spectrum (e.g., around the band
gap). The bulk AEP construction requires two simple DFT calculations of
slightly deformed elongated cells. The ensuing AEPs are given on a fine
reciprocal space grid, including the small reciprocal vector components, are
free of parameters, and involve no fitting procedure. We further show how to
connect the AEPs of different bulk materials, which is necessary to obtain
accurate band offsets. We derive a total of 20 AEPs for III-V, II-VI and group
IV semiconductors and demonstrate their accuracy and transferability by
comparison to DFT calculations of strained bulk structures, quantum wells with
varying thickness, and semiconductor alloys.Comment: 10 pages, 5 figures, submitted to PR
Landau levels in quasicrystals
Two-dimensional tight-binding models for quasicrystals made of plaquettes
with commensurate areas are considered. Their energy spectrum is computed as a
function of an applied perpendicular magnetic field. Landau levels are found to
emerge near band edges in the zero-field limit. Their existence is related to
an effective zero-field dispersion relation valid in the continuum limit. For
quasicrystals studied here, an underlying periodic crystal exists and provides
a natural interpretation to this dispersion relation. In addition to the slope
(effective mass) of Landau levels, we also study their width as a function of
the magnetic flux per plaquette and identify two fundamental broadening
mechanisms: (i) tunneling between closed cyclotron orbits and (ii) individual
energy displacement of states within a Landau level. Interestingly, the typical
broadening of the Landau levels is found to behave algebraically with the
magnetic field with a nonuniversal exponent.Comment: 14 pages, 9 figure
Crystal Structures and Electronic Properties of Haloform-Intercalated C60
Using density functional methods we calculated structural and electronic
properties of bulk chloroform and bromoform intercalated C60, C60 2CHX3
(X=Cl,Br). Both compounds are narrow band insulator materials with a gap
between valence and conduction bands larger than 1 eV. The calculated widths of
the valence and conduction bands are 0.4-0.6 eV and 0.3-0.4 eV, respectively.
The orbitals of the haloform molecules overlap with the orbitals of the
fullerene molecules and the p-type orbitals of halogen atoms significantly
contribute to the valence and conduction bands of C60 2CHX3. Charging with
electrons and holes turns the systems to metals. Contrary to expectation, 10 to
20 % of the charge is on the haloform molecules and is thus not completely
localized on the fullerene molecules. Calculations on different crystal
structures of C60 2CHCl3 and C60 2CHBr3 revealed that the density of states at
the Fermi energy are sensitive to the orientation of the haloform and C60
molecules. At a charging of three holes, which corresponds to the
superconducting phase of pure C60 and C60 2CHX3, the calculated density of
states (DOS) at the Fermi energy increases in the sequence DOS(C60) < DOS(C60
2CHCl3) < DOS(C60 2CHBr3).Comment: 11 pages, 7 figures, 4 table
Effects of large disorder on the Hofstadter butterfly
Motivated by the recent experiments on periodically modulated, two
dimensional electron systems placed in large transversal magnetic fields, we
investigate the interplay between the effects of disorder and periodic
potentials in the integer quantum Hall regime. In particular, we study the case
where disorder is larger than the periodic modulation, but both are small
enough that Landau level mixing is negligible. In this limit, the
self-consistent Born approximation is inadequate. We carry extensive numerical
calculations to understand the relevant physics in the lowest Landau level,
such as the spectrum and nature (localized or extended) of the wave functions.
Based on our results, we propose a qualitative explanation of the new features
uncovered recently in transport measurements.Comment: 15 pages, 13 figures, several pictures have been shrunk to comply
with the size requirement
The geometry of the double gyroid wire network: quantum and classical
Quantum wire networks have recently become of great interest. Here we deal
with a novel nano material structure of a Double Gyroid wire network. We use
methods of commutative and non-commutative geometry to describe this wire
network. Its non--commutative geometry is closely related to non-commutative
3-tori as we discuss in detail.Comment: pdflatex 9 Figures. Minor changes, some typos and formulation
Temperature-dependent spin gap and singlet ground state in BaCuSi2O6
Bulk magnetic measurements and inelastic neutron scattering were used to
investigate the spin-singlet ground state and magnetic gap excitations in
BaCuSi2O6, a quasi-2-dimensional antiferromagnet with a bilayer structure. The
results are well described by a model based on weakly interacting
antiferromagnetic dimers. A strongly temperature-dependent dispersion in the
gap modes was found. We suggest that the observed excitations are analogous to
magneto-excitons in light rare-earth compounds, but are an intrinsic property
of a simple Heisenberg Hamiltonian for the S=1/2 magnetic bilayer.Comment: 10 pages, 4 figures, REVTeX and PS for text, PS for figures direct
download: http://papillon.phy.bnl.gov/preprints/bacusio.htm
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