252 research outputs found
Surface States of the Topological Insulator Bi_{1-x}Sb_x
We study the electronic surface states of the semiconducting alloy BiSb.
Using a phenomenological tight binding model we show that the Fermi surface of
the 111 surface states encloses an odd number of time reversal invariant
momenta (TRIM) in the surface Brillouin zone confirming that the alloy is a
strong topological insulator. We then develop general arguments which show that
spatial symmetries lead to additional topological structure, and further
constrain the surface band structure. Inversion symmetric crystals have 8 Z_2
"parity invariants", which include the 4 Z_2 invariants due to time reversal.
The extra invariants determine the "surface fermion parity", which specifies
which surface TRIM are enclosed by an odd number of electron or hole pockets.
We provide a simple proof of this result, which provides a direct link between
the surface states and the bulk parity eigenvalues. We then make specific
predictions for the surface state structure for several faces of BiSb. We next
show that mirror invariant band structures are characterized by an integer
"mirror Chern number", n_M. The sign of n_M in the topological insulator phase
of BiSb is related to a previously unexplored Z_2 parameter in the L point k.p
theory of pure Bi, which we refer to as the "mirror chirality", \eta. The value
of \eta predicted by the tight binding model for Bi disagrees with the value
predicted by a more fundamental pseudopotential calculation. This explains a
subtle disagreement between our tight binding surface state calculation and
previous first principles calculations on Bi. This suggests that the tight
binding parameters in the Liu Allen model of Bi need to be reconsidered.
Implications for existing and future ARPES experiments and spin polarized ARPES
experiments will be discussed.Comment: 15 pages, 7 figure
Time-dependent embedding: surface electron emission
An embedding method for solving the time-dependent Schr\"odinger equation is
developed using the Dirac-Frenkel variational principle. Embedding allows the
time-evolution of the wavefunction to be calculated explicitly in a limited
region of space, the region of physical interest, the embedding potential
ensuring that the wavefunction satisfies the correct boundary conditions for
matching on to the rest of the system. This is applied to a study of the
excitation of electrons at a metal surface, represented by a one-dimensional
model potential for Cu(111). Time-dependent embedding potentials are derived
for replacing the bulk substrate, and the image potential and vacuum region
outside the surface, so that the calculation of electron excitation by a
surface perturbation can be restricted to the surface itself. The excitation of
the Shockley surface state and a continuum bulk state is studied, and the
time-structure of the resulting currents analysed. Non-linear effects and the
time taken for the current to arrive outside the surface are discussed. The
method shows a clear distinction between emission from the localized surface
state, where the charge is steadily depleted, and the extended continuum state
where the current emitted into the vacuum is compensated by current approaching
the surface from the bulk.Comment: 15 figure
Quadrupole Susceptibility and Elastic Softening due to a Vacancy in Silicon Crystal
We investigate the electronic states around a single vacancy in silicon
crystal by using the Green's function approach. The triply degenerate vacancy
states within the band gap are found to be extended over a large distance
from the vacancy site and contribute to the reciprocal
temperature dependence of the quadrupole susceptibility resulting in the
elastic softening at low temperture. The Curie constant of the quadrupole
susceptibility for the trigonal mode () is largely
enhanced as compared to that for the tetragonal mode ().
The obtained results are consistent with the recent ultrasonic experiments in
silicon crystal down to 20 mK. We also calculate the dipole and octupole
susceptibilities and find that the octupole susceptibilities are extremely
enhannced for a specific mode.Comment: 6 pages, with 5 figure
Systematic Control of Carrier Doping without Disorder at Interface of Oxide Heterostructures
We propose a method to systematically control carrier densities at the
interface of transition-metal oxide heterostructures without introducing
disorders. By inserting non-polar layers sandwiched by polar layers, continuous
carrier doping into the interface can be realized. This method enables us to
control the total carrier densities per unit cell systematically up to high
values of the order unity.Comment: 8 pages, 9 figure
A fourfold coordinated point defect in silicon
Due to their technological importance, point defects in silicon are among the
best studied physical systems. The experimental examination of point defects
buried in bulk is difficult and evidence for the various defects usually
indirect. Simulations of defects in silicon have been performed at various
levels of sophistication ranging from fast force fields to accurate density
functional calculations. The generally accepted viewpoint from all these
studies is that vacancies and self interstitials are the basic point defects in
silicon. We challenge this point of view by presenting density functional
calculations that show that there is a new fourfold coordinated point defect in
silicon that is lower in energy
Physics and chemistry of hydrogen in the vacancies of semiconductors
Hydrogen is well known to cause electrical passivation of lattice vacancies in semiconductors. This effect follows from the chemical passivation of the dangling bonds. Recently it was found that H in the carbon vacancy of SiC forms a three-center bond with two silicon neighbors in the vacancy, and gives rise to a new electrically active state. In this paper we examine hydrogen in the anion vacancies of BN, AlN, and GaN. We find that three-center bonding of H is quite common and follows clear trends in terms of the second-neighbor distance in the lattice, the typical (two-center) hydrogen-host-atom bond length, the electronegativity difference between host atoms and hydrogen, as well as the charge state of the vacancy. Three-center bonding limits the number of H atoms a nitrogen vacancy can capture to two, and prevents electric passivation in GaAs as well
Electronic and structural properties of vacancies on and below the GaP(110) surface
We have performed total-energy density-functional calculations using
first-principles pseudopotentials to determine the atomic and electronic
structure of neutral surface and subsurface vacancies at the GaP(110) surface.
The cation as well as the anion surface vacancy show a pronounced inward
relaxation of the three nearest neighbor atoms towards the vacancy while the
surface point-group symmetry is maintained. For both types of vacancies we find
a singly occupied level at mid gap. Subsurface vacancies below the second layer
display essentially the same properties as bulk defects. Our results for
vacancies in the second layer show features not observed for either surface or
bulk vacancies: Large relaxations occur and both defects are unstable against
the formation of antisite vacancy complexes. Simulating scanning tunneling
microscope pictures of the different vacancies we find excellent agreement with
experimental data for the surface vacancies and predict the signatures of
subsurface vacancies.Comment: 10 pages, 6 figures, Submitted to Phys. Rev. B, Other related
publications can be found at http://www.rz-berlin.mpg.de/th/paper.htm
First-principles study of As interstitials in GaAs: Convergence, relaxation, and formation energy
Convergence of density-functional supercell calculations for defect formation
energies, charge transition levels, localized defect state properties, and
defect atomic structure and relaxation is investigated using the arsenic split
interstitial in GaAs as an example. Supercells containing up to 217 atoms and a
variety of {\bf k}-space sampling schemes are considered. It is shown that a
good description of the localized defect state dispersion and charge state
transition levels requires at least a 217-atom supercell, although the defect
structure and atomic relaxations can be well converged in a 65-atom cell.
Formation energies are calculated for the As split interstitial, Ga vacancy,
and As antisite defects in GaAs, taking into account the dependence upon
chemical potential and Fermi energy. It is found that equilibrium
concentrations of As interstitials will be much lower than equilibrium
concentrations of As antisites in As-rich, -type or semi-insulating GaAs.Comment: 10 pages, 5 figure
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