406 research outputs found
Variability of structural and electronic properties of bulk and monolayer Si2Te3
Since the emergence of monolayer graphene as a promising two-dimensional
material, many other monolayer and few-layer materials have been investigated
extensively. An experimental study of few-layer Si2Te3 was recently reported,
showing that the material has diverse properties for potential applications in
Si-based devices ranging from fully integrated thermoelectrics to
optoelectronics to chemical sensors. This material has a unique layered
structure: it has a hexagonal closed-packed Te sublattice, with Si dimers
occupying octahedral intercalation sites. Here we report a theoretical study of
this material in both bulk and monolayer form, unveiling a fascinating array of
diverse properties arising from reorientations of the silicon dimers between
planes of Te atoms. The lattice constant varies up to 5% and the band gap
varies up to 40% depending on dimer orientations. The monolayer band gap is 0.4
eV larger than the bulk-phase value for the lowest-energy configuration of Si
dimers. These properties are, in principle, controllable by temperature and
strain, making Si2T3 a promising candidate material for nanoscale mechanical,
optical, and memristive devices.Comment: 9 pages, 4 figure
Accurate and efficient representation of intramolecular energy in ab initio generation of crystal structures. II. Smoothed intramolecular potentials
The application of Crystal Structure Prediction (CSP) to industrially-relevant molecules requires the handling of increasingly large and flexible compounds. We present a revised model for the effect of molecular flexibility on the lattice energy that removes the discontinuities and non-differentiabilities present in earlier models (Sugden et al., 2016), with a view to improving the performance of CSP. The approach is based on the concept of computing a weighted average of local models, and has been implemented within the CrystalPredictor code. Through the comparative investigation of several compounds studied in earlier literature, we show that this new model results in large reductions in computational effort (of up to 65%) and in significant increases in reliability. The approach is further applied to investigate, for the first time, the computational polymorphic landscape of flufenamic acid for Z’=1 structures, resulting in the successful identification of all three experimentally resolved polymorphs within reasonable computational time
Electron Exchange Coupling for Single Donor Solid-State Qubits
Inter-valley interference between degenerate conduction band minima has been
shown to lead to oscillations in the exchange energy between neighbouring
phosphorus donor electron states in silicon \cite{Koiller02,Koiller02A}. These
same effects lead to an extreme sensitivity of the exchange energy on the
relative orientation of the donor atoms, an issue of crucial importance in the
construction silicon-based spin quantum computers. In this article we calculate
the donor electron exchange coupling as a function of donor position
incorporating the full Bloch structure of the Kohn-Luttinger electron
wavefunctions. It is found that due to the rapidly oscillating nature of the
terms they produce, the periodic part of the Bloch functions can be safely
ignored in the Heitler-London integrals as was done by Koiller et. al. [Phys.
Rev. Lett. 88,027903(2002),Phys. Rev. B. 66,115201(2002)], significantly
reducing the complexity of calculations.
We address issues of fabrication and calculate the expected exchange coupling
between neighbouring donors that have been implanted into the silicon substrate
using an 15keV ion beam in the so-called 'top down' fabrication scheme for a
Kane solid-state quantum computer. In addition we calculate the exchange
coupling as a function of the voltage bias on control gates used to manipulate
the electron wavefunctions and implement quantum logic operations in the Kane
proposal, and find that these gate biases can be used to both increase and
decrease the magnitude of the exchange coupling between neighbouring donor
electrons. The zero-bias results reconfirm those previously obtained by
Koiller.Comment: 10 Pages, 8 Figures. To appear in Physical Review
Coupled-barrier diffusion: the case of oxygen in silicon
Oxygen migration in silicon corresponds to an apparently simple jump between
neighboring bridge sites. Yet, extensive theoretical calculations have so far
produced conflicting results and have failed to provide a satisfactory account
of the observed eV activation energy. We report a comprehensive set of
first-principles calculations that demonstrate that the seemingly simple oxygen
jump is actually a complex process involving coupled barriers and can be
properly described quantitatively in terms of an energy hypersurface with a
``saddle ridge'' and an activation energy of eV. Earlier
calculations correspond to different points or lines on this hypersurface.Comment: 4 Figures available upon request. Accepted for publication in Phys.
Rev. Let
High-temperature phonons in h-BN: momentum-resolved vibrational spectroscopy and theory
Vibrations in materials and nanostructures at sufficiently high temperatures
result in anharmonic atomic displacements, which leads to new phenomena such as
thermal expansion and multiphonon scattering processes, with a profound impact
on temperature-dependent material properties including thermal conductivity,
phonon lifetimes, nonradiative electronic transitions, and phase transitions.
Nanoscale momentum-resolved vibrational spectroscopy, which has recently become
possible on monochromated scanning-transmission-electron microscopes, is a
unique method to probe the underpinnings of these phenomena. Here we report
momentum-resolved vibrational spectroscopy in hexagonal boron nitride at
temperatures of 300, 800, and 1300 K across three Brillouin zones (BZs) that
reveals temperature-dependent phonon energy shifts and demonstrates the
presence of strong Umklapp processes. Density-functional-theory calculations of
temperature-dependent phonon self-energies reproduce the observed energy shifts
and identify the contributing mechanisms.Comment: 21 pages, 4 figures, 2 tables, 3 supplemental figures, 3 supplemental
table
Dynamic Tests of High Strength Concrete Cylinders
Idaho National Laboratory engineers collaborated
Structure and energetics of the Si-SiO_2 interface
Silicon has long been synonymous with semiconductor technology. This unique
role is due largely to the remarkable properties of the Si-SiO_2 interface,
especially the (001)-oriented interface used in most devices. Although Si is
crystalline and the oxide is amorphous, the interface is essentially perfect,
with an extremely low density of dangling bonds or other electrically active
defects. With the continual decrease of device size, the nanoscale structure of
the silicon/oxide interface becomes more and more important. Yet despite its
essential role, the atomic structure of this interface is still unclear. Using
a novel Monte Carlo approach, we identify low-energy structures for the
interface. The optimal structure found consists of Si-O-Si "bridges" ordered in
a stripe pattern, with very low energy. This structure explains several
puzzling experimental observations.Comment: LaTex file with 4 figures in GIF forma
Thermodynamic Behavior of a Model Covalent Material Described by the Environment-Dependent Interatomic Potential
Using molecular dynamics simulations we study the thermodynamic behavior of a
single-component covalent material described by the recently proposed
Environment-Dependent Interatomic Potential (EDIP). The parameterization of
EDIP for silicon exhibits a range of unusual properties typically found in more
complex materials, such as the existence of two structurally distinct
disordered phases, a density decrease upon melting of the low-temperature
amorphous phase, and negative thermal expansion coefficients for both the
crystal (at high temperatures) and the amorphous phase (at all temperatures).
Structural differences between the two disordered phases also lead to a
first-order transition between them, which suggests the existence of a second
critical point, as is believed to exist for amorphous forms of frozen water.
For EDIP-Si, however, the unusual behavior is associated not only with the open
nature of tetrahedral bonding but also with a competition between four-fold
(covalent) and five-fold (metallic) coordination. The unusual behavior of the
model and its unique ability to simulation the liquid/amorphous transition on
molecular-dynamics time scales make it a suitable prototype for fundamental
studies of anomalous thermodynamics in disordeered systems.Comment: 48 pages (double-spaced), 13 figure
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