118 research outputs found
First-principles investigation of graphene fluoride and graphane
Different stoichiometric configurations of graphane and graphene fluoride are
investigated within density functional theory. Their structural and electronic
properties are compared, and we indicate the similarities and differences among
the various configurations. Large differences between graphane and graphene
fluoride are found that are caused by the presence of charges on the fluorine
atoms. A new configuration that is more stable than the boat configuration is
predicted for graphene fluoride. We also perform GW calculations for the
electronic band gap of both graphene derivatives. These band gaps and also the
calculated Young's moduli are at variance with available experimental data.
This might indicate that the experimental samples contain a large number of
defects or are only partially covered with H or F.Comment: 6 pages, 3 figures, submitted to PR
First-principles calculations of hyperfine interaction, binding energy, and quadrupole coupling for shallow donors in silicon
Spin qubits based on shallow donors in silicon are a promising quantum information technology with enormous potential
scalability due to the existence of robust silicon-processing infrastructure. However, the most accurate theories of donor electronic
structure lack predictive power because of their reliance on empirical fitting parameters, while predictive ab initio methods have so
far been lacking in accuracy due to size of the donor wavefunction compared to typical simulation cells. We show that density
functional theory with hybrid and traditional functionals working in tandem can bridge this gap. Our first-principles approach
allows remarkable accuracy in binding energies (67 meV for bismuth and 54 meV for arsenic) without the use of empirical fitting.
We also obtain reasonable hyperfine parameters (1263 MHz for Bi and 133 MHz for As) and superhyperfine parameters. We
demonstrate the importance of a predictive model by showing that hydrostatic strain has much larger effect on the hyperfine
structure than predicted by effective mass theory, and by elucidating the underlying mechanisms through symmetry analysis of the
shallow donor charge density
Impact of electric-field dependent dielectric constants on two-dimensional electron gases in complex oxides
High-density two-dimensional electron gas (2DEG) can be formed at complex oxide interfaces such as SrTiO3/GdTiO3 and SrTiO3/LaAlO3. The electric field in the vicinity of the interface depends on the dielectric properties of the material as well as on the electron distribution. However, it is known that electric fields can strongly modify the dielectric constant of SrTiO3 as well as other complex oxides. Solving the electrostatic problem thus requires a self-consistent approach in which the dielectric constant varies according to the local magnitude of the field. We have implemented the field dependence of the dielectric constant in a Schrodinger-Poisson solver in order to study its effect on the electron distribution in a 2DEG. Using the SrTiO3/GdTiO3 interface as an example, we demonstrate that including the field dependence results in the 2DEG being confined closer to the interface compared to assuming a single field-independent value for the dielectric constant. Our conclusions also apply to SrTiO3/LaAlO3 as well as other similar interfaces
Ab initio study of hydrogenic effective mass impurities in Si nanowires
The effect of B and P dopants on the band structure of Si nanowires is studied using electronic structure calculations based on density functional theory. At low concentrations a dispersionless band is formed, clearly distinguishable from the valence and conduction bands. Although this band is evidently induced by the dopant impurity, it turns out to have purely Si character. These results can be rigorously analyzed in the framework of effective mass theory. In the process we resolve some common misconceptions about the physics of hydrogenic shallow impurities, which can be more clearly elucidated in the case of nanowires than would be possible for bulk Si. We also show the importance of correctly describing the effect of dielectric confinement, which is not included in traditional electronic structure calculations, by comparing the obtained results with those of G0W0 calculations. � 2017 IOP Publishing Ltd
Mid-infrared interference coatings with excess optical loss below 10 ppm
Low excess optical loss, combined absorption and scatter loss, is a key performance metric for any high-reflectance coating technology and is currently one of the main limiting factors for the application of optical resonators in the mid-infrared spectral region. Here we present high-reflectivity substrate-transferred single-crystal GaAs/AlGaAs interference coatings at a center wavelength of 4.54 µm with record-low excess optical loss below 10 parts per million. These high-performance mirrors are realized via a novel microfabrication process that differs significantly from the production of amorphous multilayers generated via physical vapor deposition processes. This new process enables reduced scatter loss due to the low surface and interfacial roughness, while low background doping in epitaxial growth ensures strongly reduced absorption. We report on a suite of optical measurements, including cavity ring-down, transmittance spectroscopy, and direct absorption tests to reveal the optical losses for a set of prototype mirrors. In the course of these measurements, we observe a unique polarization-orientation-dependent loss mechanism which we attribute to elastic anisotropy of these strained epitaxial multilayers. A future increase in layer count and a corresponding reduction of transmittance will enable optical resonators with a finesse in excess of 100,000 in the mid-infrared spectral region, allowing for advances in high-resolution spectroscopy, narrow-linewidth laser stabilization, and ultrasensitive measurements of various light–matter interactions
An efficient algorithm to calculate intrinsic thermoelectric parameters based on Landauer approach
The Landauer approach provides a conceptually simple way to calculate the
intrinsic thermoelectric (TE) parameters of materials from the ballistic to the
diffusive transport regime. This method relies on the calculation of the number
of propagating modes and the scattering rate for each mode. The modes are
calculated from the energy dispersion (E(k)) of the materials which require
heavy computation and often supply energy relation on sparse momentum (k)
grids. Here an efficient method to calculate the distribution of modes (DOM)
from a given E(k) relationship is presented. The main features of this
algorithm are, (i) its ability to work on sparse dispersion data, and (ii)
creation of an energy grid for the DOM that is almost independent of the
dispersion data therefore allowing for efficient and fast calculation of TE
parameters. The inclusion of scattering effects is also straight forward. The
effect of k-grid sparsity on the compute time for DOM and on the sensitivity of
the calculated TE results are provided. The algorithm calculates the TE
parameters within 5% accuracy when the K-grid sparsity is increased up to 60%
for all the dimensions (3D, 2D and 1D). The time taken for the DOM calculation
is strongly influenced by the transverse K density (K perpendicular to
transport direction) but is almost independent of the transport K density
(along the transport direction). The DOM and TE results from the algorithm are
bench-marked with, (i) analytical calculations for parabolic bands, and (ii)
realistic electronic and phonon results for .Comment: 16 Figures, 3 Tables, submitted to Journal of Computational
electronic
Transport in Silicon Nanowires: Role of Radial Dopant Profile
We consider the electronic transport properties of phosphorus (P) doped
silicon nanowires (SiNWs). By combining ab initio density functional theory
(DFT) calculations with a recursive Green's function method, we calculate the
conductance distribution of up to 200 nm long SiNWs with different
distributions of P dopant impurities. We find that the radial distribution of
the dopants influences the conductance properties significantly: Surface doped
wires have longer mean-free paths and smaller sample-to-sample fluctuations in
the cross-over from ballistic to diffusive transport. These findings can be
quantitatively predicted in terms of the scattering properties of the single
dopant atoms, implying that relatively simple calculations are sufficient in
practical device modelingComment: Submitted to Journal of Computational Electronics, presented in
IWCE-1
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