232 research outputs found
Band gap renormalization in photoexcited semiconductor quantum wire structures in the GW approximation
We investigate the dynamical self-energy corrections of the electron-hole
plasma due to electron-electron and electron-phonon interactions at the band
edges of a quasi-one dimensional (1D) photoexcited electron-hole plasma. The
leading-order dynamical screening approximation is used in the calculation
by treating electron-electron Coulomb interaction and electron-optical phonon
Fr\"{o}hlich interaction on an equal footing. We calculate the
exchange-correlation induced band gap renormalization (BGR) as a function of
the electron-hole plasma density and the quantum wire width. The calculated BGR
shows good agreement with existing experimental results, and the BGR normalized
by the effective quasi-1D excitonic Rydberg exhibits an approximate
one-parameter universality.Comment: 11 pages, 3 figure
Traveling through potential energy landscapes of disordered materials: the activation-relaxation technique
A detailed description of the activation-relaxation technique (ART) is
presented. This method defines events in the configurational energy landscape
of disordered materials, such as a-Si, glasses and polymers, in a two-step
process: first, a configuration is activated from a local minimum to a nearby
saddle-point; next, the configuration is relaxed to a new minimum; this allows
for jumps over energy barriers much higher than what can be reached with
standard techniques. Such events can serve as basic steps in equilibrium and
kinetic Monte Carlo schemes.Comment: 7 pages, 2 postscript figure
Simulation of thermal conductivity and heat transport in solids
Using molecular dynamics (MD) with classical interaction potentials we
present calculations of thermal conductivity and heat transport in crystals and
glasses. Inducing shock waves and heat pulses into the systems we study the
spreading of energy and temperature over the configurations. Phonon decay is
investigated by exciting single modes in the structures and monitoring the time
evolution of the amplitude using MD in a microcanonical ensemble. As examples,
crystalline and amorphous modifications of Selenium and are
considered.Comment: Revtex, 8 pages, 11 postscript figures, accepted for publication in
PR
Simultaneous Observation of Carrier-Specific Redistribution and Coherent Lattice Dynamics in 2H-MoTe with Femtosecond Core-Level Spectroscopy
We employ few-femtosecond extreme ultraviolet (XUV) transient absorption
spectroscopy to reveal simultaneously the intra- and interband carrier
relaxation and the light-induced structural dynamics in nanoscale thin films of
layered 2H-MoTe semiconductor. By interrogating the valence electronic
structure via localized Te 4 (39-46 eV) and Mo 4 (35-38
eV) core levels, the relaxation of the photoexcited hole distribution is
directly observed in real time. We obtain hole thermalization and cooling times
of 155 fs and 38090 fs, respectively, and an electron-hole
recombination time of 1.50.1 ps. Furthermore, excitations of coherent
out-of-plane A (5.1 THz) and in-plane E (3.7 THz) lattice
vibrations are visualized through oscillations in the XUV absorption spectra.
By comparison to Bethe-Salpeter equation simulations, the spectral changes are
mapped to real-space excited-state displacements of the lattice along the
dominant A coordinate. By directly and simultaneously probing the
excited carrier distribution dynamics and accompanying femtosecond lattice
displacement in 2H-MoTe within a single experiment, our work provides a
benchmark for understanding the interplay between electronic and structural
dynamics in photoexcited nanomaterials
Ionization degree of the electron-hole plasma in semiconductor quantum wells
The degree of ionization of a nondegenerate two-dimensional electron-hole
plasma is calculated using the modified law of mass action, which takes into
account all bound and unbound states in a screened Coulomb potential.
Application of the variable phase method to this potential allows us to treat
scattering and bound states on the same footing. Inclusion of the scattering
states leads to a strong deviation from the standard law of mass action. A
qualitative difference between mid- and wide-gap semiconductors is
demonstrated. For wide-gap semiconductors at room temperature, when the bare
exciton binding energy is of the order of T, the equilibrium consists of an
almost equal mixture of correlated electron-hole pairs and uncorrelated free
carriers.Comment: 22 pages, 6 figure
Dynamic exchange-correlation potentials for the electron gas in dimensionality D=3 and D=2
Recent progress in the formulation of a fully dynamical local approximation
to time-dependent Density Functional Theory appeals to the longitudinal and
transverse components of the exchange and correlation kernel in the linear
current-density response of the homogeneous fluid at long wavelength. Both
components are evaluated for the electron gas in dimensionality D=3 and D=2 by
an approximate decoupling in the equation of motion for the current density,
which accounts for processes of excitation of two electron-hole pairs. Each
pair is treated in the random phase approximation, but the role of exchange and
correlation is also examined; in addition, final-state exchange processes are
included phenomenologically so as to satisfy the exactly known high-frequency
behaviours of the kernel. The transverse and longitudinal spectra involve the
same decay channels and are similar in shape. A two-plasmon threshold in the
spectrum for two-pair excitations in D=3 leads to a sharp minimum in the real
part of the exchange and correlation kernel at twice the plasma frequency. In
D=2 the same mechanism leads to a broad spectral peak and to a broad minimum in
the real part of the kernel, as a consequence of the dispersion law of the
plasmon vanishing at long wavelength. The numerical results have been fitted to
simple analytic functions.Comment: 13 pages, 11 figures included. Accepted for publication in Phys. Rev.
What are the experimentally observable effects of vertex corrections in superconductors?
We calculate the effects of vertex corrections, of non-constant density of
states and of a (self-consistently determined) phonon self-energy for the
Holstein model on a 3D cubic lattice. We replace vertex corrections with a
Coulomb pseudopotential, mu*, adjusted to give the same Tc, and repeat the
calculations, to see which effects are a distinct feature of vertex
corrections. This allows us to determine directly observable effects ofvertex
corrections on a variety of thermodynamic properties of superconductors. To
this end, we employ conserving approximations (in the local approximation) to
calculate the superconducting critical temperatures, isotope coefficients,
superconducting gaps, free-energy differences and thermodynamic critical fields
for a range of parameters. We find that the dressed value of lambda is
significantly larger than the bare value. While vertex corrections can cause
significant changes in all the above quantities (even whenthe bare
electron-phonon coupling is small), the changes can usually be well-modeled by
an appropriate Coulomb pseudopotential. The isotope coefficient proves to be
the quantity that most clearly shows effects of vertex corrections that can not
be mimicked by a mu*.Comment: 28 pages, 12 figure
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De Novo Ultrascale Atomistic Simulations On High-End Parallel Supercomputers
We present a de novo hierarchical simulation framework for first-principles based predictive simulations of materials and their validation on high-end parallel supercomputers and geographically distributed clusters. In this framework, high-end chemically reactive and non-reactive molecular dynamics (MD) simulations explore a wide solution space to discover microscopic mechanisms that govern macroscopic material properties, into which highly accurate quantum mechanical (QM) simulations are embedded to validate the discovered mechanisms and quantify the uncertainty of the solution. The framework includes an embedded divide-and-conquer (EDC) algorithmic framework for the design of linear-scaling simulation algorithms with minimal bandwidth complexity and tight error control. The EDC framework also enables adaptive hierarchical simulation with automated model transitioning assisted by graph-based event tracking. A tunable hierarchical cellular decomposition parallelization framework then maps the O(N) EDC algorithms onto Petaflops computers, while achieving performance tunability through a hierarchy of parameterized cell data/computation structures, as well as its implementation using hybrid Grid remote procedure call + message passing + threads programming. High-end computing platforms such as IBM BlueGene/L, SGI Altix 3000 and the NSF TeraGrid provide an excellent test grounds for the framework. On these platforms, we have achieved unprecedented scales of quantum-mechanically accurate and well validated, chemically reactive atomistic simulations--1.06 billion-atom fast reactive force-field MD and 11.8 million-atom (1.04 trillion grid points) quantum-mechanical MD in the framework of the EDC density functional theory on adaptive multigrids--in addition to 134 billion-atom non-reactive space-time multiresolution MD, with the parallel efficiency as high as 0.998 on 65,536 dual-processor BlueGene/L nodes. We have also achieved an automated execution of hierarchical QM/MD simulation on a Grid consisting of 6 supercomputer centers in the US and Japan (in total of 150 thousand processor-hours), in which the number of processors change dynamically on demand and resources are allocated and migrated dynamically in response to faults. Furthermore, performance portability has been demonstrated on a wide range of platforms such as BlueGene/L, Altix 3000, and AMD Opteron-based Linux clusters
Glass breaks like metals, but at the nanometer scale
We report in situ Atomic Force Microscopy experiments which reveal the
presence of nanoscale damage cavities ahead of a stress-corrosion crack tip in
glass. Their presence might explain the departure from linear elasticity
observed in the vicinity of a crack tip in glass. Such a ductile fracture
mechanism, widely observed in the case of metallic materials at the micrometer
scale, might be also at the origin of the striking similarity of the
morphologies of fracture surfaces of glass and metallic alloys at different
length scales.Comment: 4 pages, 4 figures, to appear in Phys. Rev. Lett, few minor
corrections, Fig. 1b change
Molecular Dynamics for Low Temperature Plasma-Surface Interaction Studies
The mechanisms of physical and chemical interactions of low temperature
plasmas with surfaces can be fruitfully explored using molecular dynamics (MD)
simulations. MD simulations follow the detailed motion of sets of interacting
atoms through integration of atomic equations of motion, using inter-atomic
potentials that can account for bond breaking and formation that result when
energetic species from the plasma impact surfaces. This article summarizes the
current status of the technique for various applications of low temperature
plasmas to material processing technologies. The method is reviewed, and
commonly used inter-atomic potentials are described. Special attention is paid
to the use of MD in understanding various representative applications,
including tetrahedral amorphous carbon film deposition from energetic carbon
ions; the interactions of radical species with amorphous hydrogenated silicon
films; silicon nano-particles in plasmas; and plasma etching.Comment: Manuscript #271801, Accepted in J. Phys. D, November 10th, 200
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