Ab-Initio Calculations for the Electronic Spectra of Cubic and Hexagonal Boron Nitride

2004 ◽  
Vol 829 ◽  
Author(s):  
Guido Satta ◽  
Giancarlo Cappellini ◽  
Valerio Olevano ◽  
Lucia Reining
Keyword(s):  
Boron Nitride ◽  
Random Phase Approximation ◽  
Density Functional ◽  
Hexagonal Boron Nitride ◽  
Random Phase ◽  
Optical Spectra ◽  
Many Body ◽  
Self Energy ◽  
Many Body Effects ◽  
Excitonic Effects

ABSTRACTWe present state of the art fist-principles calculations for the optical spectra and the loss functions of bulk boron nitride in the cubic (c-BN) and in the hexagonal (h-BN) phases. We start from a DFT-LDA density functional Khon-Sham bandstructure to investigate the influence of many-body effects beyond the Random Phase Approximation (RPA) on the optical spectra through the inclusion of self-energy and excitonic effects by a GW calculation and the solution of the Bethe-Salpeter equation. For the loss function we only perform RPA calculations. We show to which extent the description of many-body effects is important for a meaningiful comparison with experiment, and when they can be neglected.

scholarly journals THERMALIZATION TIME OF POSITRONS IN METALS

1967 ◽  
Vol 45 (2) ◽  
pp. 387-402 ◽  
Author(s):  
J. P. Carbotte ◽  
H. L. Arora
Keyword(s):  
Random Phase Approximation ◽  
Room Temperature ◽  
Random Phase ◽  
Energy Operator ◽  
Density Parameter ◽  
Many Body ◽  
Thermalization Time ◽  
Self Energy ◽  
Many Body Perturbation Theory ◽  
Time Required

The thermalization rate of positrons in metals is computed as a function of the electron density parameter r, for the entire range of metallic density 2 < rg < 5.7. The calculation is based on the propagator technique of many-body perturbation theory. In this formalism the rate of energy loss is essentially determined by the imaginary part of the positron self-energy operator which we treat both in the random phase approximation and the Hubbard approximation. In general, we find that the time required for the positron to drop to an energy of 0.025 eV is not as short as is commonly believed, although at room temperature there can be little doubt that complete thermalization has occurred. For aluminium at 100 °K, however, the thermalization time is longer than the annihilation time and it is suggested that this effect should be detectable in an experiment similar to Stewart and Shand's recent positron effective mass experiment in sodium.

Pros and cons of time-dependent hybrid density functional approach to optical spectra of solids: a case study of CeO2

2021 ◽  
Author(s):  
Huai-Yang Sun ◽  
Shuo-Xue Li ◽  
Hong Jiang
Keyword(s):  
First Principles ◽  
Density Functional ◽  
Computational Cost ◽  
Optical Spectra ◽  
First Principles Calculation ◽  
Many Body ◽  
Many Body Perturbation Theory ◽  
Pros And Cons ◽  
Hybrid Density Functional

Prediction of optical spectra of complex solids remains a great challenge for first-principles calculation due to the huge computational cost of the state-of-the-art many-body perturbation theory based GW-Bethe Salpeter equation...

Analysis of Collective Resonances in Clusters: Metals and Carbon

1992 ◽  
Vol 272 ◽  
Author(s):  
Vitaly V. Kresin
Keyword(s):  
Random Phase Approximation ◽  
Sum Rules ◽  
Random Phase ◽  
Closed Shell ◽  
Dynamical Response ◽  
Many Body ◽  
Resonance Frequencies ◽  
Unified View ◽  
Small Clusters ◽  
Good Agreement

ABSTRACTDipole photoabsorption spectra of small clusters are analyzed. Two types of systems are considered: metal clusters and the carbon fullerenes. Both have been found to exhibit strong collective photoabsorption modes associated with the motion of delocalized electrons. We describe analytical results for the resonance frequencies in both spherical (closed-shell metallic, C60 ) and spheroidal (openshell metallic, C70) particles. The calculation is based on the techniques of many-body physics (random-phase approximation, sum rules), affords a unified view of the dynamical response of microscopic clusters, and leads to good agreement with experimental data.

scholarly journals Polymorphism of Bulk Boron Nitride

2018 ◽  
Author(s):  
Tim Gould ◽  
Claudio Cazorla
Keyword(s):  
Boron Nitride ◽  
Random Phase ◽  
Ambient Conditions ◽  
Many Body ◽  
Electronic Interactions ◽  
Fluctuation Dissipation ◽  
Cohesion Theory ◽  
High Level ◽  
Low Symmetry ◽  
Relative Thermodynamic Stability

Boron nitride (BN) is a material with outstanding technological promise because of its exceptional thermochemical stability, structural, electronic and thermal conductivity properties, and extreme hardness. Yet, the relative thermodynamic stability of its most common polymorphs (diamond-like cubic and graphite-like hexagonal) has not been resolved satisfactorily because of the crucial role played by kinetic factors in the formation of BN phases at high temperatures and pressures (experiments), and by competing bonding, electrostatic and many-body dispersion forces in BN cohesion (theory). This lack of understanding hampers the development of potential technological applications, and challenges the boundaries of fundamental science. Here, we use high-level first-principles theories that correctly reproduce all important electronic interactions (the adiabatic-connection fluctuation-dissipation theorem in the random phase approximation) to estimate with unprecedented accuracy the energy differences between BN polymorphs, and thus overcome the accuracy hurdle that hindered previous theoretical studies. We show that the ground-state phase of BN is cubic and that the frequently observed two-dimensional hexagonal polymorph becomes entropically stabilized over the cubic at temperatures slightly above ambient conditions (Tc = 63+-20'C). We also reveal a new low-symmetry monoclinic phase that is extremely competitive with the other low-energy polymorphs and which could explain the origins of the experimentally observed ``compressed h--BN'' phase. Our theoretical findings therefore should stimulate new experimental efforts in bulk BN as well as promote the use of high-level theories in modelling of technologically relevant van der Waals materials.<br>

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