Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set

Graphene is a rapidly rising star on the horizon of materials science and condensed-matter physics. This strictly two-dimensional material exhibits exceptionally high crystal and electronic quality, and, despite its short history, has already revealed a cornucopia of new physics and potential applications, which are briefly discussed here. Whereas one can be certain of the realness of applications only when commercial products appear, graphene no longer requires any further proof of its importance in terms of fundamental physics. Owing to its unusual electronic spectrum, graphene has led to the emergence of a new paradigm of 'relativistic' condensed-matter physics, where quantum relativistic phenomena, some of which are unobservable in high-energy physics, can now be mimicked and tested in table-top experiments. More generally, graphene represents a conceptually new class of materials that are only one atom thick, and, on this basis, offers new inroads into low-dimensional physics that has never ceased to surprise and continues to provide a fertile ground for applications.

The rise of graphene

There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

This article reviews the basic theoretical aspects of graphene, a one-atom-thick allotrope of carbon, with unusual two-dimensional Dirac-like electronic excitations. The Dirac electrons can be controlled by application of external electric and magnetic fields, or by altering sample geometry and/or topology. The Dirac electrons behave in unusual ways in tunneling, confinement, and the integer quantum Hall effect. The electronic properties of graphene stacks are discussed and vary with stacking order and number of layers. Edge (surface) states in graphene depend on the edge termination (zigzag or armchair) and affect the physical properties of nanoribbons. Different types of disorder modify the Dirac equation leading to unusual spectroscopic and transport properties. The effects of electron-electron and electron-phonon interactions in single layer and multilayer graphene are also presented.

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The electronic properties of graphene

The Raman shift, broadening, and relative Raman intensities of bilayer graphene are computed as functions of the electron concentration We include dynamic effects for the phonon frequencies and we consider the gap induced in the band structure of bilayer graphene by an external electric field We show that from the analysis of the Raman spectra of gated bilayer graphene it is possible to quantitatively identify the amount of charges coming from the atmosphere and from the substrate These findings suggest that Raman spectroscopy of bilayer graphene can be used to characterize the electrostatic environment of few-layers graphene

Probing the electrostatic environment of bilayer graphene using Raman spectra

We study ultrafast scattering dynamics of hot electrons photoinjected with high excess energies in the \ensuremath{\Gamma} valley of the conduction band of GaAs, using time- and angle-resolved photoemission spectroscopy and ab initio calculations. At ultrafast rates of the order of 10 fs, the packets in the \ensuremath{\Gamma} valley are transformed into hot-electron ensembles (HEEs) quasiequilibrated in momentum space but not in energy space. The energy relaxation of the HEEs takes place as a whole on a longer time scale with rates dependent only on the excess energy, irrespective of the momenta of hot electrons. Both momentum scattering and energy relaxation are ruled by the electron-phonon interaction.

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Formation of hot-electron ensembles quasiequilibrated in momentum space by ultrafast momentum scattering of highly excited hot electrons photoinjected into the Γ valley of GaAs

We present a practical method to compute the vibrational resonant Raman spectra in solids with delocalized excitations. We apply this approach to the study of tetrahedrous amorphous carbon. We determine the vibrational eigenmodes and eigenvalues using density functional theory in the local density approximation, and the Raman intensities using a tight binding approximation. The computed spectra are in good agreement with the experimental ones measured with visible and UV lasers. We analyze the Raman spectra in terms of vibrational modes of microscopic units. We show that, at any frequency, the spectra are dominated by the stretching vibrations. We identify a very rapid inversion in the relative Raman intensities of the $sp^2$ and $sp^3$ carbon sites with the frequency of the incident laser beam. In particular, the spectra are dominated by $sp^2$ atoms below 4 eV and by $sp^3$ atoms above 6 eV.

Theory of resonant Raman scattering of tetrahedral amorphous carbon

Laser without population inversion and coherent trapping

The effect of electronic dispersion over a wide variety of SiO2 polymorphs (faujasite, ferrierite, α-cristobalite,
α-quartz, coesite, and stishovite) is investigated using state-of-the-art density functional theory. Different
functionals and dispersion correction schemes are compared, ranging from the local density approximation
to fully nonlocal exchange-correlation functionals. It is shown that both empirical dispersion corrections and
fully nonlocal functionals improve the energetics and give correct volumetric data. However, the correct volume
results come from error cancellation between an overestimation of the Si-O distance and an underestimation of
the Si-O-Si angle. Quantum Monte Carlo is used to compute the quartz-cristobalite energy difference within an
accuracy of 0.2 kCal/mol per SiO2 unit. This demonstrates the feasability of achieving subchemical accuracy on
extended systems, and confirms the validity of the Slater-Jastrow ansatz for describing SiO2 polymorphs.

Francesco Mauri

Papers

Probing the electrostatic environment of bilayer graphene using Raman spectra

Formation of hot-electron ensembles quasiequilibrated in momentum space by ultrafast momentum scattering of highly excited hot electrons photoinjected into the Γ valley of GaAs

Theory of resonant Raman scattering of tetrahedral amorphous carbon

Laser without population inversion and coherent trapping

Dispersion effects in SiO 2 polymorphs: An ab initio study