2,616 research outputs found
A combined experimental and computational study of the pressure dependence of the vibrational spectrum of solid picene C_22H_14
We present high-quality optical data and density functional perturbation
theory calculations for the vibrational spectrum of solid picene
(CH) under pressure up to 8 GPa. First-principles calculations
reproduce with a remarkable accuracy the pressure effects on both frequency and
intensities of the phonon peaks experimentally observed . Through a detailed
analysis of the phonon eigenvectors, We use the projection on molecular
eigenmodes to unambiguously fit the experimental spectra, resolving complicated
spectral structures, in a system with hundreds of phonon modes. With these
projections, we can also quantify the loss of molecular character under
pressure. Our results indicate that picene, despite a \sim 20 % compression of
the unit cell, remains substantially a molecular solid up to 8 GPa, with phonon
modes displaying a smooth and uniform hardening with pressure. The Grueneisen
parameter of the 1380 cm^{-1} a_1 Raman peak () is much lower
than the effective value () due to K doping. This is an
indication that the phonon softening in K doped samples is mainly due to charge
transfer and electron-phonon coupling.Comment: Replaced with final version (PRB
A Study of the Conformers of the (Nonafluorobutanesulfonyl)imide Ion by Means of Infrared Spectroscopy and Density Functional Theory (DFT) Calculations
Pyrrolidinium-based ionic liquids with anions of the per(fluoroalkylsulfonyl)imide family
are particularly interesting for their use as electrolytes in lithium batteries. These ions have several
geometric isomers and the presence of different ion conformers and their distribution affects the ILs
(Ionic liquids) physical and chemical properties. In the present work, we report the temperature
dependence of the infrared spectra of the N-butyl-N-methyl-pyrrolidinium(trifluoromethanesulfonyl)
(nonafluorobutanesulfonyl)imide (PYR14-IM14) ionic liquid; DFT (Density Functional Theory)
calculations performed with different models provides indications about the IM14 conformers and
their vibrational spectra. Moreover the temperature dependence of the intensity of the lines identified
as markers of different conformers provide indications about the conformers’ distribution and the
difference of their enthalpy in the liquid phase
Exact exchange-correlation potential of a ionic Hubbard model with a free surface
We use Lanczos exact diagonalization to compute the exact
exchange-correlation (xc) potential of a Hubbard chain with large binding
energy ("the bulk") followed by a chain with zero binding energy ("the
vacuum"). Several results of density functional theory in the continuum
(sometimes controversial) are verified in the lattice. In particular we show
explicitly that the fundamental gap is given by the gap in the Kohn-Sham
spectrum plus a contribution due to the jump of the xc-potential when a
particle is added. The presence of a staggered potential and a nearest-neighbor
interaction V allows to simulate a ionic solid. We show that in the ionic
regime in the small hopping amplitude limit the xc-contribution to the gap
equals V, while in the Mott regime it is determined by the Hubbard U
interaction. In addition we show that correlations generates a new potential
barrier at the surface
An analysis on health care costs due to accidents involving powered two wheelers to increase road safety
Powered Two Wheelers (PTWs) provide a convenient mode for a large portion of population in many cities. At the same time PTWs present serious system problems, the most important being poorer safety if compared to other motorized modes. But even when lower safety levels are acknowledged, problems behind are far from being solved. Rome is an example: although PTWs accidents rates are not negligible, the need for a specific safety policy is still unmet. Therefore the local Mobility Agency appointed the authors of this paper for a study of PTWs accidents occurring in the urban area. An assessment of the associated health care costs was also required. The objective of the paper is to report the main outcomes of this study highlighting recurring features of PTWs accidents, the high health care costs and how to quantify the economic resources to improve safety. The methodology was based on three steps: i) an analysis of the causes of PTWs accidents, which resulted into the location of black spots and assessment of the severity of the events; ii) the estimation of health care costs after a scientific literature review; iii) the association of health care costs to black spots and accidents severity to rank interventions to improve PTWs safety. This led to a final list of roads where PTWs accidents of the highest severity occurred and the required economic resources to improve their safety level. This stressed, for the first time, the unaffordable expenditures due to PTWs accidents. In conclusion, the issue whether the awareness of such costs can be used as leverage for more mindful behaviors among the riders is addressed
Emerging giant resonant exciton induced by Ta-substitution in anatase TiO: a tunable correlation effect
Titanium dioxide (TiO) has rich physical properties with potential
implications in both fundamental physics and new applications. Up-to-date, the
main focus of applied research is to tune its optical properties, which is
usually done via doping and/or nano-engineering. However, understanding the
role of -electrons in materials and possible functionalization of
-electron properties are still major challenges. Herewith, within a
combination of an innovative experimental technique, high energy optical
conductivity, and of the state-of-the-art {\it ab initio} electronic structure
calculations, we report an emerging, novel resonant exciton in the deep
ultraviolet region of the optical response. The resonant exciton evolves upon
low concentration Ta-substitution in anatase TiO films. It is
surprisingly robust and related to strong electron-electron and electron-hole
interactions. The - and - orbitals localization, due to Ta-substitution,
plays an unexpected role, activating strong electronic correlations and
dominating the optical response under photoexcitation. Our results shed light
on a new optical phenomenon in anatase TiO films and on the possibility
of tuning electronic properties by Ta substitution
Formation and observation of a quasi-two-dimensional electron liquid in epitaxially stabilized SrLaTiO thin films
We report the formation and observation of an electron liquid in
SrLaTiO, the quasi-two-dimensional counterpart of SrTiO,
through reactive molecular-beam epitaxy and {\it in situ} angle-resolved
photoemission spectroscopy. The lowest lying states are found to be comprised
of Ti 3 orbitals, analogous to the LaAlO/SrTiO interface and
exhibit unusually broad features characterized by quantized energy levels and a
reduced Luttinger volume. Using model calculations, we explain these
characteristics through an interplay of disorder and electron-phonon coupling
acting co-operatively at similar energy scales, which provides a possible
mechanism for explaining the low free carrier concentrations observed at
various oxide heterostructures such as the LaAlO/SrTiO interface
Electronic structure of fluorides: general trends for ground and excited state properties
The electronic structure of fluorite crystals are studied by means of density
functional theory within the local density approximation for the exchange
correlation energy. The ground-state electronic properties, which have been
calculated for the cubic structures ,, , ,
, -, using a plane waves expansion of the wave
functions, show good comparison with existing experimental data and previous
theoretical results. The electronic density of states at the gap region for all
the compounds and their energy-band structure have been calculated and compared
with the existing data in the literature. General trends for the ground-state
parameters, the electronic energy-bands and transition energies for all the
fluorides considered are given and discussed in details. Moreover, for the
first time results for have been presented
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