41 research outputs found
Ab initio GW many-body effects in graphene
We present an {\it ab initio} many-body GW calculation of the self-energy,
the quasiparticle band plot and the spectral functions in free-standing undoped
graphene. With respect to other approaches, we numerically take into account
the full ionic and electronic structure of real graphene and we introduce
electron-electron interaction and correlation effects from first principles.
Both non-hermitian and also dynamical components of the self-energy are fully
taken into account. With respect to DFT-LDA, the Fermi velocity is
substantially renormalized and raised by a 17%, in better agreement with
magnetotransport experiments. Furthermore, close to the Dirac point the linear
dispersion is modified by the presence of a kink, as observed in ARPES
experiments. Our calculations show that the kink is due to low-energy single-particle excitations and to the plasmon. Finally, the GW
self-energy does not open the band gap.Comment: 5 pages, 4 figures, 1 tabl
Computational methods for 2D materials modelling
Materials with thickness ranging from a few nanometers to a single atomic
layer present unprecedented opportunities to investigate new phases of matter
constrained to the two-dimensional plane.Particle-particle Coulomb interaction
is dramatically affected and shaped by the dimensionality reduction, driving
well-established solid state theoretical approaches to their limit of
applicability. Methodological developments in theoretical modelling and
computational algorithms, in close interaction with experiments, led to the
discovery of the extraordinary properties of two-dimensional materials, such as
high carrier mobility, Dirac cone dispersion and bright exciton luminescence,
and inspired new device design paradigms. This review aims to describe the
computational techniques used to simulate and predict the optical, electronic
and mechanical properties of two-dimensional materials, and to interpret
experimental observations. In particular, we discuss in detail the particular
challenges arising in the simulation of two-dimensional constrained fermions,
and we offer our perspective on the future directions in this field.Comment: This submission does not include the third party cited figure
The bandstructure of gold from many-body perturbation theory
The bandstructure of gold is calculated using many-body perturbation theory
(MBPT). Different approximations within the GW approach are considered.
Standard single shot G0W0 corrections shift the unoccupied bands up by ~0.2 eV
and the first sp-like occupied band down by ~0.4 eV, while leaving unchanged
the 5d occupied bands. Beyond G0W0, quasiparticle self-consistency on the
wavefunctions lowers the occupied 5d bands by 0.35 eV. Globally, many-body
effects achieve an opening of the interband gap (5d-6sp gap) of 0.35 to 0.75 eV
approaching the experimental results. Finally, the quasiparticle bandstructure
is compared to the one obtained by the widely used HSE (Heyd, Scuseria, and
Ernzerhof) hybrid functional
Transport properties of molecular junctions from many-body perturbation theory
The conductance of single molecule junctions is calculated using a Landauer
approach combined to many-body perturbation theory MBPT) to account for
electron correlation. The mere correction of the density-functional theory
eigenvalues, which is the standard procedure for quasiparticle calculations
within MBPT, is found not to affect noticeably the zero-bias conductance. To
reduce it and so improve the agreement with the experiments, the wavefunctions
also need to be updated by including the non-diagonal elements of the
self-energy operator
Energy Deposition around Swift Carbon-Ion Tracks in Liquid Water
Energetic carbon ions are promising projectiles used for cancer radiotherapy. A thorough knowledge of how the energy of these ions is deposited in biological media (mainly composed of liquid water) is required. This can be attained by means of detailed computer simulations, both macroscopically (relevant for appropriately delivering the dose) and at the nanoscale (important for determining the inflicted radiobiological damage). The energy lost per unit path length (i.e., the so-called stopping power) of carbon ions is here theoretically calculated within the dielectric formalism from the excitation spectrum of liquid water obtained from two complementary approaches (one relying on an optical-data model and the other exclusively on ab initio calculations). In addition, the energy carried at the nanometre scale by the generated secondary electrons around the ion's path is simulated by means of a detailed Monte Carlo code. For this purpose, we use the ion and electron cross sections calculated by means of state-of-the art approaches suited to take into account the condensed-phase nature of the liquid water target. As a result of these simulations, the radial dose around the ion's path is obtained, as well as the distributions of clustered events in nanometric volumes similar to the dimensions of DNA convolutions, contributing to the biological damage for carbon ions in a wide energy range, covering from the plateau to the maximum of the Bragg peak
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