The new resonating valence bond method for ab-initio electronic simulations
le vendredi 24 mai 2013 à 11h00
Séminaire Conférence CPTG
Personne à contacter :
Lieu : Amphithéâtre, maison des Magistères
Résumé :
The Resonating Valence Bond theory of the chemical bond was introduced soon after the discovery of quantum mechanics and has contributed to explain the role of electron correlation within a particularly simple and intuitive approach, where the chemical bond between two nearby atoms is described by one or more singlet electron pairs.
We revisit the Pauling's resonating valence bond theory of the chemical bond
within a new formulation, introduced
by P.W. Anderson soon after the discovery of High Tc superconductivity.
It is shown that this intuitive picture
of electron correlation becomes now practical and efficient,
and allows us to perform realistic simulations with correlated wavefunctions
corresponding to several hundred atoms.
Few examples will be given: i) in the Beryllium dimer we
show the accuracy of the method for a particularly difficult case
where single determinant approaches (DFT or Hartree-Fock) miserably fail, ii) recent finite temperature realistic simulations of liquid hydrogen and liquid water.
Normal fluid phases of He 3 in two dimensions
le vendredi 31 mai 2013 à 11h00
Séminaire théorique « Philippe Nozières »
Personne à contacter :
Lieu : Amphithéâtre, Maison des Magistères
Résumé :
I will present ongoing work, based on quantum Monte Carlo simulations,
on the normal fluid phase of ³He in two dimensions, both for the strictly
2D case and for more realistic models of monolayers adsorbed on different
substrates. We find close agreement with the experiment for both the static
spin susceptibility and the dynamic structure factor. For weak enough
alkali metal substrates, we predict a gas-liquid phase transition not
found for strictly 2D ³He.
Correlated Electron-Ion Dynamics (CEID): An efficient method to model electronic (de)coherence from an atomistic point of view
le vendredi 7 juin 2013 à 11h00
Séminaire théorique « Philippe Nozières »
Personne à contacter :
Lieu : Amphithéâtre, Maison des Magistères
Résumé :
Quantum coherence between electronic and nuclear dynamics, as observed
experimentally in organic semiconductors, is the object of an intense
theoretical and computational effort. To simulate this kind of quantum coherent
dynamics, an efficient numerical scheme based on Correlated Electron-Ion
Dynamics (CEID) has been recently devised [1]. In this talk, I describe a
further generalization of CEID [2] and its practical numerical implementation
[3]. To illustrate the capability of this extended CEID scheme, an atomistic
model of the electronic decoherence of a short conjugated oligomer is presented.
Finally, I discuss convergence and scaling properties of the extended CEID
scheme along with its applicability to larger systems, e.g., to investigate the
non-radiative relaxation of photo-excited conjugated polymers [4].
[1] L. Stella et al., J. Chem. Phys. 127, 214104 (2007)
[2] L. Stella et al., J. Chem. Phys. 134, 194105 (2011)
[3] https://bitbucket.org/lstella/polyceid
[4] E.J. McEniry et al., Eur. Phys. J. B 77, 305–329 (2010)
(titre non communiqué)
le mercredi 17 juillet 2013 à 11h00
Séminaire théorique « Philippe Nozières »
Personne à contacter :
Lieu : Amphithéâtre, Maison des Magistères
Résumé :
"Quantum computing with magnetic color center in diamond"
Magnetic color centers in diamond have received a lot of interest in the
research community due to their potential use as qubits for solid-state quantum
computing or as single photon source for quantum key distribution in quantum
cryptography. Indeed, diamond's wide band gap and long spin lifetimes offers the
possibility to initialize, manipulate and readout the quantum state of the qubit
and allows the existence of more than 500 color centers with most of them yet to
be characterized. Transition metal (TM) impurities in diamond are known to
produce numerous color centers and we propose here to study TM-related defect.
After an introduction on solid-state quantum computing in diamond, we present
our results.
The electronic and magnetic properties of a neutral substitutional nickel
impurity in diamond are studied using density functional theory in the
generalized gradient approximation (GGA). The spin-one ground state consists of
two electrons with parallel spins, one located on the nickel ion in the 3d9
configuration and the other distributed among the nearest-neighbor carbons. The
exchange interaction between these spins is due to p-d hybridization and is
controllable with compressive hydrostatic or uniaxial strain. For sufficient
strain the antiparallel spin configuration becomes the ground state. Hence, the
Ni impurity forms a controllable two-electron exchange-coupled system that
should be a robust qubit for solid-state quantum information processing. The
chemical trends of neutral substitutional TMs0 impurities is calculated within
GGA. Crs0 is shown to be another potential candidate for quantum computing
applications.
We then provide an ab-initio characterization of the negatively charged
substitutional Nis- impurity in diamond using hybrid density functional calculation.
Nis- is shown to carry a spin S = 3/2. The calculated hyperfine couplings on this
defect support the identification of the W8 electron paramagnetic resonance
center with Nis- defect. We unambiguously determine the position of Nis- acceptor
level in the gap. This level is located at about 2.0 eV above the valence band
maximum and corresponds to a totally occupied triplet state responsible for the
magnetization. We calculated the excited state properties of the defect. Our
results indicate that Nis- is associated with the 3.1 eV center which has not yet
been assigned to any Ni-related defect.
Laboratoire de Physique et Modélisation des Milieux Condensés
