110 research outputs found
A fully quantum mechanical calculation of the diffusivity of hydrogen in iron using the tight binding approximation and path integral theory
We present calculations of free energy barriers and diffusivities as
functions of temperature for the diffusion of hydrogen in bcc-Fe. This is a
fully quantum mechanical approach since the total energy landscape is computed
using a new self consistent, transferable tight binding model for interstitial
impurities in magnetic iron. Also the hydrogen nucleus is treated quantum
mechanically and we compare here two approaches in the literature both based in
the Feynman path integral formulation of statistical mechanics. We find that
the quantum transition state theory which admits greater freedom for the proton
to explore phase space gives result in better agreement with experiment than
the alternative which is based on fixed centroid calculations of the free
energy barrier. We also find results in better agreement compared to recent
centroid molecular dynamics (CMD) calculations of the diffusivity which
employed a classical interatomic potential rather than our quantum mechanical
tight binding theory. In particular we find first that quantum effects persist
to higher temperatures than previously thought, and conversely that the low
temperature diffusivity is smaller than predicted in CMD calculations and
larger than predicted by classical transition state theory. This will have
impact on future modeling and simulation of hydrogen trapping and diffusion
Spectroscopy of the a^3\Sigma_u^+ state and the coupling to the X^1\Sigma_g^+ state of K_2
We report on high resolution Fourier-transform spectroscopy of fluorescence
to the a^3\Sigma_u^+ state excited by two-photon or two-step excitation from
the X^1\Sigma_g^+ state to the 2^3\Pi_g state in the molecule K_2. These
spectroscopic data are combined with recent results of Feshbach resonances and
two-color photoassociation spectra for deriving the potential curves of
X^1\Sigma_g^+ and a^3\Sigma_u^+ up to the asymptote. The precise relative
position of the triplet levels with respect of the singlet levels was achieved
by including the excitation energies from the X^1\Sigma_g^+ state to the
2^3\Pi_g state and down to the a^3\Sigma_u^+ state in the simultaneous fit of
both potentials. The derived precise potential curves allow for reliable
modeling of cold collisions of pairs of potassium atoms in their ^2S ground
state
Experimental study of the Ca2 1S+1S asymptote
The filtered laser excitation technique was applied for measuring transition
frequencies of the Ca B-X system from asymptotic levels of the
X ground state reaching . That level has an
outer classical turning point of about 20~\AA which is only 0.2 \rcm below the
molecular SS asymptote. Extensive analysis of the spectroscopic data,
involving Monte Carlo simulation, allowed for a purely experimental
determination of the long range parameters of the potential energy curve. The
possible values of the s-wave scattering length could be limited to be between
250 and 1000.Comment: 10 pages, 7 figure
Metal-insulator transition in copper oxides induced by apex displacements
High temperature superconductivity has been found in many kinds of compounds
built from planes of Cu and O, separated by spacer layers. Understanding why
critical temperatures are so high has been the subject of numerous
investigations and extensive controversy. To realize high temperature
superconductivity, parent compounds are either hole-doped, such as
{LaCuO} (LCO) with Sr (LSCO), or electron doped, such as
{NdCuO} (NCO) with Ce (NCCO). In the electron doped cuprates, the
antiferromagnetic phase is much more robust than the superconducting phase.
However, it was recently found that the reduction of residual out-of-plane
apical oxygens dramatically affects the phase diagram, driving those compounds
to a superconducting phase. Here we use a recently developed first principles
method to explore how displacement of the apical oxygen (A-O) in LCO affects
the optical gap, spin and charge susceptibilities, and superconducting order
parameter. By combining quasiparticle self-consistent GW (QS\emph{GW}) and
dynamical mean field theory (DMFT), that LCO is a Mott insulator; but small
displacements of the apical oxygens drive the compound to a metallic state
through a localization/delocalization transition, with a concomitant maximum
-wave order parameter at the transition. We address the question whether NCO
can be seen as the limit of LCO with large apical displacements, and elucidate
the deep physical reasons why the behaviour of NCO is so different than the
hole doped materials. We shed new light on the recent correlation observed
between T and the charge transfer gap, while also providing a guide towards
the design of optimized high-Tc superconductors. Further our results suggest
that strong correlation, enough to induce Mott gap, may not be a prerequisite
for high-Tc superconductivity
Self-energies in itinerant magnets: A focus on Fe and Ni
We present a detailed study of local and non-local correlations in the
electronic structure of elemental transition metals carried out by means of the
Quasiparticle Self-consistent GW (QSGW ) and Dynamical Mean Field Theory
(DMFT). Recent high resolution ARPES and Haas-van Alphen data of two typical
transition metal systems (Fe and Ni) are used as case study. (i) We find that
the properties of Fe are very well described by QSGW. Agreement with cyclotron
and very clean ARPES measurements is excellent, provided that final-state
scattering is taken into account. This establishes the exceptional reliability
of QSGW also in metallic systems. (ii) Nonetheless QSGW alone is not able to
provide an adequate description of the Ni ARPES data due to strong local spin
fluctuations. We surmount this deficiency by combining nonlocal charge
fluctuations in QSGW with local spin fluctuations in DMFT (QSGW + 'Magnetic
DMFT'). (iii) Finally we show that the dynamics of the local fluctuations are
actually not crucial. The addition of an external static field can lead to
similarly good results if non-local correlations are included through QSGW
The coupling of the X and a states of KRb
A comprehensive study of the electronic states at the 4s+5s asymptote in KRb
is presented. Abundant spectroscopic data on the \astate state were collected
by Fourier-transform spectroscopy which allow to determine an accurate
experimental potential energy curve up to 14.8 \AA . The existing data set (C.
Amiot et al. J. Chem. Phys. 112, 7068 (2000)) on the ground state \Xstate was
extended by several additional levels lying close to the atomic asymptote. In a
coupled channels fitting routine complete molecular potentials for both
electronic states were fitted. Along with the line frequencies of the molecular
transitions, recently published positions of Feshbach resonances in K
and Rb mixtures (F. Ferlaino et al. Phys. Rev. A 74, 039903 (2006)) were
included in the fit. This makes the derived potential curves capable for an
accurate description of observed cold collision features so far. Predictions of
scattering lengths and Feshbach resonances in other isotopic combinations are
reported.Comment: 14 pages, 5 figure
Interatomic potentials of van der Waals dimers and : probing discrepancies between theory and experiment
Results of new all-electron ab initio calculations and revisit of experimental studies of the interatomic potentials of lower-lying ungerade excited and ground electronic energy states of the Hg_{2} and Cd_{2} van der Waals complexes are used as probes of discrepancies between theory and experiment. From simulations of the previously and presently measured LIF excitation and dispersed emission spectra new analytical representations of the excited- and the ground-state interatomic potentials are proposed. An inverted perturbation approach was also used to improve the studied interatomic potentials. The comparison of the new ab-initio calculated potentials with the results of the analyses illustrates an improve theory-to-experiment agreement for such a demanding system like Hg_{2} or Cd_{2}
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