4,864 research outputs found
Understanding the Heavy Fermion Phenomenology from Microscopic Model
We solve the 3D periodic Anderson model via two impurity DMFT. We obtain the
temperature v.s. hybridization phase diagram. In approaching the quantum
critical point (QCP) both the Neel and lattice Kondo temperatures decrease and
they do not cross at the lowest temperature we reached. While strong
ferromagnetic spin fluctuation on the Kondo side is observed, our result
indicates the critical static spin susceptibility is local in space at the QCP.
We observe in the crossover region logarithmic temperature dependence in the
specific heat coefficient and spin susceptibility
Extended Dynamical Mean Field Theory Study of the Periodic Anderson Model
We investigate the competition of the Kondo and the RKKY interactions in
heavy fermion systems. We solve a periodic Anderson model using Extended
Dynamical Mean Field Theory (EDMFT) with QMC. We monitor simultaneously the
evolution of the electronic and magnetic properties. As the RKKY coupling
increases the heavy fermion quasiparticle unbinds and a local moment forms. At
a critical RKKY coupling there is an onset of magnetic order. Within EDMFT the
two transitions occur at different points and the disapparence of the magnetism
is not described by a local quantum critical point.Comment: 4 pages, 4 figure
Atomic oxygen adsorption and incipient oxidation of the Pb(111) surface: A density-functional theory study
We study the atomic oxygen adsorption on Pb(111) surface by using
density-functional theory within the generalized gradient approximation and a
supercell approach. The atomic and energetic properties of purely on-surface
and subsurface oxygen structures at the Pb(111) surface are systematically
investigated for a wide range of coverages and adsorption sites. The fcc and
tetra-II sites (see the text for definition) are found to be energetically
preferred for the on-surface and subsurface adsorption, respectively, in the
whole range of coverage considered. The on-surface and subsurface oxygen
binding energies monotonically increase with the coverage, and the latter is
always higher than the former, thus indicating the tendency to the formation of
oxygen islands (clusters) and the higher stability of subsurface adsorption.
The on-surface and subsurface diffusion-path energetics of atomic oxygen, and
the activation barriers for the O penetration from the on-surface to the
subsurface sites are presented at low and high coverages. In particular, it is
shown that the penetration barrier from the on-surface hcp to the subsurface
tetra-I site is as small as 65 meV at low coverage (=0.25). The other
properties of the O/Pb(111) system, including the charge distribution, the
lattice relaxation, the work function, and the electronic density of states,
are also studied and discussed in detail, which consistently show the gradually
stabilizing ionic O-Pb bond with increase of the oxygen coverage.Comment: 31 pages, 16 figure
Rotation of hydrogen molecules during the dissociative adsorption on the Mg(0001) surface: A first-principles study
Using first-principles calculations, we systematically study the potential
energy surfaces and dissociation processes of the hydrogen molecule on the
Mg(0001) surface. It is found that during the dissociative adsorption process
with the minimum energy barrier, the hydrogen molecule firstly orients
perpendicular, and then rotates to be parallel to the surface. It is also found
that the orientation of the hydrogen molecule at the transition state is
neither perpendicular nor parallel to the surface. Most importantly, we find
that the rotation causes a reduction of the calculated dissociation energy
barrier for the hydrogen molecule. The underlying electronic reasons for the
rotation of the hydrogen molecule is also discussed in our paper.Comment: 14 pages, 4 figure
Nanowrinkled Carbon Aerogels Embedded with FeN x Sites as Effective Oxygen Electrodes for Rechargeable Zinc-Air Battery.
Rational design of single-metal atom sites in carbon substrates by a flexible strategy is highly desired for the preparation of high-performance catalysts for metal-air batteries. In this study, biomass hydrogel reactors are utilized as structural templates to prepare carbon aerogels embedded with single iron atoms by controlled pyrolysis. The tortuous and interlaced hydrogel chains lead to the formation of abundant nanowrinkles in the porous carbon aerogels, and single iron atoms are dispersed and stabilized within the defective carbon skeletons. X-ray absorption spectroscopy measurements indicate that the iron centers are mostly involved in the coordination structure of FeN4, with a minor fraction (ca. 1/5) in the form of FeN3C. First-principles calculations show that the FeN x sites in the Stone-Wales configurations induced by the nanowrinkles of the hierarchically porous carbon aerogels show a much lower free energy than the normal counterparts. The resulting iron and nitrogen-codoped carbon aerogels exhibit excellent and reversible oxygen electrocatalytic activity, and can be used as bifunctional cathode catalysts in rechargeable Zn-air batteries, with a performance even better than that based on commercial Pt/C and RuO2 catalysts. Results from this study highlight the significance of structural distortions of the metal sites in carbon matrices in the design and engineering of highly active single-atom catalysts
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