3,776 research outputs found
Spin Resolution of the Electron-Gas Correlation Energy: Positive same-spin contribution
The negative correlation energy per particle of a uniform electron gas of
density parameter and spin polarization is well known, but its
spin resolution into up-down, up-up, and down-down contributions is not.
Widely-used estimates are incorrect, and hamper the development of reliable
density functionals and pair distribution functions. For the spin resolution,
we present interpolations between high- and low-density limits that agree with
available Quantum Monte Carlo data. In the low-density limit for ,
we find that the same-spin correlation energy is unexpectedly positive, and we
explain why. We also estimate the up and down contributions to the kinetic
energy of correlation.Comment: new version, to appear in PRB Rapid Communicatio
Electron Correlation and the c-axis Dispersion of Cu d_z^2: a New Band Structure for High Temperature Superconductors
Previously we showed the major effect of electron correlation in the cuprate
superconductors is to lower the energy of the Cu d_x^2-y^2/O p_sigma (x^2-y^2)
band with respect to the Cu d_z^2/O' p_z (z^2) band. In our 2D Hubbard model
for La_1.85Sr_0.15CuO_4 (LaSCO), the z^2 band is narrow and crosses the
standard x^2-y^2 band just below the Fermi level. In this work, we introduce
c-axis dispersion to the model and find the z^2 band to have considerable
anisotropic 3D character. An additional hole-like surface opens up in the z^2
band at (0,0,2pi/c) which expands with doping. At sufficient doping levels, a
symmetry allowed x^2-y^2/z^2 band crossing along the (0,0)-(pi,pi) direction of
the Brillouin zone appears at the Fermi level. At this point, Cooper pairs
between the two bands (e.g. (k uparrow x^2-y^2/k downarrow z^2)) can form,
providing the basis for the Interband Pairing Theory of superconductivity in
these materials.Comment: submitted to Phys. Rev. Lett. Related publications: Phys. Rev. B 58,
12303 (1998); Phys. Rev. B 58, 12323 (1998); cond-mat/9903088;
cond-mat/990310
Nucleation of a sodium droplet on C60
We investigate theoretically the progressive coating of C60 by several sodium
atoms. Density functional calculations using a nonlocal functional are
performed for NaC60 and Na2C60 in various configurations. These data are used
to construct an empirical atomistic model in order to treat larger sizes in a
statistical and dynamical context. Fluctuating charges are incorporated to
account for charge transfer between sodium and carbon atoms. By performing
systematic global optimization in the size range 1<=n<=30, we find that Na_nC60
is homogeneously coated at small sizes, and that a growing droplet is formed
above n=>8. The separate effects of single ionization and thermalization are
also considered, as well as the changes due to a strong external electric
field. The present results are discussed in the light of various experimental
data.Comment: 17 pages, 10 figure
Crystal Structures and Electronic Properties of Haloform-Intercalated C60
Using density functional methods we calculated structural and electronic
properties of bulk chloroform and bromoform intercalated C60, C60 2CHX3
(X=Cl,Br). Both compounds are narrow band insulator materials with a gap
between valence and conduction bands larger than 1 eV. The calculated widths of
the valence and conduction bands are 0.4-0.6 eV and 0.3-0.4 eV, respectively.
The orbitals of the haloform molecules overlap with the orbitals of the
fullerene molecules and the p-type orbitals of halogen atoms significantly
contribute to the valence and conduction bands of C60 2CHX3. Charging with
electrons and holes turns the systems to metals. Contrary to expectation, 10 to
20 % of the charge is on the haloform molecules and is thus not completely
localized on the fullerene molecules. Calculations on different crystal
structures of C60 2CHCl3 and C60 2CHBr3 revealed that the density of states at
the Fermi energy are sensitive to the orientation of the haloform and C60
molecules. At a charging of three holes, which corresponds to the
superconducting phase of pure C60 and C60 2CHX3, the calculated density of
states (DOS) at the Fermi energy increases in the sequence DOS(C60) < DOS(C60
2CHCl3) < DOS(C60 2CHBr3).Comment: 11 pages, 7 figures, 4 table
A critical assessment of the Self-Interaction Corrected Local Density Functional method and its algorithmic implementation
We calculate the electronic structure of several atoms and small molecules by
direct minimization of the Self-Interaction Corrected Local Density
Approximation (SIC-LDA) functional. To do this we first derive an expression
for the gradient of this functional under the constraint that the orbitals be
orthogonal and show that previously given expressions do not correctly
incorporate this constraint. In our atomic calculations the SIC-LDA yields
total energies, ionization energies and charge densities that are superior to
results obtained with the Local Density Approximation (LDA). However, for
molecules SIC-LDA gives bond lengths and reaction energies that are inferior to
those obtained from LDA. The nonlocal BLYP functional, which we include as a
representative GGA functional, outperforms both LDA and SIC-LDA for all ground
state properties we considered.Comment: 14 pages, 5 figure
Elastic and vibrational properties of alpha and beta-PbO
The structure, electronic and dynamic properties of the two layered alpha
(litharge) and beta (massicot) phases of PbO have been studied by density
functional methods. The role of London dispersion interactions as leading
component of the total interaction energy between layers has been addressed by
using the Grimme's approach, in which new parameters for Pb and O atoms have
been developed. Both gradient corrected and hybrid functionals have been
adopted using Gaussian-type basis sets of polarized triple zeta quality for O
atoms and small core pseudo-potential for the Pb atoms. Basis set superposition
error (BSSE) has been accounted for by the Boys-Bernardi correction to compute
the interlayer separation. Cross check with calculations adopting plane waves
that are BSSE free have also been performed for both structures and vibrational
frequencies. With the new set of proposed Grimme's type parameters structures
and dynamical parameters for both PbO phases are in good agreement with
experimental data.Comment: 8 pages, 5 figure
Split-off dimer defects on the Si(001)2x1 surface
Dimer vacancy (DV) defect complexes in the Si(001)2x1 surface were
investigated using high-resolution scanning tunneling microscopy and first
principles calculations. We find that under low bias filled-state tunneling
conditions, isolated 'split-off' dimers in these defect complexes are imaged as
pairs of protrusions while the surrounding Si surface dimers appear as the
usual 'bean-shaped' protrusions. We attribute this to the formation of pi-bonds
between the two atoms of the split-off dimer and second layer atoms, and
present charge density plots to support this assignment. We observe a local
brightness enhancement due to strain for different DV complexes and provide the
first experimental confirmation of an earlier prediction that the 1+2-DV
induces less surface strain than other DV complexes. Finally, we present a
previously unreported triangular shaped split-off dimer defect complex that
exists at SB-type step edges, and propose a structure for this defect involving
a bound Si monomer.Comment: 8 pages, 7 figures, submitted to Phys. Rev.
The Antiferromagnetic Band Structure of La2CuO4 Revisited
Using the Becke-3-LYP functional, we have performed band structure
calculations on the high temperature superconductor parent compound, La2CuO4.
Under the restricted spin formalism (rho(alpha) equal to rho(beta)), the
R-B3LYP band structure agrees well with the standard LDA band structure. It is
metallic with a single Cu x2-y2/O p(sigma) band crossing the Fermi level. Under
the unrestricted spin formalism (rho(alpha) not equal to rho(beta)), the UB3LYP
band structure has a spin polarized antiferromagnetic solution with a band gap
of 2.0 eV, agreeing well with experiment. This state is 1.0 eV (per formula
unit) lower than that calculated from the R-B3LYP. The apparent high energy of
the spin restricted state is attributed to an overestimate of on-site Coulomb
repulsion which is corrected in the unrestricted spin calculations. The
stabilization of the total energy with spin polarization arises primarily from
the stabilization of the x2-y2 band, such that the character of the eigenstates
at the top of the valence band in the antiferromagnetic state becomes a strong
mixture of Cu x2-y2/O p(sigma) and Cu z2/O' p(z). Since the Hohenberg-Kohn
theorem requires the spin restricted and spin unrestricted calculations give
exactly the same ground state energy and total density for the exact
functionals, this large disparity in energy reflects the inadequacy of current
functionals for describing the cuprates. This calls into question the use of
band structures based on current restricted spin density functionals (including
LDA) as a basis for single band theories of superconductivity in these
materials.Comment: 13 pages, 8 figures, to appear in Phys. Rev. B, for more information
see http://www.firstprinciples.co
Magnetism and structure at a vacancy in graphene
The electronic structure, bonding and magnetism in graphene containing
vacancies are studied using density-functional methods. The single-vacancy
graphene ground state is spin polarized and structurally flat. The unpolarized
state is non planar only for finite segments. Systems containing periodic
arrays of vacancies displays magnetic transitions and metal-insulator
transitions.Comment: 4 pages, four figure
Electric Field-Tuned Topological Phase Transition in Ultra-Thin Na3Bi - Towards a Topological Transistor
The electric field induced quantum phase transition from topological to
conventional insulator has been proposed as the basis of a topological field
effect transistor [1-4]. In this scheme an electric field can switch 'on' the
ballistic flow of charge and spin along dissipationless edges of the
two-dimensional (2D) quantum spin Hall insulator [5-9], and when 'off' is a
conventional insulator with no conductive channels. Such as topological
transistor is promising for low-energy logic circuits [4], which would
necessitate electric field-switched materials with conventional and topological
bandgaps much greater than room temperature, significantly greater than
proposed to date [6-8]. Topological Dirac semimetals(TDS) are promising systems
in which to look for topological field-effect switching, as they lie at the
boundary between conventional and topological phases [3,10-16]. Here we use
scanning probe microscopy/spectroscopy (STM/STS) and angle-resolved
photoelectron spectroscopy (ARPES) to show that mono- and bilayer films of TDS
Na3Bi [3,17] are 2D topological insulators with bulk bandgaps >400 meV in the
absence of electric field. Upon application of electric field by doping with
potassium or by close approach of the STM tip, the bandgap can be completely
closed then re-opened with conventional gap greater than 100 meV. The large
bandgaps in both the conventional and quantum spin Hall phases, much greater
than the thermal energy kT = 25 meV at room temperature, suggest that ultrathin
Na3Bi is suitable for room temperature topological transistor operation
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