2,899 research outputs found
Orbital Dependent Exchange-Only Methods for Periodic Systems
Various orbital-dependent exchange-only potentials are studied which exhibit
correct long-range asymptotic behaviour. We present the first application of
these potentials for polymers and by one of these potentials for molecules.
Kohn-Sham type calculations have been carried out for polyethylene in order to
make valuable comparison of these potentials with each other as well as with
Hartree-Fock and exchange-only LDA methods. The Kohn-Sham band gap obtained
with the optimized effective potetial method is corrected with the exchange
contribution to the derivative discontinuity of the exchange-correlation
potential. The corrected band gap obtained with the Slater's exchange potential
is 9.7 eV close to the experiment.Comment: 11 pages, 2 figures. Phys. Rev. B60, 1999, in pres
Lattice density functional theory at finite temperature with strongly density-dependent exchange-correlation potentials
The derivative discontinuity of the exchange-correlation (xc) energy at
integer particle number is a property of the exact, unknown xc functional of
density functional theory (DFT) which is absent in many popular local and
semilocal approximations. In lattice DFT, approximations exist which exhibit a
discontinuity in the xc potential at half filling. However, due to convergence
problems of the Kohn-Sham (KS) self-consistency cycle, the use of these
functionals is mostly restricted to situations where the local density is away
from half filling. Here a numerical scheme for the self-consistent solution of
the lattice KS Hamiltonian with a local xc potential with rapid (or
quasi-discontinuous) density dependence is suggested. The problem is formulated
in terms of finite-temperature DFT where the discontinuity in the xc potential
emerges naturally in the limit of zero temperature. A simple parametrization is
suggested for the xc potential of the uniform 1D Hubbard model at finite
temperature which is obtained from the solution of the thermodynamic Bethe
ansatz. The feasibility of the numerical scheme is demonstrated by application
to a model of fermionic atoms in a harmonic trap. The corresponding density
profile exhibits a plateau of integer occupation at low temperatures which
melts away for higher temperatures.Comment: 14 pages, 11 figure
The time-dependent exchange-correlation functional for a Hubbard dimer: quantifying non-adiabatic effect
We address and quantify the role of non-adiabaticity ("memory effects") in
the exchange-correlation (xc) functional of time-dependent density functional
theory (TDDFT) for describing non-linear dynamics of many-body systems.
Time-dependent resonant processes are particularly challenging for available
TDDFT approximations, due to their strong non-linear and non-adiabatic
character. None of the known approximate density functionals are able to cope
with this class of problems in a satisfactory manner. In this work we look at
the prototypical example of the resonant processes by considering Rabi
oscillations within the exactly soluble 2-site Hubbard model. We construct the
exact adiabatic xc functional and show that (i) it does not reproduce correctly
resonant Rabi dynamics, (ii) there is a sizable non-adiabatic contribution to
the exact xc potential, which turns out to be small only at the beginning and
at the end of the Rabi cycle when the ground state population is dominant. We
then propose a "two-level" approximation for the time-dependent xc potential
which can capture Rabi dynamics in the 2-site problem. It works well both for
resonant and for detuned Rabi oscillations and becomes essentially exact in the
linear response regime. This new, fully non-adiabatic and explicit density
functional constitutes one of the main results of the present work.Comment: 8 pages, 5 figure
A time-dependent approach to electron pumping in open quantum systems
We propose a time-dependent approach to investigate the motion of electrons
in quantum pump device configurations. The occupied one-particle states are
propagated in real time and used to calculate the local electron density and
current. An advantage of the present computational scheme is that the same
computational effort is required to simulate monochromatic, polychromatic and
nonperiodic drivings. Furthermore, initial state dependence and history effects
are naturally accounted for. This approach can also be embedded in the
framework of time-dependent density functional theory to include
electron-electron interactions. In the special case of periodic drivings we
combine the Floquet theory with nonequilibrium Green's functions and obtain a
general expression for the pumped current in terms of inelastic transmission
probabilities. This latter result is used for benchmarking our propagation
scheme in the long-time limit. Finally, we discuss the limitations of
Floquet-based schemes and suggest our approach as a possible way to go beyond
them.Comment: 14 pages, 8 figure
Highly anisotropic energy gap in superconducting Ba(FeCo)As from optical conductivity measurements
We have measured the complex dynamical conductivity, , of superconducting Ba(FeCo)As ( K) at terahertz frequencies and temperatures 2 - 30 K. In the frequency
dependence of below , we observe clear signatures of the
superconducting energy gap opening. The temperature dependence of
demonstrates a pronounced coherence peak at frequencies below 15 cm (1.8
meV). The temperature dependence of the penetration depth, calculated from
, shows power-law behavior at the lowest temperatures. Analysis of
the conductivity data with a two-gap model, gives the smaller isotropic s-wave
gap of meV, while the larger gap is highly anisotropic with
possible nodes and its rms amplitude is meV. Overall, our
results are consistent with a two-band superconductor with an gap
symmetry.Comment: 6 pages, 4 figures, discussion on pair-barking scattering and
possible lifting of the nodes is adde
Detection and Estimation Theory
Contains reports on two research projects.Joint Services Electronics Programs (U. S. Army, U. S. Navy, and U.S. Air Force)under Contract DA 28-043-AMC-02536(E
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