3,048 research outputs found
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
Dynamical Coulomb Blockade and the Derivative Discontinuity of Time-Dependent Density Functional Theory
The role of the discontinuity of the exchange-correlation potential of
density functional theory is studied in the context of electron transport and
shown to be intimately related to Coulomb blockade. By following the time
evolution of an interacting nanojunction attached to biased leads, we find
that, instead of evolving to a steady state, the system reaches a dynamical
state characterized by correlation-induced current oscillations. Our results
establish a dynamical picture of Coulomb blockade manifesting itself as a
periodic sequence of charging and discharging of the nanostructure.Comment: to appear in Physical Review Letter
BaFe_{1.8}Co_{0.2}As_2 thin film hybrid Josephson junctions
Josephson junctions with iron pnictides open the way for fundamental
experiments on superconductivity in these materials and their application in
superconducting devices. Here, we present hybrid Josephson junctions with a
BaFe_{1.8}Co_{0.2}As_2 thin film electrode, an Au barrier and a PbIn counter
electrode. The junctions show RSJ-like current-voltage characteristics up to
the critical temperature of the counter electrode of about 7.2K. The
temperature dependence of the critical current, IC, does not show an
Ambegaokar-Baratoff behavior. Well-pronounced Shapiro steps are observed at
microwave frequencies of 10-18GHz. Assuming an excess current, I_ex, of 200
{\mu}A at 4.2K we get an effective I_C R_N product of 6 {\mu}V.Comment: submitted to Appl. Phys. Let
Influence of the spreading resistance on the conductance spectrum of planar hybrid thin film SNS' junctions based on iron pnictides
To investigate the superconducting properties of iron pnictides we prepared
planar hybrid SNS' junctions in thin film technology with a pnictide base
electrode, a gold barrier layer and a lead counter electrode. Our design allows
characterization of the electrodes and the junction independently in a 4-probe
method. We show how both electrodes influence the measured spectra due to their
spreading resistance. While the Pb electrode has a constant resistance above
its , the contribution of the pnictide electrode is clearly
current-dependent and thus it needs a more advanced method to be corrected. We
present an empirical method, which is simple to apply and allows to deal with
the spreading resistance in our junctions to recalculate the actual conductance
and voltage of one junction at given temperature
Influenced of Fe buffer thickness on the crystalline quality and the transport properties of Fe/Ba(Fe1-xCox)2As2 bilayers
The implementation of an Fe buffer layer is a promising way to obtain
epitaxial growth of Co-doped BaFe2As2 (Ba-122). However, the crystalline
quality and the superconducting properties of Co-doped Ba-122 are influenced by
the Fe buffer layer thickness, dFe. The well-textured growth of the Fe/Ba-122
bilayer with dFe = 15 nm results in a high Jc of 0.45 MAcm at 12 K in
self-field, whereas a low Jc value of 61000 Acm is recorded for the
bilayer with dFe = 4 nm at the corresponding reduced temperature due to the
presence of grain boundaries
Medicaid managed care: Issues for beneficiaries with disabilities
Background: States are increasingly turning to managed care arrangements to control costs in their Medicaid programs. Historically,
such arrangements have excluded people with disabilities who use long-term services and supports (LTSS) due to their complex needs.
Now, however, some states are also moving this population to managed care. Little is known about the experiences of people with disabilities
during and after this transition.
Objective: To document experiences of Medicaid enrollees with disabilities using long-term services and supports during transition to
Medicaid managed care in Kansas.
Methods: During the spring of 2013, 105 Kansans with disabilities using Medicaid long-term services and supports (LTSS) were
surveyed via telephone or in-person as they transitioned to managed care. Qualitative data analysis of survey responses was conducted
to learn more about the issues encountered by people with disabilities under Medicaid managed care.
Results: Respondents encountered numerous disability-related difficulties, particularly with transportation, durable medical equipment,
care coordination, communication, increased out of pocket costs, and access to care.
Conclusions: As more states move people with disabilities to Medicaid managed care, it is critically important to address these identified
issues for a population that often experiences substantial health disparities and a smaller margin of health. It is hoped that the early
experiences reported here can inform policy-makers in other states as they contemplate and design similar programs
Social discounting of pain
Impatience can be formalized as a delay discount rate, describing how the subjective value of reward decreases as it is delayed. By analogy, selfishness can be formalized as a social discount rate, representing how the subjective value of rewarding another person decreases with increasing social distance. Delay and social discount rates for reward are correlated across individuals. However no previous work has examined whether this relationship also holds for aversive outcomes. Neither has previous work described a functional form for social discounting of pain in humans. This is a pertinent question, since preferences over aversive outcomes formally diverge from those for reward. We addressed this issue in an experiment in which healthy adult participants (N = 67) chose the timing and intensity of hypothetical pain for themselves and others. In keeping with previous studies, participants showed a strong preference for immediate over delayed pain. Participants showed greater concern for pain in close others than for their own pain, though this hyperaltruism was steeply discounted with increasing social distance. Impatience for pain and social discounting of pain were weakly correlated across individuals. Our results extend a link between impatience and selfishness to the aversive domain
- …