39,028 research outputs found
Assessment of density-functional approximations: Long-range correlations and self-interaction effects
The complex nature of electron-electron correlations is made manifest in the very simple but nontrivial problem of two electrons confined within a sphere. The description of highly nonlocal correlation and self-interaction effects by widely used local and semilocal exchange-correlation energy density functionals is shown to be unsatisfactory in most cases. Even the best such functionals exhibit significant errors in the Kohn-Sham potentials and density profiles
Quantum conductance of homogeneous and inhomogeneous interacting electron systems
We obtain the conductance of a system of electrons connected to leads, within
time-dependent density-functional theory, using a direct relation between the
conductance and the density response function. Corrections to the
non-interacting conductance appear as a consequence of the functional form of
the exchange-correlation kernel at small frequencies and wavevectors. The
simple adiabatic local-density approximation and non-local density-terms in the
kernel both give rise to significant corrections in general. In the homogeneous
electron gas, the former correction remains significant, and leads to a failure
of linear-response theory for densities below a critical value.Comment: for resolution of the here published results see Phys. Rev. B 76,
125433 (2007
Comment on "Dynamical corrections to the DFT-LDA electron conductance in nanoscale systems"
In a recent paper Sai et al. [1] identified a correction R^{dyn}R=R_{s}+R^{dyn}R_{s}XCR^{dyn}$ in example systems of the type they considered should be considerably reduced, once a more appropriate form for the shear electron viscosity ¿ is used
The phenomenology of electric dipole moments in models of scalar leptoquarks
We study the phenomenology of electric dipole moments (EDMs) induced in
various scalar leptoquark models. We consider generic leptoquark couplings to
quarks and leptons and match to Standard Model effective field theory. After
evolving the resulting operators to low energies, we connect to EDM experiments
by using up-to-date hadronic, nuclear, and atomic matrix elements. We show that
current experimental limits set strong constraints on the possible CP-violating
phases in leptoquark models. Depending on the quarks and leptons involved in
the interaction, the existing searches for EDMs of leptons, nucleons, atoms,
and molecules all play a role in constraining the CP-violating couplings. We
discuss the impact of hadronic and nuclear uncertainties as well as the
sensitivities that can be achieved with future EDM experiments. Finally, we
study the impact of EDM constraints on a specific leptoquark model that can
explain the recent -physics anomalies.Comment: Published versio
Rotational dynamics of a superhelix towed in a Stokes fluid
Motivated by the intriguing motility of spirochetes (helically-shaped
bacteria that screw through viscous fluids due to the action of internal
periplasmic flagella), we examine the fundamental fluid dynamics of
superhelices translating and rotating in a Stokes fluid. A superhelical
structure may be thought of as a helix whose axial centerline is not straight,
but also a helix. We examine the particular case where these two superimposed
helices have different handedness, and employ a combination of experimental,
analytic, and computational methods to determine the rotational velocity of
superhelical bodies being towed through a very viscous fluid. We find that the
direction and rate of the rotation of the body is a result of competition
between the two superimposed helices; for small axial helix amplitude, the body
dynamics is controlled by the short-pitched helix, while there is a cross-over
at larger amplitude to control by the axial helix. We find far better, and
excellent, agreement of our experimental results with numerical computations
based upon the method of Regularized Stokeslets than upon the predictions of
classical resistive force theory
Transport in Graphene Tunnel Junctions
We present a technique to fabricate tunnel junctions between graphene and Al
and Cu, with a Si back gate, as well as a simple theory of tunneling between a
metal and graphene. We map the differential conductance of our junctions versus
probe and back gate voltage, and observe fluctuations in the conductance that
are directly related to the graphene density of states. The conventional
strong-suppression of the conductance at the graphene Dirac point can not be
clearly demonstrated, but a more robust signature of the Dirac point is found:
the inflection in the conductance map caused by the electrostatic gating of
graphene by the tunnel probe. We present numerical simulations of our
conductance maps, confirming the measurement results. In addition, Al causes
strong n-doping of graphene, Cu causes a moderate p-doping, and in high
resistance junctions, phonon resonances are observed, as in STM studies.Comment: 22 pages, 5 figure
Absorption and Emission in the non-Poisson case
This letter adresses the challenging problems posed to the Kubo-Anderson (KA)
theory by the discovery of intermittent resonant fluorescence with a
non-exponential distribution of waiting times. We show how to extend the KA
theory from aged to aging systems, aging for a very extended time period or
even forever, being a crucial consequence of non-Poisson statistics.Comment: 4 pages 3 figures. accepted for publication on Physical Review
Letter
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