3,108 research outputs found
Superconducting states of pure and doped graphene
We study the superconducting phases of the two-dimensional honeycomb lattice
of graphene. We find two spin singlet pairing states, s-wave and an exotic
that is possible because of the special structure of the honeycomb
lattice. At half filling, the phase is gapless and superconductivity is
a hidden order. We discuss the possibility of a superconducting state in metal
coated graphene.Comment: 4 pages, 6 figure
Research in orbit determination optimization for space trajectories
Research data covering orbit determination, optimization techniques, and trajectory design for manned space flights are summarized
Low thrust interplanetary trajectory open loop error analysis, volume 1 Final report
Computer program for open-loop error analysis of low thrust interplanetary trajectorie
Nonlinear and adaptive estimation techniques in reentry
The development and testing of nonlinear and adaptive estimators for reentry (e.g. space shuttle) navigation and model parameter estimation or identification are reported. Of particular interest is the identifcation of vehicle lift and drag characteristics in real time. Several nonlinear filters were developed and simulated. Adaptive filters for the real time identification of vehicle lift and drag characteristics, and unmodelable acceleration, were also developed and tested by simulation. The simulations feature an uncertain system environment with rather arbitrary model errors, thus providing a definitive test of estimator performance. It was found that nonlinear effects are indeed significant in reentry trajectory estimation and a nonlinear filter is demonstrated which successfully tracks through nonlinearities without degrading the information content of the data. Under the same conditions the usual extended Kalman filter diverges and is useless. The J-adaptive filter is shown to successfully track errors in the modeled vehicle lift and drag characteristics. The same filter concept is also shown to track successfully through rather arbitrary model errors, including lift and drag errors, vehicle mass errors, atmospheric density errors, and wind gust errors
Stability of the shell structure in 2D quantum dots
We study the effects of external impurities on the shell structure in
semiconductor quantum dots by using a fast response-function method for solving
the Kohn-Sham equations. We perform statistics of the addition energies up to
20 interacting electrons. The results show that the shell structure is
generally preserved even if effects of high disorder are clear. The Coulomb
interaction and the variation in ground-state spins have a strong effect on the
addition-energy distributions, which in the noninteracting single-electron
picture correspond to level statistics showing mixtures of Poisson and Wigner
forms.Comment: 7 pages, 8 figures, submitted to Phys. Rev.
Non-Adiabatic Spin Transfer Torque in Real Materials
The motion of simple domain walls and of more complex magnetic textures in
the presence of a transport current is described by the
Landau-Lifshitz-Slonczewski (LLS) equations. Predictions of the LLS equations
depend sensitively on the ratio between the dimensionless material parameter
which characterizes non-adiabatic spin-transfer torques and the Gilbert
damping parameter . This ratio has been variously estimated to be close
to 0, close to 1, and large compared to 1. By identifying as the
influence of a transport current on , we derive a concise, explicit and
relatively simple expression which relates to the band structure and
Bloch state lifetimes of a magnetic metal. Using this expression we demonstrate
that intrinsic spin-orbit interactions lead to intra-band contributions to
which are often dominant and can be (i) estimated with some confidence
and (ii) interpreted using the "breathing Fermi surface" model.Comment: 18 pages, 9 figures; submitted to Phys. Rev.
Dynamics of compressible edge and bosonization
We work out the dynamics of the compressible edge of the quantum Hall system
based on the electrostatic model of Chklovskii et al.. We introduce a
generalized version of Wen's hydrodynamic quantization approach to the dynamics
of sharp edge and rederive Aleiner and Glazman's earlier result of multiple
density modes. Bosonic operators of density excitations are used to construct
fermions at the interface of the compressible and incompressible region. We
also analyze the dynamics starting with the second-quantized Hamiltonian in the
lowest Landau level and work out the time development of density operators.
Contrary to the hydrodynamic results, the density modes are strongly coupled.
We argue that the coupling suppresses the propagation of all acoustic modes,
and that the excitations with large wavevectors are subject to decay due to
coupling to the dissipative acoustic modes.A possible correction to the
tunneling density of states is discussed.Comment: 7 pages, Revtex, 1 figur
Valley dependent many-body effects in 2D semiconductors
We calculate the valley degeneracy () dependence of the many-body
renormalization of quasiparticle properties in multivalley 2D semiconductor
structures due to the Coulomb interaction between the carriers. Quite
unexpectedly, the dependence of many-body effects is nontrivial and
non-generic, and depends qualitatively on the specific Fermi liquid property
under consideration. While the interacting 2D compressibility manifests
monotonically increasing many-body renormalization with increasing , the
2D spin susceptibility exhibits an interesting non-monotonic dependence
with the susceptibility increasing (decreasing) with for smaller (larger)
values of with the renormalization effect peaking around .
Our theoretical results provide a clear conceptual understanding of recent
valley-dependent 2D susceptibility measurements in AlAs quantum wells.Comment: 5 pages, 3 figure
Comment on "Diffusion Monte Carlo study of jellium surfaces: Electronic densities and pair correlation functions"
In a fixed-node diffusion Monte Carlo calculation of the total energy of
jellium slabs, Acioli and Ceperley [Phys. Rev. B {\bf 54}, 17199 (1996)]
reported jellium surface energies that at low electron densities were
significantly higher than those predicted in the local-density approximation
(LDA) of density-functional theory. Assuming that the fixed-node error in the
slab and the bulk calculations cancel out, we show that their data yield
surface energies that are considerably closer to the LDA and in reasonable
agreement with those obtained in the random-phase approximation.Comment: 3 pages, 2 figures, to appear in Phys. Rev.
Zero temperature optical conductivity of ultra-clean Fermi liquids and superconductors
We calculate the low-frequency optical conductivity sigma(w) of clean metals
and superconductors at zero temperature neglecting the effects of impurities
and phonons. In general, the frequency and temperature dependences of sigma
have very little in common. For small Fermi surfaces in three dimensions (but
not in 2D) we find for example that Re sigma(w>0)=const. for low w which
corresponds to a scattering rate Gamma proportional to w^2 even in the absence
of Umklapp scattering when there is no T^2 contribution to Gamma. In the main
part of the paper we discuss in detail the optical conductivity of d-wave
superconductors in 2D where Re sigma(w>0) \propto w^4 for the smallest
frequencies and the Umklapp processes typically set in smoothly above a finite
threshold w_0 smaller than twice the maximal gap Delta. In cases where the
nodes are located at (pi/2, pi/2), such that direct Umklapp scattering among
them is possible, one obtains Re sigma(w) \propto w^2.Comment: 7 pages, 3 figure
- …