171 research outputs found
Learning Efficient Navigation in Vortical Flow Fields
Efficient point-to-point navigation in the presence of a background flow
field is important for robotic applications such as ocean surveying. In such
applications, robots may only have knowledge of their immediate surroundings or
be faced with time-varying currents, which limits the use of optimal control
techniques for planning trajectories. Here, we apply a novel Reinforcement
Learning algorithm to discover time-efficient navigation policies to steer a
fixed-speed swimmer through an unsteady two-dimensional flow field. The
algorithm entails inputting environmental cues into a deep neural network that
determines the swimmer's actions, and deploying Remember and Forget Experience
replay. We find that the resulting swimmers successfully exploit the background
flow to reach the target, but that this success depends on the type of sensed
environmental cue. Surprisingly, a velocity sensing approach outperformed a
bio-mimetic vorticity sensing approach by nearly two-fold in success rate.
Equipped with local velocity measurements, the reinforcement learning algorithm
achieved near 100% success in reaching the target locations while approaching
the time-efficiency of paths found by a global optimal control planner.Comment: 6 pages, 6 figure
Nonlinear effects in E Jahn-Teller model: Variational approach with excited phonon states and mode correlations
Interplay of nonlinear and quantum effects in the ground state of the
E Jahn-Teller model was investigated by the {\it variational
approach and exact numerical simulations}. They result in the recognition of
(i) importance of the admixture of {\it the first excited state of the
displaced harmonic oscillator} of the symmetric phonon mode in the ground state
of the system in the selftrapping-dominated regime; (ii) existence of {\it the
region of localized -undisplaced oscillator states} in the
tunneling-dominated regime. The effect (i) occurs owing to significant decrease
of the ground state energy on account of the overlapping contribution of the
symmetric phonon mode between the states of the same parity. This contribution
considerably improves variational results especially in the
selftrapping-dominated regime. Close to the Ee limit, the nonlinear
effects of {\it two-mode correlations} turn to be effective due to the
rotational symmetry of this case. In the tunneling-dominated regime the phonon
wave functions behave like the strongly localized harmonic oscillator ground
state and the effect (i) looses its significance.Comment: 28 pages,6 figure
Electronic structure, magnetism and superconductivity of MgCNi
The electronic structure of the newly discovered superconducting perovskite
MgCNi is calculated using the LMTO and KKR methods. The states near the
Fermi energy are found to be dominated by Ni-d. The Stoner factor is low while
the electron-phonon coupling constant is estimated to be about 0.7, which
suggests that the material is a conventional type of superconductor where T
is not affected by magnetic interactions. However, the proximity of the Fermi
energy to a large peak in the density of states in conjunction with the
reported non-stoichiometry of the compound, has consequences for the stability
of the results.Comment: 3 pages, 4 figure
Nesting properties and anomalous band effect in MgB2
First principle FLAPW band calculations of the new superconductor MgB2 were
performed and the polarization function P12(Q) between the two p-bands mainly
formed of boron pz-orbital was calculated. We found that P12(Q) is
substantially enhanced around Q=(0,0,p/c), which supports the two-band
mechanism of superconductivity for MgB2. P12(Q) peaks at Qz ~ 0.3(2p/c) and Qz
\~ 0.5(2p/c). These two peaks are related to the nesting of these Fermi
surfaces, but significantly deviates from the position expected from the
simplest tight-binding bands for the p-bands. From the calculations for
different lattice parameters, we have found significant dependences on the
isotopic species of B and on the pressure effect of the polarization function
in accordance with the respective changes of Tc in the above-mentioned
framework.Comment: 15 pages, 7 graphs. to be published in J. Phys. Soc. Jpn. 70_, No.
Nonradiative Electronic Deexcitation Time Scales in Metal Clusters
The life-times due to Auger-electron emission for a hole on a deep electronic
shell of neutral and charged sodium clusters are studied for different sizes.
We consider spherical clusters and calculate the Auger-transition probabilities
using the energy levels and wave functions calculated in the
Local-Density-Approximation (LDA).
We obtain that Auger emission processes are energetically not allowed for
neutral and positively charged sodium clusters. In general, the Auger
probabilities in small Na clusters are remarkably different from the
atomic ones and exhibit a rich size dependence.
The Auger decay times of most of the cluster sizes studied are orders of
magnitude larger than in atoms and might be comparable with typical
fragmentation times.Comment: 11 pages, 4 figures. Accepted for publication in Phys. Rev.
Electronic structure of undoped and potassium doped coronene investigated by electron energy-loss spectroscopy
We performed electron energy-loss spectroscopy studies in transmission in
order to obtain insight into the electronic properties of potassium
intercalated coronene, a recently discovered superconductor with a rather high
transition temperature of about 15\,K. A comparison of the loss function of
undoped and potassium intercalated coronene shows the appearance of several new
peaks in the optical gap upon potassium addition. Furthermore, our core level
excitation data clearly signal filling of the conduction bands with electrons.Comment: 15 pages, 5 figures. arXiv admin note: substantial text overlap with
arXiv:1102.328
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
Magnetic Ordering and Superconductivity in the REIrGe (RE = Y, La-Tm, Lu) System
We find that the compounds for RE = Y, La-Dy, crystallize in the tetragonal
Ibam (UCoSi type) structure whereas the compounds for RE = Er-Lu,
crystallize in a new orthorhombic structure with a space group Pmmn. Samples of
HoIrGe were always found to be multiphase. The compounds for RE = Y
to Dy which adopt the Ibam type structure show a metallic resistivity whereas
the compounds with RE = Er, Tm and Lu show an anomalous behavior in the
resistivity with a semiconducting increase in as we go down in
temperature from 300K. Interestingly we had earlier found a positive
temperature coefficient of resistivity for the Yb sample in the same
temperature range. We will compare this behavior with similar observations in
the compounds RERuGe and REBiPt. LaIrGe and
YIrGe show bulk superconductivity below 1.8K and 2.5K respectively.
Our results confirm that CeIrGe shows a Kondo lattice behavior and
undergoes antiferromagnetic ordering below 8.5K. Most of the other compounds
containing magnetic rare-earth elements undergo a single antiferromagnetic
transition at low temperatures (T12K) while GdIrGe,
DyIrGe and NdIrGe show multiple transitions. The
T's for most of the compounds roughly scale with the de Gennes factor.
which suggests that the chief mechanism of interaction leading to the magnetic
ordering of the magnetic moments may be the RKKY interaction.Comment: 25 pages, 16 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
Arbitrary Choice of Basic Variables in Density Functional Theory. II. Illustrative Applications
Our recent theory (Ref. 1) enables us to choose arbitrary quantities as the
basic variables of the density functional theory. In this paper we apply it to
several cases. In the case where the occupation matrix of localized orbitals is
chosen as a basic variable, we can obtain the single-particle equation which is
equivalent to that of the LDA+U method. The theory also leads to the
Hartree-Fock-Kohn-Sham equation by letting the exchange energy be a basic
variable. Furthermore, if the quantity associated with the density of states
near the Fermi level is chosen as a basic variable, the resulting
single-particle equation includes the additional potential which could mainly
modify the energy-band structures near the Fermi level.Comment: 27 page
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