4,131 research outputs found
Deconfinement transition and Luttinger to Fermi Liquid crossover in quasi one-dimensional systems
We investigate a system of one dimensional Hubbard chains of interacting
fermions coupled by inter-chain hopping. Using a generalization of the
Dynamical Mean Field Theory we study the deconfinement transition from a Mott
insulator to a metal and the crossover between Luttinger and Fermi liquid
phases. One-particle properties, local spin response and inter-chain optical
conductivity are calculated. Possible applications to organic conductors are
discussed.Comment: 5 page
The one-round Voronoi game replayed
We consider the one-round Voronoi game, where player one (``White'', called
``Wilma'') places a set of n points in a rectangular area of aspect ratio r
<=1, followed by the second player (``Black'', called ``Barney''), who places
the same number of points. Each player wins the fraction of the board closest
to one of his points, and the goal is to win more than half of the total area.
This problem has been studied by Cheong et al., who showed that for large
enough and r=1, Barney has a strategy that guarantees a fraction of 1/2+a,
for some small fixed a.
We resolve a number of open problems raised by that paper. In particular, we
give a precise characterization of the outcome of the game for optimal play: We
show that Barney has a winning strategy for n>2 and r>sqrt{2}/n, and for n=2
and r>sqrt{3}/2. Wilma wins in all remaining cases, i.e., for n>=3 and
r<=sqrt{2}/n, for n=2 and r<=sqrt{3}/2, and for n=1. We also discuss complexity
aspects of the game on more general boards, by proving that for a polygon with
holes, it is NP-hard to maximize the area Barney can win against a given set of
points by Wilma.Comment: 14 pages, 6 figures, Latex; revised for journal version, to appear in
Computational Geometry: Theory and Applications. Extended abstract version
appeared in Workshop on Algorithms and Data Structures, Springer Lecture
Notes in Computer Science, vol.2748, 2003, pp. 150-16
Adhesion and electronic structure of graphene on hexagonal boron nitride substrates
We investigate the adsorption of graphene sheets on h-BN substrates by means
of first-principles calculations in the framework of adiabatic connection
fluctuation-dissipation theory in the random phase approximation. We obtain
adhesion energies for different crystallographic stacking configurations and
show that the interlayer bonding is due to long-range van der Waals forces. The
interplay of elastic and adhesion energies is shown to lead to stacking
disorder and moir\'e structures. Band structure calculations reveal substrate
induced mass terms in graphene which change their sign with the stacking
configuration. The dispersion, absolute band gaps and the real space shape of
the low energy electronic states in the moir\'e structures are discussed. We
find that the absolute band gaps in the moir\'e structures are at least an
order of magnitude smaller than the maximum local values of the mass term. Our
results are in agreement with recent STM experiments.Comment: 8 pages, 8 figures, revised and extended version, to appear in Phys.
Rev.
Probing of valley polarization in graphene via optical second-harmonic generation
Valley polarization in graphene breaks inversion symmetry and therefore leads
to second-harmonic generation. We present a complete theory of this effect
within a single-particle approximation. It is shown that this may be a
sensitive tool to measure the valley polarization created, e.g., by polarized
light and, thus, can be used for a development of ultrafast valleytronics in
graphene.Comment: 5 pages, 3 figure
Local impurity effects in superconducting graphene
We study the effect of impurities in superconducting graphene and discuss
their influence on the local electronic properties. In particular, we consider
the case of magnetic and non-magnetic impurities being either strongly
localized or acting as a potential averaged over one unit cell. The spin
dependent local density of states is calculated and possibilities for
visualizing impurities by means of scanning tunneling experiments is pointed
out. A possibility of identifying magnetic scatters even by non spin-polarized
scanning tunneling spectroscopy is explained.Comment: 4 pages, 4 figure
Correlation effects on the electronic structure of TiOCl: a NMTO+DMFT study
Using the recently developed N-th order muffin-tin orbital-based downfolding
technique in combination with the Dynamical Mean Field theory, we investigate
the electronic properties of the much discussed Mott insulator TiOCl in the
undimerized phase. Inclusion of correlation effects through this approach
provides a description of the spectral function into an upper and a lower
Hubbard band with broad valence states formed out of the orbitally polarized,
lower Hubbard band. We find that these results are in good agreement with
recent photo-emission spectra.Comment: 4 pages, 3 figure
Development and validation of the crew-station system-integration research facility
The various issues associated with the use of integrated flight management systems in aircraft were discussed. To address these issues a fixed base integrated flight research (IFR) simulation of a helicopter was developed to support experiments that contribute to the understanding of design criteria for rotorcraft cockpits incorporating advanced integrated flight management systems. A validation experiment was conducted that demonstrates the main features of the facility and the capability to conduct crew/system integration research
Controlling the Kondo Effect in CoCu_n Clusters Atom by Atom
Clusters containing a single magnetic impurity were investigated by scanning
tunneling microscopy, spectroscopy, and ab initio electronic structure
calculations. The Kondo temperature of a Co atom embedded in Cu clusters on
Cu(111) exhibits a non-monotonic variation with the cluster size. Calculations
model the experimental observations and demonstrate the importance of the local
and anisotropic electronic structure for correlation effects in small clusters.Comment: 4 pages, 4 figure
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