940 research outputs found
Bistability and oscillatory motion of natural nano-membranes appearing within monolayer graphene on silicon dioxide
The recently found material graphene is a truly two-dimensional crystal and
exhibits, in addition, an extreme mechanical strength. This in combination with
the high electron mobility favours graphene for electromechanical
investigations down to the quantum limit. Here, we show that a monolayer of
graphene on SiO2 provides natural, ultra-small membranes of diameters down to 3
nm, which are caused by the intrinsic rippling of the material. Some of these
nano-membranes can be switched hysteretically between two vertical positions
using the electric field of the tip of a scanning tunnelling microscope (STM).
They can also be forced to oscillatory motion by a low frequency ac-field.
Using the mechanical constants determined previously, we estimate a high
resonance frequency up to 0.4 THz. This might be favorable for
quantum-electromechanics and is prospective for single atom mass spectrometers.Comment: 9 pages, 4 figure
Nonintegrability of the two-body problem in constant curvature spaces
We consider the reduced two-body problem with the Newton and the oscillator
potentials on the sphere and the hyperbolic plane .
For both types of interaction we prove the nonexistence of an additional
meromorphic integral for the complexified dynamic systems.Comment: 20 pages, typos correcte
Dynamical response of the nuclear pasta in neutron star crusts
The nuclear pasta -- a novel state of matter having nucleons arranged in a
variety of complex shapes -- is expected to be found in the crust of neutron
stars and in core-collapse supernovae at subnuclear densities of about
g/cm. Due to frustration, a phenomenon that emerges from the
competition between short-range nuclear attraction and long-range Coulomb
repulsion, the nuclear pasta displays a preponderance of unique low-energy
excitations. These excitations could have a strong impact on many transport
properties, such as neutrino propagation through stellar environments. The
excitation spectrum of the nuclear pasta is computed via a molecular-dynamics
simulation involving up to 100,000 nucleons. The dynamic response of the pasta
displays a classical plasma oscillation in the 1-2 MeV region. In addition,
substantial strength is found at low energies. Yet this low-energy strength is
missing from a simple ion model containing a single-representative heavy
nucleus. The low-energy strength observed in the dynamic response of the pasta
is likely to be a density wave involving the internal degrees of freedom of the
clusters.Comment: 4 pages, 3 figures, Phys Rev C in pres
Density Matrix Renormalization Group Study of the Disorder Line in the Quantum ANNNI Model
We apply Density Matrix Renormalization Group methods to study the phase
diagram of the quantum ANNNI model in the region of low frustration where the
ferromagnetic coupling is larger than the next-nearest-neighbor
antiferromagnetic one. By Finite Size Scaling on lattices with up to 80 sites
we locate precisely the transition line from the ferromagnetic phase to a
paramagnetic phase without spatial modulation. We then measure and analyze the
spin-spin correlation function in order to determine the disorder transition
line where a modulation appears. We give strong numerical support to the
conjecture that the Peschel-Emery one-dimensional line actually coincides with
the disorder line. We also show that the critical exponent governing the
vanishing of the modulation parameter at the disorder transition is .Comment: 4 pages, 5 eps figure
Two-dimensional charge order in layered 2-1-4 perovskite oxides
Monte Carlo simulations are performed on the three-dimensional (3D) Ising
model with the 2-1-4 layered perovskite structure as a minimal model for
checkerboard charge ordering phenomena in layered perovskite oxides. Due to the
interlayer frustration, only 2D long-range order emerges with a finite
correlation length along the c axis. Critical exponents of the transition
change continuously as a function of the interlayer coupling constant. The
interlayer long-range Coulomb interaction decays exponentially and is
negligible even between the second-neighbor layers. Instead, monoclinic
distortion of a tetragonal unit cell lifts the macroscopic degeneracy to induce
a 3D charge ordering. The dimensionality of the charge order in
LaSrMnO is discussed from this viewpoint.Comment: 5 pages including 6 figures, with major changes including discussion
on charge ordering phenomena in layered perovskite oxide
Probing two topological surface bands of Sb2Te3 by spin-polarized photoemission spectroscopy
Using high resolution spin- and angle-resolved photoemission spectroscopy, we
map the electronic structure and spin texture of the surface states of the
topological insulator Sb2Te3. In combination with density functional
calculations (DFT), we directly show that Sb2Te3 exhibits a partially occupied,
single spin-Dirac cone around the Fermi energy, which is topologically
protected. DFT obtains a spin polarization of the occupied Dirac cone states of
80-90%, which is in reasonable agreement with the experimental data after
careful background subtraction. Furthermore, we observe a strongly spin-orbit
split surface band at lower energy. This state is found at 0.8eV below the
Fermi level at the gamma-point, disperses upwards, and disappears at about
0.4eV below the Fermi level into two different bulk bands. Along the gamma-K
direction, the band is located within a spin-orbit gap. According to an
argument given by Pendry and Gurman in 1975, such a gap must contain a surface
state, if it is located away from the high symmetry points of the Brillouin
zone. Thus, the novel spin-split state is protected by symmetry, too.Comment: 8 pages, 10 figure
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Seasonal cycle of precipitation variability in South America on intraseasonal timescales
The seasonal cycle of the intraseasonal (IS) variability of precipitation in South America is described through the analysis of bandpass filtered outgoing longwave radiation (OLR) anomalies. The analysis is discriminated between short (10--30 days) and long (30--90 days) intraseasonal timescales. The seasonal cycle of the 30--90-day IS variability can be well described by the activity of first leading pattern (EOF1) computed separately for the wet season (October--April) and the dry season (May--September). In agreement with previous works, the EOF1 spatial distribution during the wet season is that of a dipole with centers of actions in the South Atlantic Convergence Zone (SACZ) and southeastern South America (SESA), while during the dry season, only the last center is discernible. In both seasons, the pattern is highly influenced by the activity of the Madden--Julian Oscillation (MJO). Moreover, EOF1 is related with a tropical zonal-wavenumber-1 structure superposed with coherent wave trains extended along the South Pacific during the wet season, while during the dry season the wavenumber-1 structure is not observed. The 10--30-day IS variability of OLR in South America can be well represented by the activity of the EOF1 computed through considering all seasons together, a dipole but with the stronger center located over SESA. While the convection activity at the tropical band does not seem to influence its activity, there are evidences that the atmospheric variability at subtropical-extratropical regions might have a role. Subpolar wavetrains are observed in the Pacific throughout the year and less intense during DJF, while a path of wave energy dispersion along a subtropical wavetrain also characterizes the other seasons. Further work is needed to identify the sources of the 10--30-day-IS variability in South America
Wave function mapping in graphene quantum dots with soft confinement
Using low-temperature scanning tunneling spectroscopy, we map the local
density of states (LDOS) of graphene quantum dots supported on Ir(111). Due to
a band gap in the projected Ir band structure around the graphene K point, the
electronic properties of the QDs are dominantly graphene-like. Indeed, we
compare the results favorably with tight binding calculations on the honeycomb
lattice based on parameters derived from density functional theory. We find
that the interaction with the substrate near the edge of the island gradually
opens a gap in the Dirac cone, which implies soft-wall confinement.
Interestingly, this confinement results in highly symmetric wave functions.
Further influences of the substrate are given by the known moir{\'e} potential
and a 10% penetration of an Ir surface resonanceComment: 7 pages, 11 figures, DFT calculations directly showing the origin of
soft confinment, correct identification of the state penetrating from Ir(111)
into graphen
Power-law spin correlations in pyrochlore antiferromagnets
The ground state ensemble of the highly frustrated pyrochlore-lattice
antiferromagnet can be mapped to a coarse-grained ``polarization'' field
satisfying a zero-divergence condition From this it follows that the
correlations of this field, as well as the actual spin correlations, decay with
separation like a dipole-dipole interaction (). Furthermore, a lattice
version of the derivation gives an approximate formula for spin correlations,
with several features that agree well with simulations and neutron-diffraction
measurements of diffuse scattering, in particular the pinch-point
(pseudo-dipolar) singularities at reciprocal lattice vectors. This system is
compared to others in which constraints also imply diffraction singularities,
and other possible applications of the coarse-grained polarization are
discussed.Comment: 13 pp, revtex, two figure
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