9,785 research outputs found
On the magnetic stability at the surface in strongly correlated electron systems
The stability of ferromagnetism at the surface at finite temperatures is
investigated within the strongly correlated Hubbard model on a semi-infinite
lattice. Due to the reduced surface coordination number the effective Coulomb
correlation is enhanced at the surface compared to the bulk. Therefore, within
the well-known Stoner-picture of band ferromagnetism one would expect the
magnetic stability at the surface to be enhanced as well. However, by taking
electron correlations into account well beyond the Hartree-Fock (Stoner) level
we find the opposite behavior: As a function of temperature the magnetization
of the surface layer decreases faster than in the bulk. By varying the hopping
integral within the surface layer this behavior becomes even more pronounced. A
reduced hopping integral at the surface tends to destabilize surface
ferromagnetism whereas the magnetic stability gets enhanced by an increased
hopping integral. This behavior represents a pure correlation effect and can be
understood in terms of general arguments which are based on exact results in
the limit of strong Coulomb interaction.Comment: 6 pages, RevTeX, 4 eps figures, accepted (Phys. Rev. B), for related
work and info see http://orion.physik.hu-berlin.d
Numerical Simulation of Nanoscale Double-Gate MOSFETs
The further improvement of nanoscale electron devices requires support by numerical simulations within the design process. After a brief description of our SIMBA 2D/3D-device simulator, the results of the simulation of DG-MOSFETs are represented. Starting from a basic structure with a gate length of 30 nm, the model parameters were calibrated on the basis measured values from the literature. Afterwards variations in of gate length, channel thickness and doping, gate oxide parameters and source/drain doping were made in connection with numerical calculation of the device characteristics. Then a DG-MOSFET with a gate length of 15 nm was optimized. The optimized structure shows suppressed short channel behavior and short switching times of about 0.15 ps.
Sub-milliKelvin spatial thermometry of a single Doppler cooled ion in a Paul trap
We report on observations of thermal motion of a single, Doppler-cooled ion
along the axis of a linear radio-frequency quadrupole trap. We show that for a
harmonic potential the thermal occupation of energy levels leads to Gaussian
distribution of the ion's axial position. The dependence of the spatial thermal
spread on the trap potential is used for precise calibration of our imaging
system's point spread function and sub-milliKelvin thermometry. We employ this
technique to investigate the laser detuning dependence of the Doppler
temperature.Comment: 5 pages, 4 figure
Precision spectroscopy of the 3s-3p fine structure doublet in Mg+
We apply a recently demonstrated method for precision spectroscopy on strong
transitions in trapped ions to measure both fine structure components of the
3s-3p transition in 24-Mg+ and 26-Mg+. We deduce absolute frequency reference
data for transition frequencies, isotope shifts and fine structure splittings
that are in particular useful for comparison with quasar absorption spectra,
which test possible space-time variations of the fine structure constant. The
measurement accuracy improves previous literature values, when existing, by
more than two orders of magnitude
Influence of uncorrelated overlayers on the magnetism in thin itinerant-electron films
The influence of uncorrelated (nonmagnetic) overlayers on the magnetic
properties of thin itinerant-electron films is investigated within the
single-band Hubbard model. The Coulomb correlation between the electrons in the
ferromagnetic layers is treated by using the spectral density approach (SDA).
It is found that the presence of nonmagnetic layers has a strong effect on the
magnetic properties of thin films. The Curie temperatures of very thin films
are modified by the uncorrelated overlayers. The quasiparticle density of
states is used to analyze the results. In addition, the coupling between the
ferromagnetic layers and the nonmagnetic layers is discussed in detail. The
coupling depends on the band occupation of the nonmagnetic layers, while it is
almost independent of the number of the nonmagnetic layers. The induced
polarization in the nonmagnetic layers shows a long-range decreasing
oscillatory behavior and it depends on the coupling between ferromagnetic and
nonmagnetic layers.Comment: 9 pages, RevTex, 6 figures, for related work see:
http://orion.physik.hu-berlin.d
Cluster counting: The Hoshen-Kopelman algorithm vs. spanning tree approaches
Two basic approaches to the cluster counting task in the percolation and
related models are discussed. The Hoshen-Kopelman multiple labeling technique
for cluster statistics is redescribed. Modifications for random and aperiodic
lattices are sketched as well as some parallelised versions of the algorithm
are mentioned. The graph-theoretical basis for the spanning tree approaches is
given by describing the "breadth-first search" and "depth-first search"
procedures. Examples are given for extracting the elastic and geometric
"backbone" of a percolation cluster. An implementation of the "pebble game"
algorithm using a depth-first search method is also described.Comment: LaTeX, uses ijmpc1.sty(included), 18 pages, 3 figures, submitted to
Intern. J. of Modern Physics
Poly-MTO, {(CH_3)_{0.92} Re O_3}_\infty, a Conducting Two-Dimensional Organometallic Oxide
Polymeric methyltrioxorhenium, {(CH_{3})_{0.92}ReO_{3}}_{\infty} (poly-MTO),
is the first member of a new class of organometallic hybrids which adopts the
structural pattern and physical properties of classical perovskites in two
dimensions (2D). We demonstrate how the electronic structure of poly-MTO can be
tailored by intercalation of organic donor molecules, such as
tetrathiafulvalene (TTF) or bis-(ethylendithio)-tetrathiafulvalene (BEDT-TTF),
and by the inorganic acceptor SbF. Integration of donor molecules leads to
a more insulating behavior of poly-MTO, whereas SbF insertion does not
cause any significant change in the resistivity. The resistivity data of pure
poly-MTO is remarkably well described by a two-dimensional electron system.
Below 38 K an unusual resistivity behavior, similar to that found in doped
cuprates, is observed: The resistivity initially increases approximately as
ln) before it changes into a dependence below 2 K.
As an explanation we suggest a crossover from purely two-dimensional
charge-carrier diffusion within the \{ReO\} planes at high
temperatures to three-dimensional diffusion at low temperatures in a
disorder-enhanced electron-electron interaction scenario (Altshuler-Aronov
correction). Furthermore, a linear positive magnetoresistance was found in the
insulating regime, which is caused by spatial localization of itinerant
electrons at some of the Re atoms, which formally adopt a electronic
configuration. X-ray diffraction, IR- and ESR-studies, temperature dependent
magnetization and specific heat measurements in various magnetic fields suggest
that the electronic structure of poly-MTO can safely be approximated by a
purely 2D conductor.Comment: 15 pages, 16 figures, 2 table
Purification of Single-photon Entanglement
Single-photon entanglement is a simple form of entanglement that exists
between two spatial modes sharing a single photon. Despite its elementary form,
it provides a resource as useful as polarization-entangled photons and it can
be used for quantum teleportation and entanglement swapping operations. Here,
we report the first experiment where single-photon entanglement is purified
with a simple linear-optics based protocol. Besides its conceptual interest,
this result might find applications in long distance quantum communication
based on quantum repeaters.Comment: Main article: 5 pages, 4 figure
Jump at the onset of saltation
We reveal a discontinuous transition in the saturated flux for aeolian
saltation by simulating explicitly particle motion in turbulent flow. The
discontinuity is followed by a coexistence interval with two metastable
solutions. The modification of the wind profile due to momentum exchange
exhibits a second maximum at high shear strength. The saturated flux depends on
the strength of the wind as
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