24,414 research outputs found
Systematic Mapping of the Hubbard Model to the Generalized t-J Model
The generalized t-J model conserving the number of double occupancies is
constructed from the Hubbard model at and in the vicinity of half-filling at
strong coupling. The construction is realized by a self-similar continuous
unitary transformation. The flow equation is closed by a truncation scheme
based on the spatial range of processes. We analyze the conditions under which
the t-J model can be set up and we find that it can only be defined for
sufficiently large interaction. There, the parameters of the effective model
are determined.Comment: 16 pages, 13 figures included. v2: Order of sections changed.
Calculation and discussion of apparent gap in Section IV.A correcte
Synchronous Phase Shift at LHC
The electron cloud in vacuum pipes of accelerators of positively charged
particle beams causes a beam energy loss which could be estimated from the
synchronous phase. Measurements done with beams of 75 ns, 50 ns, and 25 ns
bunch spacing in the LHC for some fills in 2010 and 2011 show that the average
energy loss depends on the total beam intensity in the ring. Later measurements
during the scrubbing run with 50 ns beams show the reduction of the electron
cloud due to scrubbing. Finally, measurements of the individual bunch phase
give us information about the electron cloud build-up inside the batch and from
batch to batch.Comment: Presented at ECLOUD'12: Joint INFN-CERN-EuCARD-AccNet Workshop on
Electron-Cloud Effects, La Biodola, Isola d'Elba, Italy, 5-9 June 201
Distinct magnetotransport and orbital fingerprints of chiral bobbers
While chiral magnetic skyrmions have been attracting significant attention in
the past years, recently, a new type of a chiral particle emerging in thin
films a chiral bobber has been theoretically predicted and
experimentally observed. Here, based on theoretical arguments, we provide a
clear pathway to utilizing chiral bobbers for the purposes of future
spintronics by uncovering that these novel chiral states possess inherent
transport fingerprints that allow for their unambiguous electrical detection in
systems comprising several types of chiral states. We reveal that unique
transport and orbital characteristics of bobbers root in the non-trivial
magnetization distribution in the vicinity of the Bloch points, and demonstrate
that tuning the details of the Bloch point topology can be used to drastically
alter the emergent response properties of chiral bobbers to external fields,
which bears great potential for engineering chiral dynamics and cognitive
computing.Comment: Supplementary available upon reques
Theoretical Analysis of a Large Momentum Beamsplitter using Bloch Oscillations
In this paper, we present the implementation of Bloch oscillations in an
atomic interferometer to increase the separation of the two interfering paths.
A numerical model, in very good agreement with the experiment, is developed.
The contrast of the interferometer and its sensitivity to phase fluctuations
and to intensity fluctuations are also calculated. We demonstrate that the
sensitivity to phase fluctuations can be significantly reduced by using a
suitable arrangement of Bloch oscillations pulses
Influence of external flows on crystal growth: numerical investigation
We use a combined phase-field/lattice-Boltzmann scheme [D. Medvedev, K.
Kassner, Phys. Rev. E {\bf 72}, 056703 (2005)] to simulate non-facetted crystal
growth from an undercooled melt in external flows. Selected growth parameters
are determined numerically.
For growth patterns at moderate to high undercooling and relatively large
anisotropy, the values of the tip radius and selection parameter plotted as a
function of the Peclet number fall approximately on single curves. Hence, it
may be argued that a parallel flow changes the selected tip radius and growth
velocity solely by modifying (increasing) the Peclet number. This has
interesting implications for the availability of current selection theories as
predictors of growth characteristics under flow.
At smaller anisotropy, a modification of the morphology diagram in the plane
undercooling versus anisotropy is observed. The transition line from dendrites
to doublons is shifted in favour of dendritic patterns, which become faster
than doublons as the flow speed is increased, thus rendering the basin of
attraction of dendritic structures larger.
For small anisotropy and Prandtl number, we find oscillations of the tip
velocity in the presence of flow. On increasing the fluid viscosity or
decreasing the flow velocity, we observe a reduction in the amplitude of these
oscillations.Comment: 10 pages, 7 figures, accepted for Physical Review E; size of some
images had to be substantially reduced in comparison to original, resulting
in low qualit
Anisotropic thermal expansion and magnetostriction of YNiBC single crystals
We present results of anisotropic thermal expansion and low temperature
magnetostriction measurements on YNiBC single crystals grown by high
temperature flux and floating zone techniques. Quantum oscillations of
magnetostriction were observed at low temperatures for starting at
fields significantly below (). Large irreversible,
longitudinal magnetostriction was seen in both, in-plane and along the c-axis,
directions of the applied magnetic field in the intermediate superconducting
state. Anisotropic uniaxial pressure dependencies of were evaluated using
results of zero field, thermal expansion measurements
Bound-to-bound and bound-to-continuum optical transitions in combined quantum dot - superlattice systems
By combining band gap engineering with the self-organized growth of quantum
dots, we present a scheme of adjusting the mid-infrared absorption properties
to desired energy transitions in quantum dot based photodetectors. Embedding
the self organized InAs quantum dots into an AlAs/GaAs superlattice enables us
to tune the optical transition energy by changing the superlattice period as
well as by changing the growth conditions of the dots. Using a one band
envelope function framework we are able, in a fully three dimensional
calculation, to predict the photocurrent spectra of these devices as well as
their polarization properties. The calculations further predict a strong impact
of the dots on the superlattices minibands. The impact of vertical dot
alignment or misalignment on the absorption properties of this dot/superlattice
structure is investigated. The observed photocurrent spectra of vertically
coupled quantum dot stacks show very good agreement with the calculations.In
these experiments, vertically coupled quantum dot stacks show the best
performance in the desired photodetector application.Comment: 8 pages, 10 figures, submitted to PR
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