627 research outputs found
Cavity-enhanced photoionization of an ultracold rubidium beam for application in focused ion beams
A two-step photoionization strategy of an ultracold rubidium beam for
application in a focused ion beam instrument is analyzed and implemented. In
this strategy the atomic beam is partly selected with an aperture after which
the transmitted atoms are ionized in the overlap of a tightly cylindrically
focused excitation laser beam and an ionization laser beam whose power is
enhanced in a build-up cavity. The advantage of this strategy, as compared to
without the use of a build-up cavity, is that higher ionization degrees can be
reached at higher currents. Optical Bloch equations including the
photoionization process are used to calculate what ionization degree and
ionization position distribution can be reached. Furthermore, the ionization
strategy is tested on an ultracold beam of Rb atoms. The beam current is
measured as a function of the excitation and ionization laser beam intensity
and the selection aperture size. Although details are different, the global
trends of the measurements agree well with the calculation. With a selection
aperture diameter of 52 m, a current of pA is
measured, which according to calculations is 63% of the current equivalent of
the transmitted atomic flux. Taking into account the ionization degree the ion
beam peak reduced brightness is estimated at A/(msreV).Comment: 13 pages, 9 figure
Direct magneto-optical compression of an effusive atomic beam for high-resolution focused ion beam application
An atomic rubidium beam formed in a 70 mm long two-dimensional
magneto-optical trap (2D MOT), directly loaded from a collimated Knudsen
source, is analyzed using laser-induced fluorescence. The longitudinal velocity
distribution, the transverse temperature and the flux of the atomic beam are
reported. The equivalent transverse reduced brightness of an ion beam with
similar properties as the atomic beam is calculated because the beam is
developed to be photoionized and applied in a focused ion beam. In a single
two-dimensional magneto-optical trapping step an equivalent transverse reduced
brightness of A/(m sr eV) was
achieved with a beam flux equivalent to nA. The
temperature of the beam is further reduced with an optical molasses after the
2D MOT. This increased the equivalent brightness to A/(m sr eV). For currents below 10 pA, for which disorder-induced
heating can be suppressed, this number is also a good estimate of the ion beam
brightness that can be expected. Such an ion beam brightness would be a six
times improvement over the liquid metal ion source and could improve the
resolution in focused ion beam nanofabrication.Comment: 10 pages, 8 figures, 1 tabl
Stripes and spin-incommensurabilities are favored by lattice anisotropies
Structural distortions in cuprate materials give a natural origin for
anisotropies in electron properties. We study a modified one-band t-J model in
which we allow for different hoppings and antiferromagnetic couplings in the
two spatial directions ( and ). Incommensurate peaks
in the spin structure factor show up only in the presence of a lattice
anisotropy, whereas charge correlations, indicating enhanced fluctuations at
incommensurate wave vectors, are almost unaffected with respect to the
isotropic case.Comment: accepted for publication on Physical Review Letters, one color figur
Optimization of Gutzwiller Wavefunctions in Quantum Monte Carlo
Gutzwiller functions are popular variational wavefunctions for correlated
electrons in Hubbard models. Following the variational principle, we are
interested in the Gutzwiller parameters that minimize e.g. the expectation
value of the energy. Rewriting the expectation value as a rational function in
the Gutzwiller parameters, we find a very efficient way for performing that
minimization. The method can be used to optimize general Gutzwiller-type
wavefunctions both, in variational and in fixed-node diffusion Monte Carlo.Comment: 9 pages RevTeX with 10 eps figure
Helicity Modulus and Effective Hopping in the Two-Dimensional Hubbard Model Using Slave-Boson Methods
The slave-boson mean-field method is used to study the two-dimensional
Hubbard model. A magnetic phase diagram allowing for paramagnetism, weak- and
strong ferromagnetism and antiferromagnetism, including all continuous and
first-order transitions, is constructed and compared to the corresponding phase
diagram using the Hartree-Fock approximation (HFA). Magnetically ordered
regions are reduced by a factor of about 3 along both the and density
axes compared to the HFA. Using the spin-rotation invariant formulation of the
slave-boson method the helicity modulus is computed and for half-filling is
found to practically coincide with that found using variational Monte Carlo
calculations using the Gutzwiller wave function. Off half-filling the results
can be used to compare with Quantum Monte Carlo calculations of the effective
hopping parameter. Contrary to the case of half-filling, the slave-boson
approach is seen to greatly improve the results of the HFA when off
half-filling. (Submitted to: Journal of Physics: Condensed Matter)Comment: 27 pages, LaTeX2e, 7 figures available upon request, INLO-PUB-10/9
Spontaneous plaquette dimerization in the Heisenberg model
We investigate the non magnetic phase of the spin-half frustrated Heisenberg
antiferromagnet on the square lattice using exact diagonalization (up to 36
sites) and quantum Monte Carlo techniques (up to 144 sites). The spin gap and
the susceptibilities for the most important crystal symmetry breaking operators
are computed. A genuine and somehow unexpected `plaquette RVB', with
spontaneously broken translation symmetry and no broken rotation symmetry,
comes out from our numerical simulations as the most plausible ground state for
.Comment: 4 pages, 5 postscript figure
Long range Neel order in the triangular Heisenberg model
We have studied the Heisenberg model on the triangular lattice using several
Quantum Monte Carlo (QMC) techniques (up to 144 sites), and exact
diagonalization (ED) (up to 36 sites). By studying the spin gap as a function
of the system size we have obtained a robust evidence for a gapless spectrum,
confirming the existence of long range Neel order. Our best estimate is that in
the thermodynamic limit the order parameter m= 0.41 +/- 0.02 is reduced by
about 59% from its classical value and the ground state energy per site is
e0=-0.5458 +/- 0.0001 in unit of the exchange coupling. We have identified the
important ground state correlations at short distance.Comment: 4 pages, RevTeX + 4 encapsulated postscript figure
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