109 research outputs found
Fixed-Node Monte Carlo Calculations for the 1d Kondo Lattice Model
The effectiveness of the recently developed Fixed-Node Quantum Monte Carlo
method for lattice fermions, developed by van Leeuwen and co-workers, is tested
by applying it to the 1D Kondo lattice, an example of a one-dimensional model
with a sign problem. The principles of this method and its implementation for
the Kondo Lattice Model are discussed in detail. We compare the fixed-node
upper bound for the ground state energy at half filling with
exact-diagonalization results from the literature, and determine several spin
correlation functions. Our `best estimates' for the ground state correlation
functions do not depend sensitively on the input trial wave function of the
fixed-node projection, and are reasonably close to the exact values. We also
calculate the spin gap of the model with the Fixed-Node Monte Carlo method. For
this it is necessary to use a many-Slater-determinant trial state. The
lowest-energy spin excitation is a running spin soliton with wave number pi, in
agreement with earlier calculations.Comment: 19 pages, revtex, contribution to Festschrift for Hans van Leeuwe
Design and experimental validation of a compact collimated Knudsen source
In this paper we discuss the design and performance of a collimated Knudsen
source which has the benefit of a simple design over recirculating sources.
Measurements of the flux, transverse velocity distribution and brightness at
different temperatures were conducted to evaluate the performance. The scaling
of the flux and brightness with the source temperature follow the theoretical
predictions. The transverse velocity distribution in the transparent operation
regime also agrees with the simulated data. The source was found able to
produce a flux of s at a temperature of 433 K. Furthermore the
transverse reduced brightness of an ion beam with equal properties as the
atomic beam reads A/(m sr eV) which is sufficient for
our goal: the creation of an ultra-cold ion beam by ionization of a
laser-cooled and compressed atomic rubidium beam
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
Человек, которого мучила бесконечность. О мудрости тургеневской
Het Zwanenwater anno 2011: heidevelden, meren, graslanden vol orchideeën, moerasplanten, wilgenstruweel en duingraslanden richting de zeereep. In hoeverre komt dit beeld overeen met dat van eind 19e eeuw, toen enkele natuurkenners na een urenlange reis dit gebied wisten te bereiken? Zijn de omstandigheden nu anders en kan aan de hand van de natuur inzicht worden verkregen in de veranderingsprocessen
Green Function Monte Carlo with Stochastic Reconfiguration
A new method for the stabilization of the sign problem in the Green Function
Monte Carlo technique is proposed. The method is devised for real lattice
Hamiltonians and is based on an iterative ''stochastic reconfiguration'' scheme
which introduces some bias but allows a stable simulation with constant sign.
The systematic reduction of this bias is in principle possible. The method is
applied to the frustrated J1-J2 Heisenberg model, and tested against exact
diagonalization data. Evidence of a finite spin gap for J2/J1 >~ 0.4 is found
in the thermodynamic limit.Comment: 13 pages, RevTeX + 3 encapsulated postscript figure
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
Enhancing the excitation gap of a quantum-dot-based Kitaev chain
Connecting double quantum dots via a semiconductor-superconductor hybrid
segment offers a platform for creating a two-site Kitaev chain that hosts a
pair of "poor man's Majoranas" at a finely tuned sweet spot. However, the
effective couplings, which are mediated by Andreev bound states in the hybrid,
are generally weak in the tunneling regime. As a consequence, the excitation
gap is limited in size, presenting a formidable challenge for using this
platform to demonstrate non-Abelian statistics of Majoranas and realizing
error-resilient topological quantum computing. In this work, we systematically
study the effects of increasing the coupling between the dot and the hybrid
segment. In particular, the proximity effect transforms the dot orbitals into
Yu-Shiba-Rusinov states, forming a new spinless fermion basis for a Kitaev
chain, and we derive a theory for their effective coupling. As the coupling
strength between the dots and the hybrid segment increases, we find a
significant enhancement of the excitation gap and reduced sensitivity to local
perturbations. Although the hybridization of the Majorana wave function with
the central Andreev bound states increases strongly with increasing coupling,
the overlap of Majorana modes on the outer dots remains small, which is a
prerequisite for potential qubit experiments. We discuss how the
strong-coupling regime shows in experimentally accessible quantities, such as
the local and non-local conductance, and provide a protocol for tuning a
double-dot system into a sweet spot with a large excitation gap.Comment: 12 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
- …