129 research outputs found
Correlations and enlarged superconducting phase of - chains of ultracold molecules on optical lattices
We compute physical properties across the phase diagram of the -
chain with long-range dipolar interactions, which describe ultracold polar
molecules on optical lattices. Our results obtained by the density-matrix
renormalization group (DMRG) indicate that superconductivity is enhanced when
the Ising component of the spin-spin interaction and the charge component
are tuned to zero, and even further by the long-range dipolar interactions.
At low densities, a substantially larger spin gap is obtained. We provide
evidence that long-range interactions lead to algebraically decaying
correlation functions despite the presence of a gap. Although this has recently
been observed in other long-range interacting spin and fermion models, the
correlations in our case have the peculiar property of having a small and
continuously varying exponent. We construct simple analytic models and
arguments to understand the most salient features.Comment: published version with minor modification
Nanoscale mapping of ultrafast magnetization dynamics with femtosecond Lorentz microscopy
Novel time-resolved imaging techniques for the investigation of ultrafast
nanoscale magnetization dynamics are indispensable for further developments in
light-controlled magnetism. Here, we introduce femtosecond Lorentz microscopy,
achieving a spatial resolution below 100 nm and a temporal resolution of 700
fs, which gives access to the transiently excited state of the spin system on
femtosecond timescales and its subsequent relaxation dynamics. We demonstrate
the capabilities of this technique by spatio-temporally mapping the
light-induced demagnetization of a single magnetic vortex structure and
quantitatively extracting the evolution of the magnetization field after
optical excitation. Tunable electron imaging conditions allow for an
optimization of spatial resolution or field sensitivity, enabling future
investigations of ultrafast internal dynamics of magnetic topological defects
on 10-nanometer length scales
Colour Passing Revisited: Lifted Model Construction with Commutative Factors
Lifted probabilistic inference exploits symmetries in a probabilistic model
to allow for tractable probabilistic inference with respect to domain sizes. To
apply lifted inference, a lifted representation has to be obtained, and to do
so, the so-called colour passing algorithm is the state of the art. The colour
passing algorithm, however, is bound to a specific inference algorithm and we
found that it ignores commutativity of factors while constructing a lifted
representation. We contribute a modified version of the colour passing
algorithm that uses logical variables to construct a lifted representation
independent of a specific inference algorithm while at the same time exploiting
commutativity of factors during an offline-step. Our proposed algorithm
efficiently detects more symmetries than the state of the art and thereby
drastically increases compression, yielding significantly faster online query
times for probabilistic inference when the resulting model is applied
Few-nm tracking of magnetic vortex orbits and their decay with ultrafast Lorentz microscopy
Transmission electron microscopy is one of the most powerful techniques to
characterize nanoscale magnetic structures. In light of the importance of fast
control schemes of magnetic states, time-resolved microscopy techniques are
highly sought after in fundamental and applied research. Here, we implement
time-resolved Lorentz imaging in combination with synchronous radio-frequency
excitation using an ultrafast transmission electron microscope. As a model
system, we examine the current-driven gyration of a vortex core in a 2
m-sized magnetic nanoisland. We record the trajectory of the
vortex core for continuous-wave excitation, achieving a localization precision
of 2nm with few-minute integration times. Furthermore, by tracking the
core position after rapidly switching off the current, we find a temporal
hardening of the free oscillation frequency and an increasing orbital decay
rate attributed to local disorder in the vortex potential
Nanoscale Magnetic Imaging using Circularly Polarized High-Harmonic Radiation
This work demonstrates nanoscale magnetic imaging using bright circularly
polarized high-harmonic radiation. We utilize the magneto-optical contrast of
worm-like magnetic domains in a Co/Pd multilayer structure, obtaining
quantitative amplitude and phase maps by lensless imaging. A
diffraction-limited spatial resolution of 49 nm is achieved with iterative
phase reconstruction enhanced by a holographic mask. Harnessing the unique
coherence of high harmonics, this approach will facilitate quantitative,
element-specific and spatially-resolved studies of ultrafast magnetization
dynamics, advancing both fundamental and applied aspects of nanoscale
magnetism.Comment: Ofer Kfir and Sergey Zayko contributed equally to this work.
Presented in CLEO 2017 (Oral) doi.org/10.1364/CLEO_QELS.2017.FW1H.
Coulomb-correlated electron number states in a transmission electron microscope beam
We demonstrate the generation of Coulomb-correlated pair, triple and
quadruple states of free electrons by femtosecond photoemission from a
nanoscale field emitter inside a transmission electron microscope. Event-based
electron spectroscopy allows a spatial and spectral characterization of the
electron ensemble emitted by each laser pulse. We identify distinctive energy
and momentum correlations arising from acceleration-enhanced interparticle
energy exchange, revealing strong few-body Coulomb interactions at an energy
scale of about two electronvolts. State-sorted beam caustics show a discrete
increase in virtual source size and longitudinal source shift for few-electron
states, associated with transverse momentum correlations. We observe
field-controllable electron antibunching, attributed primarily to transverse
Coulomb deflection. The pronounced spatial and spectral characteristics of
these electron number states allow filtering schemes that control the
statistical distribution of the pulse charge. In this way, the fraction of
specific few-electron states can be actively suppressed or enhanced,
facilitating the preparation of highly non-Poissonian electron beams for
microscopy and lithography, including future heralding schemes and correlated
multi-electron probing
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