47,979 research outputs found
4D visualization of embryonic, structural crystallization by single-pulse microscopy
In many physical and biological systems the transition from an amorphous to ordered native structure involves complex energy landscapes, and understanding such transformations requires not only their thermodynamics but also the structural dynamics during the process. Here, we extend our 4D visualization method with electron imaging to include the study of irreversible processes with a single pulse in the same ultrafast electron microscope (UEM) as used before in the single-electron mode for the study of reversible processes. With this augmentation, we report on the transformation of amorphous to crystalline structure with silicon as an example. A single heating pulse was used to initiate crystallization from the amorphous phase while a single packet of electrons imaged selectively in space the transformation as the structure continuously changes with time. From the evolution of crystallinity in real time and the changes in morphology, for nanosecond and femtosecond pulse heating, we describe two types of processes, one that occurs at early time and involves a nondiffusive motion and another that takes place on a longer time scale. Similar mechanisms of two distinct time scales may perhaps be important in biomolecular folding
Design and construction of a point-contact spectroscopy rig with lateral scanning capability
The design and realization of a cryogenic rig for point-contact spectroscopy
measurements in the needle-anvil configuration is presented. Thanks to the use
of two piezoelectric nano-positioners, the tip can move along the vertical
() and horizontal () direction and thus the rig is suitable to probe
different regions of a sample \textit{in situ}. Moreover, it can also form
double point-contacts on different facets of a single crystal for achieving,
e.g., an interferometer configuration for phase-sensitive measurements. For the
latter purpose, the sample holder can also host a Helmholtz coil for applying a
small transverse magnetic field to the junction. A semi-rigid coaxial cable can
be easily added for studying the behavior of Josephson junctions under
microwave irradiation. The rig can be detached from the probe and thus used
with different cryostats. The performance of this new probe has been tested in
a Quantum Design PPMS system by conducting point-contact Andreev reflection
measurements on Nb thin films over large areas as a function of temperature and
magnetic field.Comment: 7 pages, 7 figures, published in Rev. Sci. Instru
DC spin generation by junctions with AC driven spin-orbit interaction
An unbiased one-dimensional weak link between two terminals, subjected to the
Rashba spin-orbit interaction caused by an AC electric field which rotates
periodically in the plane perpendicular to the link, is shown to inject
spin-polarized electrons into the terminals. The injected spin-polarization has
a DC component along the link and a rotating transverse component in the
perpendicular plane. In the adiabatic, low rotation-frequency regime, these
polarization components are proportional to the frequency. The DC component of
the polarization vanishes for a linearly-polarized electric field.Comment: published versio
Work distribution for the driven harmonic oscillator with time-dependent strength: Exact solution and slow driving
We study the work distribution of a single particle moving in a harmonic
oscillator with time-dependent strength. This simple system has a non-Gaussian
work distribution with exponential tails. The time evolution of the
corresponding moment generating function is given by two coupled ordinary
differential equations that are solved numerically. Based on this result we
study the behavior of the work distribution in the limit of slow but finite
driving and show that it approaches a Gaussian distribution arbitrarily well
Mechanically Induced Thermal Breakdown in Magnetic Shuttle Structures
A theory of a thermally induced single-electron "shuttling" instability in a
magnetic nanomechanical device subject to an external magnetic field is
presented in the Coulomb blockade regime of electron transport. The model
magnetic shuttle device considered comprises a movable metallic grain suspended
between two magnetic leads, which are kept at different temperatures and
assumed to be fully spin polarized with antiparallel magnetizations. For a
given temperature difference shuttling is found to occur for a region of
external magnetic fields between a lower and an upper critical field strength,
which separate the shuttling regime from normal small-amplitude "vibronic"
regimes. We find that (i) the upper critical magnetic field saturates to a
constant value in the high temperature limit and that the shuttle instability
domain expands with a decrease of the temperature, (ii) the lower critical
magnetic field depends not only on the temperature independent phenomenological
friction coefficient used in the model but also on intrinsic friction (which
vanishes in the high temperature limit) caused by magnetic exchange forces and
electron tunneling between the quantum dot and the leads. The feasibility of
using thermally driven magnetic shuttle systems to harvest thermal breakdown
phenomena is discussed.Comment: 9 pages, 2 figure
Multifractal current distribution in random diode networks
Recently it has been shown analytically that electric currents in a random
diode network are distributed in a multifractal manner [O. Stenull and H. K.
Janssen, Europhys. Lett. 55, 691 (2001)]. In the present work we investigate
the multifractal properties of a random diode network at the critical point by
numerical simulations. We analyze the currents running on a directed
percolation cluster and confirm the field-theoretic predictions for the scaling
behavior of moments of the current distribution. It is pointed out that a
random diode network is a particularly good candidate for a possible
experimental realization of directed percolation.Comment: RevTeX, 4 pages, 5 eps figure
Subwavelength localization and toroidal dipole moment of spoof surface plasmon polaritons
We experimentally and theoretically demonstrate subwavelength scale localization of spoof surface plasmon polaritons at a point defect in a two-dimensional groove metal array. An analytical expression for dispersion relation of spoof surface plasmon polaritons substantiates the existence of a band gap where a defect mode can be introduced. A waveguide coupling method allows us to excite localized spoof surface plasmon polariton modes and measure their resonance frequencies. Numerical calculations confirm that localized modes can have a very small modal volume and a high Q factor both of which are essential in enhancing light-matter interactions. Interestingly, we find that the localized spoof surface plasmon polariton has a significant toroidal dipole moment, which is responsible for the high Q factor, as well as an electric quadrupole moment. In addition, the dispersion properties of spoof surface plasmon polaritons are analyzed using a modal expansion method and numerical calculations
Correlation Functions of Conserved Currents in N = 2 Superconformal Theory
Using a manifestly supersymmetric formalism, we determine the general
structure of two- and three- point functions of the supercurrent and the
flavour current of N = 2 superconformal field theories. We also express them in
terms of N = 1 superfields and compare to the generic N = 1 correlation
functions. A general discussion of the N = 2 supercurrent superfield and the
multiplet of anomalies and their definition as derivatives with respect to the
supergravity prepotentials is also included.Comment: 43 pages, latex, no figures, v.2: section 4.2 extende
Cosmic Mach Number as a Function of Overdensity and Galaxy Age
We carry out an extensive study of the cosmic Mach number (\mach) on scales
of R=5, 10 and 20h^-1Mpc using an LCDM hydrodynamical simulation. We
particularly put emphasis on the environmental dependence of \mach on
overdensity, galaxy mass, and galaxy age. We start by discussing the difference
in the resulting \mach according to different definitions of \mach and
different methods of calculation. The simulated Mach numbers are slightly lower
than the linear theory predictions even when a non-linear power spectrum was
used in the calculation, reflecting the non-linear evolution in the simulation.
We find that the observed \mach is higher than the simulated mean by more than
2-standard deviations, which suggests either that the Local Group is in a
relatively low-density region or that the true value of \Omega_m is ~ 0.2,
significantly lower than the simulated value of 0.37. We show from our
simulation that the Mach number is a weakly decreasing function of overdensity.
We also investigate the correlations between galaxy age, overdensity and \mach
for two different samples of galaxies --- DWARFs and GIANTs. Older systems
cluster in higher density regions with lower \mach, while younger ones tend to
reside in lower density regions with larger \mach, as expected from the
hierarchical structure formation scenario. However, for DWARFs, the correlation
is weakened by the fact that some of the oldest DWARFs are left over in
low-density regions during the structure formation history. For giant systems,
one expects blue-selected samples to have higher \mach than red-selected ones.
We briefly comment on the effect of the warm dark matter on the expected Mach
number.Comment: 43 pages, including 15 figures. Accepted version in ApJ. Included
correlation function of different samples of galaxies, and the cumulative
number fraction distribution as a fcn. of overdensity. Reorganized figures
and added some reference
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