81 research outputs found
Experimental demonstration of a high-flux capillary based XUV source in the high ionisation regime
High harmonic generation (HHG) has proven to be a fascinating and incredibly useful nonlinear optical phenomenon and has led to the realisation of tabletop sources of coherent extreme ultraviolet (XUV) radiation. Capillary based geometries in particular have attracted a great deal of attention due to their lengthy interaction regions and the potential to phase-match the HHG process leading to a large increase in XUV flux. Unfortunately due to plasma induced nonlinear and dispersive effects the simple phase-matching mechanism detailed in [1] cannot be scaled to high energy pump pulses and high gas pressures. In this work we have used a computational model [2] to design a capillary that can support a broad interaction region well-suited to quasi-phase-matching (QPM) while simultaneously reducing the effect that XUV reabsorption has on the output flux of the source. This modelling work has involved adjusting both the capillary length and gas density profile (figure 1a) in order to produce rapid oscillations in the radially integrated ionization fraction (figure 1b) coupled with a rapid decrease in gas pressure at the capillary exit. Our theory suggests that these oscillations are driven by a nonlinear self-compression process modulating the intensity of the pump pulse as it propagates through the plasma-filled waveguide [3]. Subsequent experimental work has shown an increase in XUV flux of almost 50 times over our previous capillary-based source (see figure 1c), and preliminary estimates suggest a photon flux of 1012 photons s-1 harmonic-1 in the 45 eV spectral region
Linear and nonlinear susceptibilities of a decoherent two-level system
The linear and nonlinear dynamical susceptibilities of a two level system are
calculated as it undergoes a transition to a decoherent state. Analogously to
the Glover-Tinkham-Ferrell sum rule of superconductivity, spectral weight in
the linear susceptibility is continuously transferred from a finite frequency
resonance to nearly zero frequency, corresponding to a broken symmetry in the
thermodynamic limit. For this reason, the behavior of the present model (the
Mermin model) differs significantly from the spin-boson model. The third order
nonlinear susceptibility, corresponding to two-photon absorption, has an
unexpected non-monotonic behavior as a function of the environmental coupling,
reaching a maximum within the decoherent phase of the model. Both linear and
nonlinear susceptibilities may be expressed in a universal form.Comment: 10 pages, 9 figure
Thermoelectric effect in molecular electronics
We provide a theoretical estimate of the thermoelectric current and voltage
over a Phenyldithiol molecule. We also show that the thermoelectric voltage is
(1) easy to analyze, (2) insensitive to the detailed coupling to the contacts,
(3) large enough to be measured and (4) give valuable information, which is not
readily accessible through other experiments, on the location of the Fermi
energy relative to the molecular levels. The location of the Fermi-energy is
poorly understood and controversial even though it is a central factor in
determining the nature of conduction (n- or p-type). We also note that the
thermoelectric voltage measured over Guanine molecules with an STM by Poler et
al., indicate conduction through the HOMO level, i.e., p-type conduction.Comment: 4 pages, 3 figure
Dissipation and noise in adiabatic quantum pumps
We investigate the distribution function, the heat flow and the noise
properties of an adiabatic quantum pump for an arbitrary relation of pump
frequency and temperature. To achieve this we start with the
scattering matrix approach for ac-transport. This approach leads to expressions
for the quantities of interest in terms of the side bands of particles exiting
the pump. The side bands correspond to particles which have gained or lost a
modulation quantum . We find that our results for the pump
current, the heat flow and the noise can all be expressed in terms of a
parametric emissivity matrix. In particular we find that the current
cross-correlations of a multiterminal pump are directly related a to a
non-diagonal element of the parametric emissivity matrix. The approach allows a
description of the quantum statistical correlation properties (noise) of an
adiabatic quantum pump
ThermoElectric Transport Properties of a Chain of Quantum Dots with Self-Consistent Reservoirs
We introduce a model for charge and heat transport based on the
Landauer-Buttiker scattering approach. The system consists of a chain of
quantum dots, each of them being coupled to a particle reservoir. Additionally,
the left and right ends of the chain are coupled to two particle reservoirs.
All these reservoirs are independent and can be described by any of the
standard physical distributions: Maxwell-Boltzmann, Fermi-Dirac and
Bose-Einstein. In the linear response regime, and under some assumptions, we
first describe the general transport properties of the system. Then we impose
the self-consistency condition, i.e. we fix the boundary values (T_L,\mu_L) and
(T_R,mu_R), and adjust the parameters (T_i,mu_i), for i = 1,...,N, so that the
net average electric and heat currents into all the intermediate reservoirs
vanish. This condition leads to expressions for the temperature and chemical
potential profiles along the system, which turn out to be independent of the
distribution describing the reservoirs. We also determine the average electric
and heat currents flowing through the system and present some numerical
results, using random matrix theory, showing that these currents are typically
governed by Ohm and Fourier laws.Comment: Minor changes (45 pages
Observation of focusing of 400 GeV/c proton beam with the help of bent crystals
The results of observation and studies of focusing of 400 GeV/ c proton beam with the help of bent single crystals are presented. Two silicon crystals have been used in the measurements. The focal length of the first and second crystals is found to be 1.48 m and 0.68 m, respectively. The mean square size of the horizontal profile in the focus was 3.1 and 4.3 times as small as at the exit of the crystals
Sub-arcsecond imaging with the International LOFAR Telescope I. Foundational calibration strategy and pipeline
The International LOFAR Telescope is an interferometer with stations spread across Europe. With baselines of up to ~2000 km, LOFAR has the unique capability of achieving sub-arcsecond resolution at frequencies below 200 MHz. However, it is technically and logistically challenging to process LOFAR data at this resolution. To date only a handful of publications have exploited this capability. Here we present a calibration strategy that builds on previous high-resolution work with LOFAR. It is implemented in a pipeline using mostly dedicated LOFAR software tools and the same processing framework as the LOFAR Two-metre Sky Survey (LoTSS). We give an overview of the calibration strategy and discuss the special challenges inherent to enacting high-resolution imaging with LOFAR, and describe the pipeline, which is publicly available, in detail. We demonstrate the calibration strategy by using the pipeline on P205+55, a typical LoTSS pointing with an 8 h observation and 13 international stations. We perform in-field delay calibration, solution referencing to other calibrators in the field, self-calibration of these calibrators, and imaging of example directions of interest in the field. We find that for this specific field and these ionospheric conditions, dispersive delay solutions can be transferred between calibrators up to ~1.5° away, while phase solution transferral works well over ~1°. We also demonstrate a check of the astrometry and flux density scale with the in-field delay calibrator source. Imaging in 17 directions, we find the restoring beam is typically ~0.3′′ ×0.2′′ although this varies slightly over the entire 5 deg2 field of view. We find we can achieve ~80–300 μJy bm−1 image rms noise, which is dependent on the distance from the phase centre; typical values are ~90 μJy bm−1 for the 8 h observation with 48 MHz of bandwidth. Seventy percent of processed sources are detected, and from this we estimate that we should be able to image roughly 900 sources per LoTSS pointing. This equates to ~ 3 million sources in the northern sky, which LoTSS will entirely cover in the next several years. Future optimisation of the calibration strategy for efficient post-processing of LoTSS at high resolution makes this estimate a lower limit
Sq and EEJ—A Review on the Daily Variation of the Geomagnetic Field Caused by Ionospheric Dynamo Currents
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