Enhanced collimated GeV monoenergetic ion acceleration from a shaped foil target irradiated by a circularly polarized laser pulse

Using multi-dimensional particle-in-cell (PIC) simulations we study ion acceleration from a foil irradiated by a circularly polarized laser pulse at 1022W/cm^2 intensity. When the foil is shaped initially in the transverse direction to match the laser intensity profile, the center part of the target can be uniformly accelerated for a longer time compared to a usual flat target. Target deformation and undesirable plasma heating are effectively suppressed. The final energy spectrum of the accelerated ion beam is improved dramatically. Collimated GeV quasi-mono-energetic ion beams carrying as much as 18% of the laser energy are observed in multi-dimensional simulations. Radiation damping effects are also checked in the simulations.Comment: 4 pages, 4 figure

How to Run Through Walls: Dynamics of Bubble and Soliton Collisions

It has recently been shown in high resolution numerical simulations that relativistic collisions of bubbles in the context of a multi-vacua potential may lead to the creation of bubbles in a new vacuum. In this paper, we show that scalar fields with only potential interactions behave like free fields during high-speed collisions; the kick received by them in a collision can be deduced simply by a linear superposition of the bubble wall profiles. This process is equivalent to the scattering of solitons in 1+1 dimensions. We deduce an expression for the field excursion (shortly after a collision), which is related simply to the field difference between the parent and bubble vacua, i.e. contrary to expectations, the excursion cannot be made arbitrarily large by raising the collision energy. There is however a minimum energy threshold for this excursion to be realized. We verify these predictions using a number of 3+1 and 1+1 numerical simulations. A rich phenomenology follows from these collision induced excursions - they provide a new mechanism for scanning the landscape, they might end/begin inflation, and they might constitute our very own big bang, leaving behind a potentially observable anisotropy.Comment: 15pgs, 14 figures, v2, thanks for the feedback

Absence of correlation between built-in electric dipole moment and quantum Stark effect in InAs/GaAs self-assembled quantum dots

We report significant deviations from the usual quadratic dependence of the ground state interband transition energy on applied electric fields in InAs/GaAs self-assembled quantum dots. In particular, we show that conventional second-order perturbation theory fails to correctly describe the Stark shift for electric field below $F = 10$ kV/cm in high dots. Eight-band ${\bf k}\cdot{\bf p}$ calculations demonstrate this effect is predominantly due to the three-dimensional strain field distribution which for various dot shapes and stoichiometric compositions drastically affects the hole ground state. Our conclusions are supported by two independent experiments.Comment: 4 pages, 4 figure

Unified Band Theoretic Description of Electronic and Magnetic Properties of Vanadium Dioxide Phases

The debate about whether the insulating phases of vanadium dioxide (VO2) can be described by band theory or must be described by a theory of strong electron correlations remains unresolved even after decades of research. Energy-band calculations using hybrid exchange functionals or including self-energy corrections account for the insulating or metallic nature of different phases, but have not yet successfully accounted for the observed magnetic orderings. Strongly-correlated theories have had limited quantitative success. Here we report that, by using hard pseudopotentials and an optimized hybrid exchange functional, the energy gaps and magnetic orderings of both monoclinic VO2 phases and the metallic nature of the high-temperature rutile phase are consistent with available experimental data, obviating an explicit role for strong correlations. We also report a potential candidate for the newly-found metallic monoclinic phase and present a detailed magnetic structure of the M2 monoclinic phase

Simulating the Initial Stage of Phenolic Resin Carbonization via the ReaxFF Reactive Force Field

Pyrolysis of phenolic resins leads to carbon formation. Simulating this resin-to-carbon process atomistically is a daunting task. In this paper, we attempt to model the initial stage of this process by using the ReaxFF reactive force field, which bridges quantum mechanical and molecular mechanical methods. We run molecular dynamics simulations to examine the evolution of small molecules at different temperatures. The main small-molecule products found include H_2O, H_2, CO, and C_2H_2. We find multiple pathways leading to H_2O formation, including a frequent channel via β-H elimination, which has not been proposed before. We determine the reaction barrier for H_2O formation from the reaction rates obtained at different temperatures. We also discuss the relevance of our simulations to previous experimental observations. This work represents a first attempt to model the resin-to-carbon process atomistically

Microwave (EPR) measurements of the penetration depth measurements of high-Tc superconductors

The use is discussed of electron paramagnetic resonance (EPR) as a quick and easily accessible method for measuring the London penetration depth, lambda for the high T sub c superconductors. The method uses the broadening of the EPR signal, due to the emergence of the magnetic flux lattice, of a free radical adsorbed on the surface of the sample. The second moment, of the EPR signal below T sub c is fitted to the Brandt equation for a simple triangular lattice. The precision of this method compares quite favorably with those of the more standard methods such as micro sup(+)SR, neutron scattering, and magnetic susceptibility

Unusual microwave response and bulk conductivity of very thin fese0.3te0.7 films as a function of temperature

Results of X-band microwave surface impedance measurements of FeSe1-xTex very thin film are reported. The effective surface resistance shows appearance of peak at T less and near Tc when plotted as function of temperature. The authors suggests that the most well-reasoned explanation can be based on the idea of the changing orientation of the microwave magnetic field at a SN phase transition near the surface of a very thin film. The magnetic penetration depth exhibits a power-law behavior of delta lambda proportional to T with an exponent n = 2.4 at low temperatures, which is noticeably higher than in the published results on FeSe1-xTexsingle crystal. However the temperature dependence of the superfluid conductivity remains very different from the behavior described by the BCS theory. Experimental results are fitted very well by a two-gap model with delta1/kTc=0.43 and delta2/kTc=1.22,thus supporting s(+-)- wave symmetry. The rapid increase of the quasiparticle scattering time is obtained from the microwave impedance measurements.Comment: 13 pages, 13 figure

Two-electron lateral quantum-dot molecules in a magnetic field

Laterally coupled quantum dot molecules are studied using exact diagonalization techniques. We examine the two-electron singlet-triplet energy difference as a function of magnetic field strength and investigate the magnetization and vortex formation of two- and four-minima lateral quantum dot molecules. Special attention is paid to the analysis of how the distorted symmetry affects the properties of quantum-dot molecules.Comment: 18 pages, 26 figure

Q & A Experiment to Search for Vacuum Dichroism, Pseudoscalar-Photon Interaction and Millicharged Fermions

A number of experiments are underway to detect vacuum birefringence and dichroism -- PVLAS, Q & A, and BMV. Recently, PVLAS experiment has observed optical rotation in vacuum by a magnetic field (vacuum dichroism). Theoretical interpretations of this result include a possible pseudoscalar-photon interaction and the existence of millicharged fermions. Here, we report the progress and first results of Q & A (QED [quantum electrodynamics] and Axion) experiment proposed and started in 1994. A 3.5-m high-finesse (around 30,000) Fabry-Perot prototype detector extendable to 7-m has been built and tested. We use X-pendulums and automatic control schemes developed by the gravitational-wave detection community for mirror suspension and cavity control. To polarize the vacuum, we use a 2.3-T dipole permanent magnet, with 27-mm-diameter clear borehole and 0.6-m field length,. In the experiment, the magnet is rotated at 5-10 rev/s to generate time-dependent polarization signal with twice the rotation frequency. Our ellipsometer/polarization-rotation-detection-system is formed by a pair of Glan-Taylor type polarizing prisms with extinction ratio lower than 10-8 together with a polarization modulating Faraday Cell with/without a quarter wave plate. We made an independent calibration of our apparatus by performing a measurement of gaseous Cotton-Mouton effect of nitrogen. We present our first experimental results and give a brief discussion of our experimental limit on pseudo-scalar-photon interaction and millicharged fermions.Comment: 21 pages, 13 figures, submitted to Modern Physics Letter