Search Results (185 total)

A new class of non-Gaussian curvature fluctuations ζ_pr(x)≡δN(χ_i) arises from the postinflation preheating behavior of a noninflaton field χ_i. Its billiardlike chaotic dynamics imprints regular log-spaced narrow spikes in the number of preheating e-folds N(χ_i). We perform highly accurate lattice simulations of supersymmetry-inspired quartic inflaton and coupling potentials in a separate-universe approximation to compute N(χ_i) as a function of the (nearly homogeneous) initial condition χ_i. The superhorizon modes of χ_i(x) result in positive spiky excursions in ζ_pr and hence negative gravitational potential fluctuations added to the usual sign-independent inflaton-induced perturbations, observably manifested in large cosmic structures and as (polarized) temperature cosmic microwave background cold spots.

We propose a qualitatively new mechanism for generating cosmological fluctuations from inflation. The nonequilibrium excitation of interacting scalar fields often evolves into infrared (IR) and ultraviolet cascading, resulting in an intermediate scaling regime. We observe elements of this phenomenon in a simple model with inflaton ϕ and isoinflaton χ fields interacting during inflation via the coupling g²(ϕ-ϕ₀)²χ². Isoinflaton particles are created during inflation when they become instantaneously massless at ϕ=ϕ₀, with occupation numbers not exceeding unity. Previous studies have focused on the momentary slowing down of the condensate ϕ(t) by back-reaction effects. Here, we point out that very quickly the produced χ particles become heavy and their multiple rescatterings off the homogeneous condensate ϕ(t) generates Bremsstrahlung radiation of light inflaton IR fluctuations with high occupation numbers. The subsequent evolution of these IR fluctuations is qualitatively similar to that of the usual inflationary fluctuations, but their initial amplitude is different. The IR cascading generates a bump-shaped contribution to the cosmological curvature fluctuations, which can even dominate over the usual fluctuations for g²>0.06. The IR cascading curvature fluctuations are significantly non-Gaussian, and the strength and location of the bump are model dependent, through g² and ϕ₀. The effect from IR cascading fluctuations is significantly larger than that from the momentary slowing down of ϕ(t). With a sequence of such bursts of particle production, the superposition of the bumps can lead to a new broadband non-Gaussian component of cosmological fluctuations added to the usual fluctuations. Such a sequence of particle creation events can, but need not, lead to trapped inflation.

The Linac Coherent Light Source (LCLS) is an x-ray free-electron laser project presently in a commissioning phase at the SLAC National Accelerator Laboratory. We report here on very low-emittance measurements made at low bunch charge, and a few femtosecond bunch length produced by the LCLS bunch compressors. Start-to-end simulations associated with these beam parameters show the possibilities of generating hundreds of GW at 1.5 Å x-ray wavelength and nearly a single longitudinally coherent spike at 1.5 nm with 2-fs duration.

Generation of attosecond x-ray pulses is attracting much attention within the x-ray free-electron laser (FEL) user community. Several schemes using extremely short laser pulses to manipulate the electron bunches have been proposed. In this paper, we extend the attosecond two-color enhanced self-amplified spontaneous emission scheme proposed by Zholents et al. to the long optical cycle regime using a second detuned laser and a tapered undulator. Both lasers can be about ten optical cycles long, with the second laser frequency adjustable and set to optimize the contrast between the central and side current spikes. A tapered undulator mitigates the degradation effect of the longitudinal space charge force in the undulator and suppresses the FEL gain of all side current peaks. Simulations using the Linac Coherent Light Source parameters show a single attosecond x-ray spike of ∼100  attoseconds can be produced. The second laser can also be detuned to coherently control the number of the side x-ray spikes and the length of the radiation pulse.

We propose a scheme that combines the echo-enabled harmonic generation technique with the bunch compression and allows one to generate harmonic numbers of a few hundred in a microbunched beam through up-conversion of the frequency of an ultraviolet seed laser. A few-cycle intense laser is used to generate the required energy chirp in the beam for bunch compression and for selection of an attosecond x-ray pulse. Sending this beam through a short undulator results in an intense isolated attosecond x-ray pulse. Using a representative realistic set of parameters, we show that 1 nm x-ray pulse with peak power of a few hundred MW and duration as short as 20 attoseconds (FWHM) can be generated from a 200 nm ultraviolet seed laser. The proposed scheme may enable the study of electronic dynamics with a resolution beyond the atomic unit of time (∼24 attoseconds) and may open a new regime of ultrafast sciences.

Spontaneous spatiotemporal wave activity occurs in groups of excitable particles for groups larger than a critical size. Experiments are carried out with particles loaded with the catalyst of the Belousov-Zhabotinsky reaction that are immersed in catalyst-free reaction mixture. The particles diffusively exchange activator and inhibitor species with the surrounding solution. All particles are nonoscillatory when separated from the other particles; however, target and spiral waves are exhibited in sufficiently large groups. A cellular particle model of the system also exhibits transitions from excitable steady state behavior to spatiotemporal wave activity with increasing group size.

In order to reach the high peak current required for an x-ray free electron laser, two separate magnetic dipole chicanes are used in the Linac Coherent Light Source accelerator to compress the electron bunch length in stages. In these bunch compressors, coherent synchrotron radiation (CSR) can be emitted either by a short electron bunch or by any longitudinal density modulation that may be on the bunch. In this paper, we report detailed measurements of the CSR-induced energy loss and transverse emittance growth in these compressors. Good agreement is found between the experimental results and multiparticle tracking studies. We also describe direct observations of CSR at optical wavelengths and compare with analytical models based on beam microbunching.

We demonstrate direct evidence that a single-crystal-like aragonite platelet is essentially assembled with aragonite nanoparticles. The aragonite nanoparticles are readily oriented and assembled into pseudo-single-crystal aragonite platelets via screw dislocation and amorphous aggregation, which are two dominant mediating mechanisms between nanoparticles during biomineralization. These findings will advance our understanding of nacre’s biomineralization process and provide additional design guidelines for developing biomimetic materials.

The relation between level crossings, entanglement, and Berry phases is investigated for the Breit-Rabi Hamiltonian of hydrogen and sodium atoms, describing a hyperfine interaction of electron and nuclear spins in a magnetic field. It is shown that the entanglement between nuclear and electron spins is maximum at avoided crossings. An entangled state encircling avoided crossings acquires a marginal Berry phase of a subsystem like an instantaneous eigenstate moving around real crossings accumulates a Berry phase. Especially, the nodal points of a marginal Berry phase correspond to the avoided crossing points.

Work functions of NiPt alloys with different compositions are investigated using first-principles methods based on density-functional theory. Results of our calculations reveal that surface alloy composition has a significant effect on the work function of the NiPt alloy. However, for a given surface composition, the work function is insensitive to the distributions of Ni/Pt atoms in the alloy and it is only slightly affected by alloy disorder. Our work suggests surface atomic modification as a promising way of tuning the work function of alloy metal gate.

This paper investigates the stability of wires against size variation for Stranski-Krastanow systems under the influence of an electric field generated by a patterned electric plate. The stability is determined by considering the total energy change as a function of the wire size. The results show that the wire size can be stabilized by the electric field if the system meets the viability criterion and the effective electric field effect is within the minimum and maximum limits. The minimum limit ensures that the wire formation is energetically favorable at moderate sizes, while the maximum limit enforces the suppression of the formation of large wires.

The longitudinal space-charge (LSC) force can be a major cause of the microbunching instability in the linac for an x-ray free-electron laser. In this paper, the LSC-induced beam modulation is studied using an integral equation approach that takes into account the transverse (radial) variation of the LSC field for both the coasting-beam limit and a bunched beam. Variation of the beam energy and the transverse beam size is also incorporated. We discuss the validity of this approach and compare it with other analytical analyses as well as numerical simulations.

Carbon doping of CdS is studied using first-principles calculations and Monte Carlo simulation. Our calculations predict ferromagnetism in C doped CdS, resulting from carbon substitution of sulfur. A single carbon substitution of sulfur favors a spin-polarized state with a magnetic moment of 1.22μ_B. Ferromagnetic coupling is generally observed between these magnetic moments. A transition temperature of 270  K is predicted through Monte Carlo simulation. The ferromagnetism of C doped CdS can be explained by the hole-mediated double exchange mechanism.

There is a growing interest in producing intense, coherent x-ray radiation with an adjustable and arbitrary polarization state. In this paper, we study the crossed-undulator scheme [K.-J. Kim, Nucl. Instrum. Methods Phys. Res., Sect. A 445, 329 (2000)] for rapid polarization control in a self-amplified spontaneous emission (SASE) free electron laser (FEL). Because a SASE source is a temporally chaotic light, we perform a statistical analysis on the state of polarization using FEL theory and simulations. We show that, by adding a small phase shifter and a short (about 1.3 times the FEL power gain length), 90° rotated planar undulator after the main SASE planar undulator, one can obtain circularly polarized light—with over 80% polarization—near the FEL saturation.

Ab initio study based on density functional theory is performed to study the binding energies of Mg acceptors to single oxygen in AlN and the activation energies of the resultant Mg_n-O complexes (n=2, 3, and 4). It is found that such complexes are energetically favored and have activation energies at least 0.23  eV lower than that of single Mg. The lower activation energies originate from the extra states over the valence band top of AlN induced by the passive Mg-O. By comparing to the well-established case of GaN, it is possible to fabricate Mg:O codoped AlN without MgO precipitates. These results suggest the possibility of achieving higher hole concentration in AlN by Mg:O codoping.

It is usually assumed that the space charge effects in relativistic beams scale with the energy of the beam as γ^-2, where γ is the relativistic factor. We show that for a beam accelerated in the longitudinal direction there is an additional space charge effect in free space that scales as E/γ, where E is the accelerating field. This field has the same origin as the “electromagnetic mass of the electron” discussed in textbooks on electrodynamics. It keeps the balance between the kinetic energy of the beam and the energy of the electromagnetic field of the beam. We then consider the effect of this field on a beam generated in an rf gun and calculate the energy spread produced by this field in the beam.

Recent experiments (angle-resolved photoemission spectroscopy and Raman) suggest the presence of two distinct energy gaps in high-temperature superconductors (HTSC), exhibiting different doping dependences. The results of a variational cluster approach to the superconducting state of the two-dimensional Hubbard model are presented which show that this model qualitatively describes this gap dichotomy. The antinodal gap increases with less doping, a behavior long considered as reflecting the general gap behavior of the HTSC. On the other hand, the near-nodal gap does even slightly decrease with underdoping. An explanation of this unexpected behavior is given which emphasizes the crucial role of spin fluctuations in the pairing mechanism.

In a Rayleigh-Taylor instability a dense fluid sits metastably atop a less dense fluid, a configuration that can be stabilized using a magnetic field gradient when one fluid is highly paramagnetic. On switching off the magnetic field, the instability occurs as the dense fluid falls under gravity. By affixing appropriately shaped magnetically permeable wires to the outside of the cell, one may impose arbitrarily chosen and well-controlled initial perturbations on the interface. This technique is used to examine both the linear and nonlinear growth regimes for which the perturbation amplitudes, growth rates, and nonlinear growth coefficients are obtained.

In this paper we investigate the cooperative ridge-trench formation from the theoretical point of view. Our investigation starts with numerical simulation for the morphological evolution of strained film-substrate system driven by surface diffusion. The results demonstrate that the morphological evolution of thick films leads to the unique cooperative formation of faceted trenches and ridges. The cooperative formation is further analyzed from the energy point of view by considering the two pathways of the cooperative formation, namely, the growth of the outermost structure and the gradual formation of a faceted structure adjacent to the existing one. The analyses reveal that the first pathway dictates the process initially, while the second one is more energetically favorable once the size of the outermost structure reaches a critical value. The competition of the two pathways repeats, resulting in the cooperative ridge-trench formation.

We propose a scheme to control the level of entanglement between two fixed spin-1∕2 systems by interaction with a third particle. For specific designs, entanglement is shown to be “pumped” into the system from the surroundings even when the spin-spin interaction within the system is small or nonexistent. The effect of the external particle on the system is introduced by including a dynamic spinor in the Hamiltonian. Controlled amplification of the internal entanglement to its maximum value is demonstrated. The possibility of entangling noninteracting spins in a stationary state is also demonstrated by coupling each one of them to a flying qubit in a quantum wire.

Nd_0.7Sr_0.3MnO₃ (NSMO) films 7–300  nm thick have been grown on (001)(LaAlO₃)_0.3(Sr₂AlTaO₆)_0.7 (LSAT) and (001)SrTiO₃ (STO) substrates, with lattice mismatches of 0.33% and 1.29%, respectively. The strain state evolution was examined fully by x-ray reciprocal space maps, in order to clarify its impact on the thickness-dependent properties of the films. It was found that all NSMO/LSAT films are coherently strained, having almost the same Curie (T_C) and peak resistivity (T_p) temperatures at fixed thicknesses, while the NSMO/STO films evolve from being fully strained to relaxed, showing inhomogeneous magnetic transitions and lower or higher T_C than their counterparts at 7–60  nm. The results underline that T_C (T_p) and phase separation are all controlled by the strain states of the films.

The cooperative ridge-trench (CRT) formation on heteroepitaxial systems, the process controlling the self-assembly of quantum dot molecules, is investigated by simulating the process and by analyzing the corresponding energy change. The results suggest that the CRT formation is the competition of two exclusive pathways: namely, the growth of the outermost structure and the gradual formation of a new facet structure adjacent to the existing one. The first pathway dictates the process initially, while the second one is more energetically favorable once the size of the outermost structure reaches a critical value. The competition repeats, resulting in the CRT formation.

High-gain free-electron lasers (FELs) are being developed as extremely bright sources for a next-generation x-ray facility. In this paper, we review the basic theory of the start-up, the exponential growth, and the saturation of the high-gain process, emphasizing the self-amplified spontaneous emission. The radiation characteristics of an x-ray FEL, including its transverse coherence, temporal characteristics, and harmonic content, are discussed. FEL performance in the presence of machine errors and undulator wakefields is examined. Various enhancement schemes through seeding and beam manipulations are summarized.

We propose a metamaterial with a three-layer structure based on Faraday’s law. The metamaterial is simply formed by a pair of homogeneous parallel plates separated by a thin medium. We also propose a virtual current loop with length of 2a (a is the attenuation constant) in the plates, which can be formed upon excitation of an electromagnetic field. Strong magnetic response has been observed by spectroscopic ellipsometry and the resonant frequency can be widely tuned by varying the structure dimensions. The observations are also verified by optical transfer matrix. The easy fabrication and high interfacial quality of the structure will make the applications of the magnetic response and negative refractive index metamaterials a reality.

We study the renormalization of the electron-spin-fluctuation (el-sp) vertex in a two-dimensional Hubbard model with nearest-neighbor (t) and next-nearest-neighbor (t^′) hopping by a quantum Monte Carlo calculation. We distinguish between el-sp vertices involving interacting particles and quasiparticles, i.e., we separate the renormalization of the vertex from that of the quasiparticle residue 1∕Z. We show that for t^′=0 the renormalized el-sp vertex, not dressed by 1∕Z, decreases with decreasing temperature at all momentum transfers. As a consequence, the effective pairing interaction mediated by antiferromagnetic spin fluctuations is reduced due to vertex corrections. The inclusion of a finite t^′∕t<0 increases the Landau damping rate of spin fluctuations, especially in the overdoped region. The increased damping rate leads to smaller vertex corrections, in agreement with earlier diagrammatic calculations. Still, corrections reduce the spin-fermion vertex even at finite t^′.