Phonon Transmission Rate, Fluctuations, and Localization in Random Semiconductor Superlattices: Green's Function Approach

We analytically study phonon transmission and localization in random superlattices by using a Green's function approach. We derive expressions for the average transmission rate and localization length, or Lyapunov exponent, in terms of the superlattice structure factor. This is done by considering the backscattering of phonons, due to the complex mass density fluctuations, which incorporates all of the forward scattering processes. These analytical results are applied to two types of random superlattices and compared with numerical simulations based on the transfer matrix method. Our analytical results show excellent agreement with the numerical data. A universal relation for the transmission fluctuations versus the average transmission is derived explicitly, and independently confirmed by numerical simulations. The transient of the distribution of transmission to the log-normal distribution for the localized phonons is also studied.Comment: 36 pages, Late

Phonon Universal Transmission Fluctuations and Localization in Semiconductor Superlattices with a Controlled Degree of Order

We study both analytically and numerically phonon transmission fluctuations and localization in partially ordered superlattices with correlations among neighboring layers. In order to generate a sequence of layers with a varying degree of order we employ a model proposed by Hendricks and Teller as well as partially ordered versions of deterministic aperiodic superlattices. By changing a parameter measuring the correlation among adjacent layers, the Hendricks- Teller superlattice exhibits a transition from periodic ordering, with alterna- ting layers, to the phase separated opposite limit; including many intermediate arrangements and the completely random case. In the partially ordered versions of deterministic superlattices, there is short-range order (among any $N$ conse- cutive layers) and long range disorder, as in the N-state Markov chains. The average and fluctuations in the transmission, the backscattering rate, and the localization length in these multilayered systems are calculated based on the superlattice structure factors we derive analytically. The standard deviation of the transmission versus the average transmission lies on a {\it universal\/} curve irrespective of the specific type of disorder of the SL. We illustrate these general results by applying them to several GaAs-AlAs superlattices for the proposed experimental observation of phonon universal transmission fluctuations.Comment: 16-pages, Revte

Efficacy and safety of micafungin in empiric and D-index-guided early antifungal therapy for febrile neutropenia ; A subgroup analysis of the CEDMIC trial

Objectives: The D-index is defined as the area over the neutrophil curve during neutropenia. The CEDMIC trial confirmed the noninferiority of D-index-guided early antifungal therapy (DET) using micafungin to empirical antifungal therapy (EAT). In this study, we evaluated the efficacy and safety of micafungin in these settings. Methods: From the CEDMIC trial, we extracted 67 and 113 patients who received micafungin in the DET and EAT groups, respectively. Treatment success was defined as the fulfilment of all components of a five-part composite end point. Fever resolution was evaluated at seven days after the completion of therapy. Results: The proportion of high-risk treatments including induction chemotherapy for acute leukemia and allogeneic hematopoietic stem cell transplantation was significantly higher in the DET group than in the EAT group (82.1% vs. 52.2%). The efficacy of micafungin was 68.7% (95%CI: 56.2–79.4) and 79.6% (71.0–86.6) in the DET and EAT groups, respectively. When we focused on high-risk treatments, the efficacy was 69.1% (55.2–80.9%) and 78.0% (65.3–87.7%), respectively (P = 0.30). There was no significant difference in any of the 5 components between the two groups. Conclusions: The efficacy of micafungin in patients undergoing high-risk treatment was not strongly impaired in DET compared to that in EAT

The ASTRO-H X-ray Observatory

The joint JAXA/NASA ASTRO-H mission is the sixth in a series of highly successful X-ray missions initiated by the Institute of Space and Astronautical Science (ISAS). ASTRO-H will investigate the physics of the high-energy universe via a suite of four instruments, covering a very wide energy range, from 0.3 keV to 600 keV. These instruments include a high-resolution, high-throughput spectrometer sensitive over 0.3-2 keV with high spectral resolution of Delta E < 7 eV, enabled by a micro-calorimeter array located in the focal plane of thin-foil X-ray optics; hard X-ray imaging spectrometers covering 5-80 keV, located in the focal plane of multilayer-coated, focusing hard X-ray mirrors; a wide-field imaging spectrometer sensitive over 0.4-12 keV, with an X-ray CCD camera in the focal plane of a soft X-ray telescope; and a non-focusing Compton-camera type soft gamma-ray detector, sensitive in the 40-600 keV band. The simultaneous broad bandpass, coupled with high spectral resolution, will enable the pursuit of a wide variety of important science themes.Comment: 22 pages, 17 figures, Proceedings of the SPIE Astronomical Instrumentation "Space Telescopes and Instrumentation 2012: Ultraviolet to Gamma Ray

Hitomi (ASTRO-H) X-ray Astronomy Satellite

The Hitomi (ASTRO-H) mission is the sixth Japanese x-ray astronomy satellite developed by a large international collaboration, including Japan, USA, Canada, and Europe. The mission aimed to provide the highest energy resolution ever achieved at E  >  2  keV, using a microcalorimeter instrument, and to cover a wide energy range spanning four decades in energy from soft x-rays to gamma rays. After a successful launch on February 17, 2016, the spacecraft lost its function on March 26, 2016, but the commissioning phase for about a month provided valuable information on the onboard instruments and the spacecraft system, including astrophysical results obtained from first light observations. The paper describes the Hitomi (ASTRO-H) mission, its capabilities, the initial operation, and the instruments/spacecraft performances confirmed during the commissioning operations for about a month

Large-wave-vector phonons in highly dispersive crystals: Phonon-focusing effects

The anisotropic propagation or the focusing of large-wave-vector acoustic phonons in highly dispersive crystalline Ge is studied in detail. Our analysis is entirely based on an elaborate Born—von Kármán model of the lattice dynamics which is constructed so as to reproduce the observed phonon dispersion curves very accurately. Effects of the lattice dispersion upon the phonon focusing are found to be discernible above 0.3 THz and become drastic at frequencies higher than 1 THz. These observations are made by studying complementarily the frequency dependences of the following objects: the shape of constant-frequency surfaces, the locations of phonon caustics, the distributions of phonons in real space, the angular dependences of phonon intensity, and the structure of group-velocity surfaces. A brief discussion is also given of the effects of isotopes which act to damp significantly the ballistic phonon intensity at frequencies in the 1-THz range

Numerical evidence for the bottleneck frequency of quasidiffusive acoustic phonons

A kinetic equation on quasidiffusion of phonons was recently analyzed by Esipov and he predicted the existence of a bottleneck frequency (νBN) which separates the phonons decaying from those diffusing to a detector. We have solved numerically the kinetic equation and obtained the temporal evolution of phonon concentration excited at the center of a spherical sample. We have also performed Monte Carlo simulations of phonon propagation in the same geometry. At a time much later than the ballistic arrival time of phonons, both sets of results exhibit a sharp peak in the phonon concentration around the predicted νBN. With Monte Carlo simulations we have also confirmed the same relaxation rate for the phonons of frequencies ν<νBN

Anomalously long lifetimes of high-energy surface acoustic phonons

The lifetime of Rayleigh surface phonons as opposed to decay via anharmonic three-phonon processes is calculated in the regime ℏωR≫kBT. The surface phonon decays predominantly by combining with a bulk transverse phonon to create another bulk transverse phonon. The energymomentum conservation of the processes imposes the condition that those bulk phonons which may interact with the surface phonons should possess frequencies of the same order of ωR. However, such high-frequency phonons are rarely excited thermally under the condition considered here, leading to an anomalously long lifetime of the surface phonons in proportion to ωR-4.5T(-0.5)exp(A ℏωR/kBT), where A is a constant of the order of 0.1. We also estimate numerically the magnitude of the lifetim

Localized vibrational modes in superlattices

The effect of an impurity cell on the vibrational properties of superlattices is studied theoretically. The continuous equation governing the elastic-wave motion in a superlattice is transformed exactly to a discrete form which is analogous to the equation for the displacement in one-dimensional discrete lattices. With the use of this equation isolated frequencies associated with the vibrations localized near the impurity cell are predicted in the band gaps at the center and boundary of the folded Brillouin zone of the host superlattice. Numerical calculation reveals further the existence of impurity states in the intrazone gaps due to intermode Bragg reflection. The calculated phonon transmission rate shows local enhancements due to these localized states, suggesting their observability by phonon spectroscopic experiment with a quasimonochromatic phonon detector

Isotope scattering of large-wave-vector phonons in GaAs and InSb : Deformation-dipole and overlap-shell models

Scatterings of near-zone-boundary phonons by isotopic disorder in diatomic GaAs InSb crystals are studied theoretically. In multiatomic lattices the scattering rate depends importantly on the vibrational amplitudes of constituent atoms. We find that the wave-vector dependences of these amplitudes, which are predicted by some lattice models, show quite dissimilar behaviors as compared to others. The typical examples investigated in this paper are the deformation-dipole model and the overlap-shell model. With the use of these models the frequency dependence of the relaxation time of phonons is calculated. Both models, however, predict the relaxation time of about 0.1 μsec for 1-THz TA phonons in GaAs, which is at least 1 order of magnitude shorter than the lifetime reported by Ulbrich et al