Структура электронных состояний соединений Gd-=SUB=-5-=/SUB=-Sb-=SUB=-3-=/SUB=- и Gd-=SUB=-5-=/SUB=-Ge-=SUB=-2-=/SUB=-Sb по данным зонных расчетов и оптической спектроскопии

Results of investigations of the electronic structure and optical properties of Gd5Sb3 and Gd5Ge2Sb compounds are presented. Calculations of band spectra were carried out in frame of local density approximation with a correction for strong correlation effects in 4f shell of rare-earth ion (DFT+U+SO method). Optical constants of these materials were measured by ellipsometric technique in wide wavelength interval. Energy dependencies of a number of spectral parameters were determined. The nature of quantum light absorption is discussed on the base of comparative analysis of the experimental and calculated spectra of optical conductivity.

Multi-band Antenna with CSRR Loaded Ground Plane and Stubs Incorporated Patch for WiMAX/WLAN Applications

This paper proposes a novel compact, single structure, multi-band antenna along with tested results for wireless local area networks (WLAN) and Worldwide Interoperability for Microwave Access (WiMAX) applications. In this work, modified complementary split-ring resonators (CSRR) were incorporated in the ground layer of the patch to achieve permeable bands to accommodate multi-resonance frequencies in a single device. The proposed antenna design supported the upgraded performance and led to desirable size reduction. Open stubs were incorporated at the edges of the triangle batch to get the improved reflection coefficient responses. It resulted in specific band spectra of 2.4 / 3.4 / 5.1 / 5.8GHz for WLAN/WiMAX applications. For constructive antenna design, CST microwave studio simulation software was utilized. S11 parameter was observed as -24dB at 2.4GHZ, -32dB at 3.4GHz. -15dB at 5.1GHz and -22dB at 5.8GHz bands. Field patterns of each band were observed. The parametric study of the arrangement and positioning of the CSRR unit cell was examined. Excellent consistency between the experimental and simulated results revealed the capability of the projected structure to perform with improved gain.

Bootstrapping Bloch bands

Bootstrap methods, initially developed for solving statistical and quantum field theories, have recently been shown to capture the discrete spectrum of quantum mechanical problems, such as the single particle Schrödinger equation with an anharmonic potential. The core of bootstrap methods builds on exact recursion relations of arbitrary moments of some quantum operator and the use of an adequate set of positivity criteria. We extend this methodology to models with continuous Bloch band spectra, by considering a single quantum particle in a periodic cosine potential. We find that the band structure can be obtained accurately provided the bootstrap uses moments involving both position and momentum variables. We also introduce several new techniques that can apply generally to other bootstrap studies. First, we devise a trick to reduce by one unit the dimensionality of the search space for the variables parametrizing the bootstrap. Second, we employ statistical techniques to reconstruct the distribution probability allowing to compute observables that are analytic functions of the canonical variables. This method is used to extract the Bloch momentum, a quantity that is not readily available from the bootstrap recursion itself.

Unveiling the Atomic and Electronic Structure of Stacked-Cup Carbon Nanofibers

We report results of comprehensive experimental exploration (X-ray photoemission, Raman and optical spectroscopy) of carbon nanofibers (CNFs) in combination with first-principles modeling. Core-level spectra demonstrate prevalence of sp2 hybridization of carbon atoms in CNF with a trace amount of carbon–oxygen bonds. The density functional theory (DFT)-based calculations demonstrated no visible difference between mono- and bilayers because σ-orbitals are related to in-plane covalent bonds. The influence of the distortions on π-peak is found to be significant only for bilayers as a result of π–π interlayer bonds formation. These results are supported by both experimental Raman and XPS valence band spectra. The combination of optical measurements with a theoretical modeling indicates the formation of optically active graphene quantum dots (GQDs) in the CNF matrix, with a radiative relaxation of the excited π* state. The calculated electronic structure of these GQDs is in quantitative agreement with the measured optical transitions and provides an explanation of the absence of visible contribution from these GQDs to the measured valence bands spectra.

Magneto-optical absorption coefficients in monolayer MoSe2

We study the linear, third-order nonlinear, and total absorption coefficients (OACs) caused by intra- and inter-band transitions in monolayer MoSe2 in the presence of a magnetic field by using the compact density matrix approach. The results show that the OACs display the blue-shift behaviour with an increase in the magnetic field. The Zeeman fields do not affect the peak positions but reduce peak intensities slightly. Besides, the strong spin-orbit coupling in monolayer MoSe2 causes the peaks to differ significantly due to spin-up and spin-down. The OACs due to intra-band transition display only one peak in the THz range, while the inter-band spectra show a series of peaks in the near-infrared optical range, making monolayer MoSe2 a promising candidate for novel optoelectronic applications.

Solving quasiparticle band spectra of real solids using neural-network quantum states

Establishing a predictive ab initio method for solid systems is one of the fundamental goals in condensed matter physics and computational materials science. The central challenge is how to encode a highly-complex quantum-many-body wave function compactly. Here, we demonstrate that artificial neural networks, known for their overwhelming expressibility in the context of machine learning, are excellent tool for first-principles calculations of extended periodic materials. We show that the ground-state energies in real solids in one-, two-, and three-dimensional systems are simulated precisely, reaching their chemical accuracy. The highlight of our work is that the quasiparticle band spectra, which are both essential and peculiar to solid-state systems, can be efficiently extracted with a computational technique designed to exploit the low-lying energy structure from neural networks. This work opens up a path to elucidate the intriguing and complex many-body phenomena in solid-state systems.

Detection of new O-type stars in the obscured stellar cluster Tr 16-SE in the Carina Nebula with KMOS

Context. The Carina Nebula harbors a large population of high-mass stars, including at least 75 O-type and Wolf-Rayet (WR) stars, but the current census is not complete since further high-mass stars may be hidden in or behind the dense dark clouds that pervade the association. Aims. With the aim of identifying optically obscured O- and early B-type stars in the Carina Nebula, we performed the first infrared spectroscopic study of stars in the optically obscured stellar cluster Tr 16-SE, located behind a dark dust lane south of η Car. Methods. We used the integral-field spectrograph KMOS at the ESO VLT to obtain H- and K-band spectra with a resolution of R ≈ 4000 (Δλ ≈ 5 Å) for 45 out of the 47 possible OB candidate stars in Tr 16-SE, and we derived spectral types for these stars. Results. We find 15 stars in Tr 16-SE with spectral types between O5 and B2 (i.e., high-mass stars with M ≥ 8 M⊙), only two of which were known before. An additional nine stars are classified as (Ae)Be stars (i.e., intermediate-mass pre-main-sequence stars), and most of the remaining targets show clear signatures of being late-type stars and are thus most likely foreground stars or background giants unrelated to the Carina Nebula. Our estimates of the stellar luminosities suggest that nine of the 15 O- and early B-type stars are members of Tr 16-SE, whereas the other six seem to be background objects. Conclusions. Our study increases the number of spectroscopically identified high-mass stars (M ≥ 8 M⊙) in Tr 16-SE from two to nine and shows that Tr 16-SE is one of the larger clusters in the Carina Nebula. Our identification of three new stars with spectral types between O5 and O7 and four new stars with spectral types O9 to B1 significantly increases the number of spectroscopically identified O-type stars in the Carina Nebula.

Synergistic Effects of Ca2+ and High-Valence Nb5+ co-Doping on the Structural, Optical and Magnetic Properties of BiFeO3

The structural, optical and magnetic properties of BiFeO3, BiFe0.99Nb0.01O3 and Bi1 − xCaxFe0.99Nb0.01O3 (BCFNO, 0 ≤ x ≤ 0.25) nanoparticles synthesized via sol-gel method are investigated. It has been found that a phase transition from the rhombohedral R3c structure (x ≤ 0.10) to the ideal cubic perovskite structure (x = 0.25) which can be attributed to Ca2+ doping. Increasing Ca2+ dopants results in the increase of oxygen vacancies. As doping amount x increase, the band gap of BCFNO decreases and the valence band spectra indicates that it’s a p-type semiconductor, which indicates their favorable potential in photocatalytic applications. The remnant magnetization Mr of BCFNO reaches a maximum value (0.146 emu/g about 15 times compared with pure BFO) at x = 0.10. This enhancement of magneitic properties in BCFNO can be ascribed to the synergistic effect of A and B site ions co-doping. Higher valance Nb doping cause the size effect and the magnetic polarons bounded to the impurities by Ca ions.

Estimating black hole masses in obscured AGN using X-rays

ABSTRACT Determining the black hole masses in active galactic nuclei (AGN) is of crucial importance to constrain the basic characteristics of their central engines and shed light on their growth and co-evolution with their host galaxies. While the black hole mass (MBH) can be robustly measured with dynamical methods in bright type 1 AGN, where the variable primary emission and the broad-line region are directly observed, a direct measurement is considerably more challenging if not impossible for the vast majority of heavily obscured type 2 AGN. In this work, we tested the validity of an X-ray-based scaling method to constrain the MBH in heavily absorbed AGN. To this end, we utilized a sample of type 2 AGN with good-quality hard X-ray data obtained by the Nuclear Spectroscopic Telescope Array satellite and with MBH dynamically constrained from megamaser measurements. Our results indicate that, when the X-ray broad-band spectra are fitted with physically motivated self-consistent models that properly account for absorption, scattering, and emission-line contributions from the putative torus and constrain the primary X-ray emission, then the X-ray scaling method yields MBH values that are consistent with those determined from megamaser measurements within their respective uncertainties. With this method, we can therefore systematically determine the MBH in any type 2 AGN, provided that they possess good-quality X-ray data and accrete at a moderate to high rate.