Voids and Mn-rich inclusions in a (Ga,Mn)As ferromagnetic semiconductor investigated by transmission electron microscopy

Voids adjacent to both cubic (ZnS-type) and hexagonal (NiAs-type) Mn-rich nanocrystals are characterized using aberration-corrected transmission electron microscopy in annealed Ga0.995Mn0.005As magnetic semiconductor specimen grown by molecular beam epitaxy. Nano-beam electron diffraction measurements suggest that the nanocrystals exhibit deviations in lattice parameter from that of bulk MnAs. In situ annealing inside the electron microscope is used to study the nucleation, coalescence, and grain growth of individual nanocrystals. After annealing at 903 K, the magnetic transition temperature of the specimen likely to be dominated by the presence of cubic ferromagnetic nanocrystals

Dependence of Curie Temperature on the Thickness of Epitaxial (Ga,Mn)As Film

We present the magnetotransport properties of very thin (5 to 15 nm) single (Ga,Mn)As layers grown by low temperature molecular beam epitaxy. A lower (Ga,Mn)As thickness limit of 5 nm for the ferromagnetic phase and the dependence of the Curie temperature on (Ga,Mn)As thickness are determined from electrical transport measurements. The Curie temperature is determined to be 97 K for the thinnest ferromagnetic sample and is found to decrease for increasing layer thickness. A carrier density of ~7.1$\times10^{20}$ cm$^{-3}$ for the 5 nm thick (Ga,Mn)As layer is determined from Hall measurements. Differences between magnetotransport properties of thick and thin (Ga,Mn)As layers are observed and discussed.Comment: 6 pages, 4 figure

GaAs:Mn nanowires grown by molecular beam epitaxy of (Ga,Mn)As at MnAs segregation conditions

GaAs:Mn nanowires were obtained on GaAs(001) and GaAs(111)B substrates by molecular beam epitaxial growth of (Ga,Mn)As at conditions leading to MnAs phase separation. Their density is proportional to the density of catalyzing MnAs nanoislands, which can be controlled by the Mn flux and/or the substrate temperature. Being rooted in the ferromagnetic semiconductor (Ga,Mn)As, the nanowires combine one-dimensional properties with the magnetic properties of (Ga,Mn)As and provide natural, self assembled structures for nanospintronics.Comment: 13 pages, 6 figure

Stochastic Gravitational Wave Background from Coalescing Binary Black Holes

We estimate the stochastic gravitational wave (GW) background signal from the field population of coalescing binary stellar mass black holes (BHs) throughout the Universe. This study is motivated by recent observations of BH-Wolf-Rayet star systems and by new estimates in the metallicity abundances of star forming galaxies that imply BH-BH systems are more common than previously assumed. Using recent analytical results of the inspiral-merger-ringdown waveforms for coalescing binary BH systems, we estimate the resulting stochastic GW background signal. Assuming average quantities for the single source energy emissions, we explore the parameter space of chirp mass and local rate density required for detection by advanced and third generation interferometric GW detectors. For an average chirp mass of 8.7$M_{\odot}$, we find that detection through 3 years of cross-correlation by two advanced detectors will require a rate density, $r_0 \geq 0.5 \rm{Mpc}^{-3} \rm{Myr}^{-1}$. Combining data from multiple pairs of detectors can reduce this limit by up to 40%. Investigating the full parameter space we find that detection could be achieved at rates $r_0 \sim 0.1 \rm{Mpc}^{-3} \rm{Myr}^{-1}$ for populations of coalescing binary BH systems with average chirp masses of $\sim 15M_{\odot}$ which are predicted by recent studies of BH-Wolf-Rayet star systems. While this scenario is at the high end of theoretical estimates, cross-correlation of data by two Einstein Telescopes could detect this signal under the condition $r_0 \geq 10^{-3} \rm{Mpc}^{-3} \rm{Myr}^{-1}$. Such a signal could potentially mask a primordial GW background signal of dimensionless energy density, $\Omega_{\rm{GW}}\sim 10^{-10}$, around the (1--500) Hz frequency range.Comment: 22 pages, 5 figures, 2 tables, Accepted for publication by Ap

Characteristics of high-temperature deuterium plasma in RPI-type devices

The paper presents results of the spectroscopic and corpuscular measurements of pulsed plasma streams generated within the RPI-IBIS experimental device, which was operated with the pure deuterium puffing. Particular attention was paid to time-integrated and time-resolved measurements of selected spectral lines, i.e. Da, Db and Dg, emitted from deuterium discharges. The measurements made possible an assessment of the basic plasma parameters. The achievement of the local thermal equilibrium (LTE) was investigated. Energies of emitted ions, their total numbers, as well emissions of soft X-rays and fusion-produced neutrons vary considerably with a change of gas conditions

Preventing β-amyloid fibrillization and deposition: β-sheet breakers and pathological chaperone inhibitors

Central to the pathogenesis of Alzheimer's disease (AD) is the conversion of normal, soluble β-amyloid (sAβ) to oligomeric, fibrillar Aβ. This process of conformational conversion can be influenced by interactions with other proteins that can stabilize the disease-associated state; these proteins have been termed 'pathological chaperones'. In a number of AD models, intervention that block soluble Aβ aggregation, including β-sheet breakers, and compounds that block interactions with pathological chaperones, have been shown to be highly effective. When combined with early pathology detection, these therapeutic strategies hold great promise as effective and relatively toxicity free methods of preventing AD related pathology

MnAs dots grown on GaN(0001)-(1x1) surface

MnAs has been grown by means of MBE on the GaN(0001)-(1x1) surface. Two options of initiating the crystal growth were applied: (a) a regular MBE procedure (manganese and arsenic were delivered simultaneously) and (b) subsequent deposition of manganese and arsenic layers. It was shown that spontaneous formation of MnAs dots with the surface density of 1$\cdot 10^{11}$ cm$^{-2}$ and $2.5\cdot 10^{11}$ cm$^{-2}$, respectively (as observed by AFM), occurred for the layer thickness higher than 5 ML. Electronic structure of the MnAs/GaN systems was studied by resonant photoemission spectroscopy. That led to determination of the Mn 3d - related contribution to the total density of states (DOS) distribution of MnAs. It has been proven that the electronic structures of the MnAs dots grown by the two procedures differ markedly. One corresponds to metallic, ferromagnetic NiAs-type MnAs, the other is similar to that reported for half-metallic zinc-blende MnAs. Both system behave superparamagnetically (as revealed by magnetization measurements), but with both the blocking temperatures and the intra-dot Curie temperatures substantially different. The intra-dot Curie temperature is about 260 K for the former system while markedly higher than room temperature for the latter one. Relations between growth process, electronic structure and other properties of the studied systems are discussed. Possible mechanisms of half-metallic MnAs formation on GaN are considered.Comment: 20+ pages, 8 figure

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Voids adjacent to cubic (ZnS-type) and hexagonal (NiAs-type) Mn-rich nanocrystals are characterized using aberration-corrected transmission electron microscopy in an annealed Ga 0.995 Mn 0.005 As magnetic semiconductor specimen grown by molecular beam epitaxy. Nanobeam electron diffraction measurements suggest that the nanocrystals exhibit deviations in lattice parameter as compared to bulk MnAs. After annealing at 903 K, the magnetic transition temperature of the specimen is likely to be dominated by the presence of cubic ferromagnetic nanocrystals. In situ annealing inside the electron microscope is used to study the nucleation, coalescence, and grain growth of individual nanocrystals

CALIBRATION AND APPLICATION OF NUCLEAR TRACK DETECTORS FOR HIGH-TEMPERATURE PLASMA DIAGNOSTICS

Abstract The paper reports on features of so-called solid-state nuclear track detectors (SSNTDs), their calibration measurements performed with known ion beams, and their different applications for detailed studies of charged particle emissions from various high-temperatures plasma facilities

Small and large polarons in nickelates, manganites, and cuprates

By comparing the optical conductivities of La_{1.67}Sr_{0.33}NiO_{4} (LSNO), Sr_{1.5}La_{0.5}MnO_4 (SLMO), Nd_2CuO_{4-y} (NCO), and Nd_{1.96}Ce_{0.04}CuO_{4} (NCCO), we have identified a peculiar behavior of polarons in this cuprate family. While in LSNO and SLMO small polarons localize into ordered structures below a transition temperature, in those cuprates the polarons appear to be large, and at low T their binding energy decreases. This reflects into an increase of the polaron radius, which may trigger coherent transport.Comment: File latex, 15 p. incl. 4 Figs. epsf, to appear on the Journal of Superconductivity - Proc. "Stripes 1996" - Roma Dec 199