1,646 research outputs found
XPS Study of Superconducting LiTi2O4 and LiTi2-xCuxO4 Sol-Gel Derived Powders and Thin Films
In this work X-ray photoelectron studies of lithium titanate and copper doped lithium titanate are presented. Both, powder and thin lms samples were prepared by sol gel method. After preparation, the samples were heated in argon atmosphere at various temperatures in a range from 500 ◦C to 600 ◦C for 20 h. The crystalline structure of the samples was investigated by X-ray di raction, while the oxidation states of the elements were examined by X-ray photoelectron spectroscopy method. X-ray di raction measurements con rmed spinel phase of all manufactured samples. However it is well known that electrical and superconducting properties of lithium titanate are strongly correlated not only with structure, but also with oxidation state of Ti ions. X-ray photoelectron spectroscopy investigations revealed mixture of Ti3+ and Ti4+ ions, although the Ti3+/Ti4+ ratio is much smaller than 1 2 needed for superconductivity. In this work dependence between calcination temperature as well as amount of Cu dopant and Ti3+/Ti4+ proportion are reported
Superconducting properties of VN-SiO 2 sol-gel derived thin films
In this work studies of structure and superconducting properties of VN SiO2 lms are reported. The lms
were obtained through thermal nitridation (ammonolysis) of sol gel derived V2O3 SiO2 coatings (in a proper
V2O3/SiO2 ratio) at 1200 ◦ C. This process leads to the formation of disordered structure with VN metallic grains
dispersed in the insulating SiO2 matrix. The structural transformations occurring in the lms as a result of
ammonolysis were studied using X-ray photoelectron spectroscopy (XPS). The critical superconducting parameters are obtained. The magnetoresistance at high magnetic elds has been investigated
Migratory chondrogenic progenitor cells from repair tissue during the later stages of human osteoarthritis.
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
cm and cm, 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
Ferromagnetic GaMnAs/GaAs superlattices - MBE growth and magnetic properties
We have studied the magnetic properties of (GaMnAs)m/(GaAs)n superlattices
with magnetic GaMnAs layers of thickness between 8 and 16 molecular layers (ML)
(23-45 \AA), and with nonmagnetic GaAs spacers from 4 ML to 10 ML (11-28 \AA).
While previous reports state that GaMnAs layers thinner than 50 \AA are
paramagnetic in the whole Mn composition range achievable using MBE growth (up
to 8% Mn), we have found that short period superlattices exhibit a
paramagnetic-to-ferromagnetic phase transition with a transition temperature
which depends on both the thickness of the magnetic GaMnAs layer and the
nonmagnetic GaAs spacer. The neutron scattering experiments have shown that the
magnetic layers in superlattices are ferromagnetically coupled for both thin
(below 50 \AA) and thick (above 50 \AA) GaMnAs layers.Comment: Proceedings of 4th International Workshop on Molecular Beam Epitaxy
and Vapour Phase Epitaxy Growth Physics and Technology, September 23 - 28
(2001), Warszawa, Poland, to appear in Thin Solid Films. 24 pages, 8 figure
What Developers Want and Need from Program Analysis: An Empirical Study
Program Analysis has been a rich and fruitful field of research for many decades, and countless high quality program analysis tools have been produced by academia. Though there are some well-known examples of tools that have found their way into routine use by practitioners, a common challenge faced by researchers is knowing how to achieve broad and lasting adoption of their tools. In an effort to understand what makes a program analyzer most attractive to developers, we mounted a multi-method investigation at Microsoft. Through interviews and surveys of developers as well as analysis of defect data, we provide insight and answers to four high level research questions that can help researchers design program analyzers meeting the needs of software developers.
First, we explore what barriers hinder the adoption of program analyzers, like poorly expressed warning messages. Second, we shed light on what functionality developers want from analyzers, including the types of code issues that developers care about. Next, we answer what non-functional characteristics an analyzer should have to be widely used, how the analyzer should fit into the development process, and how its results should be reported. Finally, we investigate defects in one of Microsoft's flagship software services, to understand what types of code issues are most important to minimize, potentially through program analysis
Indirect exchange in GaMnAs bilayers via spin-polarized inhomogeneous hole gas: Monte Carlo simulation
The magnetic order resulting from an indirect exchange between magnetic
moments provided by spin-polarized hole gas in the metallic phase of a GaMnAs
double layer structure is studied via Monte Carlo simulation. The coupling
mechanism involves a perturbative calculation in second order of the
interaction between the magnetic moments and carriers (holes). We take into
account a possible polarization of the hole gas due to the existence of an
average magnetization in the magnetic layers, establishing, in this way, a
self-consistency between the magnetic order and the electronic structure. That
interaction leads to an internal ferromagnetic order inside each layer, and a
parallel arrangement between their magnetizations, even in the case of thin
layers. This fact is analyzed in terms of the inter- and intra-layer
interactions.Comment: 17 pages and 14 figure
Prospects of high temperature ferromagnetism in (Ga,Mn)As semiconductors
We report on a comprehensive combined experimental and theoretical study of
Curie temperature trends in (Ga,Mn)As ferromagnetic semiconductors. Broad
agreement between theoretical expectations and measured data allows us to
conclude that T_c in high-quality metallic samples increases linearly with the
number of uncompensated local moments on Mn_Ga acceptors, with no sign of
saturation. Room temperature ferromagnetism is expected for a 10% concentration
of these local moments. Our magnetotransport and magnetization data are
consistnent with the picture in which Mn impurities incorporated during growth
at interstitial Mn_I positions act as double-donors and compensate neighboring
Mn_Ga local moments because of strong near-neighbor Mn_Ga-Mn_I
antiferromagnetic coupling. These defects can be efficiently removed by
post-growth annealing. Our analysis suggests that there is no fundamental
obstacle to substitutional Mn_Ga doping in high-quality materials beyond our
current maximum level of 6.2%, although this achievement will require further
advances in growth condition control. Modest charge compensation does not limit
the maximum Curie temperature possible in ferromagnetic semiconductors based on
(Ga,Mn)As.Comment: 13 pages, 12 figures, submitted to Phys. Rev.
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 cm 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
- …