330 research outputs found
Electronic Structure and Bulk Spin Valve Behavior in CaRuO
We report density functional calculations of the magnetic properties and
Fermiology of CaRuO. The ground state consists of ferromagnetic
bilayers, stacked antiferromagnetically. The bilayers are almost but not
exactly half-metallic. In the ferromagnetic state opposite spin polarizations
are found for in-plane and out-of-plane transport. Relatively high out of plane
conductivity is found for the majority spin, which is relatively weakly
conductive in-plane. In the ground state in-plane quantities are essentially
the same, but the out of plane transport is strongly reduced.Comment: 5 page
Investigations on finite ideal quantum gases
Recursion formulae of the N-particle partition function, the occupation
numbers and its fluctuations are given using the single-particle partition
function. Exact results are presented for fermions and bosons in a common
one-dimensional harmonic oscillator potential, for the three-dimensional
harmonic oscillator approximations are tested. Applications to excited nuclei
and Bose-Einstein condensation are discussed.Comment: 13 pages, 7 postscript figures, uses 'epsfig.sty'. Submitted to
Physica A. More information available at
http://obelix.physik.uni-osnabrueck.de/~schnack
First-Principles Calculation of Born Effective Charges and Spontaneous Polarization of Ferroelectric Bismuth Titanate
In this study, we present the results of our first-principles calculations of
the band structure, density of states and the Born effective charge tensors for
the ferroelectric (ground state B1a1) and paraelectric (I4/mmm) phases of
bismuth titanate. The calculations are done using the generalized gradient
approximation (GGA) as well as the local density approximation (LDA) of the
density functional theory. In contrast to the literature, our calculations on
B1a1 structure using GGA and LDA yield smaller indirect band gaps as compared
to the direct band gaps, in agreement with the experimental data. The density
of states shows considerable hybridization among Ti 3d, Bi 6p and O 2p states
indicating covalent nature of the bonds leading to the ferroelectric
instability. The Born effective charge tensors of the constituent ions for the
ground state (B1a1) and paraelectric (I4/mmm) structures were calculated using
the Berry phase method. This is followed by the calculation of the spontaneous
polarization for the ferroelectric B1a1 phase using the Born effective charge
tensors of the individual ions. The calculated value for the spontaneous
polarization of ferroelectric bismuth titanate using different Born effective
charges was found to be in the range of 55+/-13 C/cm2 in comparison to the
reported experimental value of (50+/-10 C/cm2) for single crystals. The
origin of ferroelectricity is attributed to the relatively large displacements
of those oxygen ions in the TiO6 octahedra that lie along the a-axis of the
bismuth titanate crystal.Comment: 36 pages, 5 figure
Level Density of a Bose Gas and Extreme Value Statistics
We establish a connection between the level density of a gas of
non-interacting bosons and the theory of extreme value statistics. Depending on
the exponent that characterizes the growth of the underlying single-particle
spectrum, we show that at a given excitation energy the limiting distribution
function for the number of excited particles follows the three universal
distribution laws of extreme value statistics, namely Gumbel, Weibull and
Fr\'echet. Implications of this result, as well as general properties of the
level density at different energies, are discussed.Comment: 4 pages, no figure
Chalcogen Height Dependence of Magnetism and Fermiology in FeTe_xSe_{1-x}
FeTexSe1-x (x=0, 0.25, 0.50, 0.75 and 1) system has been studied using
density functional theory. Our results show that for FeSe, LDA seems better
approximation in terms of magnitude of magnetic energy whereas GGA
overestimates it largely. On the other hand for FeTe, GGA is better
approximation that gives experimentally observed magnetic state. It has been
shown that the height of chalcogen atoms above Fe layers has significant effect
on band structure, electronic density of states (DOS) at Fermi level N(EF) and
Fermi surfaces. For FeSe the value of N(EF) is small so as to satisfy Stoner
criteria for ferromagnetism, (I\timesN(EF)\geq1) whereas for FeTe, since the
value of N(EF) is large, the same is close to be satisfied. Force minimization
done for FeTexSe1-x using supercell approach shows that in disordered system Se
and Te do not share same site and have two distinct z coordinates. This has
small effect on magnetic energy but no significant difference in band structure
and DOS near EF when calculated using either relaxed or average value of z for
chalcogen atoms. Thus substitution of Se at Te site decreases average value of
chalcogen height above Fe layers which in turn affect the magnetism and
Fermiology in the system. By using coherent-potential approximation for
disordered system we found that height of chalcogen atoms above Fe layer rather
than chalcogen species or disorder in the anion planes, affect magnetism and
shape of Fermi surfaces (FS), thus significantly altering nesting conditions,
which govern antiferromagnetic spin fluctuations in the system.Comment: 24 pages Text+Figs: comments/suggestions welcome
([email protected]
Optical Anisotropy in Bismuth Titanate: An Experimental and Theoretical Study
We report experimental and theoretical investigation of anisotropy in optical
properties and their origin in the ferroelectric and paraelectric phases of
bismuth titanate. Room temperature ellipsometric measurements performed on
pulsed laser deposited bismuth titanate thin films of different orientations
show anisotropy in the dielectric and optical constants, Subsequent
first-principles calculations performed on the ground state structures of
ferroelectric and high temperature paraelectric phases of bismuth titanate show
that the material demonstrates anisotropic optical behavior in both
ferroelectric and paraelectric phases. We further show that O 2p to Ti 3d
transition is the primary origin of optical activity of the material while
optical anisotropy results from the asymmetrically oriented Ti-O bonds in TiO6
octehdra in the unit cell.Comment: 13 pages, 4 figure
Superconformal Yang-Mills quantum mechanics and Calogero model with OSp(N|2,R) symmetry
In spacetime dimension two, pure Yang-Mills possesses no physical degrees of
freedom, and consequently it admits a supersymmetric extension to couple to an
arbitrary number, N say, of Majorana-Weyl gauginos. This results in (N,0) super
Yang-Mills. Further, its dimensional reduction to mechanics doubles the number
of supersymmetries, from N to N+N, to include conformal supercharges, and leads
to a superconformal Yang-Mills quantum mechanics with symmetry group
OSp(N|2,R). We comment on its connection to AdS_2 \times S^{N-1} and reduction
to a supersymmetric Calogero model.Comment: 1+28 pages, no figure; Refs added. To appear in JHE
<i>C-elegans</i> model identifies genetic modifiers of alpha-synuclein inclusion formation during aging
Inclusions in the brain containing alpha-synuclein are the pathological hallmark of Parkinson's disease, but how these inclusions are formed and how this links to disease is poorly understood. We have developed a <i>C-elegans</i> model that makes it possible to monitor, in living animals, the formation of alpha-synuclein inclusions. In worms of old age, inclusions contain aggregated alpha-synuclein, resembling a critical pathological feature. We used genome-wide RNA interference to identify processes involved in inclusion formation, and identified 80 genes that, when knocked down, resulted in a premature increase in the number of inclusions. Quality control and vesicle-trafficking genes expressed in the ER/Golgi complex and vesicular compartments were overrepresented, indicating a specific role for these processes in alpha-synuclein inclusion formation. Suppressors include aging-associated genes, such as sir-2.1/SIRT1 and lagr-1/LASS2. Altogether, our data suggest a link between alpha-synuclein inclusion formation and cellular aging, likely through an endomembrane-related mechanism. The processes and genes identified here present a framework for further study of the disease mechanism and provide candidate susceptibility genes and drug targets for Parkinson's disease and other alpha-synuclein related disorders
Compounds from an Unbiased Chemical Screen Reverse Both Er-to-Golgi Trafficking Defects and Mitochondrial Dysfunction in Parkinson's Disease Models
α-Synuclein (α-syn) is a small lipid-binding protein involved in vesicle trafficking whose function is poorly characterized. It is of great interest to human biology and medicine because α-syn dysfunction is associated with several neurodegenerative disorders, including Parkinson’s disease (PD). We previously created a yeast model of α-syn pathobiology, which established vesicle trafficking as a process that is particularly sensitive to α-syn expression. We also uncovered a core group of proteins with diverse activities related to α-syn toxicity that is conserved from yeast to mammalian neurons. Here, we report that a yeast strain expressing a somewhat higher level of α-syn also exhibits strong defects in mitochondrial function. Unlike our previous strain, genetic suppression of endoplasmic reticulum (ER)-to-Golgi trafficking alone does not suppress α-syn toxicity in this strain. In an effort to identify individual compounds that could simultaneously rescue these apparently disparate pathological effects of α-syn, we screened a library of 115,000 compounds. We identified a class of small molecules that reduced α-syn toxicity at micromolar concentrations in this higher toxicity strain. These compounds reduced the formation of α-syn foci, re-established ER-to-Golgi trafficking and ameliorated α-syn-mediated damage to mitochondria. They also corrected the toxicity of α-syn in nematode neurons and in primary rat neuronal midbrain cultures. Remarkably, the compounds also protected neurons against rotenone-induced toxicity, which has been used to model the mitochondrial defects associated with PD in humans. That single compounds are capable of rescuing the diverse toxicities of α-syn in yeast and neurons suggests that they are acting on deeply rooted biological processes that connect these toxicities and have been conserved for a billion years of eukaryotic evolution. Thus, it seems possible to develop novel therapeutic strategies to simultaneously target the multiple pathological features of PD.MGH/MIT Morris Udall Center of Excellence in Parkinson Disease Research (NS038372)Michael J. Fox Foundation for Parkinson's ResearchHoward Hughes Medical InstituteUnited States. National Institutes of Health (NS049221)American Parkinson Disease Association, Inc
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