13,668 research outputs found
Compositional disorder and its influence on the structural, electronic and magnetic properties of MgC(Ni_{1-x}Co_{x})_{3} alloys using first-principles
First-principles, density-functional based electronic structure calculations
are carried out for MgC(Ni_{1-x}Co_{x})_{3} alloys over the concentration range
0\leq x\leq1, using Korringa-Kohn-Rostoker coherent-potential approximation
(KKR CPA) method in the atomic sphere approximation (ASA). The self-consistent
calculations are used to study the changes as a function of x in the equation
of state parameters, total and partial densities of states, magnetic moment and
the on-site exchange interaction parameter. To study the magnetic properties as
well as its volume dependence, fixed-spin moment calculations in conjunction
with the phenomenological Landau theory are employed. The salient features that
emerge from these calculations are (i) a concentration independent variation in
the lattice parameter and bulk modulus at x~0.75 with an anomaly in the
variation of the pressure derivative of bulk modulus, (ii) the fixed-spin
moment based corrections to the overestimated magnetic ground state for 0.0\leq
x\leq0.3 alloys, making the results consistent with the experiments, and (iii)
the possibility of multiple magnetic states at x~0.75, which, however, requires
further improvements in the calculations
A first-principles comparison of the electronic properties of MgC_{y}Ni_{3} and ZnC_{y}Ni_{3} alloys
First-principles, density-functional-based electronic structure calculations
are employed to study the changes in the electronic properties of ZnC_{y}Ni_{3}
and MgC_{y}Ni_{3} using the Korringa-Kohn-Rostoker coherent-potential
approximation method in the atomic sphere approximation (KKR-ASA CPA). As a
function of decreasing C at%, we find a steady decrease in the lattice constant
and bulk modulus in either alloys. However, the pressure derivative of the bulk
modulus displays an opposite trend. Following the Debye model, which relates
the pressure derivative of the bulk modulus with the average phonon frequency
of the crystal, it can thus be argued that ZnCNi_{3} and its disordered alloys
posses a different phonon spectra in comparison to its MgCNi_{3} counterparts.
This is further justified by the marked similarity we find in the electronic
structure properties such as the variation in the density of states and the
Hopfield parameters calculated for these alloys. The effects on the equation of
state parameters and the density of states at the Fermi energy, for partial
replacement of Mg by Zn are also discussed.Comment: 19 pages, 15 figure
Near-Infrared Properties of Metal-poor Globular Clusters in the Galactic Bulge Direction
Aims. J, H, and K' images obtained from the near-infrared imager CFHTIR on
the Canada-France-Hawaii Telescope are used to derive the morphological
parameters of the red giant branch (RGB) in the near-infrared color-magnitude
diagrams for 12 metal-poor globular clusters in the Galactic bulge direction.
Using the compiled data set of the RGB parameters for the observed 12 clusters,
in addition to the previously studied 5 clusters, we discuss the properties of
the RGB morphology for the clusters and compare them with the calibration
relations for the metal-rich bulge clusters and the metal-poor halo clusters.
Methods. The photometric RGB shape indices such as colors at fixed magnitudes
of MK = MH = (-5.5, -5, -4, and -3), magnitudes at fixed colors of (J - K)o =
(J - H)o = 0.7, and the RGB slope are measured from the fiducial normal points
defined in the near- infrared color-magnitude diagrams for each cluster. The
magnitudes of RGB bump and tip are also estimated from the differential and
cumulative luminosity functions of the selected RGB stars. The derived RGB
parameters have been used to examine the overall behaviors of the RGB
morphology as a function of cluster metallicity. Results. The correlations
between the near-infrared photometric RGB shape indices and the cluster
metallicity for the programme clusters compare favorably with the previous
observational calibration relations for metal-rich clusters in the Galactic
bulge and the metal-poor halo clusters. The observed near-infrared magnitudes
of the RGB bump and tip for the investigated clusters are also in accordance
with the previous calibration relations for the Galactic bulge clusters.Comment: 12 pages, 9 figures, accepted for publication in Astronomy &
Astrophysic
Local realizations of contact interactions in two- and three-body problems
Mathematically rigorous theory of the two-body contact interaction in three
dimension is reviewed. Local potential realizations of this proper contact
interaction are given in terms of Poschl-Teller, exponential and square-well
potentials. Three body calculation is carried out for the halo nucleus 11Li
using adequately represented contact interaction.Comment: submitted to Phys. Rev.
The quantum critical point in CeRhIn_5: a resistivity study
The pressure--temperature phase diagram of CeRhIn_5 has been studied under
high magnetic field by resistivity measurements. Clear signatures of a quantum
critical point has been found at a critical pressure of p_c = 2.5 GPa. The
field induced magnetic state in the superconducting state is stable up to the
highest field. At p_c the antiferromagnetic ground-state under high magnetic
field collapses very rapidly. Clear signatures of p_c are the strong
enhancement of the resistivity in the normal state and of the inelastic
scattering term. No clear T2 temperature dependence could be found for
pressures above T_c. From the analysis of the upper critical field within a
strong coupling model we present the pressure dependence of the coupling
parameter lambda and the gyromagnetic ratio g. No signatures of a spatially
modulated order parameter could be evidenced. A detailed comparison with the
magnetic field--temperature phase diagram of CeCoIn_5 is given. The comparison
between CeRhIn_5 and CeCoIn_5 points out the importance to take into account
the field dependence of the effective mass in the calculation of the
superconducting upper critical field H_c2. It suggests also that when the
magnetic critical field H_(0) becomes lower than H_c2 (0)$, the persistence of
a superconducting pseudo-gap may stick the antiferromagnetism to H_c2 (0).Comment: 15 pages, 20 figures, to be published in J. Phys. Soc. Jp
Thermal Degradation of Adsorbed Bottle-Brush Macromolecules: Molecular Dynamics Simulation
The scission kinetics of bottle-brush molecules in solution and on an
adhesive substrate is modeled by means of Molecular Dynamics simulation with
Langevin thermostat. Our macromolecules comprise a long flexible polymer
backbone with segments, consisting of breakable bonds, along with two side
chains of length , tethered to each segment of the backbone. In agreement
with recent experiments and theoretical predictions, we find that bond cleavage
is significantly enhanced on a strongly attractive substrate even though the
chemical nature of the bonds remains thereby unchanged.
We find that the mean bond life time decreases upon adsorption by
more than an order of magnitude even for brush molecules with comparatively
short side chains $N=1 \div 4$. The distribution of scission probability along
the bonds of the backbone is found to be rather sensitive regarding the
interplay between length and grafting density of side chains. The life time
declines with growing contour length as ,
and with side chain length as . The probability
distribution of fragment lengths at different times agrees well with
experimental observations. The variation of the mean length of the
fragments with elapsed time confirms the notion of the thermal degradation
process as a first order reaction.Comment: 15 pages, 7 figure
Spectroscopic and Mechanistic Studies of Heterodimetallic Forms of Metallo-β-lactamase NDM-1
In an effort to characterize the roles of each metal ion in metallo-β-lactamase NDM-1, heterodimetallic analogues (CoCo-, ZnCo-, and CoCd-) of the enzyme were generated and characterized. UV–vis, 1H NMR, EPR, and EXAFS spectroscopies were used to confirm the fidelity of the metal substitutions, including the presence of a homogeneous, heterodimetallic cluster, with a single-atom bridge. This marks the first preparation of a metallo-β-lactamase selectively substituted with a paramagnetic metal ion, Co(II), either in the Zn1 (CoCd-NDM-1) or in the Zn2 site (ZnCo-NDM-1), as well as both (CoCo-NDM-1). We then used these metal-substituted forms of the enzyme to probe the reaction mechanism, using steady-state and stopped-flow kinetics, stopped-flow fluorescence, and rapid-freeze-quench EPR. Both metal sites show significant effects on the kinetic constants, and both paramagnetic variants (CoCd- and ZnCo-NDM-1) showed significant structural changes on reaction with substrate. These changes are discussed in terms of a minimal kinetic mechanism that incorporates all of the data
Quarkonium and hydrogen spectra with spin dependent relativistic wave equation
A non-linear non-perturbative relativistic atomic theory introduces spin in
the dynamics of particle motion. The resulting energy levels of Hydrogen atom
are exactly same as the Dirac theory. The theory accounts for the energy due to
spin-orbit interaction and for the additional potential energy due to spin and
spin-orbit coupling. Spin angular momentum operator is integrated into the
equation of motion. This requires modification to classical Laplacian operator.
Consequently the Dirac matrices and the k operator of Dirac's theory are
dispensed with. The theory points out that the curvature of the orbit draws on
certain amount of kinetic and potential energies affecting the momentum of
electron and the spin-orbit interaction energy constitutes a part of this
energy. The theory is developed for spin 1/2 bound state single electron in
Coulomb potential and then further extended to quarkonium physics by
introducing the linear confining potential. The unique feature of this
quarkonium model is that the radial distance can be exactly determined and does
not have a statistical interpretation. The established radial distance is then
used to determine the wave function. The observed energy levels are used as the
input parameters and the radial distance and the string tension are predicted.
This ensures 100% conformance to all observed energy levels for the heavy
quarkonium.Comment: 14 pages, v7: Journal reference adde
Ultralow Temperature Studies of Nanometer Size Semiconductor Devices
Contains a description on one research project.Joint Services Electronics Program DAAL03-89-C-000
Electrical properties of isotopically enriched neutron-transmutation-doped ^{70} Ge:Ga near the metal-insulator transition
We report the low temperature carrier transport properties of a series of
nominally uncompensated neutron-transmutation doped (NTD) ^{70} Ge:Ga samples
very close to the critical concentration N_c for the metal-insulator
transition. The concentration of the sample closest to N_c is 1.0004N_c and it
is unambiguously shown that the critical conductivity exponent is 0.5.
Properties of insulating samples are discussed in the context of Efros and
Shklovskii's variable range hopping conduction.Comment: 8 pages using REVTeX, 8 figures, published versio
- …