3,579 research outputs found
Magnetic properties of microtektites Semiannual status report, 1 Jan. - 31 Jun. 1969
Magnetic susceptibility, magnetization, and Curie constants for normal and bottle-green microtektites found in deep-sea sediment core
Preliminary ultraviolet reflectance of some rocks and minerals from 2000 angstrom to 3000 angstrom
Ultraviolet reflectance measurements of rocks and minerals from 2000 A to 3000
Melt-processed bulk superconductors: Fabrication and characterization for power and space applications
Melt-process bulk superconducting materials based on variations on the base YBa2Cu3O(x) were produced in a variety of shapes and forms. Very high values of both zero-field and high-field magnetization were observed. These are useful for levitation and power applications. Magnetic measurements show that the effects of field direction and intensity, temperature and time are consistent with an aligned grain structure with multiple pinning sites and with models of thermally activated flux motion
Hopping on the Bethe lattice: Exact results for densities of states and dynamical mean-field theory
We derive an operator identity which relates tight-binding Hamiltonians with
arbitrary hopping on the Bethe lattice to the Hamiltonian with nearest-neighbor
hopping. This provides an exact expression for the density of states (DOS) of a
non-interacting quantum-mechanical particle for any hopping. We present
analytic results for the DOS corresponding to hopping between nearest and
next-nearest neighbors, and also for exponentially decreasing hopping
amplitudes. Conversely it is possible to construct a hopping Hamiltonian on the
Bethe lattice for any given DOS. These methods are based only on the so-called
distance regularity of the infinite Bethe lattice, and not on the absence of
loops. Results are also obtained for the triangular Husimi cactus, a recursive
lattice with loops. Furthermore we derive the exact self-consistency equations
arising in the context of dynamical mean-field theory, which serve as a
starting point for studies of Hubbard-type models with frustration.Comment: 14 pages, 9 figures; introduction expanded, references added;
published versio
Random site dilution properties of frustrated magnets on a hierarchical lattice
We present a method to analyze magnetic properties of frustrated Ising spin
models on specific hierarchical lattices with random dilution. Disorder is
induced by dilution and geometrical frustration rather than randomness in the
internal couplings of the original Hamiltonian. The two-dimensional model
presented here possesses a macroscopic entropy at zero temperature in the large
size limit, very close to the Pauling estimate for spin-ice on pyrochlore
lattice, and a crossover towards a paramagnetic phase. The disorder due to
dilution is taken into account by considering a replicated version of the
recursion equations between partition functions at different lattice sizes. An
analysis at first order in replica number allows for a systematic
reorganization of the disorder configurations, leading to a recurrence scheme.
This method is numerically implemented to evaluate the thermodynamical
quantities such as specific heat and susceptibility in an external field.Comment: 26 pages, 11 figure
The absence of finite-temperature phase transitions in low-dimensional many-body models: a survey and new results
After a brief discussion of the Bogoliubov inequality and possible
generalizations thereof, we present a complete review of results concerning the
Mermin-Wagner theorem for various many-body systems, geometries and order
parameters. We extend the method to cover magnetic phase transitions in the
periodic Anderson Model as well as certain superconducting pairing mechanisms
for Hubbard films. The relevance of the Mermin-Wagner theorem to approximations
in many-body physics is discussed on a conceptual level.Comment: 33 pages; accepted for publication as a Topical Review in Journal of
Physics: Condensed Matte
Computational study of structural and elastic properties of random AlGaInN alloys
In this work we present a detailed computational study of structural and
elastic properties of cubic AlGaInN alloys in the framework of Keating valence
force field model, for which we perform accurate parametrization based on state
of the art DFT calculations. When analyzing structural properties, we focus on
concentration dependence of lattice constant, as well as on the distribution of
the nearest and the next nearest neighbour distances. Where possible, we
compare our results with experiment and calculations performed within other
computational schemes. We also present a detailed study of elastic constants
for AlGaInN alloy over the whole concentration range. Moreover, we include
there accurate quadratic parametrization for the dependence of the alloy
elastic constants on the composition. Finally, we examine the sensitivity of
obtained results to computational procedures commonly employed in the Keating
model for studies of alloys
Atomistic modeling of amorphous silicon carbide: An approximate first-principles study in constrained solution space
Localized basis ab initio molecular dynamics simulation within the density
functional framework has been used to generate realistic configurations of
amorphous silicon carbide (a-SiC). Our approach consists of constructing a set
of smart initial configurations that conform essential geometrical and
structural aspects of the materials obtained from experimental data, which is
subsequently driven via first-principles force-field to obtain the best
solution in a reduced solution space. A combination of a priori information
(primarily structural and topological) along with the ab-initio optimization of
the total energy makes it possible to model large system size (1000 atoms)
without compromising the quantum mechanical accuracy of the force-field to
describe the complex bonding chemistry of Si and C. The structural, electronic
and the vibrational properties of the models have been studied and compared to
existing theoretical models and available data from experiments. We demonstrate
that the approach is capable of producing large, realistic configurations of
a-SiC from first-principles simulation that display excellent structural and
electronic properties of a-SiC. Our study reveals the presence of predominant
short-range order in the material originating from heteronuclear Si-C bonds
with coordination defect concentration as small as 5% and the chemical disorder
parameter of about 8%.Comment: 16 pages, 7 figure
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