8,872 research outputs found
Electronic structure and magnetism in doped semiconducting half-Heusler compounds
We have studied in details the electronic structure and magnetism in M (Mn
and Cr) doped semiconducting half-Heusler compounds FeVSb, CoTiSb and NiTiSn
(XMYZ) in a wide concentration range using local-spin density
functional method in the framework of tight-binding linearized muffin tin
orbital method(TB-LMTO) and supercell approach. Our calculations indicate that
some of these compounds are not only ferromagnetic but also half-metallic and
may be useful for spintronics applications. The electronic structure of the
doped systems is analyzed with the aid of a simple model where we have
considered the interaction between the dopant transition metal (M) and the
valence band X-Z hybrid. We have shown that the strong X-d - M-d interaction
places the M-d states close to the Fermi level with the M-t states lying
higher in energy in comparison to the M-e states. Depending on the number
of available d-electrons, ferromagnetism is realized provided the d-manifold is
partially occupied. The tendencies toward ferromagnetic(FM) or
antiferromagnetic(AFM) behavior are discussed within Anderson-Hasegawa models
of super-exchange and double-exchange. In our calculations for Mn doped NiTiSn,
the strong preference for FM over AFM ordering suggests a possible high Curie
temperature for these systems.Comment: 14 pages, 6 figure
Potential inversion with subbarrier fusion data revisited
We invert experimental data for heavy-ion fusion reactions at energies well
below the Coulomb barrier in order to directly determine the internucleus
potential between the colliding nuclei. In contrast to the previous
applications of the inversion formula, we explicitly take into account the
effect of channel couplings on fusion reactions, by assuming that fusion cross
sections at deep subbarrier energies are governed by the lowest barrier in the
barrier distribution. We apply this procedure to the O +Sm and
O +Pb reactions, and find that the inverted internucleus
potential are much thicker than phenomenological potentials. A relation to the
steep fall-off phenomenon of fusion cross sections recently found at deep
subbarrier energies is also discussed.Comment: 5 pages, 3 eps figure
Probing surface diffuseness of nucleus-nucleus potential with quasielastic scattering at deep sub-barrier energies
We perform a systematic study on the surface property of nucleus-nucleus
potential in heavy-ion reactions using large-angle quasielastic scattering at
energies well below the Coulomb barrier. At these energies, the quasielastic
scattering can be well described by a single-channel potential model.
Exploiting this fact, we point out that systems which involve spherical nuclei
require the diffuseness parameter of around 0.60 fm in order to fit the
experimental data, while systems with a deformed target between 0.8 fm and 1.1
fm.Comment: 6 pages, 6 figure
Correlation effects on the electronic structure of TiOCl: a NMTO+DMFT study
Using the recently developed N-th order muffin-tin orbital-based downfolding
technique in combination with the Dynamical Mean Field theory, we investigate
the electronic properties of the much discussed Mott insulator TiOCl in the
undimerized phase. Inclusion of correlation effects through this approach
provides a description of the spectral function into an upper and a lower
Hubbard band with broad valence states formed out of the orbitally polarized,
lower Hubbard band. We find that these results are in good agreement with
recent photo-emission spectra.Comment: 4 pages, 3 figure
Third-Generation TB-LMTO
We describe the screened Korringa-Kohn-Rostoker (KKR) method and the
third-generation linear muffin-tin orbital (LMTO) method for solving the
single-particle Schroedinger equation for a MT potential. The simple and
popular formalism which previously resulted from the atomic-spheres
approximation (ASA) now holds in general, that is, it includes downfolding and
the combined correction. Downfolding to few-orbital, possibly short-ranged,
low-energy, and possibly orthonormal Hamiltonians now works exceedingly well,
as is demonstrated for a high-temperature superconductor. First-principles sp3
and sp3d5 TB Hamiltonians for the valence and lowest conduction bands of
silicon are derived. Finally, we prove that the new method treats overlap of
the potential wells correctly to leading order and we demonstrate how this can
be exploited to get rid of the empty spheres in the diamond structure.Comment: latex2e, 32 printed pages, Postscript figs, to be published in:
Tight-Binding Approach to Computational Materials Science, MRS Symposia
Proceedings No. 491 (MRS, Pittsburgh, 1998
Geometric Transition versus Cascading Solution
We study Vafa's geometric transition and Klebanov - Strassler solution from
various points of view in M-theory. In terms of brane configurations, we show
the detailed equivalences between the two models. In some limits, both models
have an alternative realization as fourfolds in M-theory with appropriate
G-fluxes turned on. We discuss some aspects of the fourfolds including how to
see the transition and a possible extension to the non-supersymmetric case.Comment: 34 pages, LaTex, 2 figures; v2: Some comments added and references
updated. Final version to appear in JHE
Equilibrium glassy phase in a polydisperse hard sphere system
The phase diagram of a polydisperse hard sphere system is examined by
numerical minimization of a discretized form of the Ramakrishnan-Yussouff free
energy functional. Crystalline and glassy local minima of the free energy are
located and the phase diagram in the density-polydispersity plane is mapped out
by comparing the free energies of different local minima. The crystalline phase
disappears and the glass becomes the equilibrium phase beyond a "terminal"
value of the polydispersity. A crystal to glass transition is also observed as
the density is increased at high polydispersity. The phase diagram obtained in
our study is qualitatively similar to that of hard spheres in a quenched random
potential.Comment: 4 pages, 4 figure
Validity of the linear coupling approximation in heavy-ion fusion reactions at sub barrier energies
The role of higher order coupling of surface vibrations to the relative
motion in heavy-ion fusion reactions at near-barrier energies is investigated.
The coupled channels equations are solved to all orders, and also in the linear
and the quadratic coupling approximations. Taking Ni + Zr
reactions as examples, it is shown that all order couplings lead to
considerably improved agreement with the experimentally measured fusion cross
sections and average angular momenta of the compound nucleus for such heavy
nearly symmetric systems. The importance of higher order coupling is also
examined for asymmetric systems like O + Cd, Sm, for
which previous calculations of the fusion cross section seemed to indicate that
the linear coupling approximation was adequate. It is shown that the shape of
the barrier distributions and the energy dependence of the average angular
momentum can change significantly when the higher order couplings are included,
even for systems where measured fusion cross sections may seem to be well
reproduced by the linear coupling approximation.Comment: Latex file, 15 pages, 6 figure
Laser induced reentrant freezing in two-dimensional attractive colloidal systems
The effects of an externally applied one-dimensional periodic potential on
the freezing/melting behaviour of two-dimensional systems of colloidal
particles with a short-range attractive interaction are studied using Monte
Carlo simulations. In such systems, incommensuration results when the
periodicity of the external potential does not match the length-scale at which
the minimum of the attractive potential occurs. To study the effects of this
incommensuration, we consider two different models for the system. Our
simulations for both these models show the phenomenon of reentrant freezing as
the strength of the periodic potential is varied. Our simulations also show
that different exotic phases can form when the strength of the periodic
potential is high, depending on the length-scale at which the minimum of the
attractive pair-potential occurs.Comment: 24 pages (including figures) in preprint forma
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