718 research outputs found
Photoelasticity of sodium silicate glass from first principles
Based on density-functional perturbation theory we have computed the
photoelastic tensor of a model of sodium silicate glass of composition
(NaO)(SiO) (NS3). The model (containig 84 atoms) is
obtained by quenching from the melt in combined classical and Car-Parrinello
molecular dynamics simulations. The calculated photoelastic coefficients are in
good agreement with experimental data. In particular, the calculation
reproduces quantitatively the decrease of the photoelastic response induced by
the insertion of Na, as measured experimentally.
The extension to NS3 of a phenomenological model developed in a previous work
for pure a-SiO indicates that the modulation upon strain of other
structural parameters besides the SiOSi angles must be invoked to explain the
change in the photoelstic response induced by Na
Maximizing nearest neighbour entanglement in finitely correlated qubit--chains
We consider translationally invariant states of an infinite one dimensional
chain of qubits or spin-1/2 particles. We maximize the entanglement shared by
nearest neighbours via a variational approach based on finitely correlated
states. We find an upper bound of nearest neighbour concurrence equal to
C=0.434095 which is 0.09% away from the bound C_W=0.434467 obtained by a
completely different procedure. The obtained state maximizing nearest neighbour
entanglement seems to approximate the maximally entangled mixed states (MEMS).
Further we investigate in detail several other properties of the so obtained
optimal state.Comment: 12 pages, 4 figures, 2nd version minor change
Basis-independent methods for the two-Higgs-doublet model II. The significance of tan(beta)
In the most general two-Higgs-doublet model (2HDM), there is no distinction
between the two complex hypercharge-one SU(2) doublet scalar fields, Phi_a
(a=1,2). Thus, any two orthonormal linear combinations of these two fields can
serve as a basis for the Lagrangian. All physical observables of the model must
therefore be basis-independent. For example, tan(beta)=/ is
basis-dependent and thus cannot be a physical parameter of the model. In this
paper, we provide a basis-independent treatment of the Higgs sector with
particular attention to the neutral Higgs boson mass-eigenstates, which
generically are not eigenstates of CP. We then demonstrate that all physical
Higgs couplings are indeed independent of tan(beta). In specialized versions of
the 2HDM, tan(beta) can be promoted to a physical parameter of the
Higgs-fermion interactions. In the most general 2HDM, the Higgs-fermion
couplings can be expressed in terms of a number of physical "tan(beta)--like"
parameters that are manifestly basis-independent. The minimal supersymmetric
extension of the Standard Model provides a simple framework for exhibiting such
effects.Comment: 56 pages, 5 tables, with Eq. (65) corrected (erratum to appear in
Physical Review D
Pressure-induced metal-insulator transition in MgV_2O_4
On the basis of experimental thermoelectric power results and ab initio
calculations, we propose that a metal-insulator transition takes place at high
pressure (approximately 6 GPa) in MgV_2O_4.Comment: 2 pages, 3 figures, accepted in Physica B (Strongly Correlated
Electron Systems '07
Eutactic quantum codes
We consider sets of quantum observables corresponding to eutactic stars.
Eutactic stars are systems of vectors which are the lower dimensional
``shadow'' image, the orthogonal view, of higher dimensional orthonormal bases.
Although these vector systems are not comeasurable, they represent redundant
coordinate bases with remarkable properties. One application is quantum secret
sharing.Comment: 6 page
Crystal structure of LaTiO_3.41 under pressure
The crystal structure of the layered, perovskite-related LaTiO_3.41
(La_5Ti_5O_{17+\delta}) has been studied by synchrotron powder x-ray
diffraction under hydrostatic pressure up to 27 GPa (T = 295 K). The
ambient-pressure phase was found to remain stable up to 18 GPa. A sluggish, but
reversible phase transition occurs in the range 18--24 GPa. The structural
changes of the low-pressure phase are characterized by a pronounced anisotropy
in the axis compressibilities, which are at a ratio of approximately 1:2:3 for
the a, b, and c axes. Possible effects of pressure on the electronic properties
of LaTiO_3.41 are discussed.Comment: 5 pages, 6 figure
Role of C in MgC_xNi_3 investigated from first principles
The influence of vacancies in the sub-lattice of , on its
structural, electronic and magnetic properties are studied by means of the
density-functional based Korringa-Kohn-Rostoker Green's function method
formulated in the atomic sphere approximation. Disorder is taken into account
by means of coherent-potential approximation. Characterizations representing
the change in the lattice properties include the variation in the equilibrium
lattice constants, bulk modulus and pressure derivative of the bulk modulus,
and that of electronic structure include the changes in the, total, partial and
-resolved density of states. The incipient magnetic properties are
studied by means of fixed-spin moment method of alloy theory, together in
conjunction with the phenomenological Ginzburg-Landau equation for magnetic
phase transition. The first-principles calculations reveal that due to the
breaking of the - bonds, some of the 3d states, which were lowered
in energy due to strong hybridization, are transfered back to higher energies
thereby increasing the itinerant character in the material. The Bloch spectral
densities evaluated at the high symmetry points however reveal that the charge
redistribution is not uniform over the cubic Brillouin zone, as new states are
seen to be created at the point, while a shift in the states on the
energy scale are seen at other high symmetry points
High Pressure X-Ray Diffraction Study of UMn2Ge2
Uranium manganese germanide, UMn2Ge2, crystallizes in body-centered
tetragonal ThCr2Si2 structure with space group I4/mmm, a = 3.993A and c =
10.809A under ambient conditions. Energy dispersive X-ray diffraction was used
to study the compression behaviour of UMn2Ge2 in a diamond anvil cell. The
sample was studied up to static pressure of 26 GPa and a reversible structural
phase transition was observed at a pressure of ~ 16.1 GPa. Unit cell parameters
were determined up to 12.4 GPa and the calculated cell volumes were found to be
well reproduced by a Murnaghan equation of state with K0 = 73.5 GPa and K' =
11.4. The structure of the high pressure phase above 16.0 GPa is quite
complicated with very broad lines and could not be unambiguously determined
with the available instrument resolution
Structural, elastic and thermal properties of cementite (FeC) calculated using Modified Embedded Atom Method
Structural, elastic and thermal properties of cementite (FeC) were
studied using a Modified Embedded Atom Method (MEAM) potential for iron-carbon
(Fe-C) alloys. Previously developed Fe and C single element potentials were
used to develop an Fe-C alloy MEAM potential, using a statistically-based
optimization scheme to reproduce structural and elastic properties of
cementite, the interstitial energies of C in bcc Fe as well as heat of
formation of Fe-C alloys in L and B structures. The stability of
cementite was investigated by molecular dynamics simulations at high
temperatures. The nine single crystal elastic constants for cementite were
obtained by computing total energies for strained cells. Polycrystalline
elastic moduli for cementite were calculated from the single crystal elastic
constants of cementite. The formation energies of (001), (010), and (100)
surfaces of cementite were also calculated. The melting temperature and the
variation of specific heat and volume with respect to temperature were
investigated by performing a two-phase (solid/liquid) molecular dynamics
simulation of cementite. The predictions of the potential are in good agreement
with first-principles calculations and experiments.Comment: 12 pages, 9 figure
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