1,319 research outputs found
Mechanism of the Verwey transition in magnetite
By combining {\it ab initio} results for the electronic structure and phonon
spectrum with the group theory, we establish the origin of the Verwey
transition in FeO. Two primary order parameters with and
symmetries are identified. They induce the phase transformation from
the high-temperature cubic to the low-temperature monoclinic structure. The
on-site Coulomb interaction between 3d electrons at Fe ions plays a crucial
role in this transition -- it amplifies the coupling of phonons to conduction
electrons and thus opens a gap at the Fermi energy. {\it Published in Phys.
Rev. Lett. {\bf 97}, 156402 (2006).}Comment: 5 pages, 3 figure
Order parameters in the Verwey phase transition
The Verwey phase transition in magnetite is analyzed on the basis of the
Landau theory. The free energy functional is expanded in a series of components
belonging to the primary and secondary order parameters. A low-temperature
phase with the monoclinic P2/c symmetry is a result of condensation of two
order parameters X_3 and \Delta_5 . The temperature dependence of the shear
elastic constant C_44 is derived and the mechanism of its softening is
discussed.Comment: 4 pages, 1 figur
Comparative study of the electronic structures of Fe3O4 and Fe2SiO4
The electronic properties of two spinels FeO and FeSiO are
studied by the density functional theory. The local Coulomb repulsion and
the Hund's exchange between the electrons on iron are included. For
, both spinels are half-metals, with the minority states at the
Fermi level. Magnetite remains a metal in a cubic phase even at large values of
. The metal-insulator transition is induced by the phonon, which
lowers the total energy and stabilizes the charge-orbital ordering.
FeSiO transforms to a Mott insulating state for eV with a gap
. The antiferromagnetic interactions induce the tetragonal
distortion, which releases the geometrical frustration and stabilizes the
long-range order. The differences of electronic structures in the high-symmetry
cubic phases and the distorted low-symmetry phases of both spinels are
discussed.Comment: 6 pages, 6 figure
The extended analog computer and functions computable in a digital sense
In this paper we compare the computational power of the Extended Analog Computer (EAC) with partial recursive functions. We first give a survey of some part of computational theory in discrete and in real space. In the last section we show that the EAC can generate any partial recursive function defined over N. Moreover we conclude that the classical halting problem for partial recursive functions is an equivalent of testing by EAC if sets are empty or not
The Extended Analog Computer and Turing machine
In this paper we compare computational power of two models of analog and classicalcomputers. As a model of analog computer we use the model proposed by Rubel in 1993 called theExtended Analog Computer (EAC) while as a model of classical computer, the Turing machines.Showing that the Extended Analog Computer can robustly generate result of any Turing machinewe use the method of simulation proposed by D.S. Graça, M.L. Campagnolo and J. Buescu [1] in2005
Three simulations of Turing machines with the use of real recursive functions
Three simulation algorithms of Turing machines by means of real recursive functions are proposed. Moore's shifting mapping GS is used to this end. The relationship between a simulation dimension and classes of rj -hierarchy is established
The von Neumann inequality for matrices in the unit Euclidean ball
It is shown that the constant in von Neumann's inequality for
d-tuples of commutative and row contractive matrices, as proved by
Hartz, Richter, and Shalit in [2], is independent of the size of the d-tuple. A
numerical estimation of the constant is provided.Comment: The article was accepted in the Israel Journal of Mathematic
First principles study of topological phase in chains of transition metals
Recent experiments have shown the signatures of Majorana bound states at the
ends of magnetic chains deposited on a superconducting substrate. Here, we
employ first principles calculations to directly investigate the topological
properties of transition metal nanochains (i.e., Mn, Cr, Fe and Co). In
contrast to the previous studies [Nadj-Perge et al. Science 346, 602 (2014) and
Ruby et al. Nano Lett. 17, 4473 (2017)], we found the exact tight binding
models in the Wannier orbital basis for the isolated chains as well as for the
surface--deposited wires. Based on these models, we calculate topological
invariant of phase for all systems. Our results for the isolated
chains demonstrate the existence of the topological phase only in the Mn and Co
systems. We considered also a non-collinear magnetic order as a source of the
non--trivial topological phase and found that this type of magnetic order is
not a stable ground state in the Fe and Co isolated chains. Further studies
showed that a coupling between the chain and substrate leads to strong
modification of the band structure. Moreover, the analysis of the topological
invariant indicates a possibility of emergence of the topological phase in all
studied nanochains deposited on the Pb surface. Therefore, our results
demonstrate an important role of the coupling between deposited atoms and a
substrate for topological properties of nanosystems.Comment: 11 pages, 7 figure
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