305 research outputs found
A study of the electronic properties of liquid alkali metals. A self--consistent approach
We study the electronic properties (density of states, conductivity and
thermopower) of some nearly--free--electron systems: the liquid alkali metals
and two liquid alloys, Li-Na and Na-K. The study has been performed within the
self-consistent second order Renormalized Propagator Perturbation Expansion
(RPE) for the self-energy. The input ionic pseudopotentials and static
correlation functions are derived from the neutral pseudoatom method and the
modified hypernetted chain theory of liquids, respectively. Reasonable
agreement with experiment is found for Na, K, Rb and Na-K, whereas for Li and
Cs and Li-Na the agreement is less satisfactoryComment: 14 pages, Latex, 1 figure, 1 tabl
Intrinsic peculiarities of real material realizations of a spin-1/2 kagome lattice
Spin-1/2 magnets with kagome geometry, being for years a generic object of
theoretical investigations, have few real material realizations. Recently, a
DFT-based microscopic model for two such materials, kapellasite Cu3Zn(OH)6Cl2
and haydeeite Cu3Mg(OH)6Cl2, was presented [O. Janson, J. Richter and H.
Rosner, arXiv:0806.1592]. Here, we focus on the intrinsic properties of real
spin-1/2 kagome materials having influence on the magnetic ground state and the
low-temperature excitations. We find that the values of exchange integrals are
strongly dependent on O--H distance inside the hydroxyl groups, present in most
spin-1/2 kagome compounds up to date. Besides the original kagome model,
considering only the nearest neighbour exchange, we emphasize the crucial role
of the exchange along the diagonals of the kagome lattice.Comment: 4 pages, 4 figures. A paper for the proceedings of the HFM 2008
conferenc
Thermodynamic Comparison and the Ideal Glass Transition of A Monatomic Systems Modeled as an Antiferromagnetic Ising Model on Husimi and Cubic Recursive Lattices of the Same Coordination Number
Two kinds of recursive lattices with the same coordination number but
different unit cells (2-D square and 3-D cube) are constructed and the
antiferromagnetic Ising model is solved exactly on them to study the stable and
metastable states. The Ising model with multi-particle interactions is designed
to represent a monatomic system or an alloy. Two solutions of the model exhibit
the crystallization of liquid, and the ideal glass transition of supercooled
liquid respectively. Based on the solutions, the thermodynamics on both
lattices was examined. In particular, the free energy, energy, and entropy of
the ideal glass, supercooled liquid, crystal, and liquid state of the model on
each lattice were calculated and compared with each other. Interactions between
particles farther away than the nearest neighbor distance are taken into
consideration. The two lattices show comparable properties on the transition
temperatures and the thermodynamic behaviors, which proves that both of them
are practical to describe the regular 3-D case, while the different effects of
the unit types are still obvious.Comment: 27 pages, 13 figure
Interchain interactions and magnetic properties of Li2CuO2
An effective Hamiltonian is constructed for an insulating cuprate with
edge-sharing chains Li2CuO2.The Hamiltonian contains the nearest and
next-nearest neighboring intrachain and zigzag-type interchain interactions.The
values of the interactions are obtained from the analysis of the magnetic
susceptibility, and this system is found to be described as coupled frustrated
chains.We calculate the dynamical spin correlation function S(q,\omega) by
using the exact diagonalization method, and show that the spectra of
S(q,\omega) are characterized by the zigzag-type interchain interactions. The
results of the recent inelastic neutron scattering experiment are discussed in
the light of the calculated spectra.Comment: 4 pages, 3 figures, RevTe
Ab initio simulations of liquid systems: Concentration dependence of the electric conductivity of NaSn alloys
Liquid NaSn alloys in five different compositions (20, 40, 50, 57 and 80%
sodium) are studied using density functional calculations combined with
molecular dynamics(Car-Parrinello method). The frequency-dependent electric
conductivities for the systems are calculated by means of the Kubo-Greenwood
formula.
The extrapolated DC conductivities are in good agreement with the
experimental data and reproduce the strong variation with the concentration.
The maximum of conductivity is obtained, in agreement with experiment, near the
equimolar composition.
The strong variation of conductivity, ranging from almost semiconducting up
to metallic behaviour, can be understood by an analysis of the
densities-of-states.Comment: LaTex 6 pages and 2 figures, to appear in J.Phys. Cond. Ma
Elementary excitations, exchange interaction and spin-Peierls transition in CuGeO
The microscopic description of the spin-Peierls transition in pure and doped
CuGeO_3 is developed taking into account realistic details of crystal
structure. It it shown that the presence of side-groups (here Ge) strongly
influences superexchange along Cu-O-Cu path, making it antiferromagnetic.
Nearest-neighbour and next-nearest neighbour exchange constants and
are calculated. Si doping effectively segments the CuO_2-chains
leading to or even slightly ferromagnetic. Strong
sensitivity of the exchange constants to Cu-O-Cu and (Cu-O-Cu)-Ge angles may be
responsible for the spin-Peierls transition itself (``bond-bending mechanism''
of the transition). The nature of excitations in the isolated and coupled
spin-Peierls chains is studied and it is shown that topological excitations
(solitons) play crucial role. Such solitons appear in particular in doped
systems (Cu_{1-x}Zn_xGeO_3, CuGe_{1-x}Si_xO_3) which can explain the
phase diagram.Comment: 7 pages, revtex, 7 Postscript figure
Ab initio simulations of liquid NaSn alloys: Zintl anions and network formation
Using the Car-Parrinello technique, ab initio molecular dynamics simulations
are performed for liquid NaSn alloys in five different compositions (20, 40,
50, 57 and 80 % sodium). The obtained structure factors agree well with the
data from neutron scattering experiments. The measured prepeak in the structure
factor is reproduced qualitatively for most compositions. The calculated and
measured positions of all peaks show the same trend as function of the
composition.\\ The dynamic simulations also yield information about the
formation and stability of Sn clusters (Zintl anions) in the liquid. In our
simulations of compositions with 50 and 57 % sodium we observe the formation of
networks of tin atoms. Thus, isolated tin clusters are not stable in such
liquids. For the composition with 20 % tin only isolated atoms or dimers of tin
appear, ``octet compounds'' of one Sn atom surrounded by 4 Na atoms are not
observed.Comment: 12 pages, Latex, 3 Figures on reques
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