149 research outputs found
Competing Ground States of the New Class of Halogen-Bridged Metal Complexes
Based on a symmetry argument, we study the ground-state properties of
halogen-bridged binuclear metal chain complexes. We systematically derive
commensurate density-wave solutions from a relevant two-band Peierls-Hubbard
model and numerically draw the the ground-state phase diagram as a function of
electron-electron correlations, electron-phonon interactions, and doping
concentration within the Hartree-Fock approximation. The competition between
two types of charge-density-wave states, which has recently been reported
experimentally, is indeed demonstrated.Comment: 4 pages, 5 figures embedded, to appear in J. Phys. Soc. Jp
Pressure-induced phase transitions of halogen-bridged binuclear metal complexes R_4[Pt_2(P_2O_5H_2)_4X]nH_2O
Recent contrasting observations for halogen (X)-bridged binuclear platinum
complexes R_4[Pt_2(P_2O_5H_2)_4X]nH_2O, that is, pressure-induced Peierls and
reverse Peierls instabilities, are explained by finite-temperature Hartree-Fock
calculations. It is demonstrated that increasing pressure transforms the
initial charge-polarization state into a charge-density-wave state at high
temperatures, whereas the charge-density-wave state oppositely declines with
increasing pressure at low temperatures. We further predict that
higher-pressure experiments should reveal successive phase transitions around
room temperature.Comment: 5 pages, 4 figures embedded, to be published in Phys. Rev. B 64,
September 1 (2001) Rapid Commu
Quantum and Thermal Phase Transitions of Halogen-Bridged Binuclear Transition-Metal Complexes
Aiming to settle the controversial observations for halogen-bridged binuclear
transition-metal (MMX) complexes, finite-temperature Hartree-Fock calculations
are performed for a relevant two-band Peierls-Hubbard model. Thermal, as well
as quantum, phase transitions are investigated with particular emphasis on the
competition between electron itinerancy, electron-phonon interaction and
electron-electron correlation. Recently observed distinct thermal behaviors of
two typical MMX compounds Pt_2(CH_3CS_2)_4I and
(NH_4)_4[Pt_2(P_2O_5H_2)_4I]2H_2O are supported and further tuning of their
electronic states is predicted.Comment: 5 pages, 3 figures embedded, to be published in J. Phys. Soc. Jpn.
Vol.70, No.5 (2001
Nickel(II) 3,4;9,10-Perylenediimide bis-Phosphonate Pentahydrate: A MetalâOrganic Ferromagnetic Dye
The new metalâorganic compound nickel(II) 3,4;9,10-
perylenediimide bis-phosphonate pentahydrate, i.e. Ni2[(PDI-BP)-
(H2O)2]·3H2O (1), has been synthesized and its structural and magnetic
properties have been studied. Reaction of 3,4;9,10-perylenediimide bisphosphonate
(PDI-BP, hereafter) ligand and nickel chloride in water resulted
in the precipitation of a red and poorly crystalline solid (1). As the solid shows
a poor crystalline organization of aggregates, the energy dispersive X-ray
diffraction analysis (EDXD) technique has been used to obtain short-range
order structural information of the single nanoaggregates by radial distribution
function analysis. The overall structure of the compound is characterized by
layers containing perylene planes shifted in the direction perpendicular to the
stacking axes in such a way that only the outer rings overlap. The edges of the
perylene planes are connected to the phosphonate groups through an imido
group. The oxygen atoms of the [âPO3]2â group and those of the water
molecules are bonded to the nickel ions resulting in a [NiO6] octahedral coordination sphere. The NiâO bond lengths are 0.21
± 0.08 nm and the NiâOâNi angles of aligned moieties are 95 ± 2°. The oxygen atoms of the water molecules and the nickel
atoms are nearly planar and almost perpendicular to the perylene planes forming chains of edge-sharing octahedra. The magnetic
properties of (1) show the presence of intrachain ferromagnetic NiâNi interactions and a long-range ferromagnetic order below
21 K with a canting angle and with a spin glasslike behavior due to disorder in the inorganic layer. Hysteresis cycles show a
coercive field of ca. 272 mT at 2 K that decreases as the temperature is increased and vanishes at ca. 20 K
Magnetodielectric coupling of a polar organic-inorganic hybrid Cr(II) phosphonate
Cr[(H(3)N-(CH(2))(2)-PO(3))(Cl)(H(2)O)] represents a rare example of a polar organic-inorganic hybrid material that exhibits a canted antiferromagnetic order below T(N)=5.5 K. The unusual coexistence of a polar crystal structure and magnetic order triggered our investigation of the magnetodielectric coupling. The coupling is evidenced by an anomaly in the temperature dependence of the dielectric constant epsilon below the Neel temperature. The magnetocapacitance is enhanced by one order of magnitude below T(N). The main characteristics of the magnetodielectic response are interpreted by Landau theoretical coupling terms
Soliton excitations in halogen-bridged mixed-valence binuclear metal complexes
Motivated by recent stimulative observations in halogen (X)-bridged binuclear
transition-metal (M) complexes, which are referred to as MMX chains, we study
solitons in a one-dimensional three-quarter-filled charge-density-wave system
with both intrasite and intersite electron-lattice couplings. Two distinct
ground states of MMX chains are reproduced and the soliton excitations on them
are compared. In the weak-coupling region, all the solitons are degenerate to
each other and are uniquely scaled by the band gap, whereas in the
strong-coupling region, they behave differently deviating from the scenario in
the continuum limit. The soliton masses are calculated and compared with those
for conventional mononuclear MX chains.Comment: 9 pages, 10 figures embedded, to be published in J. Phys. Soc. Jpn.
71, No. 1 (2002
Characterization of halogen-bridged binuclear metal complexes as hybridized two-band materials
We study the electronic structure of halogen-bridged binuclear metal (MMX)
complexes with a two-band Peierls-Hubbard model. Based on a symmetry argument,
various density-wave states are derived and characterized. The ground-state
phase diagram is drawn within the Hartree-Fock approximation, while the thermal
behavior is investigated using a quantum Monte Carlo method. All the
calculations conclude that a typical MMX compound Pt_2(CH_3CS_2)_4I should
indeed be regarded as a d-p-hybridized two-band material, where the oxidation
of the halogen ions must be observed even in the ground state, whereas another
MMX family (NH_4)_4[Pt_2(P_2O_5H_2)_4X] may be treated as single-band
materials.Comment: 16 pages, 11 figures embedded, to be published in Phys. Rev.
Experimental Evidence of a Haldane Gap in an S = 2 Quasi-linear Chain Antiferromagnet
The magnetic susceptibility of the quasi-linear chain Heisenberg
antiferromagnet (2,-bipyridine)trichloromanganese(III), MnCl_{3}(bipy), has
been measured from 1.8 to 300 K with the magnetic field, H, parallel and
perpendicular to the chains. The analyzed data yield and K. The magnetization, M, has been studied at 30 mK and 1.4 K in H up to 16
T. No evidence of long-range order is observed. Depending on crystal
orientation, at 30 mK until a critical field is achieved ( and $H_{c\bot} = 1.8\pm 0.2 T), where M increases continuously
as H is increased. These results are interpreted as evidence of a Haldane gap.Comment: 11 pages, 4 figure
Reference Force Field and CDW Amplitude of Mixed-Valence Halogen-Bridged Pt Complexes
The spectroscopic effects of electron-phonon coupling in mixed-valence
chlorine-bridged Pt chains complexes are investigated through a parallel
infrared and Raman study of three compounds with decreasing Pt-Pt distance
along the chain. The e-ph interaction is analyzed in terms of the
Herzberg-Teller coupling scheme. We take into account the quadratic term and
define a precise reference state. The force field relevant to this state is
constructed, whereas the electronic structure is analyzed in terms of a simple
phenomenological model, singling out a trimeric unit along the chain. In this
way we are able to account for all the available optical data of the three
compounds, and to estimate the relevant microscopic parameters, such as the
e-ph coupling constants and the CDW amplitude.Comment: 10 pages, compressed postscript, 6 Tables and 5 Figures also in a
compressed ps.Z file. Revision is in the submission format only (postscript
instead of tex
Self-control interventions for children under age 10 for improving self-control and delinquency and problem behaviors
Self-control improvement programs are intended to serve many purposes, most
notably improving self-control. Yet, interventions such as these often aim to reduce
delinquency and problem behaviors. However, there is currently no summary
statement available regarding whether or not these programs are effective in
improving self-control and reducing delinquency and problem behaviors. The main objective of this review is to assess the available research evidence on the
effect of self-control improvement programs on self-control and delinquency and
problem behaviors. In addition to investigating the overall effect of early selfcontrol
improvement programs, this review will examine, to the extent possible, the
context in which these programs may be most successful. The studies included in this systematic review indicate that self-control
improvement programs are an effective intervention for improving self-control and
reducing delinquency and problem behaviors, and that the effect of these programs
appears to be rather robust across various weighting procedures, and across context,
outcome source, and based on both published and unpublished data
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