92 research outputs found
High-spin low-spin transition
Temperature dependent nuclear inelastic-scattering (NIS) of synchrotron
radiation was applied to investigate both spin states of the spin-crossover
complex [Fe(tpa)(NCS)(2)] (tpa = tris(2-pyridylmethyl)amine). A remarkable
increase of the iron-ligand bond stretching upon spin crossover has
unambiguously been identified by comparing the measured NIS spectra with
theoretical simulations based on density-functional calculations
Pressure-induced changes of the vibrational modes of spin-crossover complexes studied by nuclear resonance scattering of synchrotron radiation
Nuclear inelastic scattering (NIS) spectra were recorded for the
spin-crossover complexes STP and ETP (STP =
[Fe(1,1,1-trisf[N-(2-pyridylmethyl)-N-methylamino]methylg- ethane)](ClO4)2 and
ETP =
[Fe(1,1,1-trisf[N-(2-pyridylmethyl)-N-methylamino]methylg-butane)](ClO4)2) at
30 K and at room temperature and also at ambient pressure and applied pressure
(up to 2.6 GPa). Spin transition from the high-spin (HS) to the low-spin (LS)
state was observed by lowering temperature and also by applying pressure at
room temperature and has been assigned to the hardening of iron-bond stretching
modes due to the smaller volume in the LS isomer
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
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
Pressure Tuning of the Charge Density Wave in the Halogen-Bridged Transition-Metal (MX) Solid
We report the pressure dependence up to 95 kbar of Raman active stretching
modes in the quasi-one-dimensional MX chain solid . The data
indicate that a predicted pressure-induced insulator-to-metal transition does
not occur, but are consistent with the solid undergoing either a
three-dimensional structural distortion, or a transition from a charge-density
wave to another broken-symmetry ground state. We show that such a transition
cacan be well-modeled within a Peierls-Hubbard Hamiltonian. 1993 PACS:
71.30.+h, 71.45.Lr, 75.30.Fv, 78.30.-j, 81.40.VwComment: 4 pages, ReVTeX 3.0, figures available from the authors on request
(Gary Kanner, [email protected]), to be published in Phys Rev B Rapid
Commun, REVISION: minor typos corrected, LA-UR-94-246
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.
Peierls transition in the presence of finite-frequency phonons in the one-dimensional extended Peierls-Hubbard model at half-filling
We report quantum Monte Carlo (stochastic series expansion) results for the
transition from a Mott insulator to a dimerized Peierls insulating state in a
half-filled, 1D extended Hubbard model coupled to optical bond phonons. Using
electron-electron (e-e) interaction parameters corresponding approximately to
polyacetylene, we show that the Mott-Peierls transition occurs at a finite
value of the electron-phonon (e-ph) coupling. We discuss several different
criteria for detecting the transition and show that they give consistent
results. We calculate the critical e-ph coupling as a function of the bare
phonon frequency and also investigate the sensitivity of the critical coupling
to the strength of the e-e interaction. In the limit of strong e-e couplings,
we map the model to a spin-Peierls chain and compare the phase boundary with
previous results for the spin-Peierls transition. We point out effects of a
nonlinear spin-phonon coupling neglected in the mapping to the spin-Peierls
model.Comment: 7 pages, 5 figure
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