Ground state phases and optical properties in extended Peierls–Hubbard models for halogen-bridged binuclear metal complexes

Abstract

Mechanisms of a variety of charge and lattice ordered phases observed in halogen-bridged binuclear metal complexes are theoretically studied by applying the exact diagonalization and strong-coupling expansion methods to one-and two-band extended Peierls-Hubbard models. In R 4 [Pt 2 (pop] containing charged MMX chains, three electronic phases are suggested by experiments. We find that the variation of the electronic phases originates not only from competition between site-diagonal electron-lattice and electron-electron interactions but also from competition between short-range and long-range electron-electron interactions. On the other hand, in Pt 2 (RCS 2 ) 4 I (R~CH 3 , n-C 4 H 9 ) containing neutral MMX chains, a site-off-diagonal electron-lattice interaction and the absence of counter ions are found to be crucial to produce the recently found, ordered phase. The optical conductivity spectra are also studied, which directly reflect the electronic phases. Their dependence on the electronic phase and on model parameters is clarified from the strong-coupling viewpoint.

Optical characterization of platinum-halide ladder compounds

“…Such a classical treatment of phonons is widely adopted and generally successful for mixed-valent platinum-halide compounds. 3,16,17,31,[33][34][35][36] Quantum phonons may be relatively effective in strongly correlated valence-delocalized nickelhalide chains. 4 Characterization of the brand new MX ladders is still in the early stage from both experimental 27,28 and theoretical 31,37 points of view, and therefore, little is known about the model parameters.…”

“…Metal binucleation leads to a wider variety of electronic states. 16,17 A diplatinum iodide chain compound, ͓͑C 2 H 5 ͒ 2 NH 2 ͔ 4 ͓Pt 2 ͑P 2 O 5 H 2 ͒ 4 I͔, exhibits photoand pressure-induced phase transitions, [18][19][20][21][22] whereas its analog without any counter ion, Pt 2 ͑CH 3 CS 2 ͒ 4 I, is of metallic conduction at room temperature and undergoes successive phase transitions 23,24 with decreasing temperature. There are further attempts 25,26 to bridge polynuclear and/or heterometallic units by halogens.…”

Pressure-induced phase transitions of halogen-bridged binuclear metal complexesR4[Pt2(P

“…However, a weak alternation of the lower orbital energy is much less important and the coexistent elastic forces can be renormalized into α and β. Indeed, the present model can successfully reproduce the observed optical conductivity [20], suggesting that all the essential interactions can well be described by the parameters on hand. In such circumstances that even the hopping amplitudes are not yet determined, we should minimize the number of parameters.…”

Nonlinear lattice relaxation of photoexcited diplatinum-halide chain compounds

“…Since the parameter sets (α, β, K MX ) and (aα, aβ, a 2 K MX ) are equivalent to each other with an arbitrary constant a, the specific value of K MX is insignificant. 43 The number of MMX units, denoted by N , is set equal to 300. n,↑ a n,↑ |l + l|a † n,↓ a n,↓ |l ≡ nn, l|b † n,↑ b n,↑ |l + l|b † n,↓ b n,↓ |l ≡ mn) and spin ( l|a † n,↑ a n,↑ |l − l|a † n,↓ a n,↓ |l ≡ 2sn, l|b † n,↑ b n,↑ |l − l|b † n,↓ b n,↓ |l ≡ 2tn) densities in the lth excited state and then extract their nonalternating components (nn−1 + 2nn + nn+1 ≡ −4nn, mn−1 + 2mn + mn+1 ≡ −4mn; sn−1 + 2sn + sn+1 ≡ 4sn; tn−1 + 2tn + tn+1 ≡ 4tn). In the above, the average electron density is added to the nonalternating components.…”