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We demonstrate configuration-interaction and Green’s function approaches to the Shastry-Shraiman fourth-order magnetic Raman response of Heisenberg antiferromagnets on the two-dimensional C5v Penrose and C8v Ammann-Beenker lattices. Beyond the Loudon-Fleury second-order perturbation scheme, two more symmetry species A1 and A2 become Raman-active to break the depolarized Loudon-Fleury Raman response of E2 symmetry. The A1 and A2 modes are activated by dynamic ring exchange and chiral spin fluctuations, respectively, the former of which owes much to multimagnon scatterings in a complicated manner, while the latter of which owes much to aperiodic arrangements of different rhombuses.

Photo-induced electronic dynamics in strongly correlated electron system with spin-charge coupling are studied. Motivated from recent optical pump-probe experiments in perovskite manganites and cobaltites, the double-exchange system and the spin-state transition system are theoretically analyzed. First, we focus on the double-exchange interaction in photo-excited state. The time evolutions for the spin and charge structures are examined as functions of the pump photon amplitudes. In the weak-pumping case, the initial charge-ordered antiferromagnetic insulating state tends to become the ferromagnetic metallic state. On the other hand, in the strong-pumping case, the initial antiferromagnetic correlation and insulating character are enhanced after photoirradiation. This unexpected photo-excited state is detectable by the ultrafast optical measurement. Second, in the spin-state transition system, the two-band Hubbard model is analyzed by the time-dependent mean-field approximation. By introducing photons in the low-spin band insulator, high-spin states are generated through the electron-hole pair annihilation process which is governed by the electron band width. We also take into account the relativistic spin-orbit interaction, and compare the results with and without the spin-orbit interaction.

Motivated by photoinduced magnetism in the mixed-valent bimetallic compounds with the general formula Cu2II [MoIV(CN)8]·xH2O, we investigate their ground-state properties, in comparison with those of nickel-molybdenum analogs, by employing a group-theoretical technique. These bimetallic assemblies consist of octacyanomolybdenum complexes with a square antiprism configuration and crystallize in the I4/m tetragonal symmetry. We set up a relevant three-band extended Hubbard Hamiltonian and systematically calculate its broken-symmetry solutions. An itinerant ferromagnetism extending over both the Cu and the Mo sublattices closely competes with the localized paramagnetism in Cu2[Mo(CN)8] whereas spins are rather localized in the Ni sublattice and an antiferromagnetism is likely to predominate in the ground-state phase diagram in Ni2[Mo(CN)8].

We construct a microscopic theory on the photomagnetism in the cyano-bridged bimetallic assemblies Cu2[Mo(CN)8]·𝓍H2O. Going through a grouptheoretical analysis on the ground-state properties, we simulate photoirradiation and the resultant magnetization dynamics with a time-dependent Hartree-Fock method. It is not c-axis-polarized photons but those polarized in the ab plane that induce a macroscopic magnetization. Absorption of the photon energy is double-stepped and the occurrence of a significant magnetization is simultaneous with the second stage (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoinduced spin-charge dynamics in strongly correlated electron systems is studied based on an extended double-exchange model. Solving a time-dependent Schrodinger equation with the Lanczos method, we trace the process of photoinduced melting of an antiferromagnetic charge order and analyze the excitation-density dependence on that. In the case of low density photoexcitation, both charge and spin orders are melted by photocarrier doping. This is interpreted with a conventional double-exchange mechanism. In the case of high density photoexcitation, however, the charge order is melted. The antiferromagnetic spin order is transiently weakened but after turnoff of the photoirradiation it recovers. This phenomenon strikingly differs from the weak photoexcitation case.

Nanotubes are generally prepared from their constituent elements at high temperatures, and thus it is difficult to control their size, shape and electronic states. One useful approach for synthesizing well-defined nanostructures involves the use of building blocks such as metal ions and organic molecules. Here, we show the successful creation of an assembly of infinite square prism-shaped metal-organic nanotubes obtained from the simple polymerization of a square-shaped metal-organic frame. The constituent nanotube has a one-dimensional (1D) channel with a window size of 5.9×5.9 Å(2), and can adsorb water (H(2)O) and alcohol vapours, whereas N(2) and CO(2) do not adhere. It consists of four 1D covalent chains that constitute a unique electronic structure of 'charge-density wave (CDW) quartets' on crystallization. Moreover, exchanging structural components and guest molecules enables us to control its semiconductive bandgap. These findings demonstrate the possibility of bottom-up construction of new porous nanotubes, where their degrees of freedom in both pore space and framework can be used.

We investigate the ground-state properties of polyacene in terms of an extended Peierls-Hubbard Hamiltonian with particular emphasis on its structural instability of two types: double bonds in a cis pattern and those in a trans pattern. The two Peierls-distorted states are degenerate in their energetics but quite distinct in their optics. The trans configuration is easily photoconverted into the cis one, whereas an opposite transition, if any, is hardly completed. Domain-wall (soliton) excitations play an important role in any photoinduced phase transitions, but charged and neutral ones behave quite differently.

Employing a time-dependent Hartree-Fock method, we simulate photoexcitation and the following nonlinear lattice relaxation of quasi-one-dimensional binuclear platinum complexes with bridging halogens. High-energy excitations above the electron-hole continuum tend to relax into polarons, while excitons pumped within the optical gap can be stabilized into solitons. Both neutral and charged solitons appear as stable photoproducts, which is not the case with conventional platinum-halide chains. When a pair of kink and antikink is born out of a self-trapped exciton and recombine to disappear, excess spin and charge densities alternate and oscillate on them. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

New varieties of quasi-one-dimensional transition metal (M)-halide (X) compounds, $$(\hbox{C}_8\hbox{H}_6\hbox{N}_4)[\hbox{Pt}(\hbox{C}_2\hbox{H}_8\hbox{N}_2)\hbox{Cl}]_2\hbox{Cl}(\hbox{ClO}_4)_3\cdot \hbox{H}_2\hbox{O}$$ and $$(\hbox{C}_{10}\hbox{H}_8\hbox{N}_2)[\hbox{Pt}(\hbox{C}_4\hbox{H}_{13}\hbox{N}_3)\hbox{Br}]_2\hbox{Br}_4\cdot 2\hbox{H}_2\hbox{O},$$ comprising two-leg ladders of mixed-valent platinum complexes, are theoretically investigated with particular emphasis on their photoresponses. Observations of the polarized optical conductivity prove the interchain valence arrangement to be in phase in the former but out of phase in the latter. Possible nonlinear lattice relaxation originating with a photogenerated exciton is also simulated in an attempt to further elucidate the contrast between them.

The optical conductivity of photogenerated solitons in quasi-one-dimensional halogen-bridged binuclear metal (MMX) complexes is investigated with particular emphasis on a comparison between the two family compounds R_4_[Pt_2_(P_2_O_5_H_2_)_4_X]nH_2_O (X=Cl,Br,I;R=NH_4_,Na,K,...; pop=diphosphonate=P_2_O_5_H_2_^2-^) and Pt_2_(dta)_4_I (dta=dithioacetate=CH_3_CS_2_^-^). Soliton-induced absorption spectra for the pop complexes should split into two bands, while those for the dta complex should consist of a single band.

We theoretically demonstrate photomanipulation of the ground state of a novel nanotubular assembly, [Pt(C2H8N2)(C10H8N2)I]4(NO3)8, which consists of platinum-iodide chains bound via organic bridges into square prisms. Its ground state turns out a mixture of two distinct mixed-valent phases, which have different optical features. We reveal that photoconversion between these two chargedensity-wave states is possible but directional. We propose photopurifying the mixed ground state and further predict optical observations of electron dynamics on the way.

We introduce copper molybdenum cyanides of general formula Cu2[Mo(CN)8]·nH2O, which can serve as optofunctional magnetic devices. Their ground states generally stay paramagnetic down to temperatures of the K order but exhibit a spontaneous magnetization upon photoirradiation usually below a few tens of K. To interest us still further, such a ferromagnetic stateinduced by blue-laser irradiation is demagnetized step by step through further application of red or near-infrared laser pulses. We solve this intriguing photomagnetism. The ground-state properties are fully revealed by means of a group-theoretical technique. Taking account of experimental observations, we simulate applying pump laser pulses to a likely ground state and successfully reproduce both the magnetization and demagnetization dynamics. We monitor the photorelaxation process through angle-resolved photoemission spectroscopy. Electrons are fully itinerant in any of the photoinduced steady states, forming a striking contrast to the initial equilibrium state of atomic aspect. The fully demagnetized final steady state looks completely different from the initial paramagnetism but bears good analogy to one of the possible ground states available with the Coulomb repulsion on Cu sites suppressed.

Photo-excited state in correlated electron system with spin-state degree of freedom is studied. We start from the two-orbital extended Hubbard model where energy difference between the two orbitals is introduced. Photo-excited metastable state is examined based on the effective model Hamiltonian derived by the two-orbital Hubbard model. Spin-state change is induced by photo-irradiation in the low-spin band insulator near the phase boundary. High-spin state is stabilized by creating a ferromagnetic bound state with photo-doped hole carriers. An optical absorption occurs between the bonding and antibonding orbitals inside of the bound state. Time-evolution for photo-excited states is simulated in the time-dependent mean-field scheme. Pair-annihilations of the photo-doped electron and hole generate the high-spin state in a low-spin band insulator. We propose that this process is directly observed by the time-resolved photoemission experiments., 15 pages, 16 figures

Photoirradiation effects in correlated electrons coupled with localized spins are studied based on the extended double-exchange model. In particular, we examine melting of an antiferromagnetic (AFM) charge order insulating state by varying the light intensity. When intense light is irradiated, the AFM insulating characteristics are strengthened, rather than change into the ferromagnetic metallic characteristic, which are expected from the conventional double exchange interaction when carriers are introduced by weak light irradiation or chemical doping. This provides a new principle for optically manipulating magnetism., 8 pages, 6 figures, to be published in Physical Review B

Motivated by recent chemical attempts at assembling halogen-bridged transition-metal complexes within a nanotube, we model and characterize a platinum-halide triangular prism in terms of a Peierls-Hubbard Hamiltonian. Based on a group-theoretical argument, we reveal a variety of valence arrangements, including heterogeneous or partially metallic charge-density-wave states. Quantum and thermal phase competitions are numerically demonstrated with particular emphasis on novel insulator-to-metal and insulator-to-insulator transitions under doping, the former of which is of the first order, while the latter of which is of the second order., 9 pages, 7 figures. to be published in J. Phys. Soc. Jpn. Vol. 79, No. 4

Nonlinear lattice relaxation of photoexcited diplatinum-halide chain compounds is theoretically investigated within a one-dimensional extended Peierls-Hubbard model. We first illuminate the whole relaxation scenario in terms of variational wave functions and then visualize each relaxation channel numerically integrating the Schr\"odinger equation. High-energy excitations above the electron-hole continuum tend to relax into polarons, while excitons pumped within the optical gap, unless luminescent, turn into solitonic states nonradiatively. Neutral and charged solitons coexist as stable photoproducts, which has never been observed in conventional platinum-halide chains, and they are highly resonant on the occasion of their birth and geminate recombination., Comment: 9 pages, 9 figures. to be published in J. Phys.: Condens. Matter 20 (2008) 415215

Motivated by the homometallic intertwining double-chain ferrimagnets ${A}_{3}{\mathrm{Cu}}_{3}{(\mathrm{P}{\mathrm{O}}_{4})}_{4}$ $(A=\mathrm{Ca},\mathrm{Sr},\mathrm{Pb})$, we investigate the low-energy structure of their model Hamiltonian $\mathcal{H}={\ensuremath{\sum}}_{n}[{J}_{1}({\mathbit{S}}_{n:1}+{\mathbit{S}}_{n:3})+{J}_{2}({\mathbit{S}}_{n+1:1}+{\mathbit{S}}_{n\ensuremath{-}1:3})]∙{\mathbit{S}}_{n:2}$, where ${\mathbit{S}}_{n:l}$ stands for the ${\mathrm{Cu}}^{2+}$ ion spin labeled $l$ in the $n\text{th}$ trimer unit, with particular emphasis on the range of bond alternation $0l{J}_{2}∕{J}_{1}l1$. Although the spin-wave theory, whether up to $O({S}^{1})$ or up to $O({S}^{0})$, claims that there exists a flatband in the excitation spectrum regardless of bond alternation, a perturbational treatment, as well as the exact diagonalization of the Hamiltonian, reveals its weak but nonvanishing momentum dispersion unless ${J}_{2}={J}_{1}$ or ${J}_{2}=0$. Quantum Monte Carlo calculations of the static structure factor further convince us of the low-lying excitation mechanism, elucidating similarities and differences between the present system and alternating-spin linear-chain ferrimagnets.

We investigate the optical conductivity of photogenerated solitons in quasi-one-dimensional halogen-bridged binuclear transition-metal MMX complexes with particular emphasis on a comparison among the three distinct groups: A_4_[Pt_2_(P_2_O_5_H_2_)_4_X]nH_2_O (X=Cl,Br,I; A=Na,K,NH_4_,...), Pt_2_(RCS_2_)_4_I (R=C_n_H_2n+1_) and Ni_2_(CH_3_CS_2_)_4_I, which exhibit a mixed-valent ground state with the X sublattice dimerized, that with the M_2_ sublattice dimerized and a Mott-Hubbard magnetic ground state, respectively. Soliton-induced absorption spectra for A_4_[Pt_2_(P_2_O_5_H_2_)_4_X]nH_2_O should split into two bands, while that for Pt_2_(RCS_2_)_4_I and Ni_2_(CH_3_CS_2_)_4_I should consist of a single band. The excitonic effect is significant in Ni_2_(CH_3_CS_2_)_4_I., 4 pages, 3 figures

In order to reveal the relaxation mechanism of photogenerated charge-transfer excitations in quasi-one-dimensional halogen-bridged diplatinum complexes, we calculate the low-lying adiabatic potential energy surfaces of a one-dimensional extended Peierls-Hubbard model. High-energy excitations above the electron-hole continuum may relax into polarons, while excitons pumped within the optical gap are self-localized and then either decay by luminescence or divide into solitons. Neutral solitons, charged solitons, and polarons may be simultaneously photogenerated in a diplatinum-halide chain, which has never been observed in any conventional platinum-halide chain. Optical conductivity is also simulated along the decay paths for experimental verification., Comment: 8 pages, 8 figures

Employing the one-dimensional single-band extended Peierls-Hubbard model, we investigate optical conductivity for solitonic excitations in halogen-bridged binuclear metal (MMX) complexes. Photoinduced soliton absorption spectra for MMX chains possibly split into two bands, forming a striking contrast to those for conventional mononuclear metal (MX) analogs, due to the broken electron-hole symmetry combined with relevant Coulomb and/or electron-phonon interactions., Comment: to be published in Phys. Rev. B 70, No. 7, August 15 (2004)

Intermolecular vibrations relevant to optically and thermally induced magnetic phase transitions between low temperature (LT) diamagnetic and high temperature (HT) paramagnetic phases in a strongly correlated organic radical 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA) crystal have been investigated using broadband terahertz (THz) time-domain spectroscopy. Two absorption bands with different polarizations were clearly observed at 1.3 and 4.0 THz in the LT phase, whilst absent in the HT phase. The temperature dependence of the THz absorption peak exhibits hysteretic behavior around room temperature, indicating that the observed THz absorption bands are related to the magnetic phase transition in TTTA. Considering the symmetry of the crystal using group theoretical analysis, the observed absorption bands are attributed to intermolecular vibrations with Au symmetry. We found that the absorption band at 4.0 THz disappeared upon irradiation by a laser pulse with a photon density greater than 3.0 × 10¹⁸ photons/cm². These results show that probing the intermolecular THz vibrations is indispensable for revealing the photoinduced phase transition from the LT diamagnetic to HT paramagnetic phase.

Motivated by iodo platinum complexes assembled within a quadratic-prism lattice, [Pt(C$_2$H$_8$N$_2$)(C$_{10}$H$_8$N$_2$)I]$_4$(NO$_3$)$_8$, we investigate the ground-state properties of a Peierls-Hubbard four-legged tube. Making a group-theoretical analysis, we systematically reveal a variety of valence arrangements, including half-metallic charge-density-wave states. Quantum and thermal phase competition is numerically demonstrated with particular emphasis on doping-induced successive insulator-to-metal transitions with conductivity increasing stepwise., Comment: 6 pages, 4 figures. to be published in Europhys. Lett. 87 (2009) 17006

New varieties of quasi-one-dimensional halogen (X)-bridged transition-metal (M) complexes, (C_8_H_6_N_4_)[Pt(C_2_H_8_N_2_)X]_2_X(ClO_4_)_3_\cdotH_2_O (X=Br,Cl) and (C_10_H_8_N_2_)[Pt(C_4_H_13_N_3_)Br]_2_Br_4_\cdot2H_2_O, comprising two-leg ladders of mixed-valent platinum ions, are described in terms of a multiband extended Peierls-Hubbard Hamiltonian. The polarized optical conductivity spectra are theoretically reproduced and the ground-state valence distributions are reasonably determined. The latter variety, whose interchain valence arrangement is out of phase, is reminiscent of conventional MX single-chain compounds, while the former variety, whose interchain valence arrangement is in phase, reveals itself as a novel d-p-\pi-hybridized multiband ladder material., Comment: 9 pages, 10 figures. to be published in Phys. Rev. B 76, November 15, No. 19