Charge-Transfer Excitations in One-Dimensional Dimerized Mott Insulators

We investigate the optical properties of one-dimensional (1D) dimerized Mott insulators using the 1D dimerized extended Hubbard model. Numerical calculations and a perturbative analysis from the decoupled-dimer limit clarify that there are three relevant classes of charge-transfer (CT) states generated by photoexcitation: interdimer CT unbound states, interdimer CT exciton states, and intradimer CT exciton states. This classification is applied to understanding the optical properties of an organic molecular material, 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA), which is known for its photoinduced transition from the dimerized spin-singlet phase to the regular paramagnetic phase. We conclude that the lowest photoexcited state of TTTA is the interdimer CT exciton state and the second lowest state is the intradimer CT exciton state.Comment: 6 pages, 6 figures, to be published in J. Phys. Soc. Jp

X-Band ESR Determination of Dzyaloshinsky-Moriya Interaction in 2D SrCu2_2(BO3_3)2_2 System

X-band ESR measurements on a single crystal of SrCu$_2$(BO$_3$)$_2$ system in a temperature range between 10 K and 580 K are presented. The temperature and angular dependence of unusually broad ESR spectra can be explained by the inclusion of antisymmetric Dzyaloshinsky-Moriya (DM) interaction, which yields by far the largest contribution to the linewidth. However, the well-accepted picture of only out-of-plane interdimer DM vectors is not sufficient for explanation of the observed angular dependence. In order to account for the experimental linewidth anisotropy we had to include sizable in-plane components of interdimer as well as intradimer DM interaction in addition to the out-of-plane interdimer one. The nearest-neighbor DM vectors lie perpendicular to crystal anisotropy c-axis due to crystal symmetry. We also emphasize that above the structural phase transition occurring at 395 K dynamical mechanism should be present allowing for instantaneous DM interactions. Moreover, the linewidth at an arbitrary temperature can be divided into two contributions; namely, the first part arising from spin dynamics governed by the spin Hamiltonian of the system and the second part due to significant spin-phonon coupling. The nature of the latter mechanism is attributed to phonon-modulation of the antisymmetric interaction, which is responsible for the observed linear increase of the linewidth at high temperatures.Comment: 17 pages, 4 figures, submitted to PR

Dynamics of photoexcited states in one-dimensional dimerized Mott insulators

Dynamical properties of photoexcited states are theoretically studied in a one-dimensional Mott insulator dimerized by the spin-Peierls instability. Numerical calculations combined with a perturbative analysis have revealed that the lowest photoexcited state without nearest-neighbor interaction corresponds to an interdimer charge transfer excitation that belongs to dispersive excitations. This excited state destabilizes the dimerized phase, leading to a photoinduced inverse spin-Peierls transition. We discuss the purely electronic origin of midgap states that are observed in a latest photoexcitation experiment of an organic spin-Peierls compound, K-TCNQ (potassium-tetracyanoquinodimethane).Comment: 13 pages, 13 figures, accepted for publication in PR

Bandwidth-controlled Mott transition in κ−(BEDT−TTF)2Cu[N(CN)2]BrxCl1−x\kappa-(BEDT-TTF)_2 Cu [N(CN)_2] Br_x Cl_{1-x} I. Optical studies of localized charge excitations

Infrared reflection measurements of the half-filled two-dimensional organic conductors $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_{2}$]Br$_{x}$Cl$_{1-x}$ were performed as a function of temperature ($5 {\rm K}<T<300$ K) and Br-substitution ($x=0$, 40%, 73%, 85%, and 90%) in order to study the metal-insulator transition. We can distinguish absorption processes due to itinerant and localized charge carriers. The broad mid-infrared absorption has two contributions: transitions between the two Hubbard bands and intradimer excitations from the charges localized on the (BEDT-TTF)$_2$ dimer. Since the latter couple to intramolecular vibrations of BEDT-TTF, the analysis of both electronic and vibrational features provides a tool to disentangle these contributions and to follow their temperature and electronic-correlations dependence. Calculations based on the cluster model support our interpretation.Comment: 12 pages, 12 figure

Single crystal growth and study of the magnetic properties of the mixed spin-dimer system Ba3−x_{3-x}Srx_{x}Cr2_{2}O8_{8}

The compounds Sr$_{3}$Cr$_{2}$O$_{8}$ and Ba$_{3}$Cr$_{2}$O$_{8}$ are insulating dimerized antiferromagnets with Cr$^{5+}$ magnetic ions. These spin-$\frac{1}{2}$ ions form hexagonal bilayers with a strong intradimer antiferromagnetic interaction, that leads to a singlet ground state and gapped triplet states. We report on the effect on the magnetic properties of Sr$_{3}$Cr$_{2}$O$_{8}$ by introducing chemical disorder upon replacing Sr by Ba. Two single crystals of Ba$_{3-x}$Sr$_{x}$Cr$_{2}$O$_{8}$ with $x=2.9$ (3.33\% of $mixing$) and $x=2.8$ (6.66\%) were grown in a four-mirror type optical floating-zone furnace. The magnetic properties on these compounds were studied by magnetization measurements. Inelastic neutron scattering measurements on Ba$_{0.1}$Sr$_{2.9}$Cr$_{2}$O$_{8}$ were performed in order to determine the interaction constants and the spin gap for $x=2.9$. The intradimer interaction constant is found to be $J_0$=5.332(2) meV, about 4\% smaller than that of pure Sr$_{3}$Cr$_{2}$O$_{8}$, while the interdimer exchange interaction $J_e$ is smaller by 6.9\%. These results indicate a noticeable change in the magnetic properties by a random substitution effect

Softening of Magnetic Excitations Leading to Pressure-Induced Quantum Phase Transition in Gapped Spin System KCuCl3_3

KCuCl$_3$ is a three dimensionally coupled spin dimer system, which undergoes a pressure-induced quantum phase transition from a gapped ground state to an antiferromagnetic state at a critical pressure of $P_{\rm c} \simeq 8.2$ kbar. Magnetic excitations in KCuCl$_3$ at a hydrostatic pressure of 4.7 kbar have been investigated by conducting neutron inelastic scattering experiments using a newly designed cylindrical high-pressure clamp cell. A well-defined single excitation mode is observed. The softening of the excitation mode due to the applied pressure is clearly observed. From the analysis of the dispersion relations, it is found that an intradimer interaction decreases under hydrostatic pressure, while most interdimer interactions increase.Comment: 4 pages, 5 figures, 1 table, jpsj2.cls, to be published in J. Phys. Soc. Jpn. Vol.76 (2007), the graphic problem of Fig.2 was fixe

Spin-gap opening accompanied by a strong magnetoelastic response in the S=1 magnetic dimer system Ba3BiRu2O9

Neutron diffraction, magnetization, resistivity, and heat capacity measurements on the 6H-perovskite Ba3BiRu2O9 reveal simultaneous magnetic and structural dimerization driven by strong magnetoelastic coupling. An isostructural but strongly displacive first-order transition on cooling through T*=176 K is associated with a change in the nature of direct Ru-Ru bonds within Ru2O9 face-sharing octahedra. Above T*, Ba3BiRu2O9 is an S=1 magnetic dimer system with intradimer exchange interactions J0/kB=320 K and interdimer exchange interactions J'/kB=-160 K. Below T*, a spin-gapped state emerges with \Delta\approx220 K. Ab initio calculations confirm antiferromagnetic exchange within dimers, but the transition is not accompanied by long range-magnetic order.Comment: 5 pages, 5 figures, accepted by Physical Review

Pressure-Induced Magnetic Quantum Phase Transition in Gapped Spin System KCuCl3

Magnetization and neutron elastic scattering measurements under a hydrostatic pressure were performed on KCuCl3, which is a three-dimensionally coupled spin dimer system with a gapped ground state. It was found that an intradimer interaction decreases with increasing pressure, while the sum of interdimer interactions increases. This leads to the shrinkage of spin gap. A quantum phase transition from a gapped state to an antiferromagnetic state occurs at Pc ? 8.2 kbar. For P > P c, magnetic Bragg reflections were observed at reciprocal lattice points equivalent to those for the lowest magnetic excitation at zero pressure. This confirms that the spin gap decreases and closes under applied pressure.Comment: 7 pages, 10 figures, submitted to J. Phys. Soc. Jp