Tetsuji Tokihiro

COLLECTIVE excitations resulting from many-body Coulomb interactions have been studied extensively in the solid state1: for example, the exchange interaction between the electrons in two excitons (bound electron-hole pairs) can bind the excitons together, forming a biexciton. At the other extreme, if the number of excitons is sufficiently large (∼106), they can condense into a degenerate 'liquid' phase known as an electron-hole drop. But in conventional semiconductors, intermediate bound states, consisting of more than two excitons, are not formed. We show here, both theoretically and experimentally, that bound states of multiple excitons can form in the organic charge-transfer solid anthracene-(pyromellitic acid dianhydride). Coulomb interactions along the one-dimensional stacks of this material can stabilize trains of several charge-transfer excitons, and we refer to the resulting collective excitations as exciton strings. © 1994 Nature Publishing Group.

This paper demonstrates both experimental and theoretical analysis of the Coulomb attraction of CT excitons, assembling n-string excitons in a quasi-one-dimensional charge transfer (CT) organic crystal. The excitonic strings were experimentally investigated by applying an ultrafast pump-probe study on high caliber anthracine-PMDA crystals more than 50 μm thick. It was observed that an increase in pump intensity provoked another absorption band at even lower energy. Furthermore, the experimental and theoretical spectra were adequately in agreement with each other.

In Frenkel exciton systems, conformational disorder in a molecular chain leads to randomness of orientation of transition dipole moments which simultaneously induces randomness of exciton transfer. The optical absorption spectrum is sensitive to both of randomness and represents their correlation effect. The drastic change of the spectral profile near the critical point of helix-coil phase transition is also demonstrated.

We develop ''tight-binding photon approximation'' to investigate time response of a small polariton system. This method is fully quantum mechanical and does not require an additional boundary condition (ABC). The reflection and transmission spectra of thin CuCl films and the response of several femtosecond pulses are demonstrated.

The bound states of n Frenkel excitons (n = 2,3,...), which are called excitonic n-strings, are theoretically shown to exist in contrast with a system of Wannier excitons. The electronic structure and nonlinear optical responses of these excitonic n-strings are clarified. The first evidence for the excitonic 2- and 3-strings is provided from comparison of the calculated and observed differential transmission spectrum of an Anthracene-PMDA crystal pumped by femtosecond laser pulses. © 1993.

We study the higher spin analogs of the six-vertex model on the basis of its symmetry under the quantum affine algebra U(q)(sl2). Using the method developed recently for the XXZ spin chain, we formulate the space of states, transfer matrix, vacuum, creation/annihilation operators of particles, and local operators, purely in the language of representation theory. We find that, regardless of the level of the representation involved, the particles have spin 1/2, and that the n-particle space has an RSOS type structure rather than a simple tensor product of the one-particle space. This agrees with the picture proposed earlier by Reshetikhin.