Phonons near Peierls Structural Transition in Quasi-One-Dimensional Organic Crystals of TTF-TCNQ

The Peierls structural transition in quasi-one-dimensional organic crystals of TTF-TCNQ is investigated in the frame of a more complete physical model. The two most important electron-phonon interaction mechanisms are taken into account simultaneously. One is similar of that of deformation potential and the other is of polaron type. For simplicity, the 2D crystal model is considered. The renormalized phonon spectrum and the phonon polarization operator are calculated in the random phase approximation for different temperatures. The effects of interchain interaction on renormalized acoustic phonons and on the Peierls critical temperature are analyzed.

Peierls Structural Transition in Q1D Organic Crystals of TTT<SUB>2</SUB>I<SUB>3</SUB>for Different Values of Carrier Concentration

The Peierls structural transition in the TTT2I3 (tetrathiotetracene-iodide) crystal, for different values of carrier concentration is studied in 3D approximation. A crystal physical model is applied that considers two of the most important hole-phonon interactions. The first interaction describes the deformation potential and the second one is of polaron type. In the presented physical model, the interaction of carriers with the structural defects is taken into account. This is crucial for the explanation of the transition. The renormalized phonon spectrum is calculated in the random phase approximation for different temperatures applying the method of Green functions. The renormalized phonon frequencies for different temperatures are presented in two cases. In the first case the interaction between TTT chains is neglected. In the second one, this interaction is taken into account. Computer simulations for the 3D physical model of the TTT2I3 crystal are performed for different values of dimensionless Fermi momentum kF, that is determined by variation of carrier concentration. It is shown that the transition is of Peierls type and strongly depends on iodine concentration. Finally, the Peierls critical temperature was determined.

Anomalous CDW ground state in Cu_(2)Se:A wave-like fluctuation of the dc I-v curve near 50 K

A charge density wave(CDW)ground state is observed in polycrystalline Cu_(2)Se below 125 K,which corresponds to an energy gap of 40.9 meV and an electron-phonon coupling constant of 0.6.Due to the polycrystalline structure,the Peierls transition process has been expanded to a wide temperature range from 90 K to 160 K.The Hall carrier concentration shows a continuous decrease from 2.1×10^(20)to 1.6×10^(20)cm^(-3)in the temperature range from 160 K to 90 K,while almost unchanged above 160 K and below 90 K.After entering the CDW ground state,a wave-like fluctuation was observed in theI-Vcurve near 50 K,which exhibits a periodic negative differential resistivity in an applied electric field due to the current.We also investigated the doping effect of Zn,Ni,and Te on the CDWground state.Both Zn and Ni doped Cu_(2)Se show a CDW character with increased energy gap and electron-phonon coupling constant,but no notable Peierls transition was observed in Te doped Cu_(2)Se.Similar wave-likeI-Vcurve was also seen in Cu_(1.98)Zn_(0.02)Se near 40 K.The regular fluctuation in the dcI-Vcurve was not magnetic field sensitive,but temperature and sample size sensitive.