Local Charge Density Wave Research Articles

Determination of the local effect of impurities on the charge-density-wave phase in TaS2 by scanning tunneling microscopy.

Scanning tunneling microscopy has been used to investigate the effects of titanium impurities on the charge-density-wave (CDW) phase in $1T\ensuremath{-}\mathrm{Ta}{\mathrm{S}}_{2}$. The average CDW wavelength has been directly measured and found to increase with increasing titanium concentration. However, the local CDW structure distorts significantly in response to the random lattice potential associated with the distorted titanium sites.

Magnetoquantum oscillations, magnetic breakdown, and Fermi-surface modifications in NbSe3.

The magnetoquantum oscillations in ${\mathrm{NbSe}}_{3}$ show frequency shifts and beat structures due to interaction between the pinned charge-density-wave (CDW) configuration and the normal Fermi surface (FS). Spatial variations in the phase deformation of the CDW pinned at impurities can result in changes in the local Fermi level and CDW gap, giving rise to a distribution of extremal FS areas. Magnetic breakdown (MB) between these small FS sections and the open nested sheets of FS can have striking effects on the amplitude and frequency of the quantum oscillations due to modulation of the transmission and reflection coefficients at the MB junctions. A model using a one-dimensional open-orbit network and a rectangular distribution of FS areas has been used to fit the data in the range 0--225 kG. Good fits to the beat periods and frequency shifts are obtained and provide an accurate measure of the width of the frequency distribution for different CDW configurations. The existence of these MB mechanisms also provides an explanation for hysteresis and asymmetry observed in the magnetoresistance as the CDW configuration is manipulated in electric and magnetic fields.

Electronic states of BaPb1-xBixO3 in the semiconducting phase investigated by optical measurements.

The semiconducting ${\mathrm{BaPb}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Bi}}_{\mathrm{x}}$${\mathrm{O}}_{3}$ (0.35lx\ensuremath{\le}1) is investigated in detail by optical measurements. The optical spectrum shows that ${\mathrm{BaPb}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Bi}}_{\mathrm{x}}$${\mathrm{O}}_{3}$ has a clear optical gap over the whole semiconducting compositional range. It is clearly shown by resonant Raman scattering measurements that the origin of this gap is a charge-density wave (CDW) accompanied by the breathing-mode distortion. No apparent effect of Pb substitution can be observed in the reflectivity spectra for the sample with xg0.7. For x\ensuremath{\le}0.7, the CDW gap decreases with x and simultaneously the absorption tail extends into the gap. This spectral behavior can be well interpreted in terms of the recently proposed theoretical model with emphasis on a local CDW and/or a substantial energy difference between Pb and Bi sites arising from the strong electron-phonon interaction. Thus it can be concluded that the strong electron-phonon interaction plays an essential role in the semiconducting phase of this material as well as in the metallic phase.

Effect of charge density wave fluctuations on the frequencies of optic phonons in 2H-TaSe2 and -NbSe2

The temperature dependence of the frequencies of the Raman active optic phonons between 200 and 250 cm −1 in 2H-TaSe 2 and -NbSe 2 has been measured for 2 K<T<300 K. The observed shifts are considerably larger than would be expected on the basis of measurements made on the isostructural semiconducting layer compound 2H-MoSe 2 . The anomalous temperature dependence in the charge density wave (CDW) state. The temperature dependence of the phonon frequencies for temperatures above the CDW phase transition suggests the presence of localized CDW distortions in the normal state.