No abstract

We have studied the charge density wave ͑CDW͒ repolarization dynamics in blue bronze by applying square-wave voltages of different frequencies to the sample and measuring the changes in infrared transmittance, proportional to CDW strain. The frequency dependence of the electrotransmittance was fit to a modified harmonic oscillator response and the evolution of the parameters as functions of voltage, position, and temperature are discussed. Resonant frequencies decrease with distance from the current contacts, indicating that the resulting delays are intrinsic to the CDW with the strain effectively flowing from the contact. For a fixed position, the average relaxation time has a voltage dependence given by 0 ϳ V −p , with 1 Ͻ p Ͻ 2. The temperature dependence of the fitting parameters shows that the dynamics are governed by both the force on the CDW and the CDW current: for a given force and position, both the relaxation and delay times are inversely proportional to the CDW current as temperature is varied. The long relaxation and delay times ͑ϳ1 ms͒ suggest that the strain response involves the motion of macroscopic objects, presumably CDW phase dislocation lines.

We report measurements of the scattering of electrons by helium atoms in the presence of 1.17 eV photons from a Nd:YAG laser. The incident energy of the electrons was in the range 50 -350 eV and the polarization of the laser was arranged to be parallel to electrons scattered through 135 • . Energyshifted peaks corresponding both to one-and two-photon emission were observed. Calculations using the Kroll-Watson approximation are perfectly consistent with the data.

We have studied the frequency and voltage dependence of voltage-induced torsional strains in orthorhombic TaS 3 [V. Ya. Pokrovskii, et al, Phys. Rev. Lett. 98, 206404 (2007)] by measuring the modulation of the resonant frequency of an RF cavity containing the sample. The strain has an onset voltage below the charge-density-wave (CDW) threshold voltages associated with changes in shear compliance and resistance, suggesting that the strain is associated with polarization of the CDW rather than CDW current. Measurements with square-wave voltages show that the strain is very sluggish, not even reaching its dc value at a frequency of 0.1 Hz, but the dynamics appear to be very sample dependent. By applying oscillating torque while biasing the sample with a dc current, we have also looked for strain induced voltage in the sample; none is observed at the low biases where the voltage-induced strains first occur, but an induced voltage is observed at higher biases, probably associated with strain-dependent CDW conductance.

A systematic method to measure the damping coefficient of a simple pendulum carrying out small oscillations using the resonance phenomena is considered. The method only requires a standard function generator, a simple pendulum and a loudspeaker. The results obtained from this experiment show that the damping coefficient is very small, which is confirmed by the observed beats.