Nonlinear ultrasound wave generation in quasi-two dimensional organic conductors

The amplitude of an ultrasound wave with double frequency 2ω generated by Joule heating as a heat source in the quasi-two dimensional layered organic conductor β−(BEDT-TTF)₂IBr₂ is analyzed as a function of the magnetic field strength and its orientation with respect to the plane of the layers. Angular oscillations of the ultrasound wave amplitude are correlated with the changes of electrical and thermal conductivity of the compound when the magnetic field is tilted from the normal to the layers plane due to the periodic dependence of the carriers velocity along the less conducting axis on the field orientation. The period of oscillations allows the characteristic parameters of the Fermi surface to be determined. The magnetic field dependence reveals that the effect of nonlinear generation of the ultrasound wave due to Joule heating is quadratic in proportion to the magnetic field at each field direction. The behavior of the wave amplitude is different depending on the field orientation from the normal to the layers, i.e., is it oriented at the maximum or minimum in the angular dependence. In layered organic conductors due to the small electron mean free-path the nonlinear generation of ultrasound waves can be observed for a wide range of magnetic fields providing possibilities for experimental studies using non-contact ultrasonic techniques. (author)