DEVELOPMENT OF CONDUCTION CARRIER SPECTROSCOPY AND ITS APPLICATION TO THE STUDY OF HIGH TEMPERATURE SUPERCONDUCTORS

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Our newly developed technique is a "dynamical" one, called the pulsed-laser induced "Transient Thermoelectric Effect (TTE) " method, in which after a pulsed laser is irradiated upon one end of a sample, one measures an induced voltage across both ends of the sample and analyzes its decay profile, from which we can obtain valuable information about thermal diffusions of photo-generated carriers (and phonons) drifting along the concentration and temperature gradient in the sample. This technique is particularly useful for a multicarrier system consisting of both conduction electrons and holes, ---we refer to as "conduction-carrier spectroscopy". For a more general application of this technique, we are undertaking the TTE experiments under an applied magnetic field and under a steady flow of adc electric current to get further information about transport properties of various materials, such as high T_c oxide superconductors, semiconductors and quasi-two-dimensional molybdenum oxides that show a metal-semiconductor transition due to charge-density-wave instabilities. We have improved the hitherto-made TTE apparatus with the fixed laser wavelength of 1064 nm with a Nd : YAG to a variable range of 690-1320 nm using a combined laser source of Ti : Sapphire/Foresterite with the Nd : YAG source, a detectable time range from 50 ns down to as fast as 1 ns with a digital oscilloscope, and laser intensity reduced by one tenths of the previous one as well. With this improved apparatus we have obtained valuable information about the carrier generation/recombination processes in a p-type GaAs crystal, as a test sample, due to band-band and band-EL2 centers transitions. Computer simulation of the photogenerated carrier diffusion under an ambipolar internal field for n-type GaAs has also been performed to understand the TTE process.