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

我们新开发的技术是一种“动态”技术，称为脉冲激光诱导的“瞬态热电效应（TTE）“方法，其中脉冲激光照射样品的一端后，测量样品两端的感应电压并分析其衰减曲线，从中我们可以获得关于沿着样品中的浓度和温度梯度漂移的光生载流子（和声子）的热扩散的有价值的信息。这种技术对于由导电电子和空穴组成的多载流子系统特别有用，我们称之为“导电载流子光谱学”。为了更广泛地应用这一技术，我们在外加磁场和恒定电流作用下进行了TTE实验，以获得关于各种材料输运性质的进一步信息，如高T_c氧化物超导体、半导体和准二维钼氧化物，这些材料由于电荷密度波不稳定性而表现出金属-半导体转变。我们已经改进了迄今为止制造的TTE装置，该装置具有使用Nd：YAG的1064 nm的固定激光波长，使用Ti：Sapphire/Foresterite与Nd：YAG源，可检测的时间范围从50 ns下降到1 ns的数字示波器，并且激光强度也降低了先前的十分之一。用这种改进的装置，我们已经获得了有价值的信息的载流子产生/复合过程中的p型GaAs晶体，作为测试样品，由于带-带和带-EL 2中心的转变。计算机模拟的光生载流子扩散下的双极性内场的n型GaAs也进行了了解TTE过程。

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.