DEVELOPMENT OF TRANSIENT THERMOELECTRIC EFFECT IN HIGH MAGNETIC AND ELECTRIC FIELDS AND ITS APPLICATION TO THE STUDY OF CHARGE DENSITY WAVE MATERIALS

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，从中我们可以获得有关光生载流子热扩散的有价值的信息（和声子）沿着样品中的浓度和温度梯度漂移。这种技术对于由传导电子和空穴组成的多载流子系统特别有用，我们称之为“传导载流子光谱学”。对于这种技术的更一般的应用，我们正在进行TTE实验下施加的磁场和直流电流的稳定流动下，以获得有关各种材料的输运性质的进一步信息，如准二维钼氧化物，由于电荷密度波不稳定性显示金属-半导体过渡。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，用数字示波器可检测的时间范围从50 ns到快至1 ns，用改进的装置结合常规的去输运技术，获得了n-Mo_4 O_（1 - 1）CDW材料的电子性质的有价值的信息，分子导体（DMe-DCCNQI）_2Cu及其偏钨酸盐、层状材料TaS_2、超纯补偿金属钨、合成金刚石膜和过氧化锰氧化物。此外，还研究了强磁场输运以及脉冲电场的线性和非线性响应。

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 a dc electric current to get further information about transport properties of various materials, such as 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 clown 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 combined with conventional de transport techniques, we have obtained valuable information about electronic properties of CDW materials of n-Mo_4O_1_1, molecular conductors (DMe-DCCNQI)_2Cu and its duetetated salts, layered material TaS_2, as well as ultrapure compensated metal of tungsten, synthetic diamond films, and Perovskite manganites. In addition, high magnetic field transport and linear and nonlinear response to a pulsed electric field were also studied.