Low Energy Charge Excitation in Copper Oxide Superconductors

用提拉法生长了（Y，Ca）Ba_2Cu_3O_y单晶，并通过单轴压力下氧气氛退火去孪晶，然后在O_2中进行后退火以控制载流子浓度。通过这种方式，我们已经获得了足够大的太赫兹光谱样品（1.3x3 mm^2）。在低频区反射率的测量方法上，我们从原来的以返波管为光源的常规方法，改变为采用钛宝石超快脉冲激光的时域光谱法。TDS方法通常采用透射测量的几何结构，而采用反射测量的几何结构存在许多困难。低温恒温器内样品的低温测量也带来了一些额外的困难。为了克服这些困难，我们改进了光学系统以及样品保持器。对金镜和（La，Sr）_2CuO_4超导体的实验测量得到了满意的结果，并测量了（Y，Ca）Ba_2Cu_3O_y的退孪晶远红外反射率谱，其偏振方向平行于a轴和/或b轴。a轴谱代表CuO_2面内的电荷响应。与最佳掺杂的YBa_2Cu_3O_y样品的a轴光谱相比，载流子过掺杂样品的a轴光谱即使在T = T_c处也显示出较低的反射率（<100%），这表明存在未成对载流子。在进一步过掺杂的（Y，Ca）Ba_2Cu_3O_y中，这种低反射率的趋势变得更加明显。不成对载流子的一个可能起源是载流子过掺杂导致的成对断裂，或者超导区和非超导区发生了相分离。这对理解高温超导体的电子相图具有重要意义。

Single crystals of (Y, Ca)Ba_2Cu_3O_y were grown by a pulling technique and then detwinned by annealing in oxygen atmosphere under uniaxial pressure, followed by post-annealing in O_2 for controlling carrier concentration. In this way, we have obtained samples large enough for Terahertz spectroscopy (〜3x3 mm^2). As to the method for reflectivity measurement in the low frequency region, we have changed from the original plan of usual method with use of BWO tube as a light source to the new method of time domain spectroscopy (TDS) with use of Ti-Sapphire ultra-fast pulse laser. The TDS method is usually carried out with a geometry of transmittance measurement, while there are many difficulties in measurement with a reflectance geometry. The low temperature measurement for samples within a cryostat also gives some additional difficulties. In order to overcome these difficulties, we have improved an optical system as well as a sample holder. The test measurement for a gold mirror and (La,Sr)_2CuO_4 superconductor gave us a satisfactory result.We also measured far-infrared reflectivity spectra of detwinned (Y, Ca)Ba_2Cu_3O_y with polarization parallel to a-and/or b-axes. The a-axis spectrum is expected to represent the charge response within the CuO_2-plane. Compared to the a-axis spectrum for optimally doped YBa_2Cu_3O_y, the spectrum for carrier overdoped sample was found to show lower reflectivity (<100%) even at T≪T_c, which suggests a presence of unpaired carriers. It was also found that this low reflectivity tendency becomes more pronounced in the further overdoped (Y, Ca)Ba_2Cu_3O_y. A possible origin for the unpaired carriers is that some pair-breaking is driven by carrier overdoping or that the phase separation of superconducting and non-superconducting regions takes place. This is very important findings to understand the electronic phase diagram of high-T_c superconductors.