Low Energy Charge Excitation in Copper Oxide Superconductors

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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.