2006 Fiscal Year Final Research Report Summary
Low Energy Charge Excitation in Copper Oxide Superconductors
| Project/Area Number |
17340109
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| Research Category |
Grant-in-Aid for Scientific Research (B)
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| Allocation Type | Single-year Grants |
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| Section | 一般 |
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| Research Field |
Condensed matter physics II
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| Research Institution | Osaka University |
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Principal Investigator |
TAJIMA Setsuko Osaka University, Graduate School of Science, Professor, 大学院理学研究科, 教授 (70188241)
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| Co-Investigator(Kenkyū-buntansha) |
FUJII Kenichi Osaka University, Graduate School of Science, Professor, 大学院理学研究科, 教授 (10189988)
MASUI Takahiko Osaka University, Graduate School of Science, Assistant Professor, 大学院理学研究科, 助手 (10403099)
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| Project Period (FY) |
2005 – 2006
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| Keywords | High Temperature Superconductor / Copper Oxides / Terahertz Spectroscopy / Inhomogeneous Electronic State |
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| Research Abstract |
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.
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