[论文解读] Determination of the optical properties of La{2-x}Ba{x}CuO{4} for several dopings, including the anomalous x=1/8 phase

C. C. Homes, M. Hücker+5 more|arXiv (Cornell University)|Oct 10, 2011

Physics of Superconductivity and Magnetism被引用 8

一句话总结

本研究通过红外反射率测量了三种钡掺杂浓度下La2-xBaxCuO4单晶的光学性质，包括异常的x=1/8相。结果表明，在自旋条纹有序温度（Tso ≈ 42 K）以下，x=1/8相表现出从低频到高频的光谱权重转移，暗示存在动量依赖的能隙；而超导相则显示出凝聚态形成和约瑟夫森等离子体边缘，两者均与超流密度和直流电导率的普遍标度关系一致。

ABSTRACT

The optical properties the high-temperature superconductor La{2-x}Ba{x}CuO{4} have been measured over a wide frequency and temperature range for light polarized in the a-b planes and along the c axis. Three different Ba concentrations have been examined, x=0.095 with a critical temperature T_c=32 K, x=0.125 with T_c ~ 2.4 K, and x=0.145 with T_c ~ 24 K. The in-plane behavior of the optical conductivity for these materials at high temperature is described by a Drude-like response with a scattering rate that decreases with temperature. Below T_c in the x=0.095 and 0.145 materials there is a clear signature of the formation of a SC state in the optical properties allowing the strength of the condensate (ρ_{s0}) and the penetration depth to be determined. In the anomalous 1/8 phase, some spectral weight shifts from lower to higher frequency (above 300 cm^{-1}) on cooling below the spin-ordering temperature T_{so} ~ 42 K, associated with the onset of spin-stripe order; we discuss alternative interpretations in terms of a conventional density-wave gap versus the response to pair-density-wave SC. The disappearance of the low-frequency spectral weight at low temperature may indicate the formation of a 2D SC state below the Berezinskii-Kosterlitz-Thouless transition at about 16 K prior to the onset of bulk 3D SC. The two dopings for which a SC response is observed both fall on the universal scaling line ρ_{s0}/8 ~ 4.4 σ_{dc} T_c. The optical properties for light polarized along the c axis reveal an insulating character dominated by lattice vibrations, superimposed on a weak electronic background. In the x=0.095 and 0.145 materials a Josephson plasma edge is observed in the reflectance below T_c. No Josephson plasma edge is observed in the 1/8 phase, suggesting that the presence of charge and spin order frustrates the formation of a supercurrent and bulk 3D SC.

研究动机与目标

• 研究不同钡掺杂水平下La2-xBaxCuO4的光学响应，包括异常的x=1/8相。
• 确定自旋条纹序和电荷序在调控电子光谱权重和超导相干性中的作用。
• 评估异常的x=1/8相是否表现出配对密度波（PDW）超导性的特征，或传统的电荷密度波能隙。
• 测量c轴光学响应并探测约瑟夫森等离子体边缘，以表征层间超导耦合。
• 检验高温超导体中普遍标度关系ρs0/8 ≈ 4.4σdcTc的有效性。

提出的方法

• 在宽频率和温度范围内，对x = 0.095、0.125和0.145的La2-xBaxCuO4单晶进行了红外反射率测量。
• 通过克雷默-克罗尼格分析从反射率数据中提取光学电导率，并减去声子和反共振贡献。
• 应用德鲁德模型拟合面内光学电导率，以提取散射率和等离子体频率。
• 通过Tc以下缺失的低频光谱权重确定超流密度ρs0。
• 分析c轴反射率以探测约瑟夫森等离子体边缘并评估层间耦合强度。
• 将温度依赖的光谱权重演化与PDW和传统CDW能隙的理论模型进行比较。

实验结果

研究问题

• RQ1 x=1/8相是否表现出与配对密度波（PDW）超导性一致的动量依赖能隙？
• RQ2 在Tso ≈ 42 K时自旋条纹序的形成如何影响光学响应和光谱权重分布？
• RQ3 尽管c轴电阻显著下降，为何在x=1/8相中未观测到c轴方向的约瑟夫森等离子体边缘？
• RQ4 x=0.095和x=0.145的超导相是否遵循普遍标度关系ρs0/8 ≈ 4.4σdcTc？
• RQ5 是什么原因导致在x=1/8相中低频反射率异常抑制，并使光谱权重向>300 cm⁻¹转移？

主要发现

• x=0.095和x=0.145样品表现出类似德鲁德的面内光学电导率，随温度降低而变窄，表明在Tc = 32 K和≈24 K以下形成超导凝聚态。
• 通过Tc以下缺失的低频光谱权重确定了超流密度ρs0和穿透深度，证实了超导态的形成。
• 两个超导样品均落在普遍标度线ρs0/8 ≈ 4.4σdcTc上，表明面内强耗散且与d波配对一致。
• 在x=1/8相中，低于Tso ≈ 42 K时，光谱权重从低频（<300 cm⁻¹）向高频（>300 cm⁻¹）转移，表明动量依赖能隙的形成。
• 在Tso以下，面内自由载流子响应显著变窄，且在最低温度下，自由载流子光谱权重不再可分辨，表现出类似凝聚态形成的特征，但无体超导性。
• 尽管c轴电阻显著下降，但在x=1/8相中未观测到c轴方向的约瑟夫森等离子体边缘，表明层间耦合受到抑制。