We present an experimental and theoretical study which compares the phonon anomalies and the electronic gap features in the infrared response of the weakly coupled two-leg-ladders in Sr 14−x Ca _x Cu₂₄O₄₁ (SCCO) with those of the underdoped high-T _c superconductor YBa₂Cu₃O 6+x (YBCO) and thereby reveals some surprising analogies. Specifically, we present a phenomenological model that describes the anomalous doping- and temperature-dependence of some of the phonon features in the a-axis response (field along the rungs of the ladders) of SCCO. It assumes that the phonons are coupled to charge oscillations within the ladders. Their changes with decreasing temperature reveal the formation of a crystal (density wave) of hole pairs that are oriented along the rungs. We also discuss the analogy to a similar model that was previously used to explain the phonon anomalies and an electronic plasma mode in the c-axis response (field perpendicular to the CuO₂ planes) of YBCO. We further confirm that an insulator-like pseudogap develops in the a-axis conductivity of SCCO which closely resembles that in the c-axis conductivity of YBCO. Most surprisingly, we find that the c-axis conductivity (field along the legs of the ladders) of SCCO is strikingly similar to the in-plane one (field parallel to the CuO₂ planes) of YBCO. Notably, in both cases a dip feature develops in the normal state spectra that is connected with a spectral weight shift toward low frequencies and can thus be associated with precursor superconducting pairing correlations that are lacking macroscopic phase coherence. This SCCO–YBCO analogy indicates that collective degrees of freedom contribute to the low-energy response of underdoped high T _c cuprates and it even suggests that the charges in the CuO₂ planes tend to segregate forming quasi-one-dimensional structures similar to the two-leg ladders, as predicted for the stripe-scenario or certain intertwinned states.

中文翻译：

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Sr 14−x Ca x Cu24O41 和欠掺杂 YBa2Cu3O 6+x 的红外响应中声子异常和电子间隙特征的类比

我们提出了一项实验和理论研究，比较了 Sr 中弱耦合双腿阶梯的红外响应中的声子异常和电子间隙特征 14−X 钙 _X Cu ₂₄ O ₄₁ (SCCO) 与欠掺杂高-吨 _C 超导体 YBa ₂ Cu ₃ O 6个+X (YBCO) 从而揭示了一些令人惊讶的类比。具体来说，我们提出了一个现象学模型，该模型描述了一些声子特征的异常掺杂和温度依赖性A-SCCO 的轴响应（沿梯级的场）。它假设声子耦合到阶梯内的电荷振荡。它们随温度降低而发生的变化揭示了沿梯级定向的空穴对晶体（密度波）的形成。我们还讨论了与以前用于解释声子异常和电子等离子体模式的类似模型的类比CYBCO 的轴响应（垂直于 CuO ₂平面的场）。我们进一步证实在ASCCO 的轴电导率与CYBCO 的轴电导率。最令人惊讶的是，我们发现CSCCO 的轴电导率（沿阶梯腿的场）与YBCO 的面内电导率（场平行于 CuO _{2平面）惊人地相似。}值得注意的是，在这两种情况下，在正常状态光谱中都会出现倾角特征，这与向低频的光谱权重偏移有关，因此可能与缺乏宏观相位相干性的前体超导配对相关性相关联。这种 SCCO-YBCO 类比表明集体自由度有助于欠掺杂高的低能量响应吨 _{C 铜酸盐，它甚至表明 CuO 2}平面中的电荷倾向于分离形成类似于双腿梯子的准一维结构，如条纹情景或某些相互缠绕状态所预测的那样。