[论文解读] Out-of-plane Seebeck coefficient of the cuprate La$_{1.6-x}$Nd$_{0.4}$Sr$_{x}$CuO$_4$ across the pseudogap critical point: particle-hole asymmetry and Fermi surface transformation

Adrien Gourgout, G. Grissonnanche+9 more|arXiv (Cornell University)|Jun 10, 2021

Physics of Superconductivity and Magnetism被引用 1

一句话总结

本研究测量了Nd-LSCO铜氧化物在赝能隙临界掺杂浓度p* = 0.23处的面外（S_c）和面内（S_ab）塞曼系数。在p*以下，S_c为负值，表明费米面发生重构；而在p*以上，S_ab保持正值且各向同性，表明存在与非费米液体行为相关的粒子-空穴不对称性。关键发现是，S_c符号的反转表明在p*处电子结构发生根本性变化。

ABSTRACT

We report measurements of the Seebeck effect in both the $ab$ plane ($S_{ m a}$) and along the $c$ axis ($S_{ m c}$) of the cuprate superconductor La$_{1.6-x}$Nd$_{0.4}$Sr$_{x}$CuO$_4$ (Nd-LSCO), performed in magnetic fields large enough to suppress superconductivity down to low temperature. We use the Seebeck coefficient as a probe of the particle-hole asymmetry of the electronic structure across the pseudogap critical doping $p^{\star} = 0.23$. Outside the pseudogap phase, at $p = 0.24 > p^{\star}$, we observe a positive and essentially isotropic Seebeck coefficient as $T ightarrow 0$. That $S > 0$ at $p = 0.24$ is at odds with expectations given the electronic band structure of Nd-LSCO above $p^{\star}$ and its known electron-like Fermi surface. We can reconcile this observation by invoking an energy-dependent scattering rate with a particle-hole asymmetry, possibly rooted in the non-Fermi liquid nature of cuprates just above $p^{\star}$. Inside the pseudogap phase, for $ p < p^{\star}$, $S_{ m a}$ is seen to rise at low temperature as previously reported, consistent with the drop in carrier density $n$ from $n \simeq 1 + p$ to $n \simeq p$ across $p^{\star}$ as inferred from other transport properties. In stark contrast, $S_{ m c}$ at low temperature becomes negative below $p^{\star}$, a novel signature of the pseudogap phase. The sudden drop in $S_{ m c}$ reveals a change in the electronic structure of Nd-LSCO upon crossing $p^{\star}$. We can exclude a profound change of the scattering across $p^{\star}$ and conclude that the change in the out-of-plane Seebeck coefficient originates from a transformation of the Fermi surface.

研究动机与目标

• 探究Nd-LSCO在赝能隙临界掺杂浓度p* = 0.23处电子结构中的粒子-空穴不对称性。
• 研究在p = 0.24 > p*时出现异常正值塞曼系数的原因，尽管其费米面呈电子型预期。
• 确定面外塞曼系数S_c是否反映p*处费米面拓扑结构的变化或散射机制的变化。
• 区分赝能隙转变过程中散射主导与费米面主导对塞曼效应的贡献。

提出的方法

• 在Nd-LSCO单晶中测量不同掺杂浓度x下的面内（S_ab）和面外（S_c）塞曼系数，涵盖p = 0.24 > p*和p < p*的样品。
• 施加高磁场以抑制超导性，从而在低温下探测正常态。
• 利用塞曼系数作为载流子浓度和费米面拓扑结构的探针，基于Mott公式及其各向异性扩展形式。
• 通过比较S_ab和S_c在p*附近的响应行为，推断电子结构的变化，特别是粒子-空穴不对称性和费米面重构。
• 通过比较温度和磁场依赖性，排除强散射变化作为S_c异常的根源。

实验结果

研究问题

• RQ1 为何在Nd-LSCO中，尽管费米面呈电子型，p = 0.24 > p*时塞曼系数仍为正值？
• RQ2 是什么导致面外塞曼系数S_c在p*以下变为负值？
• RQ3 S_c在p*处的变化是由费米面重构引起，还是由散射率变化引起？
• RQ4 电子结构中的粒子-空穴不对称性如何在p*附近的塞曼效应中体现？

主要发现

• 在p = 0.24 > p*时，塞曼系数S_ab为正值且各向同性，与电子型费米面的预期相矛盾，表明存在粒子-空穴不对称性。
• 在p* = 0.23以下，S_ab在低温下随温度升高而增加，与载流子浓度从n ≈ 1 + p降至n ≈ p的转变一致。
• 面外塞曼系数S_c在p*以下变为负值，这是赝能隙相中前所未有的新特征，正常态中未观察到。
• S_c在p*处的突然下降无法用散射率变化来解释，表明费米面发生了根本性重构。
• S_c符号的反转归因于电子结构的变化，特别是p*处费米面的重构。