Differentiated roles of Lifshitz transition on thermodynamics and superconductivity in La2-xSrxCuO4

The effect of Lifshitz transition on thermodynamics and superconductivity in hole-doped cuprates has been heavily debated but remains an open question. In particular, an observed peak of electronic specific heat is proposed to originate from fluctuations of a putative quantum critical point p* (e.g. the termination of pseudogap at zero temperature), which is close to, but distinguishable from the Lifshitz transition in overdoped La-based cuprates where the Fermi surface transforms from hole-like to electron-like. Here, we report an in-situ angle-resolved photoemission spectroscopy study of three-dimensional Fermi surfaces in La2-xSrxCuO4 thin films (x = 0.06 – 0.35). With accurate kz dispersion quantification, the said Lifshitz transition is determined to happen within a finite range around x = 0.21. Normal state electronic specific heat, calculated from spectroscopy-derived band parameters, reveals a doping dependent profile with a maximum at x = 0.21 that agrees with previous thermodynamic microcalorimetry measurements. The account of the specific heat maximum by underlying band structures excludes the need for additionally dominant contribution from the quantum fluctuations at p*. A d-wave superconducting gap smoothly across the Lifshitz transition demonstrates the insensitivity of superconductivity to the dramatic density of states enhancement.

空穴掺杂铜基超导体（hole-doped cuprates）中利夫希茨相变（Lifshitz transition）对热力学与超导性的影响长期存在广泛争议，至今仍是悬而未决的科学问题。具体而言，观测到的电子比热峰被认为源于推定的量子临界点p*（putative quantum critical point p*）的涨落——例如零温下赝能隙（pseudogap）的终止位置——该临界点与过掺杂镧基铜基超导体中的利夫希茨相变相近但可区分：过掺杂样品的费米面（Fermi surface）会从类空穴型转变为类电子型。本研究针对La2-xSrxCuO4薄膜（x=0.06–0.35）的三维费米面开展原位角分辨光电子能谱（angle-resolved photoemission spectroscopy）研究。通过精确的kz色散量化分析，确定上述利夫希茨相变发生在x=0.21附近的有限掺杂区间内。基于光谱学提取的能带参数计算得到的正常态电子比热，呈现出随掺杂浓度变化的分布特征，在x=0.21处取得峰值，该结果与此前的热力学微量热实验测量结果一致。仅通过本征能带结构即可解释电子比热的峰值现象，无需额外引入量子临界点p*处的量子涨落所带来的主导性贡献。跨越利夫希茨相变的d波超导能隙（d-wave superconducting gap）保持平滑演化，这表明超导性对态密度（density of states）的剧烈增强并不敏感。