Session A37: Superconductivity and fluctuations in overdoped cuprates

Unconventional spectral signature of Tc in a pure d-wave superconductor

In bulk BCS superconductors, the superconducting transition has four prominent signatures: the development of zero resistance, the onset of Meissner effect, the peak of the electronic specific heat coefficient, and the opening of the spectral gap. In the high-Tc cuprates, although the first three signatures are routinely observed, the gap is also found at temperatures above Tc and the spectral fingerprint of the phase transition remains unclear.

In this talk, I will present the missing piece of the puzzle revealed by high-precision photoemission spectroscopy [1, 2]. Focusing on overdoped Bi2212 above the pseudogap critical doping [3], we observe a pure d-wave gap that evolves smoothly across Tc free from the interference of competing orders. On the other hand, the “filling” rate of the gap is found to sharply peak at Tc and create the thermodynamic anomaly across the phase transition. I will discuss the implications of our results in the context of recent progress on overdoped cuprates.

[1] Chen, Hashimoto et al., Nature 601, 562 (2022)

[2] He, Chen, Li et al., PRX 11, 031068 (2021)

[3] Chen, Hashimoto et al., Science 366, 1099 (2019)

    

Puddle formation, persistent gaps, and non-mean-field breakdown of superconductivity in overdoped (Pb,Bi)2Sr2CuO6+δ

The cuprate high-temperature superconductors exhibit many unexplained electronic phases, but it was often thought that the superconductivity at sufficiently high doping is governed by conventional mean-field Bardeen-Cooper-Schrieffer (BCS) theory. However, a series of recent experiments show that the number of paired electrons (the superfluid density) vanishes when the transition temperature T_cgoes to zero, in contradiction to expectation from BCS theory. The origin of this anomalous vanishing is unknown. Our scanning tunneling spectroscopy measurements in the overdoped regime of the (Pb,Bi)₂Sr₂CuO_6+δ high-temperature superconductor show that it is due to the emergence of puddled superconductivity, featuring nanoscale superconducting islands in a metallic matrix. Our measurements further reveal that this puddling is driven by gap filling, while the gap itself persists beyond the breakdown of superconductivity. The important implication is that it is not a diminishing pairing interaction that causes the breakdown of superconductivity. I will argue that the gap-to-filling correlation also reveals that pair-breaking by disorder does not play a dominant role and that the mechanism of superconductivity in overdoped cuprate superconductors is qualitatively different from conventional mean-field theory.

    

Title: Superconductor-to-metal transition in overdoped cuprates

We present a theoretical framework for understanding the behavior of the normal and superconducting states of overdoped

cuprate high temperature superconductors in the vicinity of the doping-tuned quantum superconductor-to-metal transition. The

key ingredients on which we focus are d-wave pairing, a flat antinodal dispersion, and disorder. Even for homogeneous disorder,

these lead to effectively granular superconducting correlations and a superconducting transition temperature determined in large

part by the superfluid stiffness rather than the pairing scale.   

Low energy electrodynamics of the “missing superconducting electrons” in the overdoped cuprates La2-xSrxCuO4

Overdoped high-temperature cuprate superconductors have often been understood within the standard BCS framework of superconductivity. However, measurements in a variety of overdoped cuprates indicate that the superfluid density is much smaller than expected and decreases smoothly to zero as the doping is increased. In this talk, I will discuss how we combine time-domain THz spectroscopy with kHz range mutual inductance measurements on a number of as-grown and ion-irradiated overdoped La_2−xSr_xCuO₄ films to determine the total, superfluid, and uncondensed spectral weight as a function of doping and disorder. We find that a significant fraction of the carriers remains uncondensed in a wide Drude-like peak as the temperature goes to zero while the superfluid density remains linear-in-temperature. Increasing disorder causes only a small change in the superconducting transition temperature for the overdoped films, but the changes in the superfluid density are much larger. I will discuss our findings in the context of existing theories of pair-breaking in a dirty d-wave superconductor, intrinsic electronic inhomogeneity, and the possible presence of phase fluctuations. I will also briefly highlight how new THz techniques based on non-linear response might be able to shed light onto the anomalous nature of the superfluid density in overdoped cuprates.

References:

Fahad Mahmood, Xi He, Ivan Božovic, and N. P. Armitage. Phys. Rev. Lett. 122, 027003 (2019)

Fahad Mahmood, David Ingram, Xi He, J. A. Clayhold, Ivan Božovic, and N. P. Armitage. Phys. Rev. B 105, 174501 (2022)   

Understanding anomalous properties of overdoped cuprates within the Landau-BCS paradigm

New experimental data on superfluid density and terahertz conductivity of overdoped LSCO have exhibited behavior incompatible with a clean BCSsuperconductor [1]. However, it has also been suggested that the properties observed are compatible with "dirty d-wave theory" assuming weak scattering by dopant atoms. We attempt to place this theory on a more quantitative basis by performing   ab-initio calculations of dopant impurity potentials for LSCO and Tl-2201.  These potentials are more complex than the pointlike impurity models considered previously, and require calculation of forward scattering corrections to transport properties. Including realistic, ARPES-derived bandstructures and Fermi liquid renormalizations, we show that the theory can explain semiquantitatively a wide variety of results on the two most studied overdoped materials, and make predictions for future measurements.