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In most superconductors with a metallic normal state, zero-resistance state appears when Cooper pairs form below T_c. However in many optimally hole-doped high-temperature cuprate superconductors, superconducting T_c is much suppressed from the Cooper-pair formation temperature. This is usually considered a result of low dimensionality and/or low carrier density, which is further complicated by the pseudogap. Conventionally, superconducting T_c is expected to restore to the pairing temperature in sufficiently overdoped systems. Surprisingly, recent optical, magnetic and photoemission measurements show evidence of deviation from this conventional wisdom. We present direct thermodynamic and spectroscopic evidence that such superconducting fluctuation persists in heavily hole-doped cuprate Bi₂Sr₂CaCu₂O_8+d (Bi-2212 T_c =66K), which has a hole density as high as 10²² cm^-3. Supported by a sign-problem-free Quantum Monte Carlo calculation, the flat, shallow dispersion near the Brillouin boundary is found to be capable of promoting this unexpected suppression of superconducting phase coherence. Last but not least, we show evidence of weak-coupling superconductivity in this doping regime, and suggest overdoped Bi-2212 as a promising platform to advance quantitative understanding of the cuprate high-T_c mechanism.

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[2] Chen, Hashimoto et al, Science 366, 1099 (2019)
[3] He, Hashimoto et al, Science 362, 62 (2018)
[4] Kondo et al., Nat Commun 6, 7699 (2015)