Emergent normal fluid in the superconducting ground state of overdoped cuprates

Abstract

The microscopic mechanism for the disappearance of superconductivity in overdoped cuprates is still under heated debate. Here we use scanning tunneling spectroscopy to investigate the evolution of quasiparticle interference phenomenon in Bi2Sr2CuO6+δ over a wide range of hole densities. We find that when the system enters the overdoped regime, a peculiar quasiparticle interference wavevector with arc-like pattern starts to emerge even at zero bias, and its intensity grows with increasing doping level. Its energy dispersion is incompatible with the octet model for d-wave superconductivity, but is highly consistent with the scattering interference of gapless normal carriers. The gapless quasiparticles are mainly located near the antinodes and are independent of temperature, consistent with the disorder scattering mechanism. We propose that a branch of normal fluid emerges from the pair-breaking scattering between flat antinodal bands in the quantum ground state, which is the primary cause for the reduction of superfluid density and suppression of superconductivity in overdoped cuprates.

Negligible normal fluid in superconducting state of heavily overdoped Bi2Sr2CaCu2O8+δ detected by ultra-low temperature angle-resolved photoemission spectroscopy

“…The normal electrons, if they exist, may be present in the form of phase separation either in the real space or reciprocal space as shown in the STM measurement of the overdoped Bi2201 077405-4 samples. [6,7] In principle, ARPES can distinguish normal electrons from superconducting electrons because they behave differently in the superconducting state. The normal electrons will have a band that crosses the Fermi level and produce spectral weight at the Fermi level.…”

“…The normal fluid is clearly observed by STM in the superconducting state. [6,7] But in heavily overdoped Bi2212, the normal fluid is negligible in the superconducting state. In overdoped LSCO, it is found that the superfluid density decreases with increasing doping.…”

“…[4] One prominent issue arises as to whether there is normal fluid in the superconducting state due to the missing charge carriers if fewer and fewer carriers are condensed into the superconducting state in the overdoped region. [5] In Bi 2 Sr 2 CuO 6+δ (Bi2201) superconductors, normal electrons are observed in the superconducting state by scanning tunneling microscopy (STM) both in the real space [6] and in the reciprocal space [7] which are strongly affected by disorder. Whether it is universal that the normal fluid always exists in the superconducting state in the overdoped samples remains to be investigated.…”