Superconducting fluctuations probed by the Higgs mode in Bi2Sr2CaCu2O8+x thin films

Abstract

Superconducting (SC) fluctuations in cuprate superconductors have been extensively studied to gain a deep insight into preformed Cooper pairs above the SC transition temperature Tc. While the various measurements, such as the terahertz (THz) optical conductivity, Nernst effect, angle-resolved photoemission spectroscopy (ARPES), and scanning tunneling microscopy (STM) measurements, have provided the signature of the SC fluctuations, the onset temperature of the SC fluctuations depends on the measurement scheme. Here, we shed light on the Higgs mode to investigate the SC fluctuations, as it is the direct fingerprint of SC order parameter and can help elucidate the development of SC phase coherence. We perform THz pump-optical probe spectroscopy for underdoped and overdoped Bi2Sr2CaCu2O8+x (Bi2212) thin films. The oscillatory behavior in the pump-probe signal (THz Kerr signal) observed in the SC phase has been identified as the Higgs mode in single crystals in our previous work [K. Katsumi et al., Phys. Rev. Lett. 120, 117001 (2018)], but two onset temperatures are identified above Tc. Combined with the results of the single crystals in a wide range of doping, we find that the first onset T1ons is 10–30 K above Tc. T1ons coincides with that of the superfluid density Ns extracted from the THz optical conductivity. Hence, T1ons is interpreted as the onset of macroscopic SC phase stiffness. On the other hand, the second onset T2ons is identified at substantially higher than Tc, whose origin is discussed in terms of preformed Cooper pairs.

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