日時 2018年7月18日(水) 午後4時
講師 Prof. Yasutomo J. Uemura
所属 Columbia University
題目 Onset of the photo-excited transient superconductivity and Nernst effect at the emergence of local phase coherence of preformed pairs
In recent series of optical conductivity measurements after ultrafast laser photo-excitations in high-Tc cuprates and K3C60 systems, Cavalleri and co-workers found a signature similar to superconducting gapping in transient responses well above Tc, reaching up to 300–400K[1,2]. In 2000–2010, Ong and co-workers observed Nernst effect  and diamagnetic susceptibility  which develop in the underdoped region of cuprates well above Tc. In this presentation, resorting to a plot of Tc versus the superfluid density , and Tc versus the effective Fermi temperature TF [5,6], we point out that the onset of the transient gapping of optical conductivity, Nernst effect and diamagnetic susceptibility occurs at the emergence of local phase coherence of preformed pairs in the normal state well above Tc.
Since 1988, we performed Muon Spin Relaxation measurements of the superfluid density ns/m* in high Tc cuprate and many other unconventional superconductors , and presented a nearly linear relation between Tc and ns/m* at T → 0, and Tc vs TF obtained from ns/m*. The superfluid density corresponds to the gapped Drude spectral weight in optical conductivity.
From ns/m* one can derive a “hypothetical BEC energy scale” TBEC by calculating Bose-Einstein condensation temperature of ideal non-interacting Bose gas having boson density of ns/2 and mass 2m*. TBEC represents the temperature at which the thermal wave length of bosons becomes comparable to the interboson distance. For the situation with dominant pre-formed pairs in the normal state of underdoped cuprates, superconductivity should have occurred at TBEC if there were no competing states.
The actual Tc in cuprates and other unconventional superconductors is reduced from TBEC at least by a factor of 4-5 due to competition of superconducting (SC) and antiferromagnetic (AF) order. In strongly underdoped region of cuprates, however, we demonstrate that TBEC derived from MuSR agrees well with the onset temperature of Nernst effect and the diamagnetic susceptibility, and TBEC derived from the transient gapped spectral weight of optical responses agrees well with the onset temperature of the gapped optical responses.
This observation can be extended to the cases in K3C60, organic BEDT superconductors, as well as URu2Si2. These results suggest that the “superconductivity-like” responses in Nernst, diamagnetic and transient optical conductivity measurements emerge when local, dynamic and short-ranged phase coherence develop among pre-formed pairs. We also discuss the role of the magnetic resonance mode which controls Tc in the competition of the SC and AF orders, resorting to an analogy with rotons in superfluid 4He  and noticing a transient loss of the 400 cm-1 mode responses
in photo-excited optical conductivity .
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紹介教員 山地洋平 准教授、今田正俊 教授