講師:Prof. Yasutomo J. Uemura (Columbia University)
日時:2019年12月26日(木) 16:00~17:30
場所:東京大学 柏キャンパス 基盤棟物質系講義室(2B6号室)

タイトル:Dynamic Superconductivity Responses Detected by Photo-excited Optical Conductivity and Vortex Nernst Effect  — Phenomenology based on transient and equilibrium superfluid density —

High-Tc cuprate, FeAs, A3C60, and organic BEDT systems exhibit various unconventional features different from typical responses of BCS superconductors.  These include:
(1) Linear relationship between Tc and the superfluid density ns/m* (found since 1989)
(2) Anomaly in transport, magnetic susceptibility and NMR at T* suggesting formation of bosonic spin-singlet pairs in the region well above Tc (early 1990’s),
(3) Vortex-like Nernst effect and diamagnetic susceptibility observed in the normal state well above Tc but well below T* in the underdoped cuprates (2000-2010), and
(4) More recently (2012-2019), Andrea Cavalleri and co-workers performed measurements of optical conductivity after photo excitation, and found superconducting-like responses in cuprates and A3C60 systems which set in at temperatures well-above Tc.

When we plot the onset temperatures of these phenomena against the effective Fermi energy derived from the equilibrium or transient superfluid density, we find that both the photo-excited superconducting response (4) and the vortex-like Nernst effect (3) appear below the “local phase coherence temperature” TLPC at which one expects the thermal wave length of pre-formed bosonic charges becomes comparable to inter-boson distance.  In general TLPC is significantly lower than T*, as TLPC is due to many-body boson density while T* simply represents / requires two-body attractive interaction between fermions.  In the highly underdoped region of cuprates, the actual transition temperature Tc is at least a factor 4-5 lower than TLPC.  This reduction can be attributed to the competition between superconducting and antiferromagnetic order.  The competing order generates inelastic magnetic resonance mode in most of the unconventional superconductors as the lowest-energy excitable mode which controls Tc, similarly to rotons in the superfluid He controls the lambda point Tc as the inelastic mode related to competing solid HCP order.   In Cavalleri’s photo-excitation measurements, one can find a signature that the magnetic resonance mode is temporarily suppressed after the photo-excitation. This provides an explanation for the very high transient Tc as resulting from transient removal of the competing order.  Hence we have a coherent explanation [1] for all the four non-BCS behaviors (1)-(4) of unconventional superconductors.  We also discuss differences of this picture from that of Emery and Kivelson based on Kosterlitz-Thouless physics in two-dimensional systems without competing order.

[1] Y.J. Uemura, “Dynamic Superconducting Responses in Photoexcited Optical Conductivity and Nernst Effect”, Physical Review Materials 3 (2019) 104801.