<This video is meant for academic purpose only.>

►【International Young Scholar Forum】

Title：Visualizing the Cuprate Pair Density Wave State

Time: 10:00am, 6 July 2020

Speaker: Dr. Zengyi Du, Brookhaven National Lab.

Abstract: 

In this talk, I will present the demonstration the Pair Density Wave (PDW) in Bi2212 with single particle tunnelling[1]. In cuprate, the exact nature of the pseudogap state has been the focus of extensive research as a route to understanding high temperature superconductivity. Wide attention has recently focused on a PDW state [2-4] as the leading candidate to be the fundamental order parameter that characterizes the pseudogap. Although the signature of a cuprate PDW has been detected in the periodic magnitude of Cooper-pair tunneling [5] the equivalent signature in the single electron tunneling of a periodic Δ(r) modulation has never been observed. By utilizing the superconducting tip to enhance the energy-sensitivity for a spectroscopic imaging scanning tunneling microscopy (SI-STM), we have now discovered numerous signatures for the PDW. First, we have observed the spatially modulating energy-gap Δ(r) with the eight unit-cell periodicity in superconducting Bi2212. Second, the spatial analysis of Δ(r) modulations shows that they are highly unidirectional, and there is some evidence consistent with a vestigial nematic PDW state. Third, simultaneous density-of-states imaging reveals two pairs of co-existing N(r, E) modulations, at wavevectors Q and 2Q. Finally, when we visualize the topological defects in the induced N(r, E) density wave at 2Q, we have found that they were concentrated along the lines where the PDW spatial phase changes by π. All of these phenomena are canonical signatures of the PDW coexisting with the superconductivity. Therefore, our high energy sensitivity SIS imaging of Δ(r) modulations and all the consequent phenomena provide the first, strongest and most direct spectroscopic evidence of the existence of a PDW in cuprate.

[1] Zengyi et al, Nature 580, 6570 (2020).

[2] Li et al., Phys. Rev. Lett. 99, 067001 (2007).

[3] Berg et al., New J. Phys. 11, 115004 (2007).

[4] Agterberg et al., Annual Review 11:231-270 (2020).

[5] Hamidian et al., Nature 532, 343-347 (2016).

Brief CV of Dr. Zengyi Du: 

Dr. Zengyi Du got his Ph.D in 2017 from Nanjing University under the supervision of Prof. Hai-Hu Wen. His major interest was STM/S study on Iron based superconductors. Currently, he is doing postdoctoral research at Brookhaven National Laboratory with Prof. J C Seamus Davis and Dr. Kazuhiro Fujita. Working on the STM module of ‘OASIS’ platform at BNL, his lab duty includes design and maintain UHV system. And, his interest extends to the SI-STM study on the complex phase diagram of cuprate and also basic theoretical simulation on band structure or tunnelling spectroscopy.