Joan van der Waals colloquium
- Monday 9 December 2019
- Drinks afterwards.
Niels Bohrweg 2
2333 CA Leiden
- De Sitterzaal
Discovery and Exploration of the Cuprate Pair Density Wave State
Cooper-pairs, if they have finite center-of-mass momentum QP, can form a remarkable state in which the density of pairs modulates periodically in space at wavevector QP. Intense theoretical interest has recently emerged in whether such a ‘pair density wave’ (PDW) state could be a principal state in the phase diagram of underdoped cuprate superconductors. The most common model is a modulation of the electron-pair condensate with unit-cell (8a0) periodicity or wavevector QP=2πa01/8,0.
To search for a cuprate PDW at zero field, we developed a nanometer-resolution scanned Josephson tunneling microscopy (SJTM) to image Cooper-pair tunneling from a d-wave superconducting STM tip at millikelvin temperatures to the Cooper-pair condensate of underdoped Bi2Sr2CaCu2O8. The resulting images of the Cooper-pair condensate show clear modulations in magnitude of pair-density, oriented along the Cu-O bond directions with a 4a0 period .
To search for evidence of such a PDW state at high fields, we visualize the modulations in the density of electronic states N(r) within the halo surrounding vortex cores. This revealed multiple signatures of a field-induced PDW, including two sets of N(r) modulations occurring at wavevectors QP and 2QP, the amplitude of the latter decaying twice as rapidly as the former. This is in detailed agreement with theory for a field-induced primary PDW QP=2πa01/8,0; 2πa00,1/8.
Finally, using a new approach at zero field for Bi2Sr2CaCu2O8+δ, we have discovered the strong energy-gap modulations Δ(r) expected of a PDW with wavevectors QP=2πa01/8,0; 2πa00,1/8. Simultaneous imaging of the local-density-of-states N(r,E) reveals the expected modulations at wavevectors QP and 2QP, that are anticipated when the PDW coexists with superconductivity. 
Overall, these results provide compelling evidence, from multiple signatures, of a PDW state coexisting with superconductivity in a canonical cuprate.
 Nature 532, 343 (2016)
 Science 364, 976 (2019)
 Z. Du et al, Nature (2020)