Lecture

Joan van der Waals colloquium

• Séamus Davis (Oxford Universities of Oxford and Cork)

Date

Monday 9 December 2019

Time

16:15 - 17:30

Explanation

Drinks afterwards.

Location

Huygens
Niels Bohrweg 2
2333 CA Leiden

Room

De Sitterzaal

Discovery and Exploration of the Cuprate Pair Density Wave State

Cooper-pairs, if they have finite center-of-mass momentum Q_P, can form a remarkable state in which the density of pairs modulates periodically in space at wavevector Q_P. 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 [1].

            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 Q_P and 2Q_P, 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[2].

            Finally, using a new approach at zero field for Bi₂Sr₂CaCu₂O_8+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 Q_P and 2Q_P, that are anticipated when the PDW coexists with superconductivity. [3]

            Overall, these results provide compelling evidence, from multiple signatures, of a PDW state coexisting with superconductivity in a canonical cuprate.

References

[1] Nature 532, 343 (2016)

[2] Science 364, 976 (2019)

[3] Z. Du et al, Nature (2020)