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SUMMARY:Scanned Josephson Tunneling Microscopy: Visualizing Electron-Pair 
 Fluids and Density Waves - Prof J.C. Séamus Davis\, University of Oxford\
 , University College Cork\, Cornell University
DTSTART:20230517T150000Z
DTEND:20230517T160000Z
UID:TALK200722@talks.cam.ac.uk
CONTACT:Dr Nur Unal
DESCRIPTION:Superconductors are quantum fluids and pair density waves are 
 quantum crystals\, both being macroscopic quantum states of coherently con
 densed electron pairs. To visualize and explore these states directly at a
 tomic scale\, we have developed scanned Josephson tunneling microscopes (S
 JTM). Such instruments can image both the single-electron quasiparticles a
 nd\, in a different mode\, the quantum condensate of electron-pairs. Moreo
 ver\, these instruments can\, in the Andreev quasiparticle retroreflection
  regime\, be operated as scanned Andreev tunneling microscopes (SATM). To 
 visualize pair density wave (PDW) states in the conventional spin-singlet 
 superconductor NbSe2\, we implemented atomic-resolution SJTM using Nb s-wa
 ve scan tips. Here we detect three commensurate PDWs\, each whose electron
 -pair density and energy-gap modulate spatially at the wavevectors 𝑄
 𝑖=1\,2\,3 of the preexisting charge density wave (CDW) state\, but with
  a global 𝛿Φ≅±2𝜋/3 phase difference between the PDW and CDW stat
 es [1].\n\nWe searched for a pair density wave in cuprate superconductors 
 using cuprate d-wave scan tips\, and discovered a PDW state [2] exhibiting
  periodic modulations of the electron-pair density [2]\, the quasiparticle
  response to the electron-pair crystal [3]\, and of the associated electro
 n-pair binding energy [3\,4]. Most recently\, we studied the putative spin
 -triplet topological superconductor UTe2 using Nb s-wave scan tips\, where
  no Josephson pair current is detectable. However\, we can visualize the p
 airing energy-gap and find three PDWs at incommensurate wavevectors 𝑃
 𝑖=1\,2\,3 indistinguishable from the wavevectors 𝑄𝑖=1\,2\,3 of th
 e prevenient CDW. Every 𝑃𝑖:𝑄𝑖 pair is registered to each other
  spatially\, but with a relative phase 𝛿𝜙≈𝜋. From these observa
 tions\, this state is the first known spin-triplet PDW [5]. SJTM studies r
 eveal an unexplored frontier of macroscopic quantum physics with great pot
 ential for discovery.\n\nReferences:\n[1] Science 372\, 1447 (2021)\; \n[2
 ] Nature 532\, 343 (2016)\; \n[3] Science 364\, 976 (2019)\; \n[4] Nature 
 580\, 6570 (2020)\; \n[5] Nature (2023) arXiv:2209.10859.
LOCATION:Small Lecture Theatre\, Cavendish Laboratory\, J.J. Thomson Avenu
 e
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