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SUMMARY:Upper bounds on superfluid stiffness and superconducting critical 
 temperature - applications to flat bands\, FeSe/STO\, cold atoms and conne
 ctions to quantum geometry - Dr. Tamaghna Hazra\, Karlsruhe Institute of T
 echnology
DTSTART:20240207T111500Z
DTEND:20240207T123000Z
UID:TALK209908@talks.cam.ac.uk
CONTACT:Malte Grosche
DESCRIPTION:Understanding the material parameters that control the superco
 nducting (SC) transition temperature Tc is a problem of fundamental import
 ance. In many novel superconductors of interest\, fluctuations of the phas
 e of the SC order parameter determines Tc\, rather than the BCS collapse o
 f the amplitude due to pair breaking. We derive rigorous upper bounds on t
 he superfluid stiffness Ds in terms of the optical conductivity sum-rule\,
  valid in any dimension\, essentially controlled by the non-interacting ba
 nd structure. This in turn leads to rigorous upper bounds on Tc in 2D\, wh
 ich holds irrespective of the form or strength of the pairing interactions
 \, mechanism or order-parameter symmetry. We estimate the upper bounds on 
 Tc for FeSe on SrTiO3\, cold atom systems and lattice Hubbard models and f
 ind that they are quite close to the maximum observed Tc in these strongly
 -correlated systems. For magic-angle twisted bilayer graphene\, we not onl
 y calculated these bounds for lattice models of the flat bands\, but we al
 so present rigorous upper bounds on the low energy optical spectral weight
  that determines the maximum Tc in isolated flat bands in a multi-band sys
 tem. The latter bounds are directly controlled by the Marzari-Vanderbilt s
 pread functional of the flat band Wannier functions - pointing to a deep c
 onnection between the quantum geometry of the flat band eigenstates and th
 e low energy optical response.\n\n[1] Hazra\, T.\, Verma\, N. & Randeria\,
  M. Bounds on the Superconducting Transition Temperature: Applications to 
 Twisted Bilayer Graphene and Cold Atoms. Phys. Rev. X 9\, 031049 (2019).\n
 \n[2] Verma\, N.\, Hazra\, T. & Randeria\, M. Optical spectral weight\, ph
 ase stiffness\, and Tc bounds for trivial and topological flat band superc
 onductors. PNAS 118\, (2021).\n
LOCATION:Mott Seminar Room (531)\, Cavendish Laboratory\, Department of Ph
 ysics
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