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CATEGORIES:Quantum Matter Seminar
SUMMARY:Upper bounds on superfluid stiffness and supercond
ucting critical temperature - applications to flat
bands\, FeSe/STO\, cold atoms and connections to
quantum geometry - Dr. Tamaghna Hazra\, Karlsruhe
Institute of Technology
DTSTART;TZID=Europe/London:20240207T111500
DTEND;TZID=Europe/London:20240207T123000
UID:TALK209908AThttp://talks.cam.ac.uk
URL:http://talks.cam.ac.uk/talk/index/209908
DESCRIPTION:Understanding the material parameters that control
the superconducting (SC) transition temperature T
c is a problem of fundamental importance. In many
novel superconductors of interest\, fluctuations o
f the phase of the SC order parameter determines T
c\, rather than the BCS collapse of the amplitude
due to pair breaking. We derive rigorous upper bou
nds on the superfluid stiffness Ds in terms of the
optical conductivity sum-rule\, valid in any dime
nsion\, essentially controlled by the non-interact
ing band structure. This in turn leads to rigorous
upper bounds on Tc in 2D\, which holds irrespecti
ve of the form or strength of the pairing interact
ions\, mechanism or order-parameter symmetry. We e
stimate the upper bounds on Tc for FeSe on SrTiO3\
, cold atom systems and lattice Hubbard models and
find that they are quite close to the maximum obs
erved Tc in these strongly-correlated systems. For
magic-angle twisted bilayer graphene\, we not onl
y calculated these bounds for lattice models of th
e flat bands\, but we also present rigorous upper
bounds on the low energy optical spectral weight t
hat determines the maximum Tc in isolated flat ban
ds in a multi-band system. The latter bounds are d
irectly controlled by the Marzari-Vanderbilt sprea
d functional of the flat band Wannier functions -
pointing to a deep connection between the quantum
geometry of the flat band eigenstates and the low
energy optical response.\n\n[1] Hazra\, T.\, Verma
\, N. & Randeria\, M. Bounds on the Superconductin
g Transition Temperature: Applications to Twisted
Bilayer Graphene and Cold Atoms. Phys. Rev. X 9\,
031049 (2019).\n\n[2] Verma\, N.\, Hazra\, T. & Ra
nderia\, M. Optical spectral weight\, phase stiffn
ess\, and Tc bounds for trivial and topological fl
at band superconductors. PNAS 118\, (2021).\n
LOCATION:Mott Seminar Room (531)\, Cavendish Laboratory\, D
epartment of Physics
CONTACT:Malte Grosche
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