IISER Pune
INDIAN INSTITUTE OF SCIENCE EDUCATION AND RESEARCH (IISER) PUNE
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An Autonomous Institution, Ministry of Human Resource Development, Govt. of India
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Seminars and Colloquia

Physics

Upper bounds on superfluid stiffness and superconducting Tc: Applications to twisted bilayer graphene, monolayer FeSe on SrTiO3 and cold atoms 
 
Tue, Dec 03, 2019,   04:00 PM at Seminar Hall 31, 2nd Floor, Main Building

Dr. Tamaghna Hazra
Ohio State University

ABSTRACT :

We discuss fundamental limitations on the superconducting critical temperature $T_c$.  In many novel superconductors phase fluctuations determine $T_c$ ,
rather than the collapse of the pairing amplitude. We derive rigorous upper bounds on the superfluid phase stiffness for multiband systems, valid in any dimension. This in turn leads to an upper bound on $T_c$ in two dimensions, which holds irrespective of pairing mechanism, interaction strength, or order-parameter symmetry. Our bound is particularly useful for the strongly correlated regime of low-density and narrow-band systems, where mean-field theory fails. For a simple parabolic band in 2D with Fermi energy $E_F$ , we
find that $k_B T_c ≤ E_F/8$, an exact result that has direct implications for the 2D BCS-BEC crossover in ultracold Fermi gases. Applying ourmultiband bound to magic-angle twisted bilayer graphene, we find that band structure results constrain the maximum $T_c$ to be close to the experimentally observed value. Finally, we discuss the question of deriving rigorous upper bounds on $T_c$ in 3D.
 
 
 
 

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