TITLE : Obstructed Cooper Pairs in Flat Band Systems - Weakly-Coherent Superfluids and Exact Spin Liquids
ABSTRACT :
Understanding superconductivity in strongly correlated quantum materials remains one of the central challenges in modern condensed-matter physics. Systems with partially filled flat bands and consequently no Fermi surface present a particularly vexing conceptual hurdle because we cannot draw intuition from the weak-coupling Bardeen-Cooper-Schrieffer picture of a Fermi surface instability. In this talk, I will approach the strongly correlated problem of flat band superconductivity from the strong coupling limit of local attractive interactions. In this limit, the superconducting critical temperature is driven by the loss of phase coherence at an energy scale set by the kinetic energy of Cooper pairs, which is expected to scale inversely with the pair binding interaction. I will present a striking counter-example of this conventional intuition. When electrons hop on the line-graph of a lattice with strong pairing interaction, they bind into obstructed Cooper pairs whose motion is frustrated by destructive interference. As a result, the pair kinetic energy vanishes identically at the leading-order of the strong-coupling expansion, producing a flat bosonic band of compact localized pair states, zero superfluid stiffness at leading order, and an extensively degenerate many-body ground state manifold. At quarter filling, the frustrated pair dynamics maps onto a quantum dimer model at the exactly-solvable Rokhsar-Kivelson point, realizing a d-wave resonating-valence-bond spin liquid with topological ground-state degeneracy and deconfined holon excitations. This demonstrates a bridge unifying the fields of strongly correlated superconductivity and frustrated magnetism. One of the grand challenges in our understanding of quantum physics is to identify the low-energy emergent degrees of freedom that succinctly capture the physics of strongly correlated quantum materials. In this talk, we present obstructed bosons and their deconfined partons as two such exotic candidate degrees of freedom, which have no weak-coupling analogues but are grounded in exact results in the limit of infinitely strong interactions.
ABOUT THE SPEAKER :
Dr. Tamaghna Hazra is currently a Humboldt Fellow at the Karlsruhe Institute of Technology, working in the group of Joerg Schmalian since 2023. Previously, he worked on heavy fermion superconductivity and Kondo physics as a postdoc in the group of Piers Coleman at Rutgers University, after completing his PhD in 2020 with Mohit Randeria at the Ohio State University, working at the interface of superconductivity, topology and strongly correlated electron systems. His doctoral research on exact upper bounds on superconducting critical temperature was cited in his PhD advisor's Bardeen Prize and he was invited to present his postdoctoral research on a triplet pairing mechanism driven by Hund's and Kondo interactions in heavy fermion superconductors in the flagship symposium of the Division of Condensed Matter Physics at the American Physical Society March Meeting 2023.
MEETING DETAILS :
Joining Link: https://meet.google.com/toe-dxzc-afd
Speaker: Dr. Tamaghna Hazra, Humboldt Fellow, Karlsruhe Institute of Technology, Germany
Venue: A1 NKN Hall, South Campus, IIT Mandi
Date and Time: April 2, 2026, Thursday at 04:00 PM