Condensed Matter Seminar

Physics & Astronomy Condensed Matter Seminar

Title: Kinetic magnetism and unconventional pairing: moiré system and electron-doped cuprates 
 
Abstract: The magnetic and superconducting properties of correlated electron systems are traditionally attributed to exchange-driven interactions stemming from Coulomb repulsion. However, recent advances suggest that kinetic mechanisms—arising purely from the motion of doped carriers in a strongly interacting background—can drive both magnetism and unconventional pairing. We explore this unifying paradigm in two distinct systems: moiré heterostructures and electron-doped cuprates. In the moiré system, spin polarons emerge below half-filling and metallic Nagaoka  ferromagnetism appears above, with a pseudogap metal phase predicted at low doping and intermediate fields—characterized by a single-particle gap and doping-dependent magnetization plateau. 
In electron-doped cuprates, using a combination of analytical techniques and large-scale simulations in the square-lattice Hubbard model, we uncover a robust kinetic pairing mechanism enabled by frustration in charge motion. This mechanism allows d-wave superconductivity to coexist with antiferromagnetic order, as holes effectively acquire opposite "charges" on different sublattices due to spin-singlet correlations. Together, these results highlight a common origin of magnetism and superconductivity rooted in kinetic processes, and provide a broadly applicable framework for strong coupling superconductivity in doped Mott insulator.
Date:
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Location:
CP 179
Event Series:

Physics & Astronomy Condensed Matter Seminar

Title: TBA

Abstract: I will discuss the problem of strange metals, where the traditional notion of Fermi liquid quasiparticles ceases to apply. I will view the problem through the lens of a model of electrons with Hubbard-U Coulomb repulsion and a disordered Yukawa coupling to a two-dimensional bosonic bath, which can be solved in an extended dynamical mean field theory scheme. The model exhibits a quantum critical point, at which the repulsive component of the electron interactions strongly enhances the effects of the quantum critical bosonic fluctuations on the electrons, leading to a breakdown of Fermi liquid physics and the formation of a strange metal with `Planckian' quasiparticle decay rates at low temperatures, although with no holographic dual. Furthermore, the eventual Mott transition that occurs as the repulsion is increased seemingly bounds the maximum decay rate in the strange metal. I will also discuss some applications and collaborations based on this work to the iron-based superconductors and moire materials. Time permitting, I will conclude with future directions to include nonlocal effects.  

Date:
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Location:
CP 179
Event Series:

Condensed Matter Seminar: Constraints on topological order in Mott insulators

The hunt for anyonic excitations in quantum magnets is frustrated by the absence of any order parameter that could be used to detect such phases. Consequently a very important ally is the Hastings-Oshikawa-Lieb-Schultz-Mattis theorem for 2D quantum magnets, which guarantees that a fully symmetric gapped Mott insulator must be topologically ordered, though is silent on which topological order is permitted. After introducing the HOLSM theorem,  I will explain a new line of argument that constrains which topological order is permitted in a symmetric gapped Mott insulator. For example, I'll show that a fully symmetric magnet with S = 1/2 per unit cell cannot be in the double-semion topological phase. An application of our result is to the Kagome lattice quantum antiferromagnet where recent numerical calculations of entanglement entropy indicate a ground state compatible with either toric code or double semion topological order. Our result rules out the latter possibility.
 
Date:
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Location:
CP179
Event Series: