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.
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CP 179
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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.  

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CP 179
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Condensed Matter Seminar: Ahmed Abouelkomsan


Title : New aspects of fractionalization in Chern bands
 
Abstract: The discovery of the fractional quantum anomalous Hall effect in twisted MoTe2 has opened a new chapter in condensed matter physics. Much of the current understanding of topological phases in Chern bands such as fractional Chern insulators (FCIs) relies on the analogy to standard quantum Hall physics in Landau levels. A natural question then arises if there are any unique aspects in the Chern band setting that are fundamentally different and cannot be captured by a Landau level analogy and if there are novel phases that don’t have a Landau level analogue. I will highlight similarities/differences between fractionalized phases in Chern bands and Landau levels including the existence of strong particle-hole asymmetry of geometric origin that are reflected on the phase diagram. In addition, I will show that intra-Chern band excitations such as magneto-roton excitations can in principle be probed with optical absorption unlike the Landau level case where this is not allowed due to Kohn’s theorem. Moreover, I will provide examples where the intrinsic topology of Chern bands without the need for external magnetic fields opens a route towards realization of exotic non-Abelian spin Hall phases. I will conclude by showing that the gapless composite Fermi liquid phase in a Chern band is distinct from normal Fermi liquid through the vanishing of the Drude weight and possibly belongs to a larger class of "Drudeless" non Fermi liquids which are compressible yet have no Drude peak.


 

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CP-179
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Condensed Matter Seminar: Johannes Pollanen

Title: Hybrid quantum phononics with superconducting qubits*

Abstract: Superconducting qubits, and the experimental architecture of circuit quantum
electrodynamics (cQED), have emerged as not only a promising platform for quantum
computation but also for investigating fundamental and applied aspects of
synthetic/hybrid quantum systems composed of qubits coupled to other quantum systems
or degrees of freedom. In particular, the ability to leverage the properties of
superconducting qubits to investigate and manipulate phononic degrees of freedom opens
the door to exploring new regimes of circuit quantum optics using high-frequency sound.
Due to the intrinsically strong nonlinearity provided by the qubit, these types of hybrid
“quantum acoustic” systems have the potential to access a broad class of quantum states
of motion beyond what is achievable with effectively linear optomechanical or
electromechanical interactions.
In this talk I will describe some of our recent experimental results investigating the
fundamental physics of hybrid systems based on superconducting qubits coupled to
piezoelectric surface and bulk acoustic wave devices and how these systems can be used
to develop next-generation technologies for quantum sensing, computation, and
communication. As I will describe, these engineered systems, in which quantum
information stored in the qubit can be controllably coupled to the microscopic surface
and bulk phonon modes of a piezoelectric crystal, are an ideal platform for investigating
the exotic behavior of synthetic open quantum systems and phononic interference in the
quantum regime. Additionally, I will describe how these devices pave the way to exciting
new technologies ranging from quantum-limited surface sensing to phonon-based
bosonic quantum memories.


*This work was supported by the National Science Foundation via Grant No. ECCS-2142846 (CAREER)

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CP - 179
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A Tale of Two Bilayers

Speaker: Herbert Fertig. Indiana University

Title: A Tale of Two Bilayers

Abstract: Modern materials physics has made available true two-dimensional electron systems, in the form of atomic networks bonded only across a single plane.  These van der Waals systems may be formed from a variety of materials, with different electronic properties, which may be combined into bilayer heterostructures with properties not found in either layer individually.  In this talk we will describe quantum coherent states of two such systems, in which nesting plays an important role in determining the ground state phase diagram.  The first of these is a phosphorene – graphene bilayer, for which one finds Fermi surfaces with strong nesting overlaps, leading to spin-density wave ground states for sufficiently strong interactions.  The second involves an idealized bilayer in which each layer supports a particle-hole symmetric band structure, possibly with non-trivial topology.  For half-filling, we find that nesting of the Fermi surfaces on opposite layers leads to different exciton condensate states, separated by a first order transition line which ends in an unusual zero temperature critical endpoint.  We demonstrate that this endpoint is a signature of Lifshitz transitions hosted by the individual layers in the absence of interactions.   In this way their Fermi surface topologies leave an imprint in the interacting phase diagram, in regions where the states themselves are fully gapped and lack Fermi surfaces.

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CP-179
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