More information here: https://tarheels.live/rpmbt21/schedule/
Zoom link: https://unc.zoom.us/j/94760934546, p/w: 314159
YouTube live link: https://www.youtube.com/channel/UCOUN5aVy-vUWgwm4CxloGTw

September 28th, 2021: Universal behavior and strongly coupled theories
“Fermi surfaces large and small: unifying theories of the Kondo lattice and Hubbard models”
S. Sachdev – 10am

Fermi surfaces which obey the Luttinger theorem are often referred to as “large”. However, it is possible to have “small” Fermi surfaces with electron-like quasiparticles, in certain metallic states (sometimes called FL*) which evade the Luttinger theorem using emergent gauge fields. It is relatively easy to construct FL* states in Kondo lattice models, but much harder in a single band Hubbard model. I will describe a new approach which yields a variational wavefunction for FL* in the Hubbard model, and also a theory for the transition between small and large Fermi surfaces. The key idea is to avoid fractionalizing the electron, and to instead fractionalize a paramagnon into a pair of ancilla qubits. I will note applications to the phase diagram of the cuprates.

“Boundary criticality of the O(N) model in d = 3 critically revisited”
M. Metlitski – 11am
Session chair: Basar

It is known that the classical O(N) model in dimension d > 3 at its bulk critical point admits three boundary universality classes: the ordinary, the extra-ordinary and the special. The extraordinary fixed point corresponds to the bulk transition occurring in the presence of an ordered boundary, while the special fixed point corresponds to a boundary phase transition between the ordinary and the extra-ordinary classes. While the ordinary fixed point survives in d = 3, it is less clear what happens to the extra-ordinary and special fixed points when d = 3 and N is greater or equal to 2. I’ll show that formally treating N as a continuous parameter, there exists a critical value Nc > 2 separating two distinct regimes. For N < Nc the extra-ordinary fixed point survives in d = 3, albeit in a modified form: the long-range boundary order is lost, instead, the order parameter correlation function decays as a power of log r. For N > Nc there is no fixed point with order parameter correlations decaying slower than power law. I’ll discuss how these findings compare to recent Monte-Carlo studies of classical and quantum spin models with SO(3) symmetry. Based on arXiv:2009.05119.