"Interacting spin-1/2 and spin-3/2 nodal fermions: Quantum criticality and transport"

will be given by Dr. Vladimir Juričić (Nordita, KTH Royal Institute of Technology and Stockholm University, Sweden).

Abstract of the talk:

Theoretical proposals and experimental realizations of the condensed matter systems, such as topological insulators, Weyl and Dirac semimetals, displaying emergent pseudorelativistic nodal quasiparticles, have triggered a surge of activity in understanding their stability to interactions and disorder. This also motivated exploration of their universal responses at low energies, such as optical conductivity.

In this talk we will first discuss the quantum critical theory of an interacting nodal Fermi liquid of quasirelativistic pseudospin-3/2 fermions that have a noninteracting birefringent spectrum with two distinct Fermi velocities [1]. As we will show using perturbative field-theoretical renormalization group analysis, when such quasiparticles interact via either the long-range Coulomb interaction or its short range component (responsible for spontaneous symmetry breaking), in the quantum critical regime in two dimensions, the system is, respectively, described by a marginal Fermi liquid (featuring interaction-driven log-corrections to observables) or a non-Fermi liquid of relativistic spin-1/2 fermions and is always a marginal Fermi liquid in three dimensions. We will also discuss our conjecture that critical spin-1/2 excitations represent a superuniversal description of the entire family of interacting quasirelativistic fermions.

In the second part of the talk, we will discuss the quantum transport close to a relativistic quantum critical point, separating two-dimensional massless Dirac fermions from a fully gapped insulator or superconductor. In particular, we will consider the scaling of optical conductivity in the (high-frequency) collisionless regime [2]. Close to such a critical point, gapless fermionic and bosonic excitations are strongly coupled, which inside the critical regime leads to a universal suppression of the interband optical conductivity as well as of the Drude peak. These results are obtained by performing the leading order expansions in the inverse of the fermion flavor number and in the distance from the upper-critical three spatial dimensions in the problem.

[1] B. Roy, M. P. Kennett, K. Yang, and V. Juričić, Phys. Rev. Lett. 121, 157602 (2018).
[2] B. Roy and V. Juričić, Phys. Rev. Lett. 121, 137601 (2018).

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