Regensburg 2016 – wissenschaftliches Programm
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TT: Fachverband Tiefe Temperaturen
TT 35: Correlated Electrons: Quantum-Critical Phenomena - Theory
TT 35.4: Vortrag
Dienstag, 8. März 2016, 14:45–15:00, H21
Excitonic instability of three-dimensional gapless semiconductors with quadratic Fermi node — •Lukas Janssen1,2 and Igor F. Herbut2 — 1Institut für Theoretische Physik, Technische Universität Dresden, Germany — 2Department of Physics, Simon Fraser University, Burnaby, Canada
Three-dimensional gapless semiconductors with quadratic band touching, such as HgTe, α-Sn, or the pyrochlore iridate Pr2Ir2O7, are believed to display a non-Fermi-liquid ground state due to long-range electron-electron interaction. I will argue that this state is inherently unstable towards spontaneous formation of a (topological) excitonic insulator. The instability can be parameterized by a critical fermion number Nc, in formal analogy to the problem of mass generation in (2+1)-dimensional quantum electrodynamics (QED2+1). For N < Nc the rotational symmetry is spontaneously broken, the system develops a gap in the spectrum, and features a finite nematic order parameter. To the leading order in the 1/N expansion and in the static approximation, the analogy with QED2+1 yields Nc = 16/[3π(π−2)]. Taking the important dynamical screening effects into account, we find that Nc ≥ 2.6(2), and therefore safely above the physical value of N = 1. I will also discuss results of recent complementary approaches to the problem using 2+є expansion and functional renormalization group, respectively, which turn out to arrive at a similar conclusion. Some experimental consequences of the nematic ground state will be pointed out.
L. Janssen and I. F. Herbut, arXiv:1509.01737 [cond-mat.str-el]