1211.2790 (Imam Makhfudz)

Theory of Quantum Phase Transition in Iron-based Superconductor with
Half-Dirac Nodal Electron Fermi Surface    [PDF]

Imam Makhfudz

The quantum phase transition in iron-based superconductors with 'half-Dirac' node at the electron Fermi surface is a T=0 structural phase transition described in terms of nematic order. An effective low energy theory that describes half-Dirac nodal Fermions and their coupling to Ising nematic order that describes the phase transition is analyzed using renormalization group(RG) study of the large-$N_f$ version of the theory. The inherent absence of Lorentz invariance of the theory leads to RG flow structure where the velocities $v_F$ and $v_\Delta$ at the paired half-Dirac nodes ($1\bar{1}$ and $2\bar{2}$) in general flow differently under RG, implying that the nodal electron gap is deformed and the $C_4$ symmetry is broken, explaining the structural (orthogonal to orthorhombic) phase transition at the quantum critical point(QCP). The theory has Gaussian fixed point $\lambda^*=0, (v_{\Delta}/(v_F k_F))^*=0$ with stable flow lines toward it. This suggests second order quantum phase transition between Ising nematic ordered and disordered phases, in the presence of half-Dirac fermions in the background superconducting state. Interpreting the fermion-Ising nematic boson interaction as a decay process of nematic Ising order parameter scalar field into half-Dirac nodal fermions, we find that the theory behaves as systems with dynamical critical exponent $z = 1$. The nematic critical fluctuations lead to broadening of quasiparticle spectral function except at the region near the half-Dirac nodes where 'Fermi arc-like' sharp peak of spectral weight appears. The nematic critical fluctuations do not modify much the anisotropy of such peak. We however found critical point $\lambda_c$, directly related to that of $\phi$ field transition, around which the Fermi arc of spectral function collapses.

View original: http://arxiv.org/abs/1211.2790