Superconductivity from orbital nematic fluctuations

Recent experiments suggest that, besides antiferromagnetic fluctuations, nematic fluctuations may contribute to the occurrence of superconductivity in iron pnictides. Motivated by this observation, we study superconductivity from nematic fluctuations in a minimal two-band model. The employed band parameters are appropriate for iron pnictides and lead to four pockets for the Fermi line. It is shown that low-energy, long-wavelength nematic fluctuations within the pockets give rise to strong-coupling superconductivity whereas the large momenta density fluctuations between pockets are rather irrelevant. The obtained transition temperatures are similar to those typically found in the pnictides and are rather robust against repulsive Coulomb interactions. The superconducting and nematic states coexist in a large region of the phase diagram.

Figure 1

Phase diagram in the $T$-$g$ plane for $g_0=0$. The thick solid line separates the normal (N) from the orbital nematic (ON) phase, which at low temperatures is modulated, as indicated by the crosses. The dashed line is the instability line of the modulated to the homogeneous nematic state. The circles, smoothly joined by a thin solid line, separate the superconducting (SC) from the normal and nematic states. The inset shows the Fermi lines of the four pockets in the normal state.

Figure 2

Retarded pairing interactions ${\tilde{g}}_{44}$ (main diagrams) and ${\tilde{g}}_{14}$ (insets) as a function of ${\nu }_n$ for different temperatures for (a) $g=-1.7$ and (b) $g=-1.8$. In (b) the nematic state occurs below $T=T_n\approx 0.125$.

Figure 3

Temperature dependence of the largest eigenvalue using only diagonal (dotted line), nondiagonal (short- and long-dashed lines), or all pairing interactions (solid line).

Figure 4

Dependence of $T_c$ on the repulsive, instantaneous interaction $g_0$.