Interband pairing in two-band superconductors with spin-orbit and Zeeman couplings

Abstract

Interband pairing in multiband superconductors is often neglected because of its higher energetic cost compared with intraband pairing. We show that, in multiband systems, a Zeeman magnetic field can stabilize interband pairing through the near degeneracy of spin-split branches from different bands, even within a minimal on-site attractive interaction. Using hexagonal tight-binding models with locally broken inversion symmetry, we find a Zeeman-driven transition between a conventional intraband s-wave state and an interband-dominated superconducting Mixing state. The resulting quasiparticle spectrum is intrinsically gapless, leading to anomalous thermodynamic behavior, including a T-linear specific heat at low temperatures, reflecting a finite zero-energy density of states.

Figures (8)

• Figure 1: Figure showing (a) the honeycomb lattice and (b) the example of Fermi surfaces without the SOC and the external magnetic field.
• Figure 2: Figure showing lattice and Fermi surfaces. (a) Non-symmorphic two-layer hexagonal model similar to SrPtAs ( space group $D^{4}_{6 h}$ or ${\it P} 6_{3} / mmc$ ). (b) The example of Fermi surfaces without the SOC and the external magnetic field. There is a band degeneracy at $k_{z} = \pi / c$.
• Figure 3: The Fermi surfaces. (a) 2DFS1 with $h / t = 0.0$, $h / t = 0.11$. (b) 2DFS2 with $h / t = 0.0$, $h / t = 0.14$. (c) 3DFS1 with $h / t = 0.0$, $h / t = 0.115$. (d) 3DFS2 with $h / t = 0.0$, $h / t = 0.125$. The external magnetic field $h$ reduces the energy mismatch at the Fermi level between the down-spin branch of band $\xi_{\bm k 1}$ and the up-spin branch of band $\xi_{\bm k2}$.
• Figure 4: (a) Transition temperature $T_c$ as a function of the SOC strength $\alpha$ for the 2D model at $h=0$. The red (blue) curves correspond to the tight-binding parameters of 2DFS1 (2DFS2) listed in Table \ref{['ferpara']}, with all parameters fixed except for $\alpha$. (b) Corresponding results for the 3D models. The red (blue) curves represent the parameter sets 3DFS1 (3DFS2) in Table \ref{['ferpara']}, again varying only $\alpha$. In both dimensions, the transition temperature of the $s$-wave state is higher than that of the Mixing state.
• Figure 5: (a) Transition temperature $T_c$ as a function of the hopping parameter $t_c$ for the 2D model at $h=\alpha=0$. The red (blue) curves correspond to the parameter sets 2DFS1 (2DFS2) listed in Table \ref{['ferpara']}, with all parameters fixed to those values except that $\alpha$ is set to zero and $t_c$ is varied. (b) Corresponding results for the 3D models. The red (blue) curves represent the parameter sets 3DFS1 (3DFS2) in Table \ref{['ferpara']}, again with $\alpha=0$ and varying only $t_c$. For $\alpha=0$, the Mixing state reduces to purely interband pairing. In both dimensions, the transition temperature of the $s$-wave state is higher than that of the Mixing state.