Anisotropic quasiparticle coherence in nematic ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ studied with strain-dependent ARPES

The hallmark of nematic order in iron-based superconductors is a resistivity anisotropy but it is unclear to which extent quasiparticle dispersions, lifetimes, and coherence contribute. While the lifted degeneracy of the Fe $d_{xz}$ and $d_{yz}$ dispersions has been studied extensively, only little is known about the two other factors. Here, we combine in situ strain tuning with ARPES and study the nematic response of the spectral weight in ${\mathrm{BaFe}}_2{\mathrm{As}}_2$. The symmetry analysis of the ARPES spectra demonstrates that the $d_{xz}$ band gains quasiparticle spectral weight compared to the $d_{yz}$ band for negative antisymmetric strain $\mathrm{\Delta }{\epsilon }_{yy}$ suggesting the same response inside the nematic phase. Our results are compatible with a different coherence of the $d_{xz}$ and $d_{yz}$ orbital within a Hund's metal picture. We also discuss the influence of orbital mixing.

Figure 1

Changes in the spectral function due to nematicity. (a) Sketch of band structure and BZ (solid: 2Fe BZ, dashed 1Fe BZ) in the normal state. (b) Band structure and BZ in the nematic state. Sketch of the sample indicates the corresponding deformation due to uniaxial strain ${\epsilon }_{yy}$ or nematicity, respectively. Dashed lines in band structure indicate dispersion in normal state for comparison. (c) Anisotropy of the coherent spectral weight $\mathrm{\Delta }{S}_{\mathrm{nem}}$ between $d_{xz}$ and $d_{yz}$ orbitals inside the nematic state. (d) Sketch of the uniaxial strain device.

Figure 2

(a),(b) Tight-binding calculations of the orbital content of the inner and middle hole bands along $k_x$. (c),(d) Matrix elements of the Fe $3d$ orbitals calculated along $k_x$ for the experimental geometry and a photon energy of 47 eV. The role of $d_{xz}$ and $d_{yz}$ flip along $k_y$.

Figure 3

ARPES of strained ${\mathrm{BaFe}}_2{\mathrm{As}}_2$. (a) Spectra taken along $k_x$ under compression (${\epsilon }_{yy}<0$) and tension (${\epsilon }_{yy}>0$) in p-pol together with selected EDCs (a3)–(a7). This configuration probes $d_{yz}$ character as indicated in (a1),(a2). Blue (red) shading between the EDCs indicates a larger (smaller) intensity under tension $\mathrm{\Delta }{\epsilon }_{yy}>0$. The shaded area defines the change in spectral weight $\mathrm{\Delta }S$ according to Eq. (1). (b) Same as (a) for the orthogonal momentum direction $k_y$ probing $d_{xz}$ character. (c),(d) Same as (a),(b) for s-pol light. Dashed lines in (c3) and (d3) represent fits (black) with their peak (red, blue) and background (gray) contributions. The insets and the sketches on the right bottom illustrate the strain and momentum directions with respect to the BZ.

Figure 4

Strain-induced changes in the spectral weight. (a) Difference of the spectral weight $\mathrm{\Delta }S$ for the spectra taken with p-pol light shown in Figs. 3 and 3. (b) same as (a) for s-pol light. (c) Antisymmetric ($B_{2g}$) response of the spectral weight. We indicate the magnitude at $\mathrm{\Gamma }$ obtained from EDC fits instead of simple spectral weight integration.