Electronic-Structure-Driven Magnetic and Structure Transitions in Superconducting NaFeAs Single Crystals Measured by Angle-Resolved Photoemission Spectroscopy

The electronic structure of NaFeAs is studied with angle-resolved photoemission spectroscopy on high quality single crystals. Large portions of the band structure start to shift around the structural transition temperature and smoothly evolve as the temperature lowers through the spin density wave transition. Moreover, band folding due to magnetic order emerges slightly above the structural transition. Our observation provides direct evidence that the structural and magnetic transitions share the same origin and could both be driven by the electronic structure reconstruction in Fe-based superconductors instead of Fermi surface nesting. We did not observe any sign of a gap in the superconducting state, which is likely related to weakened superconductivity in the presence of the spin density wave.

Figure 1

Electronic structure of NaFeAs at 60 K. (a) Photoemission intensity map at the Fermi energy ($E_F$) integrated over [$E_F-5\mathrm{meV}$, $E_F+5\mathrm{meV}$]; the marks are measured Fermi crossings, while the curves are the fitted Fermi surfaces. (b) Photoemission intensity $I(k,\omega )$ along the $\Gamma \mathrm{\text{−}}M$ direction. (c) The second derivative [${\partial }_{\omega }^{2}I(k,\omega )$] for data in (b). The dashed lines are local minimum locus to indicate the band position. For simplicity, the bands are denoted ignoring the band crossings, and each band is represented by the same color hereafter when necessary. (d),(e) MDCs around $\Gamma$ and $M$, respectively; each MDC has been individually normalized by its integrated weight to highlight weak features. (f) Selected EDCs along the $\Gamma \mathrm{\text{−}}M$ direction. Data in (a)–(f) were taken with 19 eV circularly polarized light. (g) Photoemission intensity taken along the $\Gamma \mathrm{\text{−}}M$ direction with linear polarized 47 eV light, and their selected (h) MDCs and (i) EDCs. (j) Calculated band structure reproduced from Ref. 14.

Figure 2

SDW state electronic structure of NaFeAs measured at 10 K. (a) Photoemission intensity map at the $E_F$ integrated over [$E_F-5\mathrm{meV}$, $E_F+5\mathrm{meV}$], the marks are measured Fermi crossings, while the curves are the fitted Fermi surfaces. (b) Photoemission intensity along the $\Gamma \mathrm{\text{−}}M$ direction. (c) The second derivative of data in (b) with respect to energy. The solid lines are local minimum locus to indicate the band position. (d),(e) MDCs of data around $\Gamma$ and $M$, respectively, and each MDC has been individually normalized by its integrated weight. (f) Selected EDCs along the $\Gamma \mathrm{\text{−}}M$ direction. Data in (a)–(f) were taken with 19 eV circularly polarized light. (g) Photoemission intensity taken along the $\Gamma \mathrm{\text{−}}M$ direction with linear polarized 47 eV light, and their selected (h) MDCs and (i) EDCs. (j) The measured SDW-state band structure.

Figure 3

(a) The normal state (dashed curves) and SDW state (solid curves) band structures are compared, where the momentum positions sampled in (b)–(f) are marked. Temperature dependence of EDCs at selected momenta: (b) $k_{\parallel }=-0.28{\AA }^{-1}$, (c) $k_{\parallel }=0$ ($\Gamma$), (d) $k_{\parallel }=0.82{\AA }^{-1}$, (e) $k_{\parallel }=0.92{\AA }^{-1}$, and (f) $k_{\parallel }=1.1{\AA }^{-1}$ (the M point). The dashed curves in (d) and (e) are the reproduced 75 K EDCs that are overlaid on the low temperature ones. (g) The summary of feature shifts in (b)–(f).

Figure 4

(a) The photoemission intensity of NaFeAs along the $\Gamma \mathrm{\text{−}}M$ direction taken at 6 K (superconducting state) and (b) its second derivative with respect to energy. (c) EDCs along the $\Gamma \mathrm{\text{−}}M$ direction taken at 10 and 6 K. (d),(e) The symmetrized EDCs at the Fermi crossings. The energy resolution is 6 meV here, which would be possible to detect the sign for 1–2 meV gap opening.