Pure nematic state in the iron-based superconductor FeSe

Lattice and electronic states of thin FeSe films on ${\mathrm{LaAlO}}_3$ substrates are investigated in the vicinity of the nematic phase transition. No evidence of a structural phase transition is found by x-ray diffraction below $T^{*}\sim 90$ K, while results obtained from a resistivity measurement and angle-resolved photoemission spectroscopy clearly show the appearance of a nematic state. These results indicate the formation of a pure nematic state in the iron-based superconductor and provide conclusive evidence that the nematic state originates from the electronic degrees of freedom. This pure nematicity in the thin film implies the difference in the electron-lattice interaction from bulk FeSe crystals. FeSe films provide valuable playgrounds for observing the pure response of “bare” electron systems free from the electron-lattice interaction, and should make an important contribution to investigate nematicity and its relationship with superconductivity.

Figure 1

Comparisons of (a), (b) lattice structures, (c), (d) Fermi surfaces around the $\mathrm{\Gamma }$ and $M$ points, and (e), (f) band diagrams around the $M$ point of FeSe between the nematic state ($T<T^{*}$) and the normal state ($T>T^{*}$). The pink and blue circles in (a) and (b) indicate the appearance of electronic nematicity. The blue, red, and green curves in (c)–(f) indicate the $d_{xz}, d_{yz}$, and $d_{xy}$ orbital bands, respectively. Solid and dashed curves in (e) and (f) represent the band dispersions along the $(0,0)-(\pi ,0)$ and $(0,0)-(0,\pi )$ directions (long and short Fe-Fe directions), respectively, of the untwinned crystal [36].

Figure 2

Characterization of FeSe film on LAO. (a) XRD pattern obtained at room temperature. The number and asterisk represent peaks from the LAO substrate and the stainless-steel sample holder, respectively. (b) Rocking curve of the 001 reflection of the FeSe film. (c) Temperature dependence of the resistance $R$ of the FeSe film. The inset shows the temperature dependence of $dR/dT$ of the FeSe film. The arrow shows the nematic transition temperature $T^{*}$.

Figure 3

(a) ARPES intensity as a function of binding energy and wave vector along the $\mathrm{\Gamma }-M$ cut at $T=50\mathrm{K}$ with 56 eV photons. (b) Second-derivative intensity of (a), after division by the Fermi-Dirac distribution function convoluted with the instrumental resolution function. Magenta dashed curves are a guide for the eye to trace the band dispersions near $E_{\mathrm{F}}$. (c), (d) Same as (a) and (b), respectively, but at $T=130\mathrm{K}$. Magenta dashed curves in (d) are identical to those in (b). (e), (f) Temperature dependence of ARPES spectrum at the $M$ point and the corresponding second-derivative intensity plot, respectively. Magenta dashed curves in (e) are a guide for the eye to trace the energy position of $d_{xz}/d_{yz}$-derived bands, extracted from the peak position in (f) (red circles). (g) Magnitude of the energy difference between the $d_{xz}$- and $d_{yz}$-derived bands at the $M$ point plotted as a function of temperature. (h), (i) Same as (f) and (g), respectively, but for the $\mathrm{\Gamma }$ point.

Figure 4

(a) Result of $\theta -2\theta$ scan at the ${336}_{\mathrm{T}}$ Bragg reflection obtained at 6 K. (b) Temperature dependence of the XRD profile at the ${336}_{\mathrm{T}}$ Bragg reflection obtained with SOPHIAS. The $\theta$ angle was fixed at the maximum diffraction signal at each temperature. The horizontal axis shows the value relative to the main peak angle. (c) Temperature dependence of the width of the main peak obtained by the fitting with the Lorentz function for the XRD profiles. The vertical bars represent error bars from the fitting.