Distinctive orbital anisotropy observed in the nematic state of a FeSe thin film

Abstract

Nematic state, where a system is translationally invariant but breaks rotational symmetry, has drawn great attentions recently due to the experimental observations of such a state in both cuprates and iron-based superconductors. The origin of nematicity and its possible tie to the pairing mechanism of high-T c , however, still remains controversial. Here, we studied the electronic structure of multilayer FeSe film using angle-resolved photoemission spectroscopy (ARPES). The band reconstruction in the nematic state is clearly delineated. We found that the energy splitting between d xz and d yz bands shows non-monotonic distribution in momentum space. From the Brillouin zone center to the Brillouin zone corner, the magnitude of splitting first decreases, then increases, and finally reaches the maximum value of ∼70 meV. Moreover, besides the d xz and d yz bands, band splitting was also observed on the d xy bands with a comparable energy scale around 45 meV. Our results suggest that the electronic anisotropy in the nematic state cannot be explained by a simple on-site ferro-orbital order. Instead, strong anisotropy exists in the hopping of all d xz , d yz , and d xy orbitals, the origin of which holds the key to a microscopic understanding of the nematicity in iron-based superconductors.

Interatomic Coulomb interaction and electron nematic bond order in FeSe

“…Recent reports on 30-monolayer FeSe films, which are similar to the 35-monolayer films mentioned earlier with a nematic transition temperature around 125K [48,49], shows a superconducting T c as high as 44K [52] under K surface doping. The strongest nematic phase with T nem ≃ 180K is in fact an insulator observed in the nonsuperconducting N-phase of single and double-layer FeSe films grown on SrTiO 3 substrates [53,54].…”

“…The overarching importance of the extended Coulomb interaction V may originate from the lack of the charge reservoir layers and the shorter Fe-Fe bond in bulk FeSe when compared to Fepnictides [23]. Interestingly, electronic nematicity with similar phenomenology has been observed recently in 35-monolayer FeSe films with a larger ∆ M = 80meV and higher T nem = 125K [48,49], suggesting that further reduced screening of extended Coulomb interaction in films can result in a stronger V and an enhanced nematic response.…”

Role of the orbital degree of freedom in iron-based superconductors

“…5f and g, to fully explain the three electron bands observed in the nematic phase at the M point on this twinned sample, the shift of the d yz and d xz bands in opposite directions is insufficient, in addition, the d xy band along the longer x-direction must shift down in energy while the counterpart d xy band along the shorter y-direction must remain unshifted. 36 Furthermore, the magnitude of the energy splitting of the d xy band along two perpendicular directions is comparable to the energy splitting of the d xz /d yz orbitals, which further suggests the complexity of the orbital anisotropy in the nematic state.…”

“…25,26 The intrinsic band structure of the C 2 state was then revealed by ARPES measurements on detwinned crystals of BaFe 2 As 2 , 27-29 NaFeAs, 30,31 and FeSe, 32,33 , and inferred thereafter from twinned crystals of FeSe 34,35 and FeSe films. 36,37 Figure 5b shows an example of detwinned NaFeAs measured in the nematic state, where the d yz band (green) along the slightly longer axis is shifted up while the d xz band (red) along the shorter axis is shifted down. In the tetragonal state above T S , these two bands are degenerate in energy.…”

“…This generic upward shift of d yz orbital and downward shift of d xz orbital is observed in all measurements of FeSCs in the nematic phase. [27][28][29][30][31][32][33][34][35][36][37] Importantly, the relatively large energy scale of this orbital splitting cannot be a trivial consequence of the less than 1% orthorhombicity. 27 Hence the orbital anisotropy is unlikely to be a simple result of the lattice, but rather suggests the manifestation of electronic nematicity, consistent with the original discovery of large resistivity anisotropy in this phase.…”