# Electronic structure reconstruction: the driving force behind the magnetic and structural transitions in NaFeAs 

C. He $^{1}$, Y. Zhang$^{1}$, B. P. Xie$^{1}$, X. F. Wang$^{2}$, L. X. Yang$^{1}$, B. Zhou $^{1}$, F. Chen$^{1}$, M. Arita$^{3}$, K. Shimada $^{3}$, H. Namatame$^{3}$, M. Taniguchi$^{3}$, X. H. Chen$^{2}$, J. P. Hu4 ∗ and D. L. Feng 1 † 

$^{1}$Department of Physics, Surface Physics Laboratory (National Key Laboratory), and Advanced Materials Laboratory, Fudan University, Shanghai 200433, People’s Republic of China 2 

Department of Physics, University of science and technology of China, Hefei, Anhui 230027, People’s Republic of China 

$^{3}$Hiroshima Synchrotron Radiation Center and Graduate School of Science, Hiroshima University, Hiroshima 739-8526, Japan. and 

4 Department of Physics, Purdue University, West Lafayette, Indiana 47907, USA 

(Dated: September 18, 2018) 

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 around structural transition. Our observation provides direct evidence that the structural and magnetic transitions share the same origin, and are both driven by the electronic structure reconstruction in Fe-based superconductors, instead of Fermi surface nesting. 

Iron based high temperature superconductors, like the cuprates, are at the vicinity of an ordered magnetic phase [1–3], and magnetism has been suggested to be crucial for the superconductivity in both cases. A unique fea- ture associated with the magnetic transition in iron pnic- tides/chalcogenides is that the magnetic or spin density wave (SDW) transition is always accompanied by a struc- tural transition. Although theoretically, it has been sug- gested that the structural transition may be driven mag- netically [4], it does occur at higher temperature in many cases, and even isotope effects have been reported for the magnetic/structural transitions [5]. So far, there is no hard experimental evidence to establish the relationship between the structural transition and the magnetic tran- sition, but their ubiquitous co-occurrence makes it crucial to understand their nature and origin, in order to unveil the mystery of iron-based superconductors. 

For BaFe 2 As 2 and other parent compounds of the so called “122” series of iron pnictides and Fe 1+ $_{y}$Te, the structural transition occurs simultaneously with the magnetic transition [6–9]. It is thus hard to separate the effects of lattice and magnetic transitions there. For LaOFeAs (a representing parental compound of the “1111” series) [10] and NaFeAs (representing the “111” series) [11], the structural transition precedes the SDW transition. However, the charge redistribution on the sur- face of the “1111” series hampers direct measurements of its bulk electronic structure [12]. Without these con- straints, NaFeAs provides an ideal opportunity to ad- dress what happens to the electronic structure between the lattice and SDW transitions that has not been ac- cessed before. 

In this Letter, we report angle resolved photoemis- sion spectroscopy (ARPES) measurements of the detailed electronic structure of NaFeAs. We show that there is strong band renormalization, and the SDW is not caused by Fermi surface nesting, and rather the electronic energy is saved through band reconstruction. The entire band structure including bands well below Fermi surface par- ticipates in the reconstruction process. Such a transition of the electronic structure emerges at the structural tran- sition temperature, and it smoothly evolves through the SDW transition temperature. Moreover, band folding is observed even at the structural transition temperature, which suggests short-ranged magnetic ordering has taken place at this temperature. These results establish that both the lattice structural transition and the magnetic transition are driven by the electronic structure recon- structions in Fe-based superconductors. 

High quality NaFeAs single crystals were synthesized by a NaAs flux method. Na pieces and As powder was mixed and heated to 250 $^{◦}$C for 3 hours. The Fe and NaAs powders were loaded into an alumina crucible and then sealed in a Ta crucible. They were heated to 900 $^{◦}$C, then slowly cooled to 600 $^{◦}$C, and to room temperature after shutting off the power of the furnace. Resistivity measurements confirm that there is a superconducting transition at 8K, a SDW transition at T$_{N}$ =39K, and a structural transition at T$_{S}$ =54K [11, 13]. ARPES data were taken with circularly polarized 19 eV photons at the Beamline 9 of Hiroshima synchrotron radiation cen- ter (HSRC) with a Scienta R4000 electron analyzer; the energy resolution was 9 meV, and the angular resolution was 0.3 degree. Sample was measured within two hours after it was cleaved in situ under ultra-high-vacuum of 3 × 10 − $^{11}$torr . Aging effects are strictly monitored, so that data reflect the intrinsic electronic structure. 

The normal state photoemission intensity map at the Fermi energy is shown in Fig. 1a. Two holelike Fermi 

