We investigate the electronic reconstruction across the tetragonal-orthorhombic structural transition in FeSe by employing polarization-dependent angle-resolved photoemission spectroscopy (ARPES) on detwinned single crystals. Across the structural transition, the electronic structures around the  and M points are modified from four-fold to two-fold symmetry due to the lifting of degeneracy in d xz /d yz orbitals.The d xz band shifts upward at the  point while it moves downward at the M point, suggesting that the electronic structure of orthorhombic FeSe is characterized by a momentum-dependent sign-changing orbital polarization. The elongated directions of the elliptical Fermi surfaces (FSs) at the  and M points are rotated by 90 degrees with respect to each other, which may be related to the absence of the antiferromagnetic order in FeSe. Keywords: PACS:Most of the parent compounds of the iron-based superconductors show the tetragonal-orthorhombic structural transition at T s and the stripe-type antiferromagnetic (AFM) order below T N ( T s ) [1,2]. Near the structural transition, an orbital order defined by the inequivalent electron occupation of 3d xz (xz) and 3d yz (yz) orbitals [3][4][5], has been reported by ARPES [6,7] and X-ray linear dichroism measurements [8] in several parent compounds. Experimental and theoretical studies suggested that the structural transition is caused by the electronic nematicity of the spin [9,10] or orbital [11][12][13] degrees of freedoms. Since superconductivity develops when such complex ordered states are suppressed, it is crucial to understand how the phase transitions couple to each other.In Ba(Fe,Co) 2 As 2 , the spin-driven nematicity has been suggested from the phase diagram in which T s and T N closely follow each other as the carrier is doped [14]. The scaling behavior between the nematic fluctuation and spin fluctuation was also reported by the nuclear magnetic resonance (NMR) and shear modulus measurements [10]. On the other hand, in NaFeAs, the orbital-driven nematicity has been proposed by ARPES [11]. In this compound, the structural transition at T s = 54 K is well separated from the AFM transition at T N = 43 K. Inequivalent shift in the xz/yz orbital bands appearing above T s changes the FSs from four-fold to two-fold symmetric shape [11,15], which may be a possible trigger of the stripe type AFM order and the orthorhombicity [11,16]. The variety of iron-based

We investigate the surface electronic structures of polar 1T'-MoTe2, the Weyl semimetal candidate realized through the nonpolar-polar structural phase transition, by utilizing the laser angle-resolved photoemission spectroscopy combined with first-principles calculations. Two kinds of domains with different surface band dispersions are observed from a single-crystalline sample. The spin-resolved measurements further reveal that the spin polarizations of the surface and the bulk-derived states show the different domain-dependences, indicating the opposite bulk polarity. For both domains, some segment-like band features resembling the Fermi arcs are clearly observed. The patterns of the arcs present the marked contrast between the two domains, respectively agreeing well with the slab calculation of (0 0 1) and (0 0 -1) surfaces. The present result strongly suggests that the Fermi arc connects the identical pair of Weyl nodes on one side of the polar crystal surface, whereas it connects between the different pairs of Weyl nodes on the other side. Weyl nodes) at the Fermi level (EF). The bulk electronic structure of Weyl semimetals is characterized by the spin-polarized Weyl cone dispersions formed through the breaking of either time-reversal or space-inversion symmetry [1][2][3][4]. At the surface, on the other hand, the chiral charge associated with the Weyl nodes warrants the existence of the gapless surface states, socalled Fermi arcs that connect the two-dimensionally (2D) projected Weyl nodes. Due to these unusual bulk and surface electronic states, a variety of new magnetoelectric phenomena have been predicted [4][5][6][7][8][9]. Until now, several experimental verifications of realistic Weyl semimetal compounds have been raised (eg. the TaAs family [10][11][12] The single-crystalline 1T'-MoTe2 was synthesized as reported elsewhere [19,20].ARPES at 25 K was performed using the He-discharge lamp (21.2 eV) and the fourth harmonic generation of Ti:sapphire laser (6.43 eV, s-polarized light) [28], with a VG-Scienta R4000WALanalyzer. The total energy resolution was set to 10 and 3 meV, respectively. For the laser-SARPES at 25 K and the ARPES at 100 K, a 6.99 eV laser (s-polarized light) and a ScientaOmicron DA30Lanalyzer mounted with two sets of very-low-energy electron diffraction spin detectors were used at the Institute of Solid State Physics (ISSP), The University of Tokyo [29]. The total energy resolution was set to 30 and 3 meV, respectively. Samples were cleaved in situ at room temperature. All measurements were performed in ultrahigh vacuum better than 1×10 -10 Torr.Electronic structure calculations were performed within the context of density functional theory (DFT) using the Perdew-Burke-Ernzerhof exchange-correlation functional as implemented in the VASP program [30,31]. Relativistic effects were fully included. The structure parameters in Ref.[14] were used, and the corresponding Brillouin zone was sampled by a 20 × 10 × 5 k-mesh. To focus on the near-EF electronic structure, the ARPES images...

The phase diagram of LaFeAs1−xPxO system has been extensively studied through hole-and electron-doping as well as As/P-substitution. It has been revealed that there are three different su-

The structural flexibility at three substitution sites in LaFeAsO enabled investigation of the relation between superconductivity and structural parameters over a wide range of crystal compositions. Substitutions of Nd for La, Sb or P for As, and F or H for O were performed. All these substitutions modify the local structural parameters, while the F/H-substitution also changes band filling. It was found that the superconducting transition temperature $$T_{\text{c}}$$ T c is strongly affected by the pnictogen height $$h_{Pn}$$ h Pn from the Fe-plane that controls the electron correlation strength and the size of the $$d_{xy}$$ d xy hole Fermi surface (FS). With increasing $$h_{Pn}$$ h Pn , weak coupling BCS superconductivity switches to the strong coupling non-BCS one where electron correlations and the $$d_{xy}$$ d xy hole FS may be important.

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