Spin-orbit coupling (SOC) is a fundamental interaction in solids which can induce a broad spectrum of unusual physical properties from topologically non-trivial insulating states to unconventional pairing in superconductors. In iron-based superconductors (IBS) its role has so far been considered insignificant with the models based on spin-or orbital fluctuations pairing being the most advanced in the field. Using angle-resolved photoemission spectroscopy we directly observe a sizeable spin-orbit splitting in all main families of IBS. We demonstrate that its impact on the lowenergy electronic structure and details of the Fermi surface topology is much stronger than that of possible nematic ordering. Intriguingly, the largest pairing gap is always supported exactly by SOCinduced Fermi surfaces.In the presence of spin-orbit coupling, the electron's spin quantized along any fixed axis is no longer a good quantum number, but its total angular momentum is. This basic fact alone or in combination with a particular symmetry breaking may lead to a splitting of otherwise degenerate energy bands and is the origin of fascinating phenomena such as spin Hall effects A special role has been played by SOC in the field of superconductors. In low-dimensional or noncentrosymmetric systems it can promote and stabilize superconductivity [6], allow ferromagnetism to coexist with superconductivity [7] or even rise T c [8]. If SOC is large, some superconductors can host an elusive Fulde-Ferrell-Larkin-Ovchinnikov state [9] or topological superconductivity [4]. It is anticipated that SOC could be a very important ingredient in describing the superconducting state in Sr 2 RuO 4 [10]. Since k-dependent spin-orbit splitting is larger than the superconducting gap in this material, the SOC-induced spin anisotropy together with the orbital mixing should directly influence the orbital and spin angular momentum of the Cooper pairs. Singlet and triplet states could be strongly mixed, blurring the distinction between spin-singlet and spintriplet pairing [11].In multiband iron-based superconductors, where the low energy electronic structure is composed of different orbitals, the situation is even more complicated because of the presence of the sizeable Hund's coupling. When the electronic structure near the Fermi energy is composed of different orbitals and spins mixed via spin-orbit coupling, determination of the pairing symmetry becomes non-trivial. However, up to now SOC in iron pnictides and chalcogenides was considered weak.We start with the example of LiFeAs, which is a special representative of iron-based family of superconductors [12]. This material is one of the most studied due to its stoichiometry and non-polar surfaces. Its electronic structure is believed to be well understood from numerous angle-resolved photoemission experiments (ARPES) and the parameterization of its electronic dispersions has been used to test the most developed theoretical approaches [13][14][15]. To detect spin-orbit coupling in LiFeAs experimentally we first ne...

While recent advances in band theory and sample growth have expanded the series of extremely large magnetoresistance (XMR) semimetals in transition metal dipnictides T mPn 2 (T m = Ta, Nb; Pn = P, As, Sb), the experimental study on their electronic structure and the origin of XMR is still absent. Here, using angle-resolved photoemission spectroscopy combined with first-principles calculations and magnetotransport measurements, we performed a comprehensive investigation on MoAs 2 , which is isostructural to the T mPn 2 family and also exhibits quadratic XMR. We resolve a clear band structure well agreeing with the predictions. Intriguingly, the unambiguously observed Fermi surfaces (FSs) The emergence of novel states in condensed matter is not only classified by the typical spontaneous symmetry breaking, but also by their topology, i.e., the topologically protected quantum states [1][2][3]. The discovery of such symmetry protected states of matter in two-dimensional (2D) [4-6] and three-dimensional (3D) topological insulators [7], node-line semimetals [8,9], topological crystalline insulators [10,11], and Dirac and Weyl semimetals [12][13][14][15][16][17], has attracted tremendous interests in condensed matter physics and material science. The magnetotransport behavior of these states is often unusual, such as linear transverse magnetoresistance (MR) and negative longitudinal MR in Dirac and Weyl semimetals [18][19][20][21][22][23][24], and more generally, extremely large transverse MR (XMR) in nonmagnetic semimetals [25][26][27][28][29][30].Recently, the discovery of XMR in a class of transition metal dipnictides T mPn 2 (T m = Ta, Nb; Pn = P, As, Sb) [31][32][33][34][35][36] has sparked immense interests for understanding the underlying mechanism of quadratic XMR and exploring novel quantum states arising from nontrivial topology. Another two series of semimetals possessing quadratic XMR behavior and rich topological characteristics are the ZrSiS family [37][38][39] and LnX (Ln = La, Y, Nd, or Ce; X = Sb/Bi) series [40][41][42][43][44][45][46], whose electronic structures have been considerably studied both in theory and experiment [47][48][49][50][51][52][53]. While the band structures of the T mPn 2 series have been theoretically characterized in several work [32][33][34]54], experimental observations have not yet been reported. It is widely believed that the large positive MR in semimetals is intimately related to their underlying electronic structures. Therefore, a systematic and unambiguous experimental study on the electronic structure of the T mPn 2 family is urgently demanded. Eventually, we suggest the open-orbit Fermi surface (FS) topology as another candidate mechanism to explain the XMR, in addition to the earlier proposed origins like nontrivial band topology [40], forbidden backscattering at zero field [55], and electron-hole compensation [56].In this Letter, we employ systematic angle-resolved photoemission spectroscopy (ARPES), first-principles calculations, and magnetotransport measurements o...

Size effect on the structural, magnetic, and magnetotransport properties of electron doped manganite La0.15Ca0.85MnO3We report a comprehensive study of orbital character and tridimensional nature of the electronic structure of (Ca 0.85 La 0.15 )FeAs 2 from recently discovered "112" family of Iron-based superconductors (IBS), with angle-resolved photoemission spectroscopy. We observed that the band structure is similar to that of "122" family, namely, there are three hole-like bands at the Brillouin zone (BZ) center and two electron-like bands at the BZ corner. The bands near the Fermi level (E F ) are mainly derived from the Fe t 2g orbitals. On the basis of our present and earlier studies, we classify IBS into the three types according to their crystal structures. We show that although the bands near E F mainly originate from Fe 3d electrons, they are significantly modified by the interaction between the superconducting slabs and the intermediate atoms. V C 2015 AIP Publishing LLC.

Using angle-resolved photoemission spectroscopy (ARPES), we studied the effect of the impurity potential on the electronic structure of FeTe 0.5 Se 0.5 superconductor by substituting 10% of Ni for Fe, which leads to an electron doping of the system. We could resolve three hole pockets near the zone center and an electron pocket near the zone corner in the case of FeTe 0.5 Se 0.5 , whereas only two hole pockets near the zone center and an electron pocket near the zone corner are resolved in the case of Fe 0.9 Ni 0.1 Te 0.5 Se 0.5 , suggesting that the hole pocket having predominantly the xy orbital character is very sensitive to the impurity scattering. Upon electron doping, the size of the hole pockets decreases and the size of the electron pockets increases as compared to the host compound. However, the observed changes in the size of the electron and hole pockets are not consistent with the rigid-band model. Moreover, the effective mass of the hole pockets is reduced near the zone center and of the electron pockets is increased near the zone corner in the doped Fe 0.9 Ni 0.1 Te 0.5 Se 0.5 as compared to FeTe 0.5 Se 0.5 . We refer these observations to the changes of the spectral function due to the effect of the impurity potential of the dopants.