Perovskite SrIrO3 has long been proposed as an exotic semimetal induced by the interplay between the spin-orbit coupling and electron correlations. However, its low-lying electronic structure is still lacking. We synthesize high-quality perovskite SrIrO3 (100) films by means of oxide molecular beam epitaxy, and then systemically investigate their low energy electronic structure using in-situ angle-resolved photoemission spectroscopy. We find that the hole-like bands around R and the electron-like bands around U(T) intersect the Fermi level simultaneously, providing the direct evidence of the semimetallic ground state in this compound. Comparing with the density functional theory, we discover that the bandwidth of states near Fermi level is extremely small, and there exists a pronounced mixing between the Jeff = 1/2 and Jeff = 3/2 states. Moreover, our data reveal that the predicted Dirac degeneracy protected by the mirror-symmetry, which was theoretically suggested to be the key to realize the non-trivial topological properties, is actually lifted in perovskite SrIrO3 thin films. Our findings pose strong constraints on the current theoretical models for the 5d iridates.

The mechanism of high superconducting transition temperatures (T_{c}) in bismuthates remains under debate despite more than 30 years of extensive research. Our angle-resolved photoemission spectroscopy studies on Ba_{0.51}K_{0.49}BiO_{3} reveal an unexpectedly 34% larger bandwidth than in conventional density functional theory calculations. This can be reproduced by calculations that fully account for long-range Coulomb interactions-the first direct demonstration of bandwidth expansion due to the Fock exchange term, a long-accepted and yet uncorroborated fundamental effect in many body physics.Furthermore, we observe an isotropic superconducting gap with 2Δ_{0}/k_{B}T_{c}=3.51±0.05, and strong electron-phonon interactions with a coupling constant λ∼1.3±0.2. These findings solve a long-standing mystery-Ba_{0.51}K_{0.49}BiO_{3} is an extraordinary Bardeen-Cooper-Schrieffer superconductor, where long-range Coulomb interactions expand the bandwidth, enhance electron-phonon coupling, and generate the high T_{c}. Such effects will also be critical for finding new superconductors.

Pyrite-type PdSb 2 with a nonsymmorphic cubic structure has been predicted to host six-fold-degenerate exotic fermions beyond the Dirac and Weyl fermions. Though magnetotransport measurements on PdSb 2 suggest its topologically nontrivial character, direct spectroscpic study of its band structure remains absent. Here, by utilizing high-resolution angle-resolved photoemission spectroscopy, we present a systematic study on its bulk and surface electronic structure. Through careful comparison with first-principles calculations, we verify the existence of six-fold fermions in PdSb 2 , which are formed by three doubly degenerate bands centered at the R point in the Brillouin zone. These bands exhibit parabolic dispersion close to six-fold fermion nodes, in sharp contrast to previously reported ones in chiral fermion materials. Furthermore, our data reveal no protected Fermi arcs in PdSb 2 , which is compatible with its achiral structure. Our findings provide a remarkable platform for study of new topological fermions and indicate their potential applications.In high-energy physics, three types of fermions have been predicated to exist in our universe according to the Lanrence invarience, i.e. the Dirac, Weyl and Majorana fermions. Although the existence of Weyl and Majorana fermions as elementary particles is still under hot debate, some quasiparticles constructed out of excitations were theoretically proposed to be able to mimic all three fermions in solids. Very soon after these predictions, Dirac [1-4], Weyl [5-10] and Majorana fermions [11][12][13] were experimentally confirmed to actually exist in topological non-trivial materials. Moreover, rather than the Poincarè symmetry, it is the invariance under crystal symmetry of one of 230 space groups that constrains fermions in solids [14][15][16][17], and thus there might exist some novel fermions beyond elementary particles discussed in highenergy physics. In solids, it is well known that the topological non-trivial four-and two-fold degenerate band crossings correspond to Dirac and Wely fermions, respectively. While, allowed by crystal symmetries, some new exotic fermions showing three-, six-, or eight-fold degeneracy of band crossings have been proposed, which are particularly intriguing since there are no counterparts in high-energy physics due to Poincarè symmetry constrains [14].To date, angle-resolved photoemission spectroscopy (ARPES) experiments have verified several types of exotic fermions beyond the Dirac and Weyl ones. Unique triply degenerate points in the electronic structure of molybdenum phosphide were reported [18]. Moreover, four-, six-(double * Equal contributions † spin-1) and eight-fold (charge-2) degenerate chiral fermions were as well observed in AlPt [19] and CoSi [20-23] families with extremely long surface Fermi arcs, in sharp contrast to Weyl semimetals which have multiple pairs of Weyl nodes with only small separation. In addition, four types of symmetry-stabilized topological fermions have been observed in PdBiSe [24]. The pyrite PdSb 2 ...

Topological nodal-line semimetals with exotic quantum properties are characterized by symmetry-protected line-contact bulk band crossings in the momentum space. However, in most of identified topological nodalline compounds, these topological non-trivial nodal lines are enclosed by complicated topological trivial states at the Fermi energy (E F ), which would perplex their identification and hinder further applications. Utilizing angle-resolved photoemission spectroscopy and first-principles calculations, we provide compelling evidence for the existence of Dirac nodal-line fermions in the monoclinic semimetal SrAs 3 , which are close to E F and away from distraction of complex trivial Fermi surfaces or surface states. Our calculation indicates that two bands with opposite parity are inverted around Y near E F , which results in the single nodal loop at the Γ-Y-S plane with a negligible spin-orbit coupling effect. We track these band crossings and then unambiguously identify the complete nodal loop quantitatively, which provides a critical experimental support to the prediction of nodal-line fermions in the CaP 3 family of materials. Hosting simple topological non-trivial bulk electronic states around E F and no interfering with surface states on the natural cleavage plane, SrAs 3 is expected to be a potential platform for topological quantum state investigation and applications.Topological semimetal, as the non-trivial extension of topological classification of electronic quantum states from the insulator to metal, is a group of materials in which the conduction and valence bands cross and form nodes behaving as monopole of a Berry flux [1,2]. When nodes are close to the Fermi energy (E F ), the low-energy quasiparticle excitation would be drastically different from that of the conventional Schrödinger-type fermion and thus lead to novel transport properties, which are crucial to the further study of novel quantum states and modern quantum devices [3,4]. While some distinct point-contact nodes, e.g., 4-, 2-, and 3-fold degenerate nodes, have been confirmed in Dirac [5,6], Weyl [7][8][9][10][11][12][13][14], and triply-degenerate semimetals [15,16] in previous studies, respectively, line-contact nodes, i.e., nodal lines, with their various configurations [17] have not been fully investigated in experiments until now.The nodal ring, nodal link and nodal chain all belong to nodal-line systems in which nodes extend along onedimensional lines instead of discrete points in the threedimensional (3D) Brillouin zone (BZ) [3,[17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34]. Theories have predicted that a non-trivial Berry phase around the nodal line would generate a half-integer shift of Landau-level index [20] and result in drumhead-like surface states [19,21]. To date, nodal-line states have been theoretically proposed and * Equal contributions † yinzhiping@bnu.edu.cn ‡ then experimentally confirmed in several compounds, including CaAgX (X=P, As) [22, 23], PbTaSe 2 [24], ZrSiS [25-28], and MB 2 (M...