Observation of Dirac cone band dispersions in FeSe thin films by photoemission spectroscopy

Abstract

The electronic structure of FeSe thin films grown on SrTiO 3 substrate is studied by angle-resolved photoemission spectroscopy (ARPES). We reveal the existence of Dirac cone band dispersions in FeSe thin films thicker than 1 Unit Cell below the nematic transition temperature, whose apex are located -10 meV below Fermi energy. The evolution of Dirac cone electronic structure for FeSe thin films as function of temperature, thickness and cobalt doping is systematically studied. The Dirac cones are found to be coexisted with the nematicity in FeSe, disappear when nematicity is suppressed. Our results provide some indication that the spin degrees of freedom may play some kind of role in the nematicity of FeSe.The discovery of high temperature superconductivity in single unit-cell (UC) FeSe film grown on SrTiO 3 (STO) substrate has attracted extensively attention [1][2][3][4][5][6][7][8][9]. A superconducting gap as large as 20 meV was first discovered[1] by scanning tunneling spectroscopy (STS), which was later confirmed [3,4] by angle resolved photoemission spectroscopy (ARPES) measurements. Then, the T C above 40 K and 100 K in 1-UC FeSe films has been demonstrated by direct transport measurements [8] and in-situ electrical transport measurements [9] respectively. Until now, the superconducting mechanism for single layer FeSe is still unsolved. Another hot debate issue is the driving force of the nematicity for FeSe. Bulk FeSe undergoes a tetragonal to orthorhombic transition at Ts～90 K and at Tc～8 K superconductivity sets in [10]. Clear band splitting around the Brillioun zone corner was first discovered in multi-layer FeSe thin films [4], with a characteristic temperature much higher than the structure transition temperature(T s ), which is thought to be caused by short ranged magnetic order. Then similar band reconstruction was also found in bulk FeSe single crystal [11], which was interpreted to be triggered by the electronic nematicity. Although the existence of nematic order in FeSe is by now a well-established experimental fact, its origin remains controversial. Spin, orbital degrees of freedom or their complicated coupling are all possible candidates of the driving force proposed by experimental and theoretical researches [12][13][14][15][16][17][18][19][20][21][22].Another intriguing issue in condensed matter physics nowadays is the massless Dirac fermion states in materials, such as graphene [23], topological insulators [24,25], Weyl semimetals [26,27], as well as the parent compound of iron-based superconductors [28][29][30][31]. Dirac cone states have been theoretically predicted [28] and experimentally confirmed [29][30][31] in BaFe 2 As 2 bellow the spin density wave(SDW) transition temperature. It is well established that the formation of Dirac cone states is a consequence of the nodes of the SDW gap by complex zone foldings in bands with different parities. It is even proposed that the coexistence of highly mobile carriers and superconductivity is important for achieving high-T c superconductors.Sur...

Dirac fermions and flat bands in the ideal kagome metal FeSn

“…2j) linearly as the energy crosses ED1. The velocity of Dirac fermions in FeSn is (1.7 ± 0.2)´10 5 m/s, an order of magnitude lower than that of graphene and in close range of that of Fe3Sn2 15 and Fe-based superconductors [24][25][26] , possibly reflecting the more correlated nature of Fe-3d electrons. Overall, our ARPES data directly establish the Dirac fermiology of kagomederived bands in FeSn.…”

Two-dimensional type-II Dirac fermions in layered oxides

“…In the past decade, two-dimensional and three-dimensional type-I Dirac fermions near E F have been identified in a variety of different systems, e.g., graphene 3 , topological insulators 4 , and semimetals such as Na 3 Bi 5 , Cd 3 As 2 6 , 7 , and black phosphorus 8 . The concept of topologically protected Dirac fermions has also been applied to the band structure found in high-temperature iron-based superconductors 9 , 10 . Type-II Dirac fermions seem to be much less common.…”

Abrupt change of the superconducting gap structure at the nematic critical point in FeSe 1−x S x

“…1A, the Fermi surface in the nematic phase consists of an elliptical hole pocket at the Brillouin zone center (h1), elongated along the Γ-My line, and compensated electron pockets near the zone boundary (e1 and e2) (23). It has been reported that the e1 pocket is divided into two Dirac-like electrons in the presence of large orbital splitting (24)(25)(26), although the detailed structure of the Fermi surface is still controversial (8-13, 27, 28). The size of all of the pockets is extremely small, occupying only 1 to 3% of the whole Brillouin zone (29)(30)(31).…”