The record superconducting transition temperature (T(c)) for the iron-based high-temperature superconductors (Fe-HTS) has long been 56 K. Recently, in single-layer FeSe films grown on SrTiO3 substrates, indications of a new record of 65 K have been reported. Using in situ photoemission measurements, we substantiate the presence of spin density waves (SDWs) in FeSe films--a key ingredient of Fe-HTS that was missed in FeSe before--and we find that this weakens with increased thickness or reduced strain. We demonstrate that the superconductivity occurs when the electrons transferred from the oxygen-vacant substrate suppress the otherwise pronounced SDWs in single-layer FeSe. Beyond providing a comprehensive understanding of FeSe films and directions to further enhance its T(c), we map out the phase diagram of FeSe as a function of lattice constant, which contains all the essential physics of Fe-HTS. With the simplest structure, cleanest composition and single tuning parameter, monolayer FeSe is an ideal system for testing theories of Fe-HTS.

Ferromagnetism and the Kondo effect are crucial for 3d electrons to become spin-separated and heavy at the same time.

Heavy fermion materials gain high electronic masses and expand Fermi surfaces when the high-temperature localized f electrons become itinerant and hybridize with the conduction band at low temperatures. However, despite the common application of this model, direct microscopic verification remains lacking. Here we report high-resolution angle-resolved photoemission spec-1 arXiv:1610.06724v1 [cond-mat.str-el]

A key issue in heavy fermion research is how subtle changes in the hybridization between the 4f (5f) and conduction electrons can result in fundamentally different ground states. CeRhIn_{5} stands out as a particularly notable example: when replacing Rh with either Co or Ir, antiferromagnetism gives way to superconductivity. In this photoemission study of CeRhIn_{5}, we demonstrate that the use of resonant angle-resolved photoemission spectroscopy with polarized light allows us to extract detailed information on the 4f crystal field states and details on the 4f and conduction electron hybridization, which together determine the ground state. We directly observe weakly dispersive Kondo resonances of f electrons and identify two of the three Ce 4f_{5/2}^{1} crystal-electric-field levels and band-dependent hybridization, which signals that the hybridization occurs primarily between the Ce 4f states in the CeIn_{3} layer and two more three-dimensional bands composed of the Rh 4d and In 5p orbitals in the RhIn_{2} layer. Our results allow us to connect the properties observed at elevated temperatures with the unusual low-temperature properties of this enigmatic heavy fermion compound.

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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...

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