Z. R. Ye

Interface-induced superconductivity and strain-dependent spin density waves in FeSe/SrTiO3 thin films

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.

Nodeless superconducting gap in AxFe2Se2 (A=K,Cs) revealed by angle-resolved photoemission spectroscopy

Pairing symmetry is a fundamental property that characterizes a superconductor. For the iron-based high-temperature superconductors, an s(±)-wave pairing symmetry has received increasing experimental and theoretical support. More specifically, the superconducting order parameter is an isotropic s-wave type around a particular Fermi surface, but it has opposite signs between the hole Fermi surfaces at the zone centre and the electron Fermi surfaces at the zone corners. Here we report the low-energy electronic structure of the newly discovered superconductors, A(x)Fe(2)Se(2) (A=K,Cs) with a superconducting transition temperature (Tc) of about 30 K. We found A(x)Fe(2)Se(2) (A=K,Cs) is the most heavily electron-doped among all iron-based superconductors. Large electron Fermi surfaces are observed around the zone corners, with an almost isotropic superconducting gap of ~10.3 meV, whereas there is no hole Fermi surface near the zone centre, which demonstrates that interband scattering or Fermi surface nesting is not a necessary ingredient for the unconventional superconductivity in iron-based superconductors. Thus, the sign change in the s(±) pairing symmetry driven by the interband scattering as suggested in many weak coupling theories becomes conceptually irrelevant in describing the superconducting state here. A more conventional s-wave pairing is probably a better description.

Electronic Identification of the Parental Phases and Mesoscopic Phase Separation of KxFe2-ySe2 Superconductors

F. Chen, 1 M. Xu,1 Q. Q. Ge, 1 Y. Zhang, 1, ∗ Z. R. Ye,1 L. X. Yang,1 Juan Jiang, 1 B. P. Xie,1 R. C. Che, 2 M. Zhang, 3 A. F. Wang, 3 X. H. Chen, 3 D. W. Shen, 4 X. M. Xie,4 M. H. Jiang, 4 J. P. Hu, 5 and D. L. Feng1, † 1State Key Laboratory of Surface Physics, Department of Physics, and Advanced Materials Laboratory, Fudan University, Shanghai 200433, People’s Republic of China 2Department of Materials Science, and Advanced Materials Laboratory, Fudan University, Shanghai 200433, People’s Republic of China 3Hefei National Laboratory for Physical Sciences at Microscale and Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026, China 4State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 20005 5Department of Physics, Purdue University, West Lafayette, Indiana 47907, USA

Symmetry breaking via orbital-dependent reconstruction of electronic structure in detwinned NaFeAs

The superconductivity discovered in iron pnictides is intimately related to a nematic ground state, where the C-4 rotational symmetry is broken via the structural and magnetic transitions. We here study the nematicity in NaFeAs with polarization-dependent

Doping dependence of the electronic structure in phosphorus-doped ferropnictide superconductor BaFe2(As1−xPx)2studied by angle-resolved photoemission spectroscopy

The superconductivity in high temperature superconductors ordinarily arises when doped with hetero-valent ions that introduce charge carriers [1–4]. However, in ferropnictides, “iso-valent” doping, which is generally believed not to introduce charge carriers, can induce superconductivity as well [5–11]. Moreover, unlike other ferropnictides [12, 13], the superconducting gap in BaFe2(As1−xPx)2 has been found to contain nodal lines [14– 16]. The exact nature of the “iso-valent” doping and nodal gap here are key open issues in building a comprehensive picture of the iron-based high temperature superconductors [17–20]. With angle-resolved photoemission spectroscopy (ARPES), we found that the phosphor substitution in BaFe2(As1−xPx)2 induces sizable amount of holes into the hole Fermi surfaces, while thedxy-originated band is relatively intact. This overturns the previous common belief of “iso-valent” doping, explains why the phase diagram of BaFe2(As1−xPx)2 is similar to those of the holedoped compounds, and rules out theories that explain the nodal gap based on vanishingdxy hole pocket. BaFe2(As1−xPx)2 is a rather unique ferropnictide as its superconductivity is introduced by the iso-valent doping of P for As [5, 6]. Unlike the hetero-valent doping that alters the carrier concentration in Ba 1−xKxFe2As2, BaFe2−xCoxAs2, or LaO1−xFxFeAs [2–4], the iso-valent doping is often considered not to alter the occupation of the Fe 3 d bands, as illustrated by the density functional theory calculations of BaFe2As2 and BaFe2P2 as well [6, 7]. Yet, surprisingly, it has a similar phase diagram just like the hetero-valent dope cases: with P doping, spin density wave (SDW) is suppressed and superconductivity (SC) emerges [6]. Since P anion is smaller than As anion, and thus introduces internal strain or distortion, i.e. chemical pressure, the superconductivity introduced by iso-valent doping is associ ated with the unprecedented pressure dependence of the supercon ducting transition temperature ( Tc) generally observed in ironbased superconductors [21–24]. In fact, it is the largest am ong all superconductors in both relative and absolute scales. F or example, aTc dependency of 2-4K /GPa and sometimes even 10K/GPa is observed in BaFe 2(As1−xPx)2, LaO1−xFxFeAs, etc. [21, 22]; and an increase of Tc from 0 to above 30 K is observed in BaFe 2As2 and FeSe under pressure [23, 24]. However, these remarkable pressure e ffects are still far from understood. Theoretically, P doping is predicted to alter t h band structure and Fermi surface topology dramatically, co nsidering it changes the electron hopping terms [17, 25]. Par ticularly, it is predicted that thedz2-based band would go above the Fermi energy ( EF), while thedxy-based band would move down belowEF with P doping. Several theories further claim that nodes will appear in the superconducting gap when the dxy hole Fermi pocket disappears [17–19]. Figure 1 examines the dependence of the Fermi surfaces on the P concentration in a series of BaFe 2(As1−xPx)2, where the photoemission intensity maps near EF are shown for twokz’s. The features at the zone center ( Γ and Z) are hole pockets, and those at the zone corner (M and A) are electron pockets [26, 28]. As P doping increases, the size of the hole pockets increase significantly, while the electron pockets show neg ligible doping dependence. This indicates that the P doping could induce extra holes into the system, contradicting to t he ordinary picture of iso-valent doping. To understand such extraordinary P doping e ffect, more de-

Extraordinary Doping Effects on Quasiparticle Scattering and Bandwidth in Iron-Based Superconductors

Z. R. Ye,1 Y. Zhang,1 F. Chen,1 M. Xu,1 J. Jiang,1 X. H. Niu,1 C. H. P. Wen,1 L. Y. Xing,2 X. C. Wang,2 C. Q. Jin,2 B. P. Xie,1, ∗ and D. L. Feng1, † 1State Key Laboratory of Surface Physics, Department of Physics, and Advanced Materials Laboratory, Fudan University, Shanghai 200433, People’s Republic of China 2Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China (Dated: April 29, 2014)

Anisotropic but Nodeless Superconducting Gap in the Presence of Spin-Density Wave in Iron-Pnictide Superconductor NaFe1-xCoxAs

The coexisting regime of spin-density wave (SDW) and superconductivity in iron pnictides represents a novel ground state. We have performed high-resolution angle-resolved photoemission measurements on NaFe1-xCoxAs (x = 0.0175) in this regime and revealed its distinctive electronic structure, which provides some microscopic understandings of its behavior. The SDW signature and the superconducting gap are observed on the same bands, illustrating the intrinsic nature of the coexistence. However, because the SDW and superconductivity are manifested in different parts of the band structure, their competition is nonexclusive. Particularly, we find that the gap distribution is anisotropic and nodeless, in contrast to the isotropic superconducting gap observed in a SDW-free NaFe1-xCoxAs (x = 0.045), which puts strong constraints on theory. DOI.10.1103/PhysRevX.3.011020

Angle-resolved Photoemission Spectroscopy Study on the Surface States of the Correlated Topological Insulator YbB6

YbB6 is recently predicted to be a moderately correlated topological insulator, which provides a playground to explore the interplay between correlation and topological properties. With angle-resolved photoemission spectroscopy, we directly observed almost linearly dispersive bands around the time-reversal invariant momenta and with negligible kz dependence, consistent with odd number of surface states crossing the Fermi level in a Z2 topological insulator. Circular dichroism photoemission spectra suggest that these in-gap states possess chirality of orbital angular momentum, which is related to the chiral spin texture, further indicative of their topological nature. The observed insulating gap of YbB6 is about 100 meV, larger than that found by theoretical calculations. Our results present strong evidence that YbB6 is a correlated topological insulator and provide a foundation for further studies of this promising material.

Photoemission study of the electronic structure and charge density waves of Na₂Ti₂Sb₂O

The electronic structure of Na2Ti2Sb2O single crystal is studied by photon energy and polarization dependent angle-resolved photoemission spectroscopy (ARPES). The obtained band structure and Fermi surface agree well with the band structure calculation of Na2Ti2Sb2O in the non-magnetic state, which indicates that there is no magnetic order in Na2Ti2Sb2O and the electronic correlation is weak. Polarization dependent ARPES results suggest the multi-band and multi-orbital nature of Na2Ti2Sb2O. Photon energy dependent ARPES results suggest that the electronic structure of Na2Ti2Sb2O is rather two-dimensional. Moreover, we find a density wave energy gap forms below the transition temperature and reaches 65 meV at 7 K, indicating that Na2Ti2Sb2O is likely a weakly correlated CDW material in the strong electron-phonon interaction regime.

Weak electronic correlations and absence of heavy-fermion state in KNi2Se2

We have studied the low-lying electronic structure of a new ThCr