The electronic structure of the high-Tc superconductor Tl2Ba2CuO 6+δ is studied by ARPES. For a very overdoped Tc = 30 K sample, the Fermi surface consists of a single large hole pocket centered at (π,π) and is approaching a topological transition. Although a superconducting gap with d x 2 −y 2 symmetry is tentatively identified, the quasiparticle evolution with momentum and binding energy exhibits a marked departure from the behavior observed in under and optimally doped cuprates. The relevance of these findings to scattering, many-body, and quantum-critical phenomena is discussed.

Spin-orbit coupling has been conjectured to play a key role in the low-energy electronic structure of Sr 2 RuO 4 . By using circularly polarized light combined with spin-and angle-resolved photoemission spectroscopy, we directly measure the value of the effective spin-orbit coupling to be 130 AE 30 meV. This is even larger than theoretically predicted and comparable to the energy splitting of the d xy and d xz;yz orbitals around the Fermi surface, resulting in a strongly momentum-dependent entanglement of spin and orbital character in the electronic wavefunction. As demonstrated by the spin expectation value h ⃗ s k · ⃗ s −k i calculated for a pair of electrons with zero total momentum, the classification of the Cooper pairs in terms of pure singlets or triplets fundamentally breaks down, necessitating a description of the unconventional superconducting state of Sr 2 RuO 4 in terms of these newly found spin-orbital entangled eigenstates. DOI: 10.1103/PhysRevLett.112.127002 PACS numbers: 74.25.Jb, 74.20.Rp, 74.70.Pq, 79.60.-i After a flurry of experimental activity [1-5], Sr 2 RuO 4 has become a hallmark candidate for spin-triplet chiral p-wave superconductivity, the electronic analogue of superfluid 3 He [6][7][8]. However, despite the apparent existence of such a pairing, some later experiments [9-11] do not fully support this conclusion, as they cannot be explained within a theoretical model using spin-triplet superconductivity alone [12]. A resolution might come from the inclusion of spin-orbit (SO) coupling, which has been conjectured to play a key role in the normal-state electronic structure [13] and may be important when describing superconductivity as well. By mixing the canonical spin eigenstates, the relativistic SO interaction might play a fundamental role beyond simply lifting the degeneracy of competing pairing states [13][14][15][16][17].Thus far, the experimental study of SO coupling's effects on the electronic structure of Sr 2 RuO 4 has been limited to the comparison of band calculations against angle-resolved photoemission spectroscopy (ARPES) [13,[18][19][20][21] -no success has been obtained in observing experimentally either the strength of SO coupling or its implications for the mixing between spin and orbital descriptions. Here we probe this directly by performing spin-resolved ARPES [22], with circularly polarized light: by using the angular momentum inherent in each photon-along with electricdipole selection rules [23]-to generate spin-polarized photoemission from the SO mixed states. Combined with a novel spin-and orbitally-resolved ab initio based tightbinding (TB) modeling of the electronic structure [24], these results demonstrate the presence of a nontrivial spinorbital entanglement over much of the Fermi surface, i.e., with no simple way of factoring the band states into the spatial and spin sectors. Most importantly, the analysis of the corresponding Cooper pair spin eigenstates establishes the need for a description of the unconventional superconductivity of Sr 2 RuO 4 beyond...

By a combined angle-resolved photoemission spectroscopy and density functional theory study, we discover that the surface metallicity is polarity driven in SmB6. Two surface states, not accounted for by the bulk band structure, are reproduced by slab calculations for coexisting B6 and Sm surface terminations. Our analysis reveals that a metallic surface state stems from an unusual property, generic to the (001) termination of all hexaborides: the presence of boron 2p dangling bonds, on a polar surface. The discovery of polarity-driven surface metallicity sheds new light on the 40-year old conundrum of the low-temperature residual conductivity of SmB6, and raises a fundamental question in the field of topological Kondo insulators regarding the interplay between polarity and nontrivial topological properties.

We study Bi2Se3 by polarization-dependent angle-resolved photoemission spectroscopy (ARPES) and density-functional theory slab calculations. We find that the surface state Dirac fermions are characterized by a layer-dependent entangled spin-orbital texture, which becomes apparent through quantum interference effects. This explains the discrepancy between the spin polarization from spin-resovled ARPES -ranging from 20 to 85% -and the 100% value assumed in phenomenological models. It also suggests a way to probe the intrinsic spin texture of topological insulators, and to continuously manipulate the spin polarization of photoelectrons and photocurrents all the way from 0 to ±100% by an appropriate choice of photon energy, linear polarization, and angle of incidence.PACS numbers: 71.10.Pm, 73.20.At, 73.22.Gk Topological insulators (TIs) define a new state of matter in which strong spin-orbit interaction (SOI) leads to the emergence of a metallic topological surface state (TSS) formed by spin-nondegenerate Dirac fermions [1][2][3][4][5][6]. To capture the physics of TIs, a spin-momentum locking with 100% spin polarization is usually assumed for the TSS in time-reversal invariant models [3][4][5]. The successful realization of topological insulating behavior in quantum wells [7,8] and crystalline materials such as Bi 2 Se 3 [9-11] brings us closer to the practical implementation of theoretical concepts built upon novel topological properties. However, the large discrepancy in the degree of TSS spin polarization determined for Bi 2 Se 3 by spin-resolved ARPES (angle-resolved photoemission spectroscopy) -ranging from 20 to 85% [12-16] -challenges the hypothesis of a 100% spin polarization for real TIs. First principle density-functional theory (DFT) also indicates that the TSS spin polarization in members of the Bi 2 X 3 material family (X=Se, Te) can be substantially reduced from 100% [17,18]. Based on general symmetry arguments, it was shown that the spin polarization direction of photoelectrons in spin-resolved ARPES can be very different from that of the TSS wavefunction [19]. However, the role played by the intrinsic properties of the TSS wavefunction in defining the highest spin polarization that could be achieved, for instance in d.c. and photoinduced electrical currents, has remained elusive.We report here that the TSS many-layer-deep extension into the material's bulk -in concert with strong SOI -gives rise to a layer-dependent, entangled spin-orbital texture of the Dirac fermions in Bi 2 Se 3 . A remarkable consequence, specifically exploited in this study, is that one can gain exquisite sensitivity to the internal structure of the TSS wavefunction, Ψ TSS , via quantum interference effects in ARPES. The spin-orbital texture is captured directly in the linear-polarization dependence of the ARPES intensity maps in momentum space, and can be fully resolved with the aid of ab-initio DFT slabcalculations. This has also major consequences in the interpretation of spin-resolved ARPES results, explicitly solving ...

Single-particle spectroscopic probes, such as scanning tunneling and angle-resolved photoemission spectroscopy (ARPES), have provided us with crucial insights into the complex electronic structure of the high-T c cuprates, in particular for the under and optimally doped regimes where high-quality crystals suitable for surface-sensitive experiments are available. Conversely, the elementary excitations on the heavily overdoped side of the phase diagram remain largely unexplored. Important breakthroughs could come from the study of Tl 2 Ba 2 CuO 6+δ (Tl2201), a structurally simple system whose doping level can be tuned from optimal to extreme overdoping by varying the oxygen content. Using a self-flux method and encapsulation, we have grown single crystals of Tl2201, which were then carefully annealed under controlled oxygen partial pressures. Their high quality and homogeneity are demonstrated by narrow rocking curves and superconducting transition widths. For higher dopings, the crystals are orthorhombic, a lattice distortion stabilized by O interstitials in the TlO layer. These crystals have enabled the first successful ARPES study of both normal and superconducting-state electronic structure in Tl2201, allowing a direct comparison with the Fermi surface from magnetoresistance and the gap from thermal conductivity experiments. This establishes Tl2201 as the first high-T c cuprate for which a surfacesensitive single-particle spectroscopy and a comparable bulk transport technique have arrived at quantitative agreement on a major feature such as the normal state Fermi surface. The momentum dependence of the ARPES lineshape reveals, however, an unexpected phenomenology: in contrast to the case of under-and optimally-doped cuprates, quasiparticles are sharp near (π, 0), the antinodal region where the gap is maximum, and broad at (π/2, π/2), the nodal region where the gap vanishes. This reversed quasiparticle anisotropy past optimal doping, and its relevance to scattering, many-body, and quantum-critical phenomena in the high-T c cuprates, are discussed.