Monolayer graphene exhibits many spectacular electronic properties, with superconductivity being arguably the most notable exception. It was theoretically proposed that superconductivity might be induced by enhancing the electron-phonon coupling through the decoration of graphene with an alkali adatom superlattice [Profeta G, Calandra M, Mauri F (2012) Nat Phys 8(2):131-134]. Although experiments have shown an adatom-induced enhancement of the electron-phonon coupling, superconductivity has never been observed. Using angle-resolved photoemission spectroscopy (ARPES), we show that lithium deposited on graphene at low temperature strongly modifies the phonon density of states, leading to an enhancement of the electron-phonon coupling of up to λ ≃ 0.58. On part of the graphene-derived π*-band Fermi surface, we then observe the opening of a Δ ≃ 0.9-meV temperature-dependent pairing gap. This result suggests for the first time, to our knowledge, that Li-decorated monolayer graphene is indeed superconducting, with T c ≃ 5.9 K.graphene | superconductivity | ARPES

We study Na2IrO3 by angle-resolved photoemission spectroscopy, optics, and band structure calculations in the local-density approximation (LDA). The weak dispersion of the Ir 5d-t(2g) manifold highlights the importance of structural distortions and spin-orbit (SO) coupling in driving the system closer to a Mott transition. We detect an insulating gap Δ(gap)≃340  meV which, at variance with a Slater-type description, is already open at 300 K and does not show significant temperature dependence even across T(N)≃15  K. An LDA analysis with the inclusion of SO and Coulomb repulsion U reveals that, while the prodromes of an underlying insulating state are already found in LDA+SO, the correct gap magnitude can only be reproduced by LDA+SO+U, with U=3  eV. This establishes Na2IrO3 as a novel type of Mott-like correlated insulator in which Coulomb and relativistic effects have to be treated on an equal footing.

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...

The electronic structure of Bi(2)Se(3) is studied by angle-resolved photoemission and density functional theory. We show that the instability of the surface electronic properties, observed even in ultrahigh-vacuum conditions, can be overcome via in situ potassium deposition. In addition to accurately setting the carrier concentration, new Rashba-like spin-polarized states are induced, with a tunable, reversible, and highly stable spin splitting. Ab initio slab calculations reveal that these Rashba states are derived from 5-quintuple-layer quantum-well states. While the K-induced potential gradient enhances the spin splitting, this may be present on pristine surfaces due to the symmetry breaking of the vacuum-solid interface.

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 ...