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The Relativistic Heavy Ion Collider (RHIC), as the world's first and only polarized proton collider, offers a unique environment in which to study the spin structure of the proton. In order to study the proton's transverse spin structure, the PHENIX experiment at RHIC took data with transversely polarized beams in 2001-02 and 2005, and it has plans for further running with transverse polarization in 2006 and beyond. Results from early running as well as prospective measurements for the future will be discussed.

Despite recent advances in understanding high-transition-temperature (high-T(c)) superconductors, there is no consensus on the origin of the superconducting 'glue': that is, the mediator that binds electrons into superconducting pairs. The main contenders are lattice vibrations (phonons) and spin-excitations, with the additional possibility of pairing without mediators. In conventional superconductors, phonon-mediated pairing was unequivocally established by data from tunnelling experiments. Proponents of phonons as the high-T(c) glue were therefore encouraged by the recent scanning tunnelling microscopy experiments on hole-doped Bi2Sr2CaCu2O8-delta (BSCCO) that reveal an oxygen lattice vibrational mode whose energy is anticorrelated with the superconducting gap energy scale. Here we report high-resolution scanning tunnelling microscopy measurements of the electron-doped high-T(c) superconductor Pr0.88LaCe0.12CuO4 (PLCCO) (T(c) = 24 K) that reveal a bosonic excitation (mode) at energies of 10.5 +/- 2.5 meV. This energy is consistent with both spin-excitations in PLCCO measured by inelastic neutron scattering (resonance mode) and a low-energy acoustic phonon mode, but differs substantially from the oxygen vibrational mode identified in BSCCO. Our analysis of the variation of the local mode energy and intensity with the local gap energy scale indicates an electronic origin of the mode consistent with spin-excitations rather than phonons.

The electronic structure of NaxCoO2 revealed by recent photoemission experiments shows important deviations from band theory predictions. The six small Fermi surface pockets predicted by local-density approximation calculations have not been observed as the associated e'(g) band fails to cross the Fermi level for a wide range of sodium doping concentration x. In addition, significant bandwidth renormalizations of the t(2g) complex have been observed. We show that these discrepancies are due to strong electronic correlations by studying the multiorbital Hubbard model in the Hartree-Fock and strong-coupling Gutzwiller approximation. The quasiparticle dispersion and the Fermi surface topology obtained in the presence of strong local Coulomb repulsion are in good agreement with experiments.

Hybrid systems consisting of graphene and various two-dimensional materials would provide more opportunities for achieving desired electronic and optoelectronic properties. Here, we focus on a superlattice composed of graphene and monolayer MoS2. The geometric and electronic structures of the superlattice have been studied by using density functional theory. The possible stacking models, which are classified into four types, are considered. Our results revealed that all the models of graphene/MoS2 superlattices exhibit metallic electronic properties. Small band gaps are opened up at the K-point of the Brillouin zone for all the four structural models. Furthermore, a small amount of charge transfer from the graphene layer to the intermediate region of C–S layers is found. The band structure and the charge transfer together with the buckling distortion of the graphene layer in the superlattice indicate that the interaction between the stacking sheets in the superlattice is more than just the van der Waals interaction.

Analogs of the high-T c cuprates have been long sought after in transition metal oxides. Because of the strong spin-orbit coupling, the 5d perovskite iridates Sr 2 IrO 4 exhibit a low-energy electronic structure remarkably similar to the cuprates. Whether a superconducting state exists as in the cuprates requires understanding the correlated spin-orbit entangled electronic states. Recent experiments discovered hidden order in the parent and electron-doped iridates, some with striking analogies to the cuprates, including Fermi surface pockets, Fermi arcs, and pseudogap. Here, we study the correlation and disorder effects in a five-orbital model derived from the band theory. We find that the experimental observations are consistent with a d-wave spin-orbit density wave order that breaks the symmetry of a joint twofold spin-orbital rotation followed by a lattice translation. There is a Berry phase and a plaquette spin flux due to spin procession as electrons hop between Ir atoms, akin to the intersite spin-orbit coupling in quantum spin Hall insulators. The associated staggered circulating J eff ¼ 1=2 spin current can be probed by advanced techniques of spin-current detection in spintronics. This electronic order can emerge spontaneously from the intersite Coulomb interactions between the spatially extended iridium 5d orbitals, turning the metallic state into an electron-doped quasi-2D Dirac semimetal with important implications on the possible superconducting state suggested by recent experiments. . The canting of the inplane magnetic moments tracks the θ ≃ 11°staggered IrO 6 octahedra rotation about the c axis [3][4][5][6] due to the strong spin-orbit coupling (SOC). The AFM insulating state arises from a novel interplay between SOC and electron correlation most easily understood near the atomic limit. Ir 4þ has a 5d 5 configuration. The 5 electrons occupy the lower threefold t 2g orbitals separated from the higher twofold e g orbitals by the cubic crystal field Δ c . The strong atomic SOC λ SOC splits the t 2g orbitals into a low-lying J eff ¼ 3=2 spin-orbit multiplet occupied by 4 electrons and a singly occupied J eff ¼ 1=2 doublet. Assuming λ SOC and Δ c are sufficiently large compared to the relevant bandwidths when Sr 2 IrO 4 crystalizes, a single J eff ¼ 1=2 band is half filled and can be driven by a moderate local Coulomb repulsion U to an AFM Mott insulating state [1,2,7]. The nature of the spin-orbit entangled insulating state has been studied using the localized picture based on the J eff ¼ 1=2 pseudospin anisotropic Heisenberg model [7][8][9][10][11], the three-orbital Hubbard model for the t 2g electrons with SOC [12][13][14][15], and the microscopic correlated density functional theory such as the LDA þ U and GGA þ U [1, [16][17][18]. Moreover, carrier doping the AFM insulating state was proposed to potentially realize a 5d t 2g -electron analog of the 3d e g -electron high-T c cuprate superconductors [8,12,13,19,20].In this work, we study the hidden order in both stoichiometric and electron-doped Sr...

We study the correlation effects on the electronic structure and spin density wave order in Fepnictides. Using the multiorbital Hubbard model and Gutzwiller projection, we show that correlation effects are essential to stabilize the metallic spin density wave phase for the intermediate correlation strengths appropriate for pnictides. We find that the ordered moments depend sensitively on the Hund's rule coupling J but weakly on the intraorbital Coulomb repulsion U , varying from 0.3µB to 1.5µB in the range J = 0.3 ∼ 0.8 eV for U = 3 ∼ 4 eV. We obtain the phase diagram and discuss the effects of orbital order and electron doping, the evolution of the Fermi surface topology with the ordered moment, and compare to recent experiments. PACS numbers: 71.27.+a, 74.70.Xa, 74.25.Ha, 74.25.Jb The iron pnictides have emerged recently as another class of high-T c superconductors [1] involving the transition metal d-electrons in addition to the cuprates. In the two most studied pnictide families, the 1111 (e.g. LaO 1−x F x FeAs) and the 122 series (e.g. Ba 1−x K x Fe 2 As 2 ), the iron valence is Fe 2+ . There are six electrons occupying five Fe 3d orbitals. Their direct overlap and via As 4p orbitals produce five energy bands with a total bandwidth around 4eV [2][3][4]. This is comparable to the on-site Coulomb repulsion U = 3 ∼ 4eV [4] typical for transition metals. Thus, the Fe-pnictides are multiorbital systems where the correlation strength is intermediate and comparable to the kinetic energy. In this intermediate correlation regime lies the challenge of the many-body physics responsible for unconventional electronic ground states and emergent phenomena in condensed matter and complex materials.That the correlation effects play an important role in pnictides can be seen from the fact that despite of the orbital degeneracy, the normal state behaves quite incoherently with an enhanced magnetic susceptibility in contrast to conventional Fermi liquids [5]. At low temperatures, the observed quasiparticle dispersion [6,7] exhibits a strong bandwidth reduction due to electron correlations, as shown in a first principle calculation that includes the interaction effects in the Gutzwiller approach [8]. Appropriate treatment of the electron correlation in this intermediate regime, especially its multiorbital nature, is essential for understanding the properties, including the high-T c superconductivity, of these materials.In this paper, we investigate the correlation effects on the spin density wave (SDW) order from which the high-T c superconductivity emerges with sufficient doping. At low temperatures, semi-metallic SDW order develops in the undoped pnictides with an ordering vector Q = (π, 0) connecting the geometric centers of the electron and hole Fermi surface (FS) pockets in the unfolded Brillouin zone (BZ) containing one Fe atom per unit cell [9,10]. The atomic states for Fe 2+ is predominantly S = 2 and S = 1 in the presence of Hund's rule coupling and crystal field splitting. However, the ordered Fe moments are much s...

Superlattice provides a new approach to enrich the class of materials with novel properties. Here, we report the structural and electronic properties of superlattices made with alternate stacking of two-dimensional hexagonal germanene (or silicene) and a MoS2 monolayer using the first principles approach. The results are compared with those of graphene/MoS2 superlattice. The distortions of the geometry of germanene, silicene, and MoS2 layers due to the formation of the superlattices are all relatively small, resulting from the relatively weak interactions between the stacking layers. Our results show that both the germanene/MoS2 and silicene/MoS2 superlattices are manifestly metallic, with the linear bands around the Dirac points of the pristine germanene and silicene seem to be preserved. However, small band gaps are opened up at the Dirac points for both the superlattices due to the symmetry breaking in the germanene and silicene layers caused by the introduction of the MoS2 sheets. Moreover, charge transfer happened mainly within the germanene (or silicene) and the MoS2 layers (intra-layer transfer), as well as some part of the intermediate regions between the germanene (or silicene) and the MoS2 layers (inter-layer transfer), suggesting more than just the van der Waals interactions between the stacking sheets in the superlattices.