Spin-Dependent ππ

We present a detailed analysis of the electronic properties of graphene/Eu/Ni(111). By using angle- and spin-resolved photoemission spectroscopy and ab initio calculations, we show that the intercalation of Eu in the graphene/Ni(111) interface gives rise to a gapped freestanding dispersion of the ππ^{*} Dirac cones at the K[over ¯] point with an additional lifting of the spin degeneracy due to the mixing of graphene and Eu states. The interaction with the magnetic substrate results in a large spin-dependent gap in the Dirac cones with a topological nature characterized by a large Berry curvature and a spin-polarized Van Hove singularity, whose closeness to the Fermi level gives rise to a polaronic band.

Scaling of the Fano Effect of the In-Plane Fe-As Phonon and the Superconducting Critical Temperature in Ba_{1-x}K_{x}Fe_{2}As_{2}.

By means of infrared spectroscopy, we determine the temperature-doping phase diagram of the Fano effect for the in-plane Fe-As stretching mode in Ba_{1-x}K_{x}Fe_{2}As_{2}. The Fano parameter 1/q^{2}, which is a measure of the phonon coupling to the electronic particle-hole continuum, shows a remarkable sensitivity to the magnetic and structural orderings at low temperatures. Most strikingly, at elevated temperatures in the paramagnetic tetragonal state we observe a linear correlation between 1/q^{2} and the superconducting critical temperature T_{c}. Based on theoretical calculations and symmetry considerations, we identify the relevant interband transitions that are coupled to the Fe-As mode. In particular, we show that a sizable xy orbital component at the Fermi level is fundamental for the Fano effect and, thus, possibly also for the superconducting pairing.

Real-Time Observation of Phonon-Mediated σ-π Interband Scattering in MgB_{2}.

In systems having an anisotropic electronic structure, such as the layered materials graphite, graphene, and cuprates, impulsive light excitation can coherently stimulate specific bosonic modes, with exotic consequences for the emergent electronic properties. Here we show that the population of E_{2g} phonons in the multiband superconductor MgB_{2} can be selectively enhanced by femtosecond laser pulses, leading to a transient control of the number of carriers in the σ-electronic subsystem. The nonequilibrium evolution of the material optical constants is followed in the spectral region sensitive to both the a- and c-axis plasma frequencies and modeled theoretically, revealing the details of the σ-π interband scattering mechanism in MgB_{2}.

Unconventional Hall effect in pnictides from interband interactions.

We calculate the Hall transport in a multiband system with a dominant interband interaction between carriers having electron and hole character. We show that this situation gives rise to an unconventional scenario, beyond the Boltzmann theory, where the quasiparticle currents dressed by vertex corrections acquire the character of the majority carriers. This leads to a larger (positive or negative) Hall coefficient than what may be expected on the basis of the carrier balance, with a marked temperature dependence. Our results explain the puzzling measurements in pnictides and provide a more general framework for transport properties in multiband materials.

Gate tunable infrared phonon anomalies in bilayer graphene.

We observe a giant increase of the infrared intensity and a softening of the in-plane antisymmetric phonon mode E(u) ( approximately 0.2 eV) in bilayer graphene as a function of the gate-induced doping. The phonon peak has a pronounced Fano-like asymmetry. We suggest that the intensity growth and the softening originate from the coupling of the phonon mode to the narrow electronic transition between parallel bands of the same character, while the asymmetry is due to the interaction with the continuum of transitions between the lowest hole and electron bands. The growth of the peak can be interpreted as a "charged-phonon" effect observed previously in organic chain conductors and doped fullerenes, which can be tuned in graphene with the gate voltage.

Fermi-surface shrinking and interband coupling in iron-based pnictides.

Recent measurements of the Fermi surface with de Haas-van Alphen oscillations in LaFePO showed a shrinking of the Fermi pockets with respect to first-principle calculations, suggesting an energy shift of the hole and electrons bands with respect to local-density approximations. We show that this shift is a natural consequence of the strong particle-hole asymmetry of electronic bands in pnictides, and that it provides an indirect experimental evidence of a dominant interband scattering in these systems.

Topological change of the Fermi surface in low-density Rashba gases: application to superconductivity.

In this Letter we show how, for small values of the Fermi energy compared to the spin-orbit splitting of Rashba type, a topological change of the Fermi surface leads to an effective reduction of the dimensionality in the electronic density of states in the low charge density regime. We investigate its consequences on the onset of the superconducting instability. We show that the superconducting critical temperature is significantly tuned in this regime by the spin-orbit coupling. We suggest that materials with strong spin-orbit coupling are good candidates for enhanced superconductivity.

Spin-Hall conductivity in electron-phonon coupled systems.

We derive the ac spin-Hall conductivity sigmasH(omega) of two-dimensional spin-orbit coupled systems interacting with dispersionless phonons of frequency omega0. For the linear Rashba model, we show that the electron-phonon contribution to the spin-vertex corrections breaks the universality of sigmasH(omega) at low frequencies and provides a nontrivial renormalization of the interband resonance. On the contrary, in a generalized Rashba model for which the spin-vertex contributions are absent, the coupling to the phonons enters only through the self-energy, leaving the low-frequency behavior of sigmasH(omega) unaffected by the electron-phonon interaction.

Polaronic and nonadiabatic phase diagram from anomalous isotope effects.

Isotope effects (IEs) are powerful tools to probe directly the dependence of many physical properties on lattice dynamics. In this Letter we investigate the onset of anomalous IEs in the spinless Holstein model by employing the dynamical mean field theory. We show that the isotope coefficients of the electron effective mass and of the dressed phonon frequency are sizable also far away from the polaronic crossover and mark the importance of nonadiabatic lattice fluctuations. We draw a nonadiabatic phase diagram in which we identify a novel crossover, not related to polaronic features, where the IEs attain their largest anomalies.

High T(c) superconductivity in MgB2 by nonadiabatic pairing.

The evidence for the key role of the sigma bands in the electronic properties of MgB2 points to the possibility of nonadiabatic effects in the superconductivity of these materials. These are governed by the small value of the Fermi energy due to the vicinity of the hole doping level to the top of the sigma bands. We show that the nonadiabatic theory leads to a coherent interpretation of T(c) = 39 K and the boron isotope coefficient alphaB = 0.30 without invoking very large couplings and it naturally explains the role of the disorder on T(c). It also leads to various specific predictions for the properties of MgB2 and for the material optimization of these types of compounds.

Nonadiabatic channels in the superconducting pairing of fullerides

We show the intrinsic inconsistency of the conventional phonon mediated theory of superconductivity in relation to the observed properties of Rb3C60. The recent, highly accurate measurement of the carbon isotope coefficient alpha(C) = 0.21, together with the high value of T(c) (30 K) and the very small Fermi energy E(F) (0.25 eV), unavoidably implies the opening of nonadiabatic channels in the superconducting pairing. We estimate these effects and show that they are actually the key elements for the high value of T(c) in these materials compared to the very low values of graphite intercalation compounds.

Potential role of two novel elastase-like enzymes in processing pro-transforming growth factor-alpha.

Transforming growth factor-alpha (TGF-alpha) is a mitogenic peptide produced by tumor cells and by virally and chemically transformed cells in culture. TGF-alpha is almost certainly derived from its precursor protein (pro-TGF-alpha) by limited proteolysis, but the physiologically relevant processing enzyme(s) is(are) unknown. We now report that oncogenically transformed rat liver epithelial cells (known to secrete TGF-alpha) and Schwann cells in culture transfected with SV40 T-antigen (which are now reported to express mRNA encoding pro-TGF-alpha) contain membrane associated, neutral pH, serine proteinases which are elastase-like in their substrate specificity, but elastase is not known to be associated with these cell types. In both cell types, the enzyme is associated with a subcellular fraction enriched for microsomes and plasma membranes. Furthermore, the enzyme appears to be specifically induced 4-fold in the transformed epithelial cells as compared with the level of enzyme present in the nontransformed parental cells. The enzymes have been purified approximately 20,000-fold to near homogeneity (50-60 units/mg) and are virtually identical with regard to their molecular weights (38,000) and other physiochemical properties. Results obtained with numerous synthetic peptide substrates show the enzymes prefer nonpolar residues such as Ala and Val in the P1 and P2 positions, but promiscuity of cleavage specificity observed with long-chain peptide substrates is attributed to the absence of structure in these peptides. Thus, although these enzymes may be involved in processing pro-TGF-alpha at the plasma membrane of the cell, it is just as likely that these enzymes play other physiological roles in the parental and/or transformed cells and that there is no specific endoproteolytic processing enzyme of pro-TGF-alpha.

Characterization of liver epithelial cells transfected with myc and/or ras oncogenes.

While many liver tumors contain activated myc and ras oncogenes, the mechanisms by which these genes contribute to cellular transformation is poorly understood. Activated versions of the cellular oncogenes, c-myc and/or c-H-ras were transfected into normal rat liver epithelial cells to identify cellular pathways that are altered in the cells containing the oncogenes. The results of these and other investigations indicate that the biological properties associated with the transfection of c-myc include immortalization, reduced contact inhibition of growth, activation of phospholipase A2-mediated pathways, increased sensitivity to transformation with a ras gene, and greatly increased sensitivity to growth factors. The biological properties associated with the transfection of the ras gene include morphological transformation, anchorage-independent growth, tumorigenicity, increased phosphatidylinositol metabolism, the induction of growth-factor processing and secretion, which leads to (exogenous) growth factor-independent tumor growth, and a marked resistance to normal inhibitors of growth such as TGF-beta. It is proposed that the complementary actions of the myc and ras genes in cellular transformation may be related to the ras-induced secretion of autocrine growth factors by cells sensitized to their effects by the myc gene. The increased stimulus for growth coupled to a ras-induced insensitivity to growth inhibitors may lead to clonal expansion of these cells and tumor development.