First-principles studies of the electron-phonon coupling in graphene predict a high coupling strength for the sigma band with. values of up to 0.9. Near the top of the s band,. is found to be approximate to 0.7. This value is consistent with the recently observed kinks in the s band dispersion by angle-resolved photoemission. While the photoemission intensity from the s band is strongly influenced by matrix elements due to sublattice interference, these effects differ significantly for data taken in the first and neighboring Brillouin zones. This can be exploited to disentangle the influence of matrix elements and electron-phonon coupling. A rigorous analysis of the experimentally determined complex self-energy using Kramers-Kronig transformations further supports the assignment of the observed kinks to strong electron-phonon coupling and yields a coupling constant of 0.6(1), in excellent agreement with the calculations.

Magnetic multilayers with perpendicular anisotropy and an interfacial Dzyaloshinskii-Moriya interaction (DMI) contain chiral domain walls and skyrmions that are promising for applications. Here, we measure the temperature dependence of the DMI in Pt/CoFeB/Ir and Pt/CoB/Ir multilayers by means of static domain imaging. First, the temperature dependences of saturation magnetization (MS), exchange stiffness (A), and intrinsic perpendicular anisotropy (Ku) are determined. Then the demagnetized domain pattern in each multilayer is imaged by wide-field Kerr microscopy in the temperature range 9-290 K, and the characteristic domain period at each temperature is determined. We calculate the DMI constant D from an analytical expression for the domain wall energy density that treats the multilayer as a uniform medium. Scaling laws for Ku and D with the magnetization are established from the experiments. While the scaling of Ku is consistent with Callen-Callen theory, we find that the scaling of D is like that of A predicted theoretically (<^>1.8).

In this paper we provide a theoretical basis for the problem of Griffiths-McCoy singularities close to the quantum critical point for magnetic ordering in U and Ce intermetallics. We show that the competition between the Kondo effect and Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction can be expressed in Hamiltonian form, and that the dilution effect due to alloying leads to a quantum percolation problem driven by the number of magnetically compensated moments. We argue that the exhaustion paradox proposed by Nozieres is explained when the RKKY interaction is taken into account. We revisit the one- and two-impurity Kondo problem, and show that in the presence of particle-hole symmetry-breaking operators the system Rows to a line of fixed points characterized by coherent (clusterlike) motion of the spins. Moreover, close to the quantum critical point, clusters of magnetic atoms can quantum mechanically tunnel between different states either via the anisotropy of the RKKY interaction or by what we call the cluster Kondo effect. We calculate explicitly from the microscopic Hamiltonian the parameters which appear in all the response functions. We show that there is a maximum number N-c of spins in the clusters such that, above this number, tunneling ceases to occur. These effects lead to a distribution of cluster Kondo temperatures which vanishes for finite clusters, and therefore leads to strong magnetic response. From these results we propose a dissipative quantum droplet model wt-rich describes the critical behavior of metallic magnetic systems. This model predicts that in the paramagnetic phase there is a crossover temperature T*, above which Griffiths-McCoy-like singularities with magnetic susceptibility chi (T)proportional to T-1+lambda and specific heat C-V(T)proportional toT(lambda), with lambda <1, appear. Below T*, however, a regime dominated by dissipation occurs, with divergences stronger than power law: (T)proportional to1/[Tln(1/T)] and C-V(T)proportional to 1/ln(2)(1/T). We estimate that T* is exponentially small with N-c. Our results should he applicable to a broad class of metallic magnetic systems which are described by the Kondo lattice Hamiltonian.

By using Fourier-transform scanning tunneling spectroscopy we measure the interference patterns produced by the impurity scattering of confined Dirac quasiparticles in epitaxial graphene nanoflakes. Upon comparison of the experimental results with tight-binding calculations of realistic model flakes, we show that the characteristic features observed in the Fourier-transformed local density of states are related to scattering between different transverse modes (subbands) of a graphene nanoflake and allow direct insight into the gapped electronic spectrum of graphene. We also observe a strong reduction of quasiparticle lifetime which is attributed to the interaction with the underlying substrate. In addition, we show that the distribution of the on-site energies at flower defects leads to an effectively broken pseudospin selection rule, where intravalley backscattering is allowed.

The wave transport properties of two-dimensional stealthy hyperuniform distributions of rigid scatterers embedded in a waveguide are experimentally characterized for scalar waves in airborne audible acoustics. The nonresonant nature of the scatterers allows us to directly link these properties to the geometric distribution of points through the structure factor. The transport properties are analyzed as a function of the stealthiness x of their hyperuniform point pattern and compared to those of a disordered material in the diffusive regime, which are characterized by the Ohm's law through both the classical mean free path and the corrected mean free path by the collective approximation considering the effects of correlation. Different scattering regimes are theoretically and numerically identified showing transparent regions, isotropic band gaps, and anisotropic scattering depending on x. The robustness of these scattering regimes to losses, which are unavoidable in audible acoustics is experimentally unveiled.

Slave-boson calculations have been carried out in the three-band t-J model for the high-T-c, cuprates, with the inclusion of coupling to oxygen breathing-mode phonons. Phonon-induced Van Hove nesting leads to a phase separation between a hole-doped domain and a (magnetic) domain near half filling, with long-range Coulomb forces limiting the separation to a nanoscopic scale. Strong correlation effects pin the Fermi level close to, but not precisely at the Van Hove singularity (VHS), which can enhance the tendency to phase separation. The resulting dispersions have been calculated, both in the uniform phases and in the phase separated regime. In the latter case, distinctly different dispersions are found for large, random domains and for regular (static) striped arrays, and a hypothetical form is presented for dynamic striped arrays. The doping dependence of the latter is found to provide an excellent description of photoemission and thermodynamic experiments on pseudogap formation in underdoped cuprates. In particular, the multiplicity of observed gaps is explained as a combination of flux phase plus charge-density-wave (CDW) gaps along with a superconducting gap. The largest gap is associated with VHS nesting. The apparent smooth evolution of this gap with doping masks a crossover from CDW-like effects near optimal doping to magnetic effects (Aux phase) near half filling. A crossover from large Fermi surface to hole pockets with increased underdoping is found. In the weakly overdoped regime, the CDW undergoes a quantum phase transition (T-CDW-->0), which could be obscured by phase separation.