Electronic Structures and Optical Properties of Partially and Fully Fluorinated Graphene

In this letter we study the electronic structures and optical properties of partially and fully fluorinated graphene by a combination of abinitio G0W0 calculations and large-scale multi-orbital tight-binding simulations. We find that for partially fluorinated graphene, the appearance of paired fluorine atoms is more favorable than unpaired atoms. We also show that different types of structural disorder, such as carbon vacancies, fluorine vacancies, fluorine vacancy-clusters and fluorine armchair- and zigzag-clusters, will introduce different types of midgap states and extra excitations within the optical gap. Furthermore we argue that the local formation of $sp^3$ bonds upon fluorination can be distinguished from other disorder inducing mechanisms which do not destroy the $sp^2$ hybrid orbitals by measuring the polarization rotation of passing polarized light.Comment: Final version appeared in Phys. Rev. Let

Orbital-spin order and the origin of structural distortion in MgTi2_2O4_4

We analyze electronic, magnetic, and structural properties of the spinel compound MgTi$_2$O$_4$ using the local density approximation+U method. We show how MgTi$_2$O$_4$ undergoes to a canted orbital-spin ordered state, where charge, spin and orbital degrees of freedom are frozen in a geometrically frustrated network by electron interactions. In our picture orbital order stabilize the magnetic ground state and controls the degree of structural distortions. The latter is dynamically derived from the cubic structure in the correlated LDA+U potential. Our ground-state theory provides a consistent picture for the dimerized phase of MgTi$_2$O$_4$, and might be applicable to frustrated materials in general.Comment: 6 pages, 6 figure

Sr2_2Cu(PO4_4)2_2: A real material realization of the 1D nearest neighbor Heisenberg chain

We present evidence that crystalline Sr_2Cu(PO_4)_2 is a nearly perfect one-dimensional (1D) spin-1/2 anti-ferromagnetic Heisenberg model (AHM) chain compound with nearest neighbor only exchange. We undertake a broad theoretical study of the magnetic properties of this compound using first principles (LDA, LDA+U calculations), exact diagonalization and Bethe-ansatz methodologies to decompose the individual magnetic contributions, quantify their effect, and fit to experimental data. We calculate that the conditions of one-dimensionality and short-ranged magnetic interactions are sufficiently fulfilled that Bethe's analytical solution should be applicable, opening up the possibility to explore effects beyond the infinite chain limit of the AHM Hamiltonian. We begin such an exploration by examining some extrinsic effects such as impurities and defects

Decorated Shastry-Sutherland lattice in the spin-1/2 magnet CdCu2(BO3)2

We report the microscopic magnetic model for the spin-1/2 Heisenberg system CdCu2(BO3)2, one of the few quantum magnets showing the 1/2-magnetization plateau. Recent neutron diffraction experiments on this compound [M. Hase et al., Phys. Rev. B 80, 104405 (2009)] evidenced long-range magnetic order, inconsistent with the previously suggested phenomenological magnetic model of isolated dimers and spin chains. Based on extensive density-functional theory band structure calculations, exact diagonalizations, quantum Monte Carlo simulations, third-order perturbation theory, as well as high-field magnetization measurements, we find that the magnetic properties of CdCu2(BO3)2 are accounted for by a frustrated quasi-2D magnetic model featuring four inequivalent exchange couplings: the leading antiferromagnetic coupling J_d within the structural Cu2O6 dimers, two interdimer couplings J_t1 and J_t2, forming magnetic tetramers, and a ferromagnetic coupling J_it between the tetramers. Based on comparison to the experimental data, we evaluate the ratios of the leading couplings J_d : J_t1 : J_t2 : J_it = 1 : 0.20 : 0.45 : -0.30, with J_d of about 178 K. The inequivalence of J_t1 and J_t2 largely lifts the frustration and triggers long-range antiferromagnetic ordering. The proposed model accounts correctly for the different magnetic moments localized on structurally inequivalent Cu atoms in the ground-state magnetic configuration. We extensively analyze the magnetic properties of this model, including a detailed description of the magnetically ordered ground state and its evolution in magnetic field with particular emphasis on the 1/2-magnetization plateau. Our results establish remarkable analogies to the Shastry-Sutherland model of SrCu2(BO3)2, and characterize the closely related CdCu2(BO3)2 as a material realization for the spin-1/2 decorated anisotropic Shastry-Sutherland lattice.Comment: 16 pages, 13 figures, 2 tables. Published version with additional QMC dat

Determining the Quark Mixing Matrix From CP-Violating Asymmetries

If the Standard Model explanation of CP violation is correct, then measurements of CP-violating asymmetries in $B$ meson decays can in principle determine the entire quark mixing matrix.Comment: 8 pages (plain TeX), 1 figure (postscript file appended), DAPNIA/SPP 94-06, NSF-PT-94-2, UdeM-LPN-TH-94-18

The Lambda_b lifetime in the light front quark model

The enhancement of the Lambda_b decay width relative to B decay one due to the difference of Fermi motion effects in Lambda_b and B is calculated in the light--front quark model with the simplifying assumption that Lambda_b consists of the heavy quark and light scalar diquark. In order to explain the large deviation from unity in the experimental result for tau(Lambda_b)/tau(B), it is necessary that diquark be light and the ratio of the squares of the Lambda_b and B wave functions at the origin be \le 1.Comment: final journal version to appear in JETP Letter

Role of C in MgC_xNi_3 investigated from first principles

The influence of vacancies in the $C$ sub-lattice of $MgCNi_{3}$, on its structural, electronic and magnetic properties are studied by means of the density-functional based Korringa-Kohn-Rostoker Green's function method formulated in the atomic sphere approximation. Disorder is taken into account by means of coherent-potential approximation. Characterizations representing the change in the lattice properties include the variation in the equilibrium lattice constants, bulk modulus and pressure derivative of the bulk modulus, and that of electronic structure include the changes in the, total, partial and $\mathbf{k}$-resolved density of states. The incipient magnetic properties are studied by means of fixed-spin moment method of alloy theory, together in conjunction with the phenomenological Ginzburg-Landau equation for magnetic phase transition. The first-principles calculations reveal that due to the breaking of the $C$-$Ni$ bonds, some of the $Ni$ 3d states, which were lowered in energy due to strong hybridization, are transfered back to higher energies thereby increasing the itinerant character in the material. The Bloch spectral densities evaluated at the high symmetry points however reveal that the charge redistribution is not uniform over the cubic Brillouin zone, as new states are seen to be created at the $\Gamma$ point, while a shift in the states on the energy scale are seen at other high symmetry points

About the modern "experimental value" of W boson width.

It is shown that the methods which have been used up to now to determine the $W$ width from the $p\bar p$ data confirm the SM predictions for some combinations of various phenomenological parameters, however, they do not give an independent value for the $W$ width. Moreover, the accuracy that could be achieved in future experimental checks of SM predictions for such quantities is limited by effects which require detailed theoretical study.Comment: Latex, 8 pages