C.E.T. Gonçalves da Silva

Local density of states in a disordered chain: A renormalization group approach

Abstract We apply renormalization group techiques to evaluate the local density of phonon states for the isotopically (randomly) disordered linear chain. The method is based on a systematic decimation of atoms in the chain. Numerical studies reveal a richly structured spectrum, in reasonable agreement both with numerical simulations and with exact moments results. This is the first analytic alloy approximation which takes into account potential fluctuations of arbitrary range.

Real space renormalization of the Anderson chain in the alloy approximation

Abstract We study the local density of states for the Anderson chain of hybridized correlated and uncorrelated electronic states within Hubbards alloy approximation. The alloy problem is treated using a novel real space renormalization procedure which yields directly the quantities of interest. We discuss the cases of intra-site and inter-site hybridization, as well as the case of direct f-f hopping.

A Cluster-Bethe-Lattice approach to spin-waves in dilute ferromagnets

Abstract We study the spin-wave spectra of a dilute ferromagnet within the cluster-Bethe-lattice approximation. Short range order effects for the alloy are included. A study of finite size clusters connected at their edges to Bethe lattices of the same coordination number allows us to determine: (i) the stability condition for the magnetic system; (ii) the continuum spin-wave local density of states; and (iii) the existence of localized states below and above the continuum states.

Renormalization group treatment for the electronic spectrum of partially ordered one-dimensional alloys

Abstract A new approach is presented for the calculation of the local density of electronic states in partially ordered binary chains. The method is based on recursion relations for renormalized tight-binding Hamiltonian parameters. Diagonal, off-diagonal and environmental disorder are easily incorporated for all degrees of short-range order. The method takes into account compositional fluctuations of arbitrary range; the states are found to be localized and the calculated densities of states are richly structured. The only approximation is in the configurational averaging over species at decimated sites. The method is computationally very simple, and gives an interpolation scheme for all concentrations and degrees of short-range order, becoming exact in the ordered and dilute limits. Our results are compared with published numerical data, and schemes for improvement are discussed.

Shallow donor impurities in magnetic semiconductors

Abstract We introduce the macroscopic magnetization to treat the localized magnetic moments in a magnetic semiconductor. We obtain a set of coupled equations for the magnetization and the electronic wavefunction of a shallow donor impurity exchange coupled to the localized moments. A variational solution to the equations of motion indicates that near to the critical temperature the wavefunction contracts and the electron becomes more bound. We discuss the relevance of this model for the understanding of the activation energy for conductivity in the paramagnetic phase of EuO.

57Fe Mössbauer investigation of the ternary hydride Pd3FeHx

Abstract Ordered alloys of Pd 3 Fe are shown to readily absorbe hydrogen through electrolytic loading. The resultant ternary hydride phase is observed to retain the fcc structure of Pd 3 Fe with approximately the same lattice constant. The 57 Fe hyperfine field determined by Mossbauer spectroscopy is found to be 30% smaller in the hydride compared to Pd 3 Fe. The reduction appears to be associated with a perturbation of the Pd moment by hydrogen. The results suggest the occupation of one type of interstitial site in the structure. The absence of the site in disordered Pd 3 Fe would explain the much smaller hydrogen capacity observed for this alloy.

Magnetic susceptibility of FeSb2 : A quasi-magnetic semiconductor

Abstract We study the magnetic susceptibility of FeSb 2 within a model of highly correlated valence band states and conduction electrons Coulomb interaction. Inter-band Coulomb and Hund type couplings are also considered. The latter are strong due to the d character of both valence and conduction bands edges. We suggest that the high susceptibility of FeSb 2 ( X ≈ 6×10 −4 emu/mole) may be further enhanced by the addition of selected impurities, leading to a magnetic ground state.

Magnetic excitations near a surface of high Miller indices: Heisenberg ferromagnet

Abstract We discuss the spectral density of one-magnon states for a f.c.c. cubic ferromagnet with a (105) surface. No accounts is taken of surface relaxation and reconstruction. This surface contains terraces, with six inequivalent atoms, separated by steps of half a lattice parameter height. Our results indicate the presence of surface resonances and intrinsic surface states . The low temperature behavior of the magnetization is also presented.

Spectral density of surface states for TiO and VO: (001) surface☆

Abstract We present a calculation of the spectral density of states for crystals of TiO and VO with (001) cleaved surfaces. The transfer matrix formalism is employed, with a hamiltonian including the O 2p and the metal 3d orbitals. Slater-Koster parameters obtained by Mattheiss (1972) from a bulk calculation are used. The results are obtained at three special points in the 2D Brillouin zone for three different (001) planes: surface plane, plane immediately below it and bulk plane. In the neighborhood of the point M there is an intrinsic surface state and a surface resonance due to the hybridization between ligand and metal orbitals. The surface state has mixed e g and p z symmetry along the Σ symmetry line and lies above the Fermi level. No surface relaxation or reconstruction is considered.

Ferromagnetic alloys: A Cluster-Bethe-lattice approach☆

Abstract We apply the Cluster-Bethe-lattice method to calculate the local density of one-magnon states for a binary substitutional ferromagnetic alloy. Short range order effects for the alloy are included. Simple clusters of one atom and one atom plus its nearest neighbors are studied. We show that, for the latter type of cluster, there are localized excitations the energies of which fall outside the continuum of excitations.