The Weak Localization for the Alloy-Type Anderson Model on a Cubic Lattice

We consider alloy type random Schrödinger operators on a cubic lattice whose randomness is generated by the sign-indefinite single-site potential. We derive Anderson localization for this class of models in the Lifshitz tails regime, i.e. when the coupling parameter λ is small, for the energies E≤?Cλ ².     

Role of electron correlation effects in δ-Pu and “115”-Pu-based unconventional superconductors

Electronic structure calculations combining the local density approximation with an exact diagonalization of the Anderson impurity model show intermediate 5f⁵–5f⁶-valence ground state and delocalization of the 5f⁵ multiplet of the Pu atom 5f-shell in PuCoGa₅, and δ-Pu. The 5f-local magnetic moment is compensated by a moment formed in the surrounding cloud of conduction electrons. For PuCoGa₅ and δ-Pu the compensation is complete and the Anderson impurity ground state is a singlet. This can be important for analyses of the superconducting pairing mechanism in PuCoGa₅. It is likely that in PuCoGa₅ unconventional d-wave superconductivity is mediated by the 5f-states valence fluctuations, rather than antiferromagnetic fluctuations.     

Validity of the Anderson and Hubbard models for the description of metallic Ce and cerium heavy fermion compounds

The importance of the inclusion of inter-site f-f hybridization in electron structure calculations for metallic Ce and cerium heavy fermion compounds was studied. We demonstrate that for heavy-fermion systems such as cerium compound CeCu₂Si₂ f-f hybridization can be neglected and Anderson model application is well justified. On another hand for cerium metal f-f hybridization is strong enough to provide the contribution to hybridization function comparable to hybridization between 4f and itinerant electrons. We argue that in the case of Ce only the most general Hamiltonian combining the Hubbard and Anderson models should be used.     

Magnetic-nonmagnetic transitions in intermediate-valence systems

We investigate the coexistence of ferromagnetism and intermediate valence in 4f-systems, which have ferromagnetic ground states in the normal valence phase and fluctuate between a magnetic (J0) and a nonmagnetic (J=0) configuration in the intermediate valence phase. We use the well-knowns-f model, which is extended by a hybridization term and an intraatomicf-Coulomb-interaction, for systems with ferromagnetics-f exchange. The alloy analogy of this model is solved within a CPA-scheme. We inspect the stability of the localf-momentm _f , the valencen _f and relevant quasiparticle densities of states in terms of the Bloch bandwidthW (closely related to external pressure!), thef-level positionE _f (alloying!), and the hybridization matrix elementV.     

Sign phase transition and directed paths in random media

We consider a lattice model which corresponds to the high temperature expansions of disordered Ising and Heisenberg models and to the deeply localized regime of the disordered Anderson model. The spin correlation functions for the Ising and Heisenberg model and the amplitude of electron tunneling for the Anderson model exhibit a “sign phase transition.” At small concentration x of scatterers with a negative scattering amplitude these quantities have predictable signs while at large x their signs are unpredictable. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 4, 283–288 (25 August 1996) Published in English in the original Russian Journal. Edited by J. R. Anderson.     

Spectroscopic investigation of 4f correlations in CeAg; comparison between LDA and many-body descriptions

High resolution photoemission and bremsstrahlung isochromat spectroscopy experiments have been carried out on CeAg and LaAg. From a comparison between these two compounds, we extract the cerium 4f spectral function, we compare it with band structure calculations and with many-body model simulations based on the single-impurity Anderson Hamiltonian. We show that the localized 4f charge estimated from previous band structure calculations cannot be identified with the 4f occupation number deduced from the Anderson model.     

Anomalies in Conductance and Localization Length of Disordered Ladders

We discuss the conditions under which an anomaly occurs in conductance and localization length of Anderson model on a lattice. Using the ladder Hamiltonian and analytical calculation of average conductance we find the set of resonance conditions which complements the π-coupling rule for anomalies. We identify those anomalies that might vanish due to the symmetry of the lattice or the distribution of the disorder. In terms of the dispersion relation it is known from strictly one-dimensional model that the lowest order (i.e., the most strong) anomalies satisfy the equation E(k)=E(3k). We show that the anomalies of the generalized model studied here are also the solutions of the same equation with modified dispersion relation.     

One particle excitation spectrum of the Kondo-lattice

The renormalized time ordered perturbation approach for the (impurity -) Anderson model with respect to hybridization is combined with a random walk treatment of coherent scattering in order to calculate the one particle excitation spectrum of an Anderson lattice in the Kondo regime. It is found that the Kondo resonance at the Fermi-level splits into two narrow peaks. With decreasing temperature a gap develops in between these peaks due to coherence effects. These results furnish a more rigorous basis for phenomenological theories explaining experimental data in Kondo lattice systems like CeCu₂Si₂.     

Decay constants of heavy mesons from a QCD relativistic potential model

We compute masses and leptonic decay constants of charmed and beauty pseudoscalar and vector mesons by using a QCD-inspired relativistic potential model. We obtain results in agreement with the available experimental data; as for the pseudoscalar meson decay constants, we predict f_D=1.38f_π,f_Ds=1.51f_π,f_B=1.75f_π,f_Bs=1.86f _π.     

Quasiparticle-phonon model calculations on doubly even Zn and Ge nuclei

The doubly even isotopes of Zn and Ge have been investigated in a model in which two quasiparticle excitations, constructed in a (0f _7/2), 1p _3/2, 0f _5/2, 1P _1/2, 0g _9/2 configuration space, are coupled with quadrupole vibrations of the core, viz.⁴⁰Ca or⁵⁶Ni. The 0f _7/2 orbit is excluded in case of a⁵⁶Ni core. The spectra, except for low-lying excited 0⁺ states, could be reproduced reasonably well. The calculatedE2 strengths and quadrupole moments are in fair agreement with experiment.     

Interpretation of neutron magnetic scattering in the Kondo lattices YbPd2Si2 and YbAgCu4

We present an interpretation of published neutron inelastic scattering spectra in the Kondo lattices YbPd₂Si₂ and YbAgCu₄ obtained in terms of the Anderson impurity model, describing the hybridisation of the 4f Yb electrons with the band electrons, and also including the crystal electric field interaction. In YbPd₂Si₂, the tetragonal crystal field parameters were determined. In YbAgCu₄ the crystal field interaction was taken to exist by analogy with the isoelectronic compound YbAuCu₄ where it has been identified. Both compounds can be described by a Kondo temperature,T ₀=60 K and a Yb valency very close to 3.     

Singlet-triplet Hamiltonian for spin excitations in a Kondo insulator

In the nonsymmetric version of the periodic Anderson model for a Kondo insulator, an effective singlet-triplet Hamiltonian Ĥ _s−t with indirect antiferromagnetic f-f exchange is constructed, which makes it possible to analyze the dynamic magnetic susceptibility χ _f (k, ω) of f electrons. Hamiltonian Ĥ _s−t is used to describe the experimentally observed dispersion of the three-level spin excitation spectrum in YbB₁₂. A distinguishing feature of this analysis is the introduction of small-radius singlet and triplet collective f-d excitations that form low- and high-lying spin bands during motion over the lattice.     

Evaluation of asymmetric Mössbauer spectra of a Fe-41.3 at.% Ni invar alloy

Following a model of Billard and Chamberod the asymmetric Mössbauer spectra of a Fe-41.3 at.% Ni invar alloy have been quantitatively interpreted. In the temperature range from about 77 K up to the Curie temperature for each spectrum four parameter values have been obtained. These are the mean hyperfine field H₀, the magnetic field difference h₁ caused by the isotropic contribution of one first Fe-neighbour atom, the corresponding anisotropic difference h₂ and the strength of the anisotropic quadrupole interaction w. A physical interpretation of these parameters and their temperature dependence has been worked out. The last parameter shows an especially unexpected dependence which is strongly connected with the thermal expansion of the lattice. A mechanism seems to exist by which small changes of the lattice constant cause an enhanced variation of w.     

Long-range coulomb interaction of electrons of 4f orbitals in impurity centers Yb3+: KZnF3, CsCaF3, and Sm3+: CaF2

Expressions for calculating the matrix elements of the Coulomb interaction of f electrons of the isolated ion with an infinite crystal lattice have been obtained. The contribution of this interaction to the parameters of the crystal field in impurity centers Yb³⁺: KZnF₃, CsCaF₃, and Sm³⁺: CaF₂ has been calculated.     

Fluid friction in incompressible laminar convection: Reynolds' analogy revisited for variable fluid properties

The Reynolds' analogy between the Stanton number (St) and the skin friction coefficient (c_f) is popularly believed to hold when St increases with increasing c_f, for simple situations. In this investigation, the validity of Reynolds' analogy between St and c_f for micro-convection of liquids with variations in fluid properties is re-examined. It is found that the Sieder-Tate's property-ratio method for obtaining Nusselt number corrections is theoretically based on the validity of Reynolds' analogy. The inverse dependence of Reynolds number and skin friction coefficient is the basis for validity of the Reynolds' analogy, in convective flows with fluid property variations. This leads to the unexpected outcome that Reynolds' analogy now results in St increasing with decreasing c_f. These results and their analyses indicate that the validity of Reynolds' analogy is based on deeper foundations, and the well-known validity criterion is a special case.     

Functional integral approach to the single impurity Anderson model

Recently, a functional integral representation was proposed by Weller [1], in which the fermionic fields strictly satisfy the constraint of no double occupancy at each lattice site. This is achieved by introducing spin dependent Bose fields. The functional integral method is applied to the single impurity Anderson model both in the Kondo and mixed-valence regime. Thef-electron Green's function and susceptibility are calculated using an Ising-like representation for the Bose fields. We discuss the difficulty to extract a spectral function from the knowledge of the imaginary time Green's function. The results are compared with NCA calculations.