P. D. Sacramento

Multichannel Kondo problem and some applications

Abstract We review analytical and numerical results derived from the Bethe ansatz solution of the n-channel Kondo model of arbitrary spin S as a function of temperature, external field, impurity spin S and the number of channels n. Three situations have to be distinguished: (i) If n = 2S the conduction electrons exactly compensate the impurity spin into a singlet at low temperatures, (ii) if n 2S the impurity spin is said to be overcompensated giving rise to critical behaviour. The results are discussed in the context of magnetic impurities, e.g. Fe, Cr and Tm in simple metals, the quadrupolar Kondo effect, an impurity spin embedded in the Takhtajan-Babujian Heisenberg model and electron assisted-tunnelling of an atom in a double-well potential.

One-electron singular branch lines of the Hubbard chain

The momentum and energy dependence of the weight distribution in the vicinity of the one-electron spectral-function singular branch lines of the 1D Hubbard model is studied for all values of the electronic density and on-site repulsion U. To achieve this goal, we use the recently introduced pseudofermion dynamical theory. Our predictions agree quantitatively for the whole momentum and energy bandwidth with the peak dispersions observed by angle-resolved photoelectron spectroscopy in the quasi-1D organic conductor TTF-TCNQ.

Spinon and η-spinon correlation functions of the Hubbard chain

We calculate real-space static correlation functions related to basic entities of the one-dimensional Hubbard model, which emerge from the exact Betheansatz solution. These entities involve complex rearrangements of the original electrons. Basic ingredients are operators related to unoccupied, singly occupied with spin up or spin down and doubly occupied sites. The spatial decay of their correlation functions is determined using an approximate mean-field-like approach based on the Zou-Anderson transformation and DMRG results for the half-filled case. The nature and spatial extent of the correlations between two sites on the Hubbard chain is studied using the eigenstates and eigenvalues of the two-site reduced density matrix. PACS numbers: 71.10.Fd, 71.10.Pm, 73.90.+f

Path integrals of spin-J systems in the holomorphic representation

Abstract A path integral expression for the matrix element and the diagonal representative of the spin- J evolution operator or Boltzmann factor is obtained using Bloch coherent states in the holomorphic representation, yielding the appropriate boundary conditions. Quantum Dyson-Schwinger equations of motion are derived and used as criteria to select the appropriate action. In the case of the diagonal representative the equations of motion satisfy the commutation relations of the spin operators and are the classical limit of the Dyson-Schwinger equations, if we redefine the Lagrangian together with the integration measure. It is shown that in the case of a spin in a time-independent magnetic field the saddle-point approximation for both representatives gives the exact results. We present analytical algorithms to obtain the exponential factor and the prefactor in the saddle-point expansion, without an ad hoc reduction to an effective one-dimensional problem, but keeping instead a 2 × 2 matrix structure. We apply our results to several models.

Finite-energy Landau liquid theory for the one-dimensional Hubbard model: Pseudoparticle energy bands and degree of localization/delocalization

In this paper we consider the one-dimensional Hubbard model and study the deviations from the ground-state values of double occupation which result from creation or annihilation of holons, spinons, and pseudoparticles. These quantum objects are such that all energy eigenstates are described by their occupancy configurations. The band-momentum dependence of the obtained double-occupation spectra provides important information on the degree of localization/delocalization of the real-space lattice electron site distribution configurations associated with the pseudoparticles. We also study the band momentum, on-site electronic repulsion, and electronic density dependence of the pseudoparticle energy bands. The shape of these bands plays an important role in the finite-energy spectral properties of the model. Such a shape defines the form of the lines in the momentum-energy/frequency plane where the peaks and edges of the one-electron and two-electron spectral weight of physical operators are located. Our findings are useful for the study of the one-electron and two-electron spectral-weight distribution of physical operators.

Quantum waveguide theory of Andreev spectroscopy in multiband superconductors: The case of iron pnictides

The problem of Andreev reflection between a normal metal and a multiband superconductor is addressed. The appropriate matching conditions for the wave function at the interface are established on the basis of an extension of quantum waveguide theory to these systems. Interference effects between different bands of the superconductor manifest themselves in the conductance and the case of FeAs superconductors is specifically considered, in the framework of a recently proposed effective two-band model, in the sign-reversed s-wave pairing scenario. Resonant transmission through surface Andreev bound states is found as well as destructive interference effects that produce zeros in the conductance at normal incidence. Both these effects occur at nonzero bias voltage.

Current-induced motion of a domain wall in a magnetic nanowire

The dynamics of current-induced motion of a magnetic domain wall in a quasi-one-dimensional ferromagnet with both easy-axis and easy-plane anisotropy, is studied. We pay a special attention to the case of a sharp domain wall, and calculate the spin torque created by the electric current. The torque has two components, one of which is acting as a driving force for the motion of the domain wall while the other distorts its shape, forcing thus the magnetic moments to deviate from the easy plane.

Electronic structure of the quasi-one-dimensional organic conductor TTF-TCNQ

Abstract We present an investigation of the electronic structure of a prototypical quasi-one-dimensional conductor, the organic compound TTF-TCNQ, by means of angle-resolved photoemission spectroscopy. By comparison with LDA-band structure calculations we identify significant deviations between experiment and theory. We discuss possible origins for these deviations in terms of electronic correlations and surface effects in this material.

Curvature of Levels and Charge Stiffness of One-Dimensional Spinless Fermions

Combining the Bethe ansatz with a functional deviation expansion and using an asymptotic expansion of the Bethe ansatz equations, we compute the curvature of levels

Finite-temperature transport in finite-size Hubbard rings in the strong-coupling limit

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