70 research outputs found
Tuning the resistive switching properties of TiO2-x films
We study the electrical characteristics of TiO2-x-based resistive switching devices fabricated with different oxygen/argon flow ratio during the oxide thin film sputtering deposition. Upon minute changes in this fabrication parameter, three qualitatively different device characteristics were accessed in the same system, namely, standard bipolar resistive switching, electroforming-free devices, and devices with multi-step breakdown. We propose that small variations in the oxygen/ argon flow ratio result in relevant changes of the oxygen vacancy concentration, which is the key parameter determining the resistive switching behavior. The coexistence of percolative or non-percolative conductive filaments is also discussed. Finally, the hypothesis is verified by means of the temperature dependence of the devices in low resistance state.Fil: Ghenzi, Néstor. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área Investigaciones y Aplicaciones no Nucleares; Argentina. Universidad Nacional de San Martín. Escuela de Ciencia y Tecnología; Argentina. CIC nanoGUNE; EspañaFil: Rozenberg, M.J.. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Física; Argentina. Université Paris Sud; FranciaFil: Llopis, R.. CIC nanoGUNE; EspañaFil: Levy, Pablo Eduardo. Comisión Nacional de Energía Atómica. Gerencia del Área Investigaciones y Aplicaciones no Nucleares; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Hueso, Luis E.. Universidad del País Vasco; España. Fundación Vasca para la Ciencia; EspañaFil: Stoliar, Pablo Alberto. Universidad Nacional de San Martín. Escuela de Ciencia y Tecnología; Argentina. CIC nanoGUNE; Españ
Equation of motion approach to the Hubbard model in infinite dimensions
We consider the Hubbard model on the infinite-dimensional Bethe lattice and
construct a systematic series of self-consistent approximations to the
one-particle Green's function, . The first
equations of motion are exactly fullfilled by and the
'th equation of motion is decoupled following a simple set of decoupling
rules. corresponds to the Hubbard-III approximation. We
present analytic and numerical results for the Mott-Hubbard transition at half
filling for .Comment: 10pager, REVTEX, 8-figures not available in postscript, manuscript
may be understood without figure
Melting transition of an Ising glass driven by magnetic field
The quantum critical behavior of the Ising glass in a magnetic field is
investigated. We focus on the spin glass to paramagnet transition of the
transverse degrees of freedom in the presence of finite longitudinal field. We
use two complementary techniques, the Landau theory close to the T=0 transition
and the exact diagonalization method for finite systems. This allows us to
estimate the size of the critical region and characterize various crossover
regimes. An unexpectedly small energy scale on the disordered side of the
critical line is found, and its possible relevance to experiments on metallic
glasses is briefly discussed.Comment: 4 pages, 3 figure
Towards analytical approaches to the dynamical-cluster approximation
I introduce several simplified schemes for the approximation of the
self-consistency condition of the dynamical cluster approximation. The
applicability of the schemes is tested numerically using the
fluctuation-exchange approximation as a cluster solver for the Hubbard model.
Thermodynamic properties are found to be practically indistinguishable from
those computed using the full self-consistent scheme in all cases where the
non-interacting partial density of states is replaced by simplified analytic
forms with matching 1st and 2nd moments. Green functions are also compared and
found to be in close agreement, and the density of states computed using
Pad\'{e} approximant analytic continuation shows that dynamical properties can
also be approximated effectively. Extensions to two-particle properties and
multiple bands are discussed. Simplified approaches to the dynamical cluster
approximation should lead to new analytic solutions of the Hubbard and other
models
Quantum impurity solvers using a slave rotor representation
We introduce a representation of electron operators as a product of a
spin-carry ing fermion and of a phase variable dual to the total charge (slave
quantum rotor). Based on this representation, a new method is proposed for
solving multi-orbital Anderson quantum impurity models at finite interaction
strength U. It consists in a set of coupled integral equations for the
auxiliary field Green's functions, which can be derived from a controlled
saddle-point in the limit of a large number of field components. In contrast to
some finite-U extensions of the non-crossing approximation, the new method
provides a smooth interpolation between the atomic limit and the weak-coupling
limit, and does not display violation of causality at low-frequency. We
demonstrate that this impurity solver can be applied in the context of
Dynamical Mean-Field Theory, at or close to half-filling. Good agreement with
established results on the Mott transition is found, and large values of the
orbital degeneracy can be investigated at low computational cost.Comment: 18 pages, 15 figure
Sensitivity of the Mott Transition to Non-cubic Splitting of the Orbital Degeneracy: Application to NH3 K3C60
Within dynamical mean-field theory, we study the metal-insulator transition
of a twofold orbitally degenerate Hubbard model as a function of a splitting
\Delta of the degeneracy. The phase diagram in the U-\Delta plane exhibits
two-band and one-band metals, as well as the Mott insulator. The correlated
two-band metal is easily driven to the insulator state by a strikingly weak
splitting \Delta << W of the order of the Kondo-peak width zW, where z << 1 is
the metal quasiparticle weight. The possible relevance of this result to the
insulator-metal transition in the orthorhombic expanded fulleride NH3 K3C60 is
discussed.Comment: revtex, 15 pages including 6 ps figures. Submitted to Phys. Rev.
Charge and spin density wave ordering transitions in strongly correlated metals
We study the quantum transition from a strongly correlated metal, with heavy
fermionic quasiparticles, to a metal with commensurate charge or spin density
wave order. To this end, we introduce and numerically analyze a large
dimensionality model of Ising spins in a transverse field, coupled to two
species of fermions; the analysis borrows heavily from recent progress in the
solution of the Hubbard model in large dimensions. At low energies, the Ising
order parameter fluctuations are characterized by the critical exponent , while above an energy scale, , there is a crossover to criticality. We show that is of the order of the width of the
heavy quasiparticle band, and can be made arbitrarily small for a correlated
metal close to a Mott-Hubbard insulator. Therefore, such a correlated metal has
a significant intermediate energy range of behavior, a single
particle spectrum with a narrow quasiparticle band, and well-formed analogs of
the lower and upper Hubbard bands; we suggest that these features are
intimately related in general.Comment: 14 pages, REVTEX 3.0, 2 postscript figure
Semiclassical Analysis of Extended Dynamical Mean Field Equations
The extended Dynamical Mean Field Equations (EDMFT) are analyzed using
semiclassical methods for a model describing an interacting fermi-bose system.
We compare the semiclassical approach with the exact QMC (Quantum Montecarlo)
method. We found the transition to an ordered state to be of the first order
for any dimension below four.Comment: RevTex, 39 pages, 16 figures; Appendix C added, typos correcte
Kondo spin liquid and magnetically long-range ordered states in the Kondo necklace model
A simplified version of the symmetric Kondo lattice model, the Kondo necklace
model, is studied by using a representation of impurity and conduction electron
spins in terms of local Kondo singlet and triplet operators. Within a mean
field theory, a spin gap always appears in the spin triplet excitation spectrum
in 1D, leading to a Kondo spin liquid state for any finite values of coupling
strength (with as hopping and as exchange); in 2D and 3D cubic
lattices the spin gaps are found to vanish continuously around and , respectively, where quantum phase transitions
occur and the Kondo spin liquid state changes into an antiferromagnetically
long-range ordered state. These results are in agreement with variational Monte
Carlo, higher-order series expansion, and recent quantum Monte Carlo
calculations for the symmetric Kondo lattice modelComment: Revtex, four pages, three figures; to be published in Physical Review
B1, 1 July (2000
Fictive Impurity Models: an Alternative Formulation of the Cluster Dynamical Mean Field Method
"Cluster" extensions of the dynamical mean field method to include longer
range correlations are discussed. It is argued that the clusters arising in
these methods are naturally interpreted not as actual subunits of a physical
lattice but as algorithms for computing coefficients in an orthogonal function
expansion of the momentum dependence of the electronic self-energy. The
difficulties with causality which have been found to plague cluster dynamical
mean field methods are shown to be related to the "ringing" phenomenon familiar
from Fourier analysis. The analogy is used to motivate proposals for simple
filtering methods to circumvent them. The formalism is tested by comparison to
low order perturbative calculations and self consistent solutions
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