arXiv subjects

F. Guinea

Publications and source records attributed to F. Guinea.

At least 217 records · Page 12Linked to original sources

arXiv1998

Surface electronic Structure and magnetic properties of doped manganites

The electronic structure and magnetic properties of La_{1-x}A_{x}MnO_3 are investigated. It is assumed that, at the outermost layer, the environment of the Mn ions does not have cubic symmetry. The e_g orbitals are split and the double exchange mechanism is weakened. The charge state of the Mn ions is modified, and the magnetic ordering of the spins tends to be antiferromagnetic. The surface magnetization and the dependence of the transport properties through the resulting surface barrier on applied magnetic field and temperature is analyzed.

cond-mat.str-el↗

arXiv1998

Marginal Fermi liquid behavior from 2d Coulomb interaction

A full, nonperturbative renormalization group analysis of interacting electrons in a graphite layer is performed, in order to investigate the deviations from Fermi liquid theory that have been observed in the experimental measures of a linear quasiparticle decay rate in graphite. The electrons are coupled through Coulomb interactions, which remain unscreened due to the semimetallic character of the layer. We show that the model flows towards the noninteracting fixed-point for the whole range of couplings, with logarithmic corrections which signal the marginal character of the interaction separating Fermi liquid and non-Fermi liquid regimes.

cond-mat.str-el↗

arXiv1998

Intrinsic frustration effects in anisotropic superconductors

Lattice distortions in which the axes are locally rotated provide an intrinsic source of frustration in anisotropic superconductors. A general framework to study this effect is presented. The influence of lattice defects and phonons in $d$ and $s+d$ layered superconductors is studied.

cond-mat.supr-con↗

arXiv1998

Superconductivity, Josephson coupling and order parameter symmetry in striped cuprates

We consider a renormalization group study of the problem of coupled stripes of holes in cuprates. We use a model of a mesh of horizontal and vertical stripes and study the problemof superconductivity via the Josephson coupling. We discuss the evolution of the Fermi surface with doping and temperature, the existence of Luttinger and/or Fermi liquid behavior, the presence of pre-formed Cooper pairs and the symmetry and magnitude of the superconducting order parameter.

cond-mat.supr-con↗

arXiv1998

Phase Diagram of High-T_c Superconductors from a Field Theory Model

The renormalization group approach to correlated fermions is used to determine the phase diagram of the oxide cuprates modeled by the t-t' Hubbard model at the Van Hove filling. Spin-dependent interactions give rise to instabilities corresponding to ferromagnetic, antiferromagnetic and d-wave superconducting phases. Antiferromagnetism and d-wave superconductivity arise from the same interactions, and compete in the same region of parameter space.

cond-mat.supr-con↗

arXiv1998

Nanosized superconducting constrictions

Nanowires of lead between macroscopic electrodes are produced by means of an STM. Magnetic fields may destroy the superconductivity in the electrodes, while the wire remains in the superconducting state. The properties of the resulting microscopic Josephson junctions are investigated.

cond-mat.mes-hall↗

arXiv1998

Hole Pairs in the Two-Dimensional Hubbard Model

The interactions between holes in the Hubbard model, in the low density, intermediate to strong coupling limit, are investigated. Dressed spin polarons in neighboring sites have an increased kinetic energy and an enhanced hopping rate. Both effects are of the order of the hopping integral and lead to an effective attraction at intermediate couplings. Our results are derived by systematically improving mean field calculations. The method can also be used to derive known properties of isolated spin polarons.

cond-mat.str-el↗

arXiv1997

Spin flip scattering in magnetic junctions

Processes which flip the spin of an electron tunneling in a junction made up of magnetic electrodes are studied. It is found that: i) Magnetic impurities give a contribution which increases the resistance and lowers the magnetoresistance, which saturates at low temperatures. The conductance increases at high fields. ii) Magnon assisted tunneling reduces the magnetoresistance as $T^{3/2}$, and leads to a non ohmic contribution to the resistance which goes as $V^{3/2}$, iii) Surface antiferromagnetic magnons, which may appear if the interface has different magnetic properties from the bulk, gives rise to $T^2$ and $V^2$ contributions to the magnetoresistance and resistance, respectively, and, iv) Coulomb blockade effects may enhance the magnetoresistance, when transport is dominated by cotunneling processes.

cond-mat.mes-hall↗

arXiv1997

e-h Coherence and Charging Effects in Ultrasmall Metallic Grains

We consider a model for electron tunneling between a pair of ultrasmall metallic grains. Under appropriate circumstances, non-equilibrium final state effects can strongly enhance tunneling and produce electron-hole coherence between the grains. The model displays a quantum phase transition between a Coulomb blockaded state to a coherent state exhibiting subohmic tunneling conductance. The critical state of the junction exhibits a temperature independent resistance of order $h/e^2$. Finally we discuss the possible relevance to granular materials and quantum dots. In particular, similarities between the quantum transition in our model and the metal-insulator transition in granular wires observed by Herzog et al. are described in detail.

cond-mat.str-el↗

arXiv1997

Non Equilibrium Electronic Distribution in Single Electron Devices

The electronic distribution in devices with sufficiently small diemnsions may not be in thermal equilibrium with their surroundings. Systems where the occupancies of electronic states are solely determined by tunneling processes are analyzed. It is shown that the effective temperature of the device may be higher, or lower, than that of its environment, depending on the applied voltage and the energy dependence of the tunneling rates. The I-V characteristics become asymmetric. Comparison with recent experiments is made.

cond-mat.mes-hall↗

arXiv1997

Superconducting, ferromagnetic and antiferromagnetic phases in the t-t' Hubbard model

We apply a renormalization group approach to the determination of the phase diagram of the t-t' Hubbard model at the Van Hove filling, as function of t'/t, for small values of U/t. The model presents ferromagnetic, antiferromagnetic and d-wave superconducting phases. Antiferromagnetism and d-wave superconductivity arise from the same interactions, and compete in the same region of parameter space.

cond-mat.str-el↗

arXiv1997

Instability of Anisotropic Fermi Surfaces in Two Dimensions

The effect of strong anisotropy on the Fermi line of a system of correlated electrons is studied in two space dimensions, using renormalization group techniques. Inflection points change the scaling exponents of the couplings, enhancing the instabilities of the system. They increase the critical dimension for non Fermi liquid behavior, from 1 to 3/2. Assuming that, in the absence of nesting, the dominant instability is towards a superconducting ground state, simple rules to discern between d-wave and extended s-wave symmetry of the order parameter are given.

cond-mat.str-el↗

arXiv1997

Instabilities of the Hubbard chain in a magnetic field

We find and characterize the instabilities of the repulsive Hubbard chain in a magnetic field by studing all response functions at low frequency ωand arbitrary momentum. The instabilities occur at momenta which are simple combinations of the (U=0) σ=\uparrow ,\downarrow Fermi points, \pm k_{Fσ}. For finite values of the on-site repulsion U the instabilities occur for single σelectron adding or removing at momenta \pm k_{Fσ}, for transverse spin-density wave (SDW) at momenta \pm 2k_F (where 2k_F=k_{F\uparrow}+k_{F\downarrow}), and for charge-density wave (CDW) and SDW at momenta \pm 2k_{F\uparrow} and \pm 2k_{F\downarrow}. While at zero magnetic field removing or adding single electrons is dominant, the presence of that field brings about a dominance for the transverse \pm 2k_F SDW over all the remaining instabilities for a large domain of $U$ and density n values. We go beyond conformal-field theory and study divergences which occur at finite frequency in the one-electron Green function at half filling and in the transverse-spin response function in the fully-polarized ferromagnetic phase.

cond-mat.str-el↗

arXiv1996

Linear stability analysis of the Hele-Shaw cell with lifting plates

The first stages of finger formation in a Hele-Shaw cell with lifting plates are investigated by means of linear stability analysis. The equation of motion for the pressure field (growth law) results to be that of the directional solidification problem in some unsteady state. At the beginning of lifting the square of the wavenumber of the dominant mode results to be proportional to the lifting rate (in qualitative agreement with the experimental data), to the square of the length of the cell occupied by the more viscous fluid, and inversely proportional to the cube of the cell gap. This dependence on the cell parameters is significantly different of that found in the standard cell.

cond-mat.stat-mech↗

arXiv1996

Point-contact spectroscopy on URu$_2$Si$_2$

Tunnel and point contact experiments have been made in a URu$_2$Si$_2$ single crystal along the c-axis. The experiments were performed changing temperature and contact size in a low temperature scanning tunneling microscope. A resonance develops at the Fermi level at $T\sim 60$ K. This resonance splits and becomes asymmetric when the 17.5 K phase transition is crossed. These results are consistent with the existence of Kondo like bound states of the U$^{4+}$ ionic configurations and the conduction electrons. Below the transition, these configurations are split by the development of quadrupolar ordering. The peak separation can be interpreted as a direct measurement of the order parameter. Measurements on a policrystalline UAu_2Si_2$ sample are also reported, with a comparative study of the behavior of both materials.

cond-mat↗

arXiv1996

Ferromagnetism in the two dimensional t-t' Hubbard model at the Van Hove density

Using an improved version of the projection quantum Monte Carlo technique, we study the square-lattice Hubbard model with nearest-neighbor hopping t and next-nearest-neighbor hopping t', by simulation of lattices with up to 20 X 20 sites. For a given R=2t'/t, we consider that filling which leads to a singular density of states of the noninteracting problem. For repulsive interactions, we find an itinerant ferromagnet (antiferromagnet) for R=0.94 (R=0.2). This is consistent with the prediction of the T-matrix approximation, which sums the most singular set of diagrams.

cond-mat↗

arXiv1996

Flux flow resistivity and vortex viscosity of high-Tc films

The flux flow regime of high-T$_{\rm c}$ samples of different normal state resistivities is studied in the temperature range where the sign of the Hall effect is reversed. The scaling of the vortex viscosity with normal state resistivity is consistent with the Bardeen-Stephen theory. Estimates of the influence of possible mechanisms suggested for the sign reversal of the Hall effect are also given.

supr-con↗

arXiv1996

Unconventional quasiparticle lifetime in graphite

The influence of electron-electron scattering on quasiparticle lifetimes in graphite is calculated. In the limit when the Fermi surface is reduced to isolated points in the Brillouin Zone, the suppression of screening leads to non Fermi liquid behavior. The inverse lifetime increases linearly with energy, in agreement with recent experiments. Similar features should also be present in narrow gap semiconductors, and in carbon nanotubes.

cond-mat↗