arXiv subjects

P. S. Cornaglia

Publications and source records attributed to P. S. Cornaglia.

32 records · Page 2Linked to original sources

arXiv2005

Magnetoconductance through a vibrating molecule in the Kondo regime

The effect of a magnetic field on the equilibrium spectral and transport properties of a single-molecule junction is studied using the numerical renormalization group method. The molecule is described by the Anderson-Holstein model in which a single vibrational mode is coupled to the electron density. The effect of an applied magnetic field on the conductance in the Kondo regime is qualitatively different in the weak and strong electron-phonon coupling regimes. In the former case, the Kondo resonance is split and the conductance is strongly suppressed by a magnetic field $g mu_B B \gtrsim k_BT_K$, with $T_K$ the Kondo temperature. In the strong electron-phonon coupling regime a charge analog of the Kondo effect develops. In this case the Kondo resonance is not split by the field and the conductance in the Kondo regime is enhanced in a broad range of values of $B$.

cond-mat.str-el↗

arXiv2004

Magnetic Moment Formation in Quantum Point Contacts

We study the formation of local magnetic moments in quantum point contacts. Using a Hubbard-like model to describe point contacts formed in a two dimensional system, we calculate the magnetic moment using the unrestricted Hartree approximation. We analyze different type of potentials to define the point contact, for a simple square potential we calculate a phase diagram in the parameter space (Coulomb repulsion - gate voltage). We also present an analytical calculation of the susceptibility to give explicit conditions for the occurrence of a local moment, we present a simple scaling argument to analyze how the stability of the magnetic moment depends on the point contact dimensions.

cond-mat.mes-hall↗

arXiv2004

Many Body Effects on the Transport Properties of Single-Molecule Devices

The conductance through a molecular device including electron-electron and electron-phonon interactions is calculated using the Numerical Renormalization Group method. At low temperatures and weak electron-phonon coupling the properties of the conductance can be explained in terms of the standard Kondo model with renormalized parameters. At large electron-phonon coupling a charge analog of the Kondo effect takes place that can be mapped into an anisotropic Kondo model. In this regime the molecule is strongly polarized by a gate voltage which leads to rectification in the current-voltage characteristics of the molecular junction.

cond-mat.str-el↗

arXiv2004

On the Magnetic Nature of Quantum Point Contacts

We present results for a model that describes a quantum point contact. We show how electron-electron correlations, within the unrestricted Hartree-Fock approximation, generate a magnetic moment in the point contact. Having characterized the magnetic structure of the contact, we map the problem onto a simple one-channel model and calculate the temperature dependence of the conductance for different gate voltages. Our results are in good agreement with experimental results obtained in GaAs devices and support the idea of Kondo effect in these systems.

cond-mat.mes-hall↗

arXiv2004

Quantum transport through a deformable molecular transistor

The linear transport properties of a model molecular transistor with electron-electron and electron-phonon interactions were investigated analytically and numerically. The model takes into account phonon modulation of the electronic energy levels and of the tunnelling barrier between the molecule and the electrodes. When both effects are present they lead to asymmetries in the dependence of the conductance on gate voltage. The Kondo effect is observed in the presence of electron-phonon interactions. There are important qualitative differences between the cases of weak and strong coupling. In the first case the standard Kondo effect driven by spin fluctuations occurs. In the second case, it is driven by charge fluctuations. The Fermi-liquid relation between the spectral density of the molecule and its charge is altered by electron-phonon interactions. Remarkably, the relation between the zero-temperature conductance and the charge remains unchanged. Therefore, there is perfect transmission in all regimes whenever the average number of electrons in the molecule is an odd integer.

cond-mat.mes-hall↗

arXiv2004

Strongly correlated regimes in a double quantum-dot device

The transport properties of a double quantum-dot device with one of the dots coupled to perfect conductors are analyzed using the numerical renormalization group technique and slave-boson mean-field theory. The coupling between the dots strongly influences the transport through the system leading to a non-monotonic dependence of the conductance as a function of the temperature and the magnetic field. For small inter-dot coupling and parameters such that both dots are in the Kondo regime, there is a two-stage screening of the dot's magnetic moments that is reflected in the conductance. In an intermediate temperature regime Kondo correlations develop on one of the dots and the conductance is enhanced. At low temperatures the Kondo effect takes place on the second dot leading to a singlet ground state in which the conductance is strongly suppressed.

cond-mat.mes-hall↗

arXiv2004

Simulations of Dynamical Ordering in Pinned Vortex Systems

We model a vortex system in a sample with bulk pinning and superficial pinning generated by a magnetic decoration. We perform a sequence of finite temperature numerical experiments in which external forces are applied to obtain a dynamically ordered vortex lattice. We analyze the final structures and the behavior of the total energy of the system.

cond-mat.supr-con↗

arXiv2002

STM conductance of Kondo impurities on open and structured surfaces

We study the scanning tunneling microscopy response for magnetic atoms on open and structured surfaces using Wilson's renormalization group. We observe Fano resonances associated with Kondo resonances and interference effects. For a magnetic atom in a quantum corral coupled to the confined surface states, and experimentally relevant parameters, we observe a large confinement induced effect not present in the experiments. These results suggest that the Kondo screening is dominated by the bulk electrons rather than the surface ones.

cond-mat.mes-hall↗

arXiv2002

Transport through quantum dots in mesoscopic circuits

We study the transport through a quantum dot, in the Kondo Coulomb blockade valley, embedded in a mesoscopic device with finite wires. The quantization of states in the circuit that hosts the quantum dot gives rise to finite size effects. These effects make the conductance sensitive to the ratio of the Kondo screening length to the wires length and provide a way of measuring the Kondo cloud. We present results obtained with the numerical renormalization group for a wide range of physically accessible parameters.

cond-mat.mes-hall↗

arXiv2002

Size driven phase transitions in pinned vortex systems

We model a tridimensional vortex system in a sample with square superficial pinning in the top surface and obtain the ground state structures as a function of the sample thickness. Using a simple Frenkel-Kontorova like model and no adjustable parameters, we reproduce the experimental vortex configurations seen in the bottom surface and their range of stability. We find three phases with two transitions between them, including a continuous one from square to distorted hexagonal structure and a discontinuous one from distorted hexagonal to hexagonal structure.

cond-mat.supr-con↗

arXiv2002

Spectral densities of Kondo impurities in nanoscopic systems

We present results for the spectral properties of Kondo impurities in nanoscopic systems. Using Wilson's renormalization group we analyze the frequency and temperature dependence of the impurity spectral density for impurities in small systems that are either isolated or in contact with a reservoir. We have performed a detailed analysis of the structure of the impurity spectral density for energies around the Fermi energy for different Fermi energies relative to the intrinsic structure of the local density of states. We show how the electron confinement energy scales introduce new features in the frequency and temperature dependence of the impurity spectral properties.

cond-mat.mes-hall↗

arXiv2002

Kondo impurities in nanoscopic systems: new confinement-induced regimes

We present results for Kondo impurities in nanoscopic systems. Using exact diagonalization in small clusters and Wilson's renormalization group we analyze an isolated system and a nanoscopic system weakly coupled to a macroscopic reservoir. In the latter case, new regimes not observed in macroscopic homogeneous systems are induced by the confinement of conduction electrons. These new confinement-induced regimes are very different depending on whether the Fermi energy coincides with the energy of a resonant state or lies between two quasi-bound states.

cond-mat.mes-hall↗

arXiv2001

Quasiparticles and c-axis coherent hopping in high T_c superconductors

We study the problem of the low-energy quasiparticle spectrum of the extended t-J model and analyze the coherent hopping between weakly coupled planes described by this model. Starting with a two-band model describing the Cu-O planes and the unoccupied bands associated to the metallic atoms located in between the planes, we obtain effective hopping matrix elements describing the c-axis charge transfer. A computational study of these processes shows an anomalously large charge anisotropy for doping concentrations around and below the optimal doping.

cond-mat.str-el↗

arXiv1999

Freezing transition of the vortex liquid in anisotropic superconductors

We study the solid-liquid transition of a model of pancake vortices in laminar superconductors using a density functional theory of freezing. The physical properties of the system along the melting line are discussed in detail. We show that there is a very good agreement with experimental data in the shape and position of the first order transition in the phase diagram and in the magnitude and temperature dependence of the magnetic induction jump at the transition. We analyze the validity of the Lindemann melting criterion and the Hansen-Verlet freezing criterion. Both criteria are shown to be good to predict the phase diagram in the region where a first order phase transition is experimentally observed.

cond-mat.supr-con↗