arXiv subjects

F. Romeo

Publications and source records attributed to F. Romeo.

35 records · Page 2Linked to original sources

arXiv2011

Impurity effects on Fabry-Perot physics of ballistic carbon nanotubes

We present a theoretical model accounting for the anomalous Fabry-Perot pattern observed in the ballistic conductance of a single-wall carbon nanotubes. Using the scattering field theory, it is shown that the presence of a limited number of impurities along the nanotube can be identified by a measurement of the conductance and their position determined. Impurities can be made active or silent depending on the interaction with the substrate via the back-gate. The conceptual steps for designing a bio-molecules detector are briefly discussed.

cond-mat.mes-hall↗

arXiv2011

Scattering theory of magnetic/superconducting junctions with spin active interfaces

We formulate a generalized scattering field theory a la Buttiker describing particles transport in magnetic/superconducting heterostructures. The proposed formalism, characterized by a four- component spinorial wavefunction of the Bogoliubov de Gennes theory, allows to describe the spin flipping phenomena induced by noncollinear magnetizations in the scattering region. As a specific application of the theory, we analyze the conductance, the magnetoresistance and the generation of spin-torque produced by an applied voltage in a spin-valve system. Quantum size effects and quantum beating patterns both in the conductance and in the spin-torque are carefully described.

cond-mat.mes-hall↗

arXiv2010

Parasitic pumping currents in an interacting quantum dot

We analyze the charge and spin pumping in an interacting dot within the almost adiabatic limit. By using a non-equilibrium Green's function technique within the time-dependent slave boson approximation, we analyze the pumped current in terms of the dynamical constraints in the infinite-U regime. The results show the presence of parasitic pumping currents due to the additional phases of the constraints. The behavior of the pumped current through the quantum dot is illustrated in the spin-insensitive and in the spin-sensitive case relevant for spintronics applications.

cond-mat.mes-hall↗

arXiv2010

Memory effects in adiabatic quantum pumping with parasitic nonlinear dynamics

The charge current adiabatically pumped through a mesoscopic region coupled to a classical variable obeying a nonlinear dynamics is studied within the scattering matrix approach. Due to the nonlinearity in the dynamics of the variable, a hysteretic behavior of the pumping current can be observed for specific characteristics of the pumping cycle. The steps needed to build a quantum pump working as a memory device are discussed together with a possible experimental implementation.

cond-mat.mes-hall↗

arXiv2010

Adiabatic quantum pumping, magnification effects and quantum size effects of spin-torque in magnetic tunnel junctions

We study the adiabatic quantum pumping and quantum size effects of spin-torque in a magnetic tunnel junction within a scattering matrix approach. Quantum size effects are predicted in the presence of a dc bias as a function of the thickness of the normal metal layer inserted between two magnetic layers and of the fixed magnetic layer. In the presence of ac voltages, the results for the spin-torque show a peculiar magnification effect and advantages of spin-torque pumping in actual devices are also discussed.

cond-mat.mes-hall↗

arXiv2009

Quantum pumping in deformable quantum dots

The charge current pumped adiabatically through a deformable quantum dot is studied within the Green's function approach. Differently from the non-deformable case, the current shows an undefined parity with respect to the pumping phase ϕ. The unconventional current-phase relation, analyzed in the weak pumping regime, is due to a dynamical phase shift ϕ_D caused by the elastic deformations of the central region (classical phonons). The role of the quality factor Q of the oscillator, the effects induced by a mechanical resonance and the implications for current experiments on molecular systems are also discussed.

cond-mat.mes-hall↗

arXiv2009

Spin-torque generation by dc or ac voltages in magnetic layered structures

A general expression of the current induced spin torque in a magnetic layered structure in the presence of external dc or ac voltages is derived in the framework of the scattering matrix approach. A detailed analysis is performed for a magnetic-nonmagnetic-magnetic trilayer connected to external leads in the presence of dc voltage bias in the ballistic regime. Alternatively, the possibility of producing spin torque by means of the adiabatic ac modulation of external gate voltages (quantum pumping) is proposed and discussed.

cond-mat.mes-hall↗

arXiv2009

Adiabatic quantum pumping and rectification effects in interacting quantum dots

We derive a formula describing the adiabatically pumped charge through an interacting quantum dot within the scattering matrix and Green's function approach. We show that when the tunneling rates between the leads and the dot are varied adiabatically in time, both in modulus and phase, the current induced in the dot consists of two terms, the pumping current and a rectification-like term. The last contribution arises from the time-derivative of the tunneling phase and can have even or odd parity with respect to the pumping phase φ. The rectification-like term is also discussed in relation to some recent experiments in quantum-dots.

cond-mat.mes-hall↗

arXiv2008

Quantum pumping and rectification effects in Aharonov-Bohm-Casher ring-dot systems

We study the time-dependent transport of charge and spin through a ring-shaped region sequentially coupled to a weakly interacting quantum dot in the presence of an Aharonov-Bohm flux and spin-orbit interaction. The time-dependent modulation of the spin-orbit interaction, or of the corresponding Aharonov-Casher flux, together with the modulation of the dot-level induces an electrically pumped spin current even in absence of a charge current. The results beyond the adiabatic regime show that an additional rectification current proportional to \cos(ϕ), being ϕthe relative phase between the time varying parameters, is generated. We discuss the relevance of such term in connection with recent experiments on out-of-equilibrium quantum dots.

cond-mat.mes-hall↗

arXiv2008

The Aharonov-Bohm-Casher ring-dot as a flux-tunable resonant tunneling diode

A mesoscopic ring subject to the Rashba spin-orbit interaction and sequentially coupled to an interacting quantum dot, in the presence of Aharonov-Bohm flux, is proposed as a flux tunable tunneling diode. The analysis of the conductance by means of the nonequilibrium Green's function technique, shows an intrinsic bistability at varying the Aharonov-Bohm flux when 2U > πΓ, U being the charging energy on the dot and Γthe effective resonance width. The bistability properties are discussed in connection with spin-switch effects and logical storage device applications.

cond-mat.mes-hall↗

arXiv2007

Persistent Currents in Superconducting Quantum Interference Devices

Starting from the reduced dynamical model of a two-junction quantum interference device, a quantum analog of the system has been exhibited, in order to extend the well known properties of this device to the quantum regime. By finding eigenvalues of the corresponding Hamiltonian operator, the persistent currents flowing in the ring have been obtained. The resulting quantum analog of the overdamped two-junction quantum interference device can be seen as a supercurrent qubit operating in the limit of negligible capacitance and finite inductance.

cond-mat.mes-hall↗

arXiv2007

Phase rigidity breaking in open Aharonov-Bohm ring coupled to a cantilever

The conductance and the transmittance phase shifts of a two-terminal Aharonov-Bohm (AB) ring are analyzed in the presence of mechanical displacements due to coupling to an external can- tilever. We show that phase rigidity is broken, even in the linear response regime, by means of inelastic scattering due to phonons. Our device provides a way of observing continuous variation of the transmission phase through a two-terminal nano-electro-mechanical system (NEMS). We also propose measurements of phase shifts as a way to determine the strength of the electron-phonon coupling in NEMS.

cond-mat.mes-hall↗

arXiv2007

Persistent spin and charge currents and magnification effects in open ring conductors subject to Rashba coupling

We analyze the effect of Rashba spin-orbit coupling and of a local tunnel barrier on the persistent spin and charge currents in a one-dimensional conducting Aharonov-Bohm (AB) ring symmetrically coupled to two leads. First, as an important consequence of the spin-splitting, it is found that a persistent spin current can be induced which is not simply proportional to the charge current. Second, a magnification effect of the persistent spin current is shown when one tunes the Fermi energy near the Fano-type antiresonances of the total transmission coefficient governed by the tunnel barrier strength. As an unambiguous signature of spin-orbit coupling we also show the possibility to produce a persistent pure spin current at the interference zeros of the transmittance. This widens the possibilities of employing mesoscopic conducting rings in phase-coherent spintronics applications.

cond-mat.mes-hall↗

arXiv2006

Zero-conductance resonances and spin-filtering effects in ring conductors subject to Rashba coupling

We investigate the effect of Rashba spin-orbit coupling and of a tunnel barrier on the zero conduc- tance resonances appearing in a one-dimensional conducting Aharonov-Bohm (AB) ring symmet- rically coupled to two leads. The transmission function of the corresponding one-electron problem is derived within the scattering matrix approach and analyzed in the complex energy plane with focus on the role of the tunnel barrier strength on the zero-pole structure characteristic of trans- mission (anti)resonances. The lifting of the real conductance zeros is related to the breaking of the spin-reversal symmetry and time-reversal symmetry of Aharonov-Casher (AC)and AB rings, as well as to rotational symmetry breaking in presence of a tunnel barrier. We show that the polarization direction of transmitted electrons can be controlled via the tunnel barrier strength and discuss a novel spin-filtering design in one-dimensional rings with tunable spin-orbit interaction.

cond-mat.mes-hall↗

arXiv2006

A simple model of energy expenditure in human locomotion

A simple harmonic oscillator model is proposed to describe the mechanical power involved in human locomotion. In this framework, by taking into account the anthropometric parameters of a standard individual, we are able to calculate the speed-power curves in human walking. The proposed model accounts for the well known Margaria's law in which the cost ofthe human running (independent from the speed) is fixed to 1 Kcal/(Kg Km). The model includes the effects of a gentle slope (either positive or negative) and the effect due to the mechanical response of the walking surface. The model results obtained in the presence of a slope are in qualitative agreement with the experimental data obtained by A. Leonardi et al.

q-bio.QM↗

arXiv2006

Pumping in a mesoscopic ring with Ahronov-Casher effect

We investigate parametric pumping of spin and charge currents in a mesoscopic ring interrupted by a tunnel barrier in presence of Aharonov-Casher (AC) effect and Aharonov-Bohm (AB) flux along the axis of the same ring. Generation of a dc current is achieved by tuning the tunnel barrier strength and modulating in time either a radial(transverse) electric field or the magnetic flux. A pure spin current is generated by the interplay of breaking spin reversal symmetry, due to AC effect, and time-reversal symmetry breaking, intrinsic in parametric pumping procedure. We analyze the conditions for operating the AB-AC ring as a pure spin pump useful in spintronics and discuss generalization of our results to Rashba-gate-controlled rings.

cond-mat.mes-hall↗

arXiv2005

Time Evolution of Non-Lethal Infectious Diseases: A Semi-Continuous Approach

A model describing the dynamics related to the spreading of non-lethal infectious diseases in a fixed-size population is proposed. The model consists of a non-linear delay-differential equation describing the time evolution of the increment in the number of infectious individuals and depends upon a limited number of parameters. Predictions are in good qualitative agreement with data on influenza.

q-bio.PE↗