arXiv subjects

F. M. Souza

Publications and source records attributed to F. M. Souza.

27 records · Page 2Linked to original sources

arXiv2009

Fano-Kondo Spin Filter

We study spin-dependent conductance in a system composed of a ferromagnetic (FM) Scanning Tunneling Microscope (STM) tip coupled to a metallic host surface with an adatom. The Kondo resonance is taken into account via the Doniach-Sunjic spectral function. For short lateral tip-adatom distances and due to the interplay between Kondo physics, quantum interfering effects and the ferromagnetism of the tip, a spin-splitting of the Fano-Kondo line shape arises in the conductance. A strong enhancement of the Fano-Kondo profile for the majority spin component of the FM tip is observed. When the tip is placed on the adatom, this gives a conductance 100 % polarized for a particular range of bias voltage. The system thus can be used as a powerful generator of spin polarized currents.

cond-mat.str-el↗

arXiv2008

Spin-polarized current and shot noise in the presence of spin flip in a quantum dot via nonequilibrium Green's functions

Using non-equilibrium Green functions we calculate the spin-polarized current and shot noise in a ferromagnet--quantum-dot--ferromagnet (FM-QD-FM) system. Both parallel (P) and antiparallel (AP) magnetic configurations are considered. Coulomb interaction and coherent spin-flip (similar to a transverse magnetic field) are taken into account within the dot. We find that the interplay between Coulomb interaction and spin accumulation in the dot can result in a bias-dependent current polarization $\wp$. In particular, $\wp$ can be suppressed in the P alignment and enhanced in the AP case depending on the bias voltage. The coherent spin-flip can also result in a switch of the current polarization from the emitter to the collector lead. Interestingly, for a particular set of parameters it is possible to have a polarized current in the collector and an unpolarized current in the emitter lead. We also found a suppression of the Fano factor to values well below 0.5.

cond-mat.mes-hall↗

arXiv2008

Spin-Polarized STM for a Kondo adatom

We investigate the bias dependence of the tunneling conductance between a spin-polarized (SP) scanning tunneling microscope (STM) tip and the surface conduction states of a normal metal with a Kondo adatom. Quantum interference between tip-host metal and tip-adatom-host metal conduction paths is studied in the full range of the Fano parameter $q$. The spin-polarized STM gives rise to a splitting of the Kondo peak and asymmetry in the zero-bias anomaly depending on the lateral tip-adatom distance. For increasing lateral distances, the Kondo peak-splitting shows a strong suppression and the spin-polarized conductance exhibits the standard Fano-Kondo profile.

cond-mat.str-el↗

arXiv2008

Optical spectra of a quantum dot in a microcavity in the nonlinear regime

The optical emission spectrum of a quantum dot in strong coupling with the single mode of a microcavity is obtained in the nonlinear regime. We study how exciton-exciton interactions alter the emission spectrum of the system, bringing the linear Rabi doublet into a multiplet structure that is strongly dependent on the cavity-exciton energy detuning. We emphasise how nonlinearity can be used to evidence the genuine quantum nature of the coupling by producing satellites peaks of the Rabi doublet that originate from the quantized energy levels of the interactions.

cond-mat.mes-hall↗

arXiv2008

Spin polarized transport driven by square voltage pulses in a quantum dot system

We calculate current, spin current and tunnel magnetoresistance (TMR) for a quantum dot coupled to ferromagnetic leads in the presence of a square wave of bias voltage. Our results are obtained via time-dependent nonequilibrium Green function. Both parallel and antiparallel lead magnetization alignments are considered. The main findings include a wave of spin accumulation and spin current that can change sign as the time evolves, spikes in the TMR signal and a TMR sign change due to an ultrafast switch from forward to reverse current in the emitter lead.

cond-mat.mes-hall↗

arXiv2007

Transient Charging and Discharging of Spin-polarized Electrons in a Quantum Dot

We study spin-polarized transient transport in a quantum dot coupled to two ferromagnetic leads subjected to a rectangular bias voltage pulse. Time-dependent spin-resolved currents, occupations, spin accumulation, and tunneling magnetoresistance (TMR) are calculated using both nonequilibrium Green function and master equation techniques. Both parallel and antiparallel leads' magnetization alignments are analyzed. Our main findings are: a dynamical spin accumulation that changes sign in time, a short-lived pulse of spin polarized current in the emitter lead (but not in the collector lead), and a dynamical TMR that develops negative values in the transient regime. We also observe that the intra-dot Coulomb interaction can enhance even further the negative values of the TMR.

cond-mat.mes-hall↗

arXiv2007

Quantum Dot as a Spin--Current Diode

We report a study of spin dependent transport in a system composed of a quantum dot coupled to a normal metal lead and a ferromagnetic lead (NM-QD-FM). We use the master equation approach to calculate the spin-resolved currents in the presence of an external bias and an intra-dot Coulomb interaction. We find that for a range of positive external biases (current flow from the normal metal to the ferromagnet) the current polarization $\wp=(I_\uparrow-I_\downarrow)/(I_\uparrow+I_\downarrow)$ is suppressed to zero, while for the corresponding negative biases (current flow from the ferromagnet to the normal metal) $\wp$ attains a relative maximum value. The system thus operates as a rectifier for spin--current polarization. This effect follows from an interplay between Coulomb interaction and nonequilibrium spin accumulation in the dot. In the parameter range considered, we also show that the above results can be obtained via nonequilibrium Green functions within a Hartree-Fock type approximation.

cond-mat.mes-hall↗

arXiv2002

Screening ineffectiveness and THz emission at bare LO phonon frequencies

Within a hydrodynamic approach we investigate the dynamics of an inhomogeneous electron-hole gas coupled to phonons in Te and the corresponding THz emission. We find that the longitudinal inhomogeneity of the photogenerated electron-hole gas -- due to short absorption lengths in Te -- gives rise to screening ineffectiveness for non-zero wave-vector modes. This allows for THz dynamics and emission at the bare LO phonon frequency nu_{LO} even at high carrier densities. Screening ineffectiveness thus provides an appealingly simple explanation for the existence of bare modes at nu_{LO} in longitudinally inhomogeneous systems such as Te; no lateral inhomogeneity of the excitation spot is needed here.

cond-mat↗

arXiv2002

Current and Noise in a FM/quantum dot/FM System

Using the Keldysh nonequilibrium technique we calculate current, noise and Fano factor in a ferromagnetic(FM)-quantum dot-ferromagnetic(FM) system with Coulomb interaction and spin-flip scattering in the dot. The lead polarizations are considered in both parallel P and antiparallel AP alignments. We show that spin-flip can increase both AP-current and AP-noise, while the P-current and P-noise are almost insensible to it. This fact leads to a suppression of the tunnelling magnetoresistance with increasing spin-flip rate.

cond-mat↗