David Lopez Sanchez | Universidad de Ciencias y Artes de Chiapas (original) (raw)
Papers by David Lopez Sanchez
IEEE Latin America Transactions, 2011
This paper describes the development of an Advanced Speech Communication System for Deaf People a... more This paper describes the development of an Advanced Speech Communication System for Deaf People and its field evaluation in a real application domain: the renewal of Driver's License. The system is composed of two modules. The first one is a Spanish into Spanish Sign Language (LSE: Lengua de Signos Española) translation module made up of a speech recognizer, a natural language translator (for converting a word sequence into a sequence of signs), and a 3D avatar animation module (for playing back the signs). The second module is a Spoken Spanish generator from sign-writing composed of a visual interface (for specifying a sequence of signs), a language translator (for generating the sequence of words in Spanish), and finally, a text to speech converter. For language translation, the system integrates three technologies: an example-based strategy, a rule-based translation method and a statistical translator. This paper also includes a detailed description of the evaluation carried out in the Local Traffic Office in the city of Toledo (Spain) involving real government employees and deaf people. This evaluation includes objective measurements from the system and subjective information from questionnaires. Finally, the paper reports an analysis of the main problems and a discussion about possible solutions.
Journal of Physics: Conference Series, 2014
Physical Review B, 2003
We investigate conductances and current correlations in a system consisting of a normal multichan... more We investigate conductances and current correlations in a system consisting of a normal multichannel conductor connected to one superconducting and two ferromagnetic electrodes. For antiparallel orientation of the ferromagnet polarizations, current injection from one ferromagnet can, due to Andreev reflection, lead to a net drag of current from the second ferromagnet toward the superconductor. We present the conditions for the Andreev drag in terms of lead polarizations, contact conductances and spin-flip scattering. Remarkably, both equilibrium and nonequilibrium zero-frequency current correlations between the ferromagnets become positive even in the presence of spin relaxation. PACS numbers: 74.45.+c 72.25.-b 72.70.+m
Physical Review B, 2003
We analyze theoretically the dynamical transport through a weakly coupled semiconductor superlatt... more We analyze theoretically the dynamical transport through a weakly coupled semiconductor superlattice, under an ac potential with frequencies in the THz regime, by means of a general model for time-dependent sequential tunneling within a nonequilibrium-Green's-function framework. Highly doped superlattices present, under certain conditions, the formation of electric-field domains at a static dc voltage bias. We find that the THz signal drives the system from a stationary current toward an oscillatory time dependence as the ac intensity increases. However, the current oscillates periodically in the MHz regime, reflecting an ac-induced motion and recycling of traveling domain walls. Finally, we predict that on further increasing the intensity of the ac potential, the tunneling current undergoes a transition from a periodically time-dependent state to a stationary one in which a homogeneous electric-field distribution builds up along the sample.
Physical Review Letters, 2004
We investigate the effects of spin-polarized leads on the Kondo physics of a quantum dot using th... more We investigate the effects of spin-polarized leads on the Kondo physics of a quantum dot using the numerical renormalization group method. Our study demonstrates in an unambiguous way that the Kondo effect is not necessarily suppressed by the lead polarization: While the Kondo effect is quenched for the asymmetric Anderson model, it survives even for finite polarizations in the regime where charge fluctuations are negligible. We propose the linear tunneling magnetoresistance as an experimental signature of these behaviors. We also report on the influence of spin-flip processes. PACS numbers: 72.15.Qm, 72.25.Mk, 73.63.Kv Introduction.-Magnetic impurities embedded in metallic hosts cause anomalous resonant scattering of conduction band electrons. At the same time, the localized magnetic moments are screened at low temperature by the itinerant electron spins. This is the celebrated Kondo effect [1], which has been recently revived in mesoscopic physics . Ever since the theoretical predictions and the experimental demonstrations [5], the Kondo effect in phase-coherent systems such as quantum dots (QD's) has stimulated great interest in this field. The remarkable success behind this is the fine tunability of the parameter space (impurity level and hybridization couplings). The controlled manipulation in mesoscopic systems has not only allowed to test various aspects of the Kondo effect, which is a hard task in bulk solids, but also has posed further exciting questions. For example, when the spin-degeneracy of the impurity level is lifted by an external magnetic field, the Kondo peak in the density of states (DOS) of the dot is expected to split [6]. However, new experiments [7] and theoretical studies suggest that the situation is more subtle.
Physical Review Letters, 2003
We investigate the nonequilibrum transport properties of a quantum dot when spin flip processes c... more We investigate the nonequilibrum transport properties of a quantum dot when spin flip processes compete with the formation of a Kondo resonance in the presence of ferromagnetic leads. Based upon the Anderson Hamiltonian in the strongly interacting limit, we predict a splitting of the differential conductance when the spin flip scattering amplitude is of the order of the Kondo temperature. We discuss how the relative orientation of the lead magnetizations strongly influences the electronic current and the shot noise in a nontrivial way. Furthermore, we find that the zero-bias tunneling magnetoresistance becomes negative with increasing spin flip scattering amplitude.
Physical Review Letters, 2012
Fluctuation relations are derived in systems where the spin degree of freedom and magnetic intera... more Fluctuation relations are derived in systems where the spin degree of freedom and magnetic interactions play a crucial role. The form of the non-equilibrium fluctuation theorems relies in the assumption of a local balance condition. We demonstrate that in some cases the presence of magnetic interactions violates this condition. Nevertheless, fluctuation relations can be obtained from the micro-reversibility principle sustained only at equilibrium as a symmetry of the cumulant generating function for spin currents. We illustrate the spintronic fluctuation relations for a quantum dot coupled to partially polarized helical edges states . 73.50.Fq, 73.63.Kv Introduction. Non-equilibrium fluctuation theorems (FTs) [1-3], widely used for macroscopic systems, are based on the thermodynamics governing the physical processes when they evolve forward and backward in time. The boundary conditions for the forward and the time-reversed processes determine the balance condition for the entropy exchange and therefore the form of the fluctuation theorem . The applicability of the non-equilibrium FTs to quantum systems has become an exciting problem and, in particular, to the case of the charge transfer phenomena in mesoscopic systems in the context of the full counting statistics . Interestingly, relations akin to the fluctuation-dissipation theorem have been formulated beyond the linear response regime . These fluctuation relations relate nonequilibrium fluctuation and dissipation coefficients for phase-coherent conductors. However, the role of a genuine quantum property such as the spin degree of freedom in the fluctuation relations has not been yet investigated in detail. Our motivation is not only fundamental since the electronic spin offers enormous advantages to create devices with unusual and extraordinary new functionalities . The purpose of this work is thus to generalize the fluctuation relations for spintronic systems.
Physical Review Letters, 2013
We investigate nonlinear transport properties of quantum conductors in response to both electrica... more We investigate nonlinear transport properties of quantum conductors in response to both electrical and thermal driving forces. Within the scattering approach, we determine the nonequilibrium screening potential of a generic mesoscopic system and find that its response is dictated by particle and entropic injectivities which describe the charge and entropy transfer during transport. We illustrate our model analyzing the voltage and thermal rectification of a resonant tunneling barrier. Importantly, we discuss interaction induced contributions to the thermopower in the presence of large temperature differences.
Physical Review Letters, 2010
When a biased conductor is put in proximity with an unbiased conductor a drag current can be indu... more When a biased conductor is put in proximity with an unbiased conductor a drag current can be induced in the absence of detailed balance. This is known as the Coulomb drag effect. However, even in this situation far away from equilibrium where detailed balance is explicitly broken, theory predicts that fluctuation relations are satisfied. This surprising effect has, to date, not been confirmed experimentally. Here we propose a system consisting of a capacitively coupled double quantum dot where the nonlinear fluctuation relations are verified in the absence of detailed balance. PACS numbers: 73.23.-b 72.70.+m 73.63.Kv
Physical Review B, 2010
We investigate intersubband mixing effects in multichannel quantum wires in the presence of Rashb... more We investigate intersubband mixing effects in multichannel quantum wires in the presence of Rashba spin-orbit coupling and attached to two terminals. When the contacts are ferromagnetic and their magnetization direction is perpendicular to the Rashba field, the spin-transistor current is expected to depend in a oscillatory way on the Rashba coupling strength due to spin coherent oscillations of the travelling electrons. Nevertheless, we find that the presence of many propagating modes strongly influences the spin precession effect, leading to (i) a quenching of the oscillations and (ii) strongly irregular curves for high values of the Rashba coupling. We also observe that in the case of leads' magnetization parallel to the Rashba field, the conductance departs from a uniform value as the Rashba strength increases. We also discuss the Rashba interaction induced current polarization effects when the contacts are not magnetic and investigate how this mechanism is affected by the presence of several propagating channels.
Physical Review B, 2007
We investigate the electronic transport in a quantum wire with localized Rashba interaction. The ... more We investigate the electronic transport in a quantum wire with localized Rashba interaction. The Rashba field forms quasibound states that couple to the continuum states with an opposite spin direction. The presence of this Rashba dot causes Fano-like antiresonances and dips in the wire's linear conductance. The Fano line shape arises from the interference between the direct transmission channel along the wire and the hopping through the Rashba dot. Due to the confinement, we predict the observation of large charging energies in the local Rashba region, which lead to Coulomb-blockade effects in the transport properties of the wire. Importantly, the Kondo regime can be achieved with a proper tuning of the Rashba interaction, giving rise to an oscillating linear conductance for a fixed occupation of the Rashba dot.
Physical Review B, 2005
We analyze the spectral and transport properties of ballistic quasi one-dimensional systems in th... more We analyze the spectral and transport properties of ballistic quasi one-dimensional systems in the presence of spin-orbit coupling and in-plane magnetic fields. Our results demonstrate that Rashba precession and intersubband coupling must be treated on equal footing for wavevectors near the magnetic field induced gaps. We find that intersubband coupling limits the occurrence of negative effective masses at the gap edges and modifies the linear conductance curves in the strong coupling limit. The effect of the magnetic field on the spin textured orientation of the wire magnetization is discussed.
Physical Review B, 2013
We discuss the low-frequency response of charge and heat transport to oscillatory voltage and tem... more We discuss the low-frequency response of charge and heat transport to oscillatory voltage and temperature shifts in mesoscopic capacitors. We obtain within scattering theory generic expressions for the quantum admittances up to second order in the ac frequencies in terms of electric, thermoelectric and heat capacitances and relaxation resistances. Remarkably, we find that the thermocurrent can lead or lag the applied temperature depending on the gate voltage applied to a quantum RC circuit. Furthermore, the relaxation resistance for cross terms becomes nonuniversal as opposed to the purely electric or thermal cases.
Physical Review B, 2010
We consider a flux-threaded Aharonov-Bohm ring with an embedded quantum dot coupled to two normal... more We consider a flux-threaded Aharonov-Bohm ring with an embedded quantum dot coupled to two normal leads. The local Rashba spin-orbit interaction acting on the dot electrons leads to a spin-dependent phase factor in addition to the Aharonov-Bohm phase caused by the external flux. Using the numerical renormalization group method, we find a splitting of the Kondo resonance at the Fermi level which can be compensated by an external magnetic field. To fully understand the nature of this compensation effect, we perform a scaling analysis and derive an expression for the effective magnetic field. The analysis is based on a tight-binding model which leads to an effective Anderson model with a spin-dependent density of states for the transformed lead states. We find that the effective field originates from the combined effect of Rashba interaction and magnetic flux and that it contains important corrections due to electron-electron interactions. We show that the compensating field is an oscillatory function of both the spin-orbit and the Aharonov-Bohm phases. Moreover, the effective field never vanishes due to the particle-hole symmetry breaking independently of the gate voltage.
Physical Review B, 2009
We study the formation of local moments in quantum dots arising in quasi-one dimensional electron... more We study the formation of local moments in quantum dots arising in quasi-one dimensional electron wires due to localized spin-orbit (Rashba) interaction. Using an Anderson-like model to describe the occurrence of the magnetic moments in these Rashba dots, we calculate the local magnetization within the mean-field approximation. We find that the magnetization becomes a nontrivial function of the Rashba coupling strength. We discuss both the equilibrium and nonequilibrium cases. Interestingly, we obtain a magnetic phase which is stable at large bias due to the Rashba interaction.
Physical Review B, 2013
We investigate nonlinear heat properties in mesoscopic conductors using a scattering theory of tr... more We investigate nonlinear heat properties in mesoscopic conductors using a scattering theory of transport. Our approach is based on a leading-order expansion in both the electrical and thermal driving forces. Beyond linear response, the transport coefficients are functions of the nonequilibrium screening potential that builds up in the system due to interactions. Within a mean-field approximation, we self-consistently calculate the heat rectification properties of a quantum dot attached to two terminals. We discuss nonlinear contributions to the Peltier effect and find departures from the Wiedemann-Franz law in the nonlinear regime of transport.
Physical Review B, 2011
Ferromagnetic contacts put in proximity with carbon nanotubes induce spin and orbital polarizatio... more Ferromagnetic contacts put in proximity with carbon nanotubes induce spin and orbital polarizations. These polarizations affect dramatically the Kondo correlations occurring in quantum dots formed in a carbon nanotube, inducing effective fields in both spin and orbital sectors. As a consequence, the carbon nanotube quantum dot spectral density shows a four-fold split SU(4) Kondo resonance. Furthermore, the presence of spin-orbit interactions leads to the occurrence of an additional polarization among time-reversal electronic states (polarization in the time-reversal symmetry or Kramers sector). Here, we estimate the magnitude for the Kramer polarization in realistic carbon nanotube samples and find that its contribution is comparable to the spin and orbital polarizations. The Kramers polarization generates a new type of effective field that affects only the time-reversal electronic states. We report new splittings of the Kondo resonance in the dot spectral density which can be understood only if Kramers polarization is taken into account. Importantly, we predict that the existence of Kramers polarization can be experimentally detected by performing nonlinear differential conductance measurements. We also find that, due to the high symmetry required to build SU(4) Kondo correlations, its restoration by applying an external field is not possible in contrast to the compensated SU(2) Kondo state observed in conventional quantum dots.
Physical Review B, 2007
We discuss the calculation of evanescent states in quasi-one-dimensional quantum wires in the pre... more We discuss the calculation of evanescent states in quasi-one-dimensional quantum wires in the presence of Rashba spin-orbit interaction. We suggest a computational algorithm devised for cases in which longitudinal and transverse motions are coupled. The dispersion relations are given for some selected cases, illustrating the feasibility of the proposed computational method. As a practical application, we discuss the solutions for a wire containing a potential step.
Physical Review B, 2005
We investigate the nonequilibrium transport properties of a three-terminal quantum dot in the str... more We investigate the nonequilibrium transport properties of a three-terminal quantum dot in the strongly interacting limit. At low temperatures, a Kondo resonance arises from the antiferromagnetic coupling between the localized electron in the quantum dot and the conduction electrons in source and drain leads. It is known that the local density of states is accessible through the differential conductance measured at the (weakly coupled) third lead. Here, we consider the multiterminal current-current correlations (shot noise and cross correlations measured at two different terminals). We discuss the dependence of the current correlations on a number of external parameters: bias voltage, magnetic field and magnetization of the leads. When the Kondo resonance is split by fixing the voltage bias between two leads, the shot noise shows a nontrivial dependence on the voltage applied to the third lead. We show that the cross correlations of the current are more sensitive than the conductance to the appearance of an external magnetic field. When the leads are ferromagnetic and their magnetizations point along opposite directions, we find a reduction of the cross correlations. Moreover, we report on the effect of dephasing in the Kondo state for a two-terminal geometry when the third lead plays the role of a fictitious voltage probe.
IEEE Latin America Transactions, 2011
This paper describes the development of an Advanced Speech Communication System for Deaf People a... more This paper describes the development of an Advanced Speech Communication System for Deaf People and its field evaluation in a real application domain: the renewal of Driver's License. The system is composed of two modules. The first one is a Spanish into Spanish Sign Language (LSE: Lengua de Signos Española) translation module made up of a speech recognizer, a natural language translator (for converting a word sequence into a sequence of signs), and a 3D avatar animation module (for playing back the signs). The second module is a Spoken Spanish generator from sign-writing composed of a visual interface (for specifying a sequence of signs), a language translator (for generating the sequence of words in Spanish), and finally, a text to speech converter. For language translation, the system integrates three technologies: an example-based strategy, a rule-based translation method and a statistical translator. This paper also includes a detailed description of the evaluation carried out in the Local Traffic Office in the city of Toledo (Spain) involving real government employees and deaf people. This evaluation includes objective measurements from the system and subjective information from questionnaires. Finally, the paper reports an analysis of the main problems and a discussion about possible solutions.
Journal of Physics: Conference Series, 2014
Physical Review B, 2003
We investigate conductances and current correlations in a system consisting of a normal multichan... more We investigate conductances and current correlations in a system consisting of a normal multichannel conductor connected to one superconducting and two ferromagnetic electrodes. For antiparallel orientation of the ferromagnet polarizations, current injection from one ferromagnet can, due to Andreev reflection, lead to a net drag of current from the second ferromagnet toward the superconductor. We present the conditions for the Andreev drag in terms of lead polarizations, contact conductances and spin-flip scattering. Remarkably, both equilibrium and nonequilibrium zero-frequency current correlations between the ferromagnets become positive even in the presence of spin relaxation. PACS numbers: 74.45.+c 72.25.-b 72.70.+m
Physical Review B, 2003
We analyze theoretically the dynamical transport through a weakly coupled semiconductor superlatt... more We analyze theoretically the dynamical transport through a weakly coupled semiconductor superlattice, under an ac potential with frequencies in the THz regime, by means of a general model for time-dependent sequential tunneling within a nonequilibrium-Green's-function framework. Highly doped superlattices present, under certain conditions, the formation of electric-field domains at a static dc voltage bias. We find that the THz signal drives the system from a stationary current toward an oscillatory time dependence as the ac intensity increases. However, the current oscillates periodically in the MHz regime, reflecting an ac-induced motion and recycling of traveling domain walls. Finally, we predict that on further increasing the intensity of the ac potential, the tunneling current undergoes a transition from a periodically time-dependent state to a stationary one in which a homogeneous electric-field distribution builds up along the sample.
Physical Review Letters, 2004
We investigate the effects of spin-polarized leads on the Kondo physics of a quantum dot using th... more We investigate the effects of spin-polarized leads on the Kondo physics of a quantum dot using the numerical renormalization group method. Our study demonstrates in an unambiguous way that the Kondo effect is not necessarily suppressed by the lead polarization: While the Kondo effect is quenched for the asymmetric Anderson model, it survives even for finite polarizations in the regime where charge fluctuations are negligible. We propose the linear tunneling magnetoresistance as an experimental signature of these behaviors. We also report on the influence of spin-flip processes. PACS numbers: 72.15.Qm, 72.25.Mk, 73.63.Kv Introduction.-Magnetic impurities embedded in metallic hosts cause anomalous resonant scattering of conduction band electrons. At the same time, the localized magnetic moments are screened at low temperature by the itinerant electron spins. This is the celebrated Kondo effect [1], which has been recently revived in mesoscopic physics . Ever since the theoretical predictions and the experimental demonstrations [5], the Kondo effect in phase-coherent systems such as quantum dots (QD's) has stimulated great interest in this field. The remarkable success behind this is the fine tunability of the parameter space (impurity level and hybridization couplings). The controlled manipulation in mesoscopic systems has not only allowed to test various aspects of the Kondo effect, which is a hard task in bulk solids, but also has posed further exciting questions. For example, when the spin-degeneracy of the impurity level is lifted by an external magnetic field, the Kondo peak in the density of states (DOS) of the dot is expected to split [6]. However, new experiments [7] and theoretical studies suggest that the situation is more subtle.
Physical Review Letters, 2003
We investigate the nonequilibrum transport properties of a quantum dot when spin flip processes c... more We investigate the nonequilibrum transport properties of a quantum dot when spin flip processes compete with the formation of a Kondo resonance in the presence of ferromagnetic leads. Based upon the Anderson Hamiltonian in the strongly interacting limit, we predict a splitting of the differential conductance when the spin flip scattering amplitude is of the order of the Kondo temperature. We discuss how the relative orientation of the lead magnetizations strongly influences the electronic current and the shot noise in a nontrivial way. Furthermore, we find that the zero-bias tunneling magnetoresistance becomes negative with increasing spin flip scattering amplitude.
Physical Review Letters, 2012
Fluctuation relations are derived in systems where the spin degree of freedom and magnetic intera... more Fluctuation relations are derived in systems where the spin degree of freedom and magnetic interactions play a crucial role. The form of the non-equilibrium fluctuation theorems relies in the assumption of a local balance condition. We demonstrate that in some cases the presence of magnetic interactions violates this condition. Nevertheless, fluctuation relations can be obtained from the micro-reversibility principle sustained only at equilibrium as a symmetry of the cumulant generating function for spin currents. We illustrate the spintronic fluctuation relations for a quantum dot coupled to partially polarized helical edges states . 73.50.Fq, 73.63.Kv Introduction. Non-equilibrium fluctuation theorems (FTs) [1-3], widely used for macroscopic systems, are based on the thermodynamics governing the physical processes when they evolve forward and backward in time. The boundary conditions for the forward and the time-reversed processes determine the balance condition for the entropy exchange and therefore the form of the fluctuation theorem . The applicability of the non-equilibrium FTs to quantum systems has become an exciting problem and, in particular, to the case of the charge transfer phenomena in mesoscopic systems in the context of the full counting statistics . Interestingly, relations akin to the fluctuation-dissipation theorem have been formulated beyond the linear response regime . These fluctuation relations relate nonequilibrium fluctuation and dissipation coefficients for phase-coherent conductors. However, the role of a genuine quantum property such as the spin degree of freedom in the fluctuation relations has not been yet investigated in detail. Our motivation is not only fundamental since the electronic spin offers enormous advantages to create devices with unusual and extraordinary new functionalities . The purpose of this work is thus to generalize the fluctuation relations for spintronic systems.
Physical Review Letters, 2013
We investigate nonlinear transport properties of quantum conductors in response to both electrica... more We investigate nonlinear transport properties of quantum conductors in response to both electrical and thermal driving forces. Within the scattering approach, we determine the nonequilibrium screening potential of a generic mesoscopic system and find that its response is dictated by particle and entropic injectivities which describe the charge and entropy transfer during transport. We illustrate our model analyzing the voltage and thermal rectification of a resonant tunneling barrier. Importantly, we discuss interaction induced contributions to the thermopower in the presence of large temperature differences.
Physical Review Letters, 2010
When a biased conductor is put in proximity with an unbiased conductor a drag current can be indu... more When a biased conductor is put in proximity with an unbiased conductor a drag current can be induced in the absence of detailed balance. This is known as the Coulomb drag effect. However, even in this situation far away from equilibrium where detailed balance is explicitly broken, theory predicts that fluctuation relations are satisfied. This surprising effect has, to date, not been confirmed experimentally. Here we propose a system consisting of a capacitively coupled double quantum dot where the nonlinear fluctuation relations are verified in the absence of detailed balance. PACS numbers: 73.23.-b 72.70.+m 73.63.Kv
Physical Review B, 2010
We investigate intersubband mixing effects in multichannel quantum wires in the presence of Rashb... more We investigate intersubband mixing effects in multichannel quantum wires in the presence of Rashba spin-orbit coupling and attached to two terminals. When the contacts are ferromagnetic and their magnetization direction is perpendicular to the Rashba field, the spin-transistor current is expected to depend in a oscillatory way on the Rashba coupling strength due to spin coherent oscillations of the travelling electrons. Nevertheless, we find that the presence of many propagating modes strongly influences the spin precession effect, leading to (i) a quenching of the oscillations and (ii) strongly irregular curves for high values of the Rashba coupling. We also observe that in the case of leads' magnetization parallel to the Rashba field, the conductance departs from a uniform value as the Rashba strength increases. We also discuss the Rashba interaction induced current polarization effects when the contacts are not magnetic and investigate how this mechanism is affected by the presence of several propagating channels.
Physical Review B, 2007
We investigate the electronic transport in a quantum wire with localized Rashba interaction. The ... more We investigate the electronic transport in a quantum wire with localized Rashba interaction. The Rashba field forms quasibound states that couple to the continuum states with an opposite spin direction. The presence of this Rashba dot causes Fano-like antiresonances and dips in the wire's linear conductance. The Fano line shape arises from the interference between the direct transmission channel along the wire and the hopping through the Rashba dot. Due to the confinement, we predict the observation of large charging energies in the local Rashba region, which lead to Coulomb-blockade effects in the transport properties of the wire. Importantly, the Kondo regime can be achieved with a proper tuning of the Rashba interaction, giving rise to an oscillating linear conductance for a fixed occupation of the Rashba dot.
Physical Review B, 2005
We analyze the spectral and transport properties of ballistic quasi one-dimensional systems in th... more We analyze the spectral and transport properties of ballistic quasi one-dimensional systems in the presence of spin-orbit coupling and in-plane magnetic fields. Our results demonstrate that Rashba precession and intersubband coupling must be treated on equal footing for wavevectors near the magnetic field induced gaps. We find that intersubband coupling limits the occurrence of negative effective masses at the gap edges and modifies the linear conductance curves in the strong coupling limit. The effect of the magnetic field on the spin textured orientation of the wire magnetization is discussed.
Physical Review B, 2013
We discuss the low-frequency response of charge and heat transport to oscillatory voltage and tem... more We discuss the low-frequency response of charge and heat transport to oscillatory voltage and temperature shifts in mesoscopic capacitors. We obtain within scattering theory generic expressions for the quantum admittances up to second order in the ac frequencies in terms of electric, thermoelectric and heat capacitances and relaxation resistances. Remarkably, we find that the thermocurrent can lead or lag the applied temperature depending on the gate voltage applied to a quantum RC circuit. Furthermore, the relaxation resistance for cross terms becomes nonuniversal as opposed to the purely electric or thermal cases.
Physical Review B, 2010
We consider a flux-threaded Aharonov-Bohm ring with an embedded quantum dot coupled to two normal... more We consider a flux-threaded Aharonov-Bohm ring with an embedded quantum dot coupled to two normal leads. The local Rashba spin-orbit interaction acting on the dot electrons leads to a spin-dependent phase factor in addition to the Aharonov-Bohm phase caused by the external flux. Using the numerical renormalization group method, we find a splitting of the Kondo resonance at the Fermi level which can be compensated by an external magnetic field. To fully understand the nature of this compensation effect, we perform a scaling analysis and derive an expression for the effective magnetic field. The analysis is based on a tight-binding model which leads to an effective Anderson model with a spin-dependent density of states for the transformed lead states. We find that the effective field originates from the combined effect of Rashba interaction and magnetic flux and that it contains important corrections due to electron-electron interactions. We show that the compensating field is an oscillatory function of both the spin-orbit and the Aharonov-Bohm phases. Moreover, the effective field never vanishes due to the particle-hole symmetry breaking independently of the gate voltage.
Physical Review B, 2009
We study the formation of local moments in quantum dots arising in quasi-one dimensional electron... more We study the formation of local moments in quantum dots arising in quasi-one dimensional electron wires due to localized spin-orbit (Rashba) interaction. Using an Anderson-like model to describe the occurrence of the magnetic moments in these Rashba dots, we calculate the local magnetization within the mean-field approximation. We find that the magnetization becomes a nontrivial function of the Rashba coupling strength. We discuss both the equilibrium and nonequilibrium cases. Interestingly, we obtain a magnetic phase which is stable at large bias due to the Rashba interaction.
Physical Review B, 2013
We investigate nonlinear heat properties in mesoscopic conductors using a scattering theory of tr... more We investigate nonlinear heat properties in mesoscopic conductors using a scattering theory of transport. Our approach is based on a leading-order expansion in both the electrical and thermal driving forces. Beyond linear response, the transport coefficients are functions of the nonequilibrium screening potential that builds up in the system due to interactions. Within a mean-field approximation, we self-consistently calculate the heat rectification properties of a quantum dot attached to two terminals. We discuss nonlinear contributions to the Peltier effect and find departures from the Wiedemann-Franz law in the nonlinear regime of transport.
Physical Review B, 2011
Ferromagnetic contacts put in proximity with carbon nanotubes induce spin and orbital polarizatio... more Ferromagnetic contacts put in proximity with carbon nanotubes induce spin and orbital polarizations. These polarizations affect dramatically the Kondo correlations occurring in quantum dots formed in a carbon nanotube, inducing effective fields in both spin and orbital sectors. As a consequence, the carbon nanotube quantum dot spectral density shows a four-fold split SU(4) Kondo resonance. Furthermore, the presence of spin-orbit interactions leads to the occurrence of an additional polarization among time-reversal electronic states (polarization in the time-reversal symmetry or Kramers sector). Here, we estimate the magnitude for the Kramer polarization in realistic carbon nanotube samples and find that its contribution is comparable to the spin and orbital polarizations. The Kramers polarization generates a new type of effective field that affects only the time-reversal electronic states. We report new splittings of the Kondo resonance in the dot spectral density which can be understood only if Kramers polarization is taken into account. Importantly, we predict that the existence of Kramers polarization can be experimentally detected by performing nonlinear differential conductance measurements. We also find that, due to the high symmetry required to build SU(4) Kondo correlations, its restoration by applying an external field is not possible in contrast to the compensated SU(2) Kondo state observed in conventional quantum dots.
Physical Review B, 2007
We discuss the calculation of evanescent states in quasi-one-dimensional quantum wires in the pre... more We discuss the calculation of evanescent states in quasi-one-dimensional quantum wires in the presence of Rashba spin-orbit interaction. We suggest a computational algorithm devised for cases in which longitudinal and transverse motions are coupled. The dispersion relations are given for some selected cases, illustrating the feasibility of the proposed computational method. As a practical application, we discuss the solutions for a wire containing a potential step.
Physical Review B, 2005
We investigate the nonequilibrium transport properties of a three-terminal quantum dot in the str... more We investigate the nonequilibrium transport properties of a three-terminal quantum dot in the strongly interacting limit. At low temperatures, a Kondo resonance arises from the antiferromagnetic coupling between the localized electron in the quantum dot and the conduction electrons in source and drain leads. It is known that the local density of states is accessible through the differential conductance measured at the (weakly coupled) third lead. Here, we consider the multiterminal current-current correlations (shot noise and cross correlations measured at two different terminals). We discuss the dependence of the current correlations on a number of external parameters: bias voltage, magnetic field and magnetization of the leads. When the Kondo resonance is split by fixing the voltage bias between two leads, the shot noise shows a nontrivial dependence on the voltage applied to the third lead. We show that the cross correlations of the current are more sensitive than the conductance to the appearance of an external magnetic field. When the leads are ferromagnetic and their magnetizations point along opposite directions, we find a reduction of the cross correlations. Moreover, we report on the effect of dephasing in the Kondo state for a two-terminal geometry when the third lead plays the role of a fictitious voltage probe.