arXiv subjects

Rosa Lopez

Publications and source records attributed to Rosa Lopez.

At least 55 records · Page 3Linked to original sources

arXiv2012

Fluctuation Relations for Spintronics

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.

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arXiv2012

Transport through Majorana nanowires attached to normal leads

This paper presents a coupled channel model for transport in 2D semiconductor Majorana nanowires coupled to normal leads. When the nanowire hosts a zero mode, conspicuous signatures on the linear conductance are predicted. An effective model in second quantization allowing a fully analytical solution is used to clarify the physics. We also discuss the nonlinear current response ($dI/dV$)

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arXiv2011

Tunable Kondo effect in double quantum dots coupled to ferromagnetic contacts

We investigate the effects induced by spin polarization in the contacts attached to a serial double quantum dot. The polarization generates effective magnetic fields and suppresses the Kondo effect in each dot. The super-exchange interaction ($J_{\mathrm{AFM}}$), tuned by the inter-dot tunnelling rate $t$, can be used to compensate the effective fields and restore the Kondo resonance when the contact polarizations are aligned. As a consequence, the direction of the spin conductance can be controlled and even reversed using electrostatic gates alone. Furthermore, we study the associated two-impurity Kondo model and show that a ferromagnetic exchange coupling ($J_{\mathrm{FM}}$) leads to an effective spin-1 exchange-anisotropic Kondo model which exhibits a quantum phase transition in the presence of partially polarized contacts.

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arXiv2011

Josephson current in carbon nanotubes with spin-orbit interaction

We demonstrate that curvature-induced spin-orbit (SO) coupling induces a $0-π$ transition in the Josephson current through a carbon nanotube quantum dot coupled to superconducting leads. In the non-interacting regime, the transition can be tuned by applying parallel magnetic field near the critical field where orbital states become degenerate. Moreover, the interplay between charging and SO effects in the Coulomb Blockade and cotunneling regimes leads to a rich phase diagram with well-defined (analytical) boundaries in parameter space. Finally, the 0 phase always prevails in the Kondo regime. Our calculations are relevant in view of recent experimental advances in transport through ultra-clean carbon nanotubes.

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arXiv2011

Kramers polarization in strongly correlated carbon nanotube quantum dots

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.

cond-mat.str-el↗

arXiv2011

Many-body Correlation Effect on Mesoscopic Charge Relaxation

We investigate in a nonperturbative way the dynamics of a correlated quantum capacitor. We find that the many-body correlations do not disturb the universal low-frequency relaxation resistance per channel, $R_q(ω=0) = h/4e^2$ ensured by the Korringa-Shiba rule whereas the interpretation of the quantum capacitance $C_q$ in terms of the density of states fails when strong correlations are present. The AC resistance $R_q(ω)$ shows huge peaks (with values larger than $h/4e^2$) at $\hbarω\approx \pm Γ^*$, where $Γ^*$ is the renormalized level broadening. These peaks are merged to a single one at $ω=0$ when a finite Zeeman field is applied comparable to $Γ^*$. The observed features of $R_q$, being most evident in the Kondo regime, are attributed to the generation of particle-hole excitations in the contacts accomplished by spin-flip processes in the dot.

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arXiv2010

Kondo effect in spin-orbit mesoscopic interferometers

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.

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arXiv2010

Transport properties of a molecule embedded in an Aharonov-Bohm interferometer

We theoretically investigate the transport properties of a molecule embedded in one arm of a mesoscopic Aharonov-Bohm interferometer. Due to the presence of phonons the molecule level position ($ε_d$) and the electron-electron interaction ($U$) undergo a \emph{polaronic shift} which affects dramatically the electronic transport through the molecular junction. When the electron-phonon interaction is weak the linear conductance presents Fano-line shapes as long as the direct channel between the electrodes is opened. The observed Fano resonances in the linear conductance are originated from the interference between the spin Kondo state and the direct path. For strong enough electron-phonon interaction, the electron-electron interaction is renormalized towards negative values, {\it i.e.} becomes effectively attractive. This scenario favors fluctuations between the empty and doubly occupied charge states and therefore promotes a charge Kondo effect. However, the direct path between the contacts breaks the electron-hole symmetry which can efficiently suppress this charge Kondo effect. Nevertheless, we show that a proper tuning of the gate voltage is able to revive the Kondo resonance. Our results are obtained by using the Numerical Renormalization approximation to compute the electronic spectral function and the linear conductance.

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arXiv2010

Magnetoasymmetric transport in a mesoscopic interferometer: From the weak to the strong coupling regime

The microreversibility principle implies that the conductance of a two-terminal Aharonov-Bohm interferometer is an even function of the applied magnetic flux. Away from linear response, however, this symmetry is not fulfilled and the conductance phase of the interferometer when a quantum dot is inserted in one of its arms can be a continuous function of the bias voltage. Such magnetoasymmetries have been investigated in related mesoscopic systems and arise as a consequence of the asymetric response of the internal potential of the conductor out of equilibrium. Here we discuss magnetoasymmetries in quantum-dot Aharonov-Bohm interferometers when strong electron-electron interactions are taken into account beyond the mean-field approach. We find that at very low temperatures the asymmetric element of the differential conductance shows an abrupt change for voltages around the Fermi level. At higher temperatures we recover a smooth variation of the magnetoasymmetry as a function of the bias. We illustrate our results with the aid of the electron occupation at the dot, demonstrating that its nonequilibrium component is an asymmetric function of the flux even to lowest order in voltage. We also calculate the magnetoasymmetry of the current-current correlations (the noise) and find that it is given, to a good extent, by the magnetoasymmetry of the weakly nonlinear conductance term. Therefore, both magnetoasymmetries (noise and conductance) are related to each other via a higher-order fluctuation-dissipation relation. This result appears to be true even in the low temperature regime, where Kondo physics and many-body effects dominate the transport properties.

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arXiv2010

Josephson current in strongly correlated double quantum dots

We study the transport properties of a serial double quantum dot (DQD) coupled to two superconducting leads, focusing on the Josephson current through the DQD and the associated 0-$π$ transitions which result from the subtle interplay between the superconductivity, the Kondo physics, and the inter-dot superexchange interaction. We examine the competition between the superconductivity and the Kondo physics by tuning the relative strength $Δ/T_K$ of the superconducting gap $Δ$ and the Kondo temperature $T_K$, for different strengths of the superexchange coupling determined by the interdot tunneling $t$ relative to the dot level broadening $Γ$. We find strong renormalization of $t$, a significant role of the superexchange coupling $J$, and a rich phase diagram of the 0 and $π$-junction regimes. In particular, when both the superconductivity and the exchange interaction are in close competion with the Kondo physics ($Δ\sim J\sim T_K$), there appears an island of $π'$-phase at large values of the superconducting phase difference.

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arXiv2010

Multichannel effects in Rashba quantum wires

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.

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arXiv2009

The Two-impurity Anderson Model Revisited: Competition between Kondo Effect and Reservoir-mediated Superexchange in Double Quantum Dots

We study a series-coupled double quantum dot in the Kondo regime modeled by the two-impurity Anderson model and find a new conduction-band mediated superexchange interaction that competes with Kondo physics in the strong Coulomb interaction limit. Our numerical renormalization group results, complemented with the higher-order Rayleigh-Schrödinger perturbation theory, show that the novel exchange mechanism leads to clear experimental consequences that can be checked in transport measurements through double quantum dots.

cond-mat.mes-hall↗

arXiv2009

Localized magnetic states in Rashba dots

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.

cond-mat.str-el↗

arXiv2007

Pair Tunneling and shot noise through a single molecule in a strong electron-phonon coupling

We investigate the electronic transport through a single molecule in a strong electron-phonon coupling regime. Based on a particle-hole transformation which is made suitable for non-equilibrium situation, we treat the pair tunneling and cotunneling on an equal footing. We propose an experimental setup to enhance the visibility of pair tunneling, which has no Franck-Condon suppression. We also discuss the shot noise characteristics.

cond-mat.mes-hall↗

arXiv2007

From Coulomb blockade to the Kondo regime in a Rashba dot

We investigate the electronic transport in a quantum wire with localized Rashba interaction. The Rashba field forms quasi-bound states which 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 lineshape 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.

cond-mat.mes-hall↗

arXiv2007

Evanescent states in quantum wires with Rashba spin-orbit coupling

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.

cond-mat.mes-hall↗

arXiv2006

Kondo Effects in Carbon Nanotubes: From SU(4) to SU(2) symmetry

We study the Kondo effect in a single-electron transistor device realized in a single-wall carbon nanotube. The K-K' double orbital degeneracy of a nanotube, which originates from the peculiar two-dimensional band structure of graphene, plays the role of a pseudo-spin. Screening of this pseudo-spin, together with the real spin, can result in an SU(4) Kondo effect at low temperatures. For such an exotic Kondo effect to arise, it is crucial that this orbital quantum number is conserved during tunneling. Experimentally, this conservation is not obvious and some mixing in the orbital channel may occur. Here we investigate in detail the role of mixing and asymmetry in the tunneling coupling and analyze how different Kondo effects, from the SU(4) symmetry to a two-level SU(2) symmetry, emerge depending on the mixing and/or asymmetry. We use four different theoretical approaches to address both the linear and non-linear conductance for different values of the external magnetic field. Our results point out clearly the experimental conditions to observe exclusively SU(4) Kondo physics. Although we focus on nanotube quantum dots, our results also apply to vertical quantum dots. We also mention that a finite amount of orbital mixing corresponds, in the pseudospin language, to having non-collinear leads with respect to the orbital ''magnetization'' axis which defines the two pseudospin orientations in the nanotube quantum dot. In this sense, some of our results are also relevant to the problem of a Kondo quantum dot coupled to non-collinear ferromagnetic leads.

cond-mat.mes-hall↗

arXiv2006

Mesoscopic Charge Relaxation

We consider charge relaxation in the mesoscopic equivalent of an RC circuit. For a single-channel, spin-polarized contact, self-consistent scattering theory predicts a universal charge relaxation resistance equal to half a resistance quantum independent of the transmission properties of the contact. This prediction is in good agreement with recent experimental results. We use a tunneling Hamiltonian formalism and show in Hartree-Fock approximation, that at zero temperature the charge relaxation resistance is universal even in the presence of Coulomb blockade effects. We explore departures from universality as a function of temperature and magnetic field.

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