arXiv subjects

Rosa Lopez

Publications and source records attributed to Rosa Lopez.

At least 37 records · Page 2Linked to original sources

arXiv2015

Heat asymmetries in nanoscale conductors: The role of decoherence and inelasticity

We investigate the heat flow between different terminals in an interacting coherent conductor when inelastic scattering is present. We illustrate our theory with a two-terminal quantum dot setup. Two types of heat asymmetries are investigated: electric asymmetry $Δ_E$, which describes deviations of the heat current in a given contact when voltages are exchanged, and contact asymmetry $Δ_C$, which quantifies the difference between the power measured in two distinct electrodes. In the linear regime, both asymmetries agree and are proportional to the Seebeck coefficient, the latter following at low temperature a Mott-type formula with a dot transmission renormalized by inelasticity. Interestingly, in the nonlinear regime of transport we find $Δ_E\neqΔ_C$ and this asymmetry departure depends on the applied bias configuration. Our results may be important for the recent experiments by Lee et al. [Nature (London) 498, 209 (2013)], where these asymmetries were measured.

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arXiv2015

Cross thermoelectric coupling in normal-superconductor quantum dots

We discuss the nonlinear current of an interacting quantum dot coupled to normal and superconducting reservoirs with applied voltage and temperature differences. Due to the particle-hole symmetry introduced by the superconducting lead, the pure (subgap) thermoelectric response vanishes. However, we show that the Andreev bound states shift as the thermal gradient increases. As a consequence, the $I$--$V$ characteristic can be tuned with a temperature bias if the system is simultaneously voltage biased. This is a cross effect that occurs beyond linear response only. Furthermore, we emphasize the role of quasiparticle tunneling processes in the generation of high thermopower sensitivities.

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arXiv2015

Shiba states and zero-bias anomalies in the hybrid normal-superconductor Anderson model

We determine the phase diagram of an Anderson impurity in contact with superconducting and normal-state leads for arbitrary ratio of the gap $Δ$ to the Kondo temperature $T_K$. We observe a considerable effect of even very weak coupling to the normal lead that is usually considered as a non-perturbing tunneling probe. The numerical renormalization group results are analyzed in the context of relevant experimental scenarios such as parity crossing (doublet-singlet) quantum phase transitions induced by a gap reduction as well as novel Kondo features induced by the normal lead. We point out the important role of finite temperatures and magnetic fields. Overall, we find a very rich behavior of spectral functions with zero-bias anomalies which can emerge irrespective of whether the ground state is a doublet or a singlet. Our findings are pertinent to the tunnelling-spectroscopy experiments aiming at detecting Majorana modes in nanowires.

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arXiv2014

Nonlinear spin-thermoelectric transport in two-dimensional topological insulators

We consider spin-polarized transport in a quantum spin Hall antidot system coupled to normal leads. Due to the helical nature of the conducting edge states, the screening potential at the dot region becomes spin dependent without external magnetic fields nor ferromagnetic contacts. Therefore, the electric current due to voltage or temperature differences becomes spin polarized, its degree of polarization being tuned with the dot level position or the base temperature. This spin-filter effect arises in the nonlinear transport regime only and has a purely interaction origin. Likewise, we find a spin polarization of the heat current which is asymmetric with respect to the bias direction. Interestingly, our results show that a pure spin current can be generated by thermoelectric means: when a temperature gradient is applied, the created thermovoltage (Seebeck effect) induces a spin-polarized current for vanishingly small charge current. An analogous effect can be observed for the heat transport: a pure spin heat flows in response to a voltage shift even if the thermal current is zero.

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arXiv2014

Thermoelectric effect in the Kondo dot side-coupled to a Majorana fermion

We investigate the linear thermoelectric response of an interacting quantum dot side-coupled by one of two Majorana fermions (MFs) formed at the ends of a topological superconducting wire. We employ the numerical renormalization group technique to obtain the thermoelectrical conductance $L$ as well as the electrical conductance $G$ when the background temperature $T$ and the dot gate are tuned. We distinguish two transport regimes in which $L$ displays different features: the weak- $(Γ_{m} < T_{K})$ and strong-coupling $(Γ_{m} > T_{K})$ regimes, where $Γ_{m}$ and $T_{K}$ are the Majorana-dot coupling and the Kondo temperature, respectively. For an ideal (infinitely long) nanowire where the Majorana end states do not overlap $(ε_{m} = 0)$, the thermoelectrical conductance $L$ in the weak-coupling regime exhibits a peak at $T \sim Γ_{m}$. This peak is ascribed to the anti-Fano resonance between the asymmetric Kondo resonance and the zero-energy MF mode. Interestingly, in the strong-coupling regime, the Kondo-induced peak in $L$ is shifted due to the MF-induced Zeeman splitting in the dot. For finite but small $ε_{m} > 0$, the interference between two MFs restores the Kondo effect in the dot in a smaller energy scale $Γ^{\prime}_{m}$ and gives rise to an additional peak in $L$ at $T \sim Γ^{\prime}_{m}$, whose sign is opposite to that at $T\simΓ_{m}$. In the strong-coupling regime this additional peak can cause a non-monotonic behavior of $L$ with respect to the dot gate. Finally, we examine the case in which an ordinary spin-polarized fermion is coupled to the dot and identify the fingerprint of MFs by comparing two cases.

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arXiv2013

Dynamic thermoelectric and heat transport in mesoscopic capacitors

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.

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arXiv2013

Thermoelectrical detection of Majorana states

We discuss the thermoelectrical properties of nanowires hosting Majorana edge states. For a Majorana nanowire directly coupled to two normal reservoirs the thermopower always vanishes regardeless of the value of the Majorana hybridization. This situation changes drastically if we insert a quantum dot. Then, the dot Majorana side coupled system exhibits a different behavior for the thermopower depending on the Majorana hybridization parameter. Thermopower reverses its sign when the half fermionic state is fully developed. As long as the hybridization becomes finite the Seebeck coefficient behaves similarly to a resonant level system. The sign change of the thermopower when Majorana physics takes place and the fact that both, the electrical and thermal conductances reach, their half fermionic value could serve as a proof of the existence of Majorana edge states in nanowires. Finally, we perform some predictions about the gate dependence of the Seebeck coefficient when Kondo correlations are present in the dot.

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arXiv2013

Non-equilibrium spin-current detection with a single Kondo impurity

We present a theoretical study based on the Anderson model of the transport properties of a Kondo impurity (atom or quantum dot) connected to ferromagnetic leads, which can sustain a non-equilibrium spin current. We analyze the case where the spin current is injected by an external source and when it is generated by the voltage bias. Due to the presence of ferromagnetic contacts, a static exchange field is produced that eventually destroys the Kondo correlations. We find that such a field can be compensated by an appropriated combination of the spin-dependent chemical potentials leading to the restoration of the Kondo resonance. In this respect, a Kondo impurity may be regarded as a very sensitive sensor for non-equilibrium spin phenomena.

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arXiv2013

Proposal for a local heating driven spin current generator

We propose a two-terminal spin-orbit interferometer with a hot molecule inserted in one of its arms to generate pure spin currents. Local heating is achieved by coupling the vibrational modes of the molecule to a third (phononic) reservoir. We show that this spin calorimetric effect is due to the combined influence of spin-dependent wave interference and inelastic scattering. Remarkably, the device converts heat flow into spin-polarized current even without applying any voltage or temperature difference to the electronic terminals.

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arXiv2013

Orbital caloritronic transport in strongly interacting quantum dots

We discuss out-of-equilibrium population imbalances between different orbital states due to applied thermal gradients. This purely thermoelectric orbital effect appears quite generically in nanostructures with a pseudospin degree of freedom. We establish an orbital Seebeck coefficient that characterizes the induced orbital bias in response to a temperature difference between reservoirs coupled to a quantum conductor. We analyze a two-terminal strongly interacting quantum dot with two orbital states and find that the orbital thermopower acts as an excellent tool to describe the transition between SU(4) and SU(2) Kondo physics. Our conclusions are reinforced from a detailed comparison with the charge thermopower using numerical renormalization group calculations.

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arXiv2013

Noise and fluctuation relations of a spin diode

We consider fluctuation relations between the transport coefficients of a spintronic system where magnetic interactions play a crucial role. We investigate a prototypical spintronic device (a spin-diode) which consists of an interacting resonant level coupled to two ferromagnetic electrodes. We thereby obtain the cumulant generating function for the spin transport in the sequential tunnelling regime. We demonstrate the fulfilment of the nonlinear fluctuation relations when up and down spin currents are correlated in the presence of both spin-flip processes and external magnetic fields.

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arXiv2013

Nonlinear heat transport in mesoscopic conductors: Rectification, Peltier effect and Wiedemann-Franz law

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.

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arXiv2013

Scattering theory of nonlinear thermoelectric transport

We investigate nonlinear transport properties of quantum conductors in response to both electrical and thermal driving forces. Within 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.

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arXiv2012

Transport Measurement of Andreev Bound States in a Kondo-Correlated Quantum Dot

We report transport measurements of gate-tunable Andreev bound states in a carbon nanotube quantum dot coupled to two superconducting leads. In particular, we observe clear features of two types of Kondo ridges, which can be understood in terms of the interplay between the Kondo effect and superconductivity. In the first type (type I), the coupling is strong and the Kondo effect is dominant. Levels of the Andreev bound states display anti-crossing in the middle of the ridge. On the other hand, crossing of the two Andreev bound states is shown in the second type (type II) together with the 0-$π$ transition of the Josephson junction. Our scenario is well understood in terms of only a single dimensionless parameter, $k_BT_K^{min}/Δ$, where $T_K^{min}$ and $Δ$ are the minimum Kondo temperature of a ridge and the superconducting order parameter, respectively. Our observation is consistent with measurements of the critical current, and is supported by numerical renormalization group calculations.

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arXiv2012

Kondo Effect in a Quantum-Dot-Topological-Superconductor Junction

We investigate the dynamical and transport features of a Kondo dot side-coupled to a topological superconductor (TS). The Majorana fermion states (MFS) formed at the ends of the TS are found to be able to alter the Kondo physics profoundly: For the ideal setup where the MFS do not overlap ($ε_m=0$) a finite dot-MFS coupling $(Γ_m)$ reduces the unitary-limit value of the linear conductance by exactly a factor 3/4 in the Kondo-dominant regime $(Γ_m T_K)$, on the other hand, the spin-split Kondo resonance takes place due to the MFS-induced Zeeman splitting, which is a genuine many-body effect of the strong Coulomb interaction and the topological superconductivity. We find that the original Kondo resonance is fully restored once the MFSs are strongly hybridized ($ε_m > Γ_m$). This unusual interaction between the Kondo effect and the MFS can thus serve to detect the Majorana fermions unambiguously and quantify the degree of overlap between the MFSs in the TS.

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arXiv2012

Magnetic-field instability of Majorana modes in multiband quantum wires

We investigate the occurrence of Majorana modes in semiconductor quantum wires in close proximity with a superconductor and when both, Rashba interaction and magnetic field, are present. We consider long, but finite, multiband wires (namely, planar wires with dimensions $L_x\gg L_y$). Our results demonstrate that interband mixing coming from Rashba spin orbit term hybridizes Majorana pairs originating from different transverse modes while simultaneously closing the effective gap. Consequently, multiple Majorana modes do not coexist in general. On the contrary, Majorana physics is robust provided that only one single transverse mode contributes with a Majorana pair. Finally, we analyse the robustness of Majorana physics with respect to magnetic orbital effects.

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arXiv2012

SU(3) Kondo effect in spinless triple quantum dots

We discuss a device --- a purely capacitively coupled interacting spinless triple quantum dot system --- for the observation of the SU(3) Kondo effect. Unlike more familiar SU(2) and SU(4) Kondo effects in quantum dot devices which lead to unitary linear conductance at low temperatures, the SU(3) Kondo scenario can be easily identified by the conductance pinned to a characteristic value of 3/4 of the unitary limit. This is associated with the interesting fact that the SU(3) Kondo effect does not occur at the particle-hole symmetric point, where the system is found instead in the valence-fluctuating regime with the total dot occupancy flipping between 1 and 2, but for gate voltages in the two Kondo plateaux where the dot occupancy is pinned to an integer value, either 1 or 2. From the thermodynamic analysis in the Kondo regime we find that the effective impurity orbital moment, defined through the impurity orbital susceptibility (chi_imp) multiplied by the temperature, is Tχ_imp=1 at high temperatures and then it increases to the characteristic value of Tχ_imp=4/3 corresponding to the three-fold degenerate local-moment fixed point where the impurity entropy is S_imp=ln 3. Then, at much lower temperatures, the system flows to the non-degenerate strong-coupling fixed point in which the SU(3) Kondo effect takes place. We also report results about the robustness of the SU(3) Kondo effect against various perturbations present in real experimental setups. Finally, we describe possible mechanisms to restore the SU(3) Kondo physics by properly tuning the on-site dot potentials. We briefly comment on the spinfull case which has very different behavior and shows Kondo plateaus in conductance for all integer values of the occupancy, including at the particle-hole symmetric point.

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arXiv2012

Spin-current noise from fluctuation relations

We present fluctuation relations that connect spin-polarized current and noise in mesoscopic conductors. In linear response, these relations are equivalent to the fluctuation-dissipation theorem that relates equilibrium current--current correlations to the linear conductance. More interestingly, in the weakly nonlinear regime of transport, these relations establish a connection between the leading-order rectification spin conductance, the spin noise susceptibility and the third cumulant of spin current fluctuations at equilibrium. Our results are valid even for systems in the presence of magnetic fields and coupled to ferromagnetic electrodes.

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