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Itko R.,Jozef Stefan Institute | Itko R.,University of Ljubljana | Lim J.S.,University of the Balearic Islands | Lim J.S.,Institute Of Fisica Interdisciplinar I Of Sistemes Complexos Ifisc Csic Uib | And 4 more authors.
Physical Review Letters | Year: 2012

We investigate the effects induced by ferromagnetic contacts attached to a serial double quantum dot. Spin polarization generates effective magnetic fields and suppresses the Kondo effect in each dot. The superexchange interaction J AFM, tuned by the interdot tunneling 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. Our results demonstrate a new approach for controlling spin-dependent transport in carbon nanotube double dot devices. © 2012 American Physical Society.


Lee M.,Kyung Hee University | Lopez R.,University of the Balearic Islands | Lopez R.,Institute Of Fisica Interdisciplinar I Of Sistemes Complexos Ifisc Csic Uib | Choi M.-S.,Korea University | Martin T.,Aix - Marseille University
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

We investigate nonperturbatively the charge relaxation resistance and quantum capacitance in a coherent RC circuit in the strong-coupling regime. We find that the many-body correlations break the universality in the charge relaxation resistance: (i) The charge relaxation resistance has peaks at finite frequencies Γ*/, where Γ* is an effective level broadening, and (ii) the zero-frequency resistance deviates from the universal value when the Zeeman splitting is comparable to Γ*. This behavior becomes even more prominent in the Kondo regime. The observed features are ascribed to the generation of particle-hole excitations in the contacts accomplished by spin-flip processes in the dot. © 2011 American Physical Society.


Kim B.-K.,Korea Research Institute of Standards and Science | Kim B.-K.,Chonbuk National University | Ahn Y.-H.,Korea Research Institute of Standards and Science | Ahn Y.-H.,Korea University | And 8 more authors.
Physical Review Letters | Year: 2013

We report nonequilibrium 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 anticrossing 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, kBTKminâ¡ /Δ, where TKminâ¡ 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. © 2013 American Physical Society.


Lopez R.,University of the Balearic Islands | Lopez R.,Institute Of Fisica Interdisciplinar I Of Sistemes Complexos Ifisc Csic Uib | Lim J.S.,Institute Of Fisica Interdisciplinar I Of Sistemes Complexos Ifisc Csic Uib | Sanchez D.,University of the Balearic Islands | Sanchez D.,Institute Of Fisica Interdisciplinar I Of Sistemes Complexos Ifisc Csic Uib
Physical Review Letters | Year: 2012

Fluctuation relations are derived in systems where the spin degree of freedom and magnetic interactions play a crucial role. The form of the nonequilibrium fluctuation theorems relies on 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 microreversibility 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 edge states. © 2012 American Physical Society.


Huang S.M.,RIKEN | Huang S.M.,National Sun Yat - sen University | Badrutdinov A.O.,RIKEN | Badrutdinov A.O.,Kazan Federal University | And 10 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

We study the spin-splitting energies in low-potential-barrier quantum dots, finding splitting energies that are orbital state dependent. The theoretical analysis is done with a generalization of the Fock-Darwin states in the presence of spin-orbit interactions. We discuss experimental evidence indicating that the Rashba interaction strength in vertical InxGa1-xAs/ GaAs quantum dots is in the range 80 meV Å ≤λR≤ 120 meV Å. This enhanced spin-orbit interaction can be understood from the high penetration of the electron wave function into the quantum well with low-potential barrier. © 2011 American Physical Society.


Gelabert M.M.,University of the Balearic Islands | Serra L.,University of the Balearic Islands | Serra L.,Institute Of Fisica Interdisciplinar I Of Sistemes Complexos Ifisc Csic Uib
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

We present calculations of the g factors for the lower conductance steps of three-dimensional hole quantum wires. Our results prove that the anisotropy with magnetic field orientation, relative to the wire, originates in the Rashba spin-orbit coupling. We also analyze the relevance of the deformation, as the wire evolves from three-dimensional toward a flat two-dimensional geometry. For high enough wire deformations, the perpendicular g factors are greatly quenched by the Rashba interaction. On the contrary, parallel g factors are rather insensitive to the Rashba interaction, resulting in a high g-factor anisotropy. For low deformations, we find a more irregular behavior, which hints at a sample-dependent scenario. © 2011 American Physical Society.


Lee M.,Kyung Hee University | Lim J.S.,Institute Of Fisica Interdisciplinar I Of Sistemes Complexos Ifisc Csic Uib | Lopez R.,Institute Of Fisica Interdisciplinar I Of Sistemes Complexos Ifisc Csic Uib | Lopez R.,University of the Balearic Islands
Physical Review B - Condensed Matter and Materials Physics | Year: 2013

We investigate the dynamical and transport features of a Kondo dot side coupled to a topological superconductor (TS). The Majorana fermion states (MFSs) formed at the ends of the TS are found to be able to alter the Kondo physics profoundly: For an infinitely long wire where the MFSs do not overlap (εm=0) a finite dot-MFS coupling (Γm) reduces the unitary-limit value of the linear conductance by exactly a factor 3/4 in the weak-coupling regime (ΓmTK), 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 (εm>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. © 2013 American Physical Society.


Lim J.S.,University of the Balearic Islands | Lopez R.,University of the Balearic Islands | Lopez R.,Institute Of Fisica Interdisciplinar I Of Sistemes Complexos Ifisc Csic Uib | Aguado R.,CSIC - Institute of Materials Science
Physical Review Letters | Year: 2011

We demonstrate that curvature-induced spin-orbit coupling induces a 0-π transition in the Josephson current through a carbon nanotube quantum dot coupled to superconducting leads. In the noninteracting regime, the transition can be tuned by applying a parallel magnetic field near the critical field where orbital states become degenerate. Moreover, the interplay between charging and spin-orbit 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 ultraclean carbon nanotubes. © 2011 American Physical Society.


Lim J.S.,University of the Balearic Islands | Sanchez D.,University of the Balearic Islands | Sanchez D.,Institute Of Fisica Interdisciplinar I Of Sistemes Complexos Ifisc Csic Uib | Lopez R.,University of the Balearic Islands | Lopez R.,Institute Of Fisica Interdisciplinar I Of Sistemes Complexos Ifisc Csic Uib
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

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. © 2010 The American Physical Society.


PubMed | Korea Research Institute of Standards and Science, Korea University, Chonnam National University, University of the Balearic Islands and 2 more.
Type: Journal Article | Journal: Physical review letters | Year: 2014

We report nonequilibrium 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 anticrossing 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- transition of the Josephson junction. Our scenario is well understood in terms of only a single dimensionless parameter, k(B)T(K)(min)/, where T(K)(min) and 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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