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.

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. 73.50.Lw, 73.63.Kv, 73.50.Fq Introduction. Recent advances in nanoscale thermoelectric materials suggest novel functionalities and highly improved performances [1]. A key ingredient of thermoelectric devices is the Seebeck effect, which depends on the simultaneous existence of thermal and electric driving forces. As a result, energy conversion from waste heat is possible under the conditions of zero net current. The Seebeck coefficient S measures the amount of thermovoltage generated across a conducting sample when a thermal gradient is externally applied. Interestingly, the thermoelectric figure-of-merit is proportional to S 2 . Therefore, it is highly desirable to put forward new routes to increase S. Electron-electron interactions may dramatically enhance S in strongly correlated systems as in magnetically diluted metallic hosts [2] and artificial Kondo impurities [3].On the other hand, large temperature drops give rise, quite generally, to thermal rectification effects [4]. The possibility to apply sharp thermal gradients seems to be more feasible in nanostructured materias, as recently demonstrated in superlattices with periods spanning a few nanometers [5]. Strikingly enough, a self-consistent theory of nonlinear thermoelectric transport valid for quantum conductors is still lacking. This is the gap we want to fill in this work.Linear thermoelectric effects within the scattering approach were discussed in Ref. 6. At the same time, pioneering experiments analyzed the main properties of the thermopower at linear response in quantum point contacts [7] and quantum dots [8]. Subsequent advances have unveiled fluctuating thermopower in chaotic dots [9], large S in Andreev interferometers [10] and thermoelectric anisotropies in multiterminal ballistic microjunctions [11]. The Seebeck coefficient can also help determine the conduction character of a molecular junction [12]. Only recently has been possible a clear observation of thermal rectification effects in mesoscopic systems [13]. Thus, it is natural to ask how phase-coherent current and thermopower are affected in the nonlinear regime of transport.In the isothermal case, all terminals are held at the same background temperature T . Refs. 14 and 15 then provide a convenient theoretical framework to include nonequilibrium effects beyond linear response. The theory is based on an expansion around the equilibrium point but, importantly, the nonlinear transpo...

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. Clearly tests of non-equilibrium fluctuation relations are of fundamental interest. From a theoretical point of view, the task is to propose tests in which crucial relations valid at equilibrium fail in the non-linear regime and to demonstrate that, despite such a failure, fluctuation relations hold. For instance we have suggested experiments which test fluctuation relations for systems in the presence of a magnetic field and in a regime where the Onsager relations are already known to fail [1,7]. Such experimental tests are currently under way [8]. Here we propose to test fluctuation relations in a system where away from equilibrium we have no detailed balance. We consider two quantum dots in close proximity to each other such that they interact via long range Coulomb forces. The absence of detailed balance is manifest in a Coulomb drag [9]: the charge noise of one of the systems (the driver) drives a current through the other unbiased system [10]. Therefore, the drag current is a direct indication that this fundamental symmetry is absent. Nevertheless, we below demonstrate that there exist fluctuation relations.The interaction of two systems in close proximity to each other plays a role in many important set-ups in physics. We recall here only the interaction of a detector with a system to be measured [11] which also provides a

We investigate theoretically the non-equilibrium transport properties of carbon nanotube quantum dots. Owing to the two-dimensional band structure of graphene, a double orbital degeneracy plays the role of a pseudo-spin, which is entangled with the spin. Quantum fluctuations between these four degrees of freedom result in an SU(4) Kondo effect at low temperatures. This exotic Kondo effect manifests as a four-peak splitting in the non-linear conductance when an axial magnetic field is applied. [6]. Interestingly, the richness of the band structure of CNTs and the feasibility to attach new materials as electrodes, e.g., ferromagnetic [3] or superconducting contacts [7], allows us to explore new aspects of the Kondo effect, one of the most fundamental topics in condensed matter physics.The electronic states of a CNT form one-dimensional electron and hole sub-bands. They originate from the quantization of the electron wavenumber perpendicular to the nanotube axis, k ⊥ , which arises when graphene is wrapped into a cylinder to create a CNT. By symmetry, for a given sub-band at k ⊥ = k 0 there is a second degenerate sub-band at k ⊥ = −k 0 . Semiclassically, this degeneracy corresponds to the clockwise ( ) or counterclockwise ( ) symmetry of the wrapping modes. Linear transport measurements in the Coulomb blockade regime reveal a distinct four-fold shell filling pattern owing to the orbital and the spin degeneracies [5].In this Letter we combine several theoretical approaches, scaling theory, numerical renormalization group (NRG) [8], non-crossing approximation (NCA) [9], equation-of-motion (EOM) [10] methods, to present a unified picture of low-temperature, non-equilibrium transport through CNT quantum dots in the presence of magnetic fields. We show that quantum fluctuations between the four states { ↑, ↓, ↑, ↓} may dominate transport at low temperatures provided that both the orbital and spin indexes are conserved during tunneling. This leads to a highly symmetric SU(4) Kondo effect, and hence an enhanced Kondo temperature, in which the spin and the orbital degrees of freedom are totally entangled [11]. We also point out that the orbital degeneracy in the dot itself is not enough for having SU(4) Kondo physics. In general, SU(2) Kondo physics is possible. We show that neither an enhanced Kondo temperature nor linear