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Register a new userNon-equilibrium transport through a Josephson quantum dot
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Added by
Jan Frederik Rentrop
Publication date
2014
fields
Physics
and research's language is
English
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We study the electronic current through a quantum dot coupled to two superconducting leads which is driven by either a voltage $V$ or temperature $Delta T$ bias. Finite biases beyond the linear response regime are considered. The local two-particle interaction $U$ on the dot is treated using an approximation scheme within the functional renormalization group approach set up in Keldysh-Nambu-space with $U$ being the small parameter. For $V>0$ we compare our renormalization group enhanced results for the dc-component of the current to earlier weak coupling approaches such as the Hartree-Fock approximation and second order perturbation theory in $U$. We show that in parameter regimes in which finite bias driven multiple Andreev reflections prevail small $|U|$ approaches become unreliable for interactions of appreciable strength. In the complementary regime the convergence of the current with respect to numerical parameters becomes an issue - but can eventually be achieved - and interaction effects turn out to be smaller then expected based on earlier results. For $Delta T>0$ we find a surprising increase of the current as a function of the superconducting phase difference in the regime which at $T=0$ becomes the $pi$ (doublet) phase.
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We consider a quantum dot, affected by a local vibrational mode and contacted to macroscopic leads, in the non-equilibrium steady-state regime. We apply a variational Lang-Firsov transformation and solve the equations of motion of the Green functions in the Kadanoff-Baym formalism up to second order in the interaction coefficients. The variational determination of the transformation parameter through minimization of the thermodynamic potential allows us to calculate the electron/polaron spectral function and conductance for adiabatic to anti-adiabatic phonon frequencies and weak to strong electron-phonon couplings. We investigate the qualitative impact of the quasi-particle renormalization on the inelastic electron tunneling spectroscopy signatures and discuss the possibility of a polaron induced negative differential conductance. In the high-voltage regime we find that the polaron level follows the lead chemical potential to enhance resonant transport.
Scaling laws and universality play an important role in our understanding of critical phenomena and the Kondo effect. Here we present measurements of non-equilibrium transport through a single-channel Kondo quantum dot at low temperature and bias. We find that the low-energy Kondo conductance is consistent with universality between temperature and bias and characterized by a quadratic scaling exponent, as expected for the spin-1/2 Kondo effect. The non-equilibrium Kondo transport measurements are well-described by a universal scaling function with two scaling parameters.
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The Josephson current through an Aharonov-Bohm (AB) interferometer, in which a quantum dot (QD) is situated on one arm and a magnetic flux $Phi$ threads through the ring, has been investigated. With the existence of the magnetic flux, the relation of the Josephson current and the superconductor phase is complex, and the system can be adjusted to $pi$ junction by either modulating the magnetic flux or the QDs energy level $varepsilon_d$. Due to the electron-hole symmetry, the Josephson current $I$ has the property $I(varepsilon_d,Phi)=I(-varepsilon_d,Phi+pi)$. The Josephson current exhibits a jump when a pair of Andreev bound states aligns with the Fermi energy. The condition for the current jump is given. In particularly, we find that the position of the current jump and the position of the maximum value of the critical current $I_c$ are identical. Due to the interference between the two paths, the critical current $I_c$ versus the QDs level $varepsilon_d$ shows a typical Fano shape, which is similar to the Fano effect in the corresponding normal device. But they also show some differences. For example, the critical current never reaches zero for any parameters, while the current in the normal device can reach zero at the destruction point.
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