Динамическая восприимчивость нанокомпозита пористое стекло/Ga-In-Sn в области сверхпроводимости

Interest to studies of gallium alloys increased recently in relation to their prospective applications for self-healing superconducting connections and wires. Special attention is focused on superconductive properties of nanostructured alloys. In the present work we studied the ac susceptibility of a porous glass/Ga-In-Sn nanocomposite within the temperature range from 1.9 to 8 K at bias fields up to 5 T. Two superconducting phase transitions were revealed with temperatures of 5.6 and 3.1 K. Phase diagrams were created. Positive curvature of the parts of critical lines was demonstrated and treated within the framework of a proximity effect model. Vortex activation barriers were found from shifts of the maxima of the imaginary parts of susceptibility with changing the ac frequency. A bend was shown on the field dependence of the activation barriers.

Current Fluctuations in Hybrid-Superconductor Normal Structures with Quantum Dots

<p>Nanostructures with quantum dots in proximity to superconducting electrodes are an ideal tool to study superconducting correlations in systems with few degrees of freedom that exhibit strong Coulomb-interaction effects. Such hybrid superconductor-normal structures show rich physics due to the interplay of superconductivity, Coulomb interaction and non-equilibrium. Superconducting correlations are established on the quantum dot when it is coupled to a superconductor even in the presence of strong Coulomb repulsion and Cooper pairs can tunnel coherently between the quantum dot and the superconductor. In this thesis, we investigate theoretically electronic transport through an interacting quantum dot coupled to normal and superconducting leads. The presence of the proximity effect can be detected by the dot's current, namely the Andreev current. However, current fluctuations might reveal information on the electronic transport and the internal structure of the system which is not visible in the mean value of the current. For this reason, we study the current fluctuations through the proximized quantum dot to get access to the properties of such a hybrid quantum-dot system. In particular, we are interested in the finite-frequency fluctuations to unveil the coherent dynamics underlying the proximity effect in the quantum dot and its internal time scales. At first, we present a study of the frequency-dependent current noise for subgap transport through an interacting single-level quantum dot tunnel-coupled to normal and superconducting leads. For this purpose, we employ a non-equilibrium diagrammatic real-time approach to calculate the finite-frequency current noise. The finite-frequency noise spectrum shows a sharp dip at a frequency corresponding to the energy splitting of the Andreev bound states which is a signature of the coherent exchange of Cooper pairs between the quantum dot and the superconductor. Furthermore, in the high frequency regime, the so called quantum noise regime, the noise spectrum exhibits steps at frequencies equal to the excitation energies. These steps can be related to the effective coupling strength of the excitations. However, the statistical description of the electron transport does not stop with the noise. Current cumulants of arbitrary order can be obtained by means of full counting statistics (FCS). We set up a theory based on the diagrammatic real-time approach to calculate the finite-time FCS for quantum transport with a non-Markovian master equation that captures the initial correlations between system and reservoir. This allows us to fully describe the current fluctuations of the hybrid quantum-dot system, that is the noise and all higher order current cumulants.</p>

Current Fluctuations in Hybrid-Superconductor Normal Structures with Quantum Dots

<p>Nanostructures with quantum dots in proximity to superconducting electrodes are an ideal tool to study superconducting correlations in systems with few degrees of freedom that exhibit strong Coulomb-interaction effects. Such hybrid superconductor-normal structures show rich physics due to the interplay of superconductivity, Coulomb interaction and non-equilibrium. Superconducting correlations are established on the quantum dot when it is coupled to a superconductor even in the presence of strong Coulomb repulsion and Cooper pairs can tunnel coherently between the quantum dot and the superconductor. In this thesis, we investigate theoretically electronic transport through an interacting quantum dot coupled to normal and superconducting leads. The presence of the proximity effect can be detected by the dot's current, namely the Andreev current. However, current fluctuations might reveal information on the electronic transport and the internal structure of the system which is not visible in the mean value of the current. For this reason, we study the current fluctuations through the proximized quantum dot to get access to the properties of such a hybrid quantum-dot system. In particular, we are interested in the finite-frequency fluctuations to unveil the coherent dynamics underlying the proximity effect in the quantum dot and its internal time scales. At first, we present a study of the frequency-dependent current noise for subgap transport through an interacting single-level quantum dot tunnel-coupled to normal and superconducting leads. For this purpose, we employ a non-equilibrium diagrammatic real-time approach to calculate the finite-frequency current noise. The finite-frequency noise spectrum shows a sharp dip at a frequency corresponding to the energy splitting of the Andreev bound states which is a signature of the coherent exchange of Cooper pairs between the quantum dot and the superconductor. Furthermore, in the high frequency regime, the so called quantum noise regime, the noise spectrum exhibits steps at frequencies equal to the excitation energies. These steps can be related to the effective coupling strength of the excitations. However, the statistical description of the electron transport does not stop with the noise. Current cumulants of arbitrary order can be obtained by means of full counting statistics (FCS). We set up a theory based on the diagrammatic real-time approach to calculate the finite-time FCS for quantum transport with a non-Markovian master equation that captures the initial correlations between system and reservoir. This allows us to fully describe the current fluctuations of the hybrid quantum-dot system, that is the noise and all higher order current cumulants.</p>

Superconducting Proximity Effect in Magnetic Molecules

<p>We studied the transport through magnetic molecules (MM) coupled to superconducting (S), ferromagnetic (F) and normal (N) leads, with the aim of investigating the interplay between the magnetism and the superconducting proximity effect. The magnetic molecules were modeled using the Anderson model with an exchange coupling between the electron spins and the spin of the molecule. We worked in the infinite superconducting gap limit and treated the coupling between the molecule and the superconducting lead exactly, via an effective Hamiltonian. For the F/N-MM-S systems we used a real-time diagrammatic perturbation theory to calculate the electronic transport properties of the systems to first order in the tunnel coupling to the normal or ferromagnetic lead and then analysed the properties with respect to the parameters of these models. For these systems we found that the current maps out the excitation energies of the eigenstates of the effective Hamiltonian and that various parameters in these systems can lead to a negative differential conductance. In the N-MM-S case the current had no overall spin dependence, but when the normal lead is instead ferromagnetic there was a spin dependence and both the electronic and molecular spin expectation values could take on non-zero values. We also found that the polarisation of the ferromagnetic lead suppresses the superconducting proximity effect. Furthermore in the N-MM-S case the Fano factor indicated a transition from Poissonian transport of single electrons to Poissonian transport of electron pairs as the superconducting proximity effect goes out of resonance, however in the F-MM-S case this did not occur. For the S-MM-S systems we calculated the equilibrium Josephson current and found that in the infinite superconducting gap limit no 0 − π transition was possible. Advantages of this study compared to related ones are that we allow for arbitrarily large Coulomb interactions and we take into account coupling to the superconducting lead non-perturbatively. This is however at the expense of working in the superconducting gap limit. Recently it has been possible to couple single molecules to superconducting leads. This study therefore aims to be indicative of the transport properties that will be observed in future experiments involving single magnetic molecules coupled to leads.</p>

Superconducting Proximity Effect in Magnetic Molecules

<p>We studied the transport through magnetic molecules (MM) coupled to superconducting (S), ferromagnetic (F) and normal (N) leads, with the aim of investigating the interplay between the magnetism and the superconducting proximity effect. The magnetic molecules were modeled using the Anderson model with an exchange coupling between the electron spins and the spin of the molecule. We worked in the infinite superconducting gap limit and treated the coupling between the molecule and the superconducting lead exactly, via an effective Hamiltonian. For the F/N-MM-S systems we used a real-time diagrammatic perturbation theory to calculate the electronic transport properties of the systems to first order in the tunnel coupling to the normal or ferromagnetic lead and then analysed the properties with respect to the parameters of these models. For these systems we found that the current maps out the excitation energies of the eigenstates of the effective Hamiltonian and that various parameters in these systems can lead to a negative differential conductance. In the N-MM-S case the current had no overall spin dependence, but when the normal lead is instead ferromagnetic there was a spin dependence and both the electronic and molecular spin expectation values could take on non-zero values. We also found that the polarisation of the ferromagnetic lead suppresses the superconducting proximity effect. Furthermore in the N-MM-S case the Fano factor indicated a transition from Poissonian transport of single electrons to Poissonian transport of electron pairs as the superconducting proximity effect goes out of resonance, however in the F-MM-S case this did not occur. For the S-MM-S systems we calculated the equilibrium Josephson current and found that in the infinite superconducting gap limit no 0 − π transition was possible. Advantages of this study compared to related ones are that we allow for arbitrarily large Coulomb interactions and we take into account coupling to the superconducting lead non-perturbatively. This is however at the expense of working in the superconducting gap limit. Recently it has been possible to couple single molecules to superconducting leads. This study therefore aims to be indicative of the transport properties that will be observed in future experiments involving single magnetic molecules coupled to leads.</p>