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arXiv:0808.3885

Fractionally Quantized Cooper Pair Stair Case in Superconducting Quantum Dots

Sujit Sarkar

Published Aug 28, 2008• 0 citations
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Abstract

We study clean superconducting quantum dots (SQD) and also site dependent Josephson couplings, on site charging energies and the intersite interactions in presence of gate voltage. We predict the existence of different fractionally quantized Cooper pair stair case with many interesting physical properties. The appearance of stair case is not only due to the Coulomb blocked phenomena but also for the site dependent Josephson couplings. We also explain physically the absence of other fractionally quantized Cooper pair stair case. The physics of fractionally quantized Cooper pair stair case has close resemblance with the fractionally quantized magnetization plateau of a spin chain system under a magnetic field.

DOI
10.48550/arxiv.0808.3885

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We present a study of quantum phase analysis of inhomogeneous and homogeneous arrays of superconducting quantum dots (SQD). We observe the existance of Josephson decouple (JD) phase only at the half filling for inhomogeneous array of SQD due to the fluctuation of Josephson couplings over the sites at half filling. In JD phase superconductivity disappears even in the absence of Coulomb blockade phase. We also observe that fluctuation of on-site Coulomb charging energy produces the relevant coupling term that yields Coulomb blockade gapped phase. The presence of nearest-neighbor and next-nearest-neighbor Coulomb interaction yields the same physics for inhomogeneous and homogeneous SQD.
A quantum phase analysis of inhomogeneous arrays of superconducting quantum dots (SQD) is presented. Existence of Josephson decoupling (JD) phase is observed at the half-filling. It occurs due to fluctuations of Josephson couplings over the sites. This JD phase is the phase incoherent gapped state of the system. In JD phase superconductivity disappears even in the absence of Coulomb blockade phase. It is also observed that the inhomogeneity of on-site Coulomb charging energy produces the relevant coupling term that yields Coulomb blockade gapped phase. The presence of nearest-neighbor and next-nearest-neighbor Coulomb interaction yields the same physics for inhomogeneous and homogeneous SQD.
We analyze the quantum phases of a superconducting quantum dot array. Superconducting quantum dots are connected through Josephson junction. We study the effect of finite range Josephson coupling and also finite range interactions between the dots. Our field-theoretical study predicts different kinds of phases near the charge degeneracy point. We predict two different kinds of repulsive Luttinger liquid phases originate from different sources: one from the commensurate-incommensurate transition and the other from the XYXYXY criticality. Apart from the charge-density wave, we also predict the possibility of dimer-order density wave from the interplay of long-range interaction between the dots and Cooper-pair density. We observe different multicritical points with interesting phases and also two special points. The renormalization group flow diagram also shows many interesting phases.
We present the quantum phase diagram of a superconducting quantum dots array. Superconducting quantum dots are connected through Josephson junctions. We study the effect of finite-range Josephson coupling and also finite-range interactions between the dots. Our field-theoretical study predicts different kinds of phase near the charge degeneracy point. We predict two different kinds of repulsive Luttinger liquid phase and also the possibilities of different density wave instabilities from the inter-play of long-range interaction between the dots and Cooper pair density. We observe different multicritical points with interesting phases.
In the field of nanoelectronics, one seeks to combine quantum mechanical effects with macroscopic entities such as currents in order to find new ways of storing and manipulating information. One possibility to accomplish this is to use the quantum correlations found in superconductors. Teaming superconductors with normal metals and magnetic systems, one can obtain novel correlation and fluctuation effects. \n\nIn the first work of this thesis, we examine how charge correlations in a metallic dot are modified due to the presence of a capacitively coupled superconducting island. It is possible to overcome the Coulomb repulsion between electrons in a hybrid double-dot system consisting of a metallic island coupled capacitively to a Cooper-pair box. The superconducting island S acts as a dynamic gate on the metallic island N and in the strong coupling regime the electric charge of the Cooper pairs in S and their fluctuations are able to overscreen the repulsion between electrons in N. This produces an attractive interaction between two additional electrons in the metallic island and results in a non-monotonous charging curve of the metallic island. \n\nThe second part of the thesis considers how the dynamics of a precessing molecular magnetic moment affects the Josephson current. The precession gives rise to a time-dependent tunnel potential which not only creates different tunneling probabilities for spin-up and spin-down quasiparticles, but also introduces a time-dependent spin-flip term. In particular, we study the effects of the spin-flip term alone on the Josephson current between two spin-singlet superconductors as a function of precession frequency and junction transparency. The system displays a nonequilibrium steady-state solution which depends on the precession frequency of the classical spin.\n\n\n
The quantum phase transition in two capacitively coupled arrays of superconducting quantum dots (SQD) by considering the presence of gate voltage in each superconducting island is presented. We show explicitly that the cotunneling process associated with two coupled SQD arrays, near the maximum-charge frustration line is not sufficient to explain the correct quantum phases with physically consistent phase boundaries. We consider an extra cotunneling process along each chain to explain the correct quantum phases with physically consistent phase boundaries. There is no evidence of supersolid phase in our study. The Bethe ansatz and Abelian bosonization method is used to solve the problem.