Abstract
In the present study we investigate the selected local aspects of the metal-induced gap states (MIGSs) at the disordered metal–insulator interface, that were previously proposed to produce magnetic moments responsible for the magnetic flux noise in some of the superconducting qubit modalities. Our analysis attempts to supplement the available studies and provide new theoretical contribution toward their validation. In particular, we explicitly discuss the behavior of the MIGSs in the momentum space as a function of the onsite energy deviation, that mimics random potential disorder at the interface in the local approximation. It is found, that when the difference between the characteristic electronic potentials in the insulator increases, the corresponding MIGSs become more localized. This effect is associated with the increasing degree of the potential disorder that was earlier observed to produce highly localized MIGSs in the superconducting qubits. At the same time, the presented findings show that the disorder-induced localization of the MIGSs can be related directly to the decay characteristics of these states as well as to the bulk electronic properties of the insulator. As a result, our study reinforces plausibility of the previous corresponding investigations on the origin of the flux noise, but also allows to draw future directions toward their better verification.
Highlights
In the present study we investigate the selected local aspects of the metal-induced gap states (MIGSs) at the disordered metal–insulator interface, that were previously proposed to produce magnetic moments responsible for the magnetic flux noise in some of the superconducting qubit modalities
To investigate the localization effect of the MIGSs near the metal–insulator junction (MIJ), we concentrate our attention on the insulator region in a benchmark CsCl structure
To derive the localization characteristics of the MIGSs from the results presented in Fig. 4A, it is instructive to recall the fact that the wave function decay is given by e−κa
Summary
To investigate the localization effect of the MIGSs near the MIJ, we concentrate our attention on the insulator region in a benchmark CsCl structure. Note that the tight-binding parameters are adopted here directly from the study of Choi et al.[25], where above values were chosen in order to set the van Hove singularity away from the conduction band edge at the MIJ, keeping the density of states relatively realistic in the vicinity of the insulators band gap To this end, the U parameter describes the on-site Coulomb repulsion, with ni,σ = ci†,σ ci,σ being the number operator. The Im[k] value at this energy level is interpreted as a characteristic decay rate ( κ ) of the MIGSs within the insulator energy gap; according to the wave function decay defined as e−κa , with a being the lattice constant It provides the effective measure of how well given decaying states are localized near the MIJ. That the presented approach is not limited to the local picture and can be extended further to analyze processes of interest in the framework of a large scale calculations e.g. within the approach derived from the Anderson localization model, as presented in[25]
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