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58 records · Page 4

All sources NASA NTRS DOE OSTI

DOE OSTI2020

Vortex Matter in Confined Superconductors and Mesoscopic Hybrid Heterostructures. Final report

This program studied the fundamental features that underlie the behavior of hybrid ferromagnet/superconductor heterostructures both in the regime of magnetic coupling and proximity effect. In ferromagnet/superconductor hybrid structures, the source of superconducting correlations and ferromagnetism are spatially separated. Therefore, the interplay between ferromagnetism and superconductivity results in physical effects that are not easily observed in bulk superconductors and allows to tune the local superconducting and vortex properties, realizing, thus, a new class of materials. In the case of proximity effect the ferromagnet and the superconductor are in contact and leakage of superconducting pairs is allowed into the ferromagnetic layer. Typically, the decay length for the superconducting pair correlations in the ferromagnet is of the order of one nanometer due to the typically large ferromagnetic exchange field. However, a suitable magnetic structure in the ferromagnetic material can be used to align the spins of the superconducting pairs allowing much longer penetration of superconductivity in the magnetic material. These superconducting pairs with aligned spins can carry spin and provide the basis for superconducting spintronics devices. In the case of magnetic coupling the effect of a spatial inhomogeneous stray field produced by the ferromagnet substantially modifies the nucleation of the order parameter in the superconducting layer and can lead to a significant change in the local superconducting properties. Therefore, magnetically coupled planar ferromagnet-superconductor hybrid structures offer new avenues for manipulation of the superconductivity at the nanoscale and convenient means to control vortex dynamics.

36 MATERIALS SCIENCE↗

DOE OSTI2020

Autonomous quantum absorption refrigerators

Here, we propose a quantum absorption refrigerator using the quantum physics of resonant tunneling through quantum dots. The cold and hot reservoirs are fermionic leads, tunnel coupled via quantum dots to a central fermionic cavity, and we propose configurations in which the heat absorbed from the (very hot) central cavity is used as a resource to selectively transfer heat from the cold reservoir on the left to the hot reservoir on the right. Heat transport in the device is particle-hole symmetric; we find two regimes of cooling as a function of the dot energies—symmetric with respect to the Fermi energy of the reservoirs—and we associate them with heat transfer by electrons above the Fermi level and holes below the Fermi level. We also discuss optimizing the cooling effect by fine-tuning the energy of the dots as well as their linewidth and characterize regimes where the transport is thermodynamically reversible such that the Carnot coefficient of performance is achieved with zero cooling power delivered.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2020

Mesoscopic theory of defect ordering–disordering transitions in thin oxide films

Abstract Ordering of mobile defects in functional materials can give rise to fundamentally new phases possessing ferroic and multiferroic functionalities. Here we develop the Landau theory for strain induced ordering of defects (e.g. oxygen vacancies) in thin oxide films, considering both the ordering and wavelength of possible instabilities. Using derived analytical expressions for the energies of various defect-ordered states, we calculated and analyzed phase diagrams dependence on the film-substrate mismatch strain, concentration of defects, and Vegard coefficients. Obtained results open possibilities to create and control superstructures of ordered defects in thin oxide films by selecting the appropriate substrate and defect concentration.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2021

Precision measurement of electron-electron scattering in GaAs/AlGaAs using transverse magnetic focusing

Electron-electron (e-e) interactions assume a cardinal role in solid-state physics. Quantifying the e-e scattering length is hence critical. In this paper we show that the mesoscopic phenomenon of transverse magnetic focusing (TMF) in two-dimensional electron systems forms a precise and sensitive technique to measure this length scale. Conversely we quantitatively demonstrate that e-e scattering is the predominant effect limiting TMF amplitudes in high-mobility materials. Using high-resolution kinetic simulations, we show that the TMF amplitude at a maximum decays exponentially as a function of the e-e scattering length, which leads to a ready approach to extract this length from the measured TMF amplitudes. The approach is applied to measure the temperature-dependent e-e scattering length in high-mobility GaAs/AlGaAs heterostructures. The simulations further reveal current vortices that accompany the cyclotron orbits - a collective phenomenon counterintuitive to the ballistic transport underlying a TMF setting.

36 MATERIALS SCIENCE↗