Recent theoretical and experimental work has conclusively demonstrated that using metamaterials in dielectric response engineering can increase the critical temperature of a composite superconductor-dielectric metamaterial. This enables numerous practical applications, such as transmitting electrical energy without loss, and magnetic levitation devices. Dielectric response engineering is based on the description of superconductors in terms of their dielectric response function which is applicable as long as the material may be considered a homogeneous medium on the spatial scale, below the superconducting coherence length. With this in mind, the next logical step is to use recently developed plasmonics and electromagnetic metamaterial tools to engineer and maximize the electron pairing interaction in an artificial ‘ metamaterial superconductor’ by deliberately engineering its dielectric response function.
Researchers expect considerable enhancement of attractive electron-electron interaction in metamaterial scenarios such as epsilon-near-zero (ENZ, an artificial material engineered such that its dielectric permittivity—usually denoted as ‘ epsilon’—becomes very close to zero) and hyperbolic metamaterials (artificial materials with very strong anisotropy that behave like a metal in one direction, and a dielectric in another orthogonal direction).
They verified such phenomena in experiments with compressed mixtures of tin and barium titanate nanoparticles of varying composition. The results showed a deep connection between the fields of superconductivity and electromagnetic metamaterials. However, despite this initial success, the observed critical temperature increase was modest. We argued that the random nanoparticle mixture geometry may not be ideal because simple mixing of superconductor and dielectric nanoparticles results in substantial spatial variations of the dielectric response function throughout a metamaterial sample. Such variations lead to considerable broadening and suppression of the superconducting transition.
To overcome this issue, we considered using an ENZ plasmonic core-shell metamaterial geometry designed to achieve partial cloaking of macroscopic objects. The cloaking effect relies on mutual cancellation of scattering by the dielectric core and plasmonic shell of the nanoparticle, so that the effective dielectric constant of the nanoparticle becomes very small and close to that of a vacuum.
They have undertaken the first successful realization of an ENZ core-shell metamaterial superconductor using compressed aluminum oxide (Al2O3)-coated 18nm-diameter aluminum (Al) nanoparticles. This led to a tripling of the metamaterial critical temperature compared to the bulk aluminum. The material is ideal for proof-of-principle experiments because the critical temperature of aluminum is quite low (TcAl=1.2K), leading to a very large superconducting coherence length of ∼1600nm. Such length facilitates the metamaterial fabrication requirements. Upon exposure to the ambient conditions an ∼2nm-thick Al2O3 shell forms on the aluminum nanoparticle surface, which is comparable to the 9nm radius of the original aluminum nanoparticle.
The highest onset temperature of the superconducting transition reached 3.9K, which is more than three times as high as the critical temperature of bulk aluminum, TcAl=1.2K.
They anticipate that it may be possible to implement the same approach to other known superconductors with higher critical temperature, and our future work will focus on exploring these possibilities.
Schematic geometry of the epsilon-near-zero metamaterial superconductor based on core-shell nanoparticle geometry. The nanoparticle diameter is 18nm. The inset shows typical core-shell metamaterial dimensions. Al: Aluminum. Al2O3: Aluminum oxide.
SOURCE – Vera Smolyaninova, Towson University at SPIE