Observation of plasmon-phonons in a metamaterial superconductor using inelastic neutron scattering

Abstract

A metamaterial approach is capable of drastically increasing the critical temperature, T c, of composite metal-dielectric superconductors as demonstrated by the tripling of T c that was observed in bulk Al-Al 2 O 3 core-shell metamaterials. A theoretical model based on the Maxwell-Garnett approximation provides a microscopic explanation of this effect in terms of electron-electron pairing mediated by a hybrid plasmon-phonon excitation. We report the first observation of this excitation in Al-Al 2 O 3 core-shell metamaterials using inelastic neutron scattering. This result provides support for this novel mechanism of superconductivity in metamaterials. Recent theoretical [1-3] and experimental [4-6] work has demonstrated that many tools developed in electromagnetic metamaterial research can be used to engineer artificial metamaterial superconductors having improved superconducting properties. This connection between electromagnetic metamaterials and superconductivity research stems from the fact that superconducting properties of a material may be expressed via its effective dielectric response function  eff (q,), and the critical temperature, T c , of a superconductor is defined by the behaviour of  eff  -1 (q,) near its poles [7]. In conventional superconductors these poles are defined by the dispersion law, (q), of phonons which mediate electron-electron pairing. Recently, we have demonstrated a considerable enhancement of the attractive electron-electron interaction in such metamaterial scenarios as epsilon near zero (ENZ) [8] and hyperbolic metamaterials.[9] In both cases the inverse dielectric response function of the metamaterial may exhibit additional poles compared to the parent superconductor. The most striking example of successful metamaterial superconductor engineering was the observation of tripling of the critical temperature T c in Al-Al 2 O 3 epsilon near zero (ENZ) core-shell metamaterials compared to bulk aluminium [5]. The formation of these bulk Al-Al 2 O 3 samples enable studies that require large sample volumes, such as neutron scattering, that can reveal phonon spectral features that are not accessible in thin films.Here, we report on the use of inelastic neutron scattering to provide the first experimental evidence of an excitation that does not exist in pure Al and corresponds to the metamaterial pole of the engineered inverse dielectric response function responsible for the T c enhancement in this material. We also identify the microscopic physical origin of this additional metamaterial pole as coming from a hybrid plasmon-phonon mode which arises in a composite metal-dielectric metamaterial. The hybrid character of this mode enables efficient inelastic neutron scattering from its phonon component, which is observed in the experiment. The direct observation of plasmon-phonon modes in Al-Al 2 O 3 ENZ core-shell metamaterials using inelastic neutron scattering provides

Phonon softening in nanostructured phonon–mediated superconductors (review)

“…The diameter of Al nanoparticles was 18 nm and thickness of the dielectric shell was 2 nm. The superconducting coherence length of Al was measured as ξ ≈ 105 nm [393]. This is much larger than the size of an aluminum nanoparticle.…”

“…Because of this, in order to study the microscopic mechanism of T c growth in such metamaterials, it is possible to use INS technique, which is not applicable for nanoscale samples due to their small volume. The INS data for Al 2 O 3 /Al superconducting metamaterial [393] were compared with those for pure bulk Al [395]. The PhDOS of Al 2 O 3 /Al was obtained from these measurements.…”

Phenomenology of phonon-plasmon coupled excitations in three dimensional polar materials

“…Besides the standard Raman and the infrared spectroscopy for momenta close to the center of the Brillouin zone, an alternative with a sufficient energy resolution to capture [45] dispersions of phonon-plasmon coupled modes are experiments based on the inelastic neutron scattering. The coupling strength of a neutron to material excitations is directly proportional to the amount of ionic motion present in corresponding excitations, rendering inelastic neutron scattering a powerful method to measure phonon dispersions [46,47].…”

Hybrid acousto-electromagnetic metamaterial superconductors

“…In both cases eff -1 (q,) of the metamaterial exhibits additional poles compared to the bulk aluminium. These additional poles were identified as plasmon-phonons of the composite metamaterial structures, and they were observed using inelastic neutron scattering [5]. The hybrid acousto-electromagnetic nature of these excitations gives us a strong hint that further improvement of the superconducting properties of artificial metamaterials should arise from combined use of both acoustic metamaterial and electromagnetic metamaterial toolboxes.…”

“…Recent theoretical [1,2] and experimental [3][4][5] studies demonstrated that an electromagnetic metamaterial approach is capable of drastically increasing the critical temperature of composite metamaterial superconductors. Metamaterials are made from assemblies of multiple elements arranged in repeating subwavelength structures (which are sometimes called "artificial atoms"), and they derive their macroscopic properties from the properties of these newly designed subwavelength structures.…”