12 research outputs found
Shaping a superconducting dome: Enhanced Cooper-pairing versus suppressed phase coherence in coupled aluminum nanograins
Deterministic enhancement of the superconducting (SC) critical temperature
is a long-standing goal in material science. One strategy is engineering
a material at the nanometer scale such that quantum confinement strengthens the
electron pairing, thus increasing the superconducting energy gap , as
was observed for individual nanoparticles. A true phase-coherent SC condensate,
however, can exist only on larger scales and requires a finite phase stiffness
. In the case of coupled aluminium (Al) nanograins, can exceed that of
bulk Al by a factor of three, but despite several proposals the relevant
mechanism at play is not yet understood. Here we use optical spectroscopy on
granular Al to disentangle the evolution of the fundamental SC energy scales,
and , as a function of grain coupling. Starting from well-coupled
arrays, grows with progressive grain decoupling, causing the
increasing of . As the grain-coupling is further suppressed,
saturates while decreases, concomitantly with a sharp decline of .
This crossover to a phase-driven SC transition is accompanied by an optical gap
persisting above . These findings identify granular Al as an ideal
playground to test the basic mechanisms that enhance superconductivity by
nano-inhomogeneity.Comment: 6 + 6 pages (manuscript + supplementary material
THz conductivity of SrCaRuO
We investigate the optical conductivity of SrCaRuO across the
ferromagnetic to paramagnetic transition that occurs at . The thin films
were grown by metalorganic aerosol deposition with onto
NdGaO substrates. We performed THz frequency domain spectroscopy in a
frequency range from 3~cm to 40~cm (100~GHz to 1.4~THz) and at
temperatures ranging from 5~K to 300~K, measuring transmittivity and phase
shift through the films. From this we obtained real and imaginary parts of the
optical conductivity. The end-members, ferromagnetic SrRuO and paramagnetic
CaRuO, show a strongly frequency-dependent metallic response at
temperatures below 20~K. Due to the high quality of these samples we can access
pronounced intrinsic electronic contributions to the optical scattering rate,
which at 1.4~THz exceeds the residual scattering rate by more than a factor of
three. Deviations from a Drude response start at about 0.7~THz for both
end-members in a remarkably similar way. For the intermediate members a higher
residual scattering originating in the compositional disorder leads to a
featureless optical response, instead. The relevance of low-lying interband
transitions is addressed by a calculation of the optical conductivity within
density functional theory in the local density approximation (LDA)
Optical signatures of the superconducting Goldstone mode in granular aluminum: experiments and theory
Recent advances in the experimental growth and control of disordered thin
films, heterostructures, and interfaces provide a fertile ground for the
observation and characterisation of the collective superconducting excitations
emerging below after breaking the gauge symmetry. Here we combine
THz experiments in a nano-structured granular Al thin film and theoretical
calculations to demonstrate the existence of optically-active phase modes,
which represent the Goldstone excitations of the broken gauge symmetry. By
measuring the complex transmission trough the sample we identify a sizeable and
temperature-dependent optical sub-gap absorption, which cannot be ascribed to
quasiparticle excitations. A quantitative modelling of this material as a
disordered Josephson array of nano-grains allows us to determine, with no free
parameters, the structure of the spatial inhomogeneities induced by shell
effects. Besides being responsible for the enhancement of the critical
temperature with respect to bulk Al, already observed in the past, this spatial
inhomogeneity provides a mechanism for the optical visibility of the Goldstone
mode. By computing explicitly the optical spectrum of the superconducting phase
fluctuations we obtain a good quantitative description of the experimental
data. Our results demonstrate that nanograins arrays are a promising setting to
study and control the collective superconducting excitations via optical means