arXiv subjects

Eli Farber

Publications and source records attributed to Eli Farber.

6 recordsLinked to original sources

arXiv2021

Tunneling study in granular aluminum near the Mott metal-to-insulator transition

We find excellent agreement between tunneling and optical conductivity gap values in superconducting granular aluminum films, up to the metal-to-insulator transition. This behavior, in strong contrast with that recently reported in atomically disordered samples for which the optical gap becomes smaller than the tunneling gap, suggests that disorder is not at the origin of the transition. The large increase seen in the strong coupling ratio and a finite value of the gap at T_c near the metal-to-insulator transition are consistent with a BCS to BEC crossover.

cond-mat.supr-con↗

arXiv2020

Granular superconductors for high kinetic inductance and low loss quantum devices

Granular aluminum is a promising material for high kinetic inductance devices such as qubit circuits. It has the advantage over atomically disordered materials such as NbN_x, to maintain a high kinetic inductance concomitantly with a high quality factor. We show that high quality nano-scale granular aluminum films having a sharp superconducting transition with normal state resistivity values of the order of 1x10^5 μΩcm and kinetic inductance values of the order of 10 nH/sq can be obtained, surpassing state of the art values. We argue that this is a result of the different nature of the metal-to-insulator transition, being electronic correlations driven (Mott type) in the former and disorder driven (Anderson type) in the latter.

cond-mat.supr-con↗

arXiv2019

Optical conductivity of granular aluminum films near the Mott metal-to-insulator transition

We report measurements of the energy gap of granular aluminum films by THz spectroscopy. We find that as the grains progressively decouple, the coupling ratio $2Δ(0)/k_{B}T_{c}$ increases above the BCS weak coupling ratio $3.53$, and reaches values consistent with an approach to BCS-BEC crossover for the high resistivity samples, expected from the short coherence length. The Mattis-Bardeen theory describes remarkably well the behavior of $σ_{1,s}/σ_{1,n}$ for all samples up to very high normal state resistivities.

cond-mat.supr-con↗

arXiv2017

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 $T_c$ after breaking the $U(1)$ 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.

cond-mat.supr-con↗

arXiv2015

Shaping a superconducting dome: Enhanced Cooper-pairing versus suppressed phase coherence in coupled aluminum nanograins

Deterministic enhancement of the superconducting (SC) critical temperature $T_c$ 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 $Δ$, 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 $J$. In the case of coupled aluminium (Al) nanograins, $T_c$ 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, $Δ$ and $J$, as a function of grain coupling. Starting from well-coupled arrays, $Δ$ grows with progressive grain decoupling, causing the increasing of $T_c$. As the grain-coupling is further suppressed, $Δ$ saturates while $T_c$ decreases, concomitantly with a sharp decline of $J$. This crossover to a phase-driven SC transition is accompanied by an optical gap persisting above $T_c$. These findings identify granular Al as an ideal playground to test the basic mechanisms that enhance superconductivity by nano-inhomogeneity.

cond-mat.supr-con↗

arXiv2012

Evidence of a subenergy gap in the overdoped regime of Y_{1-x}Ca_{x}Ba_{2}Cu_{3}O_{7-δ} thin films from THz Spectroscopy

We measured the terahertz (THz) complex conductivity of Ca doped YBa_{2}Cu_{3}O_{7-δ} thin films in the frequency range of 0.1 to 3 THz (3 to 100 cm^{-1}) and at a temperature range of 20 to 300 K. The films were measured using both time domain and frequency domain THz methods. We showed evidence for the existence of a sub-gap in overdoped Y_{1-x}Ca_{x}Ba_{2}Cu_{3}O_{7-δ} samples doped with 5% and 10% Ca. Evidence for the opening of this sub-gap appears as a sharp decrease in the spectrum of the real part of conductivity at frequencies equivalent to a gap energy of 1 meV and is more prominent with increased doping. This decrease in conductivity can be explained by using d-wave pairing symmetry with an imaginary part of is or id_{xy} which suggests node removal.

cond-mat.supr-con↗