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Register a new userElectrodynamics of granular aluminum from superconductor to insulator: observation of collective superconducting modes
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Florence Levy-Bertrand
Publication date
2019
fields
Physics
and research's language is
English
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We report on a detailed study of the optical response and $T_c-rho$ phase diagram ($T_c$ being the superconducting critical temperature and $rho$ the normal state resistivity of the film) of granular aluminum, combining transport measurements and a high resolution optical spectroscopy technique. The $T_c-rho$ phase diagram is discussed as resulting from an interplay between the phase stiffness, the Coulomb repulsion and the superconducting gap $Delta$. We provide a direct evidence for two different types of well resolved sub-gap absorptions, at $omega_1simeqDelta$ and at $Deltalesssimomega_2lesssim2Delta$ (decreasing with increasing resistivity).
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The introduction of crystalline defects or dopants can give rise to so-called dirty superconductors, characterized by reduced coherence length and quasiparticle mean free path. In particular, granular superconductors such as Granular Aluminum (GrAl), consisting of remarkably uniform grains connected by Josephson contacts have attracted interest since the sixties thanks to their rich phase diagram and practical advantages, like increased critical temperature, critical field, and kinetic inductance. Here we report the measurement and modeling of circuit quantum electrodynamics properties of GrAl microwave resonators in a wide frequency range, up to the spectral superconducting gap. Interestingly, we observe self-Kerr coefficients ranging from $10^{-2}$ Hz to $10^5$ Hz, within an order of magnitude from analytic calculations based on GrAl microstructure. This amenable nonlinearity, combined with the relatively high quality factors in the $10^5$ range, open new avenues for applications in quantum information processing and kinetic inductance detectors.
High kinetic inductance materials constitute a valuable resource for superconducting quantum circuits and hybrid architectures. Superconducting granular aluminum (grAl) reaches kinetic sheet inductances in the nH/$square$ range, with proven applicability in superconducting quantum bits and microwave detectors. Here we show that the single photon internal quality factor $Q_{mathrm{i}}$ of grAl microwave resonators exceeds $10^5$ in magnetic fields up to 1T, aligned in-plane to the grAl films. Small perpendicular magnetic fields, in the range of 0.5mT, enhance $Q_{mathrm{i}}$ by approximately 15%, possibly due to the introduction of quasiparticle traps in the form of fluxons. Further increasing the perpendicular field deteriorates the resonators quality factor. These results open the door for the use of high kinetic inductance grAl structures in circuit quantum electrodynamics and hybrid architectures with magnetic field requirements.
We show that the normal state transport properties of nano-scale granular Aluminum films, near the metal to insulator transition, present striking similarities with those of Kondo systems. Those include a negative magneto-resistance, a minimum of resistance R at a temperature Tm in metallic films, a logarithmic rise at low temperatures and a negative curvature of R(T) at high temperatures. These normal state properties are interpreted in terms of spin-flip scattering of conduction electrons by local magnetic moments, possibly located at the metal/oxide interfaces. Their co-existence with the enhanced superconductivity seen in these films is discussed.
The magnetic field dependence of the superconductivity in nanocrystalline boron doped diamond thin films is reported. Evidence of a glass state in the phase diagram is presented, as demonstrated by electrical resistance and magnetic relaxation measurements. The position of the phase boundary in the H-T plane is determined from resistance data by detailed fitting to zero-dimensional fluctuation conductivity theory. This allows determination of the boundary between resistive and non-resistive behavior to be made with greater precision than the standard ad hoc onset/midpoint/offset criterion.
In certain unconventional superconductors with sizable electronic correlations, the availability of closely competing pairing channels leads to characteristic soft collective fluctuations of the order parameters, which leave fingerprints in many observables and allow to scrutinize the phase competition. Superconducting layered materials, where electron-electron interactions are enhanced with decreasing thickness, are promising candidates to display these correlation effects. For example, while bulk NbSe2 is essentially a conventional superconductor, recent experiments in the thin-film regime have shown evidence of competing unconventional nematic pairing. In this work, we report the existence of a soft collective mode in single-layer NbSe2, observed as a characteristic resonance excitation in high resolution tunneling spectra. This resonance is observed along with higher harmonics, its frequency is anticorrelated with the local superconducting gap, and its amplitude gradually vanishes by increasing the temperature and upon applying a magnetic field up to the critical values (TC and HC2), which sets an unambiguous link to the superconducting state. Aided by a microscopic model, we interpret this resonance as a collective Leggett mode that represents the fluctuation towards a proximate f-wave triplet state, due to subleading attraction in the triplet channel. Our findings demonstrate the fundamental role of correlations in superconducting 2D transition metal dichalcogenides, opening a path towards unconventional superconductivity in simple, scalable and transferable 2D superconductors.
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