WorldCat Identities

Lévy-Bertrand, Florence

Works: 5 works in 5 publications in 2 languages and 8 library holdings
Roles: Opponent, Thesis advisor
Publication Timeline
Most widely held works by Florence Lévy-Bertrand
Spectroscopie optique au sub-THz et au sub-Kelvin de supraconducteurs by Olivier Dupré( )

1 edition published in 2018 in French and held by 2 WorldCat member libraries worldwide

During my PhD-thesis, I studied superconductors thanks to a new optical spectroscopy measurements technique, based on photon detectors for astrophysics. This technique enables measurements ranging from 0 to 300 GHz with a resolution of ~1 GHz at a temperature of ~100 mK. The superconductors are lithographed into resonators whose resonance frequency depends on the superfluid density. During this thesis, I made and studied resonators from different superconducting materials in thin films.Dimensionality plays a fundamental role in superconductors. In principle, in two dimensions a system should not be superconducting but there are a lot of counterexamples like single layer of FeSe or oxide interfaces. In this context, aluminum is particularly interesting for mainly two reasons. First, the superconducting mechanism is conventional : it consists in an electron-phonon coupling. Then, it is pretty easy to modify its thickness (thin films) and its microstructure (granular aluminum).The manuscript is composed of two parts.In the first part, I studied the role played by the thickness on the superconductivity of aluminum thin films, ranging from 15 nm to 200 nm. In most superconductors, the critical temperature decreases with thickness, whereas in some materials like aluminum, it increases. Several theories may explain this phenomenon but there is currently no consensus. Thanks to combined techniques of high resolution optical spectroscopy and of resistivity measurements, I suggest that the origin of the critical temperature increase in aluminum thin films would be phonon hardening. This explanation is not among the popular ones.In the second part, we present disordered superconductors, namely indium oxide (InOx) and granular aluminum (GrAl). In these materials, we evidenced sub-gap optical absorptions. In principle, these absorptions are unexpected in superconductors. We show that they are caused by higher order resonance mode excitations combined with kinetic inductance non-linearity with the current circulating in the resonator.In granular aluminum, we studied some sub-gap excitations in two samples with a different room temperature resistivity. A radio-frequency antenna situated in front of the dilution refrigerator illuminates the resonators. We choose a resonator and we select the photon frequency in such a way that it matches with the different studied excitations. We observe the influence of the incident photon power on the resonance. We evidence non standard behaviors of resonance frequency and quality factor, according to the studied excitations
Detection of Weak Terahertz Pulsed Signals Using Kinetic Inductance Detectors by Gizem Soylu( )

1 edition published in 2018 in English and held by 2 WorldCat member libraries worldwide

Development of TiN Vacuum-Gap Capacitor Lumped-Element Kinetic Inductance Detectors by Faouzi Boussaha( )

1 edition published in 2019 in English and held by 2 WorldCat member libraries worldwide

Dissipation dans les circuits quantiques supraconducteurs by Karthik Srikanth Bharadwaj( )

1 edition published in 2021 in English and held by 1 WorldCat member library worldwide

In a world transitioning to the era of second quantum revolution, avoiding processes such as dissipation, which annihilates the quantum state unnecessarily, is crucial. Therefore, understanding the sources of loss responsible for the relaxation in a given quantum system is highly relevant. Since superconducting quantum circuits are the favorite contenders for quantum computing, two directions are followed in this work to study the sources of loss in such circuits.1.Superconducting high quality factor resonators have several applications in quantum technology. They are also the best test beds to study losses. In this work, the study of losses is carried out by realizing Aluminum resonators of different mean free path and capacitances. The behavior of the internal quality factor as a function of applied microwave power and temperature is investigated. The study focuses on the quasiparticle and dielectric losses in the resonators which have been argued to be the main cause of losses in superconducting quantum circuits. A comparative analysis of ground plane loss is also carried out in order to estimate the losses caused by the metallic ground plane. 2.Fluxonium qubit is one of the state of the art superconducting qubits. Their relaxation times are as high as 8-10 milliseconds in 3D architecture. To achieve large scalability, the qubits must be fabricated using 2D architecture. The change in architecture places new constraints on traditional sources of loss while also introducing new ones. The sources of loss causing qubit relaxation in 2D Fluxonium qubit are the area of emphasis in this work. Furthermore, the causes of noise in such a qubit are examined in order to understand the limitations on the coherence of the qubit
Modulation de la supraconductivité hors équilibre avec un STM by Thomas Jalabert( )

1 edition published in 2020 in English and held by 1 WorldCat member library worldwide

Quasiparticles dynamics often governs the ultimate performances of superconducting devices. Out-of-equilibrium superconductivity has therefore attracted a long-standing interest. In order to probe the microscopic mechanisms at play, injection of quasiparticles with the help of a tunnel junction has already been employed at the mesoscopic scale, thanks to the outstanding progress in modern nanotechnology. However, lithographed tunnel junctions lack spatial resolution and do not allow to vary the bias voltage and the tunneling current independently. In order to overcome these two limitations the novelty of this PhD work is to use a Scanning Tunneling Microscope (STM) working at very low temperature (50 mK) to tune the critical current of superconducting nanowires as a function of the tip position and the tunneling set-point.In thin niobium nanowires capped with gold, we measured a drastic reduction of the critical current by injecting a tunnelling current of quasiparticles that is six orders of magnitude lower. We interpret this observation as a local increase of the electronic temperature. We also suggest that the same mechanism is at play in superconducting Field Effect Transistors (SuFETs). The critical current depends strongly on the injection position along the nanowire, the injection rate and the energy of the quasiparticles. At large energies compared to the superconducting gap, the reduction of the critical current is controlled by the injected power. Our measurements show that the diffusion of heat by quasiparticles and phonons explains the injection power and position dependencies, and allow to probe the electron-phonon coupling in our samples. By contrast, when reducing the energy at constant injection rate, the critical current sharply decreases close to the gap energy, signalling the breakdown of the quasi-equilibrium model. We explain this behaviour as a non Fermi Dirac out of equilibrium energy distribution of the quasiparticles, and this allows to estimate the relaxation rate of the quasiparticles. We also probed the spectral properties of current carrying nanowires, and induced magnetic vortices to create spatial variations in the density of states. We thus evidenced the effect of quasiparticle trapping by vortices at the nanometer scale, which is of particular interest since until now the only experiments that allow investigating the dynamics of an inhomogeneous superconducting system necessarily probed a macroscopic volume of the superconductor, rendering explanation of the measurements in terms of the inhomogeneity difficult.Therefore, this experimental work opens a new perspective to investigate the competition between diffusion, relaxation and recombination of quasiparticles in strongly disordered superconductors with various applications such as in photon detection and superconducting electronics
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