Archive ouverte UNIGE | last documents for author 'Davide Filippo Valentinis'https://archive-ouverte.unige.ch/Latest objects deposited in the Archive ouverte UNIGE for author 'Davide Filippo Valentinis'engPropagation of shear stress in strongly interacting metallic Fermi liquids enhances transmission of terahertz radiationhttps://archive-ouverte.unige.ch/unige:151220https://archive-ouverte.unige.ch/unige:151220A highlight of Fermi-liquid phenomenology, as explored in neutral 3He, is the observation that in the collisionless regime shear stress propagates as if one is dealing with the transverse phonon of a solid. The existence of this “transverse zero sound” requires that the quasiparticle mass enhancement exceeds a critical value. Could such a propagating shear stress also exist in strongly correlated electron systems? Despite some noticeable differences with the neutral case in the Galilean continuum, we arrive at the verdict that transverse zero sound should be generic for mass enhancement higher than 3. We present an experimental setup that should be exquisitely sensitive in this regard: the transmission of terahertz radiation through a thin slab of heavy-fermion material will be strongly enhanced at low temperature and accompanied by giant oscillations, which reflect the interference between light itself and the “material photon” being the actual manifestation of transverse zero sound in the charged Fermi liquid.Mon, 26 Apr 2021 10:50:41 +0200Periodicity of superconducting shape resonances in thin filmshttps://archive-ouverte.unige.ch/unige:147103https://archive-ouverte.unige.ch/unige:147103The pairing temperature of superconducting thin films is expected to display, within the Bardeen–Cooper–Schrieffer theory, oscillations as a function of the film thickness. We show that the pattern of these oscillations switches between two different periodicities at a density-dependent value of the superconducting coupling. The transition is most abrupt in the antiadiabatic regime, where the Fermi energy is less than the Debye energy. To support our numerical data, we provide analytical expressions for the chemical potential and the pairing temperature as a function of thickness, which only differ from the exact solution at weak coupling by exponentially small corrections.Tue, 12 Jan 2021 10:50:42 +0100Electronic correlations in interacting quantum matterhttps://archive-ouverte.unige.ch/unige:108682https://archive-ouverte.unige.ch/unige:108682Cette thèse porte sur les corrélations électroniques dans la matière quantique, en particulier dans les systèmes métalliques de basse dimensionalité et à basse température. Dans ces conditions, la réponse du liquide composé par les électrons de conduction aux perturbations devient non-locale dans l’espace. En raison de ça, il est possible de décrire macroscopiquement le liquide d’électrons comme un fluide viscoélastique. Cette approche est appliquée à la réponse électromagnétique des liquides de Fermi, avec nombreuses conséquences observables pour la spectroscopie optique. Dans les supraconducteurs, les corrélations de couplage sont modifiées par la basse dimensionnalité en raison d'une basse densité électronique et du confinement quantique. On étudie ces effets analytiquement et numériquement, en obtenant une solution exacte pour la température critique d'un supraconducteur BCS multibande, dans le bulk et dans la géométrie quasi-2D typique des couches minces et des interfaces. On applique la théorie au titanate de strontium et à l'interface LAO/STO.Mon, 01 Oct 2018 16:36:26 +0200Modulation of the superconducting critical temperature due to quantum confinement at the LaAlO3/SrTiO3 interfacehttps://archive-ouverte.unige.ch/unige:97170https://archive-ouverte.unige.ch/unige:97170Superconductivity develops in bulk doped SrTiO3 and at the LaAlO3/SrTiO3 interface with a dome-shaped density dependence of the critical temperature Tc , despite different dimensionalities and geometries. We propose that the Tc dome of LaAlO3/SrTiO3 is a shape resonance due to quantum confinement of superconducting bulk SrTiO3. We substantiate this interpretation by comparing the exact solutions of a three-dimensional and quasi-two-dimensional two-band BCS gap equation. This comparison highlights the role of heavy bands for Tc in both geometries. For bulk SrTiO3, we extract the density dependence of the pairing interaction from the fit to experimental data. We apply quantum confinement in a square potential well of finite depth and calculate Tc in the confined configuration. We compare the calculated Tc to transport experiments and provide an explanation as to why the optimal Tc's are so close to each other in two-dimensional interfaces and the three-dimensional bulk material.Mon, 02 Oct 2017 09:19:18 +0200Rise and fall of shape resonances in thin films of BCS superconductorshttps://archive-ouverte.unige.ch/unige:87802https://archive-ouverte.unige.ch/unige:87802The confinement of a superconductor in a thin film changes its Fermi-level density of states and is expected to change its critical temperature Tc. Previous calculations have reported large discontinuities of Tc when the chemical potential coincides with a subband edge. By solving the BCS gap equation exactly, we show that such discontinuities are artifacts and that Tc is a continuous function of the film thickness. We also find that Tc is reduced in thin films compared with the bulk if the confinement potential is lower than a critical value, while for stronger confinement Tc increases with decreasing film thickness, reaches a maximum, and eventually drops to zero. Our numerical results are supported by several exact solutions. We finally interpret experimental data for ultrathin lead thin films in terms of a thickness-dependent effective mass.Tue, 27 Sep 2016 12:58:39 +0200BCS superconductivity near the band edge: Exact results for one and several bandshttps://archive-ouverte.unige.ch/unige:85393https://archive-ouverte.unige.ch/unige:85393We revisit the problem of a BCS superconductor in the regime where the Fermi energy is smaller than the Debye energy. This regime is relevant for low-density superconductors such as SrTiO3 that are not in the BEC limit, as well as in the problem of “shape resonances” associated with the confinement of a three-dimensional superconductor. While the problem is not new, exact results were lacking in the low-density limit. In two dimensions, we find that the initial rise of the pairing temperature Tc at low density n is nonanalytic and faster than any power of n. In three dimensions, we also find that Tc is nonanalytic, but starts with zero slope at weak coupling and infinite slope at strong coupling. Self-consistent treatment of the chemical potential and energy dependence of the density of states are crucial ingredients to obtain these results. We also present exact results for multiband systems and confirm our analytical expressions by numerical simulations.Wed, 20 Jul 2016 16:01:59 +0200Electromagnetic properties of viscous charged fluidshttps://archive-ouverte.unige.ch/unige:39150https://archive-ouverte.unige.ch/unige:39150We provide a general theoretical framework to describe the electromagnetic properties of viscous charged fluids, consisting, for example, of electrons in certain solids or plasmas. We confirm that finite viscosity leads to multiple modes of evanescent electromagnetic waves at a given frequency, one of which is characterized by a negative index of refraction, as previously discussed in a simplified model by one of the authors. In particular, we explain how optical spectroscopy can be used to probe the viscosity. We concentrate on the impact of this on the coefficients of refraction and reflection at the sample-vacuum interface. Analytical expressions are obtained relating the viscosity parameter to the reflection and transmission coefficients of light.We demonstrate that finite viscosity has the effect to decrease the reflectivity of a metallic surface, while the electromagnetic field penetrates more deeply. While on a phenomenological level there are similarities to the anomalous skin effect, the model presented here requires no particular assumptions regarding the corpuscular nature of the charge liquid. A striking consequence of the branching phenomenon into two degenerate modes is the occurrence in a half-infinite sample of oscillations of the electromagnetic field intensity as a function of distance from theTue, 05 Aug 2014 10:36:54 +0200Extensive Characterization of the 1 mm PIT Nb3Sn Strand for the 13-T FRESCA2 Magnethttps://archive-ouverte.unige.ch/unige:36164https://archive-ouverte.unige.ch/unige:36164In the framework of the EuCARD program, CERN is participating in the development of a 13 T 100-mm-aperture dipole magnet to upgrade the superconducting cable test facility FRESCA at CERN. The conductor candidates for building this magnet are two 1-mm Nb3Sn strands: the Powder In Tube (PIT) produced by Bruker-EAS and the 132/169 RRP by Oxford Superconducting Technology (OST). Recently the PIT strand has been extensively characterized by CERN in collaboration with the University of Geneva (UniGe). The critical current dependence on the magnetic field and on the axial strain has been measured at different temperatures. Furthermore, the strand magnetization has been measured at different temperature using a vibrating sample magnetometer. Finally the magneto-thermal stability of this strand was studied by measuring the quench current between 0 T and 12 T at 1.9 K and 4.3 K. The experimental results are compared with an optimized scaling law for the critical current of Nb3Sn strands. In this paper the results obtained for the PIT strand are summarized and discussed.Tue, 29 Apr 2014 14:48:44 +0200A theory of the strain-dependent critical field in Nb3Sn, based on anharmonic phonon generationhttps://archive-ouverte.unige.ch/unige:33264https://archive-ouverte.unige.ch/unige:33264We propose a theory to explain the strain dependence of the critical properties in A15 superconductors. Starting from the strong-coupling formula for the critical temperature, and assuming that the strain sensitivity stems mostly from the electron-phonon alpha^2F function, we link the strain dependence of the critical properties to a widening of alpha^2F. This widening is attributed to the nonlinear generation of phonons, which takes place in the anharmonic deformation potential induced by the strain. Based on the theory of sum- and difference-frequency wave generation in nonlinear media, we obtain an explicit connection between the widening of alpha^2F and the anharmonic energy. The resulting model is fit to experimental datasets for Nb3Sn, and the anharmonic energy extracted from the fits is compared with first-principles calculations.Wed, 15 Jan 2014 14:53:12 +0100