12 research outputs found
Spin-Orbit induced phase-shift in BiSe Josephson junctions
The transmission of Cooper pairs between two weakly coupled superconductors
produces a superfluid current and a phase difference; the celebrated Josephson
effect. Because of time-reversal and parity symmetries, there is no Josephson
current without a phase difference between two superconductors. Reciprocally,
when those two symmetries are broken, an anomalous supercurrent can exist in
the absence of phase bias or, equivalently, an anomalous phase shift
can exist in the absence of a superfluid current. We report on the
observation of an anomalous phase shift in hybrid Josephson
junctions fabricated with the topological insulator BiSe submitted to
an in-plane magnetic field. This anomalous phase shift is observed
directly through measurements of the current-phase relationship in a Josephson
interferometer. This result provides a direct measurement of the spin-orbit
coupling strength and open new possibilities for phase-controlled Josephson
devices made from materials with strong spin-orbit coupling
Quantum confinement effects in Pb Nanocrystals grown on InAs
In the recent work of Ref.\cite{Vlaic2017-bs}, it has been shown that Pb
nanocrystals grown on the electron accumulation layer at the (110) surface of
InAs are in the regime of Coulomb blockade. This enabled the first scanning
tunneling spectroscopy study of the superconducting parity effect across the
Anderson limit. The nature of the tunnel barrier between the nanocrystals and
the substrate has been attributed to a quantum constriction of the electronic
wave-function at the interface due to the large Fermi wavelength of the
electron accumulation layer in InAs. In this manuscript, we detail and review
the arguments leading to this conclusion. Furthermore, we show that, thanks to
this highly clean tunnel barrier, this system is remarkably suited for the
study of discrete electronic levels induced by quantum confinement effects in
the Pb nanocrystals. We identified three distinct regimes of quantum
confinement. For the largest nanocrystals, quantum confinement effects appear
through the formation of quantum well states regularly organized in energy and
in space. For the smallest nanocrystals, only atomic-like electronic levels
separated by a large energy scale are observed. Finally, in the intermediate
size regime, discrete electronic levels associated to electronic wave-functions
with a random spatial structure are observed, as expected from Random Matrix
Theory.Comment: Main 12 pages, Supp: 6 page
Etude par spectroscopie infrarouge de l'Ă©lectrodynamique du supraconducteur Pr2-xCexCuO4 et de la manganite La2/3 Ca1/3 Mn03
We report the temperature dependence of the infrared-visible ab plane response of Pr2-xCexCuO4. In the normal state, when varying the doping from a non-superconducting film x=0.11 to a superconducting overdoped film x=0.17, we observe a partial gap opening up to optimal doping x=0.15. A spin density wave gap model reproduces qualitatively our data. The magnitude of this gap extrapolates to zero for x=0.17 indicating the existence of a quantum critical point at this Ce concentration and the coexistence of magnetism and superconductivity. Below Tc for x=0.15 and x=0.17, the modification of the optical spectra at energies less than 2Delta=4.7kBTc is interpreted as the opening of the superconducting gap. The measured optical response of La2/3Ca1/3MnO3 is analyzed with an effective medium approximation model. An extra absorption close to the insolating/metallic transition temperature is interpreted as the signature of percolation in the sample.Nous présentons la dépendance en température de la réponse infrarouge-visible des plans ab de Pr2-xCexCuO4. Dans l'état normal, en variant le dopage de couches minces de x=0.11 (non-supraconductrice) à x=0.17 (supraconductrice surdopée), on observe l'ouverture d'un gap partiel jusqu'au dopage optimal x=0.15. Un modÚle d'onde de densité de spin reproduit qualitativement des données. En extrapolant l'amplitude de ce gap, celui-ci s'annule pour x=0.17 indiquant la présence d'un point critique quantique ainsi que la coexistence de magnétisme et de supraconductivité. En dessous de Tc, pour x=0.15 et x=0.17, la modification des spectres optiques à des énergies inférieures à 2Delta=4.7kBTc est interprétée comme la signature de l'ouverture du gap supraconducteur. La réponse optique de La2/3Ca1/3MnO3 mesurée est analysée par un modÚle de milieu effectif. L'absorption supplémentaire proche de la transition isolant/métal est interprétée comme la signature d'une percolation dans le systÚme
Etude par spectroscopie infrarouge de l'Ă©lectrodynamique du supraconducteur Pr2-xCexCuO4 et de la manganite La2/3Ca1/3MnO3
PARIS-BIUSJ-ThĂšses (751052125) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF
Shiba Bound States across the Mobility Edge in Doped InAs Nanowires
International audienceWe present a study of Andreev quantum dots fabricated with small-diameter (30 nm) Si-doped InAs nanowires where the Fermi level can be tuned across a mobility edge separating localized states from delocalized states. The transition to the insulating phase is identified by a drop in the amplitude and width of the excited levels and is found to have remarkable consequences on the spectrum of superconducting subgap resonances. While at deeply localized levels only quasiparticle cotunneling is observed, for slightly delocalized levels Shiba bound states form and a parity-changing quantum phase transition is identified by a crossing of the bound states at zero energy. Finally, in the metallic regime, single Andreev resonances are observed
Transport in a Single Self-Doped Nanocrystal
International audienceAddressing the optical properties of a single nanoparticle in the infrared is particularly challenging, thus alternative methods for characterizing the conductance spectrum of nanoparticles in this spectral range need to be developed. Here we describe an efficient method of fabricating single nanoparticle tunnel junctions on a chip circuit. We apply this method to narrow band gap nanoparticles of HgSe, which band structure combines the inverted character of the bulk semimetal with quantum confinement and self-doping. Upon tuning the gate bias, measurement reveals the presence of two energy gaps in the spectrum. The wider gap results from the interband gap, while the narrower gap results from intraband transitions. The observation of the latter near zero gate voltage confirms the doped character of the nanoparticle at the single particle level, which is in full agreement with the ensemble optical and transport measurements. Finally we probe the phototransport within a single quantum dot and demonstrate a large photogain mechanism resulting from photogating