55 research outputs found

    Direct observation of vortices in an array of holes at low temperature: temperature dependance and first visualization of localized superconductivity

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    A scanning micro superconducting quantum interference device (microSQUID) microscope is used to directly image vortices in a superconducting Al thin film. We observe the temperature dependence of the vortex distribution in a regular defect (hole) array patterned into the Al film. The first direct observation of the localized superconducting state around the holes is shown as well as the effect of the hole size on nucleation of the superconducting state

    Hybrid superconducting nanostructures: very low temperature local probing and noise

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    International audienceWe review the topic of hybrid superconducting nanostructures by introducing the basic physical concepts and describing recent key experimental results. We discuss the superconductivity nucleation in mesoscopic structures, the vortex lattice imaging in doped diamond films, the superconducting proximity effect, multiple Andreev reflection in Josephson junctions and the electronic micro-cooling in hybrid tunnel junctions. An emphasis is put on very low temperature local probes and noise measurement techniques developed in Grenoble

    MicroSQUID Force microscopy in a dilution refrigerator

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    We present a new generation of a scanning MicroSQUID microscope operating in an inverted dilution refrigerator. The MicroSQUIDs have a size of 1.21$ \ \mum\textsuperscript{2} and a magnetic flux sensitivity of 120 \mu\Phi_{0} / \sqrt{\textrm{Hz}}andthusafieldsensitivityof and thus a field sensitivity of %550^{-6} \ \Phi_{0} / \sqrt{\textrm{Hz}}550 550 \ \mu \textrm{G}/ \sqrt{\textrm{Hz}}.Thescanrangeatlowtemperaturesisabout80. The scan range at low temperatures is about 80 \mu$m and a coarse displacement of 5 mm in x and y direction has been implemented. The MicroSQUID-to-sample distance is regulated using a tuning fork based force detection. A MicroSQUID-to-sample distance of 420 nm has been obtained. The reliable knowledge of this distance is necessary to obtain a trustworthy estimate of the absolute value of the superconducting penetration depth. An outlook will be given on the ongoing direction of development

    Epitaxial rhenium microwave resonators

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    International audienceWe have fabricated rhenium microwave resonators from epitaxial films. We have used thin films of different structural quality depending on their growth conditions. The resonators were coupled to a microwave transmission line which allows the measurement of their resonance frequencies and internal quality factors. From the resonance frequency at low temperature , the effective penetration depth and the London penetration depth of the rhenium film are extracted

    Concept of an ionizing time-domain matter-wave interferometer

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    We discuss the concept of an all-optical and ionizing matter-wave interferometer in the time domain. The proposed setup aims at testing the wave nature of highly massive clusters and molecules, and it will enable new precision experiments with a broad class of atoms, using the same laser system. The propagating particles are illuminated by three pulses of a standing ultraviolet laser beam, which detaches an electron via efficient single photon-absorption. Optical gratings may have periods as small as 80 nm, leading to wide diffraction angles for cold atoms and to compact setups even for very massive clusters. Accounting for the coherent and the incoherent parts of the particle-light interaction, we show that the combined effect of phase and amplitude modulation of the matter waves gives rise to a Talbot-Lau-like interference effect with a characteristic dependence on the pulse delay time.Comment: 25 pages, 5 figure

    Niobium Nitride Thin Films for Very Low Temperature Resistive Thermometry

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    International audienceWe investigate thin film resistive thermometry based on metal-to-insulator-transition (niobium nitride) materials down to very low temperature. The variation of the NbN thermometer resistance have calibrated versus temperature and magnetic field. High sensitivity in tempertaure variation detection is demonstrated through efficient temperature coefficient of resistance. The nitrogen content of the niobium nitride thin films can be tuned to adjust the optimal working temperature range. In the present experiment, we show the versatility of the NbN thin film technology through applications in very different low temperature use-cases. We demonstrate that thin film re-sistive thermometry can be extended to temperatures below 30 mK with low electrical impedance

    Surface impedance of Ba2-xNixAs2 crystals

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    3 pagesInternational audienceMeasurements of the real and imaginary parts of the conductivity were performed in optimally doped BaFe1.9Ni0.1As2 and overdoped BaFe1.88Ni0.12As2 crystals in the frequency range 20MHz-1.5GHz using a single coil technique. The tempertaure dependence of the London penetration depth follows a T2 law. The real part of the conducivity increases with decreasing temperature below Tc in agreement with the results obtained for the optimally Co doped BaFe2-xCoxAs2 crystals. The increase of the real part of the conductivity in the superconducting state is attributed to a rapidly decrease of the quasiparticle scattering rate

    Imagerie magnétique par micro-SQUID à basse température

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    GRENOBLE1-BU Sciences (384212103) / SudocSudocFranceF

    Magnetic imaging of unconventional superconductors by scanning SQUID microscopy

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    Pendant la premi√®re ann√©e le microscope √† microSQUID √©tait mis en fonctionnement. On a avanc√© sur le plan cryogenique (dilution) et √©lectronique (programmation de boucles de r√©gulation et d'une d√©tection synchrone). Les composants √©taient test√©s √† temp√©rature ambiante et on est en train de tout tester √† basse temp√©rature. Une m√©thode √©tait con√ßu pour d√©terminer la longueur de p√©n√©tration du champ magn√©tique dans un supraconducteur avec les donn√©es qui pourront √™tre fait avec notre microscope. Ceci va √™tre utilis√© pour l'√©chantillon PrOs4Sb12. Il s'agit de trancher le d√©bat sur la nature multibande de la supraconductivit√© dans ce compos√©. En deuxi√®me ann√©e le developpement a continu√©, en particuli√®re le microscope √©tait mis √† froid. Des diff√©rents probl√®mes due aux basses temp√©ratures (mouvement de moteur, thermalisation, c√Ęblage) ont √©t√© resolues. Ensuite on a avanc√© sur le plan informatique, notamment le contr√īle de differents composants. Pendant le deuxi√®me ann√©e quelques images magn√©tique ont √©t√© faites, validant le concept. En troisi√®me ann√©e on a commence a m√©surer des domaines magnetiques d'un supraconducteur ferromagnetique (UCoGe) en Avril - Aout. On a obtenu des resultats tres interessants. Le m√™me dispositif sera ainsi op√©rationnel pour l'imagerie de domaines dans des bolom√®tres supraconducteurs.Pendant cette th√®se un microscope √† SQUID et AFM √† balayage, l'√©lectronique et les logiciels de contr√īle ont √©t√© con√ßus. Pour la calibration des mesures sur un film de niobium (avec des motifs) ont √©t√© effectu√©es, montrant la possibilit√© de faire des image de la topographie at la distribution du champ magn√©tique au dessus de l'√©chantillon simultan√©ment. On pr√©sent les premi√®res image dans l'espace r√©el de la structure de domaines dans le ferro supraconducteur UCoGe, un √©chantillon bas√© sur l'uranium (fermion lourd) avec un transition supra √† environ 0.5K √† la pression ambiante. On montre l'√©volution de la transition ferromagn√©tique en fonction de la temp√©rature. La microscope a √©t√© aussi utilis√© pour des m√©sures sur un couche mince de Rhenium, un supraconducteur conventionel. On a obtenu une estimation pour la force de pi√©geage de vortex on utilisant l'interaction entre SQUID et vortex. En plus, on a d√©termin√© la longueur de p√©n√©tration en fonction de la temp√©rature.SAVOIE-SCD - Bib.√©lectronique (730659901) / SudocGRENOBLE1/INP-Bib.√©lectronique (384210012) / SudocGRENOBLE2/3-Bib.√©lectronique (384219901) / SudocSudocFranceF

    Supraconductivit√©, Onde de Densit√© de Charge et Phonons Mous dans les dichalcog√©nures 2H-NbSe√ļ et 2H-NbS√ļ, et le compos√© interm√©tallique Lu Ir Si_

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    Cette th√®se pr√©sente une √©tude exp√©rimentale de l'interaction entre la supraconductivit√© et une onde de densit√© de charge (ODC). Dans la th√©orie standard, la temp√©rature critique d un mat√©riau supraconducteur est favoris√©e principalement par deux param√®tres : une grande densit√© d √©tats au niveau de Fermi (nF), et un fort couplage √©lectron-phonon. Cependant, un fort couplage √©lectron-phonon favorise aussi l apparition d une ODC, ce qui r√©duit nF et rivalise ainsi avec la supraconductivit√©.Notre d√©marche a consist√© √† √©tudier deux compos√©s o√Ļ supraconductivit√© et ODC coexistent, et dans lesquels on peut faire dispara√ģtre l ODC gr√Ęce √† un param√®tre externe : pression ou substitution. Le premier compos√©, 2H-NbSe2, pr√©sente une ODC en dessous de 33 K √† pression ambiante. Celle-ci coexiste avec la supraconductivit√© en dessous de 7 K. Sous pression, l ODC dispara√ģt au-dessus de 4.6 GPa, sans que la temp√©rature critique varie notablement. L ODC dispara√ģt aussi en rempla√ßant le s√©l√©nium par du soufre : 2H-NbS2 est ainsi un supraconducteur sans ODC (Tc = 6 K), et peut donc servir de compos√© t√©moin pour une √©tude comparative. Dans le second compos√©, Lu5Ir4Si10, une ODC est pr√©sente en dessous de 77 K √† pression ambiante. Celle-ci dispara√ģt sous pression au-dessus de 2 GPa, tandis que la temp√©rature critique saute simultan√©ment de 4 √† 9 K. Pour √©tudier ces compos√©s, j ai utilis√© trois techniques exp√©rimentales : la mesure de la dispersion des phonons √† basse temp√©rature (300-2 K) et sous pression (0-16 GPa) par diffusion in√©lastique des rayons X, la mesure de la d√©pendance en temp√©rature de la longueur de p√©n√©tration magn√©tique gr√Ęce √† un oscillateur √† diode tunnel et la mesure des champs critiques via des microsondes Hall.Dans la premi√®re partie, je pr√©sente la d√©pendance en temp√©rature de la dispersion des phonons dans 2H-NbS2. Nous observons la pr√©sence d un phonon mou dont l √©nergie reste toujours positive, m√™me extrapol√©e √† temp√©rature nulle. Ce compos√© est ainsi √† la limite d'une instabilit√© ODC. De plus, nous montrons qu il est relativement unique, car seuls les effets anharmoniques emp√™chent l amollissement complet des phonons. Je pr√©sente ensuite la d√©pendance en temp√©rature et en pression de la dispersion des phonons dans 2H-NbSe2. Ces exp√©riences montrent qu un mode de phonon mou persiste jusqu √† 16 GPa, m√™me quand l'√©tat √† temp√©rature nulle n'est pas l ODC. La d√©pendance en temp√©rature de ce phonon mou est alors similaire √† celle de 2H-NbS2. Dans les deux compos√©s, ces phonons mous semblent li√©s √† la pr√©sence d'un couplage √©lectron-phonon √† la fois fort et anisotrope. Nous sugg√©rons qu il s agit d un √©l√©ment essentiel pour expliquer leurs propri√©t√©s supraconductrices.Dans la seconde partie, je mesure l'anisotropie et la d√©pendance en temp√©rature de la longueur de p√©n√©tration magn√©tique dans l √©tat supraconducteur de 2H-NbS2 et Lu5Ir4Si10. La d√©pendance en temp√©rature de la densit√© superfluide dans 2H-NbS2 confirme la pr√©sence d'un gap supraconducteur r√©duit dont l'amplitude est tr√®s proche de celle mesur√©e dans 2H-NbSe2. Les phonons mous et le gap r√©duit √©tant pr√©sents dans 2H-NbS2 et 2H-NbSe2, nous prouvons exp√©rimentalement qu'il faut raisonner en termes de renforcement de la supraconductivit√© par les phonons mous plut√īt qu'en termes d interaction avec l'√©tat fondamental (ODC ou m√©tal). Nous proposons que ce renforcement soit li√© √† l'anisotropie du couplage √©lectron-phonon.En revanche, cet effet n est pas g√©n√©ral aux compos√©s o√Ļ supraconductivit√© et ODC coexistent. Les propri√©t√©s supraconductrices de Lu5Ir4Si10 sont en effet bien d√©crites par le mod√®le BCS couplage faible. Ceci est peut √™tre li√© aux caract√©ristiques de l ODC : la pr√©sence d une hyst√©r√©sis montre que la transition ODC est du premier ordre. D autre part, les mesures de diffraction X sous pression et √† basse temp√©rature r√©v√®lent que cette ODC est multiple : en plus de la p√©riodicit√© 1/7, nous observons une seconde p√©riodicit√© de 1/20.This thesis presents an experimental study of the interaction between superconductivity and a charge density wave (CDW). In the standard theory, the critical temperature of a superconductor is principally enhanced by two parameters: a large density of states at the Fermi level (nF) and a strong electron-phonon coupling. However, a strong electron-phonon coupling also favors the appearance of a CDW, which reduces nF and therefore competes with superconductivity.Our strategy was to study two compounds in which superconductivity and CDW coexist, and in which the CDW can be suppressed through an external parameter: pressure or substitution. The first compound is 2H-NbSe2, it presents a CDW below 33 K at ambient pressure. This CDW coexists with superconductivity below 7 K. Under pressure, the CDW disappears above 4.6 GPa, meanwhile the critical temperature slowly changes. The CDW also disappears when replacing selenium by sulfur: 2H-NbS2 is a superconductor without CDW (Tc=6 K), it can therefore serve as a test compound for a comparative study. The second compound is Lu5Ir4Si10, it presents a CDW below 77 K at ambient pressure. Under pressure, this CDW disappears above 2 GPa, meanwhile the critical temperature abruptly jumps from 4 to 9 K.For this study, I used three experimental techniques: inelastic x-ray scattering at low temperature (300-2 K) and under pressure (0-16 GPa) to measure the dispersion of phonons, a tunnel diode oscillator to measure the temperature dependence of the magnetic penetration depth, and Hall microprobes to measure the first and second critical fields. In the first part, I present the temperature dependence of the phonon dispersion in 2H-NbS2. We observe a soft phonon that always remains at positive energies, even extrapolated to zero temperature. Thus, this compound is on the verge of CDW instability. It is also relatively unique, since we show anharmonicity is the only effect that prevents the complete softening of the phonons.Then I present the temperature and pressure dependence of the phonon dispersion in 2H-NbSe2. These experiments show that a soft phonon persists up to 16 GPa, even if the ground state is not a CDW. The temperature dependence of this soft phonon is then similar to that of 2H-NbS2. In both compounds, these soft modes seem to be related to the strength and anisotropy of the electron-phonon coupling. We suggest this is a fundamental element to explain their superconducting properties.In the second part, I measure the anisotropy and temperature dependence of the magnetic penetration depth in the superconducting state of 2H-NbS2 and Lu5Ir4Si10. The temperature dependence of the superfluid density in 2H-NbS2 confirms the presence of a reduced superconducting gap. Its amplitude is very similar to the one measured in 2H-NbSe2. The soft modes and the reduced gap being present in both 2H-NbSe2 and 2H-NbS2, we prove experimentally that the enhancement of superconductivity is related to the soft modes rather than to the nature of the ground state (CDW or metal). We suggest this enhancement is due the anisotropy of the electron-phonon coupling.However, this effect is not general to all compounds where superconductivity and CDW coexist. The superconducting properties of Lu5Ir4Si10 are indeed well fitted by the BCS model in the weak coupling limit. This may be related to the characteristics of the CDW: the presence of hysteresis shows that the CDW transition is first order. In addition, under pressure and at low temperature, x-ray diffraction measurements indicate that the CDW is multiple: aside from the periodicity of 1/7, we observe a second periodicity of 1/20.SAVOIE-SCD - Bib.√©lectronique (730659901) / SudocGRENOBLE1/INP-Bib.√©lectronique (384210012) / SudocGRENOBLE2/3-Bib.√©lectronique (384219901) / SudocSudocFranceF
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