Quantum oscillations and upper critical magnetic field of the iron-based superconductor FeSe

Shubnikov-de Haas (SdH) oscillations and upper critical magnetic field ($H\_{c2}$) of the iron-based superconductor FeSe ($T\_c$ = 8.6 K) have been studied by tunnel diode oscillator-based measurements in magnetic fields of up to 55 T and temperatures down to 1.6 K. Several Fourier components enter the SdH oscillations spectrum with frequencies definitely smaller than predicted by band structure calculations indicating band renormalization and reconstruction of the Fermi surface at low temperature, in line with previous ARPES data. The Werthamer-Helfand-Hohenberg model accounts for the temperature dependence of $H\_{c2}$ for magnetic field applied both parallel (\textbf{H} $\|$ $ab$) and perpendicular (\textbf{H} $\|$ $c$) to the iron conducting plane, suggesting that one band mainly controls the superconducting properties in magnetic fields despite the multiband nature of the Fermi surface. Whereas Pauli pair breaking is negligible for \textbf{H} $\|$ $c$, a Pauli paramagnetic contribution is evidenced for \textbf{H} $\|$ $ab$ with Maki parameter $\alpha$ = 2.1, corresponding to Pauli field $H\_{P}$ = 36.5

Fabrication et propriétés physiques de conducteurs multifilamentaires MgB2 dopés au carbone

Des conducteurs mono- et multi-filamentaires de MgB2 dans une gaine de Nb/Cu ont été élaborés par la technique PIT (powder in tube) avec des dopages de carbone et de TiC. Les résultats montrent qu'il y a une couche de diffusion non supraconductrice à l'interface entre le Nb et MgB2 pour les échantillons traités à haute température, couche qui empêche la pénétration du courant dans le conducteur. Les traitements thermiques doivent être inférieurs à 750C. Les effets des dopages au carbone amorphe et au TiC ont été étudiés par XRD,MEB et aimantation. La substitution du bore par du carbone diminue légèrement la Tc mais augmente la piégeage des vortex, conduisant à un optimum du courant critique. Des multi-filaments de 6, 12 et 36 filaments sans dopage ont été élaborés par la technique PIT. Les propriétés mécaniques de ces conducteurs ont été renforcées en utilisant un filament central en Nb. L'assemblage MgB2/Nb/Cu est très adapté pour obtenir de grandes longueurs de conducteurs par la méthode PIT.The mono-filamentary and multi-filamentary MgB2 wires sheathed with Nb/Cu composite tube were fabricated by in-situ powder-in-tube (PIT) technique with the carbon and TiC doping. The results show that there is a diffusion layer at the interfacial region between the Nb sheath and MgB2, which is non-superconducting phase for the sample heat-treated at high temperature, and this diffusion layer obstructs the current transmission at high heat-treatment temperatures and the best heat-treatment temperature for the MgB2 wires with Nb as the barrier layer should be lower than 750C. Effects of amorphous carbon and TiC doped MgB2 wires were investigated by XRD, SEM and magnetization. The results show that substitution of B for C enhances the flux pinning but depresses Tc slightly. By controlling the processing parameters, an optimized Jc(H) performance is achieved under a partial amorphous C substitution. The Cu-clad 6-, 12-, and 36-filamentary MgB2 wires with Nb buffer layer also have been fabricated by the in-situ PIT method. To improve the strength of wires, the Nb-core was used as the central filament. The results show that the in-situ PIT technique can be used to produce the certain long length MgB2/Nb/Cu wires. The superconducting filaments of MgB2/Nb/Cu composite tubes have well workability suitable for the MgB2 wires long length and multi-filament.SAVOIE-SCD - Bib.électronique (730659901) / SudocGRENOBLE1/INP-Bib.électronique (384210012) / SudocGRENOBLE2/3-Bib.électronique (384219901) / SudocSudocFranceF

Crystal growth and characterization of the ruthenate superconducting compound: Sr2RuO4

International audienceSr2RuO4 is a copper-free layered perovskite superconductor with the tetragonal K2NiF4-type structure. The precise nature of the pairing in the superconducting state of this material is still under debate. In this paper, we report about crystal growth and characterization of this compound. The crystals were grown by a floating zone technique using a light furnace equipped with double elliptical mirrors starting from off-stoichiometric Sr2RuO4. The crystals have been checked by X-ray diffraction and microanalysis. The superconducting properties were measured by AC-susceptibility, magnetization and specific heat. High crystalline quality centimetre-sized crystals have been grown and best crystals exhibit superconducting transitions at View the MathML source (typical dimensions : 4.5 mm diameter and 70 mm length with the (0 0 1) axis perpendicular to growth direction). The unconventional superconductors are characterized by a drastic impurity effect on the superconducting properties. We try to establish a correlation between the defect concentration and the physical properties of Sr2RuO4

Thermodynamic phase diagram of Fe(Se0.5Te0.5) single crystals in fields up to 28 tesla

International audienceWe report on specific heat Cp , transport, Hall probe, and penetration depth measurements performed on Fe Se0.5Te0.5 single crystals Tc 14 K . The thermodynamic upper critical field Hc2 lines has been deduced from Cp measurements up to 28 T for both H c and H ab, and compared to the lines deduced from transport measurements up to 55 T in pulsed magnetic fields . We show that this thermodynamic Hc2 line presents a very strong downward curvature for T→Tc which is not visible in transport measurements. This temperature dependence associated to an upward curvature of the field dependence of the Sommerfeld coefficient confirms that Hc2 is limited by paramagnetic effects. Surprisingly this paramagnetic limit is visible here up to T/Tc 0.99 for H ab which is the consequence of a very small value of the coherence length c 0 4 Å and ab 0 15 Å , confirming the strong renormalization of the effective mass as compared to DMFT calculations previously observed in ARPES measurements A. Tamai, A. Y. Ganin, E. Rozbicki, J. Bacsa, W. Meevasana, P. D. C. King, M. Caffio, R. Schaub, S. Margadonna, K. Prassides, M. J. Rosseinsky, and F. Baumberger, Phys. Rev. Lett. 104, 097002 2010 . Hc1 measurements lead to ab 0 =430 50 nm and c 0 =1600 200 nm and the corresponding anisotropy is approximatively temperature independent 4 , being close to the anisotropy of Hc2 for T→Tc. The temperature dependence of both T2 and the electronic contribution to the specific heat confirm the nonconventional coupling mechanism in this system

Direct observation of the influence of the FeAs4 tetrahedron on superconductivity and antiferromagnetic correlations in Sr2VO3FeAs

International audienceWe measure the pressure dependence of the electrical resistivity and the crystal structure of iron superconductor Sr2VO3FeAs. Below ∼ 10GPa the structure compresses but remains undeformed, with regular FeAs4 tetrahedrons, and a constant Tc. Beyond 10GPa, the tetrahedron strongly distorts, while Tc goes gradually to zero. Band structure calculations of the undistorted structure show multiple-nesting features that hinder the development of an antiferromagnetic (AF) ground state, allowing the appearance of superconductivity. The deformation of the tetrahedra that breaks band degeneracy degrades multiple nesting, thus favouring one particular AF state at the expense of Tc

Effect of the isoelectronic substitution of Sb for As on the magnetic and structural properties of LaFe(As1−xSbx)O

International audienceThe antiferromagnetic (AFM) order and structural distortion in the LaFe(As1−xSbx)O system have been investigated by neutron powder diffraction and physical properties measurements. Polycrystalline samples of LaFe(As1−xSbx)O (x < 0.5) were prepared using solid state synthesis at ambient and high pressure. We find that the isoelectronic substitution of Sb for As decreases the structural and magnetic transition temperatures, but, contrary to the effects of phosphorus substitution, superconductivity is not induced. Instead a slight increase in the Fe-magnetic moment is observed

In-plane magnetic domains and N\'eel-like domain walls in thin flakes of the room temperature CrTe2_2 van der Waals ferromagnet

The recent discovery of magnetic van der Waals materials has triggered a wealth of investigations in materials science, and now offers genuinely new prospects for both fundamental and applied research. Although the catalogue of van der Waals ferromagnets is rapidly expanding, most of them have a Curie temperature below 300 K, a notable disadvantage for potential applications. Combining element-selective x-ray magnetic imaging and magnetic force microscopy, we resolve at room temperature the magnetic domains and domains walls in micron-sized flakes of the CrTe$_2$ van der Waals ferromagnet. Flux-closure magnetic patterns suggesting in-plane six-fold symmetry are observed. Upon annealing the material above its Curie point (315 K), the magnetic domains disappear. By cooling back down the sample, a different magnetic domain distribution is obtained, indicating material stability and lack of magnetic memory upon thermal cycling. The domain walls presumably have N\'eel texture, are preferentially oriented along directions separated by 120 degrees, and have a width of several tens of nanometers. Besides microscopic mapping of magnetic domains and domain walls, the coercivity of the material is found to be of a few mT only, showing that the CrTe$_2$ compound is magnetically soft. The coercivity is found to increase as the volume of the material decreases

Study of vortex pinning by development of normal-superconducting interfaces oriented on a nanometric scale within a superconducting NBTI alloy

DURANT L'ETIRAGE, DES DEFAUTS ARTIFICIELS (APC) DE NB ET DE TI5%NB INTRODUITS DANS DES FILS DE NBTI SE DEFORMENT PROGRESSIVEMENT EN RUBANS. POUR LES COMPOSITES AVEC DES DEFAUTS DE NB SOUS FORME DE BARRIERES OU D'ILES, UN PIC DE LA FORCE DE PIEGEAGE FP APPARAIT ENTRE 3 ET 5 T, ALORS QUE POUR LES APC EN COUCHES, LE PIC SE SITUE ENTRE 1 ET 2 T. LES COMPOSITES EN ILES ONT DE MEILLEURES PERFORMANCES A HAUT CHAMP A CAUSE DU PLUS GRAND NOMBRE DE PIEGES. POUR LES COMPOSITES AVEC DES APC DE TI5%NB, L'AUGMENTATION DU NOMBRE D'INCLUSIONS CONDUIT A UNE REDUCTION DE HC2 ET DE TC A CAUSE DES EFFETS DE PROXIMITE. LORSQUE LES CONDUCTEURS SONT LAMINES SOUS FORME DE RUBANS, LE COURANT CRITIQUE JC// DEPASSE 4155 A/MM2 A 5 T ET 4.2 K. POUR LES COMPOSITES AVEC DES DEFAUTS DE NB, UN TRAITEMENT THERMIQUE AUGMENTE JC A FORT CHAMP MAIS REDUIT LES PERFORMANCES A BAS CHAMP, CE QUI EST EXACTEMENT L'INVERSE POUR LES COMPOSITES AVEC DES DEFAUTS TI5%NB.GRENOBLE1-BU Sciences (384212103) / SudocSudocFranceF

Etude de l'optimisation et de la cinétique de formation de conducteurs supraconducteurs Nb3Sn obtenus par la méthode de l'étain interne

Superconductors Nb3Sn wires are one of the most applicable cryogenic superconducting materials and the best choice for high-field magnets exceeding 10T. One of the most significant utilization is the ITER project which is regarded as the hope of future energy source. The high-Cu composite designs with smaller number of sub-element and non-reactive diffusion barrier, and the RRP (Restacked Rod Process) internal-Sn technology are usually applied for the wire manufacturing. Such designed and processed wires were supplied by MSA/Alstom and WST/NIN in this research. The systematic investigation on internal-Sn superconducting wires includes the optimization of HT conditions, phase formation and its relation with superconductivity, microstructure analysis, and the phase formation kinetics. Because of the anfractuosity of the configuration design and metallurgical processing, the MF wires are not sufficient for studying a sole factor effect on superconductivity. Therefore, four sets of ME wires with different Sn ratios and different third-element addition were designed and fabricated in order to explore the relationship between phase formation and superconducting performances, particularly the A15 layer growth kinetics. Different characterization technics have been used (magnetization measurements, neutron diffraction and SEM/TEM/EDX analysis)The A15 layer thicknesses of various ME samples were measured and carried out linear and non-linear fits by means of two model equations. The results have clearly demonstrated that the phase formation kinetics of Nb3Sn solid-state reaction is in accordance with an n power relation and the n value is increased with the increase of HT temperature and the Sn ratio in the wire composite.Les fils supraconducteurs Nb3Sn sont, parmi les matériaux, ceux qui présentent le plus d applications possibles pour produire des champs magnétiques supérieurs à 10 Tesla. L utilisation la plus immédiate se place dans le cadre du projet ITER, qui est considéré comme un grand espoir de source d énergie dans le futur. La conception composite avec beaucoup de cuivre, un petit nombre de sous-éléments, une barrière de diffusion non réactive et la technologie de l étain interne RRP (Restacked Rod Process) est généralement utilisée pour la fabrication des fils. Des spécifications particulières sont nécessaires pour cette utilisation. Elles peuvent être obtenues par une conception particulière de l assemblage des filaments, la méthode de formation de la phase A15 Nb3Sn (ici le procédé de l étain interne), les traitements thermiques. De tels fils ont été fournis par les sociétés MSA/Alstom et WST/NIN pour cette étude. Pour déterminer de manière précise les conditions de formation et les cinétiques de réaction, des filaments plus simples ont été préparés (4 formulations avec des rapports Cu/Sn différents et des additifs différents). Pour cela différentes techniques de caractérisation ont été mises en œuvre, mesures d aimantation sous champ magnétique, diffraction neutronique, microscopies électroniques (MEB/EDEX, TEM). L épaisseur de la couche de la phase A15 mesurée dans différents monofilaments a permis de déterminer la loi de cinétique de formation selon deux modèles. Les résultats ont clairement montré que le modèle de diffusion en phase solide est en accord avec une loi de puissance n et que la valeur de n augmente avec la température de traitement et le taux d étain dans le fil composite.GRENOBLE1-BU Sciences (384212103) / SudocSudocFranceF

Experimental study of the competition between Kondo and RKKY interactions for Mn spins in a model alloy sytem

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