P Badica

Strong reduction of thermally activated flux jump rate in superconducting thin films by nanodot-induced pinning centers

From frequency-dependent ac susceptibility studies of (Cu,T1)BaSrCa2Cu3Oy superconducting thin films, with and without nanodot-induced artificial pinning centers, we estimated the activation energy of flux jumps. The result was that, in the film with nanodots, the pinning potential is several times higher, leading to a probability of thermally activated flux jumps several orders of magnitude lower than in the film without artificial pinning centers. We suggest that our no cost straightforward method for creating extended defects can be successfully employed for the reduction of thermal noise in superconducting electronic devices.

(Cu,Tl)Ba2Ca3Cu4Ox compositions: II. Heating rate applied to synthesis of superconducting ceramics

Superconducting samples with a starting composition Cu0.5TlxBa2Ca3Cu4O12 (x = 1–0.35) were prepared in Au tubes from oxides by using different heating rates. The optimum heating rate was 0.11 °C min−1 applied between 860 and 880 °C. These samples show the highest content of 1234 phase and the lowest of Ba–Cu–O phases as well as the highest critical temperatures. The highest values were Tc = 119.2 K and Tc0 = 115.1 K. All samples show Jc values around 6 × 105 A cm−2, at 60 K and 0.5 T. In the rest of the samples, heated by using lower or higher heating rates, a high concentration of 1223 and/or 1245 phases is detected. The content of the 1234 phase and of Ba(Ca,Tl)–Cu–O residual liquid phase, in the samples synthesized for the optimum heating rate, depends on the Tl content in the starting mixture. The influence of the oxygen content in the starting mixture on phase composition is not as strong as of Tl, but it has a major role in establishing the superconducting behaviour of the 1234 grains. Some arguments that suggest the dependence of the transport properties of the non-superconducting matrix on oxygen content will be discussed. In our technological arrangement the determined optimum heating rate can be applied to the synthesis of other single-layered superconducting phases, e.g. 1212 phase from a Cu0.25Tl0.75Ba2CaCu2O8.115 starting composition.

(Cu,C)Ba2Ca3Cu4Ox–(LiF)y: addition of LiF—an effective way to synthesize overdoped superconductor

(Cu,C)Ba2Ca3Cu4Ox superconductor with addition of y mol LiF has been synthesized by a high-pressure method. For the same synthesis conditions it was found that (almost) single-phase Cu, C-1234 samples can be synthesized for yLiF = 0–0.1 if the amount of z mol AgO oxidizer is increased linearly from zAgO = 0.45 to 0.73 and for yLiF = 0.1–0.2 if zAgO = 0.73 = constant. Transport measurements (ρ(T) and room-temperature Seebeck coefficient) have shown that these samples are overdoped: LiF is an effective addition for synthesis of overdoped Cu, C-1234 with a controlled level of carriers. LiF addition continuously decreases Tc. The critical point at yLiF = 0.1 is discussed as the solubility limit of LiF and/or the point where the doping mechanism changes. It is proposed that the reason is the reaction of extra Li with C and O to form Li2CO3, inducing a lower concentration of C in Cu, C-1234/LiF crystals, and at the same time a possible substitution of Li not only for the Cu site but also for the Ca site, resulting in formation of a higher amount of residual Ca0.828CuO2 (for yLiF>0.1). LiF induces the formation of a liquid phase and acts as a flux promoting the formation of Cu,C-12 (n−1)n with . LiF modifies to some degree the grain growth from a 3D to a 2D type (thinner platelike grains have been observed in the LiF added samples).

Electron-doped superconductivity induced by oxygen vacancies in as-grown Sr0.6Ca0.4CuO2?? infinite-layer films

Thin films of the infinite-layer compound Sr0.6Ca0.4CuO2?? have been prepared by off-axis rf magnetron sputtering on SrTiO3 (001). For an optimum level of oxygen vacancy superconductivity was observed (Tc onset = 42 K and Tc(? = 0) = 11 K). Structural and transport data suggesting n-type superconductivity are presented and discussed. Infinite-layer thin films of Sr0.6Ca0.4CuO2??, without trivalent cation doping and showing n-type superconductivity, are reported for the first time.

TlBa2Ca2Cu3Oy superconducting films on MgO with different morphologies

Abstract TlBa 2 Ca 2 Cu 3 O y (Tl-1223) superconducting films were prepared under identical conditions, on MgO substrates with different morphologies resulting from heat treatments at temperatures between 600 and 1350 °C. The superconducting films have almost the same morphology, in-plane alignment and composition, but different critical current densities J c . Critical current density J c determined at 77.3 K differs for films with various annealed MgO substrates, by a factor of 5 in zero field and 10 in 1 T. The behavior of J c is discussed in relation with the flatness of the MgO surface, and with pinning effects induced by the Ca-segregates. For the present work, the best films were obtained for the un-annealed (as-received) substrates and for substrates treated at 1350 °C. In these samples, the substrate’s flatness and morphology with regular steps are essential for high quality of the superconducting films. In the films grown on MgO annealed between 800 and 1200 °C and showing rough surface and Ca-segregates, J c was lower. However, J c was increasing with heat treatment temperature of the substrate, possibly due to the Ca-segregates inducing or acting as pinning centers. The effects of Ca-segregates are of interest for maximization of the quality of the high-temperature superconducting films. In the literature, up to now, these effects have not been considered and therefore Ca-segregates removal has been recommended.

(Cu, Tl)Ba2Ca3Cu4Ox compositions: I. The influence of synthesis time and temperature on the phase formation and evaporation-condensation mechanism

Temperature and time dependence of the phase formation in the (Cu0.5,Tl0.5)-1234 composition have been studied. The liquid–vapour deposition mechanism for the formation of 12(n − 1)n superconducting phases is presented. XRD, SEM, EDS and weight loss data are discussed. Samples were superconducting with maximum Tc and Jc (at 60 K and 0.5 T), determined from M(H) loops of 118 K and 4 × 105 A cm−2, respectively.

Fabrication of intrinsic Josephson junctions on BSCCO superconducting films grown by MOCVD

Abstract We report on the successful preparation of intrinsic Josephson junctions on high-quality BSCCO thin films grown by MOCVD. The surface of as-grown films was very smooth with the roughness of the order of a half unit cell. Mesas were effectively structured on the grown films by the liquid-nitrogen-cooled dry etching method. A typical I–V characteristic in the c-axis direction of a BSCCO film shows the hysteresis and multiple resistive branches. The number of branches is consistent with that of junctions calculated from the height of a mesa structure. This indicates that BSCCO films consist of a series array of S(CuO2 layer)/I(SrO,BiO layers)/S(CuO2 layer) junction. The formation of stacked Josephson junctions was also confirmed by the temperature dependence and the magnetic field dependence of Ic.

Effects of residual carbon on phase formation of TlBa2Can-1CunOy (n = 3 and 4) superconductors

Abstract The effects of residual carbon in precursors on phase formation of single Tl–O layer superconductors of TlBa 2 Ca n −1 Cu n O y ( n =3 and 4) have been investigated. The samples were prepared from Tl 2 O 3 and precursors of Ba 2 Ca n −1 Cu n O 2 n +1+ δ (C x ) ( n =3 and 4) with various residual carbon contents ( x =0.02–0.19). Even a small amount of carbon promotes the formation of double Tl–O layer phases of Tl 2 Ba 2 Ca n −1 Cu n O y ( n =3 and 4). Using the carbon reduced precursor ( x =0.02), a TlBa 2 Ca 2 Cu 3 O y phase is synthesized directly in a silver-sheathed tape without such elements as Pb, Bi and Sr.

Electron-doped superconductivity in Sr1−xCaxCuO2−δ infinite-layer thin films

Abstract Thin films of infinite-layer compound Sr1−xCaxCuO2−δ have been prepared by rf magnetron sputtering. For an optimum level of oxygen vacancy superconductivities were observed ( T c onset =42 and 50 K, T c (ρ=0) =11 and 20 K). Structural and transport data suggest that the doping mechanism is electron-type. The superconducting transitions were also confirmed by ac susceptibility. Further increase of doping showed to destroy superconductivity.

(Ca,Ba)CuO2 films grown by MOCVD: surface morphology and structural studies

Abstract Thin films of (Ca 1− x Ba x )CuO 2 ( x =0–1) have been grown on (0xa00xa01) SrTiO 3 by metal organic chemical vapor deposition. These films are potential candidates as barrier layer for SIS and/or SNS Josephson junctions. For this purpose, the films should be flat and smooth. In this regard we have investigated the possibility of decreasing the films roughness by the control of the Ba/Ca ratio, the substrate temperature and oxygen partial pressure during the growth of the film. The minimum mean square root roughness and regular morphology are obtained for the x =0.3–0.5. In the samples with x =0.5 the highest intensity of the X-ray diffraction peaks for the (Ca 0.5 Ba 0.5 )CuO 2 phase and the lowest for the impurity phases were attained at a substrate temperature of 750–780 °C and oxygen partial pressure of 15 Torr.