A. Crisan

Sputtered nanodots: A costless method for inducing effective pinning centers in superconducting thin films

A straightforward and cheap method for creating extended defects, strong pinning centers, in superconducting thin films is proposed. Clearly, by very short time (3–5 s) rf sputtering at suitable substrate temperatures, we deposited Ag nanodots on SrTiO3 substrates prior to the growth of superconducting thin films. The nanodots were studied by atomic force microscopy. Due to the lattice mismatch and/or chemical poisoning, on top of the nanodots the superconducting phase does not form, creating in this way extended and effective pinning centers which increase the critical current density of the film. The method was applied to (Cu, Tl)BaSrCa2Cu3Oy films grown by amorphous phase epitaxy. Thin films grown in similar conditions, with and without nanodots, were characterized by x-ray diffraction and ac susceptibility. The results show that the nanodots increased the critical current density more than one order of magnitude.

Anomalous ac susceptibility response of (Cu,C)Ba2Ca2Cu3Oy : Experimental indication of two-component vortex matter in multi-layered cuprate superconductors

We have measured the AC susceptibility response in field-cooling conditions of (Cu,C)Ba2Ca2Cu3Oy with aligned crystallites. In a certain but large range of the applied DC magnetic field, the out-of-phase susceptibility has two dissipation peaks. Due to the characteristics of the sample, the shape of the second peak and its positions field-dependence, we ruled out the possibility that the anomalous second dissipation peak could be related to intergrain dissipation. Because this multilayered cuprate is known to be a two-gap superconductor, we believe that our results are an experimental indication of a two-component vortex matter in high-critical temperature superconductors.

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.

Artificial pinning centres in YBa2Cu3O7−δ thin films by Gd2Ba4CuWOy nanophase inclusions

Artificial pinning centres produced by Gd2Ba4CuWOy (2411W) nanophase inclusions in YBa2Cu3Oy (YBCO) thin films grown by pulsed laser deposition (PLD) using a YBCO target containing 1 mol% 2411W have been investigated. For comparison, YBCO thin films have been grown under the same conditions using a pure (commercial) YBCO target. The resulting films were characterized by AC susceptibility (superconducting transition) and SQUID magnetometry (critical current density, Jc), and by x-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive x-ray (EDX) spectroscopy and atomic force microscopy (AFM) (structure, morphology and chemical composition). Jc of the film grown from the composite target is practically the same as that measured for the reference sample at 5 K over the whole range of applied field up to 4.5 T. For temperatures higher than 50 K, however, the presence of the nano-inclusions leads to increased Jc. The relative enhancement in Jc increases with further increase in temperature. Jc of the film containing nanophase inclusions is between two and three times higher than that of the reference sample at 77.3 K and 1.5 T. This can be explained by considering the dimensions of nanophase inclusions compared with the temperature-dependent coherence length. Susceptibility measurements reveal that, apart from a large and sharp transition at about 91 K, there is a secondary, smaller and broader transition between 88 and 84 K in the YBCO film containing nanophase inclusions. This can be explained by substitutions of compatible atoms between YBCO and the 2411W phase (Gd with Y), which might also create additional pinning centres of δTc-type.

Superconducting properties of the heavy-ions and neutron irradiated (Cu,C)Ba2Can−1CunO2n+4−δ (n=3, 4 and 5)

Abstract For further enhancement of critical currents density Jc and irreversibility field Hirr of (Cu1−xCx) Ba2Can−1CunO2n+4−δ ((Cu,C)-(n−1)n, n=3, 4 and 5) family, pinning centers were introduced by ion and neutron irradiation in these compounds. The polycrystalline samples were irradiated with 3.6×1017 neutrons/cm2 or with Au15+ ions (240 MeV energy) at a fluence of 1×1011 ions/cm2. Jc and Hirr were determined for the irradiated samples as well as unirradiated samples. The intragrain Jc was evaluated from M–H curves using Beans critical state model. In (Cu,C)-12(n−1)n, Jc (77 K, 1 T) shows a remarkable increase from 3.9×104 to 9.1×105 A/cm2 for neutron irradiated (Cu,C)-1234 and from 3.9×104 to 4.1×106 A/cm2 for heavy-ion irradiated (Cu,C)-1234. These results indicate the possibility of further enhancement of Jc and of achieving a very high Hirr of (Cu,C)-1234 in (Cu,C)-12(n−1)n family.

Nanodots-induced pinning centers in thin films: effects on critical current density, activation energy and flux jump rate

By very short time rf sputtering in certain deposition conditions we have grown three-dimensional Ag nanodots on the substrate prior to the growth of Tl-based superconducting films. These nanodots create pinning centers, leading to an increase in the critical current density about 10 times. From the frequency dependence of the critical current density we estimated also the activation energy for the flux jumps, which resulted to be several times higher. The rate of the thermally-activated flux jumps decreases several orders of magnitude. We suggest that our method can be used for the reduction of thermal noise in high-T/sub c/ dc SQUIDs.

Integrated nanotechnology of pinning centers in YBa2Cu3Ox films

A controlled pinning change from the ab-plane dominant to the c-axis dominant has been achieved in a novel method of nanostructured YBa2Cu3Ox (YBCO) growth. The method is a synchronous self-assembly of BaZrO3 (BZO) and Ag-assisted YBCO nanothreads. The formation of entangled nanothreads increases the critical current density while keeping the critical temperature close to that in pure YBCO films. The nanothreads extend through the whole thickness of thick films, making the method suitable for increasing total critical current density per centimeter of width (Ic − w). Two growth mechanisms, the formation of BZO nanorods and YBCO nanocolumns, complement each other, form a coherent structure and produce samples with strong correlated pinning. In addition to the increase in Ic − w, correlated pinning leads to an increase in vortex melting temperature in a wide range of magnetic fields. The films grown by this method have high Ic − w both in low magnetic fields along the c axis and high magnetic fields in the ab plane of YBCO. Such a superconductor would be suitable for both cable and magnet applications.

Second-generation quantum-well sensors for room-temperature scanning Hall probe microscopy

Scanning Hall probe microscopy is a noninvasive magnetic imaging technique with potential for having a major impact in the data storage industry if high-resolution Hall effect sensors can be developed with sufficiently low-noise figures at room temperature. To meet this requirement, we have developed a series of second-generation quantum-well Hall probes whereby the careful design of an AlGaAs∕InGaAs∕GaAs pseudomorphic heterostructure, chip layout, metal interconnects, and passivation layers has allowed a dramatic reduction of low-frequency noise sources. In addition, the Johnson noise-limited minimum detectable fields of these sensors are more than an order of magnitude lower than those used in early microscopes. The key figures-of-merit of the sensors are presented and their performance illustrated in an imaging study of a yttrium–iron–garnet thin film at room temperature.

(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.

Preparation of Tl-2212 and -1223 superconductor thin films and their microwave properties

Abstract Tl 2 Ba 2 CaCu 2 O y (Tl-2212) and Tl(Ba,Sr) 2 Ca 2 Cu 3 O y (Tl-1223) superconductor thin films have been prepared on CeO 2 buffered sapphire substrate by an ex situ process namely, amorphous phase epitaxy (APE) method. In the case of Tl-1223, an amorphous layer of TlSr 2 CaCu 2 O y (TlSr-1212) was deposited on top of CeO 2 layer to avoid chemical reaction between Tl-1223, containing Ba, and CeO 2 layers. All films are epitaxial with smooth surface and excellent crystallinity. Superconducting transition temperatures ( T c ) of Tl-2212 and 1223 are around 95 and 104 K, respectively. Critical current density ( J c ) at 77 K measured on 5×5 mm films by inductive technique is as high as 2 MA/cm 2 . Surface resistance R s of Tl-2212 films measured by a dielectric resonator technique at 38 GHz is comparable to the best YBCO films.