arXiv subjects

C. Cantoni

Publications and source records attributed to C. Cantoni.

14 recordsLinked to original sources

arXiv2016

High Tc superconductivity at the interface between the CaCuO2 and SrTiO3 insulating oxides

At interfaces between complex oxides it is possible to generate electronic systems with unusual electronic properties, which are not present in the isolated oxides. One important example is the appearance of superconductivity at the interface between insulating oxides, although, until now, with very low Tc. We report the occurrence of high Tc superconductivity in the bilayer CaCuO2/SrTiO3, where both the constituent oxides are insulating. In order to obtain a superconducting state, the CaCuO2/SrTiO3 interface must be realized between the Ca plane of CaCuO2 and the TiO2 plane of SrTiO3. Only in this case extra oxygen ions can be incorporated in the interface Ca plane, acting as apical oxygen for Cu and providing holes to the CuO2 planes. A detailed hole doping spatial profile has been obtained by STEM/EELS at the O K-edge, clearly showing that the (super)conductivity is confined to about 1-2 CaCuO2 unit cells close to the interface with SrTiO3. The results obtained for the CaCuO2/SrTiO3 interface can be extended to multilayered high Tc cuprates, contributing to explain the dependence of Tc on the number of CuO2 planes in these systems.

cond-mat.supr-con↗

arXiv2015

Evolution of competing magnetic order in the Jeff=1/2 insulating state of Sr2Ir1-xRuxO4

We investigate the magnetic properties of the series Sr2Ir1-xRuxO4 with neutron, resonant x-ray and magnetization measurements. The results indicate an evolution and coexistence of magnetic structures via a spin flop transition from ab-plane to c-axis collinear order as the 5d Ir4+ ions are replaced with an increasing concentration of 4d Ru4+ ions. The magnetic structures within the ordered regime of the phase diagram (x<0.3) are reported. Despite the changes in magnetic structure no alteration of the Jeff=1/2 ground state is observed. The behavior of Sr2Ir1-xRuxO4 is consistent with electronic phase separation and diverges from a standard scenario of hole doping. The role of lattice alterations with doping on the magnetic and insulating behavior is considered. The results presented here provide insight into the magnetic insulating states in strong spin-orbit coupled materials and the role perturbations play in altering the behavior.

cond-mat.str-el↗

arXiv2012

Electron transfer and ionic displacements at the origin of the 2D electron gas at the LAO/STO interface: Direct measurements with atomic-column spatial resolution

The discovery that the interface between two band gap insulators LaAlO3 and SrTiO3 is highly conducting has raised an enormous interest in the field of oxide electronics. The LAlO3/SrTiO3 interface can be tuned using an electric field and switched from a superconducting to an insulating state. Conducting paths in an insulating background can be written applying a voltage with the tip of an atomic force microscope, creating great promise for the development of a new generation of nanoscale electronic devices. However, the mechanism for interface conductivity in LaAlO3/SrTiO3 has remained elusive. The theoretical explanation based on an intrinsic charge transfer (electronic reconstruction) has been strongly challenged by alternative descriptions based on point defects. In this work, thanks to modern aberration-corrected electron probes with atomic-scale spatial resolution, interfacial charge and atomic displacements originating the electric field within the system can be simultaneously measured, yielding unprecedented experimental evidence in favor of an intrinsic electronic reconstruction.

cond-mat.str-el↗

arXiv2007

Suppression of vortex channeling in meandered YBa2Cu3O7-d grain boundaries

We report on the in-plane magnetic field (H) dependence of the critical current density (Jc) in meandered and planar single grain boundaries (GBs) isolated in YBa2Cu3O7-d (YBCO) coated conductors. The Jc(H)properties of the planar GB are consistent with those previously seen in single GBs of YBCO films grown on SrTiO3 bi-crystals. In the straight boundary a characteristic flux channeling regime when H is oriented near the GB plane, associated with a reduced Jc, is seen. The meandered GB does not show vortex channeling since it is not possible for a sufficient length of vortex line to lie within it.

cond-mat.supr-con↗

arXiv2005

Anisotropic, non-monotonic behavior of the superconducting critical current in thin YBa2Cu3O7-d films on vicinal SrTiO3 surfaces

The critical current density of epitaxial YBCO films grown on vicinal SrTiO3 substrates was investigated by electrical transport measurements along and across the steps of the SrTiO3 surface for a range of temperatures of 10 K to 85 K and in applied magnetic fields varying from 0 to 14 T. For vicinal angles of 4 and 8 degrees, we found evidence of enhanced pinning in the longitudinal direction at low magnetic fields for a wide region of temperatures and attribute this phenomenon to anti-phase boundaries in the YBCO film. The transverse Jc data showed a peak in the Jc(H) curve at low magnetic fields, which was explained on the basis of magnetic interaction between Abrikosov and Abrikosov-Josephson vortices. The in-plane Jc anisotropy observed for vicinal angles of 0.4 degrees was reversed with respect to the 8 degree and 4 degree samples. This phenomenon was interpreted on the basis of strain induced in the YBCO film by the stepped substrate's surface.

cond-mat.supr-con↗

arXiv2003

Self-organized current transport through low angle grain boundaries in YBa$_2$Cu$_3$O$_{7-δ}$ thin films, studied magnetometrically

The critical current density flowing across low angle grain boundaries in YBa$_2$Cu$_3$O$_{7-δ}$ thin films has been studied magnetometrically. Films (200 nm thickness) were deposited on SrTiO$_3$ bicrystal substrates containing a single [001] tilt boundary, with angles of 2, 3, 5, and 7 degrees, and the films were patterned into rings. Their magnetic moments were measured in applied magnetic fields up to 30 kOe at temperatures of 5 - 95 K; current densities of rings with or without grain boundaries were obtained from a modified critical state model. For rings containing 5 and 7 degree boundaries, the magnetic response depends strongly on the field history, which arises in large part from self-field effects acting on the grain boundary.

cond-mat.supr-con↗

arXiv2001

Electrodynamic response of MgB2 sintered pellets and thin films

We present a study of the electrodynamic response of MgB2 pellets and thin film samples exhibiting critical temperatures ranging between 26 and 38 K. We have performed accurate measurements of the surface impedance ZS =RS+iXS as a function of the temperature and of the magnitude of the electromagnetic field. The temperature variation and the field dependence of ZS was measured by a dielectric resonator cavity technique in the microwave region. In particular, the temperature variation of the magnetic penetration depth was also determined in the RF region by a single coil mutual inductance method. In the case of the films, for T<TC/2 a clear exponential behavior of the penetration depth is observed, which can be explained by a simple BCS s-wave model with a reduced value of the energy gap. On the contrary, pellets show no evidence of saturation, and the experimental results strictly follow a quadratic dependence down to the lowest temperatures. This behavior can be induced by the presence of metallic Mg inclusions that may locally depress the gap. The analysis of the field dependence of the surface impedance in the microwave region confirms that the electrodynamic response of MgB2 is dominated by different sources of dissipation, depending on the sample history, likely to be ascribed to the predominance of grain boundaries or normal regions on its surface.

cond-mat.supr-con↗

arXiv2001

Study of the microwave electrodynamic response of MgB2 thin films

We present a study on the power dependence of the microwave surface impedance in thin films of the novel superconductor MgB2. 500 nm thick samples exhibiting critical temperatures ranging between 26 and 38 K are synthesized by an ex-situ post-anneal of e-beam evaporated boron in the presence of an Mg vapor at 900 C. Preliminary results on films grown in situ by a high rate magnetron sputtering technique from stoichiometric MgB2 and Mg targets are also reported. Microwave measurements have been carried out employing a dielectrically loaded niobium superconducting cavity operating at 19.8 GHz and 4 K. The study shows that the electrodynamic response of MgB2 films is presently dominated by extrinsic sources of dissipation, appearing already at low microwave power, likely to be ascribed to the presence of grain boundaries and normal inclusions in the samples.

cond-mat.supr-con↗

arXiv2001

RHEED Studies of Epitaxial Oxide Seed-Layer Growth on RABiTS Ni(001): The Role of Surface Structure and Chemistry

The epitaxial deposition of the first oxide buffer layer (seed layer) on biaxially textured Ni tape for coated conductors is a critical step that is dependent on the atomistic surface condition of the metal. We present a study of the {100}<100> biaxially textured Ni (001) surface and seed-layer growth using in situ reflection high-energy electron diffraction (RHEED) and Auger electron spectroscopy (AES). Our observations are consistent with formation of a c(2 x 2) 2-D superstructure due to surface segregation of sulfur contained in the metal. We show that this superstructure can have a dramatic effect on the heteroepitaxial growth of oxide seed layers. In particular, the surface superstructure promotes the (200) epitaxial oxide growth of Y2O3-stabilized ZrO2 (YSZ), which is necessary for the development of high-Jc superconducting films for coated conductors.

cond-mat.supr-con↗

arXiv2001

Growth mechanism of superconducting MgB2 films prepared by various methods

The growth mechanisms of MgB2 films obtained by different methods on various substrates are compared via a detailed cross-sectional scanning electron microscopy (SEM) study. The analyzed films include (a) samples obtained by an ex-situ post-anneal at 900 degree of e-beam evaporated boron in the presence of an Mg vapor (exhibiting bulk-like Tc0 about 38.8 K), (b) samples obtained by the same ex-situ 900 degree anneal of pulsed laser deposition (PLD)-grown Mg+B precursors (exhibiting Tc0 ~ 25 K), and (c) films obtained by a low-temperature (600 - 630 degree) in-situ anneal of PLD-grown Mg+B precursors (with Tc0 about 24 K). A significant oxygen contamination was also present in films obtained from a PLD-grown precursors. On the other hand, it is clearly observed that the films obtained by the high-temperature reaction of e-beam evaporated B with Mg vapor are formed by the nucleation of independent MgB2 grains at the film surface, indicating that this approach may not be suitable to obtain smooth and (possibly) epitaxial films.

cond-mat.supr-con↗

arXiv2001

Superconducting magnesium diboride films on Silicon with Tc0 about 24K grown via vacuum annealing from stoichiometric precursors

Superconducting magnesium diboride films with Tc0 ~ 24 K and sharp transition \~ 1 K were successfully prepared on silicon substrates by pulsed laser deposition from a stoichiometric MgB2 target. Contrary to previous reports, anneals at 630 degree and a background of 2x10^(-4) torr Ar/4%H2 were performed without the requirement of Mg vapor or an Mg cap layer. This integration of superconducting MgB2 films on silicon may thus prove enabling in superconductor-semiconductor device applications. Images of surface morphology and cross-section profiles by scanning electron microscopy (SEM) show that the films have a uniform surface morphology and thickness. Energy dispersive spectroscopy (EDS) reveals these films were contaminated with oxygen, originating either from the growth environment or from sample exposure to air. The oxygen contamination may account for the low Tc for those in-situ annealed films, while the use of Si as the substrate does not result in a decrease in Tc as compared to other substrates.

cond-mat.supr-con↗

arXiv2001

Superconducting MgB(2) films via precursor post-processing approach

Superconducting MgB(2) films with Tc = 38.6 K were prepared using a precursor-deposition, ex-situ post-processing approach. Precursor films of boron, ~0.5 micrometer thick, were deposited onto Al(2)O(3) (102) substrates by e-beam evaporation; a post-anneal at 890 deg C in the presence of bulk MgB(2) and Mg metal produced highly crystalline MgB(2) films. X-ray diffraction indicated that the films exhibit some degree of c-axis alignment, but are randomly oriented in-plane. Transport current measurements of the superconducting properties show high values of the critical current density and yield an irreversibility line that exceeds that determined by magnetic measurements on bulk polycrystalline materials.

cond-mat.supr-con↗

arXiv2001

Superconducting magnesium diboride films with Tc \approx 24K grown by pulsed laser deposition with in-situ anneal

Thin superconducting films of magnesium diboride (MgB2) with Tc \approx 24K were prepared on various oxide substrates by pulsed laser deposition (PLD) followed by an in-situ anneal. A systematic study of the influence of various in-situ annealing parameters shows an optimum temperature of about 600C in a background of 0.7 atm. of Ar/4%H2 for layers consisting of a mixture of magnesium and boron. Contrary to ex-situ approaches (e.g. reacting boron films with magnesium vapor at 900C), these films are processed below the decomposition temperature of MgB2. This may prove enabling in the formation of multilayers, junctions, and epitaxial films in future work. Issues related to the improvement of these films and to the possible in-situ growth of MgB2 at elevated temperature are discussed.

cond-mat.supr-con↗

arXiv2001

Critical Current Density of YBa{2}Cu{3}O{7-d} Low-Angle Grain Boundaries in Self-Field

A study has been perfomed on the superconducting critical current density J_{c} flowing across low angle grain boundaries in epitaxial thin films of YBa{2}Cu{3}O{7-d} (YBCO). The materials studied were dual grain boundary rings deposited on SrTiO{3} and containing 2, 3, 5 and 7 degree tilt boundaries. The current density in self-field was determined by magnetometric methods at temperatures from 5 K to T_c. We conclude that at the higher temperatures of coated conductor applications, there is limited potential for improving J_{c} by reducing the grain boundary angle below about 3 degrees.

cond-mat.supr-con↗