E. Di Gennaro

Critical influence of target-to-substrate distance on conductive properties of LaGaO3/SrTiO3 interfaces deposited at 10−1 mbar oxygen pressure

We investigate pulsed laser deposition of LaGaO3/SrTiO3 at 10−1 mbar oxygen background pressure, demonstrating the critical effect of the target-to-substrate distance, dTS, on the interface sheet resistance, Rs. The interface turns from insulating to metallic by progressively decreasing dTS. The analysis of the LaGaO3 plume evidences the important role of the plume propagation dynamics on the interface properties. These results demonstrate the growth of conducting interfaces at an oxygen pressure of 10−1 mbar, an experimental condition where a well-oxygenated heterostructures with a reduced content of oxygen defects is expected.

Microwave intermodulation distortion of MgB2 thin films

The two-tone intermodulation arising in MgB2 thin films deposited in situ by planar magnetron sputtering on sapphire substrates is studied. Samples are characterized using an open-ended dielectric puck resonator operating at 8.8 GHz. The experimental results show that the third-order products increase with the two-tone input power with a slope ranging between 1.5 and 2.3. The behavior can be understood introducing a mechanism of vortex penetration in grain boundaries as the most plausible source of nonlinearities in these films. This assumption is confirmed by the analysis of the field dependence of the surface resistance, that show a linear behavior at all temperatures under test.

Film Structure of Epitaxial Graphene Oxide on SiC: Insight on the Relationship between Interlayer Spacing, Water Content, and Intralayer Structure

Chemical oxidation of multilayer graphene grown on silicon carbide yields films exhibiting reproducible characteristics, lateral uniformity, smoothness over large areas, and manageable chemical complexity, thereby opening opportunities to accelerate both fundamental understanding and technological applications of this form of graphene oxide films. Here, we investigate the vertical inter-layer structure of these ultra-thin oxide films. X-ray diffraction, atomic force microscopy, and IR experiments show that the multilayer films exhibit excellent inter-layer registry, little amount (<10%) of intercalated water, and unexpectedly large interlayer separations of about 9.35 A. Density functional theory calculations show that the apparent contradiction of “little water but large interlayer spacing in the graphene oxide films” can be explained by considering a multilayer film formed by carbon layers presenting, at the nanoscale, a non-homogenous oxidation, where non-oxidized and highly oxidized nano-domains coexist and where a few water molecules trapped between oxidized regions of the stacked layers are sufficient to account for the observed large inter-layer separations. This work sheds light on both the vertical and intra-layer structure of graphene oxide films grown on silicon carbide, and more in general, it provides novel insight on the relationship between inter-layer spacing, water content, and structure of graphene/graphite oxide materials.

A parametric study of the lensing properties of dodecagonal photonic quasicrystals

We present a study of the lensing properties of two-dimensional (2-D) photonic quasicrystal (PQC) slabs made of dielectric cylinders arranged according to a 12-fold-symmetric square-triangle aperiodic tiling. Our full-wave numerical analysis confirms the results recently emerged in the technical literature and, in particular, the possibility of achieving focusing effects within several frequency regions. However, contrary to the original interpretation, such focusing effects turn out to be critically associated to local symmetry points in the PQC slab, and strongly dependent on its thickness and termination. Nevertheless, our study reveals the presence of some peculiar properties, like the ability to focus the light even for slabs with a reduced lateral width, or beaming effects, which render PQC slabs potentially interesting and worth of deeper investigation.

Intrinsic nonlinearity probed by intermodulation distortion microwave measurements on high quality MgB2 thin films

The two-tone intermodulation distortion arising in MgB2 thin films synthesized by hybrid physical-chemical vapor deposition is studied in order to probe the influence of the two bands on the nonlinear response of this superconductor. The measurements are carried out by using a dielectrically loaded copper cavity operating at 7GHz. Microwave data on samples having critical temperatures above 41K, very low resistivity values, and residual resistivity ratio larger than 10 are shown. The dependence of the nonlinear surface losses and of the third order intermodulation products on the power feeding the cavity and on the temperature is analyzed. At low power, the signal arising from distortion versus temperature shows the intrinsic s-wave behavior expected for this compound. Data are compared with measurements performed on Nb and YBa2Cu3O7−δ thin films using the same technique.

Dual mode cross slotted filter realized with double-sided Tl2Ba2CaCu2O8 films grown by MOCVD

We report on the realization of dual mode microwave filters based on Tl2Ba2Ca1Cu2O8 (Tl-2212) double-sided superconducting films. The Tl-2212 samples have been grown ex situ on 10×10 mm2 LaAlO3 (100) substrates by a combined approach of metal-organic chemical vapour deposition (MOCVD) and thallium vapour diffusion. Their morphological, compositional and structural natures have been investigated by scanning electron microscopy, energy dispersive x-ray analyses and x-ray diffraction characterization. Both sides of the films are smooth, homogeneous in thickness and composition and show epitaxially-grown Tl-2212 phases. Typical values of Tc = 100 K and Jc = 0.05 MA cm-2 at 77 K have been inductively measured for both sides. Microwave measurements have been performed at 20 GHz by using a dielectric resonator. Two planar superconducting single-stage dual mode filters operating in the C-band with 1% and 10% fractional bandwidth have been realized. The basic element of the device is a square patch resonator diagonally crossed by unequal slots providing both a size reduction and a simple and controllable way to couple the two degenerate modes. Device response has been studied at different temperatures, performing power and intermodulation measurements. In view of practical applications, the results are compared with those obtained by a filter based on high-quality YBCO films.

Mode confinement in photonic quasicrystal point-defect cavities for particle accelerators

In this Letter, we present a study of the confinement properties of point-defect resonators in finite-size photonic-bandgap structures composed of aperiodic arrangements of dielectric rods, with special emphasis on their use for the design of cavities for particle accelerators. Specifically, for representative geometries, we study the properties of the fundamental mode (as a function of the filling fraction, structure size, and losses) via 2-D and 3-D full-wave numerical simulations, as well as microwave measurements at room temperature. Results indicate that, for reduced-size structures, aperiodic geometries exhibit superior confinement properties by comparison with periodic ones.

Effects of oxygen background pressure on the stoichiometry of a LaGaO3 laser ablation plume investigated by time and spectrally resolved two-dimensional imaging

The plume expansion dynamics strongly affects the growth and the chemistry of pulsed laser deposited thin films. The interaction with the background gas determines the kinetic energy of the species impinging on the substrate, their angular broadening, the plasma chemistry, and eventually the cations stoichiometric ratio in oxide films. Here, we exploit two-dimensional, spectrally resolved plume imaging to characterize the diverse effects of the oxygen background pressure on the expansion dynamics of La, Ga, and LaO species during pulsed laser deposition of LaGaO3. The propagation of the ablated species towards the substrate is studied for background oxygen pressures ranging from high vacuum up to ≈10−1 mbar. Our experimental results show specie-dependent effects of the background gas on the angular distribution of the precursors within the plume. These findings suggest that even in the presence of a stoichiometric ablation and of a globally stoichiometric plume, cations off-stoichiometry can take place in...

Discrete model analysis of the critical current-density measurements in superconducting thin films by a single-coil inductive method

The critical current density of a superconducting film can be easily determined by an inductive and contactless method. Driving a sinusoidal current in a single coil placed in front of a superconducting sample, a nonzero third-harmonic voltage V3 is induced in it when the sample goes beyond the Bean critical state. The onset of V3 marks the value of current beyond which the sample response to the magnetic induction is no more linear. To take into account, in a realistic way, the magnetic coupling between the film and the coil, we have developed a discrete model of the inducing and induced currents distribution. In the framework of this model the magnetic-field profile on the sample surface and the coefficient linking the current flowing in the coil and the critical current density JC of superconducting thin films are evaluated. The numerical results are checked by measuring JC of several thin films of YBa2Cu3O7−δ of known superconducting properties, used as a control material.

Study of the microwave electrodynamic response of MgB2 thin films

Abstract We present a study on the power dependence of the microwave surface impedance in thin films of the novel superconductor MgB 2 . 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 MgB 2 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 MgB 2 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.