ABSTRACT
We studied the electronic properties of a high-temperature superconductor in proximity to a ferromagnetic material in a bilayer film of La0.67Sr0.33MnO3 (LSMO)/YBa2Cu3O7 (YBCO). High-quality single-crystalline films of YBCO and LSMO/YBCO were grown epitaxially on an SrTiO3 (001) surface. Magnetization data of the LSMO/YBCO bilayer exhibit ferromagnetic transition at about 255 K, which is much smaller than the Curie temperature of bulk LSMO. Experimental data show the emergence of magnetic anisotropy with cooling, which becomes significantly stronger in the superconducting phase. The onset temperature of diamagnetism is observed at 86 K in the YBCO sample for the out-of-plane magnetization and at 89 K in the in-plane data. Interestingly, the diamagnetism sets in at about 86 K for both magnetization directions in the LSMO/YBCO film despite the presence of the ferromagnetic LSMO layer underneath. Ba 4d and Y 3d core-level spectra show different surface and bulk electronic structures. Surface contribution is reduced significantly in the LSMO/YBCO sample, suggesting enhanced bulk-like behavior due to an enhancement of electron density near the surface arising from charge transfer across the interface. These results reveal an outstanding platform for on-demand tuning of properties without affecting the superconductivity of the system for the exploration of fundamental science and applications in advanced technology.
ABSTRACT
Bulk gallium phosphide (GaP) crystallizes in the zinc-blende (ZB) structure and has an indirect bandgap. However, GaP nanowires (NWs) can be synthesized in the wurtzite (WZ) phase as well. The contradictory theoretical predictions and experimental reports on the band structure of WZ GaP suggest a direct or a pseudo-direct bandgap. There are only a few reports of the growth and luminescence from WZ and ZB GaP NWs. We first present a comprehensive study of the gold-catalyzed growth of GaP NWs via metalorganic vapor phase epitaxy on various crystalline and amorphous substrates. We optimized the growth parameters like temperature, pressure and reactant flow rates to grow WZ GaP NWs with minimal taper. These wires were characterized using electron microscopy, x-ray diffraction, Raman scattering and photoluminescence spectroscopy. The luminescence studies of bare GaP NWs and GaP/AlGaP core-shell heterostructures with WZ- and ZB-phase GaP cores suggest that the WZ-phase GaP has a pseudo-direct bandgap with weak near-band-edge luminescence intensity.
ABSTRACT
While 2D layers of WS2 have been extensively studied, there are very few investigations of WS2 nanotubes. These have usually been grown via a 2-step process involving a WO3-x intermediate. We report a simple process for the synthesis of WS2 nanotubes via the sulfurization of tungsten films under appropriate conditions and present details of their structural and optical properties that help elucidate the formation mechanism. Electron-beam evaporated films of tungsten are sulfurized under flowing N2 gas at 950-1000 °C temperature under atmospheric pressure to obtain WS2 nanotubes. High-resolution scanning and transmission electron microscopy studies show that 2D WS2 flakes curl up and wrap around themselves to form nanotubes. Interlayer spacings in both 'a' and 'c' directions are slightly smaller than the corresponding values in bulk and thin film WS2. Micro-photoluminescence and micro-transmission studies show a resonance that seems intrinsic to the WS2 nanotubes since it cannot be related to the known optical characteristics of WS2 flakes.
ABSTRACT
In this work, we demonstrate a simple technique to grow high-quality whiskers of Bi2 Sr2 CaCu2 O8+δ - a high T c superconductor. Structural analysis shows the single-crystalline nature of the grown whiskers. To probe electrical properties, we exfoliate these whiskers into thin flakes (~50 nm thick) using the scotch-tape technique and develop a process to realize good electrical contacts. We observe a superconducting critical temperature, T c , of 86 K. We map the evolution of the critical current as a function of temperature. With 2-D materials emerging as an exciting platform to study low-dimensional physics, our work paves the way for future studies on two-dimensional high-T c superconductivity.
ABSTRACT
Despite the numerous reports on the metal-catalyzed growth of GaN nanowires, the mechanism of growth is not well understood. Our study of the nickel-assisted growth of GaN nanowires using metalorganic chemical vapor deposition provides key insights into this process. From a comprehensive study of over 130 nanowires, we observe that as a function of thickness, the length of the nanowires initially increases and then decreases. We attribute this to an interplay between the Gibbs-Thomson effect dominant in very thin nanowires and a diffusion induced growth mode at larger thickness. We also investigate the alloy composition of the Ni-Ga catalyst particle for over 60 nanowires using energy dispersive X-ray spectroscopy, which along with data from electron energy loss spectroscopy and high resolution transmission electron microscopy suggests the composition to be Ni2Ga3. At the nanowire growth temperature, this alloy cannot be a liquid, even taking into account melting point depression in nanoparticles. We hence conclude that Ni-assisted GaN nanowire growth proceeds via a vapor-solid-solid mechanism instead of the conventional vapor-liquid-solid mechanism.
ABSTRACT
Crystalline hydrogen titanate (H2Ti3O7) nanowires were irradiated with N(+) ions of different energies and fluences. Scanning electron microscopy reveals that at relatively lower fluence the nanowires are bent and start to adhere strongly to one another as well as to the silicon substrate. At higher fluence, the nanowires show large-scale welding and form a network of mainly 'X' and 'Y' junctions. Transmission electron microscopy and Raman scattering studies confirm a high degree of amorphization of the nanowire surface after irradiation. We suggest that while ion-irradiation induced defect formation and dangling bonds may lead to chemical bonding between nanowires, the large scale nano-welding and junction network formation can be ascribed to localized surface melting due to heat spike. Our results demonstrate that low energy ion irradiation with suitable choice of fluence may provide an attractive route to the formation and manipulation of large-area nanowire-based devices.
