Viscous fingering of miscible fluids in an anisotropic radial hele-shaw cell: coexistence of kinetic and surface-tension dendrite morphology types and an exploration of small-scale influences

The evolution of viscous fingering morphology is examined for the case of a system of miscible fluids in an anisotropic radial Hele-Shaw cell. It is shown that dendritic morphologies similar to the kinetic and surface-tension morphology types coexist for this case. The critical role of the means of introducing anisotropy in the Hele-Shaw cell is established, and an explanation of the pattern behavior is offered on the basis of shape discontinuities of the individual elements of the lattice used to induce anisotropy. The ramifications of such an explanation are experimentally verified by demonstrating a clear difference in the morphology evolution in two halves of a single Hele-Shaw cell, one half of which contains square lattice elements, and the other half of which contains circular lattice elements

Surface modifications of ferrocene using argon ion implantation and γ-irradiation

The effect of Ar+-ion implantation (energy 100 keV) has been studied on ferrocene pellets at 1×1014 and 5×1014 ions/cm2 doses. The characterization techniques used were infrared spectroscopy, small-angle X-ray diffraction and bulk X-ray diffraction. At the lower dose, formation of a metastable alkene occurred; at the higher dose a metastable alkane was formed. The X-ray data showed that metastable phases like Fe-C, (Fe-C)H and carbon phases were produced at the surface. Not being observable on the surface, the organic counterparts are expected to be volatile in nature. When treated with varying energy doses, ferrocene showed bulk damage; the IR spectra showed the loss of fine structure and only a single peak persisted, which was due to metal-ring vibration

Electric Field Effect in Diluted Magnetic Insulator Anatase Co:TiO2

An external electric field induced reversible modulation of room temperature magnetic moment is achieved in an epitaxial and insulating thin film of dilutely cobalt-doped anatase TiO2. This first demonstration of electric field effect in any oxide based diluted ferromagnet is realized in a high quality epitaxial heterostructure of PbZr0.2Ti0.8O3/Co:TiO2/SrRuO3 grown on (001) LaAlO3. The observed effect, which is about 15% in strength in a given heterostructure, can be modulated over several cycles. Possible mechanisms for electric field induced modulation of insulating ferromagnetism are discussed.Comment: 14 pages, 4 figure

Interface characterization of all-perovskite oxide field effect heterostructures

All-oxide devices consisting of Niobium-doped Strontium Titanate (Nb:STO)/Strontium Titanate (STO)/Lanthanum Strontium Cuprous Oxide (LSCO) heterostructures were fabricated and characterized electrically for their interface properties through capacitance-voltage (C-V) and current-voltage (I-V) techniques, in the context of electric field effect studies. The C-V studies establish the occurrence of charge modulation in the LSCO channel. Absence of hysteresis in the C-V characteristic when the voltage is retraced suggests the absence of mobile ions in the gate oxide and slow interface traps. This is further corroborated by the absence of drift in the C-V characteristic and shift in the flat band voltage (V FB) when the device is subjected to temperature-bias aging. The interface state density obtained from V FB is ~1012/cm2. The uncompensated hole concentration in the LSCO channel calculated from the measured room temperature C-V data is ~1020/cm3 and is in good agreement with the expected hole concentration in LSCO. Current-time and current-voltage plots are invariant with respect to the polarity of the applied voltage up to ~5 V. This, in a structure with asymmetric interfaces, indicates that the electrical contacts to STO are non-blocking and the conduction through STO is bulk-limited in this voltage regime. Thickness dependent current and capacitance studies also corroborate the bulk-limited nature of conduction through the device in this voltage regime. However, I-V characteristic shows a rectifying nature beyond ~8 V indicating that the mechanism in this voltage regime could be interface limited

Temperature dependent optical studies of Ti1−x_{1-x}Cox_xO2_2

We present the results of Raman and photoluminescence (PL) studies on epitaxial anatase phase Ti$_{1-x}$Co$_x$O$_2$ films for $x$ = 0-0.07, grown by pulsed laser deposition. The low doped system ($x$=0.01 and 0.02) shows a Curie temperature of ~700 K in the as-grown state. The Raman spectra from the doped and undoped films confirm their anatase phase. The photoluminescence spectrum is characterized by a broad emission from self-trapped excitons (STE) at 2.3 eV at temperatures below 120 K. This peak is characteristic of the anatase-phase TiO$_2$ and shows a small blueshift with increasing doping concentration. In addition to the emission from STE, the Co-doped samples show two emission lines at 2.77 eV and 2.94 eV that are absent in the undoped film indicative of a spin-flip energy.Comment: 8 pages, 4 figure

Occurrence of coexisting dendrite morphologies: immiscible fluid displacement in an anisotropic radial hele-shaw cell under a high flow rate regime

Viscous fingering morphologies during the displacement of a high viscosity fluid by a low viscosity immiscible fluid in a radial fourfold anisotropic Hele-Shaw cell are examined. By using the kerosene-glycerin system for which the µ/T ratio (µ being the relative viscosity and T the interfacial tension between the fluids) is about ten times higher than that for the commonly used air-glycerin system, we have been able to access the hitherto unexplored Nca 1 regime (capillary number Nca=Uµ/T, U being the advancing fingertip velocity). Within the anisotropy-dominated regime, and when flow rates are significantly high (capillary number well beyond Nca=1), a new phase is seen to evolve wherein the dendrites grow simultaneously along the channels and along the directions making an angle of 45° with the channels, both being kinetically driven. This new phase resembles the one observed in a miscible fluid system at all flow rates of the displacing fluid

Chemical passivation of unstable FeO - a Mossbauer study

Highly unstable FeO is chemically passivated by incorporating Cr3+ ions by solid solution technique and forming FexO:Cr3+ single phase material. XRD, chemical analysis and Mossbauer spectroscopy are used for the characterization of the freshly prepared as well as samples aged in the desiccator for nearly three months. Optimum concentration range - 0.25 to 0.75 mole% - of Cr2O3 has been found to be necessary for stabilizing FexO:Cr3+. x is determined by chemical analysis. Mossbauer and XRD studies have confirmed the chemical passivation of unstable FeO

Superconducting MgB2 thin films by pulsed laser deposition

Growth of MgB2 thin films by pulsed laser deposition is examined under ex situ and in situ processing conditions. For the ex situ process, Boron films grown by PLD were annealed at 900 C with excess Mg. For the in situ process, different approaches involving ablation from a stoichiometric target under different growth conditions, as well as multilayer deposition involving interposed Mg layers were examined and analyzed. Magnetic measurements on ex situ processed films show TC of ~39 K, while the current best in situ films show a susceptibility transition at ~ 22 K.Comment: 3 pages, PD

Charge states and magnetic ordering in LaMnO3/SrTiO3 superlattices

We investigated the magnetic and optical properties of [(LaMnO3)n/(SrTiO3)8]20 (n = 1, 2, and 8) superlattices grown by pulsed laser deposition. We found a weak ferromagnetic and semiconducting state developed in all superlattices. An analysis of the optical conductivity showed that the LaMnO3 layers in the superlattices were slightly doped. The amount of doping was almost identical regardless of the LaMnO3 layer thickness up to eight unit cells, suggesting that the effect is not limited to the interface. On the other hand, the magnetic ordering became less stable as the LaMnO3 layer thickness decreased, probably due to a dimensional effect.Comment: 17 pages including 4 figures, accepted for publication in Phys. Rev.

Hall effect in cobalt-doped TiO2−δ_{2-\delta}

We report Hall effect measurements on thin films of cobalt-doped TiO$_{2-\delta}$. Films with low carrier concentrations (10$^{18}$ - 10$^{19}$) yield a linear behavior in the Hall data while those having higher carrier concentrations (10$^{21}$ - 10$^{22}$) display anomalous behavior near zero field. In the entire range of carrier concentration, n-type conduction is observed. The appearance of the anomalous behavior is accompanied by a possible structural change from rutile TiO$_{2}$ to Ti_[n}O$_{2n-1}$ Magneli phase(s)