Sheela K. Ramasesha

Electronic structure of high-T c superconductors and related compounds

We study a 5-band Hubbard model for the CuO2 planes in cuprate super-conductors using Hartree-Fock mean-field theory including spiral spin density waves. For the half-filled case we recover a ZSA-like phase diagrambut with an additional new region characterized by strong covalency effects, which we call a covalent insulator region. We also provide a nonperturbative calculation ofJeff, the effective in-plane antiferromagnetic interaction, as a function of parameters of the model. We suggest that the high-Tc cuprates are in or very close to the covalent insulator region and within this we show that a consistent explanation of apparently conflicting high energy spectroscopic and magnetic measurements of the high-Tc cuprates can be given.

An explanation of the phenomenon of polytypism

Based on the analogy between polytypes and spin-half Ising chains, polytypes can be considered as different phases of a spin-half Ising system with competing nearest neighbour and next nearest neighbour interactions operating in a single direction. It is known that such an Ising system exhibits extremely rich and complicated phase behaviour. This behaviour is shown to be very similar to the phase behaviour exhibited by polytypes.

Anodization of Aluminium using a fast two-step process

AbstractUltra-fast two-step anodization method is developed for obtaining ordered nano-pores on aluminium (Al) foil. First anodization was carried out for 10 min, followed by 3 min of second anodization at high voltage (150 V) compared to previous reports of anodization times of 12 h (40-60 V). The pore dimensions on anodized alumina are 180 nm for pore diameter and 130 nm for inter-pore distance. It was evident that by increasing the anodization voltage to 150 V, the diameter of the pores formed was above 150 nm. The electrolyte and its temperature affect the shape and size of the pore formation. At lower anodization temperature, controlled pore formation was observed. The anodized samples were characterized using the field emission scanning electron microscope (FE-SEM) to determine the pore diameter and inter-pore distance. Using UV-Visible spectroscopy, the reflectance spectra of anodized samples were measured. The alumina (Al2O3) peaks were identified by x-ray diffraction (XRD) technique. The x-ray photo electron spectroscopy (XPS) analysis confirmed the Al 2p peak at 73.1 eV along with the oxygen O 1s at 530.9 eV and carbon traces C 1s at 283.6 eV. Graphical AbstractFast two-step anodization process using oxalic acid and phosphoric acid as electrolytes is reported. Uniform nano-holes were formed on the Al foil with diameter of 180 nm and wall thickness of 130 nm. The nanoporous alumina was characterized using FE-SEM, UV-Visible spectra, XRD and XPS.

EMF of bielectrolyte solid state cells with different mobile ions in each phase

Abstract An expression for the emf of an isothermal solid state bielectrolyte cell is developed starting from the basic equations of transport for each solid electrolyte. If the two different migrating species react to form a compound at well defined activity at the interface between the two solid electrolytes, interfacial chemical potentials and the emf are unambiguously defined. The interfacial chemical potentials are determined by the transport properties of the two solid electrolytes. The emf of the bielectrolyte cell will be less than that calculated from the Gibbs energy change for the virtual cell reaction when the interfacial or electrode chemical potentials lie outside the electrolytic conduction domain ( t ion >0.99) of the solid electrolytes. Using estimated values for electonic and hole conductivities in Na β-alumina, the emf of a bielectrolyte combination consisting of β-alumina and (CaO)ZrO 2 is computed for different chemical potentials of sodium and oxygen at the electrodes. The chemical potential gradient for sodium across β-alumina is found to be very small because the conductivity of β-alumina is much larger than that of (CaO)ZrO 2 . When two cationic or anionic conductors with different mobile species are coupled together, the chemical potentials at junction of the solid electrolytes and hence the emf are usually ill defined. A definite relationship between ionic fluxes in each electrolyte is necessary to mathematically define the interfacial chemical potentials.

Design of temperature-compensated reference electrodes for non-isothermal galvanic sensors

The criterion for the design of a temperature-compensated reference electrode for non-isothermal galvanic sensors is deduced from the basic flux equations of irreversible thermodynamics. It is shown that when the Seebeck coefficient of the non-isothermal cell using a solid oxygen ion-conducting electrolyte under pure oxygen is equal to the relative partial molar entropy of oxygen in the reference electrode divided by 4F, then the EMF of the non-isothermal cell is the same as that of an isothermal cell with the same electrodes operating at the higher temperature. By measuring the temperature of the melt alone and the EMF of the non-isothermal galvanic sensor, one can derive the chemical potential or the concentration of oxygen in a corrosive medium. The theory is experimentally checked using sensors for oxygen in liquid copper constructed with various metal+oxide electrodes and fully stabilised (CaO)ZrO2 as the electrolyte. To satisfy the exact condition for temperature compensation it is often necessary to have the metal or oxide as a solid solution in the reference electrode.

Continuous-cooling method of specific heat measurement in the temperature range 100-300 K

The authors have developed a simple continuous-cooling method to determine specific heat of liquids and solids in the temperature range 100-300 K. The technique employs very simple instrumentation and continuously records the sample temperature as it cools to the bath temperature through a calibrated heat link. They have obtained specific heat values which agree with the reported data to within 3% for the samples investigated. This method also facilitates easy detection of abrupt changes in specific heat, as demonstrated in the observation of glass transition in some organic glass-forming systems. The method is sensitive to the study of relaxing heat capacity in supercooled liquids.

Fabrication of back contacts using laser writer and photolithography for inscribing textured solar cells

Semiconductor fabrication process begins with photolithography. Preparing a photo mask is the key process step in photolithography. The photo mask was fabricated by inscribing patterns directly onto a soda lime glass with the help of a laser beam, as it is easily controllable. Laser writer LW405-A was used for preparing the mask in this study. Exposure wavelength of 405 nm was used, with which 1.2 μm feature size can be written in direct write-mode over the soda lime glass plate. The advantage of using the fabricated mask is that it can be used to design back contacts for thin film Photovoltaic (PV) solar cells. To investigate the process capability of LW405-A, same pattern with different line widths was written on soda lime glass samples at different writing speeds. The pattern was inscribed without proximity effect and stitching errors, which was characterized using optical microscope and field emission scanning electron microscope (FE-SEM). It was proven that writing speed of a mask-writer is decided according to the intended feature size and line width. As the writing speed increases, the edges of the patterns become rougher due to uneven scattering of the laser beam. From the fabricated mask, the solar cell can be developed embedding both the contacts at the bottom layer, to increase the absorption of solar radiation on the top surface effectively by increasing light absorption area.

Thermoelectric power of YBa2Cu3O7−δ under pressure up to 9 GPa

Thermoelectric power (TEP) of two YBa2Cu3O7−δ compounds (with δ=0·17 and 0·21) was measured as a function of quasi-hydrostatic pressure up to 9GPa at 300K on samples with low porosity. In both cases TEP decreases with increasing pressure, at a rate ∼ 0·8 μVK−1/GPa. The data obtained under hydrostatic pressure up to 3 GPa are in good agreement with those under quasi-hydrostatic pressure. The TEP of both compositions is found to decrease linearly at a rate 0·8 μVK−1/GPa above 1·5 GPa.

Effect of hole localization on Madelung potential of YBa2Cu3O7

The Madelung potential and formation energy of the superconducting compound YBa2Cu3O7 have been computed for hole localization at different sites in the crystal. The cases considered include Cu3+ ion at Cu(1) and Cu(2) sites, O− ion at O(1), O(2), O(3) and O(4) sites and combinations of O− and Cu3+ ions at O(4) and Cu(1) and O(2,3) and Cu(2) sites. The two lowest-energy configurations correspond to Cu3+ ion at Cu(1) site and O− ion at O(4) site. The difference in formation energy between those configurations is relatively small. The next preferred configuration corresponds to simultaneous partial localization of the hole at Cu (1) site and O(1) site. Other configurations are much less stable. The results suggest a resonating or fluctuating valence model for YBa2Cu3O7.

Patterning of nanopillars-based CdS/CdTe thin films for photonic applications

ABSTRACT In this study we propose a new design to place the contacts for both p-type and n-type semiconductors at the bottom of solar cell to increase photons entering the absorber layer. The fabricated device contains periodic nanopillars of ∼243 nm radius and centre to centre pillar distance of ∼550 nm. In addition to the fabrication, thin films of n-CdS and p-CdTe have been characterised for their material properties at different annealing temperatures in this study. The X-ray diffraction pattern for CdTe and CdS revealed that the growth is preferentially along the (111) plane with a cubic zinc blend and cubic structure, respectively. Optimum absorption of the solar spectrum for a single junction solar cell occurs with the optical band gap of 2.36 eV for CdS and 1.45 eV for CdTe. The significance of the work creates a new beginning for the fabrication of textured thin film photovoltaic cell.