T. Venkatappa Rao

Ground state properties of the periodic Anderson model

Abstract The periodic Anderson model is investigated in the limit of infinite Coulomb correlation. The self-consistent equations for the occupancy of the 4ƒ-level and for the chemical potential have been evaluated. The condition for the magnetic order is obtained. The relation between the ground state magnetic susceptibility and valence is investigated for different values of the total number of electrons per lattice site n. The results are interpreted by looking at the density of states at the Fermi-level. For n = 1 and n > 1 the phase diagrams have been constructed to show the parameter space in which the model system displays the magnetic order.

REENTRANT-LIKE BAND JAHN–TELLER EFFECT AND ITS FIELD DEPENDENCE

In the presence of the electron–lattice interaction where the lattice strain is coupled to the difference in orbital occupancy, the metallic system whose Fermi level lies in an orbitally degenerate eg band undergoes a structural transition to a lower symmetric state with distortion. The distortion is the consequence of the band Jahn–Teller (J–T) effect, and results from the gain of the electronic energy against the increase in the elastic energy. The extent of distortion depends on the nature of the density of states (DOS) and carrier concentration. These effects are examined for a system described by a different model DOS, and an orbitally degenerate eg band with the J–T interaction. In a certain region of parameter space, the temperature dependence of distortion exhibits a reentrant-like behavior, and the magnetic field augments the distortion in this region. In the parameter space where there is no reentrant-like behavior, the field suppresses the distortion, and the normalized distortion becomes a universal function of the normalized field. A phase diagram is obtained in the magnetic field–temperature plane.

Anomalous behaviour of the coefficient of thermal expansion in intermediate valence systems

Abstract The electronic contribution to the coefficient of thermal expansion α for intermediate valence systems is related to the temperature derivative of 4⨍-level occupancy. Using the Greens function technique, the occupancy of the 4⨍-level and its temperature derivative have been evaluated self-consistently with the periodic Anderson model in the limit of infinite Coulomb repulsion and upto quadratic terms in hybridization interaction. The influence of the 4⨍-level position with respect to the conduction band and the strength of hybridization interaction on the temperature dependence of the coefficient of thermal expansion is investigated. It has been noted that α displays a peak-like behaviour as a function of temperature and also can be both positive and negative as observed in some experiments.

Effect of electron beam irradiation on thermal and mechanical properties of poly (lactic acid)/poly (ethylene-co-glycidyl methacrylate) blend

Physiochemical blend of Poly (lactic acid) (PLA)/Poly (ethylene-co-glycidyl methacrylate) (PEGMA) has been prepared by using twin-screw Micro compounder. The weight ratio of the blends was fixed at 80:20 (PLA: PEGMA). After that, the multipurpose test specimens -ASTM D638 of resulting blend and pristine PLA were prepared by injection moulding with mould temperature 32˚C. Furthermore, some test Specimens -ASTM D256 also prepared for notch impact test. The prepared samples were exposed to electron beam irradiation at different doses. These samples (un-irradiated and irradiated) were tested for mechanical and thermal properties. A detailed study is made by observing the improvement in the mechanical and thermal properties of the prepared blends with and without electron beam irradiation.Physiochemical blend of Poly (lactic acid) (PLA)/Poly (ethylene-co-glycidyl methacrylate) (PEGMA) has been prepared by using twin-screw Micro compounder. The weight ratio of the blends was fixed at 80:20 (PLA: PEGMA). After that, the multipurpose test specimens -ASTM D638 of resulting blend and pristine PLA were prepared by injection moulding with mould temperature 32˚C. Furthermore, some test Specimens -ASTM D256 also prepared for notch impact test. The prepared samples were exposed to electron beam irradiation at different doses. These samples (un-irradiated and irradiated) were tested for mechanical and thermal properties. A detailed study is made by observing the improvement in the mechanical and thermal properties of the prepared blends with and without electron beam irradiation.

Finite temperature magnetic properties of the highly correlated Anderson Lattice

Abstract A major success of the periodic Anderson model (PAM) is in explaining a variety of properties of systems containing an array of rare earth (RE) ions. The magnetic properties of these systems in particular have also been investigated both theoretically and experimentally. In an earlier publication the magnetic properties of PAM have been reported for T = 0. The calculations were performed by assuming that the double occupation of the states localized on RE ions is forbidden. The present paper extends the calculation to finite temperature. From the study of the normalized spontaneous magnetization m f and the magnetic susceptibility χ, as a function of temperature and the model parameters, several interesting features emerge. These features are explained using the interplay of the various interactions in the model. Further, the results are compared with some of the experimental results. For example, the peak in χ, the linear correlation between χ( T = 0) and the peak temperature T 0 and a peak in Curie temperature as a function of chemical composition which are observed experimentally are obtained in the present calculation. Finite temperature calculations for the same model are available but in the weak correlation limit. The results of the present calculation are compared with these calculations and the significant differences are pointed out.

The effect of electron beam irradiation on physicochemical properties of potato starch

The paper presents the effect of electron beam (e-beam) irradiation on potato starch. Starch powder was exposed to e-beam source of doses 30, 60 and 90 kGy and the effect of irradiation treatment on structural, thermal, morphological and viscosity properties of starch granules were studied. Signals were traced from electron spin resonance (ESR) spectroscopy indicating that free radicals are formed on irradiation. The results have showed that the viscosity of starch solution decreases whereas solubility increases on irradiation. Gelatinization temperature decreased following irradiation from differential scanning calorimetry studies. From fourier transform infrared (FTIR) studies, the intensity of absorption band 999 cm−1 increases on irradiation which indicates an increase in amorphous nature of starch. The X-ray diffraction results are in good agreement with the FTIR studies regarding crystallinity. The surface structure of the starch granules have changed significantly at high dose of irradiation from scanning electron microscopy.The paper presents the effect of electron beam (e-beam) irradiation on potato starch. Starch powder was exposed to e-beam source of doses 30, 60 and 90 kGy and the effect of irradiation treatment on structural, thermal, morphological and viscosity properties of starch granules were studied. Signals were traced from electron spin resonance (ESR) spectroscopy indicating that free radicals are formed on irradiation. The results have showed that the viscosity of starch solution decreases whereas solubility increases on irradiation. Gelatinization temperature decreased following irradiation from differential scanning calorimetry studies. From fourier transform infrared (FTIR) studies, the intensity of absorption band 999 cm−1 increases on irradiation which indicates an increase in amorphous nature of starch. The X-ray diffraction results are in good agreement with the FTIR studies regarding crystallinity. The surface structure of the starch granules have changed significantly at high dose of irradiation from s...

Low-cost and high-resolution interrogation scheme for LPG-based temperature sensor

A low-cost and high-resolution interrogation scheme for a long-period fiber grating (LPG) temperature sensor with adjustable temperature range has been designed, developed and tested. In general LPGs are widely used as optical sensors and can be used as optical edge filters to interrogate the wavelength encoded signal from sensors such as fiber Bragg grating (FBG) by converting it into intensity modulated signal. But the interrogation of LPG sensors using FBG is a bit novel and it is to be studied experimentally. The sensor works based on measurement of shift in attenuation band of LPG corresponding to the applied temperature. The wavelength shift of LPG attenuation band is monitored using an optical spectrum analyser (OSA). Further the bulk and expensive OSA is replaced with a low-cost interrogation system that employ an FBG, photodiode and a transimpedance amplifier (TIA). The designed interrogation scheme makes the system low-cost, fast in response, and also enhances its resolution up to 0.1°C. The measurable temperature range using the proposed scheme is limited to 120 °C. However this range can be shifted within 15-450 °C by means of adjusting the Bragg wavelength of FBG.A low-cost and high-resolution interrogation scheme for a long-period fiber grating (LPG) temperature sensor with adjustable temperature range has been designed, developed and tested. In general LPGs are widely used as optical sensors and can be used as optical edge filters to interrogate the wavelength encoded signal from sensors such as fiber Bragg grating (FBG) by converting it into intensity modulated signal. But the interrogation of LPG sensors using FBG is a bit novel and it is to be studied experimentally. The sensor works based on measurement of shift in attenuation band of LPG corresponding to the applied temperature. The wavelength shift of LPG attenuation band is monitored using an optical spectrum analyser (OSA). Further the bulk and expensive OSA is replaced with a low-cost interrogation system that employ an FBG, photodiode and a transimpedance amplifier (TIA). The designed interrogation scheme makes the system low-cost, fast in response, and also enhances its resolution up to 0.1°C. The measurable temperature range using the proposed scheme is limited to 120 °C. However this range can be shifted within 15-450 °C by means of adjusting the Bragg wavelength of FBG.

Spectroscopic and Thermal Studies on Irradiated Poly (Ethylene Maleic Anhydride)

Radiation effects in Poly (ethylene maleic anhydride) (MANPE) have been investigated by Electron spin resonance (ESR), Fourier transform infrared (FTIR) and Differential scanning calorimeter (DSC) techniques. The ESR spectra of MNAPE under conditions like radiation dose and temperature have been recorded on irradiation. Alkyl group formed in MANPE which abstract oxygen and convert to peroxy radical on increase of radiation dose. More number of chain cleavages occur producing more number of free radicals which resulted increase in intensities of ESR spectrum. With the increase of temperature the free radical formed in MANPE may react with themselves or with other radical, resulting in the decay of ESR signals at high temperature. The FTIR spectra of irradiated MANPE possess absorption bands at 3500‐3000cm−1 and 1700cm−1 positions indicating the presence of OH and C = O groups. The DSC data suggests that on irradiation the melting point slightly increases due to the formation of network structure.

Optical absorption and fluorescence characteristics of Pr3+: Li2O-MO (Nb2O5, MoO3 and WO3)-B2O3 glass systems

With in glass forming region, a particular composition of the glass system 1Pr3+: 40Li2O-4MO (Nb2O5, MoO3 and WO3)-55B2O3 (all are in mol %) is chosen. The optical absorption spectra of these glasses explores the J-O parameters in the order Ω4 > Ω6 > Ω2, by performing least square fitting analysis. The fluorescence spectra of these glasses show highest branching ratio for the transition 3P0 → 3H4. From the analysis of these results, it has been identified that WO3 mixed glass network is more favourable environment for rare earth ions to give more luminescence efficiencies when compared with other two systems.