Syamaprasad, U; Vallabhan, C P G(Institute of Physics, 1981)
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Abstract:
Four distinct peaks are observed at 140, -26, -132 and -140°C in the sigma x* against T-1 plot between 200 and - 196°C for (NH4)3H(SO4)2, corresponding to four different phase
transitions of which the one at -26°C is reported here for the first time. Data on doped
samples reveal the charge transport mechanism in the crystal.
Syamaprasad, U; Vallabhan, C P G(Elsevier, May , 1981)
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Abstract:
Anomalous variations of d.c. electrical conductivity with temperature are observed in ammonium sulphate single crystals, suggesting a possible phase transition at 150°C. Measurements of thermally stimulated current also support these results. The mechanism of electrical conduction is explained on the basis of studies made on doped and quenched crystals.
Measurements of dc conductivity and dielectric constant show that deuteration causes an upward shift of the high temperature phase transition point from 186.5 to 191°C and a downward shift of the low temperature transition point from 10 to -1.5°C in LiNH4SO4. Mechanisms of phase transitions and of electrical transport in the crystal are discussed.
Pillai, S M; Vallabhan, C P G(Solid State Communications, 1983)
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Abstract:
ZnS: Cu: Cl phosphor prepared under a vacuum firing process is found to
give blue electroluminescence with emission peak at 460 nm which remams
unaltered with the frequency of the excitation voltage. Addition of excess
chlorine in the phosphor gives blue, green and red emission at 460, 520
and 640 run. The intensity of the blue band decreases and It fmally
disappears as chlorine concentration is increased. A scheme involving
three energy levels attributed to Cu2+, Cu+ and Cl- centres in Zns
explains the experimental results completely.
Subhadra, V K; Syamaprasad, U; Vallabhan, C P G(American Institute of Physics, 1983)
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Abstract:
Results of axiswise measurements of the electrical conductivity (dc and ac) and dielectric constant of NH4H2PO4 confirm the occurrence of the recently suggested high‐temperature phase transition in this crystal (at 133 °C). The corresponding transition in ND4D2PO4 observed here for the first time takes place at 141.5 °C. The mechanism involved in these transitions and those associated with the electrical conduction and dielectric anomalies are explained on the basis of the motional effects of the ammonium ions in these crystals. Conductivity values for deuterated crystals give direct evidence for the predominance of protonic conduction throughout the entire range of temperatures studied (30–260 °C).
Jayaraj, M K; Vallabhan, C P G(IOP Publishing, 1989)
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Abstract:
The method of preparation of ZnS phosphors doped with praseodymium and copper is given. The electroluminescence (EL) spectrum of ZnS:Pr,Cl has two broad bands at 470 and 570 nm. ZnS:Cu,Pr,Cl gives white emission with spectral peaks at 470, 520, 570 and 640 nm. The EL spectra of both types of phosphor exhibit a conspicuous colour shift as the frequency of the excitation voltage is varied. Detailed investigations show that the relative intensities of spectral peaks are strongly dependent on the frequency of the excitation voltage. The colour shift is explained on the basis of the Schon-Klasens model.
Wilson,K J; Vallabhan, C P G(IOP Publishing, 1989)
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Abstract:
A simple and inexpensive linear magnetic field sweep generating system suitable for magnetic resonance experiments is described. The circuit, utilising a modified IC bootstrap configuration, generates field sweep over a wide range of sweep durations with excellent sweep linearity.
Navil Kumar, R; Vallabhan, C P G(IOP Publishing, 1989)
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Abstract:
DC and AC electrical conductivity measurements in single crystals of diammonium hydrogen phosphate along the c axis show anomalous variations at 174, 246 and 416 K. The low-frequency dielectric constant also exhibits peaks exactly at these temperatures with a thermal hysteresis of 13 degrees C for the peak at 416 K. These specific features of the electrical properties are in agreement with earlier NMR second-moment data and can be identified with three distinct phase transitions that occur in the crystal. The electrical conductivity values have been found to increase linearly with impurity concentration in specimens doped with a specific amount of SO42- ions. The mechanisms of the phase transition and of the electrical conduction process are discussed in detail.
Padmaja, G; Ravi Kumar, A V; Vidyalal, V; Radhakrishnan, P; Nampoori, V P N; Vallabhan, C P G(IOP Publishing, 1989)
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Abstract:
In order to characterise the laser ablation process from high-Tc superconductors, the time evolution of plasma produced by a Q-switching Nd:YAG laser from a GdBa2Cu3O7 superconducting sample has been studied using spectroscopic and ion-probe techniques. It has been observed that there is a fairly large delay for the onset of the emission from oxide species in comparison with those from atoms and ions of the constituent elements present in the plasma. Faster decay occurs for emission from oxides and ions compared with that from neutral atoms. These observations support the view that oxides are not directly produced from the target, but are formed by the recombination process while the plasma cools down. Plasma parameters such as temperature and velocity are also evaluated.
Padmaja, G; Ravi Kumar, A V; Vidyalal, V; Radhakrishnan, P; Nampoori, V P N; Vallabhan, C P G(Springer, May , 1989)
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Abstract:
Laser induced plasma emission spectra from highT c superconducting samples of YBa2Cu3O7 and GdBa2Cu3O7 obtained with 1.06µm radiation from a Q switched Nd:YAG laser beam has been analysed. The results clearly show the presence of diatomic oxides in addition to ionized species of the constituent metals in the plasma thus produced.
Jayaraj, M K; Vallabhan, C P G(Elsevier, October , 1989)
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Abstract:
The dielectric properties of vacuum-deposited europium oxide films have been investigated in the frequency range from 1 kHz to 1 MHz at various temperatures (300-543 K). The dielectric constant is found to depend on film thickness and it attains a constant value beyond 1000 Å. Films deposited at higher substrate temperatures (above 423 K) exhibit improved dielectric properties owing to the recovery of stoichiometry. The frequency variation of the loss factor exhibits a minimum which increases with rise in temperature. The breakdown field strength (about 106V cm-1) is found to be thickness dependent and it varies in accordance with the Forlani-Minnaja relation. The films exhibit ohmic conduction with an activation energy of 0.86 eV at low electric fields but at higher fields the conductivity becomes space charge limited. X-ray studies show that the films are amorphous in nature. The a.c. conductivity is proportional to ω at low frequency, whereas a square law dependence is observed at higher frequencies. The optical constants n, α and k and optical band gap are calculated from the UV-visible-near-IR spectra.
Ravi Kumar, A V; Padmaja, G; Nampoori, V P N; Vallabhan, C P G(Springer, November , 1989)
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Abstract:
Absorption spectra of formaldehyde molecule in the gas phase have been recorded using photoacoustic (PA) technique with pulsed dye laser at various power levels. The spectral profiles at higher power levels are found to be different from that obtained at lower laser powers. Two photon absorption (TPA) is found to be responsible for the photoacoustic signal at higher laser power while the absorption at lower laser power level is attributed to one photon absorption (OPA) process. Probable assignments for the different transitions are given in this paper.
Jayaraj, M K; Vallabhan, C P G(IOP Publishing, 1990)
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Abstract:
AC thin film electroluminescent devices of MIS and MISIM have been fabricated with a novel dielectric layer of Eu2O3 as an insulator. The threshold voltage for light emission is found to depend strongly on the frequency of excitation source in these devices. These devices are fabricated with an active layer of ZnS:Mn and a novel dielectric layer of Eu2O3 as an insulator. The observed frequency dependence of brightness-voltage characteristics has been explained on the basis of the loss characteristic of the insulator layer. Changes in the threshold voltage and brightness with variation in emitting or insulating film thickness have been investigated in metal-insulator-semiconductor (MIS) structures. It has been found that the decrease in brightness occurring with decreasing ZnS layer thickness can be compensated by an increase in brightness obtained by reducing the insulator thickness. The optimal condition for low threshold voltage and higher stability has been shown to occur when the active layer to insulator thickness ratio lies between one and two.
Sathy, P; Reji, Philip; Nampoori, V P N; Vallabhan, C P G(Elsevier, January 1, 1990)
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Abstract:
Two-photon absorption in Rhodamine 6G using the second harmonic of a pulsed Q-switched Nd:YAG laser has been studied by photoacoustic technique. It is observed that there is a competition between one-photon and two-photon absorption processes. At lower concentration the two-photon process is predominant over the one-photon process.
Sathy, P; Reji, Philip; Nampoori, V P N; Vallabhan, C P G(Springer, June , 1990)
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Abstract:
Experimental method for measuring photoacoustic(PA) signals generated by a pulsed laser beam in liquids is described. The pulsed PA technique is found to be a convenient and accurate method for determination of quantum yield in fluorescent dye solutions. Concentration dependence of quantum yield of rhodamine 6G in water is studied using the above method. The results indicate that the quantum yield decreases with increase in concentration in the quenching region in agreement with the existing reports based on radiometric measurements.
Jayaraj, M K; Vallabhan, C P G(Thin Solid Films, 1991)
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Abstract:
The dielectric properties of electron beam evaporated Sm2O3 films have been
investigated in the frequency range from 1 kHz to 1 MHz at various temperatures
(300 K-453 K). The dielectric constant is found to depend on film thickness and it
attains a constant value beyond 1000 A. The present electron beam evaporated
Sm2O3 films have a high dielectric constant of 43. The frequency dependence of
and tan teeta at various temperatures is also studied.
Thomas, Baby; Ramachandran, T; Radhakrishnan, P; Nampoori, V P N; Vallabhan, C P G(IOP Publishing, 1991)
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Abstract:
A low inductance, triggered spark gap switch suitable for a high-current fast discharge system has been developed. The details of the design and fabrication of this pressurized spark gap, which uses only commonly available materials are described. A transverse discharge Blumlein-driven N2 laser incorporating this device gives a peak output power of 700 kW with a FWHM of 3 ns and an efficiency of 0.51%, which is remarkably high for a pulsed nitrogen laser system.
Santhakumari, N C; Vallabhan, C P G(Elsevier, April , 1991)
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Abstract:
dc and ac electrical conductivities, dielectric constant and dielectric loss factor in single crystals of ethylenediammonium dinitrate (EDN) have been measured axiswise as a function of temperature. All the above properties exhibit anomalous variations at 404 K thereby confirming the occurence of a phase transition in EDN at this temperature. Electrical conductivity parameters have been evaluated and possible conduction mechanisms are discussed. The role of protons in electrical trasport phenomenon is established by chemical analysis.
Sasi Kumar, P R; Padmaja, G; Ravi Kumar, A V; Nampoori, V P N; Vallabhan, C P G(Springer, April , 1991)
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Abstract:
Observation of laser induced two-photon photoemission optogalvanic (TPPOG) effect from tungsten electrode in a discharge cell using 564 nm radiation obtained from a pulsed dye laser is described. The magnitude of the POG signal is studied as a function of laser energy under various discharge parameters. Competition between one-photon and two-photon processes has been observed when nitrogen gas is used in the discharge cell.