The present measurement of positron annihilation lifetimes for the high-(Formula presented) superconductor (Formula presented) or (Bi, Pb)-2223, as a function of temperature, has been inspired by our recent DBPARL (Doppler-broadened positron annihilation radiation line shape) finding of a step and minima in the region around (Formula presented) K, in the variation with temperature, of the fraction of low momentum electrons at the annihilation sites. The positron annihilation lifetime in the superconductor probes the number density of electrons at the annihilation sites. The present work observes a reduction of lifetime components in the (Formula presented) region. Significant changes in the lifetime components in other temperature regions have also been observed and discussed. © 1998 The American Physical Society.

D SANYAL

D BANERJEE

U DE

University of Calcutta (informally known as Calcutta University or CU) is a collegiate public state university located in Kolkata, West Bengal, India.&nbsp;Within India it is recognized as a &quot;Five-Star University&quot; and accredited &quot;A&quot; Grade by National Assessment and Accreditation Council.

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The University of Calcutta has secured the Fifth position among the Universities of India in the prestigious &quot;Indian University Ranking 2019&quot; list, released by the NIRF of the Ministry of Human Resource Development, Government of India.

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It was established on 24 January 1857, and was one of the first institutions in Asia to be established as a multidisciplinary and Western-style university. Its alumni and faculty include several heads of state, heads of government, social reformers, prominent artists, only academy award winner and Dirac medal winner in India, many Fellows of the Royal Society and five Nobel laureates as of 2019 which is highest in South Asia.

University Of Calcutta

Physical Review B - Condensed Matter and Materials Physics

Nanocrystalline (average crystal size ∼ 3-4 nm) ZnO have been prepared by chemical route. Employing positron annihilation techniques these nanocrystalline ZnO have been characterized. Positron annihilation lifetime spectroscopy indicates the presence of large number of vacancy defects at the surface of these nanocrystaline ZnO. A large percentage of positronium formation has been observed in this nanocrystalline wide band gap semiconductor. © 2012 Elsevier Ltd.

Vacuum

Positron annihilation characterization of nanocrystalline ZnO

Positron annihilation technique is a well known technique to characterize the defects in a material. These defects can be identified by positron annihilation lifetime and coincidence Doppler broadening of positron annihilation radiation measurement. In this chapter we report the room temperature positron annihilation lifetime for single crystalline ZnO. From our study it is confirmed that the present crystal contains VZn-hydrogen complexes with low open volumes. Another important nuclear solid technique is the Mossbauer Spectroscopic technique which has been used to probe the local magnetic properties of a solid. Here we have summarized Mossbauer spectroscopic studies on ferrites. © 2012 Trans Tech Publications, Switzerland.

Materials Science Forum

Probing materials by positron annihilation technique and mossbauer spectroscopy-review

In this short topical review, a brief account of the evolution of defects due to controlled changes in polycrystalline zinc oxide has been presented. X-ray diffraction, Positron annihilation spectroscopy and optical absorption spectroscopy has been employed to understand various defective states of ZnO. Thermogravimetric analysis, room temperature resistivity and photoluminescence measurements (just mentioned) have been used to throw more light on this topic. A coherent scenario in the light of previous works in this field has been discussed. At the end discussion on the magnetic studies on ZnO-based systems has been added in short. © 2008 Elsevier Ltd. All rights reserved.

Progress in Materials Science

Role of defects in tailoring structural, electrical and optical properties of ZnO

Mechanical milling and subsequent annealing in air at temperatures between 210 and 1200 °C have been carried out on high purity ZnO powder to study the defect generation and recovery in the material. Lowering of average grain size (from 76±1 to 22±0.5 nm) as a result of milling has been estimated from the broadening of x-ray lines. Substantial grain growth in the milled sample occurs above 425 °C annealing temperature. Positron annihilation lifetime (PAL) analysis of the samples shows a distinct decrease of the average lifetime of positrons very near the same temperature zone. As indicated from both x-ray diffraction (XRD) and PAL results, high temperature (&gt;700 °C) annealed samples have a better crystallinity (or lower defect concentration) than even the nonmilled ZnO. In contrast, the measured optical band gap of the samples (from absorption spectroscopy) does not confirm lowering of defects with high temperature annealing. Thermally generated defects at oxygen sites cause significant modification of the optical absorption; however, they are not efficient traps for positrons. Different thermal stages of generation and recovery of cationic as well as anionic defects in granular ZnO are discussed in the light of XRD, PAL, and optical absorption studies. © 2006 American Institute of Physics.

Fulltext

Journal of Applied Physics

Annealing effect on nano-ZnO powder studied from positron lifetime and optical absorption spectroscopy

As-supplied polycrystalline ZnO samples (purity 99.9% from Sigma-Aldrich, Germany) have been annealed at different temperatures and subsequently characterized by positron annihilation spectroscopy, x-ray-diffraction (XRD) analysis, thermogravimetric analysis (TGA), and resistivity measurements. Positron annihilation lifetime analysis and coincidence Doppler-broadened electron-positron annihilation γ -radiation (CDBEPAR) line-shape measurements have been employed at a time to identify the nature of defects in differently annealed ZnO materials. Annealing up to 300 °C, an increase of defect lifetime (τ2) as well as shape parameter (S parameter) has been observed. Further annealing causes a large decrease of τ2 and S parameter. TGA study shows considerable mass loss from ZnO as the annealing temperature is increased above 300 °C. This is possibly due to oxygen evaporation from the sample. The c -axis lattice parameter, extracted from the XRD spectra, shows an increase due to annealing above 600 °C, which is a signature of the presence of a huge number of oxygen vacancies. Resistivity variations of the annealed samples are also consistent with the TG and XRD analyses. The ratio curve analysis of the CDBEPAR spectra successfully probes the change in zinc-related vacancy defects in annealed ZnO. © 2005 American Institute of Physics.

Defect dynamics in annealed ZnO by positron annihilation spectroscopy

Shape parameter S determined from the Doppler broadening of the positron annihilation radiation line-shape measurement (DBPARL), gives the fraction of suitably defined low momentum electrons among the annihilating electrons. Since BCS pairing in momentum space is accepted for the so-called Conventional superconductors, such studies of possible changes in the electron momentum distribution as a function of temperature across the superconducting transition in high-temperature superconductors (HTSC) may help clarify the yet unknown mechanism of superconducting pairing in HTSC. Our S vs T data at close intervals (down to 1 K) of temperature revealed one minimum at Tc in Bi-2212 pellets and two minima in the Tc region in (Bi,Pb)-2223 pellets, in addition to the somewhat better known steplike change at Tc. Similar-DBPARL·measurements presently on Y-123 single crystals show the S vs T minima in these as well as in previous (Bi,Pb)-2223 pellets to be at Tc and 0.95Tc. These observations can imply a redistribution of electron momentum similar to the better discussed charge redistribution.

Probing single-crystalline YBa2Cu3O7 across the superconducting transition temperature by positron annihilation measurements

For single-phase Bi-2212, Doppler broadened positron annihilation radiation line shape (DBPARL) parameter S has been measured from 15 to 300 K. S represents the fraction of low momentum electrons involved in the annihilation process. It shows a broad dip in S over 58 to 90 K in contrast to the 90-110 K complex dip in (Bi, Pb)-2223. The shift of the dip to lower temperatute for the lower-Tc superconductor proves that our observed decrease of the fraction of low momentum electrons in the Tc-region is linked to superconducting pairing.

Physica B: Condensed Matter

Electron momentum re-distribution due to superconducting transition in Bi-2212 and (Bi, Pb)-2223 oxides

High Tc superconductors like (Bi,Pb)-2223 show an abrupt change, across Tc, in the S-parameter, calculated from Doppler broadened positron annihilation radiation lineshape (DBPARL) data. The S-parameter gives the fraction of low momentum electrons. Here we report the new feature of a sharp fall in the S-parameter on cooling the sample to its Tc (R = 0) and a subsequent complex but partial rise on further cooling. The detailed observations and possible implications are discussed.

Physics Letters, Section A: General, Atomic and Solid State Physics

Complex variation of electron momentum distribution in (Bi,Pb)-2223 oxides across the superconducting transition

Positron annihilation lifetime (PAL) measurements on well-characterized pellets of Bi-2212, (Bi,Pb)-2212 and (Bi,Pb)-2223 superconductors indicate the presence of a very long (around 1500 ps) third lifetime. This τ3 is attributed to positronium formation in voids even in these conducting oxides and subsequent o-Ps to p-Ps conversion. Ignoring τ3 in the fit leads to altered values of τ1 and τ2. © 1995.

Journal	Physical Review B - Condensed Matter and Materials Physics
Publisher	-
ISSN	1098-0121