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.

U DE

D SANYAL

S CHAUDHURI

P M G NAMBISSAN

TH WOLF

H WÃ¼HL

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

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

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.

Fulltext

Journal of Applied Physics

Defect dynamics in annealed ZnO by positron annihilation spectroscopy

Background-suppressed Doppler broadening spectroscopy, using two detector coincidence technique of the electron-positron annihilation γ-radiation, has been employed on tetragonal La 0.7Y 0.3Ca 0.5Ba 1.5Cu 3O z polycrystalline high-Tc superconductor to study the changes in the electron momentum distributions due to superconducting transition. The temperature-dependent (298 K to 60 K) S-parameter graph shows a sharp peak at the superconducting transition temperature, Tc, region. The spectra at different sample temperatures have been compared with the spectra of defect-free 99.9999% pure Al and Cu metals by constructing ratio-curves. The results indicate a decrease of the annihilation of positrons with 3d electrons of Cu ions just above Tc. © 2004 Elsevier B.V. All rights reserved.

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

Doppler broadening measurements of the electron-positron annihilation radiation in La 0.7Y 0.3Ca 0.5Ba 1.5Cu 3O z superconductor

Doppler broadening of the electron positron annihilation γ-radiation spectra of single crystalline Bi2Sr2CaCu2O 8-δ high Tc superconducting sample along the two different crystallographic orientations (c-axis and a-b-plane) have been analyzed by constructing ratio-curves with the reference spectra of defect free 99.9999% pure Al and Cu, respectively. The results indicate that the momentum component (pL) of the 2p electrons of oxygen ions and the 3d electrons of the Cu ions is relatively more towards the c-axis than the a-b-plane. © 2004 Elsevier B.V. All rights reserved.

Physica C: Superconductivity and its Applications

Doppler broadening measurements of positron annihilation in single crystalline Bi2Sr2CaCu2O8+Î´ high Tc superconductor along two different crystallographic directions

Background suppressed Doppler broadening spectroscopy using a two detector coincidence technique of the electron-positron annihilation γ-radiation has been employed on single crystalline Bi2Sr2CaCu 2O8+δ high Tc superconductor. The spectra at different sample temperatures have been compared with the room temperature spectrum by constructing ratio-curve. The results indicate an effective shift of oxygen ions towards the Bi-O plane from the Cu-O plane just above the superconducting transition temperature. © 2004 Elsevier B.V. All rights reserved.

Studies of coincidence Doppler broadening of the electron-positron annihilation radiation in the single crystalline Bi2Sr 2CaCu2 O8+Î´ superconductor

Our recent measurements of positron lifetime (related to electron density at the annihilating sites) components or τi and Doppler broadening of positron annihilation radiation lineshape or S-parameter (denoting the fraction of low-momentum electrons among the annihilating electrons) on Bi-2212 and (Bi,Pb)-2223 pellets and Y-123 single crystals firstly confirm step-like changes across Tc. Our second and new observation is that S versus T graph in the superconducting transition region shows a double minimum for 2223 and 123 and a single minimum for 2212. These steps and minima, not seen for superconductors like Nb3Sn, are linked to such features of HTSC electronic structure as are not found in conventional superconductors.

Physica B: Condensed Matter

Unconventional features of oxide HTSC from positron annihilation measurements

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.

Probing superconductors from 30 to 300 K by positron-lifetime 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.

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.

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

For YNi2B2C, a typical borocarbide superconductor, we have carried out positron annihilation lifetime measurements, before and after a 40 MeV α-irradiation to a fluence of 2X1016 particles /cm2. A 3-lifetime fit, giving improved variance, showed a third lifetime longer than 1 ns and of low (1 to 2%) intensity. This lifetime is due to positronium formation and o-Ps to p-Ps conversion in the porous samples. The irradiation increases the bulk lifetime, τB, calculated from τ1 and τ2, from 181 ps to 213 ps. © 1996 Plenum Publishing Corporation.

Journal	Physical Review B - Condensed Matter and Materials Physics
Publisher	AMER PHYSICAL SOC
ISSN	0163-1829