* Corresponding author. Fax: # 91-33-3346871. E-mail address: ude@veccal.ernet.in (U. De) Physica B 281&282 (2000) 928}930 Unconventional features of oxide HTSC from positron annihilation measurements D. Sanyal, P.M.G. Nambissan, Th. Wolf, Udayan De* Department of Physics, University of Calcutta, 92 APC Road, Calcutta 700009, India Saha Institute of Nuclear Physics, 1/AF Bidhan Nagar, Calcutta 700064, India Forschungszentrum Karlsruhe, Institut fur Technische Physik, Postfach 3640, D-76021, Karlsruhe, Germany Variable Energy Cyclotron Centre, 1/AF Bidhannagar, Calcutta 700064, India Abstract Our recent measurements of positron lifetime (related to electron density at the annihilating sites) components or   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 "rstly con"rm step-like changes across ¹  . Our second and new observation is that S versus ¹ 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 Nb  Sn, are linked to such features of HTSC electronic structure as are not found in conventional superconductors.  2000 Elsevier Science B.V. All rights reserved. Keywords: High-¹  cuprates; Superconductivity; Electron momentum distribution 1. Introduction Dynamical consequences of electron correlations and interactions need to be considered for a better under- standing of exotic systems like cuprate superconductors, widely known as high-temperature superconductors or HTSC. Pairing of electrons in momentum space explains BCS superconductivity. Whether HTSC is BCS-like or not [1,2], it is of interest to investigate experimentally the temperature dependence of the momentum distribution of electrons in various HTSCs, particularly, across ¹  , the superconducting transition temperature. Partial but important [3}7] information on this distribution has been conveniently obtained from Doppler broadened positron annihilation radiation lineshape (DBPARL) in- vestigation of Bi-2212 [6] and (Bi,Pb)-2223 [7] HTSC pellets. Here, by extending the DBPARL investigation to Y-123 single crystals, we establish a systematics that may be helpful in understanding the mechanism of HTSC. 2. Experimental outline and results The lineshape-parameter, S, is de"ned [3}7] as the ratio of the area of a suitably selected central region of the energy spectrum N(E) versus E of the annihilation radiations to the area under the full spectrum. Due to Doppler shift, the oppositely emitted E  "511 keV gam- mas appear to be of energy E"[E  $pc/2] in the labor- atory frame where p is the momentum of the annihilating electron in the direction of detection. Consequently, S gives the fraction of chosen low momentum (corre- sponding to 510.4}511.6 keV energy in the present analy- sis) electrons among the electrons participating in the annihilations. While step-like changes of S and of   across ¹  have been claimed [1}3] since 1987, an extra feature of either one minimum Fig. 2 of Ref. [3], for Bi-2212 pellets [6] or two minima Fig. 1(a), for Physb1=17411=KGM=Venkatachala=BG 0921-4526/00/$ - see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 1 0 6 0 - 1