Evidence for pseudogap and phase-coherence gap separation by Andreev reflection experiments.pdfVIP

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Evidence for pseudogap and phase-coherence gap separation by Andreev reflection experiments.pdf

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Evidence for pseudogap and phase-coherence gap separation by Andreev reflection experiments

a r X i v : c o n d - m a t / 0 1 0 1 2 0 9 v 2 [ c o n d - m a t .s u p r - c o n ] 1 3 A p r 2 0 0 1 Evidence for pseudogap and phase-coherence gap separation by Andreev reflection experiments in Au/La2?xSrxCuO4 point-contact junctions R.S. Gonnelli,? A. Calzolari, D. Daghero, L. Natale, and G.A. Ummarino INFM - Dipartimento di Fisica, Politecnico di Torino, 10129 Torino, Italy V.A. Stepanov P.N. Lebedev Physical Institute, Russian Academy of Sciences, 117924 Moscow, Russia M. Ferretti Dipartimento di Chimica e Chimica Industriale, Universita? di Genova, 16146 Genova, Italy (Dated: February 1, 2008) We present new Au/La2?xSrxCuO4 (LSCO) point-contact conductance measures as a function of voltage and temperature in samples with 0.08 ≤ x ≤ 0.2. Andreev reflection features disappear at about the bulk Tc, giving no evidence of gap for T Tc. The fit of the normalized conductance at any T Tc supports a (s + d)-wave symmetry of the gap, whose dominant low-T s component follows the Tc(x) curve in contrast with recent angle-resolved photoemission spectroscopy and quasi- particle tunneling data. These results prove the separation between pseudogap and phase-coherence superconducting gap in LSCO at x ～ 0.2. PACS numbers: 74.50.+r, 74.25.Dw, 74.72.Dn In a recent paper, G. Deutscher claimed the existence of two distinct energy scales - that is, two distinct gaps - in high-Tc superconductors (HTS) [1]. According to his discussion, one of these gaps should appear at T ? Tc in optimally-doped and underdoped samples and could be due to an incoherent pairing between charge carri- ers (whose physical origin is still under discussion) which leads to a pair pre-formation. This gap, ?p, would coin- cide with the pseudogap observed by angle-resolved pho- toemission spectroscopy (ARPES) and tunneling exper- iments. The second gap, ?c, would appear at Tc and would be associated to the achievement of the phase co- herence by the pre-formed pairs and, consequently, to the onset o