Unusual Strong-Coupling Effects in the Tunneling Spectroscopy of Optimally Doped and OverdopedBi2Sr2CaCu2

Abstract

Tunneling spectroscopy measurements are reported on single crystals of Bi 2 Sr 2 CaCu 2 O 81d using vacuum tunneling and point-contact methods. A reproducible dip feature in the tunneling conductance is found near jeV j 2D, observed for both voltage polarities in the best resolved spectra. With overdoping the position of the dip continues to scale with D, and its magnitude decreases as D decreases. These results indicate that the dip feature arises from a strong-coupling effect whereby the quasiparticle lifetime is decreased at a characteristic energy of ϳ2D, consistent with an electron-electron pairing interaction. [S0031-9007(97)04908-9] PACS numbers: 74.50. + r, 74.62.Dh, 74.72.Hs A provocative feature that has commonly been observed in the superconductor-insulator-normal (SIN) metal tunneling conductances of Bi 2 Sr 2 CaCu 2 O 81d (Bi2212) is a dip at jeV j ϳ 2D that is very large for voltage polarities which correspond to the removal of quasiparticles from the superconductor [1][2][3]. This feature has generated much interest due to its similarity to a dip found in the angle-resolved photoemission (ARPES) [4,5] spectra of Bi2212 and to strong-coupling effects in general [6]. But in contrast to the tunneling phonon structures observed in strong-coupled, low-T c superconductors [6], the dip is often highly asymmetric with bias voltage. For voltages corresponding to electron injection, the dip has appeared as a shoulder in early point-contact tunneling (PCT) measurements [1] and is scarcely observable in some scanning tunneling microscope (STM) measurements [2]. We report here that the dip feature is indeed observed for both bias voltage polarities in the best resolved SIN spectra obtained from both STM and PCT methods. This points toward a strong-coupling interpretation. As T c and D are reduced by overdoping (the latter from 37 to 15 meV in this study), the dip location continues to scale with D and its magnitude is reduced. The coupling to D is quite unusual and suggests that the dip arises from a pairing interaction that is purely electronic so that the superconducting gap feeds back into the excitations which mediate the pairing. This is also consistent with recent interpretations of the dip in ARPES spectra as arising from the coupling of quasiparticles to collective excitations [7,8].SIN tunneling spectroscopy is a unique probe of high temperature superconductors (HTS) in that it can, in principle, reveal the quasiparticle excitation spectrum directly with an energy resolution better than 1 meV. In conventional, s-wave superconductors, strong-coupling effects due to quasiparticle emission of bosons of frequency v produce a dip feature [6] in the tunneling conductance, dI͞dV , near a voltage, eV D 1 "v. More precisely, it is the maximum negative slope if dI͞dV vs V that pinpoints the boson frequency. In d-wave superconductors, which have gap nodes, quasiparticle decay processes which turn on at some threshold energy (e.g., 2D) display a dip in the density of states (DOS) near that energy [9], unshifted...

Quasiparticle and Josephson tunneling of overdopedBi2Sr2CaCu2<

“…A possible explanation for the origin of the dip feature in terms of strong coupling effects is given in Ref. 2, but here we simply note that it is observed in the most overdoped crystals studied to date. The zero-bias conductances vary from 20% to 40% of the estimated normal-state conductance, but in a few cases we have seen values as low as 10%-15% in overdoped crystals.…”

“…31 Such a peak in the band structure DOS is a natural explanation for the consistent observation of the decreasing background in both PCT and STM. 2,9 When the superconducting dynamic conductance is divided by n * , we obtain the normalized conductances ͑dots in Fig. 3͒, which can then be compared to a theoretical DOS to find the energy gap of the superconductor.…”

“…The reason for these large values is probably an indication that there is a significant coherent part to the tunneling matrix element; 39 that is, k must equal kЈ for quasiparticle states on either side of the junction. In this case the signs of the order parameters are multiplied, i.e., (ϩ1) 2 or (Ϫ1) 2 , and this leads to a nonzero Josephson current for d-wave superconductors. It should be noted that recent measurements of the Josephson current for intrinsic c-axis junctions in Bi-2212 mesas 40 are also consistent with d-wave gap symmetry and coherent tunneling processes.…”

“…Better control of the purity of the crystal narrows the spectrum of obtained energy gap values for a given doping level, and now it can be stated with some confidence that for optimally doped Bi-2212 with a T c onset of 93-95 K, ⌬ is around 38 meV. 2,9,10 This improved reproducibility has allowed the observation by PCT and break junctions of a remarkable effect whereby the superconducting gap in Bi-2212 changes substantially and monotonically with doping over a narrow doping region where the T c has a maximum. 1 On the overdoped side this effect has been seen by STM as well.…”

“…1 On the overdoped side this effect has been seen by STM as well. 2,15,16,22 The overdoped Bi-2212 single crystals examined here have T c values of 82, 78, and 62 K, with the lowest T c samples forming the bulk of this study. A few of the results have been published in Ref.…”