Predominantly Superconducting Origin of Large Energy Gaps in UnderdopedBi2Sr2CaCu

Abstract

New tunneling data are reported in Bi 2 Sr 2 CaCu 2 O 81d which show quasiparticle excitation gaps, D, reaching values as high as 60 meV for underdoped crystals with T c 70 K. These energy gaps are nearly 3 times larger than those of overdoped crystals with similar T c . Despite the large differences in gap magnitude, the tunneling spectra display qualitatively similar characteristics over the entire doping range. Detailed examination of the spectra, including the Josephson I c R n product measured in break junctions, indicates that these energy gaps are predominantly of superconducting origin. , and thus if the gap has a superconducting origin it strongly suggests that the pseudogap state is due to some type of precursor superconductivity [1,4,5]. However, the smooth dependence on doping may also originate from a quasiparticle gap that evolves from superconducting character in the overdoped phase to another type (e.g., charge density wave) in the underdoped phase [6]. In support of this picture are some measurements that suggest a superconducting order parameter (or coherence gap) that scales with T c [7][8][9]. Thus, a critical question is whether the large energy gap found in tunneling originates entirely from superconducting pairing or has a contribution from some other electronic effect. Here we address the nature of the gap measured by tunneling and report new data in more heavily underdoped Bi2212. Energy gaps, D, of 51 6 2, 54 6 2, and 58 6 2 meV are observed for three underdoped crystals with T c 77, 74, and 70 K, respectively, extending the previously reported trend [2] further into the underdoped regime. Detailed examination of the tunneling spectra over a wide doping range, including the Josephson I c R n product, show that these energy gaps are predominantly of superconducting origin.Superconductor-insulator-superconductor (SIS) junctions provide an accurate measure of the gap from the peaks in tunneling conductance (at a bias voltage jeV p j 2D) which are only weakly affected by thermal smearing or quasiparticle scattering [2,10]. However, the large magnitudes of energy gaps observed here lead to such extraordinarily large values of 2D͞kT c (as high as 20) that it is necessary to examine carefully the entire tunneling spectrum to clarify their physical origin. Most theoretical models of HTS stress the importance of electronic correlations which lead to spin density waves [11,12], or charge density waves in the underdoped phase [6,13]. These correlations give rise to gaps (or pseudogaps) in the electronic excitation spectrum, D c ͑k͒, that are distinct from those arising from superconductivity, D s ͑k͒. Since these other correlation gaps are used to explain pseudogap phenomena above T c , our investigation here has a direct bearing on this issue. The experimental goal is to determine whether the single-particle excitation gap as measured in tunneling has contributions from both D s and D c . We argue that if two distinct gaps exist, (i) they should have different magnitudes as well as different dopi...

Evidence of Doping-Dependent Pairing Symmetry in Cuprate Superconductors

“…In Bi-2212, the spectral "dip/hump" features were widely observed [14][15][16][17][18][19] and attributed to quasiparticle damping via interactions with collective spin fluctuations [27,28]. In the strong coupling limit, the spectral dip is expected to appear at the energy v dip D 1 V res , where D is the measured gap, and V res is associated with the resonance of collective spin excitations [27,28].…”

“…Assuming the position where a local maximum intensity of a resonant peak occurred as an impurity site, we found that the peak persisted over several lattice constants for displacement along either ͕100͖ or K ra sn o v e t a l . 20 M iya ka w a e t a l. 14,15 R e n n e r e t a l . ͕010͖, and the peak intensity decreased rapidly within the Fermi wavelength, as shown in the inset of Fig.…”

“…3(b). On the other hand, experiments on Bi-2212 found an averaged gap D ‫ء‬ that scaled with the pseudogap temperature T ‫ء‬ , 2D ‫ء‬ ϳ 6.6k B T ‫ء‬ [14][15][16][17][18][19], and that D ‫ء‬ increased with decreasing p [14][15][16][17][18][19]. We speculate that the superlattice and charge-density-wave modulations of the BiO layers in Bi-2212 may have complicated the quasiparticle spectra [14][15][16][17][18][19][20][21].…”

“…Early experiments on the Bi-2212 system reported a measured energy gap D ‫ء‬ ͑p͒ that increased monotonically with decreasing p and was nearly independent of temperature [14][15][16][17][18][19]. However, the low-temperature spectra of the optimally doped and underdoped Bi-2212 appeared to consist of a sharp peak feature on top of a broad "hump."…”

Modelling Cuprate Gaps in a Composite Two-Band Model

“…Spectrally l must be related to the hump-feature. Eventually it remains preserved for T > T c (insert in Fig.1) at the dopings where T c is optimized [52] and it is shifted to larger energies with reduced doping [54]. The common manifestation of two underdoped state pseudogaps is expected theoretically.…”