Measurement of an Anisotropic Energy Gap in Single PlaneBi2Sr2−xLa

Abstract

We report angle-resolved photoemission spectra both above and below T c in the single-plane cuprate superconductor Bi 2 Sr 22x La x CuO 61d . The superconducting state measurements show a highly anisotropic excitation gap with a maximum magnitude smaller than that of the bilayer compound Bi 2 Sr 2 CaCu 2 O 8 by a factor of 3. For a range of doping, the gap persists well above T c , behavior previously associated with underdoped bilayer cuprates. The anisotropy and magnitude of the normalstate gap are very similar to the superconducting state gap, indicating that the two gaps may have a common origin in a pairing interaction.[S0031-9007(97) A central issue in the physics of high-T c superconductivity is the role of coupling between the two-dimensional copper-oxygen planes in producing superconductivity. The T c of these materials tends to increase with the number of layers per unit cell. It is currently an open question whether the superconducting state order parameter symmetry will be the same in one-layer and the more strongly coupled two-layer compounds. Angle-resolved photoemission spectroscopy (ARPES) has the potential to resolve this issue since it is able to measure directly the anisotropy of the superconducting state gap (the magnitude of the order parameter). In the two-plane material Bi 2 Sr 2 -CaCu 2 O 81d (Bi2212), the gap was found by ARPES to be highly anisotropic and consistent with a d x 2 2y 2 order parameter [1,2]. We report measurements of the one-plane material Bi2201 that show a similarly large anisotropy with a smaller overall gap magnitude.In underdoped Bi2212, ARPES measurements have shown that the anisotropic gap persists well above T c [3,4], consistent with many other experiments that have shown a pseudogap or spin gap in the normal state of cuprate superconductors [5]. Current evidence for the normalstate gap in one-plane materials is much weaker than in two-plane materials, and its existence is controversial [6]. Our results show a clear normal-state gap up to high temperatures in optimally doped and underdoped Bi2201, but not in overdoped Bi2201.Single crystal samples of Bi 21x Sr 22͑x1y͒ La y CuO 61d were grown using a floating zone method, and for comparison by a self-flux method. X-ray scattering confirms that the crystals are single-phase Bi2201, and electron-probe microanalysis was used to measure the atomic ratios of the cations. Substitution of trivalent La or Bi for divalent Sr reduces the hole concentration in the CuO 2 planes. The effect of La doping goes beyond changing the carrier density [7], however, and raises the maximum T c from 10 to 30 K.A roughly parabolic dependence of T c on ͑x 1 y͒ has been observed [8]. Our optimally doped crystals (T c 29 K) come from substituting La 0.35 for Sr. With no La substitution, Bi͞Sr ratios of 2.3͞1.7 and 2.1͞1.9 give the underdoped samples with T c , 4 K and the overdoped samples with T c 8 K, respectively. The transition temperatures were taken as the zero resistance values and confirmed by SQUID magnetization measurements. The tr...

Determination of the superconducting gap in near optimally doped Bi2Sr2−xLaWang1, Liu2, Lin3 et al. 2011Phys. Rev. B

“…In an early ARPES experiment, 8 an antinodal superconducting gap of 10±2 meV was observed, which shows good agreement with the LTSH result. This agreement confirms that the superconducting gap essentially follows the simple d-wave form because in ARPES the ∆ 0 is measured directly at the antinodes while in LTSH it is derived from the nodal gap slope v ∆ with the assumption ∆ = ∆ 0 cos(2φ).…”

“…Note that although the dwave pairing has been probed for La-Bi2201 by other experimental techniques such as ARPES, [8][9][10][11][12] present LTSH findings confirm that it is a bulk property of the sample.…”

“…4 we have plotted the ∆ 0 for near optimal-doped La-Bi2201 inferred from LTSH [according to Eq. (4)] and aforementioned ARPES and STM studies, 8,[10][11][12] where the doping level of the sample (p ∼ 0.16) is determined by the empirical relation 27,28 between p and the reported La content. Unlike the above-compared ARPES experiments,…”

Metal-to-insulator crossover and pseudogap in single-layerBi2+xSr2−xCu1+<

“…In a compound, the optimal cation states for Sr, La and Bi, are Sr 2+ , La 3+ and Bi 3+ , respectively. Therefore, the substitution of triva- 35 and Harris et al 36 found that as the Bi/Sr ratio increases, and one moves toward the bottom of the phase diagram of the solid solution, the number of holes doped into the system decreases, which thus pushes the system towards the hole-underdoped regime. The lower T c , together with the larger residual resistivity of Bi2201 in comparison with BSLCO (the maximum T c is 38 K 18 ) apparently suggests that the disorder due to (Sr,Bi) substitution is stronger in Bi2201 than the disorder due to (Sr,La) substitution 37 .…”

Concepts in High Temperature Superconductivity

“…Furthermore, ∆ o (T ) is largely undiminished in going from T = 0 to T = T c in underdoped samples, and the size and shape of the gap are basically unchanged through the transition. Add to this the contravariance of T c with the low temperature magnitude of the gap as the doping is changed, and it appears the gap and T c are simply independent energies [134,288]. The gap decreases with overdoping, which may be responsible for the depression of T c in that region, so that the transition may be more conventional on the overdoped side.…”