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Sumner, Michael J.; Kim, Jin‐Tae; Lemberger, Thomas R.

doi:10.1103/physrevb.47.12248

Cited by 35 publications

(13 citation statements)

References 17 publications

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“…This result is consistent with a recent reflectivity study on Ni-doped YBa 2 Cu 3 O 7−δ films [19]. The absence of fully developed s-wave gap is in accordance with the the results of tunneling studies on the same material [20].…”

supporting

confidence: 82%

No far-infrared-spectroscopic gap in clean and dirty high-Tcsuperconductors

et al. 1993

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We report far infrared transmission measurements on single crystal samples derived from Bi2Sr2CaCu2O8. The impurity scattering rate of the samples was varied by electron-beam irradiation, 50MeV 16 O +6 ion irradiation, heat treatment in vacuum, and Y doping. Although substantial changes in the infrared spectra were produced, in no case was a feature observed that could be associated with the superconducting energy gap. These results all but rule out "clean limit" explanations for the absence of the spectroscopic gap in this material, and provide evidence that the superconductivity in Bi2Sr2CaCu2O8 is gapless.PACS numbers: 74.70Vy, 78.30ErThe existence of a superconducting energy gap in the high-T C superconductors has been hotly debated. A simple s-wave BCS model has a complete gap of width 2∆ around the Fermi energy with 2∆/k B T C = 3.5 for weak coupling or higher for stronger coupling. This energy gap is evident in the far-infrared -microwave range for low-T C superconductors [1]. Infrared studies [2][3][4][5][6][7] on high-T C materials reveal a feature at ∼ 8−12k B T C , originally thought to be the gap. However there is increasing evidence to the contrary [7]. In high T c superconductors the gap might not show up in the infrared spectrum for several reasons. First, the scattering rate of the charge carriers may be low relative to the superconducting energy gap (1/τ << 2∆), and the infrared spectroscopy can not distinguish between a near perfect conductor and a true superconductor [3]. Second, there seems to be a temperature independent contribution to the oscillator strength in this freaquency range, overlapping and possibly masking the weak gap feature. Third, the gap feature may be broad, either due to anisotropy or lifetime effects. It is also possible that the description of the superconducting state cannot be put in a BCS framework.We can experimentally test the first possibility by enhancing the impurity scattering rate to produce optimum circumstances for the observation of the spectroscopic gap. In this Letter, we present far-infrared data on ultrathin single crystals of the Bi 2 Sr 2 CaCu 2 O 8 family. We first measured the "pure" samples' far infrared transmission spectra. The samples were then made "dirty" by electron-beam irradiation, 50MeV 16 O +6 ion irradiation, or heat treatment in vacuum to drive out oxygen. The infrared measurements were then repeated and compared with the "pure" measurements. In another set of experiments we looked at Y doped samples of composition Bi 2 Sr 2 Ca 1−x Y x Cu 2 O 8 , with x = 0.0 − 0.35. In spite of the enhanced scattering rate, the results from all samples show no spectroscopic gap. We present a simple analysis and conclude that an s-wave BCS gap should lead to significant features in the infrared spectrum.The far-infrared measurements were made at beamline U 4IR at the National Synchrotron Light Source, Brookhaven National Laboratory [16]. The samples were mounted on the cold-finger of an LT −3−110A Heli-Tran liquid transfer refrigeration system and were m...

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supporting

confidence: 82%

No far-infrared-spectroscopic gap in clean and dirty high-Tcsuperconductors

et al. 1993

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“…For instance, it has been proposed that impurity doping in the CuO 2 planes of the high-T c superconductors may also allow a distinction between d wave and the various s-wave models. [9][10][11] A comparison between the effects of Ni and Zn doping in YBa 2 Cu 3 O 7Ϫ␦ shows that while both Ni and Zn are effective scatters in the normal state and preserve the linear temperature dependence of the resistivity, [12][13][14][15] Zn reduces T c about three times more quickly than Ni. 16 There is some evidence that the rapid reduction of T c due to Zn doping may be related to magnetic effects.…”

Section: Introductionmentioning

confidence: 98%

“…22 A larger body of work exists of studies of Ni and Zn-doped YBa 2 Cu 3 O 6.95 thin films. 12,13 In this paper, we report on the effects of Ni doping on the optical properties of twinned, single crystals of YBa 2 (Cu 1Ϫx Ni x ) 3 O 6ϩy with optimal oxygen doping (y ϭ0.95) over a wide frequency range, above and below T c for a pure system (xϭ0), a material with a light Ni concentration (xϭ0.0075), and a heavy Ni concentration (x ϭ0.014). In addition, the crystal with the lighter Ni concentration has been mechanically detwinned and studied at both the optimal oxygen doping, and in the underdoped (y ϭ0.60) regime, for light polarized along the a and b axes and is compared to nominally pure material with the same oxygen content.…”

Section: Introductionmentioning

confidence: 99%

Effect of Ni impurities on the optical properties ofYBa2Cu3O6<

et al. 1999

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The optical properties of twinned single crystals of YBa 2 (Cu 1Ϫx Ni x ) 3 O 6ϩy have been determined for a nominally pure system and for Ni concentrations of xϭ0.0075 and 0.014 with oxygen dopings of yϭ0.60 ͑underdoped͒ and 0.95 ͑optimal͒ over a wide frequency range above and below T c . The optical conductivity of the twinned materials shows a rapid increase in the residual conductivity at low frequency with increasing Ni concentration, and an unusual feature appears below T c at Ϸ300 cm Ϫ1 (37 meV), which is absent in the pure system and is not sensitive upon oxygen content. In a detwinned crystal of the optimally doped system this Ni-induced feature is observed only along the b axis, suggesting that Ni is doping into the chains. Since the feature is observed in the normal state, it is not associated with the superconducting transition. The free-carrier response in the CuO 2 planes is quite different in the optimally doped and underdoped materials. The presence of Ni in these materials has little effect upon the in-plane dynamics, but the low-frequency conductivity along just the chains is destroyed. This suggests that Ni acts either as a localization site, or an impurity scatterer and strong pair breaker, destroying the superconductivity along the chains and strongly reducing the anisotropy of the system at low frequency. The feature at Ϸ300 cm Ϫ1 along the chain direction is most likely due to the Ni impurities acting as strong scattering barriers, resulting in thermally activated hopping. However, the proximity of this feature to the estimated value of the c-axis pseudogap in the underdoped materials also suggests that strong scattering out of the chains may be coupling to the c-axis dynamics. ͓S0163-1829͑99͒00537-8͔

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“…This x dependence of T c agrees with the results in the previous reports. 6,7,9,10,14,15,17,18,21,23,24 The oxygen contents, 7-␦, for the as-synthesized samples are 6.92-6.95, indicating that almost full oxygenation was attained for the samples. The obtained results of lattice parameters, T c and oxygen content are summarized in Table I.…”

Section: Resultsmentioning

confidence: 97%