Gap in the infrared response ofHgBa2Ca2Cu3

Abstract

The ab-plane optical spectra of one underdoped and one nearly optimally doped single crystal of HgBa2Ca2Cu3O 8+δ were investigated in the frequency range from 40 to 40,000 cm −1 . The frequency dependent scattering rate was obtained by Kramers Kronig analysis of the reflectance. Both crystals have a scattering rate gap of about 1000 cm −1 which is much larger than the 700 cm −1 gap seen in optical studies of several cuprates with maximum Tc around 93 K. There appears to be a universal scaling between scattering rate gap and maximum Tc for the cuprate superconductors.PACS numbers: 74.25. Gz, 74.72.Gr, 78.20.Ci Although high temperature superconductivity in the copper oxides was discovered over a decade ago, an understanding of the mechanism that gives rise to the high transition temperature, T c , is still elusive. Most of the research has been directed towards the one-and twolayer systems which have a maximum T c in the 93 K range. Recently, however, high quality single crystals of the three-layer Hg based materials with a maximum T c ≈ 135 K have become available [1] making it possible to examine, spectroscopically, oxide superconductors that have significantly higher T c .Of particular interest is the magnitude of the normal state pseudogap. This is a partial suppression of the density of low energy excitations seen well above T c in all underdoped high temperature superconductors. A pseudogap has been seen with a variety of techniques such as angle-resolved photoemission (ARPES), tunneling spectroscopy, specific heat, dc resistivity, nuclear magnetic resonance and optical spectroscopy [2]. Measurements of dc resistivity [3] and NMR 63 Cu T 1 [4] on the threelayer underdoped HgBa 2 Ca 2 Cu 3 O 8+δ (Hg-1223) show evidence of a pseudogap with onset temperatures, T * , of 320 K and 230 K respectively.The size of the pseudogap in the one-and two-layer materials is of the order of 9.5k B T c at optimal doping and essentially independent of temperature. As T c is reduced below optimal doping the pseudogap increases in size [5][6][7].In ab-plane infrared response, the pseudogap is most clearly seen in the frequency-dependent scattering rate which can be calculated from the reflectance after Kramers-Kronig analysis. Here the pseudogap is a suppression of scattering below a characteristic energy which is taken to be a measure of the "size" of the pseudogap.

Crystal growth and in-plane optical properties ofTl2Ba2Can−1CunC.1, Wang2 2005Phys. Rev. B172110View full textAdd to dashboardCite

“…In fact, the energy scales of those features continue to shift up in compounds with higher superconducting transition temperature. This can be seen very clearly from optical work done on Hg-1223 with T c ∼130 K [21]. The characteristic features were widely believed to result from the interaction of electrons with a collective mode in combination with a superconducting gap that appears below T c [22,23,24,25,26].…”

“…The data for Hg-1223 crystal taken from ref. [21] are also plotted. In order to perform second derivative, the curves were fit with a high-order (40 orders) polynomial.…”

Role of the electron-phonon interaction in the strongly correlated cuprate superconductors

“…For the much higher ∆ max in Hg1223 there is the additional possibility of a resonance between the phonon and the superconducting gap, being the two energies close to each other. Optics experiment saw in fact a drop in 1/τ in Hg1223 system at a frequency 30% higher than the one observed in Bi2212 46 , which scales exactly with the pairing gap (and T c ) difference. The reason for the difference in Hg1223 is maybe the reduced strain on the CuO 2 sheets by other layers in this material, a factor related to the µ * problem as we will discuss later.…”

The mysterious pseudogap in high temperature superconductors: an infrared view

“…Infrared spectroscopy on the ab-plane that has been performed by McGuire et al [76] shows that the material behaves very much like other cuprates with one notable exception: the energy scale for the gap in abplane scattering is 40 % higher than in materials with a T c of 93 K, in the exact ratio of their transition temperatures. Energy scales measured with other probes such as the B 1g Raman intensity [77] and the copper hyperfine coupling constant measured by NMR [78] are also much higher.…”