a-b plane anisotropy of single-domain crystals of Bi2SrzCaCu208

The polarized reflectance of the a-b plane of single-domain BizSrzCaCu208 crystals is anisotropic above and below T~. The normal-state infrared conductivity is higher for E l[ a whereas the high-frequency conductivity is higher along b, particularly for transitions associated with the B i O layers. Below Tc there is a definite anisotropy to the far-infrared absorption, with a finite absorption for E I] b down to ~ 20 meV. This anisotropy of the a-b plane could be due either to anisotropy of the superconducting gap or to anisotropy of the midinfrared component to the conductivity. PACS: 74.30.Gn; 74.70.Vy; 78.47.+p; 84.40.Cb Superconductivity in the copper-oxide materials is associated with the quasi-two-dimensional CuO 2 planes. The anisotropy between the c axis and these a-b planes is large and well established. What is less well understood is the anisotropy within the a-b plane. There are two important questions about anisotropy: the anisotropy of the 2-d electronic structure in the normal state and the anisotropy of the superconducting order parameter. The anisotropy of the superconducting order parameter is closely related both to the pairing mechanism and to the description of the normal state of these materials. Models like the marginal Fermi liquid picture lead to swave pairing [ 1,2]. In the strong-coupling limit (or dirty limit) this s-wave gap is expected to be isotropic, even for strongly anisotropic materials [3]. Other models, mostly those which rely on antiferromagnetism for the pairing interaction, give a d-wave (essentially dx2_y2 ) character to the order parameter. (See, for example, Refs. 4 and 5 and references therein). Recently, analyses of tunneling [6], photoemission [7], and penetration depth [8] have been advanced in support of such dx2_y2 pairing. Other types of pairing, such as p-wave [9] and combinations of s and d wave states [10], have also been proposed. Infrared spectroscopy is a direct probe of the anisotropic order parameter in unconventional superconductors [11 ]. However, in not every case does the anisotropy of the order parameter lead to an anisotropic a-b-plane optical conductivity. The conductivity for pure dx2_y2 pairing is isotropic in the a-b plane (although the spectrum would of course be different from the case of s-wave pairing). In contrast, a p-wave component, a dxz pairing, or a combination of sand d-symmetries would give anisotropic a-b-plane optical conductivity. Most previous infrared measurements of a-b-plane anisotropy have been for the 90-K transition-temperature YBa2Cu3OTa material [ 12-16]. Much of the observed anisotropy of YBa2Cu307_ a can be attributed to the quasi-one-dimensional CuO chains; their presence prevents determining whether the CuO 2 planes themselves have any intrinsic anisotropy. The Bi-based materials are better candidates for investigating the issue of a-b-plane anisotropy because there are no chains in these compounds. In the so-called 2212 phase, Bi2Sr2CaCu208 (T~=85 K), the CuO 2 planes are separated by Bi-O double layers, which are believed to act as a charge reservoir. There is an orthorhombic distortion of the nearly tetragonal ab-plane on account of a long-range superlattice modulation of the Bi-O layers [17]. Several authors [18-27] have reported optical studies of Bi2Sr2CaCu208. Only a few measurements have addressed the polarization dependence within the a-b plane. Ellipsometric measurements [19] in the near-infrared-ultraviolet find a strong anisotropy, with a transition at 3.8 eV stronger and sharper for light polarized along the modulation direction, a-b-plane anisotropy has also been observed in the far-infrared transmittance of free standing single crystals [24]. In this paper we present a detailed study of the a-b-plane anisotropy in the optical reflectance for untwinned samples of BizSr2CaCu208 in a wide frequency range (Far-IR-UV) and for temperatures above and below the superconducting transition temperature. The single crystals used in this study were grown as described by Han et al. [28]. Typical crystals are thin rectangular platelets with a surface area of a few milli-