GROWTH, STRUCTURE AND PROPERTIES OF LADDER-TYPE CUPRATE SINGLE CRYSTALS

L.Shvanskaya¹, L.Leonyuk¹, G.-J.Babonas², A.Reza², R.Szymczak³, V.Rybakov¹, V.Maltsev¹

¹ Moscow State University, 119899 Moscow, Russia
² Semiconductor Physics Institute, Gostauto 11, LT-2600 Vilnius, Lithuania
³ Institute of Physics, Al. Lotnikow 32/46, PL 02-668, Warsaw, Poland

Keywords: ladder-type cuprates, single crystals, growth, structure, properties

The interest to study cuprates with ladder-type structure was caused by the prediction of superconductivity [1] in these compounds. The superconducting properties were indicated in [2,3]. In the present work the results of recent investigations of the growth, structure and physical properties of the single crystals of ladder-type compounds are summarized.

Single crystals of cuprates were grown by a self-flux method from non-stoichiometric melts of the systems Sr-CaO(RE₂O₃)-CuO-Bi₂O₃(PbO) (I) and RE₂O₃-BaO(SrO, CaO)-CuO (II). The decanting method was used in the Bi-containing system in order to separate high-melting phases of ladder-type structure from the Bi-2212-type phases. In the RE-Ba series II the single crystals were obtained at Ba-substitution by Sr and Ca in equal amount.

The structure of the single crystals was studied by X-ray spectral and phase analyses. The superconducting properties were indicated by the measurements of resistance and magnetic susceptibility. Magnetic properties were studied making use of a SQUID magnetometer in the temperature range 2-300 K in magnetic fields up to 5 T. The optical properties were investigated by spectroscopic ellipsometry in the region 0.5-5.0 eV.

The basic structural element of the compounds under consideration is a so-called ladder-ribbon CuO₂ consisting of the Cu-O squares sharing both corners and edges. The ladder-type ribbons are joint into the ladder-type planes Cu₂O₃. The ladder-planes are stacked alternatively with cation layers in the structural type SrCu₂O₃. The latter compound was synthesized only at high pressures [4]. At normal conditions two phases can be formed. In the first structural type SrCuO₂ the ladder-ribbons CuO₂ are separated by cations. The Sr-atoms can be substituted isomorphically up to 50% by Ca. The structure of the second phase [M₂Cu₂O₃]m[CuO₂]n can be represented by a stack along the b-axis of two fragments MCu₂O₃ (structural type SrCu₂O₃) and M_1-xCuO₂ (structural type LiCuO₂) with similar parameters a and b and incommensurate parameters c1 and c2. The values of modulation coefficients m and n depend on the cation composition M. Taking into account the Cu-O distances, the phase m/n=7/10 is most stable. The superconductivity was indicated in the phases m/n=5/7 and 1/1.

In order to determine the growth conditions of a given phase, the metastability of ladder-type cuprates was studied and the characteristic temperature-concentration intervals were defined. The phases [M₂Cu₂O₃]m[CuO₂]n, SrCuO₂ and Ca₂CuO₃ were determined to be characterized by similar crystallization temperatures. Taking into account the data obtained, an epitaxial intergrowth of the related phases was excluded. The incommensurate-type phase m/n=5/7 was shown to be formed from the Bi-Sr-Ca-Cu melt of a composition 4334 only at equal amounts of Sr and Ca. The ratio of the Sr and Ca concentrations 3.2/2.8 led to the crystallization of the phase m/n=1/1 whereas the ratio 3/1 resulted in the formation of (Sr,Ca)CuO₂ of the structural type SrCuO₂. An increase of Ca content up to 1/3 caused a crystallization of (Ca,Sr)₂CuO₃ of the structural type Ca₂CuO₃.

The physical properties of [M₂Cu₂O₃]m[CuO₂]n-type crystals depend primarily on the cation composition. The hole doping of the CuO2 ribbons was indicated [5] in the incommensurate-type compounds by a substitution of La for Sr. Depending on the cation composition the single crystals of the incommensurate phase m/n=5/7 [2] as well as the commensurate-type compound m/n=1/1 [6] were found to be superconducting with T_c values in the range 72-85 K.

A strong anisotropy of magnetic susceptibility and optical response is to be noticed which is in a good agreement with the structure of incommensurate type compounds. The superconducting state thermodynamic parameters were derived from the measurements of the reversible dc magnetization M(H,T) in magnetic fields H_c1<<H<<H_c2 applied along the orthorhombic axes. The penetration depth l_i(T) and the coherence length x_i(T) for i=a, b and c were determined [2]. The obtained data confirmed the expectations that the observed superconductivity has a quasi-one-dimensional character and should be related to the ladders or chains. The high values of a Ginsburg-Landau parameter k_I=l_i/x_i indicate that the superconductivity in the ladder materials under consideration should be described as an extreme type-II limit.

The optical response for the light polarised along the b-axis was characteristic for the excitation of bound electrons, while for the c-polarisation a very strong contribution of a free carrier system is pronounced [7]. A fine structure of the optical spectra in (M₂Cu₂O₃)_m(CuO₂)_n -type compounds as in the other high-T_c cuprates can be interpreted as the charge transfer d⁹-d¹⁰L (L is the hole in the p⁶-shell of ligands O^2-), "interband" d¹⁰-d¹⁰sL and dipole allowed 3d-4p Cu⁺ transitions. The data on incommensurate-type cuprates show that the edge of the fundamental absorption is similar for various members of the series (M₂Cu₂O₃)_m(CuO₂)_n and can be correlated with the optical response in the ladder plane M₂Cu₂O₃. On the other hand, a large anisotropy of the dielectric tensor components e_c and e_a allows one to assume a significant contribution of the excitations in the plane of the Cu-O ribbons at a higher photon region. A decrease of the e_a component in the region 3-4 eV for the superconducting (M₂Cu₂O₃)(CuO_2+g)_1+d -type crystal as well as the difference between superconducting and non-superconducting (M₂Cu₂O₃)₅(CuO₂)₇ crystals indicate a reconstruction of the plane of the Cu-O ribbons in various compounds of the incommensurate type. The systematic analysis of the data for (M₂Cu₂O₃)_m(CuO₂)_n, Y-123, (Ca,Sr)₂CuO₃, LiCu₃O₃, CuO and SrCuO₂ has shown that the optical spectra of related cuprates can be considered in the model analogues to the modular approximation in the structural analysis. In this case the electronic excitations in particular structural units can be distinguished and the general regularities in optical response can be revealed.

[1] T.M.Rise, S.Gopalan, M.Sigrist: Physica B 199&200 (1994) 378.
[2] L.Leonyuk et al.: Czechosl. J. Phys. 46, suppl. S3 (1996) 1457
[3] M.Uehara et al.: J. Phys. Soc. Japan 65 (1996) 2764.
[4] M.Takano: J. Superconductivity 7 (1994) 49.
[5] S.A.Carter et al.: Phys. Rev. Letters 77 (1996) 1378.
[6] L.Leonyuk et al.: Z. Kristall. 213 (1998) 000.
[7] G.-J.Babonas et al.: Supercond. Sci. Technol. 10 (1997) 496.