ORAL SESSION 3E4: Ladder Cuprates

Wednesday, Feb. 23, 10:00 a.m. – 12:00 p.m., Room 301D (GRB)

Chairs: T. Tohyama (Tohoku U), A. Weber (NSF)

3E4.1 Cupric Spin Ladder Oxides: Synthesis, Spin Gap, Impurity Effects

Mikio Takano, Zenji Hiroi, Masaki Azuma, Takashi Terashima, Takahito Terashima, and Yutaka Hurubayashi, Institute for Chemical Research, Kyoto University, Uji, Kyoto-fu 611-0011, Japan

Presenting Author: M. Takano

SrCu₂O₃ and Sr₂Cu₃O₅ are the simplest two-legged and three-legged cupric spin ladder oxides, respectively. The synthesis and single crystal growth of these oxides under high pressure and also their electronic properties including the presence and absence of a spin gap and quite remarkable impurity effects studied through measurements of NMR, m SR, inelastic neutron scattering done in collaboration with several physics groups will be reported.

The growth of a single crystalline film of the most heavily hole-doped two-legged ladder system, Ca₁₄Cu₂₄O₄₁, and its strongly anisotropic resistivity will also be reported.

3E4.2 Superconductivity, Spin Liquid State and Antiferromagnetism in the Spin Ladder Compound (Sr,Ca)₁₄Cu₂₄O₄₁

Jun Akimitsu, Department of Physics, Aoyama-Gakuin University, Setagaya-ku, Tokyo 157-8572, Japan

Presenting Author: J. Akimitsu

Recently, much attention has been paid for the spin ladder compound, which is an intermediate material between one and two dimensional systems. The ground state of this system is given by spin singlet in so-called "rungs" which connect between the one dimensional chains. Most interesting point of this system is an expectation of the superconductivity in the doped spin ladder system. Two years ago, we found the superconductivity in the doped spin ladder compound (Sr,Ca)₁₄Cu₂₄O₄₁ under high pressure. We report the present experimental situation of the superconducting and magnetic properties of the (Sr,Ca)₁₄Cu₂₄O₄₁ single crystal. In particular, special attention has been focused on the coexistence between antiferromagnetism and the spin gap state at ambient pressure. We will also discuss the hole concentration dependence of the new four-leg ladder compound.

3E4.3 Dynamics in the low-dimensional ladder cuprates Sr_14-xCa_xCu₂₄O₄₁ (x=0, 5 and 12)

B. Ruzicka 1, L. Degiorgi 1, U. Ammerahl 2, and A. Revcolevschi 2. 1 Laboratorium für Festkörperphysik, ETH Zürich, CH-8093 Zürich, Switzerland. 2 Lab. de Thermodynamique et Physicochimie des Materiaux, Universite Paris-Sud, F-91405 Orsay, France.

Presenting Author: L. Degiorgi

It is predicted that even-leg ladders have a spin liquid ground state with a spin gap and that doping into the spin ladders results in the pairing of the doped holes which might lead to the formation of bipolarons or superconducting Copper pairs. The role played by the low-dimensionality effects, associated with the anisotropic crystallographic structure, as well as the effects induced by hole doping the ladders on the electrodynamic response can be revealed by optical investigations. We will review our measurements of the optical reflectivity as a function of temperature from the far-infrared up to the ultra violet (i.e., from 2 meV up to 12 eV) and with light polarized along the three crystallographic directions. Our findings reveal first of all an important anisotropy in the electrodynamic response, when measuring along the a, b and c (axis parallel to the ladders) directions. Moreover, upon doping there is transfer of spectral weight from high to low frequency which manifests itself in an enhancement of the metallic component in the reflectivity. For the whole series of compounds a progressive metal-insulator transition is seen with decreasing temperature, in accordance with the transport data. The talk will also stress the possible similarities with another class of correlated low-dimensional systems, namely the organic Bechgaard salts.

3E4.4 Electronic phase diagram of the two-leg ladder compound Sr_14-xCa_xCu₂₄O₄₁ as investigated by the transport property measurements

Hiroshi Eisaki 1,2, Naoki Motoyama 1, Kenji M. Kojima 1, Shin-ichi Uchida 1, Nao Takeshita 3, and Nobuo Mori 3. 1 Department of Superconductivity, The University of Tokyo, Hongo 7-3-1, Tokyo 113-8656, Japan. 2 Department of Applied Physics, Stanford University, Stanford, CA 94305-5457. 3 Institute for Solid State Physics, The University of Tokyo, Roppongi 7-22-1, Tokyo 108-8666, Japan

Presenting Author: H. Eisaki

We study the charge transport properties of the two-legged ladder compound, Sr_14-xCa_xCu₂₄O₄₁, with various Ca concentrations. For 0<x<3, the charge dynamics of this compound is characterized by the strong suppression in the optical conductivity (s (w )) as decreasing temperature, which is not observed for higher x’s. The decrease in s (w ) is responsible for the insulating behavior in these compounds and may be associated with the ordering of hole carriers within the ladders. This possible charge ordering is suppressed by applying high pressure and the system becomes metallic for 0<x<6, although its behavior is distinct from that for x=12 where the superconducting ground state is realized under high pressure. It is considered that the charge ordered state observed for x=0 is another ground state in this system which compete with superconductivity, and that Ca substitution destabilize the charge ordered state.

3E4.5 X-ray Absorption Spectroscopy of (Sr,Ca,Y,La)₁₄Cu₂₄O₄₁ Ladder Compounds

N. Nücker 1, M. Merz 1, C.A. Kuntscher 1, S. Gerhold 1, S. Schuppler 1, R. Neudert 2, M.S. Golden 2, J. Fink 2, D. Schild 3, V. Chakarian 4, J. Freeland 4, S. Stadler 4, Y. U. Idzerda 4, K. Conder 5, M. Uehara 6, T. Nagata 6, J. Goto 6, J. Akimitsu 6, S. Uchida 7, U. Ammerahl 7,8, and A. Revcolevschi 8. 1 Forschungszentrum Karlsruhe, INFP, P.O. Box 3640, D - 76021 Karlsruhe, FRG. 2 Institut für Festkörper-und Werkstofforschung Dresden, P.O. Box 270016, D - 01171 Dresden, FRG. 3 Forschungszentrum Karlsruhe, INE, P.O. Box 3640, D - 76021 Karlsruhe, FRG. 4 Naval Research Laboratory, Code 6345, Washington, DC 20375, USA. 5 Laboratorium für Festkörperphysik, ETH, CH - 8093 Zürich, Switzerland. 6 Department of Physics, Aoyama-Gakuin University, Chitosedai, Setagaya-ku, Tokyo, 157, Japan. 7 Department of Superconductivity, The University of Tokyo, Bunkyo.ku, Tokyo 113, Japan. 8 Laboratoire de Chimie des Solides, Université de Paris-Sud, F - 91405 Orsay CEDEX, France. 9 II. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, D - 50937 Köln, FRG.

Presenting Author: N. Nuecker

The unoccupied electronic structure of the ladder compound family Sr₁₄Cu₂₄O₄₁ has been investigated for various partial substitutions of Sr by Ca, keeping the total hole count constant, and by Y and La, reducing the total hole count. With polarization-dependent O 1s x-ray absorption spectroscopy, hole states on both the chain and ladder sites could be studied. In the compounds Y₃Sr₁₁Cu₂₄O₄₁ and La₃Sr₃Ca₈Cu₂₄O₄₁, both with 3 holes per unit formula, the spectral weight representing holes on oxygen sites is found to be symmetric with respect to O 2p_x and O 2p_z character, showing that at this doping level, all holes reside on the chain sites. For the fully doped (6 holes) compound Sr₁₄Cu₂₄O₄₁, the spectral weight near threshold is not symmetric, since now holes appear on ladder sites as well. This effect is further enhanced upon Ca substitution (Sr_14-xCa_xCu₂₄O₄₁).

3E4.6 n-leg Ladders and the High T_c Problem

Douglas Scalapino, University of California, Santa Barbara, CA 93106-9530

Presenting Author: D. Scalapino

The n-leg ladder cuprate materials not only respresent interesting systems in their own right, but also in addition offer new insight into the 2D layered cuprate high T_c materials. In this talk we will review our present understanding of n-leg ladders and discuss what this suggests about pairing and stripes in the high T_c problem.