Single-crystal growth and magnetic properties of La₂CuO ₄ and related compounds (T, T*, and T' phases) (invited) (abstract)

We have performed magnetic, transport, and optical measurements on well-characterized crystals of (un)doped La₂CuO₄-related compounds, so-called T, T* and T' phases, in order to understand the physical properties of quasi-2D S=1/2 antiferromagnet with/without added carriers. Single crystals of the (un)doped single-layer cuprates with large in-plane dimension and optically flat surface have been grown by the flux technique with CuO excess. Ways to control and measure dopant level in the crystals will be discussed. Our various measurements of magnetism in the pristine compounds indicate that magnetic phenomena in the insulating layer systems are qualitatively understandable in terms of classical spin models of 2D S=1/2 antiferromagnet, that is, quantum fluctuation effects, expected to be large in S=1/2 systems, are only perturbative. In this context, we discuss results of various investigations such as studies of magnetic dilution effects, field-induced transition, two-magnon scattering, and inelastic neutron scattering. Optical spectra have been measured over a wide range of energies and carrier densities. In undoped crystals, one simple excitation is the electron-hole pair at the edge of a charge transfer gap, whose energy scales with the Cu-O spacing. With a few added carriers, additional excitations appear due to the formation of impurity bands. We discuss a possible relationship between the excitations and spin dynamics.