P. J. Picone

Phase diagram and single crystal growth of pure and Sr doped La2CuO4

Abstract Single crystals of pure and Sr doped La 2 CuO 4 have been grown from excess CuO with a top seeded solution growth technique. Crystals have been grown both in air and in oxygen. Growth in air leads to an oxygen deficiency as evidenced by a reduction in the lattice parameters. Differential Thermal Analysis measurements in oxygen on this system reveal a eutectic temperature of 1075°C for the composition 87.5 mol% CuO. Measurements in air show a reduction in the eutectic temperature to 1040°C and the formation of a 3 component system with the partial reduction of CuO to Cu 2 O.

Top seeded solution growth of La4CuO4

Abstract Single crystals of La 2 CuO 4 have been grown from a flux with a top seeded solution growth technique. X-ray and chemical analyses indicate the presence of some flux impurities that are detrimental to superconductive properties. Strontium addition was also attempted to produce superconductive crystals.

Frequency dependence of the conductivity and dielectric constant of La2CuO4+y near the insulator-metal transition

Abstract Measurement are reported of the conductivity and the dielectric constant (ɛ 1 ) at frequencies (ω) up to 20 MHz and magnetic fields up to 15 T for a single crystal of La 2 CuO 4+y with y varied in the range 0.001 – 0.01. The frequency dependence of the conductivity at low temperatures follows a power law, which is typical of conventional doped semiconductors. ɛ 1 at high ω is independent of ω and grows as the insulator to metal transition (IMT) is approached, but only for the electric field in the CuO 2 layers, indicating that the IMT is two-dimensional. For the highest oxygen concentration, this excess ɛ 1 was reduced by a factor of 2 at a magnetic field of 15 T perpendicular to the CuO 2 layers.

Polarized reflectivity studies of single-crystal La 2 CuO 4

Room-temperature polarized reflectivity measurements are reported for single-crystal La2CuO4 for the (E ⊥ ĉ) and (E || ĉ) polarizations, where ĉ denotes the tetragonal axis. A Kramers–Kronig analysis is performed to obtain the frequency-dependent dielectric functions. Frequencies for the seven zone-center IR-active phonons are obtained and the longitudinal-optic–transverse-optic splittings are used to evaluate the dynamic charge on the oxygen atoms. The reflectivity spectrum for the (E ⊥ ĉ) polarization is decidedly metallic and is fitted accordingly with an overdamped Drude model and a plasma frequency of 95 meV. In the high-frequency region of the spectrum, selection rules are observed for allowed and forbidden electronic transitions. The importance of this study to the interpretation of optical measurements on high-Tc, polycrystalline pellets of La2CuO4 and related materials is also discussed.

Lattice instability in single-crystal La2−xSrxCuO4

Abstract The structural phase transition from the tetragonal to the orthorhombic phase of doped and undoped samples of La2−xSrxCuO4 has been investigated by using inelastic neutron scattering techniques. The rotational nature of the soft mode leads to moderate electron-phonon coupling and the mode is unlikely to enhance significantly conventional phonon mediated superconductivity.

Magnetoresistance and the spin flop transition in pure single - Crystal La2CuO4+y

Abstract Magnetoresistance (MR) measurements are reported for pure single crystals of La 2 CuO 4+y at magnetic fields up to 23T. With the magnetic field →H‖→c, parallel to the staggered moment, the MR shows evidence of a spin flop transition, which provides a measure of both the antisymmetric and the symmetric anisotropies of the nearest-neighbour exchange. The MR, which arises from the interlayer hopping conductivity, is proportional to M † + , the order parameter of the antiferromagnetism (AF) with propagation vector →gt‖→c.

Weak localization effects in conductance measurements of La2−xSrxCuO4−y

Abstract Conductivity data from many La 2−x Sr x CuO 4−y crystals is presented. All sufficiently conductive crystals (with conductances of order e 2 /h per square per CuO 2 layer) show a logarithmic temperature dependent correction to the conductance, of order e 2 /h, predicted by weak localization theory.

Two Dimensional Quantum Spin Fluid — Progenitor of High Temperature Superconductivity —

Neutron scattering probed novel two dimensional antiferromagnetic spin correlations in pure La2CuO4 which is the mother compound of high temperature superconductors of doped La2-xMxCuO4. Cu2+ moments are ordered instantaneously over very large areas in the CuO2 plane but there is no measurable time averaged staggered moment. Since the dynamical behavior observed is much akin to that in the quantum antigerromagnetic chain, we concluded that spin fluctuations observed at finite temperatures are quantum spin liquid state like that in quantum antiferromagnetic chain at T = 0.

NEUTRON SCATTERING STUDIES OF MAGNETIC EXCITATIONS IN La2−xSrxCuO4

The results of recent neutron scattering studies on La 2−x Sr x CuO 4 are reviewed. For × = 0 the material is an antiferromagnetic insulator, and the spins in the CuO 2 sheets are well-described by the 2D S = 1/2 square lattice Heisenberg model. With increasing × the Neel state is destroyed and is replaced at low temperatures, at least for samples which are nonmetallic or have low superconducting T c s, by a spin glass. The instantaneous correlations as well as those of the glass are incommensurate with the crystal lattice. The spin-spin correlation length in the doped CuO 2 sheets equals the average separation between the holes which ultimately carry the supercurrent.

Effect of Oxygen Annealing on the Magnon-Pair Raman Scattering in La 2 CuO 4-y

Polarized Raman scattering spectra at 300K and 20K are reported for a single crystal of La 2 CuO 4-y before and after annealing in oxygen at 1000°C for 24 hours. Oxygenation strongly quenches the temperature sensitive line at ∼1430 cm -1 (for T ∼ 300K) in the “as grown” crystal, previously identified with magnon-pair excitation associated with two-dimensional antiferromagnetic ordering. In addition, the phonon spectrum is greatly modified by the addition of oxygen, indicating a more uniform local crystalline environment after oxygenation. Raman scattering is thus a sensitive probe of the effect of oxygenation on the near surface magnetic and phonon properties of La 2 CuO 4-y .