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We have undertaken a study of the thermal conductivity of single crystals of pure and strontium doped lanthanum copper oxide in the temperature range 0.1–6 K. For in plane conduction, all samples show approximately a T 2.7 dependence, whereas for conduction across the planes for x = 0 we observe κ ∼ T 2 . This suggests that dimensionality may play an important role in the lattice heat conduction properties of these materials at very low temperature. more...

Measurements are reported of the magnetoresistance (MR) for fields up to 23 T in ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ single crystals, which order antiferromagnetically at ${T}_{N}$\ensuremath{\sim}240 K, and in which the conductivity at low temperature is characterized by hopping in localized states. Using the MR, the phase diagram of the spin-flop transition, observed when the magnetic field is applied parallel to the zero-field staggered magnetization, is mapped out. Two transitions of the background ${\mathrm{Cu}}^{2+}$ spins are observed, which are governed by the symmetric and antisymmetric anisotropic components of the superexchange tensor. The antiferromagnetic propagation vector changes from \ensuremath{\tau}\ensuremath{\parallel}a at zero field to \ensuremath{\tau}\ensuremath{\parallel}c at the highest fields. This subtle change in the ordering of the ${\mathrm{Cu}}^{2+}$ spins is accompanied by a large enhancement of the interlayer hopping conductivity up to a factor 2. We show that the magnetoconductance is proportional to the three-dimensional staggered moment with \ensuremath{\tau}\ensuremath{\parallel}c direction. In an appendix we discuss the possible relevance of these results to the behavior of superconducting ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathrm{x}}$(Sr,Ba${)}_{\mathrm{x}}$${\mathrm{CuO}}_{4}$. more...

We report epitaxial growth of the simple cubic perovskites Ba1-xKxBiO3 and Ba1-xRbxBiO3 on Ba3LaNb3O12, LiBaF3, BaF2, and Al2O3. Films with excellent crystallinity are obtained on the perovskite substrates, LiBaF3, and Ba3LaNb3O12. (111) or (110) films can be obtained on {111} BaF2, depending on the nucleation temperature. Non-epitaxial superconducting films have been grown on Si and on GaAs. At the low growth temperatures used (200-450°C), reactions with the substrates do not pose problems. The choice of substrate not only determines the microstructure of the film, but also plays a role in allowing the film to change its oxygen stoichiometry. Stoichiometry shifting is a pervasive necessity for the synthesis of high Tc thin films. more...

We have measured the thermal conductivity of a single crystal of ${\mathrm{La}}_{1.96}$${\mathrm{Sr}}_{0.04}$${\mathrm{CuO}}_{4}$ in directions both parallel and perpendicular to the copper-oxygen planes. While the effects of electron scattering for in-plane heat transport are strong, the transfer of heat across the ${\mathrm{CuO}}_{2}$ planes is limited primarily by sheetlike faults with spacings on the order of 100 A\r{}. The resulting very high out-of-plane thermal resistivity masks the effects of electron scattering in this direction and prevents a careful study of the anisotropy of the electron-phonon coupling in this superconductor. more...

Longitudinal magnetostriction ($\frac{\ensuremath{\Delta}l}{l}$) experiments have been used to probe the magnetic phase diagram of single-crystal ${\mathrm{La}}_{2}$Cu${\mathrm{O}}_{4}$ at 4.2, 77, 145, and 191 K. The geometry used is ($l\ensuremath{\parallel}\mathrm{H}\ensuremath{\parallel}\stackrel{^}{\mathrm{b}}$), where $\stackrel{\mathrm{^}}{\mathrm{b}}$ is a unit vector along the orthorhombic $b$ axis, perpendicular to the Cu${\mathrm{O}}_{2}$ planes. The observed magnetic transition is associated with an induced alignment, along the field direction, of the small out-of-plane canting of the ${\mathrm{Cu}}^{2+}$ spins. At all temperatures, the transition is marked by a small increase in $l$, with $\frac{\ensuremath{\Delta}l}{l}$ about 2 to 3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}8}$. A large hysteresis was observed at 4.2 K, with the transition field, ${H}_{t}$, in the up trace being 60.1 kOe, and in the down trace 45.8 kOe. This hysteresis strongly decreases with increasing temperature. At 191 K the jump in $l$ is still quite sharp indicating that the transition is still first order. Our values of ${H}_{t}$ are in good agreement with those obtained on similar crystals using different techniques. more...

${\mathrm{La}}_{2}$Cu${\mathrm{O}}_{4}$ possesses an orthorhombic superstructure based upon a tetragonal ${\mathrm{K}}_{2}$Ni${\mathrm{F}}_{4}$-type subcell for which a number of different space groups have been reported. A single-crystal neutron diffraction study, performed with large flux-grown crystals, confirms the nature of the substructure and provides $a=5.359(1)$, $b=5.363(1)$, and $c=13.127(2)$ \AA{}A. The orthorhombic strain, $\ensuremath{\epsilon}=\frac{(b\ensuremath{-}a)}{\frac{1}{2}(a+b)}$, is 7.64\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$, an order of magnitude smaller than for pure ${\mathrm{La}}_{2}$Cu${\mathrm{O}}_{4}$ as a result of incorporation of Li, derived from the flux, in the Cu site. The apparent space group is Pccn, but systematic examination of structural models in all possible centrosymmetric orthorhombic subgroups which may accommodate a ${\mathrm{K}}_{2}$Ni${\mathrm{F}}_{4}$-type substructure led to satisfactory refinement with positive-definite temperature factors only in space group Bmab-${D}_{2h}^{18}$. A substantial number of very weak reflections were detected which violate the required systematic absences of this space group. Through structure-factor calculation, after reindexing based on a probable twin law, and through dark-field transmission electron microscopy, the "forbidden" reflections are shown to arise from a 0.302(2) volume fraction of domains in twinned orientation which otherwise escaped detection because of the small value of $\ensuremath{\epsilon}$. Final agreement indices for 346 reflections with $I\ensuremath{\ge}\ensuremath{\sigma}(I)$ are $R(F)=3.22%$, $R({F}^{2})=2.57%$, ${R}_{w}({F}^{2})=3.07%$; the goodness of fit, $S$, is 1.13. The La and oxygen sites are found to be fully occupied within standard deviations of 0.4 at.%. The occupancy of the Cu site is ${\mathrm{Cu}}_{0.950}$${\mathrm{Li}}_{0.050(3)}$, in excellent accord with the results of a bulk chemical analysis. more...

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. more...

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. more...

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. more...

Using a flux technique we have grown sizable single crystals of ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Sr}}_{\mathrm{x}}$${\mathrm{CuO}}_{4}$. With x rays and neutrons we have studied both the static and dynamic aspects of the tetragonal to orthorhombic structural phase transition; classic soft-phonon behavior is observed at the ((1/2, 1) / 2 , 0) zone boundary involving rotations of ${\mathrm{CuO}}_{6}$ octahedra. The pure and lightly-doped single crystals show hopping conductivity, ln\ensuremath{\rho}\ensuremath{\sim}(${T}_{0}$/T${)}^{1/4}$, indicating that the electronic states at the Fermi energy are localized. more...

We report elastic, quasielastic (F dE), and inelastic neutron-scattering studies of the instantaneous and dynamic spin fluctuations in as-grown and doped ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$. Four samples have been studied: (A) as-grown ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ with ${T}_{N}$=195 K, (B) oxygenated ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ with ${\mathit{T}}_{\mathit{N}}$\ensuremath{\simeq}100 K, (C) ${\mathrm{La}}_{2}$${\mathrm{Cu}}_{0.95}$${\mathrm{Li}}_{0.05}$${\mathrm{O}}_{4}$, and (D) ${\mathrm{La}}_{1.97}$${\mathrm{Sr}}_{0.03}$${\mathrm{Cu}}_{0.95}$${\mathrm{Li}}_{0.05}$${\mathrm{O}}_{4}$. All crystals exhibit variable-range-hopping conductivity behavior. At room temperature each sample exhibits two-dimensional (2D) antiferromagnetic instantaneous correlations in the ${\mathrm{CuO}}_{2}$ sheets with correlation length varying from \ensuremath{\sim}200 A\r{} in crystal A to \ensuremath{\sim}14 A\r{} in crystal D. The integrated intensity and therefore the effective moment is, however, constant to within the experimental error. In samples A and B the 2D correlation length becomes sufficiently large with decreasing temperature that the interplanar coupling is able to drive a transition to 3D long-range order. The spin dynamics have been studied in detail in crystals A and B and quite unusual behavior is observed. In contrast to previously studied planar antiferromagnets, there is no significant E\ensuremath{\simeq}0 component for temperatures \ensuremath{\ge}${T}_{N}$ and instead the 2D response function is highly inelastic. The effective dispersion of the spin excitations is \ensuremath{\ge}0.4 eV A\r{}. This large energy scale for the spin fluctuations gives credence to models of the superconductivity in doped ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ in which the pairing is magnetic in origin. more...

Measurements are reported of the conductivity, its anisotropy, and the in-plane Hall coefficient for a single crystal of La/sub 2/CuO/sub 4+/ital y// with /ital y/ varied from 0%--1.5%. The conductivity is thermally activated with a single activation energy at high temperatures and with a continuously decreasing activation energy below /similar to/50 K. The anisotropy is as large as 500 at high temperatures, falls sharply to 10--20 at /similar to/50 K, and then is only weakly /ital T/ dependent at lower temperatures. The Hall coefficient is simply activated above /similar to/50 K (with an activation energy similar to that of the in-plane conductivity), and has a peak at lower temperatures. All these features are evidence for a change in the transport mechanism near 50 K. The low-temperature conductivity is consistent with thermally assisted tunneling between localized states at the Fermi energy. The high-temperature process arises from thermal activation of carriers into a band of highly anisotropic states. The low mobility associated with these states makes it impossible, however, to determine whether they arise from impurities or are states near a band or mobility edge. more...

We have carried out elastic, quasielastic (..integral.. dE), and inelastic neutron-scattering studies of the antiferromagnetic spin correlations in La/sub 2-//sub x/Sr/sub x/CuO/sub 4/ with x varying between 0.02 and 0.18. The crystals, which were grown in three different laboratories, exhibit behavior that varies smoothly with x. In all cases, antiferromagnetic correlations with a scattering amplitude corresponding to a fully occupied Cu/sup 2+/ square lattice are observed. However, the Neel state is destroyed by the doping and spin-spin correlation lenght xi is quite short, varying from approx.35 to approx.8 A as x varies between 0.02 and 0.18; the local order is, however, the same as in pure La/sub 2/CuO/sub 4/. The fluctuations are dynamic in character as in La/sub 2/CuO/sub 4/ above the Neel temperature T/sub N/. To a first approximation, xi = 3.8/ ..sqrt..x A, the average separation between the holes introduced by the Sr/sup 2+/ doping. The x = 0.08 sample exhibits superconductivity with T/sub c/ = 10 K and with a Meissner fraction exceeding 15% at 5 K; no important differences in the magnetic scattering are observed in the normal and superconducting states. In an appendix we present additional data on the spin dynamics in pure La/sub 2/CuO/submore » 4/ at T = 300 K in a sample with T/sub N/ = 235 K.« less more...

Raman spectra are presented of pristine and doped (Li,Sr) single crystals of ${\mathrm{La}}_{2}\mathrm{Cu}{\mathrm{O}}_{4\ensuremath{-}y}$. Based on polarization measurements, we assign the 424-${\mathrm{cm}}^{\ensuremath{-}1}$ line to the symmetric ${A}_{1g}$ vibration of the out-of-plane oxygen atoms along the tetragonal axis. The line at 231 ${\mathrm{cm}}^{\ensuremath{-}1}$ is identified as the ${A}_{1g}$ vibration of the La atoms along the tetragonal axis and the line at 238 ${\mathrm{cm}}^{\ensuremath{-}1}$ as the ${E}_{g}$ vibration of the out-of-plane oxygen atoms parallel to the Cu-O layers. A symmetry-forbidden Raman line is observed at 680 ${\mathrm{cm}}^{\ensuremath{-}1}$ which, by comparison with infrared measurements, can be tentatively assigned to an in-plane oxygen vibration. The dependence of the Raman spectra upon Li and Sr doping is presented and discussed. Especially noteworthy are two strong and sharp Raman lines (at 757 and 773 ${\mathrm{cm}}^{\ensuremath{-}1}$ at room temperature) in the Sr-doped crystals, because of their higher frequencies, polarization sensitivities, and unusual temperature dependences. more...

Measurements are reported of the conductivity and dielectric constant at frequencies up to 10 MHz for a single crystal of La{sub 2}CuO{sub 4+{ital y}} with {ital y}{similar to}0.001 to 0.01. The reported behavior is typical of conventional doped semiconductors, with dielectric constant (at {ital y}=0) and effective mass that are not exceptionally large. The dielectric constant grows as the insulator-to-metal transition is approached, but only for the electric field in the CuO{sub 2} layers, indicating that the transition is two dimensional. The crossover to weak localization occurs close to the antiferromagnetic phase boundary. more...

The resistivity of nonmetallic La/sub 2-//sub y/Sr/sub y/Cu/sub 1-//sub x/Li/sub x/O/sub 4-//sub delta/ single crystals and ceramics accurately follows the functional form exp((T/sub 0//T)/sup 1/4/), characteristic of variable-range hopping. For each of the crystals grown from Li-containing flux and CuO flux, pure reduced ceramics, and ceramics containing 0.025 to 0.2 mol % Li, the values of T/sub 0/ are in the range 0.3--7 x 10/sup 6/ K. The hopping conductivity shows that the crystals, which also manifest the two-dimensional quantum spin fluid state, antiferromagnetism, and the tetragonal-to-orthorhombic transition, are nonmetallic because the electronic states at the Fermi energy are localized. No evidence of a large gap is observed, and all samples, including the reduced ceramic, are p type, leading to the suggestion that in the nonmetallic state as well as the superconductor, the Fermi energy lies near the top of the band of singly occupied states (lower Hubbard band). The suppression of superconductivity by Li impurities is discussed. more...

The dispersion of the low-lying phonon branches of several doped and undoped single crystals of ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ has been investigated by using inelastic-neutron-scattering techniques. The zone-center modes are in reasonable agreement with Raman measurements. The reported peaks in the phonon density of states show up at energies that correspond to extrema in the dispersion curves of the transverse and longitudinal acoustic branches near the zone boundary. The tetragonal-to-orthorhombic phase transition is caused by a softening of a transverse-optic-phonon mode at the $X$ point. 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. We did not observe any evidence for the predicted breathing-mode instability near the zone boundary. more...

Neutron-scattering studies provide direct evidence for a field-driven magnetic transition which originates from the canting of spins out of the Cu${\mathrm{O}}_{2}$ planes: At the transition between antiferroand ferromagnetic ordering of the canted component of the spins in the layers, the (100) Bragg peak vanishes while the (201) peak appears. Detailed measurements and analysis suggest that the phase transition can be described by the mean-field theory of Thio et al. only close to the N\'eel temperature. more...

The soft optical phonons associated with the tetragonal-to-orthorhombic structural phase transition in La/sub 2-//sub x/Sr/sub x/CuO/sub 4-//sub y/ have been investigated using inelastic-neutron-scattering techniques. Single crystals with Sr concentration of x = 0, 0.08, and 0.14 were studied. Both branches of the doubly degenerate X-point phonon which go soft at the tetragonal-to-orthorhombic transition have been observed in the orthorhombic phase. In all three samples one branch hardens, while the other remains relatively soft with behavior indicative of a further incipient structural phase transition. We identify the hard and soft branches of the orthorhombic phase as zone center and zone-boundary phonons, respectively. The zone-center mode has A/sub g/ symmetry and is, consequently, Raman active. The relation of these measurements to other structural properties and to the superconductivity in La/sub 2-//sub x/Sr/sub x/CuO/sub 4-//sub y/ is discussed. more...

We report the results of phonon relaxation processes obtained from picosecond transient thermoreflectance measurements of single‐crystal La2CuO4+y (y∼0.06). Nonequilibrium heating was found to occur on a picosecond time scale. The time scale of the nonequilibrium heating is a sensitive measurement of the modest electron‐phonon interaction (λ≤0.1) in the nonsuperconducting La2CuO4+y. more...

Single crystals of Nd 2 CuO 4 have been grown from excess CuO by a top seeded solution growth technique. Ce-doped Nd 2 CuO 4 platelet-like single crystals have also been grown by slow cooling of the melt with nucleation sites provided by a platinum wire. more...

Pulsed room‐temperature laser action at 2.06 μm has been achieved using Ho3+:YLF (LiF4) sensitized with Er3+ and Tm3+. An unoptimized cavity gave a threshold of 35 J and a slope efficiency of 1.3% in normal mode with an output mirror of 62% T. A slope efficiency of 2% and a 65‐J threshold were observed in Q‐switched operation along with Q‐switched energy of >500 mJ from a 0.9×⅛‐in. rod. The sensitizer ions have broad absorption bands and transfer times (to the active ion) of 150–200 μsec. more...

Presentation d'analyses de thermoreflectance transitoire de l'ordre de la picoseconde du monocristal La 2 CuO 4 , entre 80 et 300 L. Etude de l'influence de la direction de polarisation de la lumiere more...

Growth and laser performance of the laser material Er:YLF was investigated. This is a 4 level emitting at 0.85 microns and operated in the flash pumped mode at room temperature. Large, uniform diameter boules of Er:YLF have been grown under automatic diameter control in an argon atmosphere. 0.25 x 3 inch rods with scattering loss less than 0.5% 1/cm and optical path distortion of less than 1 fringe per inch have been routinely grown on this program. The rod yield is limited only by external geometry; there are no cores in this material. Boule uniformity and boule-boule reproducibility was excellent. Two feed purification procedures were employed in this program. Comparative laser testing showed a linear increase in laser efficiency with Er concentration from 2 - 5% Er. At 10 Hz, long pulse, 1.1 watts output were measured from a 0.25 x 3 inch rod with a slope efficiency of 0.5% (overall efficiency 0.3%) at room temperature. Higher output powers were constrained by flashlamp peak current limitations. Q-switched outputs at 1/2 the long pulse efficiency were obtained. more...

Single crystals of pure and Sr doped La2CuO4 have been grown with a top seeded solution growth technique. Crystals grown from a lithium borate flux incorporate impurities detrimental to superconductive properties while those grown from excess copper oxide are free from this difficulty. more...

The raw materials and clean-room processing techniques required to produce a low alpha-particle emitting insulating material have been investigated with the goal of producing a dense fired alumina ceramic with a reproducible, low alpha-particle flux (≤ 0.01 α/cm2hr). Various sources of A12 O3, MgO, and SiO2, the primary constituents of the alumina body, were tested for the alpha-particle emitting elements, U and Th, and for total alpha-particle flux. Many materials contain U concentrations with corresponding alpha-particle fluxes of 0.01α/cm2hr. Die pressing, filter casting and isopressing have been used under class 100 clean-room conditions to process powder compacts and the benefits and problems of each were evaluated. Analysis of the material before, at various stages during processing, and after firing has shown that dense fired Al2O3 ceramics can be produced with no detectable increase above that of the source powder in the U and Th contents, or the alpha-particle flux. more...

Polarized Raman scattering spectra at 300K and 20K are reported for a single crystal of La2CuO4-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 La2CuO4-y. more...

Fluoride crystals have some general characteristics that are desirable for a laser host. These include a wide transparency range, low refractive index and nonlinear index. They can in general be grown with a better optical quality than most oxide, crystals. LiYF4 is an example of a very successful fluoride host crystal and, even though it does not have quite as good mechanical and thermal properties as YAG. in many applications it performs as well or better than YAG. The principal conditions that must be met if a compound is to be useful are that it is the primary phase crystalizing from the melt, that the yield be practicably large, that the crystal does not undergo a phase-transformation upon being cooled to room temperature and that the melt be sufficiently stable (with regard to evaporation and oxidation state of dopants) over the several days normally required for crystal growth. more...

This results of the first six months of a fourteen month program entitled Er(3+): YLF Solid State Laser Development are reported. The program goals are: Growth of 0.25 inch diameter by 3-inch length laser rods with scattering less than 0.5% cm-1; Q-switched operation at a PRF of 10 Hz with an output power in excess of 1 watt; beam divergence less than 5 milliradians at full angel, 90% energy points; and operation at an overall efficiency of 0.5% in the Q-switched mode. more...

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. more...

Lanthanum chloride was synthesized by reacting La2O3 with CCl4. Purification carried out by zone refining resulted in reduction of Nd and Eu concentrations by at least a factor of 103. Impurity segregation coefficients were calculated from the concentration profiles in the charge measured by neutron activation analysis. Values of 0.85, 0.5, and 0.15 were found for Ce, Nd and Eu while a value of 0.65 was obtained for Pr from a plot of measured distribution coefficient versus ionic radius. These results show the feasibility of removing rare earth impurities to the ppb level from LaCl3. After purification, LaC13 is converted to LaF3 more...

Dramatic implantation-induced changes in the Raman and infrared spectra of polycrystalline samples of La2CuO4-y are reported for implantation of Sr in the fluence range 0.5 to 1.5 × 1016/cm2. The Raman and ir-reflectance spectra of the Sr implanted material reveal disorder-induced lines corresponding to breakdown of selection rules and to non-zone-center phonon modes. Whereas the main features in the Raman spectra of the implanted sample are broad disorder-induced lines centered at ∼ 560 ± 5 cm−1, reststrahlen lineshapes are observed above ∼ 800 cm−1 in the ir-reflectance spectra. After thermal annealing (550°C), the multiphonon lines in the ir spectra diminish, and the disorder-induced lines in the Raman spectra split to form well-resolved, sharper lines at 530 and 580 cm−1, identified with maxima in the phonon density of states. The sensitivity and complementarity of Raman and infrared spectroscopy to implantation-induced disorder and subsequent partial recovery by thermal annealing suggest that these techniques can be used to characterize sensitively disorder in ion implanted La2CuO4-y high Tc, materials. more...

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. more...

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. more...

Magnetoresistance (MR) measurements are reported for pure single crystals of La2CuO4+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. more...

Variably valent iron impurities have been suggested as the source of photorefractive charge carriers in BaTiO3. High-purity BaTiO3 crystals were grown with transition-metal impurity levels below the 1016 cm−3 baseline estimated for photorefractivity. Iron-doped crystals were grown with concentrations of 5, 50, 250, 500, 750, and 1000 parts in 106 of iron. Changes in iron valence monitored by optical absorption in the PO2 range 1–10−4 atm were found to be consistent with a defect-chemical model indicating Fe3+ and Fe4+ to be the stable valence states in this range. Photorefractive characterization of high-purity BaTiO3 suggests that variably valent iron ions are not the dominant photorefractive species in these crystals, whereas the role of iron centers in doped crystals is complicated by large absorption losses. more...

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. more...