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The normal-state magnetotransport properties of ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Ba}}_{x}{\mathrm{CuO}}_{4}$ single crystals with $x=0.095$ are measured; at this composition, a structural transition to a low-temperature tetragonal (LTT) phase occurs without suppression of superconductivity. None of the measured properties (in-plane and out-of-plane resistivity, magnetoresistance, and Hall coefficient) shows any sudden change at the LTT phase transition, indicating that the occurrence of the LTT phase does not necessarily cause an immediate change in the electronic state such as the charge-stripe stabilization.

Single-crystalline films of La 2− x Sr x CuO 4 were prepared on nonsuperconducting substrates of Zn-doped La 2 CuO 4 single crystals by the liquid phase epitaxial (LPE) technique. The La 2− x Sr x CuO 4 single-crystalline films, less than 50 μm thick, were subgrain- and crack-free. The Sr content of the La 2− x Sr x CuO 4 films was determined to be 0.07–0.11, but the films were contaminated with less than 3 at.% aluminum as a result of the use of alumina crucibles. Some of the La 2− x Sr x CuO 4 films exhibited superconductivity with T c onset ≈30 K and Δ T c ≈20 K.

Single crystals of La2−xMxCuO4 (MCa, Sr, Ba) were grown by the TSFZ method with an infrared heating furnace. In the TSFZ growth of La2−xMxCuO4, the Sr content was controlled within a wide range as compared with the other dopants. The La2−xMxCuO4 grown crystals had high quality with a sharp superconducting transition.

The electron tunneling spectroscopy is performed on PbILa 1− x Sr x CuO 4 (LSCO) junctions fabricated on high quality LSCO single crystals grown by traveling solvent floating zone (TSFZ) method. For the junctions fabricated parallel to ab -planes, periodic sharp spikes in the spectrum are observed at ∼ 6 × (2 n + 1) mV, where n is a natural number, while for the junctions fabricated on the (001) plane only gap structures corresponding to Pb and LSCO are observed. We ascribe the spike structure to subbands in the electron density of states caused by the Andreev reflections on the periodically modulated gap potential along the c -axis of LSCO.

We report on vector magnetization studies of a La{sub 1.9}Sr{sub 0.1}CuO{sub 4} single crystal, cut in the form of a disk with the anisotropy (c) axis parallel to the disk plane. We observe a sharp threshold in the angular dependences of the isothermal remanent magnetization. When the magnetic field is applied within a threshold angle to the ab plane, the remanent moment is strictly parallel to ab, and also the c-axis component of magnetization is reversible. The threshold angle scales with temperature in the same manner as the lower critical field parallel to the c axis, H{sub c1}{sup c}. These observations give direct support to the presence of a vortex lock-in transition to the CuO planes in our crystal. {copyright} {ital 1997} {ital The American Physical Society}

In order to study these inhomogeneities, Ce-doped and undoped strontium-barium niobate (SBN) fibers were grown by a laser-heated pedestal growth (LHPG) technique in various Ce:Sr x Ba 1− x Nb 2 O 6 compositions along both c [001] and a [100] crystallographic axes. Specific optical inhomogeneities distributed in a radial manner (core effects) are observed in such crystals. Two-dimensional scanning electron microprobe analysis (SEPMA) indicated Ba enrichment and Sr decrease in the cores. An approximate control segregation coefficient (' k oA ) was introduced to characterize the segregation in the fibers. For different cations ' k oSr = 0.98, ' k oBa = 1.04 and ' k oNb = 0.99 were obtained which both determine and demonstrate the complexity of the inhomogeneities in the core in the radial direction.

For the first time the growth of La 2− x Ca x CuO 4 single crystals with 0 ≤ x ≤ 0.1 by the traveling solvent floating zone (TSFZ) method is reported. The grown crystals were homogeneous in composition and free from inclusions. The Ca content in the grown crystals became saturated at about x ≈ 0.1 even though the Ca contents in the feeds and solvents were increased further. Magnetic susceptibilities and electrical resistivities in the ab -plane and along the c -axis reveal significant anisotropy.

We report detailed experimental results on the anisotropic electric resistivity ϱ ( T , x ) for a series of La 2− x Ba x CuO 4−δ single crystals ( x = 0.06, 0.08, 0.09) grown by the TSFZ method. In all as-grown crystals, a semiconducting electric conduction was observed in the normal state both in the ab plane and along the c -axis. After annealing in oxygen, ϱ ab ( T ) became metallic and ϱ c ( T ) remained semiconducting above the superconducting transition temperature. Though the change in T c was small after the annealing, the normal-state resistivity changed significantly. A possible reason for such behaviors was discussed. The resonating valence bond (RVB) model and power law were adopted to fit the experimental data.

Purple Au-Al alloy films were deposited on glass substrates using a radio frequency ion plating method. Au-Al alloys were used as source materials. The effects on the film of the source alloy composition and the evaporation time were studied. The color tones of the deposited films were measured using spectral reflectance and their crystal structures were studied by X-ray diffraction. As a result, a composition of Au 80%-Al 20% was chosed as the most suitable source alloy. The color tone of a film deposited from a single evaporation source changed from white to purple as the evaporation time increased. The intensity of the X-ray diffraction pattern of the AuAl2 sharpened as the color tone of the film became more purpler. The purple color of the film became brighter after 1hour of heat annealing at 550°C in an atmosphere of Ar (8×103Pa).

Single crystals of Ba 1− x Sr x TiO 3 were grown by the floating zone (FZ) method using an infrared heating furnace. The grown crystals of Ba 1− x Sr x TiO 3 , typically 8 mm in diameter and 4 mm in length, were crack-free and yellowish transparent. Substitution of more than 1.2 at% Sr for Ba in BaTiO 3 was effective in preventing the formation of the hexagonal phase. It was found that Ba 1− x Sr x TiO 3 melts congruently at 1.5 at% Sr.

The oxygen annealing effect on the temperature-dependent electrical resistivity has been studied in La 1.91 Ca 0.09 CuO 4− y single crystals grown by the TSFZ method. In as-grown crystals, semiconducting-like electrical conduction has been observed, both in the ab -plane and the c -axis at the non-superconducting state. The onset transition temperature T c-onset was about 17.5 K. After annealing in oxygen, ϱ ab ( T ) becomes metallic and shows a resistivity minimum at a certain temperature T min that separates regions of metallic behavior at T > T min from semiconducting behavior at T T min , while ϱ c ( T ) remains semiconducting. In the low-temperature region, an upward curvature of the resistivity (ϱ ab ( T )) has been observed with decreasing temperature below T min . This is an indication of the occurence of weak electron localization due to the randomness of the oxygen distribution the crystal lattice caused by oxygen deficiency that might still be present in the annealed samples. The Anderson-Zou fit to the data works well in metallic samples and the variable range hopping law (VRH) in non-metallic samples.

Nd 1.85 Ce 0.15 CuO 4 single crystals of improved quality have been growth by the TSFZ method at an oxygen partial pressure in ambient atmosphere of 0.2, 0.1, 0.08 and 0.01 atm. Superconductivity in the as-grown state has been observed in the crystals grown at 0.08 and 0.01 atm. The improvement of the Ce distribution has been achieved due to a decreased value of the Ce distribution coefficient at low oxygen partial pressure in ambient atmosphere. The probability to obtain the same characteristics of the superconducting transition in the as-grown state as that in the crystals after post-growth high-temperature treatment seems to be very low because of oxidation during the cooling of the grown crystals from the crystallization temperature to room temperature.

Nd 2− x Ce x CuO 4 crystals have been grown by the TSFZ process at an oxygen partial pressure in the growth atmosphere of about 2.0, 0.2 and 0.1 atm. The corresponding values of the cerium effective distribution coefficient are 9.1, 3.4 and 2.4. It has been found that the Ce concentration in the central part of the crystals is slightly lower than that in the peripheral part, although the average Ce concentration has the same value as that in the feed rods. This phenomenon is discussed from the point of view of the different stirring conditions in the corresponding parts of the molten zone. The difference in the Ce concentration between the central and peripheral parts of the crystals becomes lower at decreased oxygen partial pressure in the growth atmosphere. Optimization of the growth conditions at the air atmosphere (oxygen partial pressure 0.2 atm) is pointed out as the promising way for quality improvement of the Nd 2− x Ce x CuO 4 single crystals.

We present the results of neutron-diffraction experiments on a series of La2−xSrxCuO4+δ single crystals at temperatures between 540 K and 22 K. There is an intrinsic misfit between the size of the LaO9 polyhedra and the CuO6 octahedra, which seems to cause the structural transition from the high-temperature tetragonal (HTT) to the low-temperature orthorhombic (LTO) phase. Within this misfit model it is possible to explain almost the entire phenomenology of the HTT-LTO transition. The electronic structure is related to this phase transition via the length of the CuO bonds. One of the effects of the transition on the structure is to lengthen these CuO bonds and hence to alter the electronic band structure. The introduction of additional charge by doping leads to a decrease of the equilibrium CuO distances, and hence to a reduction of the misfit. In this way the charge-carrier concentration determines the structural transition. We always observe strongly anisotropic thermal parameters, which confirm the instability of the La2−xSrxCuO4 structure against a rotation of the CuO6 octahedra around any axis. In the case of an undoped crystal (x = 0.0), we find a good agreement between the experimental mean square displacements and the results of a lattice dynamics model which is based on the known phonon dispersion curves. In a doped (x = 0.13) and superconducting crystal, the anharmonic effects related to the structural transition lead to significant changes in the temperature dependence of the thermal parameters.

The phase transition from the tetragonal to the orthorhombic structure in La2−x Sr x CuO4+δ has been studied combining elastic and inelastic neutron scattering and specific heat measurements on the same well characterized crystals. This analysis focusses on the comparison between undoped and doped compounds. The proportionality between the orderparameter and the orthorhombic strain, predicted by the Landau theory, can be confirmed in La2−x Sr x CuO4+δ in a wide concentration and temperature range. We observe different values for the critical exponent β for doped superconducting and undoped insulating crystals. The temperature dependent study of the soft modes on a metallic superconducting crystal reveals an extremely large temperature range where this mode is strongly anharmonic. The comparison with previously published data shows that the frequencies of the soft modes at low temperatures decrease with doping, i.e. with increasing charge carrier concentration.

A strong dependence of the cerium distribution coefficient ( K Ce ) on oxygen partial pressure P [O 2 ] has been shown in direct TSFZ experiments by changing the oxygen pressure during Nd 2−x Ce x CuO 4 single crystal growth. The ratio of K 0.1 atm Ce / K 0.2 atm Ce has been found to be equal to ∼0.62. Absolute values of K Ce measured from zone compositions have been found to be about 2.42, 3.37 and 9.06 at P[O 2 ]=0.1, 0.2 and 2.0 atm, respectively. Our experiment shows good agreement between measured and calculated K Ce values. We have shown that the quite large difference in K Ce obtained in a number of investigations is explained by a strong dependence of K Ce on P [O 2 ]. We have also shown that a reason for this dependence is the variable valence of Ce. Cerium effectively incorporates in a Nd 2−x Ce x CuO 4 single crystal in the form of Ce +4 ion.

Neutron scattering experiments have been performed to study the dynamic spin properties of single-crystal La 1.86 Sr 0.14 Cu 0.988 Zn 0.012 O 4 ( T C =16 K). The width of S ( Q , ω) in momentum space decreases at low ω and at low temperatures. The intensity at low energies at the incommensurate peak position (π(1-δ), π)) shows a broad maximum as a function of temperature around the superconducting transition temperature T C . This behavior is qualitatively similar to that observed by Thurston et al . in La 1.85 Sr 0.15 CuO 4 ( T C =33 K).

The infrared (IR) reflection spectra were measured on single crystals of La 2− x Sr x CuO 4− δ ( x =0, 0.09, 0.14 and 0.19) by using specially made far-IR and IR microspectrophotometers. The spectra show the Drude, the phonon and the mid-IR exciton bands. The near-IR to UV reflection spectra were also measured. The dielectric functions were calculated by Kramers-Kronig transformation of the reflection spectra. The plasma frequency of the Drude band is relatively small (4100 cm −1 for x =0.14). We found all predicted phonon bands and several other phonon bands which appeared by the lowering of the D 4h symmetry of the CuO 4 cluster. We confirmed the presence of the mid-IR exciton bands and the 1.7 eV band which appeared by the doping and assigned them to the transition from the deeply lying O 2p energy bands to the hole in the uppermost O 2p energy band. The mid-IR exciton bands show an anomalously strong intensity, which comes from the coupling of the exciton and the charge fluctuation of the electron.

Magnetization curves of large single crystals of (La1-xSrx)2CuO4 (x=0.065 and 0.07) were measured at temperatures from 4.2 to 30K under magnetic fields of up to 9T. The crystals utilized had long dimensions in either the c-axis or the a-axis direction along which the magnetic field was applied, in order to minimize the demagnetization effect. The analysis of the hysteresis data revealed strong anisotropy in flux pinning and critical current density, Jcab(H//c), Jcab (H//ab) and Jcc (H//ab). It was found that the field and temperature dependence was most pronounced for Jcab(H//c) and discussion will be made for this particular case in terms of the conventional scaling law.

Flux jumps have been observed in a large (138 mg) single crystal of La 1.86 Sr 0.14 CuO 4 . Both complete and incomplete flux jumps are observed with the energy dissipated by complete flux jumps consistent with temperature rises to nearly T c . The systematics of these flux jumps with respect to field history, temperature and the time duration of the hysteresis measurements are reported. The field at which flux jumps first occur and their subsequent spacing are not consistent with a simple adiabatic theory of flux jumps for our experimental conditions. Instead, the crystal is observed to be stabilized against flux jumps by flux creep which occurs during the time interval over which field steps are accomplished as well as during imposed time intervals between field steps.

The ionic-electronic conducting compounds of lithium vanadium bronzes, copper vanadium bronzes, and Mo doped vanadium bronzes were obtained from molten LiVO 3 − V 2 O 5 system, V 2 O 5 − Li 3 PO 4 system, V 2 O 5 − CuCl system, and V 2 O 5 − Li 2 MoO 4 system by electrolysis. The electrolysis conditions were determined from the cyclic voltammograms of electrolytic bath at 650°C to 800°C. The mixture melts of V 2 O 5 containing 9.1 mol% Li 3 PO 4 were used as the ideal composition of electrolytic bath for β-Li z0.36 V 2 O 5 crystallization. The lithium contents of β-Li 0.36 V 2 O 5 cystals were controlled with current density and composition of electrolytic bath. The crystals of Li 0.29 V 1.82 Mo 0.18 O were obtained from electrolysis of V 2 O 5 melts containing 25 mole% Li 2 MoO 4 . The activation energy of conductivity for Mo doped lithium vanadium bronzes was less than the non Mo doped bronzes.

Phase diagrams of the Nd2O3-CuO and (Nd. Ce)2O3-CuO systems were investigated by differential thermal analysis. It was found that Nd2CuO4 and Nd2-xCexCuO4 melt incongruently at 1270±5 °C and 1315±5 °C respectively, and the solids of Nd2CuO4 and Nd2-xCexCuO4 exist in equilibrium with the liquid of 79–91 mol% CuO, respectively. On the base of these results, Nd1.86Ce0.14CuO4 single crystals were grown by the traveling solvent, floating zone (TSFZ) method. The as-grown crystals were 5 mm in diameter and 50 mm in length with a lusterless black, and had a few subgrains. The Meissner signal was not observed from the as-grown crystals, but it was observed below 19 K from the crystals annealed in N2 at 900 °C for 70 hours.

A novel resonant ultrasound technique is used to measure all elastic constants and their temperature dependence in single-crystal La{sub 1.86}Sr{sub 0.14}CuO{sub 4}. The in-plane shear modulus {ital c}{sub 66} drops by nearly half upon cooling to 227 K, an effect attributed to order-parameter fluctuations (of 2D Gaussian character) near the tetragonal-orthorhombic phase boundary. Implications for current models of the structural phase transition are discussed, as well as the effect of domains in the low-temperature orthorhombic state.

Single crystals of beryl grown by slow cooling from an impurity doped flux without a seed crystal have been grown and their habit modifications studied by SEM and optical microscopic observation. Transition metal doping using Fe 2 O 3 , Cr 2 O 3 , Mn 2 O 3 and alkali metal doping experiments were also carried out. Two characteristics, (1) the ratio (H/W), height to width of the crystals and (2) combination and relative development of crystal faces, were used as a measure of habit modifications. In general, as impurity doping increased, the crystals became slender and the second order prism disappeared. The ratio H/W was less in the larger crystals. This tendency can be attributed to the selective incorporation of impurity atoms on certain faces.

Single crystals of SrTa 2 0 6 and Ba 0.1 Sr 0.9 Ta 2 O 6 were grown by the floating zone method using an infrared heated furnace. The grown crystals of SrTa 2 0 6 and Ba 0.1 Sr 0.9 Ta 2 O 6 were about 5 mm in diameter and up to 35 mm long. The crystals of SrTa 2 O 6 showed a transition from the tetragonal structure to the orthorhombic structure at about 600 °C, whereas the crystals of Ba 0.1 Sr 0.9 Ta 2 O 6 exhibited the tetragonal structure without a phase transition. The dielectric constants ∈ of the SrTa 2 O 6 single crystals were independent of frequency and anisotropic with ∈ α =108 and ∈ c =128. However, ∈ c decreased as frequency increased for the Ba substituted crystals.

We measured Raman scattering below 400 cm−1 in Cu1−xZnxGeO3 (x = 0, 0.013, 0.017, and 0.0212) to study the Zn-doping effect in the spin-Peierls phase. When Zn atoms (S = 0) are doped, a two-magnetic-exciton bound state disappears in the spin-Peierls phase. In the sample of x = 0.0212, the folded phonon mode at 370 cm−1 and the two-magnetic-exciton Raman spectra below 250 cm−1 were observed even in the uniform phase. In pure CuGeO3, the quasi-elastic scattering was observed at high temperatures. The origin of this scattering is considered as a spin fluctuation interacting with lattice.

We have measured two phonon-dispersion curves above 50 meV in a superconducting (T c = 33 K) single crystal of La 1.85 Sr 0.15 CuO 4 . In the normal phase (T = 45 K), we show that the Λ 1 mode compatible with the Raman A 1g (425 cm -1 ) mode is weakly dispersive, falling from 53.7 meV at the zone center to about 51 meV at the zone boundary along the tetragonal (0,0,ξ) direction. For the Λ 1 mode compatible with the infra-red A 2u (480 cm -1 ) mode, we observe a significant dispersion of the phonon energy which drops monotonically from 60 to 56 meV. The global features of these dispersion curves are similar to those previously reported in studies of both pure and 10% Sr doped La 2 CuO 4 . However significant differences in the shape of the dispersion of the highest Λ 1 branch is shown, as well as a significant softening with increasing carrier concentration.

Pyrochlore type Na0.39Bi3.47O7 containing Bi5+ was prepared by low temperature hydrothermal reaction using hydrated sodium bismuth oxide, NaBiO3⋅nH2O, as a starting material. The mean valence of Bi in the pyrochlore is 3.94. The X-ray powder pattern for the pyrochlore was indexed in the cubic system with a=10.943(8)A. The crystal structure was refined with the X-ray powder Rietveld method. The final R-factors, RWP, RP and RB were 13.3, 9.5 and 12.8%, respectively. The pyrochlore decomposes above 380°C with two steps of weight losses caused by complete reduction from Bi5+ to Bi3+. The Na-pyrochlore exhibited high electric resistivity (-107Ω⋅cm) at room temperature.

Surface of La2CuO4 single crystals were oxidized by KMnO4 aqueous solutions. The oxidation conditions such as KMnO4 concentration, soaking time and solution temperature of KMnO4, were systematically controlled. The magnetic properties of the crystals were drastically dependent on the oxidation conditions. The superconducting transition properties under the optimum condition, 48 h, 150°C and 5 wt%, were T c = 28 K and ∆T c = 2K. The chemical oxidation was limited only to the surface region of the crystals. Therefore, chemical oxidation may be useful for the preparation of superconducting single-crystalline films.

La2-xBaxCuO4 (LBCO) single crystals were grown under oxygen partial pressure (Po2) of 1~200 kPa by the traveling solvent floating zone (TSFZ) method. The Ba solubility in the LBCO crystals was maximum for growth under about 100 kPa, and was decreased with deviating Po2 from 100 kPa. The Ba solubility in the LBCO grown crystals was increased up to 0.19 under 100 kPa, but the grown crystals contained subgrains and inclusions of La4BaCu5O13 and CuO. The subgrain- and inclusion- free single crystals of LBCO with the Ba concentration x≤0.113, the typical size of 5 mm in diameter and 30 mm in length, were obtained under 100 kPa.

Precision lattice parameter in La1.85Sr0.15CuO4 single crystal superconductor has been measured in a wide temperature range between 4.2 K and room temperature by the X-ray Bond’s method. An anomalous lattice contraction associated with the appearance of the superconductivity was found in c-plane. Along c-direction, on the contrary, an anomalous expansion of the lattice was observed. The relative spontaneous change in the lattice parameter in c-plane was 2.3xl0­-5 and that of c-axis was 1.5×l0-5 at 4.2 K. The temperature dependence of the spontaneous lattice change was compared with currently existing theories for high Tc superconductivity.

We report normal-state electrical resistivity of La2-xBaxCuO4-δ single crystals with x=0.06, 0.08 and 0.09, grown by TSFZ method. The resistivity is non-metallic both in ab-plane and along c-axis in as-grown crystals. In-plane metallic resistivity evolved by annealing in oxygen. This result indicates the oxygen deficiency in as-grown crystals. The perturbation of the CuO2 sheet potentials due to oxygen-vacancies in the crystal lattice possibly induces semiconducting behavior in as-grown crystals. Filling the vacancies by annealing in oxygen relieve the perturbation of the sheet potentials, results in metallic resistivity in the ab-plane. A good fit of the experimental data found with RVB model.

By measuring the ultrasound velocity for various modes, we found unusual temperature dependence of the elastic coefficients in single-crystalline La2−xSr x CuO4 with x = 0.09, 0.14 and 0.19. The longitudinal elastic coefficients show a rapid drop in La2−xSr x CuO4 at Tc, as is seen in conventional superconductors. As the temperature decreases, however, the lattice shows gradual hardening in La2−xSr x CuO4 both for the longitudinal and transverse modes of ultrasound in contrast to most of the conventional superconductors, in which the lattice continues to soften in the superconducting state. According to thermodynamic analysis, the temperature dependence of the elastic coefficients below Tc suggests that the second strain derivative of the superconducting condensation energy d2 O /d e 2 is negative in La2−xSr x CuO4, whereas the sign is positive in the conventional superconductors. An important possibility is that this unusual lattice hardening in the superconducting state is a common character of the high-Tc copper oxides.

Phase relations between La2-xSrxCuO4 solidus and liquidus were determined by TG-DTA and a slow-cooling experiment. Single crystals of high Sr-doped La2-xSrxCuO4 (x>0.15) were grown by the TSFZ method on the basis of the phase relations. The grown crystals, 5 mm in diameter and about 30 mm in length, were crack-free and black with a metallic luster. The grown crystals of x=0.20 had no inclusions and subgrains. However, the grown crystals of x=0.25 contained subgrains, and those of x=0.30 had inclusions of La2-xSr1+xCu2O6+δ.

We studied the superconducting properties of an electron irradiated La1.85Sr0.15CuO4 single crystal. Point-like defects are introduced into the crystal with the irradiation. The irradiation decreases the correlation between CuO2 layers. However, both before and after the irradiation, the flux creep activation energy Ueff (J,T,H) can be expressed the same formula as Ueff (J,T,H) = const.*In(J)*(1-T/Tc)n/H, where n is a constant determined from the irreversibility line.

Bulks of YBCO superconductor were prepared by melt process with seeding single grains of YBCO and SmBCO superconductors and single crystals of SrTiO3 and MgO. The seedeing of YBCO single grain and MgO single crystal was effective for the growth of single-grained bulk of YBCO superconductor. The maximum size of the single-grained YBCO bulk was 45 mm in diameter and 15 mm in thickness.

In order to estimate the magnetic shielding effect of YBCO bulk at macroscale, spatial distribution of magnetic field was measured with a Holl-probe. The shielding effect of zero field cooled sample was measured under magnetic fields (0.5,1,2 kOe). Crystallographic analysis were carried out by Laue method and optical microscope. The YBCO bulk prepared by MMTG method has domain structure. Both low-angle (misorientation of 10°) grain boundaries among macrograins (~cm) are observed. Our experimental results show that high-angle grain boundaries make week link and hence magnetic flux invades along this boundaries.

La2CuO4 single crystals grown at an oxygen pressure of 0.2 MPa by TSFZ method are superconducting below 32 K, and show a semiconducting behavior in nonsuperconducting state. The single crystals of La2Cu04 are changed from superconductors to semiconductors by annealing in argon, and are returned to superconductors by annealing at ambient pressure of oxygen. Therefore, superconductivity of the La2CuO4 single crystals is due to excess oxygen.

Magnetic susceptibility of superconducting single-crystal La2−x Sr x:CuO4 (x = 0.12, 0.14 and 0.19) was measured with a SQUID magnetometer from 30 to 300 K under the magnetic fields (H) of 1 ~ 5 T. The field- dependent susceptibility is evident below 40 K under H perpendicular to the CuO2 planes and is assigned to the superconducting fluctuation-induced diamagnetism (SFD). The field-independent part was subtracted by taking the difference of the susceptibilities for the different H. The field-dependent part was fitted by using the two-dimensional form of the Lawrence-Doniach model for SFD. The magnetic-field dependence of the superconducting-transition temperature was obtained by this analysis. The values of the in-plane coherence length at 0 K are determined as 28.3, 29.2 and 32.5 A for the samples of x = 0.12, 0.14 and 0.19, respectively.

Tunneling spectroscopy studies have been performed on differently doped la 2−x Sr x CuO 4−δ single crystals by break junction technique. By a fine adjustment of the mechanically controlled spacing between the two fractured electrodes, we were able to investigate highly stable conductance characteristics along the ab -planes in various tunneling regimes. In the weak link regime, Josephson effect was clearly observed with critical current decreasing monotonically with temperature and vanishing below the crystal's T c . By increasing the electrode's distance, in the quasiparticle regime, a well developed zero bias conductance peak and smeared gap-like structures were observed. All these features are consistent with the d-wave model for high T c superconductors.

We studied La 1.85 Sr 0.15 CuO 4− δ single crystals by tunneling spectroscopy both in planar and in break junction configurations. In the c -axis oriented planar junctions, with natural barriers and evaporated Pb film counterelectrodes, we observed at low-temperature gap-like structures around ±7–10 meV. In the a , b oriented break junctions we found conductance structures at ±15–20 mV, an increasing background at high biases and a conductance peak at zero voltage. The presence of this peak can be interpreted in terms of d-wave component for the order parameter in this material.

Magnetization curves of (La1−xSrx)2CuO4 (x=0.065−0.07) single crystals were measured at temperatures from 4.2 to 35K under magnetic field of up to 9T. The field was applied along the a- or the c-axis of the two types of the crystals having different macroscopic geometry. The analysis of the hysteresis data revealed strong anisotropy in flux pinning and critical current density. At 4.2 K and 5 T, for example, Jc//ab-plane(H//c-axis), Jcc(H//ab) were determined to be 2.1×105, and 3.0×104 A/cm2, respectively and Jcab(H//ab) was estimated to be 4.2×105 A/cm2. It was also found that the field and temperature dependence was quite appreciable in Jc//ab(H//c), as expected from the generally observed behavior in cuprate-based HTSC oxides.

Single crystals of La2CuO4 5 mm in diameter and 50 mm in length were grown at an oxygen pressure of 0.2 MPa by the traveling solvent floating zone method (TSFZ method). The as-grown crystals of La2CuO4 were superconducting, but were a mixture of a superconducting phase and an antiferromagnetic phase. The single crystals of La2CuO4 were changed reversibly from superconductors to nonsuperconductors by annealing. In the nonsuperconducting state, the single crystals of La2CuO4 were semiconducting and had a highly anisotropic resistivity, where the resistivity ratio ρ ⊥/ρ ‖ was about 28,000 at RT.

Ultrasonic measurements were performed on single crystals of La 2− x Sr x CuO 4 (LSCO) with x = 0.09, 0.14 and 0.19. We found that the elastic constants of LSCO increase well below the superconducting transition temperature T c , in contrast with softeniing observed in conventional superconductors. The difference originates from the change in superconducting condensation energy o under the lattice strain e i induced by a sound wave.

Nd 1.85 Ce 0.15 CuO 4 single crystals of improved quality were grown by TSFZ method at low oxygen partial pressure of 0.2, 0.1, 0.08 and 0.01 atm. Superconductivity in as-grown state was observed in the crystals grown at 0.08 and 0.01 atm. The improvement of Ce distribution was achieved due to decreased value of Ce distribution coefficient at low oxygen partial pressure in ambient atmosphere. The probability to obtain the same characteristics of the superconducting transition in as-grown state as that in the crystals after post-growth high temperature treatment seems to be very low because of oxidation during the cooling of the grown crystals from the crystallization temperature to the room temperature.