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Alkali metal fullerides (A x C 60 ) are a subject of considerable current interest because of the occurrence of superconductivity for A x C 60 at temperatures surpassed only by the high T c copper oxides. The preparation and characterization of A x C 60 (A = alkali metal, x = 2,3,4,6) by powder X-ray diffraction, NMR ( 13 C, 23 Na and 87 Rb), and d.c. magnetization are reported. The structures are described as intercalation compounds of the FCC structure of pristine c 60 or of hypothetical BCC or BCT structures. The structures and phase diagrams can be rationalized on the basis of ion size and electrostatic considerations. Only the A 3 C 60 compounds are metallic (and superconducting). The superconducting T c increases nearly linearly with unit cell size. EHT (Extended Huckel Theory) calculations and 13 C NMR relaxation measurements indicate higher densities of states for the higher T c compositions.

Isochronal anneal sequences have been carried out on pnp and npn transistors irradiated with fast neutrons at a variety of fluences. The evolution of base and collector currents was utilized to characterize the annealing behavior of defects in both the emitter-base depletion region and the neutral base. Various annealing biases, theoretical modeling, as well as previous deep level transient spectroscopy (DLTS) data, were used to assign the relative magnitude of each of the important defects to the total recombination current. We find that donor-vacancy pairs in the neutral n-type base of our pnp transistors are responsible for about 1/3 of the postdamage lifetime degradation, while the remaining recombination currents can be largely attributed to a cluster-related divacancylike defect which has no shallow state DLTS emission peak. This latter defect anneals gradually from 350 to 590 K. Generation/recombination currents in the base-emitter junctions in both types of devices were found to anneal in a simila...

We have explored defect annealing in radiation damaged silicon in a regime characterized by defect clusters and higher doping. Several types of pnp and npn Si bipolar transistors have been irradiated with ions and neutrons, then isochronally annealed from 300 to 600 K to study the evolution of deep level transient spectroscopy (DLTS) defect signatures. Variations in these data with radiation environment, Fermi level, annealing temperature, and doping density have been used to separate the contributions of three dominant defects to the DLTS defect spectra. We find that the normal Si divacancy and a divacancylike defect with similar properties make similar contributions to a DLTS peak normally associated with transitions from the single minus charge state of the divacancy. However the latter defect is clearly associated with the presence of defect clusters. The vacancy-donor center can also contribute to this high temperature DLTS signature, and its relative importance can be quantitatively assessed by vary...

Single-crystal epitaxial thin films of the isotropic metallic oxides Sr1-xCaxRuO(3) (0/= x/= 1) were grown on miscut SrTiO(3)(100) substrates in situ by 90 degrees off-axis sputtering. These thin films exhibit low isotropic resistivities, excellent chemical and thermal stability, good surface smoothness, and high crystalline quality. Furthermore, the lattice parameters and magnetic properties can be varied by simply changing the strontium/calcium ratio. These epitaxial thin films, and their multilayer structures with other oxide materials, can be used for the fabrication of superconducting, ferroelectric, magneto-optic, and electro-optic devices.

Previous studies looking at the sensitivity, specificity, and predictive accuracy of single photon emission computed tomography (SPECT) have been based upon the results obtained by visual interpretation of coronary arteriograms. Since the results of visual and quantitative determination of percent diameter stenosis have been shown to be statistically different, the results obtained from SPECT imaging when compared to quantitative methods for assessing coronary artery disease would be expected to provide a more correct assessment of sensitivity, specificity, and predictive accuracy. To determine the "true" sensitivity, specificity, and predictive accuracy of SPECT in diagnosing coronary artery disease, this study compared the results obtained in 44 SPECT images (20 thallium and 24 teboroxime) with the results obtained when quantitative coronary arteriography was used to analyze the coronary arteriograms. These 44 cases were then compared against 8 different definitions of significant coronary artery disease, varying from 30 to 80%, to yield 352 comparisons. The maximum specificity and predictive accuracy was found when 45% diameter stenosis was used to define the presence or absence of significant disease. At 45% diameter stenosis, SPECT imaging demonstrated an 86% sensitivity, 78% specificity, and 94% predictive accuracy with only 6% false positives. In 100% of the cases where 45% diameter stenosis was used to define the presence of disease and exercise failed to demonstrate ST segment changes or angina, when SPECT imaging demonstrated a perfusion defect(s), quantitative coronary arteriography agreed with SPECT imaging results.(ABSTRACT TRUNCATED AT 250 WORDS)

The fullerenes C60 and C70 act as spherical building blocks in crystalline solids to form a variety of crystal structures. In many cases, the icosahedral molecular symmetry of C60 appears to play little role in determining the crystal structure. In this chapter we discuss our results on the crystallography of pure and solvated fullerenes and some general features of fullerenes as building units in crystalline solids. For pure C60 or C70, the face-centered cubic arrangement is preferred. In solvated crystals and compounds, the packing readily adapts to form non-close-packed structures.

Thirty subjects underwent single photon emission computed tomography (SPECT) imaging with technetium-99m teboroxime (TEBO). Of these, 26 underwent thallium SPECT imaging and 25 underwent quantitative coronary arteriography (QCA). Twenty-one of the subjects underwent all three studies. SPECT images were reviewed by two independent observers blinded to the clinical data. Stenoses were considered significant if there was a greater than or equal to 50% diameter narrowing as defined by QCA analysis of the coronary arteriograms. The overall sensitivity and specificity was 78% and 78%, respectively, for thallium-201. The overall sensitivity and specificity for teboroxime was 72% and 80%, respectively. The results obtained for these two tracers were not statistically different. Some of the false-positive results obtained from teboroxime imaging appear to have been due to the 10-min acquisition protocol and can be reduced with the use of new software programs using a continuous 3-min acquisition and dipyridamole. Teboroxime's rapid biologic half-life allows completion of SPECT imaging within 60-90 min, compared with the minimum of 4 h required for thallium SPECT imaging.

The results of (13)C nuclear magnetic resonance (NMR) measurements on alkali fullerides KxC(60) are reported. The NMR spectra demonstrate that material with 0x3 is in fact a two-phase system at equilibrium, with x = 0 and x = 3. NMR lineshapes indicate that C(3-)(60) ions rotate rapidly in the K(3)C(60) phase at 300 K, while C(6)-(60) ions in the insulating K(6)C(60) phase are static on the time scale of the lineshape measurement. The temperature dependence of the (13)C spin-lattice relaxation rate in the normal state of K(3)C(60) is found to be characteristic of a metal, indicating the important role of the C(3-)(60) ions in the conductivity. From the relaxation measurements, an estimate of the density of electronic states at the Fermi level is derived.

We have evaluated the biodistribution of human low-density lipoprotein (LDL) radiolabeled with 99mTc or with 123I-tyramine cellobiose in rabbits and in rhesus monkeys. Biodistribution was assessed after intravenous injection of radiolabeled LDL by quantitative analysis of scintigrams, counting of excreta, and counting of tissues at necropsy. Both rabbits and monkeys showed lower renal uptake (123I:99mTc approximately 1:3, as regional percent injected activity corrected for physical decay) and excretion (1:2 to 1:4), but higher hepatic (1.5:1 to 2:1) and cardiac (1.7:1 to 4:1) uptake of 123I than of 99mTc. Adrenals were visualized in normolipemic animals with 123I-tyramine cellobiose-LDL but not with 99mTc-LDL. Hyperlipemic animals showed increased cardiac (up to six-fold) and decreased hepatic activity (by 50%-60%) of both radionuclides. We conclude that 123I-tyramine cellobiose-LDL is better suited than 99mTc-LDL for dynamic studies of LDL metabolism in vivo.

Proton irradiation of single-crystal Ba2YCu3O7−δ reproducibly increases the critical current tenfold, but does not alter the activation energy for flux creep. This can be accounted for in terms of a conventional flux pinning model. Based on this interpretation, as well as circumstantial structural evidence from TEM and X-ray studies, we propose that the chief effect of low-fluence proton irradiation is to create a higher density of oxygen point defects (which exist even in undamaged crystals). This conclusion is supported by observations of the effect of irradiation on electronic parameters.

Two deep level transient spectroscopy (DLTS) electron emission signatures, previously labeled E4 and E5, have been shown to be bistable with respect to minority carrier injection at room temperature. These result from two charge state transitions of the same defect. We have performed DLTS measurements as function of annealing between 350 and 680 K, using minority carrier injection after each annealing stage to make E4 and E5 visible. We show that the E4–E5 pair is associated with defect clusters which dominate after neutron or ion damage with annealing characteristics that closely parallel to those of silicon divacancies found in damage clusters. At annealing temperatures above 500 K, the E4–E5 pair ceases to be bistable and exists after anneals in thermal equilibrium. We show that the stable E4 peak appears to be the same emission signature previously labeled the L center. The transformation of the E4–E5 bistable pair into the stable L center and a stable E5 companion level occurs at the same temperature...

We have grown crystals of the carbon structure C60 by sublimation. In contrast to solution-grown crystals, the sublimed crystals have long range order with no evidence of solvent inclusions. Sublimed C60 forms three dimensional, faceted crystals with a close-packed, face-centered cubic unit cell. We have refined a crystal structure using the “soccer ball” model of the C60 molecule. The results indicate that the C60 molecule has the expected spherical shape, however the data are not sufficiently accurate to unambiguously determine atomic positions.

We have exposed silicon bipolar transistors to fast neutrons and characterized the properties of the resulting defects using capacitance-based spectroscopy of the n-type collector. We have performed low-temperature electron capture measurement of the divacancy (=/−) and vacancy-oxygen (−/0) defects after the samples were annealed from 350–500 K. We show from a simple rate equation analysis that one can define an unambiguous test for cluster-induced reductions of defect level occupation due to slow capture. This allows easy identification of deep level transient spectroscopy (DLTS) levels where the capture is inhibited due to band bending. Our measurements show extremely long, temperature-dependent capture times for the doubly charged state of the divacancy. We have modeled the capture dynamics as a function of annealing using a simple electrostatic band-bending approach coupled with a realistic simulation of the cluster size and shape distribution as estimated from computer simulation of the damage cascad...

Using deep level transient spectroscopy, the authors have measured the defect spectrum in the collector of a n-p-n bipolar transistor following fast neutron irradiation as well as the gain on the same device. They show that a slow change observed in both the gain and deep level traps in the n-type collector at 300K are bistable. The transistor gain and the defects can be returned to the postirradiation condition by forward bias at room temperature, i.e., by operating the transistor (gain) or injection through the base-collector diode (defect spectrum).

Deuterated p-type GaN(Mg,H2) films were irradiated at room temperature with 1MeV protons to create native point defects with a concentration approximately equal to the Mg doping (5×1019cm−3). The samples were then annealed isothermally at a succession of temperatures while monitoring the infrared absorption due to the H local mode of the MgH defect. As the samples were annealed, the MgH absorption signal decreased and a new mode at slightly higher frequency appeared, which has been associated with the approach of a mobile nitrogen interstitial. We used the time dependence of the MgH absorption to obtain a diffusion barrier of the nitrogen interstitial in p-type GaN of 1.99eV. This is in good agreement with theoretical calculations of nitrogen interstitial motion in GaN.

We show that fee C60 is converted to at least two different structures by high temperature and pressure. These new phases are stabilized by temperature-quenching. The synthesis condition of P=5GPa and 300°C

This work is a systematic study of carbon incorporation in Ta2O5 and its effect on the material and electrical properties of Ta2O5, a promising replacement for silicon oxide in embedded dynamic random access memory applications. Using pulsed-dc reactive and rf-magnetron sputtering of Ta2O5 performed in an argon/oxygen/carbon-dioxide plasma, we have methodically doped the Ta2O5 films with carbon. In thick (70 nm) Ta2O5 films, an optimal amount (0.8–1.4 at. %) of carbon doping reduced the leakage current to 10−8 A/cm2 at +3 MV/cm, a four orders of magnitude reduction compared to a leakage current of 10−4 A/cm2 in an undoped Ta2O5 film grown in similar conditions without CO2 in the plasma. This finding suggests that carbon doping can further improve the dielectric leakage property at an optimal concentration. X-ray Photoemission Spectroscopy analysis showed the presence of carbonate (carbon bonded to three oxygen) in these electrically improved carbon-doped films. Analysis by high-resolution transmission ele...

We report the synthesis by molecular beam epitaxy of a new insulating phase in the barium bismuth oxide system, BaBi 2 O y , and its use as a building block for bismuth oxide heterostructures.

THE discovery of conductivity1 in AxC60 (where A represents an alkali metal) and superconductivity2 in KxC60 has been followed by reports of superconductivity in other alkali-metal-doped full-erides with transition temperatures as high as 33 K (ref. 3). Elucidation of phase diagrams and understanding the relationship between structure and superconducting properties is essential to a detailed understanding of superconductivity in these systems. So far, structural data have been reported only for the non-conducting, intercalated body-centred cubic (b.c.c.) structures A6C60 (where A is K or Cs; ref. 4), a body-centred tetragonal structure for A6C60 (where A is K, Rb, Cs; ref. 5) and the superconducting, intercalated face-centred cubic (f.c.c.) material K3C60 (ref. 6). Here we report the preparation of a series of single-phase, isostructural f.c.c. superconductors with composition A3C60 (where A is K, Rb, Cs or a mixture of these), and show that Tcc increases monotonically with the size of the unit cell. Extended Hiickel band-structure calcula-tions also show a monotonic increase in the density of states at the Fermi level, N(EF), with lattice parameter. The primary impli-cation of these results is that all A3C60 superconductors have the same structure and that changes in Tc can be accounted for by changes in N(EF).

We report extended Huckel theory (EHT) band structure calculations for the high-temperature polymorph of α-6T (α-sexithiophene) and C60, two organic materials that have shown promising device characteristics, as the active element in thin-film transistors (TFTs). We also report calculations for α-3T and κ-(ET)2Cu(NCS)2[where ET = bis(ethylenedithio)tetrathiafulvalene] for comparison purposes. Both α-6T/HT and C60 show well developed band dispersions. Whereas C60 is an isotropic three-dimensional (3D) molecular solid, α-6T/HT shows a strongly 2D electronic structure, and the band structure is quite similar to that of the ET organic superconductors such as κ-(ET)2Cu(NCS)2. Factors important in determining the mobilities of organic semiconductors are reviewed.

We have investigated the properties of NiFe2O4 thin films prepared by laser ablation of a stoichiometric NiFe2O4 target. Textured polycrystalline films were obtained on a‐SiO2 as well as on various substrates with Au, Ag, Pt, and MgF2 buffer layers. Epitaxially oriented films were obtained on MgO, (1102)‐oriented Al2O3, (1120)‐oriented Al2O3, Y‐stabilized ZrO2 (YSZ), and SrTiO3, although the crystalline quality of the films varied. Contamination by diffusion from the substrate and strains induced by both lattice constant mismatch and differential thermal expansion degraded the magnetic properties of the films, and in some cases decreased the electrical resistivity as well. By choosing the right substrate (YSZ), temperature (600 °C), and PO2 (0.01 mT), we are able to prepare epitaxial films with bulk saturation magnetization (Ms=270 G) and fairly low anisotropy (K∼105 erg/cm3) as inferred by torque magnetometry. These films and bilayers are expected to be useful in a variety of fundamental investigations...

Nature 352, 701-703 (1991). THE positions of the alkali atoms, originally reported as (0,0.5, z) with z = 0.28, should read (x,0.5,0) with x = 0.22. These are the positions used in the calculations for Table 1 and the positions illustrated in Fig. 2.

The synthetic conditions for the preparation of KCu4S3, K3Cu8S6, and KCu3S2 have been carefully defined. K3Cu8S6 is apparently a kinetic phase and must be trapped with a minimum amount of either KCu4S3 or KCu3S2. Magnetic susceptibility and electrical resistivity data display behavior typical of CDW systems. In view of the low-dimensional metallic nature of K3Cu8S6 and preliminary X-ray data, a second order reversible transition at T sub 1 = 152 + or - lK has been identified as the onset of an incommensurate lattice. In the region of 50-60K, a first order transition with considerable hysteresis seems to be a structural transition to a commensurate superlattice state.

A detailed study has been made of interdiffusion in (GaAs)n(AlAs)m multilayer structures grown by molecular beam epitaxy. The subscripts n and m indicate that the structure is a repeating sequence of n monolayers of GaAs followed by m monolayers of AlAs. The time dependence of the Fourier components of the composition modulation at a constant annealing temperature was obtained by measuring the intensities of the superlattice satellites by x‐ray diffraction. For a sample with (n,m) = (12,9) at a temperature of 860 °C, the data can be fit by a composition‐dependent diffusion coefficient of the form D (C) =8.8×10−20 exp(αC) cm2/sec, where α=2.06 and C is the gallium concentration. The parameter α is proportional to the difference in the diffusion activation energy in GaAs and AlAs. An α of 2.06 at 860 °C corresponds to an energy difference of 0.201 eV. Calculations which assume vacancy diffusion via the arsenic sublattice give an energy difference of 0.25 eV in reasonable agreement with the experimental valu...

The compounds Ta2NiS5 and Ta2NiSe5 have a novel layered structure: within the layers are rows of tantalum atoms separated by rows of nickel atoms. The lower dimensional structure of these compounds as well as the expected d1 non-bonding electron configuration of the tantalum atom leads to the expectation that classical charge density wave behavior would be observed in their properties. An examination of the electrical, magnetic and structural properties shows that this is not the case; rather, the properties are more consistent with the materials being small band gap semiconductors. Further, both compounds show structural phase transitions: in Ta2NiS5 a first-order transition occurs at 28 K and in Ta2NiSe5 a second-order transition occurs at 328 K. The latter transition is shown by electron microscopy to be martensitic-like.

We have obtained anisotropic thermal parameters for ${\mathrm{Ba}}_{2}$Y${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ from a dual-wavelength x-ray crystal structure determination on a single crystal with a small amount of twinning. Anharmonic thermal motion was found for both Ba and Cu atoms with the largest anharmonic feature attributed to a Ba-Ba breathing mode. The anisotropic and anharmonic thermal parameters are consistent with the coordination polyhedra as well as the bonding distances involved.

X-ray scattering studies of Nb${\mathrm{Se}}_{3}$ show the formation of two independent, incommensurate charge-density waves (CDW) with wave vectors ${\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}}}_{1}=(0, 0.243, 0)$ and ${\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}}}_{2}=(0.5, 0.263, 0.5)$ at ${T}_{1}=144$ K and ${T}_{2}=59$ K, respectively. Electric fields that suppress the resistive anomaly associated with the lower-temperature CDW have no measurable effect on either the CDW amplitude or wave vector. The field required for suppression appears to scale roughly with the defect concentration as measured by the residual-resistance ratio. These results are consistent with the Fr\"ohlich sliding-CDW model with impurity pinning.

Ambient temperature measurements of the crystallographic cell parameters for oxygen deficient Ba 2 YCu 3 O 7−δ prepared by gettered annealing indicate the presence of microscopic differences in the oxygen configuration at fixed δ dependent on annealing temperature. The loss of superconductivity with increasing oxygen deficiency is shown to be due to a step-like increase in length of the plane copper-apical oxygen bond, and not to the orthorhombic to tetragonal transition. The crystal structure of the 84K superconductor Bi 2 Sr 2 CaCu 2 O 8+δ is described, as is the stabilization of 110K superconductivity via partial Pb/Bi substitution with long time annealing.

We report high-resolution synchrotron x-ray-scattering measurements on two compounds with charge-density-wave (CDW) phase transitions, $2H$-${\mathrm{TaSe}}_{2}$ and ${\mathrm{NbSe}}_{3}$. In these materials the scattering from the CDW superlattice is typically more than 3 orders of magnitude smaller than that of the host lattice. The increased flux available with a synchrotron source allows one to study weak CDW scattering with a $Q$ resolution on the order of ${10}^{\ensuremath{-}4}$ ${\mathrm{\AA{}}}^{\ensuremath{-}1}$, which is 1 to 2 orders of magnitude better than that attainable with a rotating-anode source. In the case of $2H$-${\mathrm{TaSe}}_{2}$, we were able to measure finite widths of the CDW superlattice peaks and to determine precisely the temperature dependence of the CDW incommensurability near the commensurate-incommensurate transition. We have established a lower bound for the CDW coherence length in ${\mathrm{NbSe}}_{3}$, and we have shown that the sum of the two CDW wave vectors is not commensurate as previously suspected. We have also looked with higher precision for possible effects of the electric field on the CDW structure in ${\mathrm{NbSe}}_{3}$.

We have observed evidence of finite-range order in quenched crystals of nominal composition ${\mathrm{Ba}}_{2}$${\mathrm{YCu}}_{3}$${\mathrm{O}}_{6.7}$. X-ray scattering shows diffuse peaks with a reduced wave vector of (1/2,0,0) in the oxygen-deficient compound but not in fully oxidized ${\mathrm{Ba}}_{2}$${\mathrm{YCu}}_{3}$${\mathrm{O}}_{7}$. Although the crystals are twinned, we find that the diffuse satellites occur symmetrically around (h,0,0) peaks rather than (0,k,0) peaks indicating a doubling of the unit cell along the a axis (transverse to the Cu-O chains). The width of the diffuse peaks yields coherence lengths of 21, 16, and 9 A\r{} for the ${a}_{0}$, ${b}_{0}$, and ${c}_{0}$ directions, respectively.

High-pressure x-ray scattering studies of $2H$-Ta${\mathrm{Se}}_{2}$ reveal reentrant behavior of the charge-density-wave (CDW) lock-in transition. As pressure is increased the normal-to-incommensurate CDW transition changes less than 5 K; however, the lock-in transition, initially suppressed to $T\ensuremath{\rightarrow}0$ K at \ensuremath{\sim}1.7 GPa, reappears at \ensuremath{\sim}2.5 GPa. At the highest pressures (4.4 GPa) the temperature dependence of the incommensurability approaches the universal curve given by Landau theory for a single-$\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}}$ CDW.

The magnetic structures of several single‐crystal, magnetic rare‐earth superlattice systems grown by molecular‐beam epitaxy are reviewed. In particular, the results of recent neutron diffraction investigations of long‐range magnetic order in Gd‐Y, Dy‐Y, Gd‐Dy, and Ho‐Y periodic superlattices are presented. In the Gd‐Y system, an antiphase domain structure develops for certain Y layer spacings, whereas modified helical moment configurations are found to occur in the other systems, some of which are commensurate with the chemical superlattice wavelength. References are made to theoretical interaction mechanisms recently proposed to account for the magnetic states of these novel materials.

We have mapped the temperature and composition dependence of the tetragonal-orthorhombic phase-transition temperature ${T}_{d}$ in the high-temperature superconductor ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Sr}}_{\mathrm{x}}$${\mathrm{CuO}}_{4\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ using high-resolution powder x-ray diffraction and measurements of the thermal expansion. The maximum volume fraction of ideal diamagnetism occurs in the composition range where ${T}_{d}$ is approaching the superconducting transition. In the composition range xg0.20 where no orthorhombic distortion is observed above 12 K, no bulk superconductivity is observed in ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Sr}}_{\mathrm{x}}$${\mathrm{CuO}}_{4\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$.

The monoclinic blue-bronze, K0.30MoO3, is an anisotropic metal that undergoes a charge density wave (CDW)-driven phase transition at 180 K, below which nonlinear current-voltage characteristics are observed. In this paper, we examine the anisotropy of the electric and magnetic properties of K0.30MoO3 and their relationship to observed CDW phenomena. We show that the conductivity is highest along the monoclinic b axis and 10 and 100 times lower in two directions perpendicular to b (in the ( 2 0 1 plane and perpendicular to these planes, respectively). In the CDW state, the conductivity along the b axis is nonlinear when the applied electric field exceeds a small threshold on the order of 50 mV/cm, but is field-independent in perpendicular directions. A periodic response to a dc bias was observed in one small K0.30MoO3 sample, however, the observation of an NbSe3-like response in K0.30MoO3 may depend on sample size or the quality of the electrical contacts to the sample.

We report the Raman spectrum of single crystals of Bi/sub 2.2/Ca/sub 0.8/Sr/sub 2/Cu/sub 2/O/sub 8+//sub delta/. The most prominent features are at 120, 295, 462, and 625 cm/sup -1/. The 462-cm/sup -1/ mode is tentatively assigned to axial O(3) symmetric stretching. The 625-cm/sup -1/ mode is assigned to planar Cu-O(1) bond stretching. The 120-cm/sup -1/ mode shows a strong a-b anisotropy. The effect of the b-axis superstructure upon the Raman spectrum is emphasized.

Superconducting Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-//sub x/ films were produced by metal molecular-beam epitaxy with one electron beam and two thermal cells and a molecular oxygen-gas source. The films were epitaxially grown on SrTiO/sub 3/(100) with an a-axis orientation perpendicular to the plane, and b and c axes ordered in the plane. A sharp resistive superconducting transition with T/sub c/(R = 0) at 82 K was obtained. A lower limit of the average J/sub c/ in the plane is 10/sup 3/ A/cm/sup 2/ at 77 K 5 x 10/sup 4/ A/cm/sup 2/ at 70 K, and 10/sup 6/ A/cm/sup 2/ at 4.2 K.

The dissipation observed at the superconducting transition of the high-${T}_{c}$ superconductor ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$Ca${\mathrm{Cu}}_{2}$${\mathrm{O}}_{8}$ is explained quantitatively by the Kosterlitz-Thouless theory of vortex-antivortex pair excitations within the Cu${\mathrm{O}}_{2}$ planes. This conclusion is drawn from the observation of an exponential square-root singularity in the resistivity and a power-law dependence of the resistivity on magnetic field. The Kosterlitz-Thouless phase transition (${T}_{c}=84.7$ K) is at the $\ensuremath{\rho}=0$ point, and the mean-field Ginzburg-Landau transition (${T}_{c0}=86.8$ K) at \ensuremath{\sim}25% of the normal-state resistivity.

Two phase transitions have previously been identified in K/sub 3/Cu/sub 8/S/sub 6/, a second-order transition at 153 K and a first-order transition at 55 K. In this x-ray scattering study we show that the 153-K transition, which is associated with a decrease in the conductivity, corresponds to the onset of an incommensurate charge-density wave (CDW) with a wave vector of q = (0,(1-delta)/2,0). The 55 K transition, which is accompanied by an increase in the conductivity, results in a significant modification of the host structure and the creation of a superlattice with a commensurate periodicity given by q = ((1/2),(1/2),0). The temperature dependence of the CDW order parameter is unusual. First, at temperatures just above the first-order transition, oscillations in the order parameter as a function of temperature are observed. Similar oscillations have previously been observed in the conductivity and the magnetic susceptibility. Second, like the transport properties, the order parameter shows a large hysteresis (approx.60 K) as the temperature is scanned through the first-order transition.

Measurements of the nonlinear conductivity in Nb${\mathrm{Se}}_{3}$ resulting from the high-temperature charge-density wave (CDW) have been made. The results show that the electronic properties of Nb${\mathrm{Se}}_{3}$ in the temperature regime where only one CDW exists are qualitatively the same as those in the the region where two CDW's coexist. In both cases a threshold field for non-Ohmic conduction and quasiperiodic noise are observed. These results rule out mechanisms for noise production which rely on the coexistence of two CDW's, such as beat phenomena. Measurements of the temperature dependence of the two threshold fields show a separate depinning of each of the two CDW's at temperatures below 59 K. No evidence for a lock-in of the two threshold fields to a common value has been seen.

Temperature-induced phase transitions in TlReO 4 have been investigated by Raman spectroscopy and by x-ray powder diffraction. The data reveal the occurrence of a hitherto unreported phase transition from orthorhombic to the tetragonal scheelite-type structure near 200 K and this is a first-order transition. At temperatures above 400 K the orthorhombic phase is known to transform to the scheelite-type structure. These phase changes present a unique situation. The lattice parameters of the low-temperature (LT) scheelite phase are a 0 =5.798 A and c 0 =12.920 A at 70 K and for the tetragonal high-temperature (HT) scheelite phase are a 0 =5.761 A, and c 0 =13.33 A at 400 K. The corresponding unit-cell volumes are 434 A 3 and 442 A 3 . For the orthorhombic phase a 0 =5.623, b 0 =5.791, c 0 =13.295, and V=432 A 3 at 293 K. The orthorhombic-LT-scheelite and the orthorhombic-HT-scheelite phase boundaries drawn on the basis of the ΔV's at the transition point intersect at negative pressures in a P-T diagram. This intersection appears to satisfy the criteria required for the occurrence of an "isolated point" in the P-T plane; namely a point at which two first-order phase boundaries intersect to become a continuous transition. This is probably the first experimental indication for such a possibility.

We have observed a periodic superlattice modulation in Nd 2 CuO 4 and in the “electron-doped” Nd 2 CuO 4−x F x and Nd 2−x Ce x− CuO 4 by electron diffraction and microscopy. The superlattice modulation characterized by a wave vector q=( 1 4 1 4 ,0) parallel to the Cu-O plane was found in both superconducting and non-superconducting samples, but most of the samples examined were not homogeneous with respect to the superlattice. The orthorhombic superlattice phase is found to exist in four possible variants with its c-axis parallel to the four equivalent [111] zone-axes in the underlying tetragonal lattice. Samples quenched from high temperature invariably showed no superlattice or superconductivity; bulk superconductivity and regions of superlattice were observed on annealing in the range 250–750°C. We believe that oxygen migration is responsible for this disproportionation into superlattice and nonsuperlattice regions.

We report the crystal chemical properties of solid solutions obtained by substitution of lanthanum for barium in the high-T/sub c/ superconductor Ba/sub 2/YCu/sub 3/O/sub 7/. An orthorhombic-to-tetragonal symmetry change occurs at x = 0.4 in Ba/sub 2-//sub x/La/sub x/YCu/sub 3/O/sub 7//sub +- //sub delta/ with the tripled perovskite supercell maintained. Charge compensation is achieved through reduction of the formal copper valence (delta = 0) in the orthorhombic phase and by a mixture of excess oxygen accommodation (delta>0) and formal valence reduction in the tetragonal phase. The resistivity increases by several orders of magnitude, even though the average formal copper valence changes very little.