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A package-integrated, compact, low-cost, athermal wavelength locker is introduced. It uses a novel etalon material, Schnaelite, to reduce environmental temperature sensitivity well over an order of magnitude compared to commercially available fused silica based lockers. Thermo-optical properties of temperature insensitive etalons are presented. Results demonstrating frequency stability of devices incorporating Schnaelite etalon based discriminators show optical frequency shifts below 1 GHz for large changes in case and laser temperature. This enabling technology promotes future small form-factor, low power consumption, multichannel laser sources.

We report the synthesis and structural characterization of a new oxoanion halide, NaCa 2 GeO 4 F. Crystal data: FW=235.74, Pnma (No. 62), orthorhombic, a =5.362(2), b =7.328(3), c =12.681(4) A, V =498.3(3) A 3 , Z =4, R ( F )=0.044, R w ( F )=0.046 for 404 averaged reflections. Compositionally, this material can be viewed as the end-member of the hypothetical solid solution Ca 2 GeO 4 ·(CaO) 1− x ·(NaF) x . However, in this unusual oxyfluoride, the fluoride ion orders and fully occupies one anion site. The structure consists of complicated condensed chains of six-coordinated cations centered by fluoride ions and linked by chains of isolated germanium oxide tetrahedra. The structure is best described in terms of the anion packing rather than the packing of cation-centered coordination polyhedra.

Quasi-low-dimensional (quasi-low-D) inorganic materials are not only ideally suited for angle resolved photoemission spectroscopy (ARPES) but also they offer a rich ground for studying key concepts for the emerging paradigm of non-Fermi liquid (non-FL) physics. In this article, we discuss the ARPES technique applied to three quasi-low-D inorganic metals: a paradigm Fermi liquid (FL) material TiTe 2 , a well-known quasi-1D charge density wave (CDW) material K 0.3 MoO 3 and a quasi-1D non-CDW material Li 0.9 Mo 6 O 17 . With TiTe 2 , we establish that a many body theoretical interpretation of the ARPES line shape is possible. We also address the fundamental question of how to accurately determine the k F value from ARPES. Both K 0.3 MoO 3 and Li 0.9 Mo 6 O 17 show quasi-1D electronic structures with non-FL line shapes. A CDW gap opening is observed for K 0.3 MoO 3 , whereas no gap is observed for Li 0.9 Mo 6 O 17 . We show, however, that the standard CDW theory, even with strong fluctuations, is not sufficient to describe the non-FL line shapes of K 0.3 MoO 3 . We argue that a Luttinger liquid (LL) model is relevant for both bronzes, but also point out difficulties encountered in comparing data with theory. We interpret this situation to mean that a more complete and realistic theory is necessary to understand these data.

Parallel synthesis together with high-throughput screening was used to identify candidate materials for integrated circuit applications that demand a superior high permittivity dielectric thin film. Specifically, we developed a continuous-composition spread (CCS) technique to synthesize much of a pseudoternary oxide system in a single deposition and used this in conjunction with a high-throughput measurement protocol, thereby allowing each chemical system to be deposited and evaluated in about 24 h. This approach led to the identification of compositions in the Zr-Sn-Ti-O system with promising properties. The same technique was used to determine the optimum compositions as a function of processing parameters. Films with the composition Zr .2 Sn .2 Ti .6 O 2 were then prepared using a conventional synthetic technique (on-axis sputtering) and were verified to have excellent properties. Thus, the CCS technique has demonstrated utility in rapidly identifying and developing a useful new material.

We describe the use of the laser-heated pedestal growth (LHPG) technique to prepare single-crystal fibers of potential solid-state laser and optical amplifier materials. Compared with other crystal growth approaches, including Czochralski and flux growth, the LHPG approach, which has very large thermal gradients at the growth interface, has fewer dislocations, resulting in crystals with improved optical quality. We discuss the use of the LHPG technique for the growth of spinel MgAl2O4:Ni2+ crystals. Such crystals are interesting as candidate materials for a 1.3 μm optical amplifier.

Barium and potassium hydroxide have been investigated as fluxes for the growth of cuprate single crystals. The relatively high solubility of transition metals and lanthanoids in these salt fluxes at moderate temperatures allows significant lowering of the growth temperatures required for many phases. Also, phases not stable at high temperatures become accessible. Two new cuprates have been prepared in the Ba–Ca–Cu–O and Ba–Y–Cu–O systems from a Ba(OH)2 · H2O flux. The compounds Ba3(Y0.23Cu0.77)2O5.78 and Ba3(Ca0.24Cu0.76)2O4.43 crystallize in a tetragonal (space group I4/mmm) oxygen deficient Sr3Ti2O7-type structure with lattice parameters a = 4.069(2) Å, 4.022(1) Å and c = 21.61(2) Å, 21.63(2) Å, respectively. The compound (Ba0.92Sr0.08) (Ca0.38Cu0.62)O2.1 crystallizes with a doubled perovskite unit cell along all three axes, a = 8.116(4) Å. In addition, single crystals of Ba2Ycu3O7–δ have been prepared from a KOH flux at 750 °C.

We have used a dc superconducting quantum interference device to measure the spectral density of magnetic flux noise, {ital S}{sub {Phi}}({ital f}), generated by vortex motion in crystals of YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} (YBCO) both before and after irradiation with 3.1-MeV protons and 1-GeV Au ions. In addition, we have studied the effects of irradiation on the critical current {ital J}{sub {ital c}} of the same samples. Both types of irradiation reduced {ital S}{sub {Phi}}({ital f}) at most temperatures and magnetic fields and increased {ital J}{sub {ital c}} at all temperatures and magnetic fields. By measuring {ital S}{sub {Phi}}({ital f}) versus temperature, we extract the distribution of vortex pinning energies, {ital D}({ital U}{sub 0}). Both crystals have peaks in {ital D}({ital U}{sub 0}) near 0.1 eV and 0.2 eV before irradiation, and a much reduced peak near 0.1 eV after irradiation. The noise level after either kind of irradiation was substantially higher than in an unirradiated YBCO film. {copyright} {ital 1996 The American Physical Society.}

The Fermi surfaces and single-particle spectral functions of several low-dimensional materials have been measured as part of an effort to assess the occurrence of non-Fermi-liquid behaviour in non-cuprate materials.

We present angular resolved photoemission data of the quasi-one-dimensional metals K 0.3 MoO 3 and (TaSe 4 ) 2 I. The measured conduction band dispersions reflect the highly anisotropic nature of these materials. However, no clear Fermi-Dirac cutoff is observed in the spectra above the Peierls transition, being due to either charge density wave (CDW) fluctuations or due to electronic correlation effects in one dimension. The experimental conduction band width of (TaSe 4 ) 2 I agrees well with a tight-binding calculation, suggesting a description as a conventional Peierls system with CDW fluctuations suppressing the spectral weight at the Fermi energy. In contrast, the spectra of K 0.3 MoO 3 show an excitation which along the direction of highest conductivity (“easy axis”) disperses 5 times faster than expected from band theory. This anomalous increase cannot be explained by electron-electron or electron-phonon interactions in a Fermi liquid scenario, but can be accounted for as holon dispersion in a Luttinger liquid framework, which need not conflict with the material's CDW properties.

The magnetic flux noise generated by films and crystals of B${\mathrm{i}}_{2}$S${\mathrm{r}}_{2}$CaC${\mathrm{u}}_{2}$ ${\mathrm{O}}_{8+y}$ and YB${\mathrm{a}}_{2}$C${\mathrm{u}}_{3}$ ${\mathrm{O}}_{7\ensuremath{-}x}$, up to 30 \ensuremath{\mu}m thick and cooled in nominally zero magnetic field, has been measured at opposing surfaces by two dc superconducting quantum interference devices. For both materials, the noise sources at the two surfaces were highly correlated at specific temperatures in a given cooldown. This result suggests that the observed vortices moved as rigid rods. At other temperatures, the noise was mostly uncorrelated, suggesting that the relevant vortices were pinned at more than one point along their length.

We have investigated the crystallization of amorphous oxides that are considered likely candidates to replace amorphous SiO2 as the gate dielectric in advanced field-effect transistors. To avoid crystallization, the mole fraction of main-group oxide in the Zr–Si–O, Zr–Al–O, and Hf–Si–O systems must be greater than 83%, 65%, and 78%, respectively, leading to a maximum useful dielectric constant of only 6.9, 12.7, and 6.6, respectively. We conclude that the silicate systems are not likely to be useful as replacements for SiO2, while aluminates are more promising.

We report the synthesis and initial characterization of Ba 9 Sc 2 (SiO 4 ) 6 , the first example of a new oxide structure-type accommodating a large alkaline earth ion, Ba 2+ , a large octahedral ion, Sc 3+ , and tetrahedral silicate units. Transparent crystals which crystallize in the rhombohedral space group, R3, with a H = 9.8716(2)A, c H = 21.9376(7)A were grown from a silica-rich eutectic melt. The structure contains layers made up of ScO 6 octahedra linked by SiO 4 tetrahedra. The Ba 2+ ions occupy large high-coordination-number interlayer sites.

We report on transport measurements in the mixed state of YBa[sub 2]Cu[sub 3]O[sub 7] single crystals using the dc flux transformer geometry. In the vortex fluid regime we have observed the onset of a nonlocal conductivity. At lower temperatures, close to but above the phase transition temperature the vortices become effectively three dimensional in character. This is in sharp contrast to recent flux transformer experiments on Bi[sub 2]Sr[sub 2]CaCu[sub 2]O which have failed to see a nonlocal conductivity, confirming the two-dimensional character at high temperatures of the vortices in that system. These observations suggest that in a finite temperature interval, the vortex state in YBa[sub 2]Cu[sub 3]O[sub 7] is a line liquid.

Transport measurements show that the critical current in the c direction, J c c , of Bi 2 Sr 2 CaCu 2 O 8 single crystals increases with temperature for low temperature zero field cooled (ZFC) measurements. As a consequence, the c direction electrical resistance of the ZFC critical state, at a given dc current, is finite, goes to zero with increasing temperature, and becomes finite again at higher temperatures. The results are consistent with a loss of the c axis long range correlation induced by the critical current flowing in the ab planes, J c ab , and an increase of that correlation with temperature, as a consequence of the corresponding decrease of J c ab

63 Cu(2) NMR linewidths in Zn-doped YBa 2 Cu 3 O 7 exhibit a predominant component proportional to T -1 in the normal state. We attribute this result to spin-exchange scattering of carriers from localized moments associated with Zn impurities in the CuO 2 planes. The estimated moment-carrier exchange, J eff =20 meV, is more than an order of magnitude too small to account for the suppression of T c in this system via Abrikosov-Gorkov pair breaking. A suggested alternative explanation is pair breaking by strong potential scattering from the Zn impurity sites which requires d-wave pairing

The YCuO2 and LaCuO2 delafossites consist of alternating triangular planes of Ln3+ octahedra and O -Cu1+-O sticks. Here it is shown that under a limited set of conditions they may be oxidized to form new delafossite-derived phases YCuO2.5+x, 0 ≤ x ≤ 0.05, and LaCuO25+x, 0.07 ≤ x ≤ 0.18. Electron microscopy and powder X-ray diffraction are employed to characterize the resulting structures. Temperature-dependent resistivities and magnetic susceptibilities are reported. Characterization by soft X-ray photoabsorbtion spectroscopy, which shows them to be electronically unlike cuprates such as La2-xSrxCuO4, is also reported.

The preparation and solid-state characterization of the 1,3,5-triazine-bridged tris(1,2,3,5-ditliiadiazolyl) complex 4,4',4''-(2,4,6-(1,3,5-triazinetriyl))tris(1,2,3,5-dithiadiazolyl) [N 3 C 3 (CN 2 S 2 ) 3 ] are described. The crystals belong to the monoclinic space group C2/c, with a=20.759(7) A, b=10.525(3) A, c=17.554(3) A, β=140.18(4) o , fw=390.5, Z=8, and V=2456.2(12) A 3 . The crystal structure consists of layers of interlocked dimers. Alternate layers are oriented in an antiparallel fashion, thereby precluding a stacked structure similar to that found in the related 1,3,5-benzene-based triradical. There are two dimerization environments

A series of double perovskites Ba{sub 2}MBiO{sub 6} (M=Ce to Lu) was prepared and characterized chemically and by powder X-ray diffraction. Crystal structures of Ba{sub 2}MBiO{sub 6}(M = Ce, Pr, Nd, Tb, Yb) were determined by neutron diffraction and magnetic susceptibilities were measured for M = Ce, Pr, Nd, and Tb. 26 refs., 11 figs., 11 tabs.

We report the synthesis and initial characterization of BaMnTi 5 O 12 , the first example of an oxide displaying an interesting tripled hollandite-type structure. Shiny black crystals which crystallize in the monoclinic space group, P2/n, with a =9.921(3) A, b =8.8716(22) A, c =10.309(3) A, and β =91.24(3)°, were grown from a borate-based melt in vacuo. The structure contains layers made up of edge-shared pairs of TiO 6 octahedra forming a hollandite framework. The Ba 2+ ions occupy large high coordination number channel sites. Oxidation states of the transition metal ions are such that a charge balance of one half barium ion per three framework units with an ideal stoichiometry Ba 0.5 M 3 O 6 , produces the tripling. In addition, crystals of Ba 2 MnTi 5 O 13 , monoclinic, space group C2/m, a =15.075(1) A, b =3.9470(2) A, c =9.0985(5) A, and β =98.23(1)°, which is isostructural with the mineral jeppeite, K 2 Ti 6 O 13 , were also grown.

We report transport measurements in the mixed state of single crystals of ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{\mathit{x}}$ using a modified version of the dc flux transformer geometry pioneered by Giaever in low-${\mathit{T}}_{\mathit{c}}$ materials. We find that over a wide range of the phase diagram the vortices are effectively two dimensional in character. Our results suggest that vortex cutting is an efficient process and that it is unlikely that the vortices are strongly entangled in this region of the phase diagram.

Several scaling relations are inferred from magnetization data taken in fields up to 20 T on single crystal YBa 2 Cu 3-x Ni x O 7 where 0≤x≤0.03. A Bean-like critical state exists in the samples at high fields (≤20 T) and low temperatures (4.2 K) regardless of x. Strong systematics are also displayed in the temperature dependent data: it is found that beyond an applied field breakpoint, which itself scales with temperature as (1-T/T c ) 3/2 , all the scaled hysteresis loops decrease in a universal manner

We have carried out optical and transport measurements of the free carriers and optical measurements of states above the semiconducting energy gap in the layered crystal ${\mathrm{Bi}}_{2}$${\mathrm{Te}}_{3}$. We find that cooling causes a substantial amount of spectral weight to condense from the above-gap states into the metallic states.

Precise $^{63}\mathrm{Cu}$ NMR shift results are reported for both copper sites in oriented powder samples of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7.0}$, in which the weak normal-state temperature dependences are resolved. Magnetic-susceptibility data are also given, which, in spite of the presence of small Curie-like terms, are shown to have the same intrinsic temperature dependence as data recently presented by Lee, Klemm, and Johnston. A combined analysis of shift and susceptibility data is described, in which we obtain (1) a partition of the spin paramagnetism between the chain and plane sites, (2) a full description of the susceptibility and hyperfine interaction tensors based only on static measurements, and (3) extracted values for temperature-dependent diamagnetism under field orientation both along the c axis and in the ab plane. The c-axis diamagnetism is large and decays with temperature in a fashion consistent with the asymptotic ${\mathit{T}}^{\mathrm{\ensuremath{-}}1}$ behavior predicted in recent theories. In the ab plane, the diamagnetism sets on at T\ensuremath{\sim}125 K in what appears to be a dimensional crossover from purely two-dimensional behavior at high temperatures. The analysis also suggests that the actual degree of bulk susceptibility anisotropy is slightly higher than experimentally reported values. The hyperfine parameters are not markedly changed from those given earlier by Mila and Rice.

The solid-state characterization of a second or β-phase of the 1,3-phenylene-bridged diselenadiazolyl diradical 1,3-[(Se2N2C)C6H4(CN2Se2)] is reported. Crystals of this β-phase are monoclinic, space group P21/n, with a = 9.108 (6), b = 15.233 (13), c = 16.110 (5) Å, β = 103.37 (5)°, Z = 8. The crystal structure consists of chain-like arrays of discrete dimers (4 per unit cell), although one intradimer Se-Se linkage is notably longer (3.411 Å) than the other three (3.125, 3.196, 3.204 Å). The dimer units lie in chains that are linked by a complex three-dimensional array of Se-Se contacts. Variable-temperature single-crystal conductivity measurements on this phase indicate a band gap of 0.77 eV. Consistent with the conductivity measurements, extended Hückel band structure calculations suggest a relatively isotropic electronic structure.

In studies of the optical spectrum of lightly doped semiconducting crystals containing ${\mathrm{CuO}}_{2}$ planes, we have observed absorption maxima that occur at the same energies as excitations in the superconducting phase of these materials. One structure occurs just above the phonon energies and near the antiferromagnetic exchange energy.

We have observed the occurrance of superconductivity at temperatures between 40 and 50K for multiple phase samples of Sr 2 Ln 1−x Ca x GaCu 2 O 7 treated at 950–1000°C at 25 atmospheres oxygen pressure. We have not been able to find conditions at oxygen pressures of 25 atmospheres or below which make single phase samples superconducting. We also report the characterization of the properties of the simple double-layer 60K superconductor La 2−x Sr x CaCu 2 O 6±δ . The optimal properties occur at x = 0.35 and δ = 0.0.

We determined by X-ray diffraction the structure of a slightly K-doped BaBiO 3 single-crystal. The best refinements were obtained by describing the structure in the monoclinic I2/m space group similarly to the structure of the undoped compound. In this description the Bi cations occupy two crystallographically independent sites. We found that at this low K concentration, the K atoms are not, as expected, substituting for the Ba cations but are located on one of the two Bi sites. The two octahedral Bi sites are alternatively compressed and elongated along the c -axis. The formal valences of the two Bi cations deduced from the Brown and Altermatt formula are 3.79 and 4.44, respectively. This indicates that these cations are disproportionated into Bi 3+ and Bi 5+ ; however, they are only partially ordered.

We have studied the tunneling characteristics of planar junctions made on YBCO single crystals with T c =90 K and T c =60 K . Natural barriers of good quality have been obtained on both compounds. In comparison with the 90 K phase, the superconducting conductance curves of the 60 K phase show gap-like structures reduced in amplitude and shifted towards higher energies. For the higher T c material, a gap opening, at about 90 K, is observed. Normal state conductances, measured at T > T c , are highly linear in voltage on both compounds and have steeper slopes on the 60 K phase. The slopes are quite insensitive to temperature variations, and do not depend on the junction resistances nor on the materials used as counterelectrodes.

We have formed planar tunnel junctions between Pb films and YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} single crystals with different surface oxygen stoichiometries. Oxygen losses have been induced by heating the crystals at 400 {degree}C in vacuum for different times. Low-leakage tunnel junctions have also been obtained in the case of the most degraded'' samples. As the oxygen deficiency increases, the Y-Ba-Cu-O gaplike structure gradually broadens and shifts towards higher energies. For thermally untreated samples, tunneling spectroscopy gives information on the local {ital T}{sub {ital c}}.

The synthesis and structural characterization of a new type of homologous series of lanthanum cuprates, with the general formula La 4+4 n Cu 8+8 n O 14+8 n , is reported. The n =2 and n =3 members, La 2 Cu 2 O 5 and La 8 Cu 7 O 19 , synthesized in bulk, are stable over very narrow temperature ranges in air and oxygen. Higher members ( n > 3) have been observed by electron microscopy. The crystal structure of La 2 Cu 2 O 5 consists of thin slabs of La 2 CuO 4 sandwiched between CuO planes of complex low-dimensional geometry. The temperature dependent magnetic susceptibilities of La 2 Cu 2 O 5 and La 8 Cu 7 O 19 are characteristic of low-dimensional Heisenberg antiferromagnets with a remarkably low exchange coupling energy.

Precise magnetization measurements of YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} single crystals have found evidence for a bulk phase transition a few degrees below the superconducting critical temperature. The phase transition is indicated by a sudden drop of the lower critical field, {ital H}{sub {ital c}1}({ital T}), separating a full coherent state at low fields and temperatures, the Meissner state, from a region at higher temperatures where magnetic excitations are induced at {ital H}=0.

The preparation and solid-state characterization of the 1,3-phenylene-bridged bis(dithiadiazolyl) and bis(diselenadiazolyl) diradicals 1,3-[(E2N2C)C6H4(CN2E2)] (E = S, Se) are reported. The isomorphous crystals of 1,3-[(E2N2C)C6H4(CN2E2)] so obtained are tetragonal, space group I41/a. Stacks of diradical molecules, linked vertically in a zigzag fashion through alternaie ends by long E- - -E contacts (mean 3.140/3.284 Å, for E = S/Se), are arranged in pinwheellike clusters about the 41 and ¯4 axes, producing complex patterns of interstack E- - -E contacts. Both compounds show the presence of spin defects in the lattice, and there is a very large enhancement in the paramagnetism of the sulfur compound at high temperatures. The selenium compound is a semiconductor, with a room temperature conductivity of 2 × 10-4 S cm-1. Solid-state NMR experiments find enhanced relaxation times, which have their origin in the presence of a mobile paramagnetic defect. Extended Hückel band structure calculations show the materials to be semiconductors, with band gaps of about 1.0/0.8 eV for E = S/Se. Although the compounds adopt a columnar structure, the calculations indicate significant interactions between the stacks and the materials exhibit well-developed three dimensionality. The enhanced paramagnetism in the sulfur compound is attributed to the presence of thermally generated phase kinks in the lattice, whereas the selenium compound is classified as an intrinsic semiconductor.

Transport and magnetization measurements are presented for YBa/sub 2/Cu/sub 3-x/Ni/sub x/O/sub 7/ single crystals, where x ranges from 0.0 to 0.03. The presence of nickel systematically sharpens the resistive transition of the material in a high magnetic field, but it does not correlate with the critical current as determined from magnetization measurements. The breadth of the transition for both pure and doped samples, although different, scales according to the model proposed by M. Tinkham Phys. Rev. Lett., (vol.61, p.1658, 1988).

Using a SQUID picovoltmeter. we have measured current-voltage curves of YBa2Cu3O7 microtwinned crystals as a function of temperature in fields from I to 6 T. The data constitute evidence for a finite-temperature phase transition in the vortex state. Exponents derived within the framework of the vortex-glass model are found to be similar to thin-film values. Strongly temperature-dependent correlations in-volving up to 105 vortices near the transition. combined with the qualitative failure of thermal-activation models to fit our data. support the existence of a phase transition.

An insulating polymorph with a composition close to Bi2Sr2CuO6 complicates the synthesis of the single copper layer superconductor Bi2+xSr2−yCuO6±δ. We concentrate on the region where the material is single phase in air synthesis (2.18 < x < 2.29, 1.44 < y < 1.7). The physical properties from insulating to marginally conducting as the composition moves toward x = y = 0. Like to two-layer compound, the material has a superlattice due to a buckling of the planar structure, however the superlattice coherence is degraded by defects at finite values of x and y. Density measurements indicate that the nonstoichiometry of Bi2+xSr2−y CuO6±δ is not accompanied by the formation of a large number of vacancies. The insulating polymorph, Bi2Sr2CuO6, can be suppressed by synthesis in modest oxygen pressures (above 5–10 atm). The result is a bulk superconductor (Tc ≈ 10 K).

High dielectric constant Hf–Sn–Ti–O thin-film materials were identified using a compositional-spread approach. Thin films of composition Hf0.2Sn0.05Ti0.75O2 prepared at 250 °C have excellent dielectric properties: 40–70-nm-thick films with a dielectric constant of 40–60 were obtained, depending on the processing conditions, yielding a specific capacitance of 9–17 fF/μm2. Breakdown fields were measured to be about 3–4 MV/cm, yielding a figure of merit ee0Ebr∼19 μC/cm2. Leakage currents, measured at 1 MV/cm, were in the range 10−7−10−6 A/cm2.

The results of a systematic study of the optical properties of the ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathit{x}}$-based insulators and superconductors are reported. Specifically, we present measurements and analysis of the optical reflectivity R of a series of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathit{x}}$ crystals in the frequency range from 30 to 20 000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ (4 meV to 2.5 eV), and temperature range from 10 to 270 K. From R we obtain the real part of the frequency-dependent optical conductivity \ensuremath{\sigma}(\ensuremath{\omega}) by Kramers-Kronig analysis. In our discussion, we emphasize the development of structure and spectral weight in \ensuremath{\sigma}(\ensuremath{\omega}) as the compounds change from insulators to high-${\mathit{T}}_{\mathit{c}}$ superconductors with varying O content or Al doping. We identify the free carrier, and interband components of \ensuremath{\sigma}(\ensuremath{\omega}), and focus on the free-carrier component.The free-carrier component is analyzed by calculating \ensuremath{\sigma}(\ensuremath{\omega},T) within a model in which carriers scatter from a spectrum of dispersionless oscillators parametrized by ${\mathrm{\ensuremath{\alpha}}}^{2}$F(\ensuremath{\omega}) (where \ensuremath{\alpha} is the coupling constant and F(\ensuremath{\omega}) is the density of modes). For \ensuremath{\omega}50 meV, \ensuremath{\sigma}(\ensuremath{\omega},T) is well described by weak coupling (\ensuremath{\lambda}\ensuremath{\simeq}0.4) to an F(\ensuremath{\omega}) which is broad on the scale of ${\mathit{k}}_{\mathit{B}}$T. From the fit we obtain the inelastic scattering rate as a function of T, and the spectral weight in the translational, or Drude mode, of the quasiparticles. Above 50 meV, \ensuremath{\sigma} cannot be fit by this scattering model, with any ${\mathrm{\ensuremath{\alpha}}}^{2}$F(\ensuremath{\omega}), which suggests a two-component picture of \ensuremath{\sigma}(\ensuremath{\omega},T). As T is lowered, a ``knee'' in R(\ensuremath{\omega}), and a threshold in the corresponding \ensuremath{\sigma}(\ensuremath{\omega}), is resolved, which we associate with the low-frequency edge of this second component. In addition, a second threshold in the range 15--20 meV is seen at low T, although the magnitude of the change in R is close to our detection limit of 1%. We compare the properties of these thresholds with expectations for a superconducting energy gap as described in BCS theory. Finally, we discuss the implications of other experiments, which also probe the spectrum of low-energy excitations in the cuprate superconductors, on the interpretation of \ensuremath{\sigma}(\ensuremath{\omega},T).

Field-dependent microwave absorption in high-quality YBa{sub 2}Cu{sub 3}O{sub 7} single crystals shows that even very low magnetic fields penetrate into the sample. In the temperature interval of a few degrees below {Tc}, a broad hysteretic signal is observed. We interpret it in terms of a dense structure of weak links, whose origin is in twin boundaries and other thin defect planes. As the temperature is reduced, the critical currents of these junctions increase rapidly, and approach the bulk value. The broad signal then vanishes. At low temperatures one can observe narrow discrete absorption lines due to occasional defects of a different kind, which are thicker and, therefore, maintain a weak coupling. The dense structure of weak links, active just below {ital T}{sub {ital c}}, may preclude the measurements of some intrinsic bulk superconducting properties close to {Tc}, and invalidate a straightforward comparison of experimental data with theoretical predictions.

The structure/property relationships in the Bi-Sr-Cu-O system have been investigated. This system includes Bi2+xSr2-yCuO6±δ, the single Cu layer compound that is one member of the Bi-Sr-Ca-Cu-O family of superconductors. This family includes Bi2Sr2CaCu2O8, a 95K superconductor. We have determined that two structurally related phases exist near the composition Bi2Sr2CuO6. One phase is a solid solution, Bi2+xSr2-yCuO6±5, with a wave modulated structure. The second phase, Bi2Sr2CuO6, has a structure with a step dislocation. The former phase exhibits superconductivity while the latter phase is an insulator. The transition between structures is driven, atleast in part, by a change in cation ratios.

We report measurements of entropy transport due to vortex motion in single-cyrstal ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$. From these magnetothermal measurements we are able to determine thermodynamic properties, such as the transport line energy and superconducting phase boundary ${\mathit{T}}_{\mathit{c}}$(H), as well as transport parameters associated with flux motion. Below ${\mathit{T}}_{\mathit{c}}$, the data are compared with the microscopic theory of Caroli and Maki. Interestingly, we also find entropy transport in the fluctuation regime above ${\mathit{T}}_{\mathit{c}}$, which grows with applied field and for which no theory exists.

We report observation of circular dichroism in reflection from various cuprate superconducting materials, including ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{2}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{x}}$ films, an etched ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ crystal, and a ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8}$ crystal. This signal develops as T is lowered below 200 K and exhibits little change near ${\mathit{T}}_{\mathit{c}}$. The exact onset temperatures differ for the various materials.

{sup 63}Cu NMR shifts and spectra are presented for both Cu(1) and Cu(2) sites in a specimen of YBa{sub {ital z}}Cu{sub 3}O{sub 6.64} ({ital T}{sub {ital c}}{similar to}60 K). For {bold H} along the {ital c} axis, the Cu(2) shift is not distinguishable from that reported for YBa{sub 2}Cu{sub 3}O{sub 7.0}. For {bold H} in the {ital a}-{ital b} plane, the spin component of Cu(2) shift declines sharply as {ital T}{r arrow}{ital T}{sub {ital c}}. The Cu(1) shift behaves similarly, showing also a dramatic broadening at low temperatures in clearly nonmetallic behavior. Combined with anomalous {ital T}{sub 1} data reported earlier for the 60-K phase of YBa{sub 2}Cu{sub 3}O{sub 6+{ital x}}, these results are shown to be consistent with strong and monotonically increasing antiferromagnetic correlations as {ital T} is decreased.