Magnetic Phase Diagram of Ca1?xMnxO

Alternating current susceptibility and direct current magnetization have been studied for polycrystalline Ca_1–x Mn _x O. On increasing the Mn content, magnetic ordering changes from spin glass behavior for 0.25 x 0.4 to antiferromagnetic order. The paramagnetic/antiferromagnetic transition is of second order for 0.5 x 0.65 and of first order for x 0.7. For low Mn concentrations, the high-temperature alternating current susceptibility can be described by a diluted Heisenberg magnet model developed for diluted magnetic semiconductors.     

Energy Scales in the High-Tc Superconductor YBa2Cu3O6+x

The optical conductivity sum rule is used to examine the evolution of the spectral weight N() in both the normal and superconducting states of optimally and underdoped YBa₂Cu₃O_6+x along the a axis. Differences in N() above and below T _c allow the strength of the superconducting condensate _s to be determined. In the optimally-doped material, _s is fully formed at energies comparable to the full superconducting gap maximum (0.1 eV), while in the underdoped material the energy scale for convergence is considerably higher (0.6 eV). This difference is discussed in terms of normal-state properties.     

Isochoric Heat Capacity of Heavy Water at Subcritical and Supercritical Conditions

The heat capacity of heavy water was measured in the temperature range from 294 to 746 K and at densities between 52 and 1105 kg·m^–3 using a high-temperature, high-pressure adiabatic calorimeter. The measurements were performed at 14 liquid and 9 vapor densities between 52 and 1105 kg·m^–3. Uncertainties of the measurements are estimated to be within 3% for vapor isochores and 1.5% for the liquid isochores. In the region of the immediate vicinity of the critical point (0.97T/T _c1.03 and 0.75/_c1.25), the uncertainty is 4.5%. The original C _V data were corrected and converted to the new ITS-90 temperature scale. A parametric crossover equation of state was used to represent the isochoric heat capacity measurements of heavy water in the extended critical region, 0.8T/T _c1.5 and 0.35/_c1.65. The liquid and vapor one- and two-phase isochoric heat capacities, temperatures, and saturation densities were extracted from experimental data for each measured isochore. Most of the experimental data are compared with the Hill equation of state, and the overall statistics of deviations between experimental data and the equation of state are given.     

Hc2 of MgCNi3 Determined from the Specific Heat and Resistivity Measurements

H _c2 of MgCNi₃ has been determined from the specific heat C and resistivity measurements in the same sample. The results from are nearly identical with those determined from the anomaly in C. Furthermore, utilizing the relation (H) H and the value of d/dH, the obtained value of H _c2 is the same as that by the WHH model, if the spin paramagnetic effect and the spin–orbit interaction are taken into account. The results of this comparison have strong implications on the order parameter of MgCNi₃.     

d-Hole localization and the suppression of superconductivity in YBa2(Cu1-xZnx)3O7?y

The temperature and Zn concentration dependence of the electrical resistivity, specific heat, magnetic susceptibility, and electron paramagnetic resonance (EPR) spectra of YBa₂(Cu_1–x Zn _x )₃O_7–y withy0.1 has been measured forx0.16. In addition, the temperature and field dependence of the magnetization has been measured for 2<T<300K and 0<H<9.0T, along with the temperature and quasihydrostatic pressure dependence of the electrical resistivity for selected samples for 0<P<13 GPa. The substitution of Zn for Cu in YBa₂Cu₃O_7–y causes a rapid and nearly linear depression of the superconducting transition temperature,T _c , withT _c going to 0 K forx 0.10. YBa₂(Cu_1–x Zn _x )₃O_7–y retains the YBa₂Cu₃O_7-y orthorhombic structure forx0.16 for both the superconducting and nonsuperconducting samples. Initially, the unit cell volume increases nearly linearly with Zn content; however, an abrupt change occurs in the vicinityx=0.8–0.10. Forx<0.10, the temperature dependence of the electrical resistivity,(T), is metallic-like (d/dT>0) and increases gradually with increasing Zn content. However, forx 0.10,(T) becomes semiconductor-like, with a very rapid increase of the resistivity with increasingx. The electrical resistivity, magnetic susceptibility, EPR spectra, and specific heat all indicate that thed-holes associated with the Cu ions become localized in the nonsuperconducting phase,x>-0.10.     

Optical Properties, Electronic Structure and Magnetism of α′-NaxV2O5

The optical properties of sodium-deficient -Na _x V₂O₅ (0.85 x 1.00) single crystals are analyzed using ellipsometry, and infrared reflectivity techniques. In sodium deficient samples, the optical absorption peak associated to the fundamental electronic gap develops in the middle of the pure -NaV₂O₅ gap at 0.44 eV, and the material remains insulating up to the maximal achieved hole concentration of about 15%. Nonmetallic behavior under hole doping provoked reinterpretation of the -NaV₂O₅ optical spectra. We argue that the absorption peak at about 0.9 eV corresponds to the photoionization energy of a large polaron.     

The Anomalous Optical Conductivity of Metallic La2?xSrxCuO4

The in-plane optical conductivity of three metallic La_2–x Sr _x CuO₄ single crystals with 0.10 x 0.15 has been studied between 30 and 295 K. Strong peaks in the far-infrared are observed, which cannot be explained by Drude-like models. Their similarity with peaks reported in Cu–O ladders with one-dimensional charge-ordering, their extremely low frequency, and their behavior with temperature allows us to conclude that those anomalous features are excitations of charge stripes in the Cu–O planes.     

Thermodynamic Properties of Sulfur Hexafluoride

We present new vapor phase speed-of-sound data u(P, T), new Burnett density–pressure–temperature data (P, T), and a few vapor pressure measurements for sulfur hexafluoride (SF₆). The speed-of-sound data spanned the temperature range 230 KT460 K and reached maximum pressures that were the lesser of 1.5 MPa or 80% of the vapor pressure of SF₆. The Burnett (P, T) data were obtained on isochores spanning the density range 137 mol·m^–34380 mol·m^–3 and the temperature range 283 KT393 K. (The corresponding pressure range is 0.3 MPaP9.0 MPa.) The u(P, T) data below 1.5 MPa were correlated using a model hard-core, Lennard–Jones intermolecular potential for the second and third virial coefficients and a polynomial for the perfect gas heat capacity. The resulting equation of state has very high accuracy at low densities; it is useful for calibrating mass flow controllers and may be extrapolated to 1000 K. The new u(P, T) data and the new (P, T) data were simultaneously correlated with a virial equation of state containing four terms with the temperature dependences of model square-well potentials. This correlation extends nearly to the critical density and may help resolve contradictions among data sets from the literature.     

Single-crystal growth and anisotropic electrical properties of (La1?xCax)2CaCu2O6

Large single crystals of (La_1–x Ca _x )₂CaCu₂O₆ were synthesized by the travelling solvent floating zone (TSFZ) technique. The dimension of the grown boules was typically 4 mm×30 mm long with thec-axis perpendicular to the growth direction. After oxygen loading at 1080°C in 400 atm of O₂, they became superconductive with a sharp transition at around 50 K. The resistivity was metallic both along thec-axis and within theab-plane, with an anisotropy ratio of ²( _c/ _ab ) 50 which is almost independent of temperature. Details of the crystal growth, heat treatment procedures, and the results of electrical and magnetic property measurements are presented.     

X-Ray Diffraction Studies on LaBa2Cu3Ox Cuprates with Iron Substitution

LaBa₂Cu_3–y Fe _y O _x ceramic samples with y = 0.00–1.50 are synthesized by the solid-state reaction technique. Rietveld analysis for X-ray diffraction is performed on these iron-doped samples. A BaCuO₂ impurity phase and a ceramic cuprate phase coexist in each sample. An orthorhombic-to-tetragonal (OT) phase transition occurs in the doping range of 0.03y0.06, and a tetragonal-to-orthorhombic (TO) one occurs in the doping level of 0.10y0.25. There is a jump in the structural parameters due to the iron doping. The occupancy of oxygen at the O(4) site, which is in the La plane at z = 1/2, increases with increase in iron content. These results may relate to the iron preferential occupancy for the Cu(1) site at the lower doping level, and for Cu(2) sites at the higher doping level.     

Theoretical study of isotope effect in La-doped Y-123

The recent oxygen isotope effect measurement made in the system YBa_2-x La _x Cu₃O₇ (0x0.5) is analyzed using the two-carrier model which describes a superconducting system as a mass anisotropic system consisting of charge carriers with two different masses as a result of lattice defects or oxygen deficiency. The extent to which Ba, Cu, and O atoms participate in the phonon-mediated Cooper pairing interaction is elucidated. The relationship between the theoretically deduced mass anisotropic factor andT _c agrees closely with the experimental results of Uemuraet al. [28]. Furthermore, the superconducting energy gap toT _c ratio is found to be close to 3.5. Excellent agreement is also achieved between the theoretically calculated oxygen isotopic constant and the experimental results of Bornemann and Morris [12].     

Anisotropy in the resistive superconducting transition under magnetic fields in single crystal Pb2Sr2Ho0.5Ca0.5Cu3O8

Anisotropie properties of the single crystal Pb₂Sr₂Ho_0.5Ca_0.5Cu₃O₈ have been investigated by measuring the electrical resistivity in theab-plane _ab (H, ,T), which depends on the angle between theab-plane and the magnetic-field direction, in various constant fieldsH perpendicular to the current direction. All the angle-dependent values of _ab (H, ,T) at a constant temperature are scaled to be on one curve as a function of reduced field. The anisotropic parameter (m _c ^* /m _ab ^* )^1/2 is estimated as 12–13, which is larger than that of YBa₂Cu₃O₇ and much smaller than that of Bi₂Sr₂CaCu₂O₈. It has been concluded that the anisotropy does not always depend on the thickness of the blocking layer but seems to depend on the overlap of the electronic wave functions along thec-axis. Anisotropy in the pinning potential has also been discussed from the resistive tail in the temperature dependence of _ab (H,,T).     

The Interplay of Electronic and Nuclear Magnetism in PtFe x at Milli-, Micro-, and Nanokelvin Temperatures

We have measured ac susceptibility, nuclear magnetic resonance, and nuclear heat capacity of two PtFe _x samples with concentrations of magnetic impurities x = 11 ppm and 41 ppm at magnetic fields (0 ± 0.05) mTB248 mT. The susceptibility data have been measured at temperatures of 0.3 KT100 mK, no hint for nuclear magnetic ordering could be detected to a temperature of 0.3 K. The nuclear heat capacity data taken at 1.4 KT10 mK show enhanced values which scale with x at low polarization. This effect is described by a model assuming an internal magnetic field caused by the impurities. No indication for nuclear magnetic ordering could be detected to 1.4 K. The nuclear magnetic resonance experiments have been performed on these samples at 0.8 KT0.5 mK and 2.5 mTB22.8 mT as well as on three other samples with x = 5, 10, 31 ppm in a different setup at 40 KT0.5 mK and at 5.4 mTB200 mT. Spin-lattice and effective spin-spin relaxation times ₁and ₂ ^* of ¹⁹⁵ Pt strongly depend on x and on the external magnetic field. No temperature dependence of ₁and ₂ ^* could be detected and the NMR data, too, give no hint for nuclear magnetic ordering to 0.8 K.     

Surface resistance of large-area Tl2Ba2CaCu2O8 thin films at microwave and millimeter wave frequencies measured by three noncavity techniques

The surface resistanceR _s of Tl₂Ba₂CaCu₂O₈ films fabricated on LaAlO₃ wafers up to 3 inches (7.6 cm) in diameter through a post-deposition anneal process was measured over the frequency range 5.55–94.1 GHz by the following techniques: 5.55 and 27.5 GHz high-temperature superconductor (HTS)-sapphire resonators, 10 GHz parallel plate resonator, and 94.1 GHz scanning confocal resonator.R _s was found to exhibit a quadratic dependence on frequencyf at 77 K:R _s f ^2.0±0.1. The highest-quality films yieldR _s =145±15 at 10 GHz and 77 K. Scanning confocal resonator mapping ofR _s across a 2-inch (5.1 cm) diameter wafer yielded a base value forR _s of 16±1 m at 77 K and 94.1 GHz (equivalent to 180±10 at 10 GHz) and good uniformity inR _s across the wafer. HTS-sapphire resonator measurements ofR _s for fifteen 1.2 cm square parts cut from a 3-inch diameter wafer yieldedR _s values scaled to 10 GHz of 196±10 at 80 K. Similar values were measured for Tl₂Ba₂CaCu₂O₈ films prepared on both sides of a 2-inch diameter wafer.Rs values at 10 GHz and 80 K of 147–214 were maintained over the course of 40 independent and successive deposition runs and corresponding anneals under nominally identical film fabrication conditions. Surface resistance at 5.55 GHz remained below 80 for maximum rf magnetic fields up to 85 Oe at 4.2 K and 7 Oe at 80 K, respectively. Results are compared with predictions of the two-fluid model. The relative advantages and disadvantages of the different techniques for measuring surface resistance are discussed.     

Comparison of Superconductivity and Structure for Lanthanum Replacing for Barium and Copper in YBa2Cu3Od

X-ray diffraction studies and the resistivity measurements are used to characterize the structure and the superconductivity of the nominal composition of YBa₂Cu_{3 – x} La _x O _d (YBCLO) cuprates with x 0.30. There was a BaCuO₂ impurity phase detected with x 0.10. The structure of the main phase (123) has the orthorhombic form with Pmmm symmetry in the whole doping range. With increasing content of lanthanum, x, the lattice constants increase for x < 0.04, and decrease for x 0.04. Rietveld refinements for X-ray diffraction show that the dopant of lanthanum substitutes for copper in the lower doping level, and replaces for both barium and copper in the high doping level. The zero-resistance temperature T _c0 first increases with the increase of the content of lanthanum in YBCLO as x 0.04 and then decreases with x as x 0.04. We compared the results with those of La-doped YBa_{2 – z} La _z Cu₃O _y cuprates. The different relationship in superconductivity dependence of lanthanum content may result from the strains due to the different occupancy of lanthanum in the unit cell of YBa₂Cu₃O _d .     

Bulk thermal expansion studies of BiCaSrCu2O x and Bi2CaSr2Cu2O x

The compounds BiCaSrCu₂O _x and Bi₂CaSr₂Cu₂O _x were prepared by ceramic techniques and characterized by X-ray powder diffractometry (XRD) and microthermogravimetry (TG) and their bulk thermal expansion measurements were carried out using dilatometry in the temperature range 298T1073 K in air. The results have been analyzed and are compared with those obtained earlier for YBa₂Cu₃O₇. The XRD analysis shows that both BiCaSrCu₂O _x and Bi₂CaSr₂Cu₂O _x are single phase in nature, having an orthorhombic symmetry. The TG analysis carried out in oxygen, air, and nitrogen shows negligible weight loss (R~0.1%) on heating to 1073 K, indicating that these two compounds, unlike YBa₂Cu₃O₇, are quite stable. The analysis of bulk thermal expansion data reveals that the average linear thermal expansion coefficient ( ₁) for both BiCaSrCu₂O _x and Bi₂CaSr₂Cu₂O _x is almost the same ( ₁ 10.5×10^–6 K^–1) and is found to be nearly half of that for YBa₂Cu₃O₇ ( ₁ 18×10^–6 K^–1), suggesting that the interatomic bonding in both BiCaSrCu₂O _x and Bi₂CaSr₂Cu₂O _x is stronger as compared to YBa₂Cu₃O₇.     

Synthesis and dielectric properties of pyrochlore solid solutions in the Bi2O3-ZnO-Nb2O5-TiO2 system

Phase studies of solid solutions based on (Bi_1.5Zn_0.5)(Ti_1.5Nb_0.5)O₇ revealed extended regions of pyrochlore formation in the Bi₂O₃-ZnO-TiO₂-Nb₂O₅ system. At room temperature and 1 MHz (Bi_1.5Zn_0.5)(Ti_1.5Nb_0.5)O₇ has a high permittivity ( = 200), low dielectric loss (tan<1·10^–4) and a temperature coefficient of the permittivity, , = –1300ppm/K. Pyrochlore solid solutions based on (Bi_1.5Zn_0.5)(Ti_1.5Nb_0.5)O₇ can be formed with (Bi_2–xZn_x)(Ti_{2– x} Nb_x)O₇ (0.35x1.0) and with (Bi_1.5 Zn_0.5–y/3Ti_1.5+yNb_0.5–2y/3) O₇ (–1.5y0.75). Investigations of the dielectric characteristics showed that the high temperature dependence of the permittivity of (Bi_1.5 Zn_0.5)(Ti_1.5Nb_0.5)O₇ can be significantly modified by changing the composition of the pyrochlore within these regions of solid solubility. Below room temperature several of these compositions also exhibit a diffuse frequency dependent maximum in their permittivity characteristic of a transition to a relaxor type ferroelectric state. A third region of high permittivity pyrochlores with (Bi_1.5+2zZn_0.25–z Ti_2.25–z)O₇ (0.0z<0.15) was also identified in the Bi₂O₃-ZnO-TiO₂ sub system.     

Normal-State Optical Response Functions of MgB2 Superconductor

The reflectivity of superconducting MgB₂ (T _c = 39 K) has been measured on a randomly oriented thin film at room temperature over a wide-range of frequencies, 20 < 100000 cm^–1. The conductivity shows highly metallic behavior but cannot be explained with a simple Drude model alone. The electronic contribution is analyzed by a generalized Drude model. The scattering rate 1/() and the mass renormalization ratio m*()/m = 1 + () exhibit clear frequency dependence. The electron–phonon coupling strength is estimated to be 1.5 ± 0.5 while the plasma frequency _p is 2.4 eV.     

Dynamic Behavior of a Pure Fluid at and Near Its Critical Density Under Microgravity and 1g

A new sample cell allowing accurately measurable density quenches was developed for further and systematic investigation of dynamic temperature propagation, or the piston effect. Several experiments were performed under 1g and microgravity during and in preparation for the Perseus mission in 1999. The starting temperatures ranged in the one-phase state between 1 and 1000 mK above T _c, while the density varied between 0.7 _c<<1.3 _c. The method for the determination of the isentropic difference coefficients (/T)_S, (/p)_S, and (T/p)_S is explained. The coefficients are in reasonable agreement with the equation of state for SF₆, and the difference between the ground and the microgravity experiments is discussed. The advantage of the density quench method in contrast to the temperature quench method is demonstrated, particularly with regard to the influence of convection.     

Mean polarizabilities and second optical and density virial coefficients of CH3OH and CCl2F2 between 300 and 355 K

Mean dipole polarizabilities ₀(, T) as well as second optical (or refractive index) virial coefficients b _R(, T) and second density virial coefficients B(T) of gaseous CH₃OH and CCl₂F₂ have been determined by precise measurements of the refractive index n(, T, p) [543 nm 633 nm, 300 K T 355 K, p<0.25 bar (CH₃OH) and p<3 bar (CCl₂F₂)]. ₀ critically compared with the few data in literature. The b _R of these gases was measured for the first time with the cyclic-expansion method. The values of ¦B¦ and b _R=3160(25) cm³ · mol^–1 measured for CH₃OH are considerably greater than the values calculated by Buckingham's statistical-mechanical expressions for a Stockmayer interaction potential. This difference is discussed by assuming dimerization via H bonds, with result H ₂ ⁰ –(28 ... 33) kJ · mol^–1 and S ₂ ⁰ –(116 133) J · mol^–1 · K^–1 for the dimerization enthalpy and entropy for standard conditions, respectively. On the other hand, Buckingham's formulae can be used with success to estimate b _R and B of CCl₂F₂.Dedicated to Prof. Dr. F. Kohler on the occasion of his 65th birthday