Pressure effect on the low temperature tetragonal phase and the superconductivity in (La1−xBax)2CuO4

The resistivity of (La_1−xBa_x)₂CuO₄ was measured under pressure up to 20 kbar. The resistive increment at low temperatures, associated with the structural phase transition to low temperature tetragonal phase, in x=0.06 was suppressed completely under pressures above 10 kbar. Dip of the superconducting transition temperature around x=0.06 is not observable under high pressures where the resistive increment at low temperatures disappears. The results give an evidence that the low temperature structural phase transition does suppress the superconductivity.     

Spin fluctuations of Y(Mn1−xAlx)2 and Y1−xScxMn2

The thermal expansion and low temperature and low temperature specific heat were measured for Y_1−xScxMn2. The results are discussed in terms of spin fluctuations and compared with those of Y(Mn_1−xAl_x)₂, which show al local moment character. It is revealed that Y_1−xSc_xMn₂ is a typical nearly antiferromagnet in which giant spin fluctuations are thermally excited.     

Thermoelectric power of Y(Mn1−xAlx)2

Electrical resistivity and thermoelectric power of Y(Mn_1−xAl_x2 and (Y_1−xSc_x)Mn₂ were measured from 2 to room temperature. Anomalous behavior of the thermoelectric power of YMn₂ disappears rapidly with the increase of Al content in Y(Mn_1−xAl_x)₂. Based upon spin fluctuations of the itinerant antiferromagnetism of the samples, the anomaly of the thermoelectric power is discussed.     

Thermogalvanic power and fast ion conduction in δ-Bi2O3 and δ-(Bi2O3)1−x(R2O3)x with R = Y, Tb---Lu

The thermogalvanic power (Seebeck coefficient) of O^2- conducting δ-Bi₂O₃ and δ-(Bi₂O₃)_1−x(Y₂O₃)_x has been measured directly as a function of temperature and partial oxygen pressure in N₂---O₂ mixtures. The of δ-(Bi₂O₃)_0.75(R₂O₃)_0.25 with R = Tb---Lu was indirectly determined using an isothermal concentration cell technique. Except for pure δ-Bi₂O₃, the heat of transport is much smaller than the activation energy for O^2- conduction for all materials. The vibrational freedom of O²⁻ ions in all δ-stabilized materials is reflected in their IR spectra at room temperature. Two prototypes of a thermogalvanic P_O2 meter were tested.     

Preparation and superconductivity of the co-doped superconductor Nd2−x−yCayCexCuO4 with T′ structure

The preparation and superconductivity of the co-doped superconductor Nd_2−x−yCa_yCe_xCuO₄ with T′ structure were studied by XRD and superconductivity measurements. The results indicate that the effect of the co-doping of Ca²⁺ and Ce⁴⁺ on T_O and the carrier type is not significant, but the effect on the range of the superconducting compositions in this system is obvious. The study of co-doped superconductors could be important for an understanding of the effect of co-doping on the carrier concentration and to explore new superconductors.     

Itinerant metamagnetic transition in Y(Co1−xFex)2

High field magnetization measurements have been performed to examine the existence of itinerant metamagnetism in exchange-enhanced systems related to YCo₂ together with Fe_1−x Co_xSi. In the Y(Co_−xFe_x)₂ system, the meta magnetism inherent in YCo₂ has been observed in 0.04 x0.07. The transition is not as sharp as in the Y(Co_1−xAl_x)₂ system. Other exchange-enhanced paramagnets Y(Co_−xCu_x)₂ and Y_1−xLa_xCo_x2 and weakly itinerant ferromagnet Fe_1−xCo_xSi exhibit no metamagnetic transition up to 430 kOe.     

Defects in Nd2CuO4 and Nd1.815Ce0.185CuO4 single crystals

We describe here four kinds of defects found in Nd₂CuO₄ and Nd_1.815Ce_0.185CuO₄ single crystals grown by the traveling solvent floating zone method. These defects include: (1) cracks, (2) inclusions, (3) gas bubbles and (4) uneven distribution of Ce⁴⁺. The formation mechanism of these defects and the measures of eliminating these defects are discussed. It is thought that the preparation of raw materials and stringent growth conditions are necessary to obtain high quality of Nd₂CuO₄ and Nd_1.815Ce_0.185CuO₄ single crystals.     

Magnetic properties of the intermetallic compounds Gd(Mn1−xNix)2

The magnetic properties of pseudobinary compounds Gd(Mn_1−xNi_x)₂ were studied to understand the type of magnetic interaction among the constituent atoms of the parent binary compound. The Arrott plot for GdMn₂ indicates no spontaneous magnetization. The magnetization and the Curie temperature increase with increasing concentration of Ni on the Mn rich side. The collapse of the Mn moment makes the Gd moment dominant in magnetization with increasing Ni concentration.     

The anomalous behaviour of Gd atoms (Fe1−xMnx)2 compounds

The educed Gd atoms in the X-structure (Th₆Mn₂₃-type) of Gd(Fe_1−xMn_x)₂ were magnetically investigated by comparing with Gd₆(Fe_1−yMn_y)₂₃ whose structure is Th₆Mn₂₃-type. The magnetic properties of Gd(Fe_1−xMn_x)₂ (0.4≤x≤0.7) were observed to be quite similar to those of Gd₆(Fe_1−yMn_y)₂₃ (0.4≤y≤0.7).     

Itinerant electron ferromagnetism in the Laves phase compounds Se(Co1−xAlx)2 and Lu(Co1−yAly)2

Typical weak itinerant ferromagnetism has been found in Sc(Co_1−xAl_x)₂, which a magnetism and higher Curie temperatures. Magnetic for X = 0.10. Lu(Co_1−yAl_y)₂ also shows ferromagnetism with much larger moments anad higher Curie temperatures. Magnetic and nonmagnetic Co atoms coexist in 0.06 < y < 0.12.     

Origin of the metal-insulator transition in the superconducting series Bi2Sr2Ca1−xYxCu2O8+δ

The metal-insulator transition in the solid solution Bi₂Sr₂Ca_1−xY_xCu₂O_8+δ (0≤x≤1) has been investigated by TGA (oxygen content) and by X-ray absorption spectroscopy (Bi and Cu valence states). Resistivity and AC magnetic susceptibility measurements have shown that the superconducting properties and the metallic behavior vanish for x>0.55. The oxygen content δ is larger than x/2 for x≤0.3 and smaller than x/2 for x≥0.6. For x=0, the Cu K edge shows a shift towards high energy with respect to the Cu(II) oxide La₂CuO₄; this shift decreases with increasing x in agreement with the decrease of the doping hole density and the variations of the physical properties. For 0≤x≤0.3, the Bi L₃ edge shows a shift of 1 eV towards low energy with respect to the Bi(III) oxide Bi₂O₃ in agreement with the charge transfer between [CuO₂]_∞ and [BiO]_∞ planes. This shift also decreases with increasing x, but is still present for the x=0.6 composition for which δ is smaller than x/2. A model of the metal-insulator transition in this series is proposed based on the fact that the intercalation of excess oxygen raises the bottom of the Bi-O band with respect to the Fermi level and decreases the contribution of the Bi-O electron pocket to the hole density.     

On the appearance of ferromagnetism in Y(Co1−xAlx)2 alloys

The appearance of ferromagnetism in Y(Co_1−xAl_x)₂ is discussed in terms of a d band model. The approximate d bands for YCo₂ and Y(Co_1−x)₂ are calculated and the decrease in the electronic energy due to magnetization of the spin of estimated. The energy decrease is the largest in YCo₂, and it gradually decreases as the Al content increases, if the lattice constant is fixed, while this energy decrease increases if the lattice constant increases with increasing Al content. These results of calculations give a good account of the appearance of ferromagnetism in Y(Co_1−xAl_x)₂ around x = 0.15. The ferromagnetism in Sc(CO_1−xAl_x)₂ is also discussed, leading to the appearance of ferromagnetism between x = 0.15 and 0.30.     

Structure and superconductivity of HgBa2CuO4+δ

We have used neutron powder diffraction to investigate the defect structure of HgBa₂CuO_4+δ. An interstitial oxygen defect in the Hg plane is the primary doping mechanism. A superconducting transition temperature, T_{c onset}, of 95 K is achieved when ≈0.06 oxygen atoms per formula unit are incorporated at this site by annealing the sample at 500°C in pure oxygen. Annealing in argon at 500°C lowers the oxygen content in this site to ≈0.01 and results in a T_c of 59 K. The neutron powder diffraction data give evidence for a second defect in the Hg plane which we conclude involves the substitution of copper for about 8% of the mercury and the incorporation of additional oxygen (≈0.1 atoms per formula unit), presumably bonded to the copper defects. In the present samples, the concentration of this defect does not vary with synthesis conditions and its contribution to doping is, therefore, unclear. The structure of the compound is the same at room temperature and superconducting temperatures.     

Structural and transport properties of the triple-layer compounds Ba4(Pb1−xBix)3O10 (0≤ x<0.3)

A series of new compounds Ba₄(Pb_1−xBi_x)₃O₁₀ with 0≤x<0.3 has been prepared and characterized by X-ray and electron diffraction. The d.c. resistivity vs. temperature data for different x values are presented and structural features are compared to those of the superconducting phase Ba(Pb_1−xBi_x)O₃. The “mother-compound” Ba₄Pb₃O₁₀ is tetragonal, with lattice constants A=0.4280(1) and C=3.017(1) nm. Its structure can be regarded as a stacking of Ba(PbBi)O₃ triple-layers, separated by single BaO layers. Electron diffraction reveals the presence of a weak superstructure with the a-axis related to that of the perovskite by a=√2a_p. Frequent intergrowth of this phase with the perovskite structure is observed. The conductivity of the samples Ba₄Pb₃O₁₀ is nearly independent of temperature, while Bi-doped samples exhibit semiconductor-like behaviour at low temperature. No superconducting transition is observed down to 2K.     

Superconductivity and crystallographic properties of La2 − xMxCuO4 − δ (M = Na, K)

Superconductivity and crystallographic properties of La_{2 − x}M_xCuO_{4 − δ} (M = Na, K) are studied. In the La_{2 − x}M_xCuO_{4 − δ} system, superconductivity is detected for x 0.2. Oxygen content analysis shows that the system has more oxygen vacancies than the La_{2 − x}Sr_xCuO_{4 − δ} system. These oxygen vacancies may reduce the hole concentration, and high Na-doping is needed to produce superconductivity. In the La_{2 − x}K_xCuO_{4 − δ} system, superconductivity is observed for the first time. Resistivity and magnetic susceptibility measurements show that T_c(onset) is 40 K and the Meissner volume fraction is about 4% for x = 0.7. The system changes from orthorhombic to a tetragonal K₂NiF₄ structure at x ≈ 0.3 and only tetragonal samples show superconductivity.     

Superconductivity magnetism and low temperature specific heat in YBa2(Cu1−xFex)3O7−δ with δ0 and δ1

Magnetic and low temperature specific heat measurements have been performed on iron doped YBa₂(Cu_1−xFe_x)₃O_7−δ samples with different oxygen contents (δ0 and δ1). Iron doping induces an orthorhombic to tetragonal transition and a decrease of both T_c and diamagnetic signal. Low temperature specific heat measurements reveal a Schottky type anomaly for δ0 samples with x=0.01 (1.8 K) and x=0.02 (3 K). This anomaly is attributed to magnetic interactions within iron limited chains. A numerical analysis of this effect is proposed.     

Line Positions and Intensities of the ν1+ ν2+ 3ν3, ν2+ 4ν3, and 3ν1+ 2ν2Bands of Ozone

Using a Fourier transform spectrometer, we have recorded the spectra of ozone in the region of 4600 cm⁻¹, with a resolution of 0.008 cm⁻¹. The strongest absorption in this region is due to the ν₁+ ν₂+ 3ν₃band which is in Coriolis interaction with the ν₂+ 4ν₃band. We have been able to assign more than 1700 transitions for these two bands. To correctly reproduce the calculation of energy levels, it has been necessary to introduce the (320) state which strongly perturbs the (113) and (014) states through Coriolis- and Fermi-type resonances. Seventy transitions of the 3ν₁+ 2ν₂band have also been observed. The final fit on 926 energy levels withJ_max= 50 andK_max= 16 gives RMS = 3.1 × 10⁻³cm⁻¹and provides a satisfactory agreement of calculated and observed upper levels for most of the transitions. The following values for band centers are derived: ν₀(ν₁+ ν₂+ 3ν₃) = 4658.950 cm⁻¹, ν₀(3ν₁+ 2ν₂) = 4643.821 cm⁻¹, and ν₀(ν₂+ 4ν₃) = 4632.888 cm⁻¹. Line intensities have been measured and fitted, leading to the determination of transition moment parameters for the two bands ν₁+ ν₂+ 3ν₃and ν₂+ 4ν₃. Using these parameters we have obtained the following estimations for the integrated band intensities,S_V(ν₁+ ν₂+ 3ν₃) = 8.84 × 10⁻²²,S_V(ν₂+ 4ν₃) = 1.70 × 10⁻²², andS_V(3ν₁+ 2ν₂) = 0.49 × 10⁻²²cm⁻¹/molecule cm⁻²at 296 K, which correspond to a cutoff of 10⁻²⁶cm⁻¹/molecule cm⁻².     

Field-induced pinning effect of Nd(Ba1−xNdx)2Cu3O7−δ single crystals grown by the traveling-solvent floating-zone method

We have studied the superconducting properties of Nd(Ba_1−xNd_x)₂Cu₃O_7−δ (Nd123, x ≈ 0.1) single crystals grown by the traveling-solvent floating-zone method under 0.1% O₂ in Ar atmosphere. The enhancement of the magnetization with increasing field is observed in the hysteresis (M-H) loop in fields both parallel and perpendicular to the c-axis of the Nd123 single crystals as well as in the bulk crystals prepared by the oxygen-controlled-melt-growth (OCMG) method. The composition variation of Ba/Nd is observed in the matrix of Nd123 crystals by an analytical TEM equipped with a cold field-emission gun. It turns out that the enhancement is due to the field-induced pinning effect ascribed to the weak superconducting Nd---Ba substitution regions in the Nd123 matrix.     

A new layered cuprate superconductor: GaSr2(Y, Ce)2Cu2O9−δ

A new layered cuprate compound with a nominal composition of GaSr₂Y_2−xCe_xCu₂O_9−δ has been prepared. It crystallizes in a tetragonal lattice with cell parameters: a = 3.812 Å, c = 28.16 Å. The structure of the compound belongs to the same family of 1222 phase and is derived from that of GaSr₂LnCu₂O₇ by replacing the single Ln³⁺ layer with a double fluorite (Y, Ce)₂O₂ layer. Like other parent cuprate compounds of superconductors, the as-prepared samples showed antiferromagnetic and semiconducting behavior. After treatment under high oxygen pressure, the samples exhibited bulk superconductivity with transition temperatures between 12–14 K.     

High TcYBa2Cu3O7−δ superconducting transition-edge microbolometers

High T_c superconducting thin film YBa₂Cu₃O_7−δ (YBCO) bolometers have been fabricated on various substrates such as MgO, LaAlO₃, YSZ and Si using laser ablation technique. Performance of these IR bolometers operating with a Joule-Thomson refrigerator has been investigated. Measurements of the responsivity and low frequency noise near T_c in the current biased YBCO bolometers show that reliable devices can be fabricated. Measured noise equivalent power (NEP), for YBCO/YSZ bolometer, reaches 6 × 10⁻¹⁰W/Hz^1/2 at 165 Hz and has a responsivity of 60 V/W with a blackbody source. This performance is comparable to that of the optimized pyroelectric detectors. The characteristics of YBCO films deposited on Si substrates reveal that superconducting thin film multi-elements or focal plane array with silicon integrated readout circuit are feasible. Such bolometers exhibit NEP of 7 × 10⁻⁹W/Hz^1/2, and significant improvement appears possible. Electrical measurements show no noticeable film degradation after the bolometer is exposed to atmosphere for three months.