Search

In large cities, pollution composed of many different chemical components and small particles is an important public health problem that affects especially children and people presenting breathing difficulties.

The possibility to manufacture single devices allowing the 10 GHz threshold to be largely overcome has been demonstrated using Electron-beam lithography. In this paper, we propose an original alternative to lithographic techniques based on the Focused Ion Beam etching technique to fabricate SAW devices by directly “writing” the electrode pattern in the metal overlay. We also explore the RF capabilities of epitaxial Pb(Zr 0.20 Ti 0.80 )O 3 (PZT) thin films deposited onto SrTiO 3 single crystal substrates. We found that guided waves can be efficiently excited and detected at frequencies ranging from 3 to 5 GHz. Theory/experiment assessment shows a good predictability of the waves characteristics.

This paper describes the system of the Swiss national metrological institute (IRA-METAS) for the absolute standardisation of radon gas. This method relies on condensing radon under vacuum conditions within a specified cold area using a cryogenerator, and detecting its alpha particles with an ion-implanted silicon detector, through a very accurately defined solid angle. The accuracy of this defined solid angle standardisation technique was corroborated by another primary measurement method involving 4πγ NaI(Tl) integral counting and Monte Carlo efficiency calculations. The 222Rn standard submitted by IRA-METAS to the Systeme International de Reference (SIR) at the Bureau International des Poids et Mesures (BIPM) has also been found to be consistent with an analogous standard submitted by the German national metrological institute (PTB). IRA-METAS is able to deliver radon standards, with activities ranging from a few kBq to 350 kBq, in NIST-Type ampoules, and glass or steel containers usable for calibrating radon-measuring instruments.

A systematic study of texturing mechanisms in pure Ag is presented, the goal being to obtain a cube textured {100}001 tape that can be used as a substrate for superconducting coated conductors. It is shown that a sharp cube texture can be produced after pre-heated deformation and recrystallization. Optimal parameters are detailed. 99.99% pure Ag powder is used as starting material. Fine grained (15 µm) Ag ingots are pre-heated at 100 °C before standard cold rolling. The effect of pre-heating on deformation texture is to change the main texture from the brass orientation {110}112 to the Goss {110}001 one. The presence of a copper-type texture with relatively high values of the ODF (oriented distribution function) after the 100 °C pre-heated deformation can be understood as a slight increase of the stacking fault energy of Ag during the pre-heated deformation. The optimal annealing is achieved at 700 °C for 30 min in a primary vacuum. This leads to a sharp cube texture. The FWHM values of the three x-ray pole figures are, at no more than 10°, the smallest ever reported for cube textured Ag tapes. The formation of cube texture is interpreted in terms of orientation nucleation and orientation growth theories that play a common role during the texturing process. The stability of cube texture at high temperature up to 900 °C indicates that the tapes are suitable for epitaxial growth of superconducting oxides.

The magnetisation curves M ( H , T ) of the most anisotropic high temperature superconductors cross in a single point M * ( T * ), independently of the magnetic field H that is applied perpendicular to the superconducting planes. This property may be very generally derived from the singular part of the free energy of a two-dimensional (2D) superconductor with fluctuations at T c . Correspondingly, three-dimensional (3D) superconductors have a crossing point in the quantity M / H 1/2 at T c , with an amplitude proportional to the anisotropy ratio γ ≡( m c / m ab ) 1/2 .

The irreversibility of the magnetization and the low temperature specific heat of high purity YBa2Cu3O7−δ crystals are investigated. We find that the heat treatment that suppresses the magnetic “fishtail” effect also suppresses the Schottky component of the heat capacity. Both effects originate from the distribution of oxygen states.

The ( Tl ,Pb,Bi) 1 ( Sr ,Ba) 2 Ca 2 Cu 3 O 9 (Tl-1223) superconducting phase is characterized by means of X-ray diffraction, SEM and optical micrographs, AC susceptibility, resistivity, magnetization M ( T , H ) both in the reversible and in the irreversible regime up to 5 T, and specific heat near T c up to 14 T. Reversible magnetization data are analyzed using the London model in the low field limit, and the lower critical field is determined from the first flux penetration. Values are given for the upper critical field, μ 0 H c2c (0) = 75 T, the thermodynamic critical field, μ 0 H c (0) = 0.81 T, the lower critical field, μ 0 H c1c (0) = 0.037 T, the penetration depth, λ ab (0) = 1370 A , the Ginzburg-Landau parameter, κ ( T c ) = 45, κ (0) = 65, and for the mean-field specific heat jump, ΔC / T c = 18 mJ/(K 2 mol). The variation of the entropy with the field is measured near T c and agrees with magnetization data, thus demonstrating equilibrium conditions. Large magnetic hysteresis is observed only at temperatures below 20 K. The irreversibility line in the high temperature regime follows the functional dependence H irr ( T )∞(1− T / T c ) 2 . A systematic departure from this behaviour is observed below 70 K, possibly as a result of a phase transformation in the vortex system.

The mixed-state magnetization M(H||c,T) of a Bi-2212 single crystal has been investigated with high resolution using a SQUID magnetometer. In the high-temperature region (50 K < T < Tc = 80.2 K), we found that the slope ∂M/∂H|T vs. H shows a positive step at Htrans(T) ≈ H0×(1 − T/Tc)n with H0 = 2340 Oe and n = 1.28. This observation is compatible with a first-order phase transition with a distribution of internal fields, and is attributed to the melting of the 3D vortex lattice. The estimated entropy jump is 1 kB/vortex/layer CuO. However, when T is lower than 50 K, we observe radical changes in M(H); the 3D melting line divides into a decoupling line at a temperature-independent field and the onset of the irreversibility.

The stability of the ${\mathrm{BaCuO}}_{\mathit{x}}$ (x\ensuremath{\ge}2) phase has been mapped over a wide range of temperature (300--1100 \ifmmode^\circ\else\textdegree\fi{}C) and oxygen pressure (${10}^{\mathrm{\ensuremath{-}}5}$--${10}^{3}$ bar). At ambient pressure and temperature, ${\mathrm{BaCuO}}_{\mathit{x}}$ is found to be in a metastable state: long annealing at 450 \ifmmode^\circ\else\textdegree\fi{}C tends to decompose the phase into ${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{5}$ and ${\mathrm{BaO}}_{2}$. Having obtained the phase stability domain in the [T,p(O${)}_{2}$] plane we were able to prepare single-phase samples of ${\mathrm{BaCuO}}_{\mathit{x}}$ with different oxygen contents suitable for precise intrinsic thermodynamical measurements. We show that the behavior of the low-temperature specific heat (1.1\ensuremath{\le}T\ensuremath{\le}32 K) and its dependence on the magnetic field (0\ensuremath{\le}B\ensuremath{\le}14 T) may be understood by taking into account a many-level magnetic system directly related to the ${\mathrm{Cu}}_{6}$${\mathrm{O}}_{12}$ and ${\mathrm{Cu}}_{18}$${\mathrm{O}}_{24}$ structural blocks of ${\mathrm{BaCuO}}_{\mathit{x}}$. Depending on the oxygen concentration, competition between antiferromagnetic (AF) ordering and the many-level system is observed. With increasing oxygen content, the N\'eel temperature decreases whereas amplitude of the many-level system increases. The zero-field AF transition belongs to the three-dimensional isotropic Heisenberg universality class.

The specific heat of Tl 2 Ba 2 CuO 6 (“2201”) superconducting ceramics with T c ≈ 90 K is measured in magnetic fields of 0, 0.1, 1, 3, 5, and 14 T. The very small anomaly at the superconducting transition can be described in B = 0 by 2D fluctuations alone, without any measurable mean-field component. Its amplitude is found to decrease rapidly for small fields, and very slowly at large fields while its shape remains symmetrical. The temperature of the top of the anomaly ( “T c ” ) is field independent. These properties show that the dimensional nature of the phase transition in Tl 2 Ba 2 CuO 6 is more closely related to that of Bi 2 Sr 2 CaCu 2 O 8 than to that of YBa 2 Cu 3 O 7 . The behavior of the specific-heat anomaly with the field is thermodynamically consistent with previous magnetization measurements.

We investigate the reversible magnetization in the mixed state of a batch of Bi 2.12 Sr 1.90 Ca 1.02 Cu 1.96 O 8 + δ (Bi-2212) ceramics with different oxygen concentrations δ and superconducting transition temperatures T c between 50 and 92 K. Additional data for a single crystal ( T c = 85 K) demonstrate the validity of the effective-mass model with essentially infinite anisotropy. The magnetization curves M ( T ) for different fields cross in a single point { M * (δ); T * (δ)} for all T c 's≥ 65K. The saturation magnetization M * (δ) is found to be proportional to T * (δ). This agrees with existing theories if the superconducting volume fraction, presumably 100%, does not vary with δ. The systematic suppression of the Meissner effect that is observed with decreasing T c is therefore attributed to enhanced pinning. For T c = 50 K, there is no single crossing point. This is attributed to a crossover to 3D in the strongly overdoped state. The penetration depth λ( T ) is evaluated from lim T →0 ∂ M /∂ln H . The systematic dependence of λ(0) versus δ indicates that the carrier density increases with oxygen content, as expected.

We present a kinetic and thermodynamic analysis of the phase transitions in stoichiometric Tl2Ba2CuO6 samples prepared under a pressure of 100 bar. We have found a first-order transformation which maintains the orthorhombic structure from high pressure to 1 bar. The as-prepared tetragonal structure in an inert gas converts to orthorhombic symmetry when annealed in an oxygen flux. The activation energies have been deduced from a dynamic analysis of weight losses during the phase changes. The enthalpy and entropy for the formation of oxygen vacancies have been determined.

We present calculations of the electron-positron momentum density for the high-Tc superconductor Tl2Ba2CuO6, together with some preliminary two-dimensional angular correlation of the annihilation radiation (2D-ACAR) measurements. The calculations are based on the first-principles electronic structure obtained using the full-potential linearized augmented plane wave (FLAPW) and the linear muffin-tin orbital (LMTO) methods. We also use a linear combination of the atomic orbitals-molecular orbital method (LCAO-MO) to discuss orbital contributions to the anisotropies. Some agreement between calculated and measured 2D-ACAR anisotropies encourage sample improvement for further Fermi surface investigations. Indeed, our results indicate a non-negligle overlap of the positron wave function with the CuOo2 plane electrons. Therefore, this compound may be well suited for investigating the relevant CuO2 Fermi surface by 2D-ACAR.

The work presented here is a new and precise measurement of the half-life of (166m)Ho by determining the activity concentration, using an ionisation chamber calibrated for this nuclide, and measuring the number of (166m)Ho atoms using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). Since the isotope (166)Er interferes with the mass spectrometric measurement, Er has to be eliminated from the (166m)Ho radioactive solution. The elimination was achieved using ion-exchange chromatography with the cation exchange resin Dowex AG 50W-X8 and 2-Hydroxybutanoic acid as the mobile phase. After a first transit through the chromatographic column, the purified (166m)Ho eluate was spiked with natural Er to get a resulting Er isotopic composition close to that of natural Er at better than 99.5%, and then it underwent two further separations to eliminate the Er. The activity concentration of this Er-free radioactive (166m)Ho solution was measured in our reference ionisation chamber calibrated for this nuclide by means of the 4πβ(PC)-γ and 4πβ(PS)-4πγ coincidence techniques and integral counting with a well-type NaI(Tl) detector and Monte Carlo efficiencies. An aliquot of this standardized solution was sent to the Paul Scherrer Institute (PSI) for mass concentration determination using an isotope dilution MC-ICP-MS approach. The mass concentration of (166m)Ho in this solution was determined with 0.25% relative standard uncertainty. This value was corroborated by two other independent measurements. The new half-life of (166m)Ho, 1132.6(39) years (k=1), is compatible with the value determined in 1965, but is 5.6% shorter and about 43 times more precise.

A homogeneous batch of YBa2Cu3Ox porous ceramics was studied in detail as a function of the annealing temperature Ta. X-ray diffraction patterns and optical micrographs show definitely one single phase. Well-separated double superconducting transitions are, however, observed; most clearly in the specific heat curves, except for Ta≈460°C. Initial values are Tc1=87 K and Tc2=92 K for Ta=300°C. Upon increasing the annealing temperature Ta to 500°C, Tc1 increases to 93.5 K while Tc2 remains almost constant. Beyond Ta=500°C, both Tc1 and Tc2 decrease. The essentially reversible splitting of the superconducting transition appears to be due to the complex kinetics of the oxygen uptake. The two critical temperatures Tc1 and Tc2 are attributed to the high oxygen content external layer of the grains and to the low oxygen content grain cores, respectively.

We have performed magnetic measurements of the critical current density ( J c ) in powders of the (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O x material. Considering flux relaxation, and applying the same criteria used for resistive J c measurements, we arrive at J c ≈2×10 4 A / cm 2 (77 K, 0 T). This value is considerably less than the highest values obtained in silver sheathed tapes of the same material, indicating that deformation induced flux pinning and possibly even grain boundary pinning may play some role in determining the J c in tapes.

The specific heat of a single crystal of YBa2Cu3O7 was measured by adiabatic calorimetry between 40 and 150 K. The typical scatter of the data is well below 0.1%. Fields up to 20 T were applied either parallel or perpendicular to the c-axis. The shape of the anomaly at Tc changes with the magnetic field in the same manner for both directions, within a scale factor that determines the bulk anisotropy ratio, 5.5 ± 0.5, for (dHc2/dT)Tc. The superconducting transition is still observable in 20 T. A growing mixed-state Maki term is present. The characteristic non-classical behavior of YBa2Cu3O7 in a field, featuring a considerable smearing of the transition and an apparently field-independent onset, may be qualitatively understood as a phenomenological consequence of the small and anisotropic value of the coherence length ξ alone, using the relevant models of field-induced critical 1D fluctuations on the one hand, and a London-like regime for the mean-field part of the transition on the other hand.

The reversible mixed state of the Tl 2 Ba 2 CuO 6+x high Tc superconductor with Tc of 90K has been investigated by DC SQUID measurements of the magnetization M(H, T) and by direct measurement of the AC differential susceptibility χ ' H which reduces to the equilibrium dM/dH when, as it is actually the present case in the reversible regime, the flux flow viscous contribution to χ ' H could be neglected. The two methods give quite similar results: a strong (field enhanced) contribution from diamagnetic fluctuation above Tc, while below Tc one observes deviation from the Abrikosov mean field theory of the mixed state which is presumably generated by thermal fluctuation enhanced effect on the vortex density and distorsions in the layered structure. This behavior appears to be common to all the high Tc superconductors as we have shown in an earlier work [1, 2, 3] for the YBa 2 Cu 3 O 7−x , 2223 Tl and 2212 Bi based compounds. Moreover, these deviations prevent any reliable determination of the H C2 critical field line by the standard method of linear extrapolation of M(H, T) curves or by the step anomaly on χ ' H expected to occur at Tc(H) if the mean field theory was to be followed in the mixed state.

We have attempted to substitute calcium on the yttrium and barium sites in the “247” superconducting phase, i.e. (Y1 - x Cax)2Ba4 Cu7O15 + δ and Y2(Ba1 - xCax)4Cu7O15 + δ. X-Ray diffraction and microprobe analyses indicate that calcium enters the structure but preferentially occupies the barium site. The solubility of calcium in the “247” phase is clearly lower than 10% (x < 0.1). The samples are not single phase even for small substitutions (x = 0.05). Higher values of x result in a higher amount of impurity phases. The (Y1 - xCax)2Ba4Cu7O15 + δ samples contain BaCuO2 as the major impurity, whereas in Y2 (Ba1 - xCax)4Cu7O15 + δ a new phase is found. This new phase was identified by microprobe analysis to have the composition YCaCu2O4.5 ± δ. No increase is observed in the critical temperature Tc as a function of calcium substitution. This is in contrast with the calcium-substituted “124” superconductor (Y1 - xCax)Ba2Cu4O8 where Tc increases from 80 K (x = 0) to about 90 K (x = 0.1).

We report on the growth of SrCuO 2 thin films prepared by RF reactive magnetron sputtering onto (100) SrTiO 3 substrates. Epitaxial (001) SrCuO 2 films are obtained at 550°C in a sputtering pressure of 1–10m Torr of Ar. Off axis x-ray analysis reveals perfect in plane ordering. We find, depending on the preparation conditions, changes in the c-axis value, which may be due to a change in the Sr-Cu-O concentrations. These modifications are correlated to changes in the resistivity of the films.

We have attempted to substitute calcium on the yttrium and barium sites in the “247” superconducting phase, i.e. (Y1 - x Cax)2Ba4 Cu7O15 + δ and Y2(Ba1 - xCax)4Cu7O15 + δ. X-Ray diffraction and microprobe analyses indicate that calcium enters the structure but preferentially occupies the barium site. The solubility of calcium in the “247” phase is clearly lower than 10% (x < 0.1). The samples are not single phase even for small substitutions (x = 0.05). Higher values of x result in a higher amount of impurity phases. The (Y1 - xCax)2Ba4Cu7O15 + δ samples contain BaCuO2 as the major impurity, whereas in Y2 (Ba1 - xCax)4Cu7O15 + δ a new phase is found. This new phase was identified by microprobe analysis to have the composition YCaCu2O4.5 ± δ. No increase is observed in the critical temperature Tc as a function of calcium substitution. This is in contrast with the calcium-substituted “124” superconductor (Y1 - xCax)Ba2Cu4O8 where Tc increases from 80 K (x = 0) to about 90 K (x = 0.1).

High-quality Bi-2212 ceramic samples were obtained by a solid-state reaction method. At the starting composition Bi2.12Sr1.90Ca1.02Cu1.96Oy, X-ray diffraction, optical micrographs and scanning electron microscopy investigations show that the level of impurity phases is less than 1% by volume. Synthesis at a few degrees below the melting point favours grain growth. A final annealing treatment at controlled temperature and oxygen pressure, followed by rapid quenching, yields sharp superconducting transitions, as observed by measurement of the a.c. susceptibility and Meissner effect, and specific heat experiments. The Meissner fraction in 2 mT approaches 50%.

A great number of pure and homogeneous Y2Ba4Cu7Oz samples were investigated by AC susceptibility, field cooling magnetization measurements, X-ray diffraction, microprobe analysis and microstructural examinations in a light microscope and a scanning electron microscope. The oxygen content z between 14.1 and 15.3 was determined by the reduction of the samples in a hydrogen flow. To reach the (z-T-p) equilibrium, well-crystallized samples were annealed in pure oxygen (10−6 ⩽p(O2)⩽102 bar) between 300 and 900°C and subsequently quenched. The specimens produced in this way show sharp χAC transitions in the entire range of critical temperatures (30 K ⩽ Tc ⩽ 95 K). The correlation Tc versus the orthorhombicity 2(b−a)/(b+a) is almost linear, similar to that observed in YBa2Cu3Ox. This is contrast to the distinct behaviour of the functions Tc versus z and Tc versus x in these two compounds. We present a stability-decomposition map 0for Y2Ba4Cu7Oz over a wide range of (T, p) coordinates.

Dimagnesium ruthenium tetrahydride (Mg2RuH4) and its deuteride were synthesized by the reaction of magnesium with ruthenium under a hydrogen (deuterium) pressure and characterized by X-ray and neutron powder diffraction. The compound crystallizes in the orthorhombic space group Cmcm. The structure contains a new 16-electron low spin [RuD4]4 complex anion of C2v symmetry which derives from the octahedral 18-electron [RuDe]4- complex anion in Mg2RuD6 by removal of two cis deuterium ligands.

Under suitable preparation conditions, Y2Ba4Cu7O15 (“247”) has the highest critical temperature of the YBCO superconductor family. We present an extensive characterization of 247 samples with Tc onsets as high as 95K in bulk ceramic form. The experiments performed include metallographic examinations, X-ray diffraction, microprobe analysis, resistivity, AC susceptibility, Meissner and normal-state susceptibility, magnetic determinations of the slope of the critical field at Tc and of the critical current density, and specific heat from 1 to 330 K. Several properties (e.g., normal-state susceptibility, specific heat jump at Tc, slope of Hc2(T) at Tc, resistivity) may be considered as an approximate average between those of YBa2Cu4O8 (“124”) and those of YBa2Cu3O7 (“123”). The signature of fluctuations in magnetic properties is enhanced with respect to both 124 and 123. The behavior of the specific heat close to T = 0 is similar to that of the other members of the family, with the presence of a quasi linear T-term and the absence of any upturn in C/T. The maximum Tc appears to be related to optimal doping, consistent with the behavior observed in 123 as a function of the oxygen concentration.

More than 100 samples in the system Y-Ba-Cu-O were investigated by X-ray diffraction, microprobe analysis and microstructural examination in a light microscope and a scanning electron microscope. The samples were prepared under a pressure of p(O2) = 90 bar at temperatures between 900 and 1300 °C. The stable phases under these conditions are discussed in detail. In particular we observed the two new compounds Ba2Cu3Ox and Y2Ba9Cu6Oz.

We have mapped the critical temperature of Bi 2 Sr 2 CaCU 2 O 8+δ (2212) in the ( p, T ) plane for 10 −5 ≤ p ≤ 80 bar (O 2 ) and 300 ≤ T ≤ 840°C. Equilibrium conditions are frozen by quenching the samples into liquid gallium. The DC diamagnetic transitions remain sharp, and span an unusual temperature range for the 2212 phase (51.5 K to 94.4 K). This variation is attributed to a measured 2.5% change of the oxygen content ( Δδ ⋍ 0.2 ) in the structure. At the same time, we do not observe any significant variation of the lattice parameters. The Meissner effect, measured in an external magnetic field of 20 Oe, increases with T c . The slope of the normal-state susceptibility ∂ χ / ∂T becomes markedly less negative with decreasing oxygen concentration. It vanishes for the optimum doping that leads to T c ⋍ 94 K .

The specific heat of four samples of the 81 K-superconductor Yba 2 Cu 4 O 8 prepared under high oxygen pressure is investigated. The low-temperature specific heat is sample-dependent. For the most homogeneous sample, the coefficient of the linear term at T ⪡ T c is γ ∗ =4.9±0.2mJ/(K 2 mole) and the initial Debye temperature is θ (0)=350±10K. A small specific heat jump Δ C / T c =12 to 16 mJ/(K 2 mole) is observed at the superconducting transition; it is only weakly correlated with the field cooling Meissner fraction (14 to 55% in 20 Oe). A tentative separation of the total specific heat into electron and phonon terms is obtained by a least-squares fit of high-temperature data using a model of the phonon density of states. The electron contribution is found to be much smaller than for YBa 2 CU 3 O 7 . Thermochemical data are tabulated.

We have carried out extensive characterization and measurements on a high purity Y 1 Ba 2 Cu 4 O 8±δ (124) sample. Metallographic investigations, X-ray diffraction and microprobe analysis have been performed and reveal a single phase with traces of CuO. Resistivity measurements give a resistivity ratio R (300 K)/ R (100 K) of ≈ 6.1, and a critical temperature T co of 82.5 K. The AC susceptibility shows a sharp transition at about same temperature. The Meissner effect, with a 55% fraction (at 4.2 K with H =20.3 Oe) confirms the bulk nature superconductivity. The specific heat difference C (0, T )− C ( H , T ) has been estimated from magnetization measurements in the vicinity of T c for μ 0 H =8 T and compared to the direct calorimetric measurement. Both methods indicate that the specific heat jump amounts to approximately 15 mJ/K 2 mol. Fluctuation contributions are found up to several degrees above T c . Inter- and intra-grain critical currents were determined at 4.2 K by AC inductive measurements and by measuring the irreversibility of the magnetization curve at a constant temperature (Bean's method). The results shows that there is no significant difference between the critical currents of the 124 phase and those of the 123 phase (Y 1 Ba 2 Cu 3 O 7 ).

A great number of YBa2Cu3Ox (if6.2 ⩽ x ⩽ 7.0) samples were investigated by a.c. susceptibility and field cooling magnetization measurements, X-ray and microprobe analyses, microstructural examinations in a light microscope and a scanning electron microscope. The oxygen content x was determined by the reduction of the samples in a hydrogen flow. To reach the (x-T-p) equilibrium, well-oxidized YBCO samples were annealed in pure oxygen flow (10−1 atm ⩽ P(O2) ⩽ 1 atm) at temperatures between 200 and 1000 °C and subsequently quenched into liquid gallium. Other samples were prepared under 80 atm O2 (200 °C ⩽ T⩽ 1000 °C). We were able to produce macroscopically very homogeneous oxygen deficient material with sharp χa.c. transitions in the entire range of critical temperatures. The evolution of the Meissner fraction is non-monotonic with a relative maximum near x = 6.6, a concentration where the orthorhombicity 2 (b − a) (a + b) also deviates from monotonic behaviour.

Strain relaxation and the ferroelectric critical temperature were investigated in a series of epitaxial Pb(Zr0.20Ti0.80)O-3 thin films of different thicknesses grown on metallic 0.5% Nb-doped SrTiO3 substrates. Detailed x-ray diffraction studies reveal that strain relaxation progressively occurs via misfit dislocations as the film thickness is increased from fully coherent films (for films below 150 A) to essentially relaxed films (for thicknesses above typically 800 A). It is found that this change in the strain state does not modify the ferroelectric critical temperature which is found for all the samples to be around 680 degrees C, a value much higher than the bulk.

The radioactive concentrations of (18)F, (88)Y and (152)Eu solutions and the activity of (222)Rn gas ampoules are measured using a primary method involving 4pigamma NaI(Tl) integral counting with a well-type NaI(Tl) detector and efficiencies computed by Monte Carlo simulations. The simulations use the GEANT code coupled with a routine (sch2for), which generates randomly the decay paths and emissions depending on the decay scheme parameters. The resulting radioactive concentrations of (88)Y, (152)Eu and (18)F are found to agree with those measured with other primary measurement methods, such as 4pi (beta, e, X)-gamma coincidence counting or liquid scintillation counting. Results of the determination of the activity of (222)Rn gas ampoules by this method also match the results of an absolute standardisation technique in which radon is condensed onto a cold surface and its alpha-emissions are detected through an accurately specified solid angle.

Using a SQUID magnetometer, the magnetic field-induced magnetization has been measured on a ferroelastically `poled’ orthorhombic ferroelectric/ferroelastic single domain of Ni3B7O13Br. Measurements were performed along the three principal orthorhombic crystallographic axis directions at 0.5, 1.0, 3.0, and 5.0 kOe in the range 4–40 K, and at 10 kOe in the range 4–300 K. The corresponding magnetic susceptibilities have been derived. The sequence of magnetic phases at low temperature, their magnetic point groups and transition temperatures Tc1=30 K and Tc2=21 K have been confirmed: mm21′⇐Tc1⇒m′m2′⇐Tc2⇒1. In the weakly ferromagnetic/ferroelectric/ferroelastic orthorhombic phase with Shubnikov–Heesch point group m′m2′ in the range 21 K M s lies along the orthorhombic b0-axis, with a value of M s =0.71 emu/g at 25 K. In the triclinic phase with point group 1, below Tc2=21 K, M s assumes a temperature-dependent crystallographic general direction lying nearly in the pseudo-orthorhombic (0 0 1)0-plane, for example at 5 K with a value of M s =(2.7 a 0 +1.76 b 0 +0.34 c 0 ) emu/g.

The olivine Mn2SiS4 is antiferromagnetic (AF) up to 83 K, behaves as a weak ferromagnet (WF) from 83 to 86 K, and becomes paramagnetic (P) at higher temperatures. Its counterpart Fe2GeS4 undergoes similar transitions in a reversed order: WF up to 69 K, AF up to 144 K and then P. These transitions are characterized by susceptibility measurements and investigated by high resolution specific heat experiments. The transition to the paramagnetic state is of the critical type and belongs to the universality class of the Heisenberg model for Mn2SiS4 and the 3D Ising model for Fe2GeS4. The WF/AF or AF/WF transition is of first order with a latent heat ΔH = 0.09 J/mol for Mn2SiS4 and ΔH = 105 J/mol for Fe2GeS4. The lattice is found to be stiffer for the Mn thiosilicate than for the Fe thiogermanate.

International audience; We have investigated the reversible magnetization below Tc in high-quality YBa2Cu307_d (Y-123), YBa2Cu4Os (Y-124), Y2Ba4Cu7O15+x (Y-247) and Bi2Sr2CaCu2O8+x (Bi-2212), Tl2Ba2Cu106+d (Tl-2201) and Tl2Ba2CalCu2O8+d (T1-2212) ceramics. Except for the stoichiometric Y-124 phase, the oxygen concentration was optimized in order to obtain the highest value of the critical temperature for which the normal-state susceptibility becomes temperature independent. Using the simple London model, we are able to fit the reversible magnetization M(T, H) outside the region near Tc with good accuracy for the nearly three-dimensional YBaCuO phases. For the very anisotropic BiSrCaCuO and TlBaCaCuO phases, we have to include an additional term to take into account the fluctuations of vortices. An important result is that Y-123 exhibits a critical field clearly higher than those of the BiSrCaCuO or TlBaCaCuO phases. We obtain for the Y-123 phase a slope at Tc μodHC2,C/dT = -4.3 T/K and an extrapolated μoHC2,C (0) = 280 T.

The reversible magnetization M(T,H) in the mixed state of the quasi-two-dimensional superconductor Bi2Sr2CaCu2O8 appears to factorize into a product f(T)g(H). It was recently argued that this implies a puzzling temperature-independent critical field Hc2, at least in the region where scaling is observed. We propose an alternative explanation in which this scaling property is approximate and results from the presence of vortex fluctuations, keeping a conventional temperature dependence for Hc2(T). New data in the BiSrCaCuO (2212) and TlBaCaCuO (2201, 2212) systems that obey to a good approximation the factorization M(T,H)=f(T)g(H) are presented.

A 0.25 g single crystal of Bi 2 Sr 2 CaCu 2 O 8 with a sharp superconducting transition at 85 K is characterized by AC susceptibility, X-ray diffraction, Meissner effect, normal-state susceptibility, magnetization in the mixed state, and specific heat from 1 to 300 K. The residual linear term γ ∗ T in the specific heat at low T is small and field independent. High-resolution ( t c show that: (a) there is no mean-field specific heat jump at T c ; (b) a symmetrical logarithmic ‘divergence’ occurs at the transition; (c) its amplitude decreases with B -2 3 ; (d) its width scales with B 1 2 ; (e) T c does not depend on the field, and coincides with the crossing point of the magnetization curves M ∗ ( H, T ∗ ); (f) a crossing point C x (H, T x ) is found in the specific heat 10 K below T c ; (g) the effect of a field along the CuO planes is at least 50 times smaller than that of a field along the c axis; (h) no transition is observed at the irreversibility line. An empirical formula for ϑ( C/T )/ϑ H that accounts for C ( H, T ) and M ( H, T ) data over a wide temperature and field range is proposed.

A serie of the Tl-2201 cuprate with T c ranging from ∼ 40 K to 92 K with tetragonal as well as orthorhombic symmetry has been prepared and their differential magnetic susceptibility χ' H measured under static field up to 2 Tesla. Independently of the criteria defining the irreversibility onset, we show that the extension of the reversible regime in the (H,T/T c ) diagram increases with T c . Thus, contrary to what has been observed for YBa 2 Cu 3 O 6+x where the reversibility domain was enlarged with the T c decreasing, here for low T c the reversibility domain is more limited and shifted toward H c2 (T) line. These results are discussed taking into consideration the evolution of λ and the possible evolution of ξ with T c in this Cu-O monolayer overdoped HTS.

Homogeneous Tl 2 Ba 2 CuO x ceramics were synthesized by a novel route allowing tight control of thallium losses, starting from Tl 2 Ba 2 O 5 precursors and using high gas pressures. Annealing followed by fast quenching into gallium allowed us to map the critical temperature (0 to 92 K) and the crystal symmetry (orthorhombic or tetragonal) in the (T;p(O 2 )) plane. The orthorhombic phase is found to be stoichiometric, whereas the tetragonal phase has a homogeneity domain in the thallium concentration. The critical temperature, determined by the oxygen concentration, is not sensitive to the crystal structure. Selected physical properties of Tl 2 Ba 2 CuO x compounds are presented.

The stability of the Y2Ba4Cu7Ox phase (Y-247) has been investigated under pure oxygen pressures up to 1700 bar. The samples, as synthesized at 1000°C and p(O2)=20 bar, show a critical temperature Tonsetc of 94K. However, the positive temperature derivative of the normal state susceptibility above the fluctuation region suggests that the optimum hole doping is not yet reached. Although at temperature higher than 600°C and p(O2) in the 1500 bar range, the 247 phase decomposes into YBa2Cu4O8 as a major phase, the metastable 247 phase may be further oxygenated at lower temperatures. The effect on the susceptibility is as expected (χ becomes temperature independent) but surprisingly, Tc saturates at 95K . Very long annealing times broaden the superconducting transition, decrease the Meissner fraction and finally induce decomposition even at temperatures as low as 300°C.

The reversible mixed state of the Tl 2 Ba 2 CaCu 2 O 8 (Tl-2212) high T c superconductor with T c of 107K has been investigated by direct measurement of the AC differential susceptibility ρ' H which reduces to the equilibrium dM/dH when, as it is actually the present case in the reversible regime, the flux flow viscous contribution to ρ' H could be neglected. The following results have been obtained: a strong (field enhanced) contribution from diamagnetic fluctuation above T c , while below T c one observes deviation from the Abrikosov mean field theory of the mixed state which is presumably generated by thermal fluctuation enhanced effect on the vortex density and distortions in the layered structure. This behaviour appears to be common to all the high T c superconductors as we have shown in an earlier work for the YBa 2 Cu 3 O 7−x , Tl-2201, Tl-2223 and Bi-2212 compounds. Moreover, these deviations prevent any reliable determination of the H c2 critical field line by the standard method of linear extrapolation of M(H,T) curves by the step anomaly on ρ' H expected to occur at T c (H) if the mean field theory was to be followed in the mixed state.

We have investigated the reversible magnetization below Tc of the quasi two-dimensional superconducting compound Tl2Ba2CaCu2O8. All magnetization data collapse on a single curve when plotted as M(T,H)/M(T',H) vs T and M(T,H)/M(T,H') vs H, where T' and H' are freely chosen in the reversible domain. This scaling implies a factorization M M(T,H) = f(T)g(H). In the frame of the London model, one would conclude that Hc2(T) is constant in the scaling region, a puzzling issue. We show that if we take into account additionally the vortex fluctuation term that explains the crossing point {T∗;M∗} where ∂M/∂H|T vanishes, one can explain the scaling using a conventional temperature dependence of Hc2(T).

We have investigated the reversible magnetization M(T,H) of YBaCuO ceramics. The three superconducting phases of this system (Y-123: YBa2Cu3O7; Y-247: Y2Ba4Cu7O15 and Y-124: YBa2Cu4O8) present a small reversible regime below Tc extending over 10 to 25 K, depending on the external magnetic field (3 to 55 kOe, respectively). Within these temperature and field limits, the magnetization can be described satisfactorily by the London model. Using additionally the Werthamer-Helfand-Hohenberg (WHH) temperature dependence for Hc2(T) and the 2-fluid parabolic law for Hc(T), a fit of all reversible data yields μ o ∂ H c2,c /∂ T | Tc ≈−4.3 T / K , ξ ab (0)≈10.8 A and λ ab (0)≈1650 A for the Y-123 phase. The critical field appears to be significantly higher than that obtained by other methods. Lower critical fields are obtained for Y-124, and intermediate values are found for Y-247, as expected from its crystallographic structure that consists of alternating blocks of Y-123 and Y-124.

Single crystals of KNiPO4 were synthesized in a KCl flux. The crystals have the orthorhombic space group Pna21 at room temp. with a = 8.6333(5), b = 9.2565(5) and c = 4.9064(9)[.ANG.]. Ferroelectric 180 Deg domains, parallel to [001] were revealed by chemical etching. Magnetization measurements along all three orthorhombic axis, made on a SQUID magnetometer, show a magnetic phase transition to the antiferromagnetic state at TN(25 +- 0.5) K with a weak ferromagnetic behavior. All possible coeffs. of the linear magnetoelec. effect have been tested, only a12 and a21 were found to be non zero. These components of the linear magnetoelec. effect are consistent with magnetic point group mm2 which was also found by neutron diffraction studies. However, this magnetic point group does not allow weak ferromagnetism, which is also obsd. The pyroelec. nature of this compd. has also been put into evidence by measuring a change of the spontaneous polarization DPs = 0.61 [mC cm-2] between 4 and 300 K. The total pyroelectric coefficient at 300 K is p3 = 2.75 x 10-5 [Cm-2] K-1].

We have synthesized high quality ceramic Tl 2 Ba 2 Cu 1 O 6+δ (2201) samples using the high pressure, high temperature route. At high oxygen content, the structure is orthorhombic and the samples are metallic but non-superconducting. Upon lowering the oxygen content, the symmetry changes from orthorhombic to tetragonal or pseudo-tetragonal. In the latter phase, the maximum superconducting critical temperature reaches 92 K. Optical micrographs show large 2201 grains and some traces of impurity phases like Tl 2 Ba 2 O 5 , Ba 2 Cu 3 O x and CuO. X-ray diffraction shows only the 2201 phase. Plasma emission spectroscopy indicates that the global sample stoichiometry is Tl:Ba:Cu=2:2:1. This analysis proves that the high pressure route effectively prevents the thallium evaporation. HREM investigations exclude the possibility of cation vacancies. Microprobe analyses (EDAX) show no variation of the cation stoichiometry between the 2201 grains. X-ray diffraction on a superconducting single crystal yields a refined composition Tl 1.94 Ba 2 Cu 1.06 O 6 .

We have measured magnetic properties of high quality Tl2Ba2Cu1O6+δ 90 K tetragonal superconducting and orthorhombic metallic non-superconducting ceramics. The AC susceptibility shows distinct intra- and intergrain transitions down to H=0.004 Oe rms. The field cooling measurement at 20 Oe shows a sharp transition at 90 K and 50% of full flux exclusion at 5 K. At T>150 K, i.e. above the region where superconducting fluctuations contribute, the normal-state susceptibility is temperature independent and diamagnetic with a value of −5×10−8 emu/g. The intergrain critical current densities determined at B=1 T, using Bean's critical state model, are very low, 6×103 A/cm2 at 5 K and below 1 A/cm2 at 40 K.

International audience; High-quality Tl2Ba2CuO6+delta (2201) compounds were synthesized using a high-pressure route. Starting from Tl2Ba2O5 and CuO, the reactions yielding either the orthorhombic or the tetragonal (superconducting) phases were carried out under 100 bar of oxygen or argon and helium, respectively. Preferential thallium losses are prevented and high sintering temperatures can be used to obtain pure samples with large 2201 grains. The experiments performed include metallographic examinations, X-ray diffraction, microprobe analysis, plasma emission spectroscopy, HRTEM, AC susceptibility, Meissner field cooling, hysteresis loops, normal-state susceptibility and specific heat measurements.

We present high resolution heat capacity data in the temperature range 30–300 K for several homogeneous samples of the superconductor Tl2Ba2CuO6 (“2201”) with transition temperatures in the vicinity of 90 K. The specific heat peaks that appear close to the superconducting transition in some samples are identified as being due to gases trapped in the pores of the ceramics. The very small intrinsic anomaly at Tc, of the order of 1% of the total specific heat, is best observed in samples prepared under He atmosphere. The data are analysed using a model that includes a parametrized phonon spectrum, a mean-field jump at Tc, and a fluctuation contribution of the Lawrence-Doniach type. The fit yields an average coherence length of 7.5 A (sample with Tc =92 K) and 8.2 A (sample with Tc=87 K), and a vanishingly small mean-field jump. The homogeneity of the electronic subsystem is questioned. The data are compared with new results for Bi2Sr2CaCu2O8 (“2212”, Tc=93 K) and YBa2Cu3O7 (“123”, Tc=91 K). The fluctuation component is similar for all three, but YBa2Cu3O7 stands out with its relatively large mean-field jump.