Showing papers on "Transition temperature published in 1993"

Superconductivity above 130 K in the Hg–Ba–Ca–Cu–O system

Abstract: THE recent discovery1 of superconductivity below a transition temperature (Tc) of 94 K in HgBa2CuO4+δ has extended the repertoire of high-Tc superconductors containing copper oxide planes embedded in suitably structured (layered) materials. Previous experience with similar compounds containing bismuth and thallium instead of mercury suggested that even higher transition temperatures might be achieved in mercury-based compounds with more than one CuO2 layer per unit cell. Here we provide support for this conjecture, with the discovery of superconductivity above 130 K in a material containing HgBa2Ca2Cu3O1+x (with three CuO2 layers per unit cell), HgBa2CaCu2O6+x (with two CuO2 layers) and an ordered superstructure comprising a defined sequence of the unit cells of these phases. Both magnetic and resistivity measurements confirm a maximum transition temperature of ∼ 133 K, distinctly higher than the previous established record value of 125–127 K observed in Tl2Ba2Ca2Cu3O10 (refs 2,3). The discovery in 1986 of the first copper oxide superconductor stimulated an explosion of research activity that continues to the present day. The early years of high-temperature superconductivity were characterized by the rapid discovery of many new materials with increasingly high transition temperatures. The record now stands at ~133 K, attributed to a mercury-containing compound reported by Schilling et al. in 1993, although the dream of achieving room-temperature superconductivity has yet to be fulfilled.

Observation of the spin-Peierls transition in linear Cu2+ (spin-1/2) chains in an inorganic compound CuGeO3

Abstract: The magnetic susceptibility of single-crystal ${\mathrm{CuGeO}}_{3}$, a linear ${\mathrm{Cu}}^{2+}$ (spin-1/2) chain compound, was measured. The susceptibilities in all the directions rapidly drop to small constant values with decreasing temperature below a phase transition temperature near 14 K. The magnetic-field dependence of the transition temperature quantitatively agrees with both theoretical predictions and experimental results for organic spin-Peierls systems. This Letter is the first report of an unambiguous determination of the existence of the spin-Peierls transition in an inorganic compound.

Superconductivity at 94 K in HgBa 2 Cu0 4+δ

Abstract: FOLLOWING the discovery1 of high-transition-temperature (high-Tc) superconductivity in doped La2CuO4, several families of related compounds have been discovered which have layers of CuO2 as the essential requirement for superconductivity: the highest transition temperatures so far have been found for thallium-bearing compounds2. Recently the mercury-bearing compound HgBa2Rcu2O6+δ (Hg-1212) was synthesized3 (where R is a rare-earth element), with a structure similar to the thallium-bearing superconductor TlBa2CaCu2O7 (Tl-1212), which has one T1O layer and two CuO2 layers per unit cell, and a Tc of 85 K (ref. 2). But in spite of its resemblance to Tl-1212, Hg-1212 was found not to be superconducting. Here we report the synthesis of the related compound HgBa2CuO4+δ (Hg-1201), with only one CuO2 layer per unit cell, and show that it is superconducting below 94 K. Its structure is similar to that of Tl-1201 (which has a Tc of < 10 K)4, but its transition temperature is considerably higher. The availability of a material with high Tc but only a single metal oxide (HgO) layer may be important for technological applications, as it seems that a smaller spacing between CuO2 planes leads to better superconducting properties in a magnetic field5.

Superconductivity above 150 K in HgBa2Ca2Cu3O8+δ at high pressures

Abstract: RECENTLY, superconductivity at >130 K was reported1,2 in multiphase samples of the compound system HgBa2Can−1CunO2n+2+δ (Hg-12(n−l)n) with n = 1, 2, 3,.... Single-phase Hg-1223 was subsequently synthesized and found to be superconducting at a record temperature of 135 K (ref. 3). It remains to be seen if a much higher transition temperature (Tc) than 135K can be achieved in this compound system. The application of pressure generally increases the Tc of underdoped layered cuprates, with a pressure derivative of Tc that decreases with increasing doping4,5. Preliminary studies on Hg-12236 showed a pressure-induced increase in Tc without any sign of saturation up to 17 kbar. Here we report the observation of superconductivity at up to 153 K in Hg-1223 at 150 kbar, with a main transition at 147 K. This observation provides an indication that superconductivity at >150 K may be possible in this system at ambient pressure, if suitable forms of chemical substitution can be found.

The effect of oxidation on the Verwey transition in magnetite

Abstract: At the Verwey transition (Tv≈110–120 K), magnetite transforms from monoclinic to cubic spinel structure. It has long been believed that magnetic remanence and susceptibility would change markedly at Tv in the case of coarse grains but only slightly or inappreciably in the case of fine (<1 µm) grains. We find on the contrary that remanence changes at Tv by 50–80% in both large and small crystals, if they are stoichiometric. However, minor surface oxidation suppresses the transition, and the fact that fine grains oxidize more readily leads to an apparent size dependence. Our experiments used submicron magnetite cubes with mean sizes of 0.037, 0.076, 0.10 and 0.22 µm which were initially non-stoichiometric (oxidation parameter z from 0.2–0.7). A saturation isothermal remanent magnetization (SIRM) given in a 2.5 T field at 5 K decreased steadily during zero-field warming to 300 K with little or no indication of the Verwey transition. After the oxidized surface of each crystal was reduced to stoichiometric magnetite, the SIRM decreased sharply during warming by 50–80% around 110 K. The change in SIRM for the 0.22 µm grains was almost identical to that measured for a 1.5 mm natural magnetite crystal. Thus a 1012 change in particle volume does not materially affect the remanence transition at Tv but oxidation to z=0.3 essentially suppresses the transition. The effect of the degree of oxidation on Tv provides a sensitive test for maghemitization in soils, sediments and rocks.

A ferromagnetic transition at 1.48 K in an organic nitroxide

Abstract: IN one of the pioneering studies of magnetism at the microscopic level, Heisenberg1 concluded that ferromagnetism could not exist in compounds consisting only of light elements. A few well defined2, purely organic materials (consisting only of the elements carbon, hydrogen, oxygen and nitrogen) have subsequently been found3–5 to exhibit evidence of ferromagnetic interactions at low tem-peratures, with a small number of these showing a transition to a true ferromagnetic state. For example, the p-nitrophenyl nitronyl nitroxide radical has a Curie (transition) temperature of 0.60 K (ref. 6), and a possible ferromagnetic transition has been identified7,8 in a C60-based ionic complex. More recently, it was reported9 that the magnetization and magnetic susceptibility behaviour of a crystalline nitroxide biradical, N,N′-dioxy-l,3,5,7-tetramethyl-2,6-diazaadamantane, are indicative of ferromagnetic interactions. Here we report the existence of a ferromagnetic transition in this system, with a Curie temperature of 1.48 ± 0.02 K. As yet, this is the highest transition temperature found for a purely organic, non-ionic material.

Phase transition in PbTiO3 ultrafine particles of different sizes

Abstract: The size effect on the ferroelectric phase transition in PbTiO3 ultra-fine particles is reported. Samples with particle sizes from 20 to 200 nm were prepared by a sol-gel process followed by calcining at different temperatures. The particle size was determined by X-ray diffraction from the integrated width of diffractions. The soft-mode frequency at room temperature was measured by Raman scattering. It decreases with decreasing particle size. The ferroelectric phase transition was traced by specific-heat measurement. The transition temperature decreases and the transition becomes diffused as the particle size decreases. The size dependence of TC can be described by TC(D)=766-256/(D-8.8) (K), where 766 K is the TC for bulk PbTiO3 and D (nm) is the particle size. This equation gives a critical size of 9.1 nm below which ferroelectricity disappears.

Shear-induced isotropic-to-lamellar transition.

Abstract: Reciprocating shear is found to increase the isotropic-to-lamellar transition temperature in a symmetric diblock copolymer melt. The temperature at which the isotropic state becomes unstable rapidly approaches the ordering transition as the shear rate increases. Shear-induced ordering results in lamellae orientation with wave vectors directed normal to the shear and velocity gradient directions. These results are anticipated by a recent theory that accounts for the suppression of fluctuations by a symmetry breaking field in this class of weakly first-order phase transitions.

Magnetic phase diagram of the spin-Peierls cuprate CuGeO3

Abstract: The magnetisation of the spin-Peierls cuprate CuGeO 3 was measured in magnetic fields up to 25 T at constant temperatures from 3.5 to 20.3 K. The transition temperature without the field is 14.0 K. The characteristic nonlinearity of the magnetisation was observed below 13.4 K, which means the transition was from a dimerised phase to other phases. This transition is of first and second order below and above 10.0 K, respectively. The critical field of the transition between dimerised and magnetic phases is 12-13 T. The magnetic phase diagram of CuGeO 3 agrees qualitatively with both experimental results of typical organic spin-Peierls materials and a theoretical calculation

Calculations of dielectric properties from the superparaelectric model of relaxors

Abstract: An ideal superparaelectric is treated as an ensemble of independent, coherently polarizing regions, of linear dimension lambda , each of which behaves as a 'Devonshire ferroelectric', with transition temperature T0, and local polarization, Ps approximately (T-T0)12/. For sufficiently small lambda , the direction of the local polarization vector can fluctuate with thermal energies, giving rise to a static permittivity, epsilon s, which follows a modified Curie law: Es approximately (T-T0) lambda 3/T. In addition, the peak in permittivity at T0 is suppressed due to spatially uniform thermal fluctuations in the magnitude of the local polarization. The activation energy for the directional fluctuations increases with decreasing temperature: Ea approximately (T-T0)2 lambda 3, giving rise to Debye-type relaxation and peaks in the real and imaginary permittivity around Tm, where T0-Tm increases with decreasing lambda . For a fictitious superparaelectric, based on Pb(Zr0.7Ti0.3)O3, the effects described above become important for lambda

Increased transition temperature in superconducting Na2CsC60 by intercalation of ammonia

Abstract: A WIDE variety of alkali-metal fullerides (AxC60) have been prepared, with stoichiometries varying between 1⩽=x⩽=11 (refs 1,2). Most of the compounds with x = 3 are superconductors with unusually high transition temperatures (Tc) which increase with the size of the face-centred cubic unit cell3. The increase in Tc on lattice expansion is attributed to a decrease in the conduction band width, and therefore one ultimately expects a saturation or downturn of Tc followed by a metal–insulator transition. We have sought to explore this regime by expanding the A3C60 structure through intercalation of neutral molecules capable of solvating the A+ ions. Here we report that the reaction of ammonia with Na2CsC60 (Tc = 10.5K; lattice constant α = 14.132 A) produces the compound (NH3)4Na2CsC60, which has an expanded unit cell (a = 14.473 A) and an increased Tc of 29.6 K. The structure contains the Na(NH3)+4 cation on the octahedral site.

Prewetting in a binary liquid mixture.

Abstract: We present evidence for the existence of a prewetting transition in the mixed region of the phase diagram of the binary liquid mixture methanol-cyclohexane. Our measurements reveal the existence of a first order thin to thick film transition as the temperature is decreased towards the separation temperature T S at a temperature T pw above T S and distinct from it. This transition is strongly hysteretic. The determined prewetting line approaches the coexistence curve tangentially. Also, the prewetting critical point is found to display 2D Ising-like criticality

Ellipsometry Study of 2D Crystallization of 1-Alcohol Monolayers at the Water Surface

Abstract: We investigate monolayers of 1-alcohols (C8 to C14) obtained by placing an excess of material on top of a clean water surface. The excess is collected in a lens which constitutes a reservoir of molecules for the monolayer. We observe a first-order transition (2D crystallization) with temperature. Ellipsometry measurements show a narrow transition in temperature with no measurable hysteresis corresponding to a jump of the monolayer thickness. For each sample, simultaneous surface tension measurements show a break of slope at the same transition temperature. Both below and above the transition we observe large positive coefficients for the temperature variation of the surface tension.

Effect of superconducting fluctuations on the transverse resistance of high-Tc superconductors.

Abstract: Superconducting fluctuations in strongly anisotropic planar superconductors substantially affect the resistivity above the transition temperature. In a wide temperature range above T(c), the main effect of these fluctuations is to create a virtual gap in the electronic spectrum, causing an increase in resistivity. As a result the resistivity shows a maximum close to the transition temperature. Closer to the transition temperature the correction to the resistivity becomes negative and grows.as 1/(T - T(c))2, WhiCh is followed by the usual (T - T(c))1/2 law still closer to the transition temperature. This observation may help to understand the unusual behavior of the resistivity in the c direction in high-T(c) cuprates.

Stoichiometric YBa2Cu3O7 is overdoped

Abstract: There has been much disagreement about the origin of the 90 K transition temperature plateau in YBa 2 Cu 3 O 7−δ . It is argued that the “90 K plateau” for YBa 2 Cu 3 O 7−δ is simply the peak of the parabolic curve of T c as a function of hole concentration and that when δ ≈0 then 1–2–3 is slightly overdoped by 0.03 to 0.04 holes per copper. This is demonstrated from studies on altervalent substitutions, oxygen deficiency, comparison with 1–2–4 and 2–4–7, thermoelectric power, normal state susceptibility, structural data and bond valence sums, muon spin relaxation, Raman studies on the δ-dependence of the superconducting energy gap, and the pressure dependence of T c . It is also shown that cation-stoichiometric LaBa 2 Cu 3 O 7 should have T c max ≈100 K.

Origin of tweed texture in the simulation of a cuprate superconductor

Abstract: The origin of metastable tweed texture (microstructure) is studied by computer simulation. A two-dimensional model of 99*99 unit cells represents a layer with an oxygen deficit of the high-Tc superconductor YBa2Cu3O7- delta and exhibits the ferroelastic tetragonal-orthorhombic phase transition. The tweed texture is known to be important for flux pinning. In the model, the local ordering of the oxygen atoms produces long-range strain fields, which have been studied computationally by a molecular dynamics technique. The system has a strong tendency to form the tweed texture, as observed experimentally. Well above the structural phase transition temperature, the strain fluctuations show well developed embryos of the tweed texture, whose temperature dependence agrees with theoretical estimates obtained using the independent-site approximation. On quenching to below the transition temperature, the texture first becomes more regular in spacing and coarsens before the system orders macroscopically: the kinetics behaviour is quite different from the traditional model of nucleation and growth.

The glassy state and accelerated aging of soybeans

Abstract: Deteriorative changes in seeds may be expected to reflect changes in physical state as well as in chemical composition. In the present study, measurements of changes in glass transition, lipid phase transition and sugar content were made during accelerated aging of two cultivars of soybeans (Glycine max Merrill cv. Chippewa 64 and cv. Hodgson 78). The glass transition in axes, as measured by the thermally stimulated depolarization current method, showed gradual decreases in both magnitude and transition temperature during accelerated aging, and eventually, axes of seeds lost their ability to enter the glassy state. Sucrose, raffinose and stachyose contents in seed axes showed little or no change during the aging treatment. Membrane lipids in aged axes retained the liquid crystalline phase during aging. These data suggest that the changes of glass transition during accelerated aging occurred without associated changes in soluble sugar contents or changes in the liquid crystalline state of membrane lipids. The loss of the glass transition during accelerated aging could be a consequence of the annealing effect due to elevated temperature and moisture content. We propose that a loss of the glassy state during accelerated aging leads to an enhanced rate of subsequent deteriorative reactions in seeds and accelerates the loss of viability.

Reentrant Superconductivity of CeRu2

Abstract: Magnetization curves [MC] and temperature dependence of the AC susceptibility [TS] were measured in the superconducting phase of CeRu 2 with T c =6.55 K in zero field. For fields higher than 0.8 T in [TS], the superconducting susceptibility behaves normally up to the lowest transition temperature T cL where it decreases suddenly; the field-induced reentrant superconductivity occurs at T cL and breaks very sharply to the normal state at T cH (the highest transition temperature). In [MC], an anomaly appears when the magnetization reaches to the extrapolated magnetization of the normal state at H R (reentrant field). Magnetization drops suddenly to negative in the field-increasing process for temperatures below 3.9 K, and it only increases for temperatures above 4.0 K. Reentrant superconductivity occurs between, H R and H c2 . The reentrant phase has positive magnetization in the background.

Synthesis and neutron powder diffraction study of the superconductor HgBa2CaCu2O6+δ before and after heat treatment

Abstract: The crystal structure of the superconductor HgBa2CaCu2O6+δ has been analyzed by neutron powder diffraction techniques at room temperature and at 10 K. The compound crystallizes with the symmetry of space group P4/mmm and lattice parameters a=3.8526(2), c=12.6367(8) A. The structure is made of the sequence of layers …[(BaO)(CuO2)(Ca)(CuO2)(BaO)(HgOδ)]… The compound, prepared by a solid-state reaction between HgO and the precursor Ba2CaCu2Ox at 18 kbar and 880°C, has an oxygen content corresponding to δ=0.35(2) and a transition temperature of T c ⋍ 104 K . Annealing in O2 (300°C for 20 h) reduces the oxygen content to δ=0.28(2). A sample annealed under the same conditions or in N2 (250°C for 12 h) was found to have a Tc of 123 K. Additional annealing in N2 at 400°C for 18 h lowers Tc to 110 K. Our results indicate that the material with δ=0.35 is overdoped. The oxygen above the O6 stoichiometry is located on the Hg layers of the structure, at the positions 1 2 , 1 2 , 0. No evidence has been found of the substitution of mercury by copper. The copper and oxygen atoms of the (CuO2) layers are almost exactly coplanar in this compound.

Stability domain of the ice VIII proton‐ordered phase at very high pressure and low temperature

Abstract: The transition line between ice VII and ice VIII has been determined by Raman scattering for H2O and D2O. Above 15 GPa the transition temperature decreases linearly with pressure, and then drops to 0 K around 62 and 72 GPa for H2O and D2O, respectively. An analysis, using the pseudospin formalism, shows that the linear regime is due to the decrease with pressure of the dipolar interaction, while the proton tunnelling remains negligible. The proton disorder becomes efficient only from 60 GPa for H2O (or 72 for D2O) where it limits the stability range of the ordered phase VIII. Our analysis suggests that ice X, the so‐called ice with hydrogen at the midpoint of the O‐‐‐O bonds, would be obtained above 80 GPa (H2O) or 100 GPa (D2O).

Mapping the stiffness-temperature-moisture relationship of solid biomaterials at and around their glass transition

Abstract: Mechanical changes in biomaterials at and around their glass transition are key factors in their functionality and/or stability. They are described in terms of a relationship betwen a relative stiffness R(T,M) defined as the ratio between a modulus or storage modulus at a temperature T, and moisture M, and its magnitude in the glassy state. The relationship, in turn, is expressed by the model R(T,M) = 1/[1+exp [{T-Tc,(M))/a(M)]} where Tc(M) is a critical temperature identifying the transition temperature range and a(M) a constant representing the relationship's slope. The proposed model correctly accounts for the downward concavity of the stiffness vs temperature relationship at the transition onset. Published data on biosolids indicate that Tc(M) can be described by a single exponential decay term, and so most probably also a(M). Incorporation of these terms into the model enables the creation of realistic three-dimensional maps of the relative stiffness-temperature-moisture relationship at and in the neighborhood of the glass transition region. In principle, the same method can also be used to describe the effect of plasticizers other than water if their influence on Tcand the steepness parameter can be formulated as an algebraic expression.

Spatially modulated antiferromagnetic order in CoO/NiO superlattices.

Abstract: Neutron diffraction measurements confirm the presence of long-range antiferromagnetic order in superlattices of cobalt oxide and nickel oxide. A single transition temperature between the values for bulk CoO and NiO is evident for two samples with 36 A periods as previously reported, but the separate Ni and Co order parameters in a 72 A period superlattice approximate bulk behavior. In the latter sample the magnetic order remains coherent across ostensibly paramagnetic CoO interlayers above 400 K. Mean field analysis accounts for the layer thickness dependence of the ordering temperature

The nematic to isotropic transition of a liquid crystal in porous media

Abstract: An experimental study of the effect of disorder on the nematic to isotropic (N–I) transition of a liquid crystal is reported. Differential scanning calorimetry is used to monitor the N–I transition of p‐azoxyanisole in a series of controlled pore glasses. An increase and a decrease in the N–I transition temperature is observed with varying pore sizes. The direction of the temperature shift depends on the pore size. The observed increase in the transition temperature can be attributed to surface induced ordering, while the observed decrease can be explained as a result of the interruption of the orientational correlation of the liquid crystalline order due to confinement in pores. These results agree, qualitatively, with Monte Carlo results showing that quenched disorder can lower the transition temperature, round and lower the heat capacity peak, and change the order of the transition.

Characterisation of superconducting Hg-Ba-Ca-Cu-oxides: Structural and physical aspects

Abstract: Some of the recently discovered high- T c superconducting phases of Hg-Ba-Ca-Cu-oxides have been investigated and characterised b by analytical and high-resolution transmission electron microscopy. The highest measured critical temperature T c of 133.5 K can be assigned to the Hg 1 Ba 2 Ca 2 Cu 3 O 8+ x phase (Hg-1223). Besides Hg-1212 and Hg-1223 phases, different perfectly-ordered polytypes with large c -axes up to 19 nm consisting of different stackings of Hg-1212 and Hg-1223 have been observed. The investigated oxide materials contain also several different stacking defects consisting of intercalated layers of the related Hg-1234 and Hg-1245 structures.

Chemical, structural, and electronic properties of sulfur‐passivated InP(001) (2×1) surfaces treated with (NH4)2Sx

Abstract: Some chemical, structural, and electronic properties of (NH4)2Sx‐treated InP(001) surfaces have been studied using x‐ray photelectron spectroscopy, x‐ray photoelectron diffraction and reflection high‐energy electron diffraction A (2×1) surface reconstruction is observed for substrates heated in vacuum at a transition temperature of about 200–350 °C Sulfur atoms are only bonded to indium atoms and the exchange between phosphorus and sulfur occurs in the first five atomic planes leading to the formation of an InP1−xSx pseudomorphic overlayer The sulfur surface concentration varies from about 085±015 after annealing at 350 °C to 05±015 monolayer at 550 °C The sulfidation treatment results in (2×1) reconstructed surfaces of high thermal stability up to 560 °C and of high chemical stability Unpinned ultraclean surfaces, free of carbon and oxygen, are obtained after vacuum annealing at 550 °C