Showing papers on "Transition temperature published in 1991"

Superconductivity at 18 K in potassium-doped C60

Abstract: THE synthesis of macroscopic amounts of C60 and C70 (fullerenes)1 has stimulated a variety of studies on their chemical and physical properties2,3. We recently demonstrated that C60 and C70 become conductive when doped with alkali metals4. Here we describe low-temperature studies of potassium-doped C60 both as films and bulk samples, and demonstrate that this material becomes superconducting. Superconductivity is demonstrated by microwave, resistivity and Meissner-effect measurements. Both polycrystalline powders and thin-film samples were studied. A thin film showed a resistance transition with an onset temperature of 16 K and essentially zero resistance near 5 K. Bulk samples showed a well-defined Meissner effect and magnetic-field-dependent microwave absorption beginning at 18 K. The onset of superconductivity at 18 K is the highest yet observed for a molecular superconductor.

Apex oxygen and critical temperature in copper oxide superconductors: Universal correlation with the stability of local singlets

Abstract: Electronic-energy-level structures of the layered Cu oxide compounds are drawn within the ionic model. Correlations between the level separations obtained and the superconducting critical temperature ${\mathit{T}}_{\mathit{c}}$ are examined for all families of the Cu oxide superconductors. It is proposed that the position of the energy level of the apex oxygen atoms is of primary importance for the electronic states of the ${\mathrm{CuO}}_{2}$ plane and governs the optimum ${\mathit{T}}_{\mathit{c}}$'s of all families of hole-carrier suprconductors. The pressure effect of ${\mathit{T}}_{\mathit{c}}$ is discussed within this framework. Based on cluster-model calculations, where the covalency neglected in the ionic model is fully taken into account, we argue that the role of the apex oxygen atoms is to prescribe the stability of local singlet states made of two holes in the Cu 3${\mathit{d}}_{\mathit{x}}^{2}$-${\mathit{y}}^{2}$ and O 2${\mathit{p}}_{\mathrm{\ensuremath{\sigma}}}$ orbitals of the ${\mathrm{CuO}}_{2}$ plane, and thereby to control the maximum ${\mathit{T}}_{\mathit{c}}$ of each family.

Order-parameter saturation and low-temperature extension of Landau theory

Abstract: The temperature evolution of the structural order parameter in displacive phase transitions with high transition temperatures (e.g. quartz, As2O5, LaAlO3, CaCO3, NaNO3, Pb3(PO4)2 etc.) follows a Landau-type behaviour over large temperature intevals. Below a saturation temperatureT s , however, the order parameter tends to become temperature independent. A quantitative analysis shows the correlationk B T s =ħΩ/4, where Ω is the frequency of the relevant local excitation atT s . A general Landau-type expression for the Gibbs energy is given which includes the effects of order-parameter saturation.

Effect of 5.3-GeV Pb-ion irradiation on irreversible magnetization in Y-Ba-Cu-O crystals.

Abstract: We report a dramatic change in the irreversibility line of Y-Ba-Cu-O crystals after irradiation with Pb ions. Near the transition temperature, following irradiation, the irreversibility temperature increases and the curvature of the irreversibility line changes sign. These changes are accompanied by a strong enhancement of critical current density and a decrease in flux creep rate. Pb irradiation induces damage in the form of amorphous tracks which penetrate throughout the thickness of the sample. We maintain that these defects are most efficient in terms of flux trapping and are responsible for the observed changes in irreversible magnetic features in the irradiated sample.

Anisotropic gels and plasticized networks formed by liquid crystal molecules

Abstract: Two component mixtures containing liquid crystalline diacrylates and liquid crystal molecules with no reactive groups have been made. Upon photopolymeriz-ation of the mixtures anisotropic networks containing liquid crystal molecules which were not chemically attached to the networks were created. In these systems the behaviour of the liquid crystal molecules was found to be dominated by the network molecules. Even when the liquid crystal molecules in these networks were heated above their isotropic transition temperature they remained partly oriented, thus contributing to the anisotropy of the system.

Relation between superconducting transition temperature and oxygen ordering in YBa 2 Cu 3 0 6+x

Abstract: THE superconducting transition temperature, Tc, of the ceramic high-temperature superconductor YBa2Cu3O6+xis known to depend not only on the oxygen stoichiometry x (0 < x < 1) but also on the specific ordering of the oxygen atoms in the basal CuO planes1–7. Here we present computer simulations of the formation of oxygen-ordered domains of orthorhombic structure in the basal CuO plane using a microscopic model of the oxygen ordering. Together with a minimal-model assumption for the charge transfer, our calculations strongly suggest that it is these domains that are responsible for the characteristic variation of Tc(x). Our results lead to a theoretical prediction of Tc(x) that is in close quantitative agreement with experiments.

Off-center displacement of the Nb ions below and above the ferroelectric phase transition of KTa0.91Nb0.09O3

Abstract: We have measured the off-center displacements of the Nb ions in ${\mathrm{KTa}}_{091}$${\mathrm{Nb}}_{009}$${\mathrm{O}}_{3}$ as a function of temperature both below and above the ferroelectric transition temperature ${\mathit{T}}_{\mathit{c}}$=856 K The purpose of these measurements was to elucidate the basic nature of the ferroelectric phase transitions in mixed perovskites The off-center displacements were determined by x-ray-absorption fine-structure (XAFS) measurements The data were analyzed by calculating the XAFS spectra theoretically as a function of structural and other parameters and by refining these parameters to obtain the best fit with the experimental results up to the third coordination shell In these calculations we used curved photoelectron wave functions and took into account multiple scattering up to and including, the third-order collinear scattering It is found that these scattering contributions are both necessary and sufficient in order to account for all the scattering contributions with a total scattering path length of 8 \AA{} or less At 70 K the off-center displacement, relative to the oxygen octahedra, is found to be 0145 \AA{} in the [111] direction These displacements change by less than 20% from 70 K to room temperature, namely, from below to far above the phase transition These results lead to the conclusion that the magnitude of the displacements hardly changes over a very wide temperature range Thus, the ferroelectric transition is not displacive and the main change taking place at the phase transition is from a disordered to orientationally ordered arrangement of displacements

Cure Parameters and Phase Behavior of An Ultraviolet-Cured Polymer-Dispersed Liquid Crystal

Abstract: Using calorimetry and scanning electron microscopy we have investigated the formation energetics and kinetics as well as the phase behavior of an ultraviolet-cured polymer-dispersed liquid crystal (PDLC) system. The purpose of this work was to determine those cure conditions which lead to liquid crystal microdroplet formation. We have found that a range of cure temperatures exists in which the degree of polymer cure is highest. In this range phase separation of liquid crystal from the polymer is maximized, and the best droplet microstructure occurs. Samples cured in the optimum temperature range also have the highest nematic-isotropic transition temperature. Results of calorimetric and electron microscopic studies are presented.

Superconductivity at 45 K in Rb/Tl Codoped C60 and C60/C70 Mixtures

Abstract: The appearance of superconductivity at relatively high temperatures in alkali metal—doped C60 fullerene provides the challenge to both understand the nature and origin of the superconductivity and to determine the upper limit of the superconducting transition temperature (Tc). Towards the latter goal, it is shown that doping with potassium-thallium and rubidium-thallium alloys in the 400 to 430°C temperature range increases the Tc of C60/C70 mixtures to 25.6 K and above 45 K, respectively. Similar increases in Tc were also observed upon analogous doping of pure C60. Partial substitution of potassium with thallium in interstitial sites between C60 molecules is suggested by larger observed unit cell parameters than for the K3C60 and K4C60 phases. Contrary to previous results for C60 doped with different alkali metals, such expansion does not alone account for the changes in critical temperature.

(RbxK1-x)3C60 Superconductors: Formation of a Continuous Series of Solid Solutions.

Abstract: By means of an approach that employs alkali-metal alloys, bulk single-phase (Rb x K 1– x ) 3 C 6O superconductors have been prepared for all x between 0 and 1. For x = 1 it is shown that the maximum superconducting fraction, which approaches 100% in sintered pellets, occurs at a Rb to C 60 ratio of 3:1. More importantly, single-phase superconductors are formed at all intermediate values of x , and it is shown that the transition temperature ( T c ) increases linearly with x in this series of materials. The formation of a continuous range of solid solutions demonstrates that the rubidium- and potassium-doped C 60 superconducting phases must be isostructural, and furthermore, suggests that the linear increase in T c with x results from a chemical pressure effect.

Differential scanning calorimetry studies of NaCl effect on the inverse temperature transition of some elastin‐based polytetra‐, polypenta‐, and polynonapeptides

Abstract: and (~-Val'-L-Pro~-L-Ala~-~-Val~-Gly~),; the polytetrapeptide (~-Val'-~-Pro~-Gly~-Gly~), and its more hydrophobic analogue ( ~-1le'~-Pro*-Gly~-Gly~),,; and the polynonapeptide (a pentatetra hybrid), (~-Val'-~-Pro~-G ly~~-Val~-Gly~-~-Val~-~-Pro~-Gly~-G ly~)~. Previous physical characterizations of the polypentapeptides have demonstrated the occurrence of an inverse temperature transition since increase in order of the polypentapeptide, as the temperature is raised from below to above that of the transition, has been repeatedly observed using different physical characterizations. In the present experiments, it is observed that the transition temperatures of the polypeptides studied are linearly dependent on NaCl concentration. The molar effectiveness of NaCl in shifting the transition temperature AT,/ [ N] , is about 14"C/[ N] , with the dependence on peptide hydrophobicity being fairly small. Interestingly, however, the 6AQ/ [ N] does depend on the hydrophobicity of a polypeptide.

A spectroscopic study of phase-transition kinetics in syndiotactic polystyrene

Abstract: The degree and kinetics of the transformation of crystallites containing a helical chain conformation into those containing an all-trans chain conformation in solution cast syndiotactic polystyrene films are studied. The amount of phase transformation above the transition temperature is highly dependent upon sample morphology and therefore their thermal history. Imperfections in films cast from solution cannot be removed by annealing at low temperatures. Extensive phase transformation was observed to occur very rapidly above a critical temperature of 192 o C for cast films annealed at 150 o C

Comparative study of implantation-induced damage in GaAs and Ge: Temperature and flux dependence

Abstract: Damage accumulation during ion implantation of 100 keV Si+ into GaAs and Ge has been investigated. A comparison is made of the amount of damage created in GaAs and Ge and its dependence on dose, temperature, and dose rate. General similarities are observed in the dependence of damage in the two materials on dose and temperature. Both materials exhibit a well‐defined transition temperature above which the damage decreases dramatically. This transition occurs near room temperature in GaAs and approximately 112 °C higher in Ge. Furthermore, a strong dose‐rate effect on damage growth is demonstrated in both Ge and GaAs near their respective transition temperatures. The temperature dependence of the damage yield in both materials is compared to that given by the model of Morehead and Crowder [Rad. Eff. 6, 27 (1970)] for a range of ion doses.

An efficient and reproducible approach for attaining superconductivity at 128 K in Tl2Ba2Ca2Cu3O10-δ

Abstract: We report an efficient, and highly reproducible, method for the preparation of almost single phase Tl-2223 material, which is characterized by high-temperature superconductivity with T c midpoint = 128 K and T c zero = 126 K as measured by electrical resistivity, and a diamagnetic onset temperature [ T c mag ] of 128 K as measured by AC susceptibility. This procedure involves synthesizing a material with nominal stoichiometry Tl 1.6 Ba 2 Ca 2.4 Cu 3 O 10-δ , which was sintered at 910°C for 3 h, annealed at 750°C (10 days) in an evacuated quartz tube, and finally annealed at 600°C (2 h) in a 0.2% oxygen/nitrogen gas mixture. Based on these results, it appears that the optimization of the hole concentration in Tl-2223, achieved here by controlling the oxidizing/reducing power of the annealing atmosphere, appears to be crucially important in attaining such a high transition temperature.

"Intrinsic" response of polymer liquid crystals in photochemical phase transition

Abstract: Time-resolved measurements were performed on the photochemically induced isothermal phase transition of polymer liquid crystals (PLC) with mesogenic side chains of phenyl benzoate (PAPB3) and cyanobiphenyl (PACB3) under conditions wherein the photochemical reaction of the doped photoresponsive molecule (4-butyl-4-{prime}-methoxyazobenzene, BMAB) was completed within {approximately} 10 ns, and the subsequent phase transition of the matrix PLC from nematic (N) to isotropic (I) state was followed by time-resolved measurements of the birefringence of the system. Formation of a sufficient amount of the cis isomer of BMAB with a single pulse of a laser lowered the N-I phase transition temperature of the mixture, inducing the N-I phase transition of PLCs isothermally in a time range of {approximately} 200 ms. This time range is comparable to that of low molecular weight liquid crystals, indicating that suppression in mobility of mesogens in PLCs does not affect significantly the thermodynamically controlled process.

Local oxygen ordering in superconducting YBa 2 Cu 3 0 6.4 observed by neutron diffraction

Abstract: THE superconducting transition temperature, Tc, in the YBa2Cu3O6+x system is known to depend not only on the oxygen stoichiometry1, but also on the degree of oxygen order2,3. Observations of oxygen ordering structures by electron microscopy4,5 have led to the suggestion that the '60-K plateau' near x = 0.5 in the curve of Tc versus x arises from an ordered structure (the 'ortho-II phase') in which Cu–O and Cu chains alternate in the basal plane6. Until now, however, the ortho-II phase has been seen only by electron diffraction, except for one observation at an oxygen stoichiometry of x = 6.7 by X-ray diffraction7. Compared with true bulk methods such as X-ray and neutron diffraction, electron diffraction suffers from the disadvantage that only a very small portion of the sample is probed. Here we report the observation, by neutron diffraction, of local ortho-II ordering in an YBa2Cu3O6.4 crystal (Tc = 38 K). The observed correlation lengths along the principal axes are 24b (along the chains), 10a and 2c. Our observation of the existence of the ortho-II phase for an oxygen stoichiometry close to the minimum for which superconductivity is observed (x≈0.4) supports existing theoretical models8,9, in particular one8 that relates Tc to the presence of two types of ordered domain.

Paraelectric-antiferroelectric phase transition in titanite, CaTiSiO5. I, A high temperature X-ray diffraction study of the order parameter and transition mechanism

Abstract: The paraelectric to antiferroelectric phase transition in titanite at ∼500 K involves a displacement of the titanium atom from the center of the [TiO6] octahedron in the paraelectric phase (A2/a) to an off-center position in the antiferroelectric (P2 1/a) phase. We have carried out a detailed single crystal high temperature x-ray diffraction study of the phase transition including structure refinements at 294, 350, 400, 430, 440, 450, 500, 600, and 700 K. The unit cell dimensions show a pronounced hysteresis effect in the 450–500 K range on heating and cooling during the first cycle along with a reduction of the transition temperature, T c from 495 ± 5 K on heating to 445 ± 5 K on cooling. The hysteresis effect disappears on further heating and the superstructure reflections show residual intensities above T c (445 K). An order parameter treatment of the phase transition is presented in terms of Landau theory and induced representation theory. The Ti-displacements parallel and antiparallel to a are taken as the primary order parameter η, which transforms as the Y 2 + representation. A coupling of Y 2 + with T 1 + results in the linear-quadratic coupling of the spontaneous strain components, ɛ ij with η. The Ti-displacements are coupled linearly to the Cadisplacements. Both sets of displacements predicted from induced representation theory are observed experimentally. The phase transition is initially driven by the soft mode at the zone boundary point Y 2 + ; near T c critical fluctuations set in and an order-disorder mechanism finally drives the phase transition, whereby parallel and antiparallel Ti-displacements related by [0, 1/2, 1/2] in adjacent domains are dynamically interchanged. Immediately above T c , the high temperature (A2/a) phase is a statistical average of small dynamic antiphase domains of the low temperature (P2 1/a) phase. Vacancies and defects pinning the domain boundaries may drastically alter the transition behavior and affect the domain mobility.

Monte Carlo study of phase transitions in ferromagnetic bilayers

Abstract: We have used Monte Carlo computer simulations to study the behavior of an Ising model consisting of two ferromagnetic layers with different interaction constants coupled weakly together. For the range of lattice sizes studied it appears as though the system undergoes a single transition at the transition temperature of an isolated layer with the stronger coupling, but substantial changes in the thermodynamic properties also occur near the transition temperature of an isolated layer with the weaker of the two couplings.

Influence of lattice imperfections on the transition temperatures of structural phase transitions: The plateau effect

Abstract: The influence of defects on the apparent phase transition temperature has been studied over a large range of defect concentrations. An interval was found for low concentrations (around 1 mol%) in which the transition temperature depends only weakly on the number of defects and where defect tails occur. This interval is called the plateau. For higher defect concentrations we find that chemical mixing leads to strong variations of the transition temperature which are well described in the approximation of a homogeneous crystal.

Magnetic order and moment stability in YMn2

Abstract: High-resolution neutron powder diffraction and transverse field mu SR measurements have been used to examine the nature of the magnetic and structural transition at 100 K in YMn2. The neutron measurements indicate a first-order hysteretic transition occurring at 92.5 K on cooling and 108 K on warming. The low-temperature phase is tetragonal (c/a=0.995) and antiferromagnetic with localized Mn moments of 2.8 mu B ordering in a long-wavelength helix characterized by an antiferromagnetic translation vector ( tau , tau ',1) where tau =0.018 and tau '=0.003. The high-temperature phase is cubic and Pauli paramagnetic. The collapse of the Mn moment at the transition is accompanied by a 5% decrease in the cell volume. In the paramagnetic state of YMn2 measurement of the muon transverse relaxation rate reveals an underlying second-order transition which the authors associate with the critical slowing of longitudinal spin fluctuations as the transition temperature is approached.

Crystallization and properties of Sr-Ba aluminosilicate glass-ceramic matrices

Abstract: Powders of roller quenched (Sr,Ba)O-Al2O3-2SiO2 glasses of various compositions were uniaxially pressed into bars and hot isostatically pressed at 1350 C for 4 hours or cold isostatically pressed and sintered at different temperatures between 800 to 1500 C for 10 or 20 hours. Densities, flexural strengths, and linear thermal expansion were measured for three compositions. The glasss transition and crystallization temperatures were determined by Differential Scanning Calorimetry (DSC). The liquidus and crystallization temperature from the melt were measured using high temperature Differential Thermal Analysis (DTA). Crystalline phases formed on heat treatment of the glasses were identified by powder x ray diffraction. In Sr containing glasses, the monoclinic celsian phase always crystallized at temperatures above 1000 C. At lower temperatures, the hexagonal analog formed. The temperature for orthorhombic to hexagonal structure transformation increased monotonically with SrO content, from 327 C for BaO-Al2O3-2SiO2 to 758 C for SrO-Al2O3-2SiO2. These glass powders can be sintered to almost full densities and monoclinic celsian phase at a relatively low temperature of 1100 C.

Self-similar light-scattering spectra of β relaxation near the liquid-glass transition

Abstract: We report a light-scattering observation of β relaxation in [Ca(NO 3 ) 2 ] 0.4 −(KNO 3 ) 0.6 in the vicinity of the liquid-glass transition. The spectra exhibit self-similarity with I(ω)=A/ω 1−a over a spectral range of nearly 3 orders of magnitude in both the liquid and glass phases. This result agrees with predictions of mode-coupling theories of the glass transition. However, the value of a is strongly temperature dependent, a result no predicted by the theory. The spectra are strongly depolarized with a depolarization ratio of ∼0.75

Raman spectrum of Sr0.61Ba0.39Nb2O6

Abstract: The Raman spectra of a single crystal of strontium barium niobate were recorded in the temperature range–193 to 140°C. The configurations studied were X(ZZ)Y, X(YZ)Y, X(YX)Y, Z(XX)Y, Z(YZ)Y and Z(XZ)Y. The behavior of integrated reduced intensities, band widths and band centers with temperature indicated that the ferro-electric transition temperature was near 65°C. The Raman data were not consistent with a classical Landau transition. Disorder-induced first-order spectra were observed and attributed to the disordered nature of the crystal Evidence was found for a possible phase transition near 120°C.

Possibility of orbital magnetic phase transitions in mesoscopic metallic rings.

Abstract: It is shown phase a sufficiently pure mesoscopic metallic ring will exhibit a magnetic phase transition to a low-temperature state with a persistent orbital current in zero externalfield (to a flux-phase state). The transition temperature and the spontaneous persistent current depend strongly on the geometry of the ring and of the Fermi surface

The anisotropy of r2fe17cx (r = sm and tm)

Abstract: The temperature dependence of the anisotropy field of Sm2Fe17C(x) and Tm2Fe17C(x) has been determined over the entire uniaxial ferro- or ferrimagnetic temperature range using the singularity point detection technique. The temperature dependence of the ac initial susceptibility was measured on these two systems from 4.2 to 300 K. The Curie temperature, lattice constants, spin-reorientation temperature (of Tm2Fe17C(x)) and the magnetic anisotropy field are increasing with increasing carbon concentration. A "magnetic-phase-like transition" is observed in Sm2Fe17C(x) for x < 0.7 at low temperatures, which is proved not to be a spin reorientation. Additionally, the transition temperature increases systematically with increasing carbon concentration. However, the nature of this transition is not yet well understood.

Lower temperature post‐annealing of thin films of YBa2Cu3O7 at lower oxygen partial pressure

Abstract: Thin films of YBa2Cu3O7 formed by ambient temperature deposition and furnace post‐annealing have been obtained at annealing temperatures around 750 °C and an oxygen partial pressure of 29 Pa. The zero resistance transition temperature of these smooth films on LaAlO3 was 89 K, and a critical current density in excess of 1 MA cm−2 at 77 K was found by transport measurements.