Showing papers on "Transition temperature published in 1997"

Magnetic localization in mixed-valence manganites

Abstract: The metal-insulator transition is mixed-valence manganites of the ~La0.7Ca0.3!MnO3 type is ascribed to a modification of the spin-dependent potential J HsnS associated with the onset of magnetic order at TC . Here JH is the on-site Hund’s-rule exchange coupling of an e g electron with s51/2 to the t 2g ion core with S 53/2. Above TC, the e g electrons are localized by the random spin-dependent potential and conduction is by variable-range hopping. Over the whole temperature range, the resistivity varies as ln( r/r ‘) 5@T0$12( M/ MS) 2 %/T# 1/4 , where M/ MS is the reduced magnetization. The temperature and field dependence of the resistivity deduced from the molecular-field theory of the magnetization reproduces the experimental data over a wide range of temperature and field. @S0163-1829~97!04513-X# Interest in mixed-valence manganites of the ~La0.7Ca0.3!MnO3 type has revived 1 with the observations of large negative magnetoresistive effects, 2,3 especially in suitably annealed thin films. 4 The magnetoresistance is greatest in the vicinity of the Curie point TC of ferromagnetic compositions which exhibit ‘‘metallic’’ ~temperatureindependent! conduction at low temperatures and thermally activated conduction above TC . These compositions have a structure which is a variant of the cubic perovskite cell where the Mn-O bond lengths are unequal and Mn-O-Mn bond angles differ from 180 °. 5 Their electronic properties are re

Influence of ion energy and substrate temperature on the optical and electronic properties of tetrahedral amorphous carbon (ta-C) films

Abstract: The properties of amorphous carbon (a-C) deposited using a filtered cathodic vacuum arc as a function of the ion energy and substrate temperature are reported. The sp3 fraction was found to strongly depend on the ion energy, giving a highly sp3 bonded a-C denoted as tetrahedral amorphous carbon (ta-C) at ion energies around 100 eV. The optical band gap was found to follow similar trends to other diamondlike carbon films, varying almost linearly with sp2 fraction. The dependence of the electronic properties are discussed in terms of models of the electronic structure of a-C. The structure of ta-C was also strongly dependent on the deposition temperature, changing sharply to sp2 above a transition temperature, T1, of ≈200 °C. Furthermore, T1 was found to decrease with increasing ion energy. Most film properties, such as compressive stress and plasmon energy, were correlated to the sp3 fraction. However, the optical and electrical properties were found to undergo a more gradual transition with the deposition temperature which we attribute to the medium range order of sp2 sites. We attribute the variation in film properties with the deposition temperature to diffusion of interstitials to the surface above T1 due to thermal activation, leading to the relaxation of density in context of a growth model.

Magnetic quantum phase transition in MnSi under hydrostatic pressure

Abstract: The crossover from a spin-polarized to nonpolarized state as a function of pressure ( p) at low temperature (T) has been investigated in MnSi via high-precision measurements of the electrical resistivity r and magnetic susceptibility x. In the magnetic phase ( p, pc.14.6 kbar!, r}T 2 at low T as expected for a Fermi liquid in a weakly polarized state. In the nonmagnetic phase ( p. pc), r vs T is consistent with the predictions for a marginal Fermi liquid model in which nearly critical spin fluctuations of long wavelength lead to a singular quasiparticle interaction. The transition is second order for p, p*.12 kbar and weakly firstorder in the range p*, p, pc , where the transition temperature Tc lies below a peak of x vs T. The variation of Tc with p and of both r and x with T and p may be understood in terms of a model of quantum critical phenomena. @S0163-1829~97!04213-6# I. INTRODUCTION

Superconductivity at 10–17 K in compressed sulphur

Abstract: Recent high-pressure studies of condensed matter at extreme densities have uncovered various new phenomena in simple molecular and elemental substances1. One of the most significant pressure-induced changes in materials properties is the transformation of insulators into metals and superconductors. Previous studies of compressed sulphur indicated transitions to metallic phases at 90 GPa (ref. 2) and 162 GPa (ref. 3). Here we demonstrate that at 93 GPa, elemental sulphur transforms not only to a metal, but also to a superconductor with a transition temperature, T c, of 10.1 K. Using a highly sensitive magnetic susceptibility technique adapted for megabar-pressure diamond anvil cells, we find that T c increases linearly with pressure up to 157 GPa. This contrasts with the negative d T c/dP observed4 (at much lower pressures) in the heavier superconducting chalcogenides Se and Te. Moreover, at the transformation in sulphur to a higher pressure metallic phase near 160 GPa, T c increases from 14 to 17 K. These are the highest reported transition temperatures for an elemental solid. As such, these results may provide crucial tests of mechanisms of superconductivity.

High Temperature Superconductivity in Metallic Hydrogen:Electron-Electron Enhancements

Abstract: We investigate the possibility of superconductivity in a dense phase of hydrogen which becomes metallic while retaining diatomic character. Correlated fluctuations between electrons and holes in the ensuring band-overlap state can lead to significant enhancements in the transition temperature (compared with monatomic phases) principally through a reduction in the associated Coulomb pseudopotential. The effective electron-electron interaction is determined by a method which treats electrons and phonons on an equivalent footing, an approach which confirms that monatomic phases also remain candidates for high temperature superconductivity. {copyright} {ital 1996} {ital The American Physical Society}

Incommensurate Spin Fluctuations in High-Transition Temperature Superconductors

Abstract: Neutron scattering experiments have revealed a fascinating interplay between the hole doping, the spin fluctuations, and the superconductivity of the cuprate superconductors. Recently, electrochemical techniques have been used to produce large single crystals of La2CuO4+ y , which has mobile oxygen dopants. Staging behavior of the excess oxygen has been demonstrated, and the low-energy spin fluctuations in stage 6 La2CuO4+ y have been measured. The spin fluctuations are incommensurate with the lattice and have spatial, energy, and temperature dependencies very much like those in La2− x Sr x CuO4, with similar high transition temperature. This establishes the universality of the incommensurate spin fluctuations among the La2CuO4-based superconductors.

Effects of Gd, Co, and Ni doping in La 2 / 3 Ca 1 / 3 MnO 3 : Resistivity, thermopower, and paramagnetic resonance

Abstract: The effect of elemental substitutions on the properties of the ferromagnetic, conducting, highly magnetoresistive compound ${\mathrm{La}}_{2/3}{\mathrm{Ca}}_{1/3}{\mathrm{MnO}}_{3}$ has been studied. The results of Co doping and Ni doping on the Mn site and Gd doping on the La site are reported. These compounds were investigated by x-ray diffraction, magnetization measurements, resistivity measurements, thermopower measurements, and by paramagnetic resonance. The result of replacing La by Gd atoms is to lower the ferromagnetic (or metal-insulator) transition temperature, an effect which is shown to be due to bond bending caused by the lattice adjusting to the size differential between the La and Gd ions. On the other hand, the reduction of the magnetic transition temperature when Mn ions are replaced with Co or Ni ions is not attributed to changes in the size of the ions. Instead, we ascribe the lowering of the ferromagnetic transition temperature to a weakening of the double-exchange interaction between two unlike ions. The resistivity and Seebeck coefficient in these materials have been investigated as a function of elemental substitution. The magnetic polaron theory of Zhang is used, phenomenologically, to provide a quantitative explanation of these transport properties. In addition, the effect of these elemental substitutions on the linewidths of the paramagnetic resonances is studied and is discussed in terms of exchange narrowing and spin-lattice relaxation.

Local structure, transport, and rare-earth magnetismin the ferrimagnetic perovskite Gd 0.67 Ca 0.33 MnO 3 s

Abstract: Bulk, single crystal, and metal-organic chemical-vapor deposition thin-film samples of ${\mathrm{Gd}}_{0.67}$ ${\mathrm{Ca}}_{0.33}$ ${\mathrm{MnO}}_{3}$ were prepared and examined for their electrical, magnetic, and structural properties. ${\mathrm{Gd}}_{0.67}$ ${\mathrm{Ca}}_{0.33}$ ${\mathrm{MnO}}_{3}$ is ferrimagnetic with a transition temperature between 50 and 80 K and a compensation temperature of about 15 K. A molecular field model with a ferromagnetic manganese sublattice antiparallel to the gadolinium sublattice qualitatively explains the magnetism data. A large high-field susceptibility is observed at 5 K, suggesting a sublattice rotation. The resistivity and the magnetoresistance show no anomaly near the ferrimagnetic transition. There is no noticeable change in the structure, as seen from the x-ray-absorption fine structure between 40 and 69 K, indicating that there is no structural discontinuity across the paramagnetic insulator to ferromagnetic insulator phase boundary. The resistivity of ${\mathrm{Gd}}_{0.67}$ ${\mathrm{Ca}}_{0.33}$ ${\mathrm{MnO}}_{3}$ is consistent with small polaron hopping at high temperatures (up to 1100 K), and possibly by a different mechanism at low temperatures.

Superconducting T c and Electron-Phonon Coupling in Nb to 132 GPa: Magnetic Susceptibility at Megabar Pressures

Abstract: We have measured superconducting T{sub c} using a highly sensitive magnetic susceptibility technique to megabar pressures ({gt}100 GPa). We observed anomalies in T{sub c}(P) for Nb at 5{endash}6GPa and 60{endash}70GPa, at which pressure T{sub c} increases by 0.7K and decreases by about 1K, respectively. In contrast, T{sub c} in Ta remains nearly constant up to 45GPa. We suggest that the anomalies in Nb arise from stress-sensitive electronic topological transitions. Between 70 and 132GPa, T{sub c} for Nb drops continuously to 4.7K, which is related to the decrease in density of states at the Fermi level with increasing pressure. {copyright} {ital 1997} {ital The American Physical Society}

Relationship between polymer chain conformation and phase boundaries in a supercritical fluid

Abstract: We investigate the solvent density driven changes in polymer conformation and phase behavior that occur in a supercritical fluid, with a particular emphasis on conditions near the lower critical solution temperature (LCST) phase boundary. Using continuous space Monte Carlo simulations, the mean square end-to-end distance (R) and radius of gyration (Rg) are calculated for a single chain with 20 Lennard-Jones segments in a monomeric solvent over a broad range of densities and temperatures. The chains collapse as temperature increases at constant pressure, or as density decreases at constant temperature. A minimum in R and Rg occurs at a temperature slightly above the coil-to-globule transition temperature (C-GTT), where the chain adopts a quasi-ideal conformation, defined by the balance of binary attractive and repulsive interactions. Expanded ensemble simulations of finite-concentration polymer–solvent mixtures reveal that the LCST phase boundary correlates well with the single chain C-GTT. At temperatures well above the LCST, the chain expands again suggesting an upper critical solution temperature (UCST) phase boundary above the LCST.

Ordered Phases in Aqueous Solutions of Diblock Oxyethylene/Oxybutylene Copolymers Investigated by Simultaneous Small-Angle X-ray Scattering and Rheology

Abstract: The phase behavior of gels of E 40 B 10 in 0.2 mol dm -3 aqueous K 2 SO 4 was studied as a function of temperature and concentration. E 40 B 10 is a diblock copolymer of poly(oxyethylene) (E) and poly(oxybutylene) (B), where the subscripts denote the number of repeats. The phase of the material was characterized by both simultaneous rheology and small-angle X-ray scattering, (SAXS). Depending on polymer volume fraction in the range 23-38% a body-centered cubic (bcc) structure or a face-centered cubic (fcc) structure was observed at low temperature, and at high temperature a hexagonally packed rod structure was formed. The phase transitions were shown to be characterized by discontinuous changes in the values of the dynamic shear moduli. A bcc-fcc transition was observed at high concentration, the corresponding transition temperature increasing with increasing polymer concentration. The effects of reciprocating shear were shown to increase the degree of order, manifested as a sharpening of the diffraction peaks in the SAXS pattern. The dynamic moduli decreased rapidly on the application of oscillatory shear and recovered equally rapidly when the deformation ceased. The decrease in moduli was shown, via the SAXS patterns acquired simultaneously, to be correlated to structural changes within the gel.

Rheo-Optical Evidence of a Flow-Induced Isotropic-Nematic Transition in a Thermotropic Liquid-Crystalline Polymer

Abstract: A direct rheo-optical characterization of the flow-induced isotropic-nematic (I-N) transition in a semiflexible thermotropic liquid-crystalline polymer (TLCP) was investigated, using a specially constructed apparatus enabling in-situ optical microscopic observations at elevated temperatures, along with cone-and-plate rheometry. For the investigation, an aromatic polyester, poly((phenylenesulfonyl)- p-phenylene 1,10-decamethylenebis(4-oxybenzoate)) (PSHQ10), was synthesized via solution polymeri- zation. Above the equilibrium isotropic-nematic transition temperature for this polymer, T ) 170.5 °C, application of steady-state shear flow above a certain critical value of shear rate, A o c, produces a first- order I-N transition, with A o c increasing with temperature. This transition is evidenced by the formation of elongated nematic (birefringent) domains in the isotropic matrix, accompanied by a drastic decrease in shear viscosity ( ). Remarkably, the nematic domains that form for A o> A o care optically uniform under cross-polarized optical microscopy; i.e., they are apparently free of disclinations (defects), typical of textured TLCPs. The flow-induced I-N transition in PSHQ10 is found to be reversible; i.e., upon cessation of shear flow, the domains melt to the original isotropic phase and the dynamic moduli rise toward the pretransition values. The observed flow-induced I-N transition may find important applications, such as envisaging new routes for processing TLCPs with better mechanical properties and helping to understand bioprocesses such as silk thread spinning.

Scaling of current-voltage curves in superconducting Bi-2223 silver-sheathed tape wires

Abstract: The current-voltage characteristics are measured for two Bi-2223 tape specimens with different critical current densities at various temperatures under the magnetic field parallel to the c -axis. It is found that the current-voltage curves are scaled on two master curves by normalizing as predicted in the vortex glass-liquid transition theory. However, the obtained dynamic critical indices increased, and the static critical indices decreased appreciably with increasing magnetic field. These experimental results are compared with the numerical calculation based on the flux creep and flow model, taking the distribution of the flux pinning strength into account. Agreement is obtained between the experimental and theoretical results not only on the scaling curve but also on the critical indices and the transition line. The static critical index showing a divergence of the correlation length near the transition temperature can also be explained from the temperature dependence of the pinning correlation length. These results suggest that the scaling behavior is not correlated to the phase transition of fluxoids assumed in the vortex glass-liquid transition theory but seems to be correlated to a kind of transition governed by the flux pinning.

Colossal magnetoresistance in epitaxial la(1-x-y)naymno3 thin film

Abstract: We have synthesized La083Na011MnO293 by heating La2O3 and MnCO3 in NaCl melt at 900 °C The exact composition was arrived by analyzing each ion by an independent chemical method The compound crystallized in a rhombohedral structure and showed an insulator-to-metal transition at 290 K Epitaxial thin films were fabricated on LaAlO3 (100) using a pulsed laser deposition technique The film also showed an insulator-to-metal transition at 290 K Magnetoresistance [ΔR/R0=(RH−R0)/R0] was −71% near the insulator-to-metal transition temperature of 290 K at 6 T magnetic field

Temperature- and field-induced valence transitions of

Abstract: The magnetic properties of the system were studied in which an intermediate-valence state of Eu is realized at around x = 0.75. It was found that the compounds with show a temperature-induced valence transition below room temperature, while those with are antiferromagnetic with a stable state. A first-order valence transition induced by high magnetic field was observed for . Such valence transitions against temperature or field are discussed on the basis of the interconfigurational fluctuation (ICF) model. It is shown that the observed linear relation between the transition field and the transition temperature is explained by the present model.

Effect of magnetic and nonmagnetic impurities (Ni, Zn) substitution for Cu in Bi 2 (SrCa) 2+n (Cu 1-x M x ) 1+n O y whiskers

Abstract: We have measured the change in the resistive ${T}_{c}$ of ${\mathrm{Bi}}_{2}(\mathrm{SrCa}{)}_{2+n}({\mathrm{Cu}}_{1\ensuremath{-}x}{M}_{x}{)}_{1+n}{\mathrm{O}}_{y}$ whiskers due to substitution of $M$ = Ni (magnetic) or $M$ = Zn (nonmagnetic) for Cu. These whisker crystals show resistance drops for both the 2212 and 2223 $(n=1,2)$ phases, with transition widths (10--90 %) of 1 K. We checked the effect of oxygenation at several values of $x$, and found that ${T}_{c}$ is maximum for the $y$ obtained by growing and annealing in 1 atm of ${\mathrm{O}}_{2}$. We observed a linear depression of ${T}_{c}$ for both 2212 and 2223 phases with ${D=dT}_{c}/dx\ensuremath{\approx}\ensuremath{-}800$ K for both Ni and Zn. This is in contrast with YBCO in which ${D}_{\mathrm{Zn}}\ensuremath{\approx}{3D}_{\mathrm{Ni}}.$

Ferroelectric properties of SrTiO 3 -PbTiO 3 solid solutions

Abstract: Dielectric properties of ceramic samples of Sr1−x PbxTiO3 solid solutions for x varying from 0 to 0.3 have been studied. The ferroelectric phase transition in this system has been established to persist down to x=0.005. Within the x values of 0.002 to 0.05, the ferroelectric transition temperature is shown to follow the relation T c=A(x−x c)1/2, with A=440 K and x c=0.002.

Analysis of the thermally stimulated discharge current around glass-rubber transition temperature in polyethylene terephthalate

Abstract: The nature of the thermally stimulated discharge current (TSDC) for polyethylene terephthalate samples in the temperature range from room temperature to above glass-rubber transition temperature of the amorphous phase is analyzed. The well conditioning of the sample is strictly necessary in order to have a good reproducibility and accuracy of results. A main peak was observed whose maximum temperature moves towards a lower value with the decreasing of the amount of charge that flows through the sample during polarization. The peak position changes as well, if the sample is polarized in air or in oxygen and the nature of change is more important in the case of oxygen. The shape of the peak is complex and at least four shoulders have been identified around 85, 90, 105, and 125 °C using the cleaning technique. The activation energy tends to increase with repetition of the TSDC runs, in the glass-rubber transition temperature range, in the case when the cleaning technique is used for the peaks separation. For...

Crystal structure and electronic transport property of perovskite manganese oxides with a fixed tolerance factor

Abstract: The structural and electronic transport properties of ABO3-type compounds La2/3−xRxCa1/3MnO3(R=Pr, Nd, Sm, Eu, Gd, Tb, Dy, Y, Er, and Tm) with a fixed tolerance factor of t=0.91137 is studied. Similar structure deformation, characterized by the nearly constant average Mn–O, A–O distances and Mn–O–Mn bond angles, is observed in different compounds. The metal–semiconductor transition shows a strong R dependent feature. The transition temperature decreases monotonously from 187 to 77.6 K as R varies from Tm to Pr, with a corresponding maximum resistivity ranging from 2.34 to 6.17×104 Ω cm. Very different magnetoresistance effects are also observed in this series of compounds. By assuming the presence of inhomogeneous local lattice distortions due to ions with different sizes at A sites and their mismatch with B ions, the lattice effects can be understood qualitatively.

Observation of Pressure-Induced Superconductivity of Sulfur.

Abstract: The electrical resistance of sulfur has been measured under pressures up to 100 GPa and temperatures down to 70 mK. Under pressures approaching 100 GPa, sulfur shows metallic reflection comparable to that of used Pt electrodes. The temperature dependence of the resistance reveals metallic behavior, and the sharp drop of the resistance at around 15 K and 100 GPa is due to superconducting transition as the transition temperature shows typical magnetic field dependence.

Growth and properties of c-axis textured La0.7Sr0.3MnO3−δ films on SiO2/Si substrates with a Bi4Ti3O12 template layer

Abstract: c-axis textured La0.7Sr0.3MnO3−δ (LSMO) films were fabricated on SiO2/Si(001) substrates using a Bi4Ti3O12 (BTO) template layer. Electrical and magnetic properties of LSMO were investigated. The LSMO/BTO layer of this structure has no in-plane alignment. Even though a ferromagnetic transition temperature, TC, is as high as that of the epitaxial LSMO film (360 K), a resistivity peak temperature, TP, is about 140 K lower than TC. The resistivity behavior as a function of temperature for LSMO/BTO/SiO2/Si films is found to be dominated by grain boundary effects. Low field sensitive magnetoresistance which suggests spin tunneling through the grain boundaries is also observed at room temperature.