Showing papers on "Transition temperature published in 1999"

Mechanical, Thermal, and Oxidation Properties of Refractory Hafnium and zirconium Compounds

Abstract: The thermal conductivity, thermal expansion, Youngs Modulus, flexural strength, and brittle–plastic deformation transition temperature were determined for HfB2, HfC0·98, HfC0·67, and HfN0·92 ceramics. The oxidation resistance of ceramics in the ZrB2–ZrC–SiC system was characterized as a function of composition and processing technique. The thermal conductivity of HfB2 exceeded that of the other materials by a factor of 5 at room temperature and by a factor of 2·5 at 820°C. The transition temperature of HfC exhibited a strong stoichiometry dependence, decreasing from 2200°C for HfC0·98 to 1100°C for HfC0·67 ceramics. The transition temperature of HfB2 was 1100°C. The ZrB2/ZrC/SiC ceramics were prepared from mixtures of Zr (or ZrC), SiB4, and C using displacement reactions. The ceramics with ZrB2 as a predominant phase had high oxidation resistance up to 1500°C compared to pure ZrB2 and ZrC ceramics. The ceramics with ZrB2/SiC molar ratio of 2 (25 vol% SiC), containing little or no ZrC, were the most oxidation resistant.

Spatially Inhomogeneous Metal-Insulator Transition in Doped Manganites

Abstract: Scanning tunneling spectroscopy was used to investigate single crystals and thin films of La1– x Ca x MnO3(with x of about 0.3), which exhibit colossal magnetoresistance. The different spectroscopic signatures of the insulating (paramagnetic) and metallic (ferromagnetic) phases enable their spatial extent to be imaged down to a lateral scale of the order of 10 nanometers. Above the bulk transition temperature T c, the images show mostly insulating behavior. Below T c, a phase separation is observed where inhomogeneous structures of metallic and more insulating areas coexist and are strongly field dependent in their size and structure. Insulating areas are found to persist far below T c. These results suggest that the transition and the associated magnetoresistance behavior should be viewed as a percolation of metallic ferromagnetic domains.

Vanishing of phase coherence in underdoped Bi 2 Sr 2 CaCu 2 O 8+δ

Abstract: Although the binding of electrons into Cooper pairs is essential in forming the superconducting state, its remarkable properties—zero resistance and perfect diamagnetism—require phase coherence among the pairs as well. When coherence is lost at the transition temperature T c, pairing remains, together with phase correlations which are finite in space and time. In conventional metals, Cooper pairs with short-range phase coherence survive no more than 1 K above T c. In underdoped high-T c copper oxides, spectroscopic evidence for some form of pairing is found up to a temperature T *, which is roughly 100 K above T c (refs 1,2,3). How this pairing and Cooper-pair formation are related is a central problem in high-T c superconductivity. The nature of this relationship hinges on the extent to which phase correlations accompany pairing in the normal state4. Here we report measurements of high-frequency conductivity that track the phase-correlation timeτ in the normal state of the Bi2Sr2CaCu2O8+δ family of underdoped copper oxide superconductors. Just above T c, we find that τ reflects the motion of thermally generated topological defects in the phase, or vortices5,6. However, vortex proliferation reduces τ to a value indistinguishable from the lifetime of normal-state electrons at 100 K, well below T *.

Substrate temperature effects on splat formation, microstructure development and properties of plasma sprayed coatings Part I: Case study for partially stabilized zirconia☆

Abstract: In recent years it has been observed that the substrate surface temperature during thermal spray deposition has a profound effect on the morphology of the impacted droplet (splat) and consequently on the microstructure and properties of the deposits. In this set of two papers (one for metal and one for ceramic), the substrate temperature effects have been studied in an integrated manner relating the initial splat formation to microstructure development and eventually to the properties of the deposit. Isolated impacted splats have been obtained on polished steel substrates at two different temperatures (high and low) and these have been analyzed quantitatively for their shape factors and thicknesses. The deposits have been formed nominally at these two different temperatures and their microstructures and properties have been analyzed. The results confirm that there exists a threshold transition temperature for the substrate surface beyond which the splat morphology changes from a fragmented (splashed) to a more contiguous (disk-shaped) morphology. In the case of zirconia this temperature appears to be in the range of about 250‐300°C, which is roughly 10% of the melting temperature of zirconia. It has been further observed that the splat‐substrate and inter-splat contact is significantly improved at higher temperatures, leading to reduced porosity, increased thermal conductivity and strength. These results are assimilated to develop an integrated structure‐property relationship and preliminary arguments are presented as to the reason for such transitions. © 1999 Elsevier Science S.A. All rights reserved.

Superconducting magnetoresistive memory element using controlled exchange interaction

Abstract: A magnetoresistive memory element utilizing an enhanced proximity effect of ferromagnetic layers (ML) in a tri-layer configuration. A oscillatory decay characteristic of the pair wavefunction is expanded by using materials for the ML with a low Curie temperature and a small exchange field. As a result, the sum magnetic field of the alternetingly parallel and anti-parallel magnetized ML changes the critical temperature of the superconductor below and above the operational device temperature such that a passing read pulse remains unaltered at one of the two logical conditions stored within the device.

Structure-properties phase diagram for La 1 − x Sr x MnO 3 ( 0.1 x 0 . 2 )

Abstract: By combining the results of magnetization, resistivity, and neutron powder-diffraction data for stoichiometric ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3},$ we have constructed a phase diagram that describes the magnetic, transport, and structural properties and the relationships among them as a function of composition $(0.1l~xl~0.2)$ and temperature (10--340 K). We show that, with increasing Sr content, the Curie temperature increases linearly, while the temperature of an orbital ordering transition to a state with a large coherent Jahn-Teller (JT) distortion decreases. These two phase-transition lines cross at $x=0.145$ and $T=210\mathrm{K}.$ When the transition to the ferromagnetic state occurs in a phase that has a large coherent JT distortion $(xl0.145),$ a strong magnetolattice coupling is observed; the coherent JT distortion is dramatically reduced and the incoherent distortion is enhanced in the ferromagnetic phase. For $xg0.145,$ where the coherent JT distortion is small above Curie temperature, magnetolattice coupling reduces the incoherent distortion at ${T}_{C}$ and strongly suppresses the transition to a phase with a large coherent JT distortion. These observations are consistent with a competition between ferromagnetism and JT distortion that is mediated by a colossal spin-charge-lattice coupling. A metallic state occurs below the Curie temperature when both coherent and incoherent JT distortions are suppressed.

Studies of the Thermal Volume Transition of Poly(N-isopropylacrylamide) Hydrogels by High-Sensitivity Differential Scanning Microcalorimetry. 2. Thermodynamic Functions

Abstract: We report the first accurate measurements of the partial heat capacity of poly(N-isopropylacrylamide) hydrogels with varying cross-link density. When the cross-link density is increased, the transition broadens and the transition temperature decreases, while the enthalpy, entropy, and heat capacity increment of the transition do not practically change. The transition heat capacity increment is negative, Δtcp = −0.63 ± 0.04 J/g/K. This indicates the formation of a hydrophobic core of the gel upon the transition. The partial heat capacity of polymer network in the gel approaches the partial heat capacity of the unfolded linear poly(N-isopropylacrylamide) at low temperatures, indicating a complete disordering of the gel under these conditions. On the basis of the calorimetric data, thermodynamic functions of the transition were calculated from 0 to 150 °C. They allow one to compare enthalpic and entropic contributions to the stabilization of the collapsed gel. This state is found to be most stable at about 1...

Crystalline and Amorphous Phases in the Poly(ethylene oxide)−LiCF3SO3 System

Abstract: A detailed vibrational spectroscopic study of the ionic species present in the (PEO)xLiTf system (x = 3−100) indicates that “free” ions and ion pairs are the dominant species present in the dilute salt region at room temperature. When the salt concentration is increased to an O:M ratio of about 40:1, a band due to an aggregate species first appears and is assigned to the triflate ion vibrating in the (PEO)3LiCF3SO3 compound. The amorphous phase of (PEO)3LiCF3SO3 was studied by examining the temperature dependence of the spectra as the compound was heated through the melting transition at about 176 °C. Spectral behavior of the CH2 rocking modes and the COC stretching mode indicated that disordering above the melting temperature resulted in a broad distribution of −O−C−C−O− torsional angles. However the basic lithium-polyether backbone interaction was not been significantly changed for at least 15 °C above the transition temperature. Additional effects of thermal disordering were noted in the CF3 symmetric ...

Epitaxial thin films of ordered double perovskite Sr2FeMoO6

Abstract: Epitaxial thin films of Sr2FeMoO6, that are ordered double perovskite with half-metallic nature, have been successfully prepared on SrTiO3 (001) and (111) substrates by pulsed laser deposition in a narrow window of the temperature and oxygen pressure From the surface morphology analysis for the atomic scale step-and-terrace structures, the film growth is concluded to take place with the chemical formula as the growth unit when the ordering direction is normal to the surface The films showed metallic conduction with ferromagnetic transition temperature above 400 K and intergrain tunneling type magnetoresistance even at room temperature

Scaling of transition temperature and CuO2 plane buckling in a high-temperature superconductor

Abstract: A characteristic feature of the high-temperature superconductors is the existence of a chemical composition that gives a maximum transition temperature, Tc, separating the so-called under-doped and over-doped regimes1, 2. This behaviour is thought to be universal for high-temperature superconductors. In practice, there are only a few high- Tc compounds for which the composition can be varied continuously throughout the entire doping range. Here we report a study of correlations between structure and Tc in a compound with the ‘123’ structure in which both the under-doped and over-doped regimes can be accessed. We observe a clear scaling between Tc and the buckling of the copper oxide planes; both go through a maximum at the same oxygen composition (and hence doping level), so implying a common origin. Previous work has shown that, for a fixed chemical composition, increased CuO2 plane buckling lowers the transition temperature3,4,5,6,7,8,9,10,11. Thus the observation of a maximum in the buckling at the maximum Tc indicates that, as the composition is changed to increase Tc, there is a structural response that competes with superconductivity.

Improved properties of La2/3Ca1/3MnO3 thin films by addition of silver

Abstract: Thin films of Ag-added La2/3Ca1/3MnO3 exhibit enhancement of several desirable characteristics over the pristine counterparts. We find that the addition of Ag results in a pronounced increase in the insulator–metal transition temperature (Tp) and ferromagnetic transition temperature (Tc). There is also a remarkable improvement in the magnetic and electrical homogeneity of the samples as indicated by narrower ferromagnetic resonance linewidths and narrower resistive transitions, respectively. The observed improvement in properties is inferred to be largely associated with improved oxygen stoichiometry of the films although microstructural effects are not ruled out.

Growth and magnetoresistive properties of (LaMnO3)m(SrMnO3)n superlattices

Abstract: Artificial (LaMnO3)m(SrMnO3)n superlattices, n/(m+n)=0.26, were grown with pulsed-laser deposition, and the superlattice period Λ was varied. Their structural characteristics were investigated using x-ray diffraction and electron microscopy. When Λ

Transport and magnetic properties of laser ablated La0.7Ce0.3MnO3 films on LaAlO3

Abstract: La07Ce03MnO3 is a relatively new addition to the family of colossal magnetoresistive manganites, in which the cerium ion is believed to be in the Ce4+ state In this article, we report the magnetotransport properties of laser ablated La07Ce03MnO3 films on LaAlO3, and the effect of varying the ambient oxygen pressure during growth and the film thickness We observe that the transport and magnetic properties of the film depend on the oxygen pressure, surface morphology, film thickness, and epitaxial strain The films were characterized by x-ray diffraction using a four-circle goniometer We observe an increase in the metal-insulator transition temperature with decreasing oxygen pressure This is in direct contrast to the oxygen pressure dependence of La07Ca03MnO3 films and suggests the electron doped nature of the La07Ce03MnO3 system With decreasing film thickness we observe an increase in the metal-insulator transition temperature This is associated with a compression of the unit cell in the a-b plane due to epitaxial strain On codoping with 50% Ca at the Ce site, the system (La07Ca015Ce015MnO3) is driven into an insulating state suggesting that the electrons generated by Ce4+ are compensated by the holes generated by Ca2+, thus making the average valence at the rare-earth site 3+ as in the parent material LaMnO3La07Ce03MnO3 is a relatively new addition to the family of colossal magnetoresistive manganites, in which the cerium ion is believed to be in the Ce4+ state In this article, we report the magnetotransport properties of laser ablated La07Ce03MnO3 films on LaAlO3, and the effect of varying the ambient oxygen pressure during growth and the film thickness We observe that the transport and magnetic properties of the film depend on the oxygen pressure, surface morphology, film thickness, and epitaxial strain The films were characterized by x-ray diffraction using a four-circle goniometer We observe an increase in the metal-insulator transition temperature with decreasing oxygen pressure This is in direct contrast to the oxygen pressure dependence of La07Ca03MnO3 films and suggests the electron doped nature of the La07Ce03MnO3 system With decreasing film thickness we observe an increase in the metal-insulator transition temperature This is associated with a compression of the unit cell in the a-b

X-ray scattering from monolayers of F(CF2)10(CH2)2OH at the water–(hexane solution) and water–vapor interfaces

Abstract: Synchrotron x-ray reflectivity is used to study the structure of a monolayer of F(CF2)10(CH2)2OH self-assembled at the liquid–liquid interface from a solution in hexane placed in contact with water It is demonstrated that this monolayer is in a high density (solid) phase below a transition temperature This is in contrast to the conventional expectation that soluble surfactants form disordered monolayers at the liquid–liquid interface Above the transition temperature the monolayer desorbs into the hexane solution, leaving behind an interface with a very low density of surfactants Hysteresis in the formation of the monolayer occurs when the temperature is scanned through the transition temperature The success of these measurements relied upon the development of a novel technique to flatten the liquid–liquid interface to the extent required for x-ray reflectivity The measurements of F(CF2)10(CH2)2OH at the liquid–liquid interface are compared to x-ray surface diffraction measurements of monolayers of t

Spin-dependent tunneling and properties of ferromagnetic (Ga,Mn)As (invited)

Abstract: Low-temperature molecular beam epitaxy allows one to dope GaAs with Mn over its solubility limit, making it possible to realize a III–V-based diluted magnetic semiconductor (Ga,Mn)As. Magnetization measurements revealed that (Ga,Mn)As is ferromagnetic at low temperatures. The ferromagnetic transition temperature Tc can be as high as 110 K for 5% Mn. Magnetotransport properties of (Ga,Mn)As were found closely related to its magnetic properties. The temperature and magnetic-field B dependence of resistivity ρ can be understood in terms of spin-dependent scattering. The p–d exchange determined from the B dependence of ρ is shown to be consistent with the exchange expected from Tc based on the Ruderman–Kittel–Kasuya–Yosida interaction. The anomalous Hall effect dominates the Hall resistance up to room temperature, allowing one to determine the magnetic properties from the magnetotransport measurements alone. Since (Ga,Mn)As can be grown pseudomorphically on GaAs, one can introduce ferromagnetism in GaAs-based...

Hierarchical Pattern Formation in Thin Film Diblock Copolymers above the Order−Disorder Transition Temperature

Abstract: Thin symmetric poly(styrene-b-methyl methacrylate) (PS-b-PMMA) diblock copolymer films at temperatures higher than the bulk order−disorder transition temperature TODT are shown to dewet silicon sub...

Thermodynamic theory of stress distribution in epitaxial Pb(Zr, Ti)O3 thin films

Abstract: A phenomenological thermodynamic model has been developed to account for the effects of the film thickness on various properties of ferroelectric thin films. To this end, we have suitably incorporated a position-dependent stress distribution function into the elastic Gibbs function. Various physical properties can be predicted as a function of the film thickness using this modified thermodynamic formalism. A comparison of the theoretical predictions with experimental values of the average strain and the para-ferro transition temperature indicates that the tensile stress caused by the cubic-tetragonal displacive phase transition dominates over the compressive thermal stress in the epitaxially oriented tetragonal Pb(Zr, Ti)O3 thin films.

Electronic-structure evolution through the metal-insulator transition in RNiO3

Abstract: We performed a photoemission investigation of the electronic structure of PrNiO3 and NdNiO3 through the first order (paramagnetic) metal to (antiferromagnetic) insulator transition. Surprisingly, the data reveal a temperature-dependent loss of spectral weight near the chemical potential, which extends well below the coincident metal-insulator (T-MI) and magnetic (T-Neel) transition temperatures. This is in contrast with the behavior in SmNiO3 and EuNiO3, where the two transitions are separate. The spectral properties clearly indicate two distinct regimes for RNiO3 with T-MI = T-Neel and T-MI > T-Neel.

Ionic-to-neutral phase transformation induced by photoexcitation of the charge-transfer band in tetrathiafulvalene-p-chloranil crystals

Abstract: The transformation from ionic to neutral phases in tetrathiafulvalene-p-chloranil crystals is induced by exciting the charge-transfer absorption band below the transition temperature. The transformation takes place only above a threshold intensity of excitation, and the efficiency of generating neutral-phase domains is lower than the case induced by the excitation in intramolecular optical transitions of tetrathiafulvalene molecules. The mechanism of the photoinduced phase transition in this crystal is discussed based on these results.

Superconductivity and the c Axis Spectral Weight of High-Tc Superconductors

Abstract: The temperature dependence of the c axis spectral weight (frequency integral of the interplane conductivity) of high transition temperature (high-Tc) superconductors is shown to be a probe of thermal and quantal fluctuations of the phase of the superconducting order parameter. The behavior of underdoped cuprates is shown to be a natural consequence of superconducting pairing without long-ranged phase coherence. Very underdoped cuprates are found to have strong phase fluctuations, even for temperatures much less than the transition temperature.

Fatigue in sol-gel derived barium titanate films

Abstract: The effect of fatigue on the electrical properties of sol-gel derived ferroelectric barium titanate (BaTiO3) thin films have been investigated. At room temperature, the resistivity, breakdown field, dielectric constant (e), and loss tangent (tan δ) decreases after fatigue while the coercive field increases. Fatigue also influences the dielectric properties of the film at high temperatures. The sharpness and magnitude of the peak in the dielectric constant at the transition temperature decreases after fatigue. The loss tangent of the virgin film showed a peak at the transition temperature while it is absent in the case of the fatigue film. The phenomena are explained in terms of (i) the accumulation of point charge defect, (ii) the pinning of domain wall mobility, and (iii) the space charge layer models of fatigue.