Showing papers on "Transition temperature published in 2001"

Superconductivity at 39 K in magnesium diboride

Abstract: In the light of the tremendous progress that has been made in raising the transition temperature of the copper oxide superconductors (for a review, see ref. 1), it is natural to wonder how high the transition temperature, Tc, can be pushed in other classes of materials. At present, the highest reported values of Tc for non-copper-oxide bulk superconductivity are 33 K in electron-doped CsxRbyC60 (ref. 2), and 30 K in Ba1-xKxBiO3 (ref. 3). (Hole-doped C60 was recently found4 to be superconducting with a Tc as high as 52 K, although the nature of the experiment meant that the supercurrents were confined to the surface of the C60 crystal, rather than probing the bulk.) Here we report the discovery of bulk superconductivity in magnesium diboride, MgB2. Magnetization and resistivity measurements establish a transition temperature of 39 K, which we believe to be the highest yet determined for a non-copper-oxide bulk superconductor.

Magnetic-field-induced superconductivity in a two-dimensional organic conductor

Abstract: The application of a sufficiently strong magnetic field to a superconductor will, in general, destroy the superconducting state. Two mechanisms are responsible for this. The first is the Zeeman effect, which breaks apart the paired electrons if they are in a spin-singlet (but not a spin-triplet) state. The second is the so-called 'orbital' effect, whereby the vortices penetrate into the superconductors and the energy gain due to the formation of the paired electrons is lost. For the case of layered, two-dimensional superconductors, such as the high-Tc copper oxides, the orbital effect is reduced when the applied magnetic field is parallel to the conducting layers. Here we report resistance and magnetic-torque experiments on single crystals of the quasi-two-dimensional organic conductor lambda-(BETS)2FeCl4, where BETS is bis(ethylenedithio)tetraselenafulvalene. We find that for magnetic fields applied exactly parallel to the conducting layers of the crystals, superconductivity is induced for fields above 17 T at a temperature of 0.1 K. The resulting phase diagram indicates that the transition temperature increases with magnetic field, that is, the superconducting state is further stabilized with magnetic field.

High critical current density and enhanced irreversibility field in superconducting MgB2 thin films

Abstract: The discovery of superconductivity at 39 K in magnesium diboride1 offers the possibility of a new class of low-cost, high-performance superconducting materials for magnets and electronic applications. This compound has twice the transition temperature of Nb3Sn and four times that of Nb-Ti alloy, and the vital prerequisite of strongly linked current flow has already been demonstrated2,3,4,5. One possible drawback, however, is that the magnetic field at which superconductivity is destroyed is modest. Furthermore, the field which limits the range of practical applications—the irreversibility field H*(T)—is approximately 7 T at liquid helium temperature (4.2 K), significantly lower than about 10 T for Nb-Ti (ref. 6) and ∼20 T for Nb3Sn (ref. 7). Here we show that MgB2 thin films that are alloyed with oxygen can exhibit a much steeper temperature dependence of H*(T) than is observed in bulk materials, yielding an H* value at 4.2 K greater than 14 T. In addition, very high critical current densities at 4.2 K are achieved: 1 MA cm-2 at 1 T and 105 A cm-2 at 10 T. These results demonstrate that MgB2 has potential for high-field superconducting applications.

MgB2 superconducting thin films with a transition temperature of 39 kelvin.

Abstract: We fabricated high-quality c axis-oriented epitaxial MgB2 thin films using a pulsed laser deposition technique. The thin films grown on (1 i 0 2) Al2O3 substrates have a transition temperature of 39 kelvin. The critical current density in zero field is approximately 6 x 10(6) amperes per cubic centimeter at 5 kelvin and approximately 3 x 10(5) amperes per cubic centimeter at 35 kelvin, which suggests that this compound has potential for electronic device applications, such as microwave devices and superconducting quantum interference devices. For the films deposited on Al2O3, x-ray diffraction patterns indicate a highly c axis-oriented crystal structure perpendicular to the substrate surface.

Finite-Temperature Magnetism of Transition Metals: An ab initio Dynamical Mean-Field Theory

Abstract: We present an ab initio quantum theory of the finite-temperature magnetism of iron and nickel. A recently developed technique which combines dynamical mean-field theory with realistic electronic structure methods successfully describes the many-body features of the one electron spectra and the observed magnetic moments below and above the Curie temperature.

Effects of Disorder on Ferromagnetism in Diluted Magnetic Semiconductors

Abstract: We present results of a numerical mean-field treatment of interacting spins and carriers in doped diluted magnetic semiconductors, which takes into account the positional disorder present in these alloy systems. Within our mean-field approximation, disorder enhances the ferromagnetic transition temperature for metallic densities not too far from the metal-insulator transition. Concurrently, the ferromagnetic phase is found to have very unusual temperature dependence of the magnetization as well as specific heat as a result of disorder. Unusual spin and charge transport is implied.

Superconductivity at 39 K in Magnesium Diboride.

Abstract: In the light of the tremendous progress that has been made in raising the transition temperature of the copper oxide superconductors (for a review, see ref. 1), it is natural to wonder how high the transition temperature, Tc, can be pushed in other classes of materials. At present, the highest reported values of Tc for non-copper-oxide bulk superconductivity are 33 K in electron-doped CsxRbyC60 (ref. 2), and 30 K in Ba1-xKxBiO3 (ref. 3). (Hole-doped C60 was recently found4 to be superconducting with a Tc as high as 52 K, although the nature of the experiment meant that the supercurrents were confined to the surface of the C60 crystal, rather than probing the bulk.) Here we report the discovery of bulk superconductivity in magnesium diboride, MgB2. Magnetization and resistivity measurements establish a transition temperature of 39 K, which we believe to be the highest yet determined for a non-copper-oxide bulk superconductor.

Micro-Raman scattering and dielectric investigations of phase transition behavior in the BaTiO3–BaZrO3 system

Abstract: In this study, the phase transition behavior of the BaTiO3–BaZrO3 system was studied using micro-Raman scattering and dielectric measurement techniques. BaZrxTi1−xO3 ceramics were prepared for x=0.00, 0.05, 0.08, 0.15, 0.20, and 1.00 compositions using a solid-state reaction technique. A single-phase perovskite structure of the ceramics was identified by the x-ray diffraction technique. The basic phase transition temperatures in these compositions were studied in the temperature range of 70–575 K. The tetragonal to cubic transition temperature was found to decrease with increasing Zr content. The orthorhombic to tetragonal transition temperature that increases with an initial increase in Zr content merges with the tetragonal–cubic transition for x⩾0.15 compositions. Raman spectra of rhombohedral and orthorhombic phases could not be distinguished. Excellent agreement between the crystallographic transition temperatures obtained by both techniques suggested that Zr substituted octahedra were uniformly distr...

Mobile silver ions and glass formation in solid electrolytes

Abstract: Solid electrolytes are a class of materials in which the cationic or anionic constituents are not confined to specific lattice sites, but are essentially free to move throughout the structure. The solid electrolytes AgI and Ag2Se (refs 1, 2, 3, 4, 5, 6, 7) are of interest for their use as additives in network glasses8,9,10,11,12, such as chalcogenides and oxides, because the resulting composite glasses can show high electrical conductivities with potential applications for batteries, sensors and displays. Here we show that these composite glasses can exhibit two distinct types of molecular structures—an intrinsic phase-separation that results in a bimodal distribution of glass transition temperatures, and a microscopically homogeneous network displaying a single glass transition temperature. For the first case, the two transition temperatures correspond to the solid-electrolyte glass phase and the main glass phase (the ‘base glass’), enabling us to show that the glass transition temperatures for the AgI and Ag2Se phases are respectively 75 and 230 °C. Furthermore, we show that the magnitude of the bimodal glass transition temperatures can be quantitatively understood in terms of network connectivity, provided that the Ag+ cations undergo fast-ion motion in the glasses. These results allow us to unambiguously distinguish base glasses in which these additives are homogeneously alloyed from those in which an intrinsic phase separation occurs, and to provide clues to understanding ion-transport behaviour in these superionic conductors.

Microscopic theory of heterogeneity and nonexponential relaxations in supercooled liquids.

Abstract: Recent experiments show that supercooled liquids around the glass transition temperature are "dynamically heterogeneous" [H. Sillescu, J. Non-Cryst. Solids 243, 81 (1999)]. Such heterogeneity is expected from the random first order transition theory of the glass transition. Using a microscopic approach based on this theory, we derive a relation between the departure from Debye relaxation as characterized by the beta value of a stretched exponential response function, phi(t) = e(-(t/tau(KWW))beta), and the fragility of the liquid. The beta value is also predicted to depend on temperature and to vanish as the ideal glass transition is approached at the Kauzmann temperature.

Volumetric studies of aqueous polymer solutions using pressure perturbation calorimetry: A new look at the temperature-induced phase transition of poly(N-isopropylacrylamide) in water and D2O

Abstract: We report the first application of pressure perturbation calorimetry (PPC) to determine the hydration properties of poly(N-isopropylacrylamide) (PNIPAM) in H2O and in D2O as the solutions undergo a temperature-induced phase transition. The technique, which measures the heat change resulting from a pressure change above a solution of PNIPAM placed in a microcalorimeter cell, yields the temperature dependence of the coefficient of thermal expansion, αp, of the polymer in solution and the change in volume of the solvation layer around the polymer chain. In the temperature ranges below and above the phase transition, αp of PNIPAM in H2O increased linearly with temperature. It underwent a sharp increase at the transition temperature, Tmax, then rapidly decreased. The phase transition was accompanied by an increase in the partial specific volume of the hydrated polymer. This increase was significantly higher for solutions of PNIPAM in D2O, compared to H2O. A study by PPC of the phase transition of hydrophobical...

Critical cooling rate and thermal stability of Zr–Ti–Cu–Ni–Be alloys

Abstract: The critical cooling rate as well as the thermal stability are measured for a series of alloys in the Zr–Ti–Cu–Ni–Be system. Upon cooling from the molten state with different rates, alloys with compositions ranging along a tie line from (Zr70Ti30)55(Ni39Cu61)25Be20 to (Zr85Ti15)55(Ni57Cu43)22.5Be27.5 show a continuous increase in the critical cooling rate to suppress crystallization. In contrast, thermal analysis of the same alloys shows that the undercooled liquid region, the temperature difference between the glass transition temperature and the crystallization temperature, is largest for some compositions midway between the two endpoints, revealing that glass forming ability does not correlate with thermal stability. The relationship between the composition-dependent glass forming ability and thermal stability is discussed with reference to a chemical decomposition process.

High-temperature superconductivity in lattice-expanded C60.

Abstract: C60 single crystals have been intercalated with CHCl3 and CHBr3 in order to expand the lattice. High densities of electrons and holes have been induced by gate doping in a field-effect transistor geometry. At low temperatures, the material turns superconducting with a maximum transition temperature of 117 K in hole-doped C60/CHBr3. The increasing spacing between the C60 molecules follows the general trend of alkali metal–doped C60 and suggests routes to even higher transition temperatures.

On transition temperatures in the plasticity and fracture of semiconductors

Abstract: Recent experiments on deformation of semiconductors show an abrupt change in the variation in the critical resolved shear stress τY with temperature T. This implies a change in the deformation mechanism at a critical temperature T c. In the cases examined so far in our laboratories (Case Western Reserve University and Poitiers) and elsewhere, this critical temperature appears to coincide approximately with the brittle-to-ductile transition temperature T BDT. In this paper, new deformation experiments performed on the wide-bandgap semiconductor 4H-SiC over a range of temperatures at two strain rates are described together with a transmission electron microscopy characterization of induced dislocations below and above T c. Based on these, and results recently reported on a few III–V compound semiconductors, a new model for the deformation of tetrahedrally coordinated materials at low and high temperatures is proposed, and the relation of the transition in deformation mode to the transition in fract...

Diamagnetic Activity Above Tc as a Precursor to Superconductivity in La2-xSrxCuO4 Thin Films

Abstract: Superconductors show zero resistance to electric current, and expel magnetic flux (the Meissner effect) below the transition temperature (Tc). In conventional superconductors, the 'Cooper pairs' of electrons that are responsible for superconductivity form only below Tc. In the unconventional high-Tc superconductors, however, a strong electron correlation is essential for pair formation: there is evidence that some pairs are formed above Tc in samples that have less than the optimal density of charge carriers (underdoped) and an energy gap-the 'pseudogap'-appears to be present. Moreover, excitations that look like the vortices that carry magnetic flux inside the superconducting state have been reported above Tc (refs 6, 7). Although the origin of the pseudogap remains controversial, phase fluctuations above Tc, leading to some form of local superconductivity or local pairing, seem essential. Here we report magnetic imaging (scanning SQUID microscopy) of La2-xSrxCuO4 thin films. Clear quantized vortex patterns are visible below Tc (18-19 K), and we observe inhomogeneous magnetic domains that persist up to 80 K. We interpret the data as suggesting the existence of diamagnetic regions that are precursors to the Meissner state.

Superconducting Mg-B films by pulsed laser deposition in an in-situ two-step process using multi-component targets

Abstract: Superconducting thin films have been prepared in a two-step in-situ process, using the Mg-B plasma generated by pulsed laser ablation. The target was composed of a mixture of Mg and MgB2 powders to compensate for the volatility of Mg and therefore to ensure a high Mg content in the film. The films were deposited at temperatures ranging from room temperature to 300 degrees C followed by a low-pressure in-situ annealing procedure. Various substrates have been used and diverse ways to increase the Mg content into the film were applied. The films show a sharp transition in the resistance and have a zero resistance transition temperature of 22-24 K.

Evidence for another low-temperature phase transition in tetragonal Pb(Zr x Ti 1-x )O 3 (x=0.515,0.520)

Abstract: Results of dielectric and resonance frequency (f r ) measurements below room temperature are presented for Pb(Zr x Ti 1-x )O 3 , x=0.515 and 0.520. It is shown that the temperature coefficient of f r changes sign from negative to positive around 210 and 265 K for x=0.520 and 200 and 260 K for x=0.515. Anomalies in the real part of the dielectric constant (e') are observed around the same temperatures at which the temperature coefficient of f r changes sign because of the electrostrictive coupling between the elastic and dielectric responses. Low-temperature powder x-ray-diffraction (XRD) data, however, reveal only one transition from the tetragonal to monoclinic phase similar to that reported by Noheda et al. [Phys. Rev. B, 61, 8687 (2000)]. Electron-diffraction data, on the other hand, reveal yet another structural transition at lower temperatures corresponding to the second anomaly in the e' vs T and f r vs T curves. This second transition is shown to be a cell-doubling transition not observed by Noheda et al. in their XRD studies. The observation of superlattice reflections raises doubts about the correctness of the Cm space group proposed by Noheda et al. for the monoclinic phase of Pb(Zr x Ti 1-x )O 3 below the second transition temperature.

Diluted Magnetic Semiconductors in the Low Carrier Density Regime

Abstract: This paper, based on a presentation at the Spintronics 2001 conference, provides a review of our studies on II-VI and III-V Mn-doped Diluted Magnetic Semiconductors. We use simple models appropriate for the low carrier density (insulating) regime, although we believe that some of the unusual features of the magnetization curves should qualitatively be present at larger dopings (metallic regime) as well. Positional disorder of the magnetic impurities inside the host semiconductor is shown to have observable consequences for the shape of the magnetization curve. Below the critical temperature the magnetization is spatially inhomogeneous, leading to very unusual temperature dependence of the average magnetization as well as specific heat. Disorder is also found to enhance the ferromagnetic transition temperature. Unusual spin and charge transport is implied.

A Temperature- and Molar Mass-Dependent Change in the Crystallization Mechanism of Poly(1-butene): Transition from Chain-Folded to Chain-Extended Crystallization?

Abstract: The crystallization behavior of poly(1-butene) (P1b) was investigated by polarized light microscopy (PLM), atomic force microscopy (AFM), wide-angle X-ray scattering (WAXS), dilatometry, and also by time- and temperature-resolved small-angle X-ray scattering experiments (SAXS). Observations in the PLM indicate a temperature-dependent change in the mechanism of crystallization. When crossing a certain critical crystallization temperature, the morphology changes from spherulites to quadratic, platelike single crystals. Investigations of samples with different molar mass show that the transition temperature is molar mass-dependent; on decreasing the molar mass the transition shifts to lower temperatures. As proved by WAXS, both the spherulites and the single crystals are of the metastable form II. The morphological change is also observed in AFM images obtained after a rapid cooling of the samples to room temperature; the difference in the morphological appearance is preserved through the transformation from...

Crystal Growth Shape of Whewellite Polymorphs: Influence of Structure Distortions on Crystal Shape

Abstract: The theoretical and experimental crystal growth shape has been determined for the two whewellite polymorphs. Theoretical studies consisted of PBC analysis, Donnay−Harker, Ising temperature, and Attachment Energy calculations. These calculations were based on accurate energy values, which were specially derived for the whewellite crystal structure by ab initio quantum mechanics. The morphology of crystals grown below and above the polymorph transition temperature is similar. However, theoretical morphologies are different, due to the doubling of the unit cell dimensions in the low-temperature structure. These results suggest a refinement of the thickness rule in Hartman-Perdock (H−P) theory. The Attachment Energy method is the most accurate of the three theoretical morphology methods used in this report. The high performance of the Ising model for this ionic crystal is probably due to the directionality of Ca−oxalate bonds. It has been found that crystal shape changes substantially along growth, massive pe...

Temperature-Dependent Electron Transport through Silver Nanocrystal Superlattices

Abstract: Temperature-dependent electron transport was measured through three-dimensional close-packed alkanethiol-stabilized silver nanocrystal arrays using interdigitated array electrodes. Nanocrystals ranging from 35 to 77 A in diameter with Coulomb blockade energies well above kT were studied. The nanocrystal superlattices exhibit linear current−voltage behavior for temperatures as low as 70 K. Ordered face-centered cubic (fcc) superlattices exhibit a positive temperature coefficient of resistivity (TCR), characteristic of a metal, at temperatures above approximately 225 to 245 K, depending on the particle size. The values of the conductivity, on the order of 10-6 to 10-7 Ω-1 cm-1, however, are characteristic of semiconductors. Below the transition temperature, the TCR for the size-monodisperse nanocrystal arrays becomes negative, characteristic of an insulator and the conductance G, of the ordered arrays scales exponentially with temperature as G ∝ exp[−(To/T)ν]. The exponent ν, ranges from 0.67 to 1.34 for na...

Ferroelectricity and ferrimagnetism in iron-doped BaTiO3

Abstract: The structural and physical properties of pseudocubic thin-film BaTi1−xFexO3(0.5⩽x⩽0.75) grown by pulsed-laser deposition are reported. This material is of interest because the corresponding bulk compounds have hexagonal structure for comparable x, and because the films are both ferroelectric and ferrimagnetic well above room temperature. A substantial increase of the ferroelectric transition temperature relative to that of bulk BaTiO3 is attributed to lattice expansion induced by Fe doping.

Electron and Hole Mobilities in High Purity Anthracene Single Crystals

Abstract: Using ultrapurified, highly perfect anthracene crystals electron and hole mobility data were measured by the time-of-flight method over a wide temperature range along 5 different crystallographic directions. From these results full tensor data (principal axes mobilities, tensor orientations and tensor rotations with temperature) have been determined. The highest principal electron mobility component is close to the a axis, reaching 35cm2/Vs at 20K, the highest hole component along b, reaching 50cm2/Vs at 35K. For electrons moving along the crystallographic c direction a transition from a nearly temperature-independent mobility (400K-100K) to a mobility increasing upon further cooling has been found, with very similar absolute values and transition temperature as previously reported for naphthalene and explained as a hopping to band transition. A non-Ohmic, sublinear velocity–electric field relation, reflecting hot charge carriers, has been obtained between 20 and 36K for electrons drifting along ...

Effect of silica aerosil dispersions on the dielectric properties of a nematic liquid crystal.

Abstract: The static and dynamic aspects of (4-n-heptyl-4'-cyanobiphenyl) (7CB) perturbed by the dispersion of 70-A-diameter hydrophilic silica aerosil spheres have been investigated using dielectric spectroscopy. Results on five mixtures of 7CB plus silica aerosil are presented in order to probe systematically the disorder introduced by the silica aerosil network on the 7CB molecules. Measurements on homeotropically aligned samples have been made from 75 kHz to 30 MHz in the temperature range 30-60 degrees C. It was found for the mixtures that the nematic-isotropic transition temperature is lower than that of the bulk 7CB. Bulklike relaxation processes due to the rotation of the molecules around the short axis have been analyzed, and were found to follow Arrhenius-type behavior in the nematic phase except in the vicinity of the nematic-isotropic transition temperature. These processes are slightly faster in the mixture than in the free phase. A dielectric process in the low frequency range, absent in the bulk, has been observed in samples with higher silica concentration. All the observed relaxation processes in the mixtures are of non-Debye type.