Showing papers on "Transition temperature published in 2002"
TL;DR: An ab initio calculation of the superconducting gaps in MgB2 and their effects on measurable quantities is reported, suggesting comparable or higher transition temperatures may result in layered materials based on B, C and N with partially filled planar orbitals.
Abstract: Magnesium diboride differs from ordinary metallic superconductors in several important ways, including the failure of conventional models to predict accurately its unusually high transition temperature, the effects of isotope substitution on the critical transition temperature, and its anomalous specific heat. A detailed examination of the energy associated with the formation of charge-carrying pairs, referred to as the 'superconducting energy gap', should clarify why MgB(2) is different. Some early experimental studies have indicated that MgB(2) has multiple gaps, but past theoretical studies have not explained from first principles the origin of these gaps and their effects. Here we report an ab initio calculation of the superconducting gaps in MgB(2) and their effects on measurable quantities. An important feature is that the electronic states dominated by orbitals in the boron plane couple strongly to specific phonon modes, making pair formation favourable. This explains the high transition temperature, the anomalous structure in the specific heat, and the existence of multiple gaps in this material. Our analysis suggests comparable or higher transition temperatures may result in layered materials based on B, C and N with partially filled planar orbitals.
740 citations
TL;DR: The effect of uniaxial stress along the c axis on the metal-insulator transition of VO2 has been studied in the form of epitaxial thin films grown on TiO2 (001) and (110) substrates as mentioned in this paper.
Abstract: The effect of uniaxial stress along the c axis on the metal–insulator transition of VO2 has been studied in the form of epitaxial thin films grown on TiO2 (001) and (110) substrates. A large reduction in the transition temperature TMI from 341 K for a single crystal to 300 K has been observed in the film on TiO2 (001) where the c-axis length is compressed owing to an epitaxial stress, while the TMI has been increased to 369 K in the film on TiO2 (110) where the c-axis length is expanded. The correlation between the c-axis length and TMI is suggested: the shorter c-axis length results in the lower TMI.
481 citations
TL;DR: The dielectric and piezoelectric properties of the new perovskite solid solution system (1-x)BiScO3-xPbTiO3 were investigated in this paper.
Abstract: The dielectric and piezoelectric properties of the new perovskite solid solution system (1-x)BiScO3–xPbTiO3 were investigated. This system is representative of a new group of high temperature piezoelectrics that includes Bi(Me)O3–PbTiO3, where Me+3 is a relatively large cation, Sc, Y, Yb, In, etc., and combinations thereof. In the (1-x)BiScO3–xPbTiO3 series, perovskite stability was achieved for x>50 mol% PbTiO3 being ferroelectric rhombohedral and transforming to ferroelectric tetragonal in the region x=64 mol% PbTiO3, designated as the morphotropic phase boundary (MPB). Analogous to (1-x)PbZrO3–xPbTiO3 (PZT), the dielectric and piezoelectric properties were enhanced for compositions near the MPB. Piezoelectric coefficient d33 values reached 450 pC/N, comparable to soft PZT's with a transition temperature of 450°C, more than 100°C higher than commercial PZT. The combination of high TC and excellent piezoelectric activity make (1-x)BiScO3–xPbTiO3 materials candidates for high temperature, and temperature stable actuators and transducers.
480 citations
TL;DR: In this article, the authors describe an atomic physics experiment that can provide critical insight into the origin of high-temperature superconductivity in cuprates, using standing wave light.
Abstract: Fermionic atoms confined in a potential created by standing wave light can undergo a phase transition to a superfluid state at a dramatically increased transition temperature Depending upon carefully controlled parameters, a transition to a superfluid state of Cooper pairs, antiferromagnetic states or d-wave pairing states can be induced and probed under realistic experimental conditions We describe an atomic physics experiment that can provide critical insight into the origin of high-temperature superconductivity in cuprates
351 citations
TL;DR: In this paper, size effects in the structural phase transition of submicron vanadium dioxide precipitates in silica were observed in terms of heterogeneous nucleation statistics with a phenomenological approach in which the density of nucleating defects is a power function of the driving force.
Abstract: We have observed size effects in the structural phase transition of submicron vanadium dioxide precipitates in silica. The ${\mathrm{VO}}_{2}$ nanoprecipitates are produced by the stoichiometric coimplantation of vanadium and oxygen and subsequent thermal processing. The observed size dependence in the transition temperature and hysteresis loops of the semiconductor-to-metal phase transition in ${\mathrm{VO}}_{2}$ is described in terms of heterogeneous nucleation statistics with a phenomenological approach in which the density of nucleating defects is a power function of the driving force.
323 citations
TL;DR: The ferromagnetic domains almost disappear at a temperature T P2 higher than T P1, showing a local magnetic hysteresis in agreement with the resistivity hysteResis, indicating magnetic inhomogeneity.
Abstract: Upon cooling, the isolated ferromagnetic domains in thin films of La0.33Pr0.34Ca0.33MnO3start to grow and merge at the metal-insulator transition temperatureTP1, leading to a steep drop in resistivity, and continue to grow far below TP1. In contrast, upon warming, the ferromagnetic domain size remains unchanged until near the transition temperature. The jump in the resistivity results from the decrease in the average magnetization. The ferromagnetic domains almost disappear at a temperature TP2higher than TP1, showing a local magnetic hysteresis in agreement with the resistivity hysteresis. Even well above TP2, some ferromagnetic domains with higher transition temperatures are observed, indicating magnetic inhomogeneity. These results may shed more light on the origin of the magnetoresistance in these materials.
307 citations
TL;DR: In this paper, the synthesis of room-temperature ferromagnetic semiconductors, Zn1−xFexO, was reported, and the essential ingredient in achieving room temperature ferromagnetism in bulk Zn0.94Fe0.05Cu0.01O was found to be additional Cu doping.
Abstract: Successful synthesis of room-temperature ferromagnetic semiconductors, Zn1−xFexO, is reported. The essential ingredient in achieving room-temperature ferromagnetism in bulk Zn1−xFexO was found to be additional Cu doping. A transition temperature as high as 550 K was obtained in Zn0.94Fe0.05Cu0.01O; the saturation magnetization at room temperature reached a value of 0.75μB per Fe. A large magnetoresistance was also observed below 100 K.
290 citations
TL;DR: It is shown that Li becomes superconducting at pressures greater than 30 GPa, with a pressure-dependent transition temperature (Tc) of 20 K at 48 GPa, which is the highest observed Tc of any element; it confirms the expectation that elements with low atomic numbers will have high transition temperatures, and suggests that metallic hydrogen will have a very high Tc.
Abstract: Superconductivity at high temperatures is expected in elements with low atomic numbers, based in part on conventional BCS (Bardeen-Cooper-Schrieffer) theory. For example, it has been predicted that when hydrogen is compressed to its dense metallic phase (at pressures exceeding 400 GPa), it will become superconducting with a transition temperature above room temperature. Such pressures are difficult to produce in a laboratory setting, so the predictions are not easily confirmed. Under normal conditions lithium is the lightest metal of all the elements, and may become superconducting at lower pressures; a tentative observation of a superconducting transition in Li has been previously reported. Here we show that Li becomes superconducting at pressures greater than 30 GPa, with a pressure-dependent transition temperature (T(c)) of 20 K at 48 GPa. This is the highest observed T(c) of any element; it confirms the expectation that elements with low atomic numbers will have high transition temperatures, and suggests that metallic hydrogen will have a very high T(c). Our results confirm that the earlier tentative claim of superconductivity in Li was correct.
284 citations
TL;DR: In this article, the authors reported Curie temperatures up to 150 K in annealed Ga1-xMnxAs epilayers with a relatively low As:Ga beam equivalent pressure ratio.
Abstract: We report Curie temperatures up to 150 K in annealed Ga1-xMnxAs epilayers grown with a relatively low As:Ga beam equivalent pressure ratio. A variety of measurements (magnetization, Hall effect, magnetic circular dichroism and Raman scattering) show that the higher ferromagnetic transition temperature results from an enhanced free hole density. The data also indicate that, in addition to the carrier concentration, the sample thickness limits the maximum attainable Curie temperature in this material - suggesting that the free surface of Ga1-xMnxAs epilayers is important in determining their physical properties.
245 citations
TL;DR: It is reported that, for a material in the pseudogap state, left-circularly polarized photons give a different photocurrent from right- Circularly polarization photons, which shows that time-reversal symmetry is spontaneously broken below T*, which therefore corresponds to a phase transition.
Abstract: A change in ‘symmetry’ is often observed when matter undergoes a phase transition—the symmetry is said to be spontaneously broken. The transition made by underdoped high-transition-temperature (high-Tc) superconductors is unusual, in that it is not a mean-field transition as seen in other superconductors. Rather, there is a region in the phase diagram above the superconducting transition temperature Tc (where phase coherence and superconductivity begin) but below a characteristic temperature T* where a ‘pseudogap’ appears in the spectrum of electronic excitations1,2. It is therefore important to establish if T* is just a cross-over temperature arising from fluctuations in the order parameter that will establish superconductivity at Tc (refs 3, 4), or if it marks a phase transition where symmetry is spontaneously broken5,6,7,8,9,10. Here we report that, for a material in the pseudogap state, left-circularly polarized photons give a different photocurrent from right-circularly polarized photons. This shows that time-reversal symmetry is spontaneously broken11 below T*, which therefore corresponds to a phase transition.
238 citations
TL;DR: It is deduced from the DFT calculation that the relative intensity of the bands at 1173 and 1155 cm-1 in the amide III region reflects the population change in the gauche and trans conformations in the main chain during the coil-globule transition.
Abstract: A conformational change in the coil−globule transition of poly(N-isopropylacrylamide) (PNiPA) was investigated by Fourier transform infrared (FT-IR) spectroscopy with attenuated total reflection (ATR) method and density functional theory (DFT) calculations. ATR/IR spectra of PNiPA in an aqueous solution change dramatically in the vicinity of the coil−globule transition temperature (θ temperature). Below the θ temperature, unimodal peaks are observed at 1624 cm-1 in the amide I region and at 1562 cm-1 in the amide II region, respectively. Above the θ temperature, a new peak appears abruptly near 1653 cm-1 in the amide I region and the amide II band shifts gradually to a lower frequency by 6 cm-1. In the amide III region, the relative intensity of a band at 1173 cm-1 is weaker than that of a band at 1155 cm-1 at lower temperatures, but it becomes larger during the coil−globule transition of PNiPA. DFT calculation for dimer models of PNiPA suggests that the amide I band at 1624 cm-1 is assigned mainly to a s...
TL;DR: The superconducting phase transition in heavy fermion CeCoIn5 becomes first order when the magnetic field H parallel [001] is greater than 4.7 T, and the transition temperature is below T0 approximately 0.31T(c).
Abstract: The superconducting phase transition in heavy fermion CeCoIn5 (T(c)=2.3 K in zero field) becomes first order when the magnetic field H parallel [001] is greater than 4.7 T, and the transition temperature is below T0 approximately 0.31T(c). The change from second order at lower fields is reflected in strong sharpening of both specific heat and thermal expansion anomalies associated with the phase transition, a strong magnetocaloric effect, and a steplike change in the sample volume. This effect is due to Pauli limiting in a type-II superconductor, and was predicted theoretically in the mid-1960s.
TL;DR: In this paper, a review of a micro-Raman scattering study on various lead and barium-based ABO3 perovskites (A = Pb, La, Ba, Sr, Ce, Gd, Nd and B = Ti, Zr), Aurivillius-type Bi-layered compounds (SrBi2Ta2O9, Bi4Ti3O12) and their solid solutions useful in a broad range of device applications.
Abstract: This paper provides a review of a systematic micro-Raman scattering study on various lead- and barium-based ABO3 perovskites (A = Pb, La, Ba, Sr, Ce, Gd, Nd and B = Ti, Zr), Aurivillius-type Bi-layered compounds (SrBi2Ta2O9, Bi4Ti3O12) and their solid solutions useful in a broad range of device applications. Various factors that influence the material properties such as particle size, stresses, stoichiometry, compositional homogeneity and their effects on phase transition were investigated. The processing conditions, A- and B-site substitution, size-dependent Raman spectra and the structure–property correlations are discussed in the bulk, thin film and nano-crystalline forms of these materials. A film thickness dependence stress study on lead titanate (PT) and lead zirconate titanate (PZT) films indicated that the nature of stress depends strongly on the lattice parameters of the film and substrate. The size effect was found to decrease the ferroelectric transition temperature in lead-based perovskite materials. Both ionic charge and radii induced changes in the Raman spectra of A- and B-site substituted perovskites and layered compounds were studied. A-site substitution in strontium bismuth tantalite (SBT) was found to induce a relatively linear variation of transition temperature compared with the B-site substituted SBT. Raman spectra of layered compounds and their solid solutions exhibited a strong dependence on dopants resulting in structural modifications. Copyright © 2002 John Wiley & Sons, Ltd.
TL;DR: The sol-to-gel transition temperature of polymer mixture aqueous solutions was well described by an empirical equation of miscible blends, indicating miscibility of the two polymer systems in water on the molecular level.
TL;DR: In this article, a model for estimating the glass transition temperature of covalent amorphous materials from their enthalpy of atomisation is presented, which can be used for the development of phase-change materials for rewritable optical recording.
Abstract: A model is presented for estimating the glass transition temperature of covalent amorphous materials from their enthalpy of atomisation. The enthalpy of atomisation is calculated by summing all individual bond enthalpies taking account of the coordination numbers of the constituting elements. A consistent set of bond enthalpies has been derived from the enthalpies of atomisation of stoichiometric compounds. The model is applicable to covalent amorphous materials consisting of the elements from group IB–VIB and for which the average number of valence electrons is no less than 4. It is shown that the model can be used for the development of phase-change materials for rewritable optical recording. Since the glass transition temperature is a lower limit for the crystallisation temperature, the model can predict the stability against spontaneous crystallisation of amorphous phase-change materials.
TL;DR: In this article, the authors used spin-resolved spectroscopy to study the ferromagnetic semiconductor EuO and observed large changes in the electronic structure across the Curie and metal-insulator transition temperature.
Abstract: High quality thin films of the ferromagnetic semiconductor EuO have been prepared and were studied using a new form of spin-resolved spectroscopy. We observed large changes in the electronic structure across the Curie and metal-insulator transition temperature. We found that these are caused by the exchange splitting of the conduction band in the ferromagnetic state, which is as large as 0.6 eV. We also present strong evidence that the bottom of the conduction band consists mainly of majority spins. This implies that doped charge carriers in EuO are practically fully spin polarized.
TL;DR: In this paper, a coarse-grained bead spring model of short polymer chains is studied by constant pressure molecular dynamics simulations, and a cooling-rate-dependent glass transition temperature Tg(Γ) by cooling the model system from T = 0.6 down to T=0.3, applying cooling rates from Γ ≥ 10−3 to Γ≈10−6.
Abstract: A coarse-grained bead spring model of short polymer chains is studied by constant pressure molecular dynamics (MD) simulations. Due to two competing length scales for the length of effective bonds and the energetically preferred distance between nonbonded beads, one observes a glass transition when dense melts are cooled down (as shown in previous work, at a pressure p=1 the mode coupling critical temperature is at Tc≈0.45 and the Vogel–Fulcher temperature is T0≈0.33, in Lennard-Jones units). The present work extends these studies, estimating a cooling-rate-dependent glass transition temperature Tg(Γ) by cooling the model system from T=0.6 down to T=0.3, applying cooling rates from Γ≈10−3 to Γ≈10−6 (in MD time units), and attempting to identify Tg(Γ) from a kink in the volume versus temperature or potential energy versus temperature curves. It is found that Tg(Γ) lies in the range 0.43⩽Tg(Γ)⩽0.47, for the cooling rates quoted, and the variation of Tg(Γ) for Γ is compatible with the expected logarithmic variations. We will show why a detailed distinction between competing theories on these cooling rate effects would need an excessive amount of computer time. To estimate also the melting transition temperature Tm of this model, the sytem was prepared in a crystalline configuration as an initial state and heated up. The onset of diffusion, accompanied by an isotropization of the pressure tensor was observed for Tm≈0.77. This implies that the model is suitable for studying deeply supercooled melts.
01 Mar 2002
TL;DR: High quality thin films of the ferromagnetic semiconductor EuO have been prepared and were studied using a new form of spin-resolved spectroscopy, finding that large changes in the electronic structure across the Curie and metal-insulator transition temperature are caused by the exchange splitting of the conduction band.
Abstract: High quality thin films of the ferromagnetic semiconductor EuO have been prepared and were studied using a new form of spin-resolved spectroscopy. We observed large changes in the electronic structure across the Curie and metal-insulator transition temperature. We found that these are caused by the exchange splitting of the conduction band in the ferromagnetic state, which is as large as 0.6 eV. We also present strong evidence that the bottom of the conduction band consists mainly of majority spins. This implies that doped charge carriers in EuO are practically fully spin polarized.
TL;DR: In this article, the effects of the vanadium dioxide nanocrystal size, nanocrystals morphology, and particle/host interactions on the VO2 semiconductor-to-metal phase transition were characterized.
Abstract: Vanadium dioxide single-crystal precipitates with controlled particle sizes were produced in an amorphous, fused SiO2 host by the stoichiometric coimplantation of vanadium and oxygen ions and subsequent thermal processing. The effects of the vanadium dioxide nanocrystal size, nanocrystal morphology, and particle/host interactions on the VO2 semiconductor-to-metal phase transition were characterized. VO2 nanoparticles embedded in amorphous SiO2 exhibit a sharp phase transition with a hysteresis that is up to 50 °C in width—one of the largest values ever reported for this transition. The relative decrease in the optical transmission in the near-infrared region in going from the semiconducting to the metallic phase of VO2 ranges from 20% to 35%. Both the hysteresis width and the transition temperature are correlated with the size of the precipitates. Doping the embedded VO2 particles with ions such as titanium alters the characteristics of the phase transition, pointing the way to control the hysteresis beha...
TL;DR: In this paper, the influence of cation order-disorder phenomena on the crystal structure, magnetic and electrical transport properties of new CMR perovskites for LBaMn 2 O 6 - γ (L=Pr, Nd, Sm, Eu, Gd, Tb) series has been investigated.
Abstract: Influence of cation order-disorder phenomena on the crystal structure, magnetic, and electrical transport properties of new CMR perovskites for LBaMn 2 O 6 - γ (L=Pr, Nd, Sm, Eu, Gd, Tb) series has been investigated. For each rare-earth ion three compounds have been synthesized by the topotactic reduction-oxidation method. Structural investigations have shown the oxygen-stoichiometry LBaMn 2 O 6 compound obtained in air to be cubic with disordered L 3 + and Ba 2 + cations whereas the oxygen-deficient LBaMn 2 O 5 is tetragonal with ordered L 3 + and Ba 2 + and alternate stacking of rare earth and barium containing layers along c. This crystal structure is similar to the YBaCuFeO 5 related one. Another form of oxygen-stoichiometry LBaMn 2 O 6 compound obtained by reoxidation of oxygen-deficient LBaMn 2 O 5 is also tetragonal and retains the ordering of L 3 + and Ba 2 + cations. It is notable that the reoxidized EuBaMn 2 O 6 compound has an orthorhombic unit cell. It is observed that this type of cation ordering leads to considerable increase of transition temperature to paramagnetic state. For example, disordered EuBaMn 2 O 6 compound has magnetic properties similar to spin glass and shows freezing temperature of magnetic moments T f 40 K while ordered EuBaMn 2 O 6 is an inhomogeneous ferromagnet with Curie point T C 260 K. Electrical resistivity behavior correlates with magnetization. Below the T C the Pr, Nd, Sm based compounds undergo a transition to metallic state and demonstrate a peak of magnetoresistance. It is supposed that the remarkable changing of the magnetic and electrical properties of the reoxidized compounds is a consequence of the L/Ba ordering and can be explained on the base of the Goodenough-Kanamori rules for 180° indirect superexchange interactions taking into account an ion size effect in A sublattice of perovskite.
TL;DR: In this paper, a combination of counter-balanced Si and C co-substitution for B, leading to a large number of intragranular dislocations and the dispersed nano-size impurities induced by the substitution.
Abstract: By doping MgB2 superconductor with SiC nano-particles, we have successfully introduced pinning sites directly into the crystal lattice of MgB2 grains (intra-grain pinning). It became possible due to the combination of counter-balanced Si and C co-substitution for B, leading to a large number of intra-granular dislocations and the dispersed nano-size impurities induced by the substitution. The magnetic field dependence of the critical current density was significantly improved in a wide temperature range, whereas the transition temperature in the sample MgB2(SiC)x having x = 0.34, the highest doping level prepared, dropped only by 2.6 K.
TL;DR: In this paper, the thickness dependence of the magnetotransport properties of La 2 / 3 Ca 1 / 3 MnO 3 thin films epitaxially grown on SrTiO 3, LaAlO 3, and NdGaO 3 single-crystalline substrates was studied.
Abstract: We have studied the thickness dependence of the magnetotransport properties of La 2 / 3 Ca 1 / 3 MnO 3 thin films epitaxially grown on SrTiO 3 , LaAlO 3 , and NdGaO 3 single-crystalline substrates. When thickness decreases, aglobal disruption of the magnetoelectronic properties occurs, namely the resistivity and the low-temperature magnetoresistance increase while the metal-to-insulator transition temperature (Tp) is lowered. We state that the electronic properties of these films, especially close to the film/substrate interface, differ from those of the bulk material. This is confirmed by nuclear-magnetic-resonance measurements which provide evidence that these films have an inhomogeneous magnetoelectronic nanostructure with distinguishable regions containing localized charges. These regions are scattered within the films, with a higher density close to interfaces in the case of La 2 / 3 Ca 1 / 3 MnO 3 films on SrTiO 3 but more homogeneously distributed for films grown on NdGaO 3 . Since our manganite films have a virtually unrelaxed crystal structure, the thickness dependence of Tp can neither be related to the strain states nor to dimensional effects. Alternatively, we show that the coexistence of different electronic phases leads to a modification of the carrier density in the metallic regions and, presumably, to an enhancement of the disorder in the Mn-O bond length and Mn-O-Mn angles. We will argue that the conjunction of both factors promotes a decrease of the double exchange transfer integral and, consequently, accounts for the reduction of the Curie temperature for the thinnest films. The possible mechanisms responsible for this phase separation are discussed in terms of the microstructure of the interfaces between the manganite and the insulating perovskite.
TL;DR: In this article, high-oriented V2O5 thin films were prepared by sol-gel method on SiO2/Si substrate and they were found to convert to VO2 polycrystalline thin films under a temperature above 400°C and a pressure below 2ÕPa in air.
TL;DR: By doping MgB_2 superconductor with SiC nano-particles, the authors successfully introduced pinning sites directly into the crystal lattice of Mb_2 grains (intra-grain pinning).
Abstract: By doping MgB_2 superconductor with SiC nano-particles, we have successfully introduced pinning sites directly into the crystal lattice of MgB_2 grains (intra-grain pinning). It became possible due to the combination of counter-balanced Si and C co-substitution for B, leading to a large number of intra-granular dislocations and the dispersed nano-size impurities induced by the substitution. The magnetic field dependence of the critical current density was significantly improved in a wide temperature range, whereas the transition temperature in the sample MgB_2(SiC)_x having x = 0.34, the highest doping level prepared, dropped only by 2.6 K.
TL;DR: In this article, the temperature of transition from γ-to α-phase was determined using differential scanning calorimetry, and it was observed that all the grown crystals except the one grown from a mixture of water and sodium hydroxide change from −1 to −2 at 168°C.
TL;DR: In this article, temperature-dependent DC transport measurements on self-assembled Ag nanoparticle monolayers have been obtained as a function of particle size distribution induced disorder and a transition temperature between the transport regimes of simple, activated transport to variable range hopping is observed to be in tune with increasing disorder of the monolayer.
Abstract: Temperature-dependent DC transport measurements on self-assembled Ag nanoparticle monolayers have been obtained as a function of particle size distribution induced disorder. A transition temperature between the transport regimes of simple, activated transport to variable range hopping is observed to be in tune with increasing disorder of the monolayer. The overall transport behavior is described in terms of a mobility gap in the Anderson localized regime and an Efros-Shklovskii variable range hopping between localized states in the gap below some temperature characterized by the degree of disorder. From these results, we predict a true insulator−metal transition at narrow but still finite (<3%) size distributions.
TL;DR: In this article, the microstructure of thin films of vanadium dioxide has been studied on glass substrates and it is found that nearly monophasic, monoclinic VO2(M) films are formed in the narrow temperature range 475-520°C, films formed outside this range comprising significant proportions of other vanadium oxide phases.
Abstract: Thin films of vanadium dioxide have been deposited on glass by low pressure metal-organic chemical vapour deposition using the β-diketonate complex, vanadyl acetylacetonate, as the precursor. It is found that nearly monophasic, monoclinic VO2(M) films are formed in the narrow temperature range 475–520 °C, films formed outside this range comprising significant proportions of other vanadium oxide phases beside VO2(M). The microstructure of these well-crystallized films varies significantly with temperature in this range. Films grown at 475 °C are dense and have a very strong (200) orientation. At 520 °C, films are somewhat porous, and display little preferred orientation. Film microstructure influences the semiconductor–metal transition noticeably. Films deposited at 475 °C have a large change in resistance at 66 °C, and display a small temperature hysteresis
in the transition. The transition temperature in films grown at 520 °C is higher (72 °C), whereas the change in resistance is smaller and the hysteresis larger. An attempt has been made to understand the unusual microstructure of VO2 films grown on glass substrates. The variation in the phase transition characteristics is interpreted in terms of the observed film microstructure. The thermal properties of the CVD precursor are also reported.
TL;DR: In this article, the hardness and indentation cracking at room temperature as well as the yield and creep at high temperatures in the range 800 − 1400 °C have been studied for various Laves phases AB2 with the transition metals A=Zr, Nb, Ta and B=Cr, Fe, Co with partial substitution of the B elements Fe and Co by Al, which effects a transition from cubic C15 structure to hexagonal C14 structure in the case of Nb(Co,Al)2.
TL;DR: Diblock copolymers formed from polystyrene covalently linked to poly (n-pentylmethacrylate), P(S-b-nPMA), which have only weak segmental interactions, are shown to exhibit a closed-loop phase behaviour over a narrow range of molecular weight.
Abstract: Closed-loop phase diagrams are known in systems with specific intermolecular interactions. In weakly interacting systems, however, such behaviour has never been observed. Here, diblock copolymers formed from polystyrene covalently linked to poly (n-pentylmethacrylate), P(S-b-nPMA), which have only weak segmental interactions, are shown to exhibit a closed-loop phase behaviour over a narrow range of molecular weight. The endothermic transitions from the disordered to ordered and back to the disordered state, as a function of increasing temperature, are dominantly entropic in origin. The morphology and rheological properties of P(S-b-nPMA) undergo characteristic changes at the transitions. Whereas the disorder-to-order transition temperature increases with decreasing molecular weight, the order-to-disorder transition temperature decreases. At a limiting molecular weight, the closed-loop vanishes and no ordering occurs. These findings provide quantitative insight into an elusive transition in weakly interacting multicomponent systems.
TL;DR: The system Mg x B 2 has been studied to investigate possible nonstoichiometry in MgB 2, a line compound with a possible Mg vacancy content of about 1% as discussed by the authors.
Abstract: The system Mg x B 2 has been studied to investigate possible nonstoichiometry in MgB 2 . When synthesized at 850 °C, MgB 2 is a line compound with a possible Mg vacancy content of about 1%. Small changes in lattice constants as a function of starting composition result from grain-interaction stresses, whose character is different in the Mg-rich, near-stoichiometric, and Mg-deficient regimes. A small linear decrease of the superconducting transition temperature, T c , in the Mg-rich regime results from accidental impurity doping.