Phonon renormalization,specific-heat anomaly and why some aspects of mean-field theory can be relevant for high-T c superconductivity

Abstract

Phonon anomalies due to superconductivity in YBa₂Cu₃O₇ are correlated with the superconducting order parameter. The phonon part of the excess specific heat follows the same temperature evolution as the measured macroscopic excess heat capacity. The electronic part of the transformation entropy corresponds to about 10% of the total carrier concentration. No excess entropy was found at temperatures well above T_c. The effect of phonon contributions to the stabilization energy of the superconducting phase is discussed.     

Structural investigations on Nd-doped silica nanocomposites: effect of sintering temperature and dopant concentration

Neodymium-doped silica nanocomposites were prepared from an acid-catalysed sol–gel solution followed by heat treatment. The structural and microstructural properties of the prepared samples as a function of sintering temperature and Nd concentration are reported. Fourier transform infrared spectra show that phase separation occurs during heat treatment. The presence of Nd₂O₃ and α-Nd₂Si₂O₇ phases in the samples was established by X-ray diffraction (XRD), and transmission electron microscopy (TEM) micrographs revealed the microstructure of the nanocomposites. From XRD patterns, the crystallite size was determined using the Debye–Scherrer formula, while the particle size was estimated from TEM micrographs. The results suggest that sintering at high temperature enhances the crystallinity and density of Nd₂O₃–SiO₂ nanocomposites, while the high concentration of neodymium prevents the crystallization of SiO₂.     

Similar improvements in inhibitory control following low-volume high-intensity interval exercise and moderate-intensity continuous exercise

ObjectivesWe previously demonstrated that a traditional high-volume (HV) high-intensity interval exercise (HIIE) was more effective in improving post-exercise inhibitory control (IC) than moderate-intensity continuous exercise (MCE). Nevertheless, because HV-HIIE is performed with a higher volume and longer duration, it may result in decreased adherence to this exercise for some individuals. In this study, we compared the effect of a practical low-volume (LV) HIIE to that of MCE on post-exercise IC improvements.DesignTwenty healthy males performed both LV-HIIE and MCE on a cycle ergometer in a crossover design.MethodLV-HIIE was consisted of ten 1-min bouts at 90% of the peak oxygen consumption (VO_{2 peak}) with 1-min active recovery at 30% of VO_{2 peak}, which had a total duration of 20 min. MCE was performed for 40 min at 60% of VO_{2 peak}. To evaluate IC, the Stroop test was administered before exercise, immediately after exercise, and every 10 min during the 30-min post-exercise recovery period.ResultsIC significantly improved immediately after LV-HIIE and MCE compared with that before each exercise (both Ps < 0.05). The improved IC remained significant until the 20-min post-exercise recovery period for both protocols (all Ps < 0.05). The degrees of post-exercise IC improvements throughout the 30-min post-exercise recovery period did not differ significantly between protocols.ConclusionsThese findings demonstrated that despite lower exercise volume and shorter exercise duration, LV-HIIE could improve post-exercise IC similar to MCE.     

The formation of cubic and monoclinic Y2O3 nanoparticles in a gas-phase flame process

Synthesis of phase-pure cubic and monoclinic Y₂O₃ nanoparticles (16–90?nm) was achieved in a gas-phase flame process. The effect of process parameters on the crystal structure of the Y₂O₃ nanoparticles was systematically investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Fuel gases, H₂ or C₂H₄, at various flow rates, two yttrium precursors, an oxidant stream with varying degrees of N₂ or Ar dilution and different burner diameters were used to examine the effects of chemical atmosphere, flame temperature, residence time and precursor concentration on the crystal structure of the synthesized Y₂O₃ nanoparticles. Regardless of the other process parameters, at diluent/O₂ ratios of 0.25 or lower, monoclinic Y₂O₃ nanoparticles were obtained, whereas at diluent/O₂ ratios of 1 or higher, cubic Y₂O₃ nanoparticles were obtained. A lower diluent/O₂ ratio was related to higher flame temperature. Thermodynamic analyses suggest that high temperatures likely favour the formation of monoclinic Y₂O₃ nanoparticles in this flame process.     

Effect of network connectivity on thermal properties of As30Te70– x Tl x glasses

Alternating differential scanning calorimetry (ADSC) studies were undertaken to investigate the effect of Tl addition on the thermal properties of As₃₀Te_{70– x} Tl _x (6?≤?x?≤?22 at%) glasses. These include parameters such as glass-transition temperature (T _g), changes in specific heat capacity (ΔC _p) and relaxation enthalpy (ΔH ^NR) at the glass transition. It was found that T _g of the glasses decreased with the addition of Tl, which is in contrast to the dependence of T _g in As–Te glasses on the addition of Al and In. The change in heat capacity ΔC _p through the glass transition was also found to decrease with increasing Tl content. The addition of Tl to the As–Te matrix may lead to a breaking of As–Te chains and the formation of Tl⁺Te^?AsTe_2/2 dipoles. There was no significant dependence of the change of relaxation enthalpy, through the glass transition, with composition.     

Morphological and magnetic response of copper-substituted nickel ferrite nanoparticles

Ferrite nanoparticles are interesting materials owing to their unique physical and chemical properties. The metal-doped ferrites have well-defined structures and magnetic response, such as high permeability for a specific frequency range. In this study, copper-substituted nickel ferrite (Ni_1?xCu_xFe₂O₄) nanoparticles with a compositional range of 0?≤?x?≤?0.3 were synthesised through a co-precipitation technique. Metal chlorides were used as precursors and NaOH as a precipitating agent for the growth of ferrite nanoparticles. To minimise the internal stresses and maximise the magnetic response, ferrite nanoparticles were annealed in a furnace at 700°C for 6 h. The structural and magnetic response of Ni_1?xCu_xFe₂O₄ ferrite with different values of x were investigated using Scanning Electron Microscopy (SEM), Fourier Transform Infrared spectroscopy (FT-IR), Vibrating Sample Magnetometer (VSM) and X-ray Diffraction (XRD) techniques. XRD analysis confirmed the formation of cubic spinel structure of single phase for all the compositions. The lattice constant decreased with increase in the value of x. FT-IR study showed two main metal oxygen bonds in the range 500–700 cm^?1 confirming the formation of a single-phase cubic inverse structure of Cu-substituted Ni ferrite. VSM results revealed the formation of ferrimagnetic nanoparticles. The optical and magnetic response of the ferrite nanoparticles changed with Cu content.     

HIGH-Tc superconductivity in perovskite oxides of the Y─Ba─Cu─O system

Abstract

Several biphasic compositions of the type Y_3?xBa_3+xCu₆O₁₄ show an onset of superconductivity in the 90-115K range, attaining zero resistance in the 70-85K range. The phase responsible for superconductivity in these compositions is a perovskite oxide of composition YBa₂Cu₃ O₇. This oxide annealed in oxygen shows the onset of superconductivity at 120K and zero resistance at 87K. YBa₂,Cu₃O₇ shows the highest Meissner effect of all oxide superconductors. The superconducting behaviour of the two perovskite oxides, Y_0.95Ba_1.95,Cu₃O₇ and Y_1.05Ba_1.95Cu₃O₇ show interesting features; a marked decrease in resistivity is observed from room temperature itself in the former oxide with zero resistance at 89K. Electron microscopy and infrared spectra of these oxides are briefly discussed.     

Experimental investigations of TB2 alloy by pack boriding with rare-earth oxides

ABSTRACT

An orthogonal test with four factors, namely temperature, time, type of rare-earth oxides (REOs) and REO content, was performed to obtain the optimised boriding parameters of TB2 alloy by pack boriding with REOs. It is found that temperature has the strongest effect on the boride layer thickness, while time has the strongest effect on the surface hardness and coefficient of friction. The optimum parameters for pack boriding of TB2 alloy with REO are a temperature of 1373?K, a time of 20?h, La₂O₃ as the REO with a content of 4?wt.%.     

Thermophysical properties of substantially undercooled liquid Ti-Al-Nb ternary alloy measured by electromagnetic levitation

The thermophysical properties of undercooled liquid alloys at high temperature are usually difficult to measure by experiment. Here, we report the specific heat of liquid Ti₄₅Al₄₅Nb₁₀ ternary alloy in the undercooled state. By using electromagnetic levitation technique, a maximum undercooling of 287?K (0.15 T _L) is achieved for this alloy. Its specific heat is determined to be 32.72?±?2.51?J?mol^?1 K^?1 over a broad temperature range of 1578–2010?K.     

Specific heat,enthalpy, and density of undercooled liquid Fe–Si–Sn alloy

The specific heat of liquid Fe₉₀Si_6.95Sn_3.05 alloy has been investigated by electromagnetic levitation drop calorimetery in the temperature range of 1390–2140 K. The enthalpy of this liquid alloy increases linearly with the rise of temperature. Furthermore, the enthalpy obtained from molecular dynamics calculation shows a similar trend to the experimental results in a broader temperature range of 1000–2200 K. The calculated specific heat is 39.7 J mol^?1 K^?1, which agrees well with the experimental result of 39.9 J mol^?1 K^?1. The density of this liquid alloy decreases as a quadratic function of temperature.     

Reversible phase transition in vanadium oxide films sputtered on metal substrates

Vanadium oxide films, deposited on aluminium (Al), titanium (Ti) and tantalum (Ta) metal substrates by pulsed RF magnetron sputtering at a working pressure of 1.5 x10^?2 mbar at room temperature are found to display mixed crystalline vanadium oxide phases viz., VO₂, V₂O₃, V₂O₅. The films have been characterized by field-emission scanning electron microscopy, X-ray diffraction, differential scanning calorimetry (DSC) and X-ray photoelectron spectroscopy, and their thermo-optical and electrical properties have been investigated. Studies of the deposited films by DSC have revealed a reversible-phase transition found in the temperature range of 45–49 °C.     

Nd2O3 : Eu3+ nanocrystalline phosphor–a new potential thermoluminescing material for dosimetry

Nanoparticles of trivalent Eu³⁺-doped Nd₂O₃ phosphors have been prepared using a low-temperature solution combustion method with metal nitrate as precursor and oxalyldihydrazide as a fuel at a fairly low temperature (<500°C) and in a very short time (<5 min). A powder X-ray diffraction pattern reveals that cubic Nd₂O₃ : Eu³⁺ crystallites are directly obtained without the requirement of further calcinations. The crystallite size, evaluated from Scherer's formula, was found to be in the range of 20–30 nm. The microstructure and morphology were studied by scanning electron microscopy, which showed the phosphor to be foamy and fluffy in nature. Thermoluminescence characteristics of the Nd₂O₃ : Eu³⁺ have been studied using gamma irradiation. These demonstrate that the phosphor is suitable for use as a dosimeter.     

Microstructure and superconductivity of bulk BPSCCO/LPMO composite

Bulk superconducting (SC) ceramics containing BPSCCO and LPMO (Lanthanum/Lead – manganite phase) have been produced. The initial components were prepared by a low-temperature Pechini method. The submicron powders in weight ratio 90/10 Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O_z/La_0.6Pb_0.4MnO₃ (nominal composition) were heat treated at 840°C. The duration of the heat treatment effect (60 and 100?h) of the composite on the transformation of the microstructure was studied. The obtained composites were analyzed by scanning electron microscopy (SEM) and by the method of energy-dispersive X-ray spectroscopy (EDX). They contain several phases. It was established that the SC 2212 phase is predominate in the composite. The phase La_0.6Pb_0.4MnO₃ transforms in solid solution with preliminary composition La_0.5(Sr?+?Ca)_0.5Mn_{1? z} Cu _z O₃, which after full replacement of the La and Mn ions leads to the appearance of phases with nominal composition Sr_{1? x} Ca _x CuO _y . AC and DC magnetization measurements were used to study the SC and magnetic properties of the samples. Both samples are SC with critical temperatures 75 and 77?K, respectively. It was concluded that the SC and magnetic phases stably coexist in the composite sintered at 60?h heat treatment at 840°C.     

Internet-Delivered Acceptance and Values-Based Exposure Treatment for Fibromyalgia: A Pilot Study

Background: Acceptance and commitment therapy (ACT) is a promising treatment option for fibromyalgia (FM). Studies have shown that many cognitive behavioral protocols can be transferred to the Internet with sustained efficacy. However, no study has investigated the effect on an Internet-delivered ACT-based protocol for FM. This study evaluated the efficacy, acceptability, and the health economic effects of an Internet-delivered acceptance and values-based exposure treatment for FM. Methods: This open pilot trial included 41 self-referred women with a FM diagnosis. The 10-week Internet-delivered treatment included acceptance, mindfulness, work with life-values, and systematic exposure to FM symptoms and FM-related situations. Participants also had regular contact with an assigned online therapist. Assessments were made at pretreatment, post-treatment, and 6-month follow-up. Results: The treatment was completed by 70% of the participants. Attrition rates were low, with 98% completing the post-treatment assessment and 90% completing the 6-month follow-up assessment. Multiple imputations were used to replace missing values. Pre- to post-treatment within-group effect sizes were in the moderate to large range (Cohen's d = 0.62–1.56) on measures of FM symptoms and impact, disability, quality of life, depression, anxiety, fatigue, and psychological flexibility. All improvements were maintained at follow-up. Economical analyses revealed significant societal cost reductions that offset the treatment costs within 2 months of treatment completion. Conclusions: An Internet-delivered psychological treatment based on acceptance and exposure principles seems to be an efficacious, acceptable, and cost-effective treatment for FM. Randomized controlled trials are needed to confirm these results.     

Dislocation configurations in twin-free melt-textured YBa2Cu3O7 processed at high pressure and temperature

Melt-textured YBa₂Cu₃O₇ samples were deformed under high pressure–high temperature (HP–HT) conditions (P?=?2?GPa, T?=?900 and 800°C, t?=?15?min). During the HP–HT treatment, samples were in contact with annealed zirconia, preventing oxygen loss, in such a way that the orthorhombic superconductor phase was preserved at the end of the experiment. The deformed microstructure, investigated by transmitted electron microscopy, exhibited original features characterized by a very low twin density, a memory effect on the dislocations shape and a non-classical faulted loops distribution in the matrix. A mechanism, based on the interaction between twin and gliding dislocations, is proposed to explain the nucleation of these aligned loops.     

Effects of temperature and electron energy on the electron-irradiation-induced decomposition of sapphire

The effects of temperature and electron energy on the electron-irradiation-induced decomposition of sapphire have been investigated by in situ transmission electron microscopy (TEM). It was found that the decomposition rate of α-Al₂O₃ increased with increasing irradiation temperature and decreased with increasing acceleration voltage. The core-hole Auger decay process (K–F model), rather than knock-on displacement, is responsible for the decomposition of α-Al₂O₃ under electron irradiation.     

Analysis of the relation between structure and the superconducting transition in YBa2 Cu3 O∼7

Abstract

It is shown that in the high-T_c YBa₂ Cu₃ O_～7 superconductor the critical temperature is a function of the orthorhombic distortion (b–a)/a of the unit cell. From the extrapolation of the (b - a)/a ratio against T_c , a maximum critical temperature of 66 K for the tetragonal phase of YBa₂ Cu₃ O_～7 was predicted. From the correlation between the transition width δT_c and the orthorhombic distortion there was deduced an upper limit for T_c , in the orthorhombic phase of YBa₂ Cu₃ O_～7 of 94·5K.     

Thermodynamic fluctuations in magnetic states: Fe3Pt as a prototype

To understand the Invar anomalies, such as negative thermal expansion and spontaneous magnetization, we have applied our recently developed thermodynamic framework for a system with itinerant-electron magnetism to the ordered Fe₃Pt. The framework has coherently predicted the finite temperature intermixing between the fully ferromagnetic (FM) configuration and the spin-flipping configurations (SFCs). We have also discovered a tri-critical point at which a high-temperature second-order phase transition, between the fully ordered FM phase and the paramagnetic phase which is disordered due to SFCs, becomes first order at low temperatures.     

Formation and structure of misfit dislocations at the La2Zr2O7-Y2O3 -stabilized ZrO2 (001) reaction front during vapour-solid reaction

La₂ Zr₂ O₇ (LZO)-based pyrochlore islands were grown on a Y₂ O₃ -stabilized ZrO₂ (YSZ) (001) single-crystal surface by the reaction between La₂O₃ vapour and the crystal. A network of interfacial edge dislocations with line directions [100] and [010] and Burgers vectors ( a_s/2)[101] and ( a_s/2)[011] respectively ( a_s being the lattice parameter of the YSZ) was observed at the moving LZO-YSZ reaction front. The interface-parallel component of the Burgers vectors accommodates the LZO-YSZ lattice mismatch of +5.0%, while the perpendicular component causes a slight tilt of the LZO lattice with respect to the YSZ lattice. The dislocation half-loops nucleate and glide on inclined {101} planes at the edges of the four corners of the growing islands.     

Frequency-temperature dependent dynamics of dielectrics in ferric oxoborate Fe3BO6 of cocktail structure of nanorods

Oxoborates have both dielectric and magnetic properties useful for magnetodielectric devices, sensors, or biological tools. Such compounds Fe₂BO₄, Fe₃BO₅, or Fe₃BO₆ are known to grow easily as single crystals in a liquid flux. A polycrystalline phase forms only on controlled conditions of a solid state reaction of the basic oxides. In this study, we report highly dielectric Fe₃BO₆ when grown in a specific shape of nanorods (～200?nm diameter and 50–100?µm length) from an iron borate glass, which offers devisable shapes of sheets, discs, and fibers. Frequency (f)-temperature reliant dynamics of dielectric constant ε_r is studied over 25–300°C at 0.1–10^3?kHz frequencies. At low frequency such as 100?Hz, a large ε_r -value 40,000, better than most of high ε_r -value ferroelectrics, incurs at room temperature. At f?≥?50?kHz, although only an order of diminished ε_r -value lasts, it increases steadily with temperature, possibly due to increasing electrical conductivity in a specific resistor–capacitor network. Suppressed dielectric relaxation and spin-flops share a merely weak spin-reorientation transition near 160°C. A stable power loss ≤0.5 lasts at f?>?10?kHz useful for possible applications of magnetodielectric materials.