Shift Of Tc Research Articles

Specific Heat and Magnetocaloric Effect of LaFe11.2 – xMnxCo0.7Si1.1 (x = 0, 0.1, 0.2, 0.3)

The specific heat and the magnetocaloric effect of intermetallic LaFe11.2 – xMnxCo0.7Si1.1 (x = 0, 0.1, 0.2, 0.3) compounds have been measured in the temperature range 80–300 K in magnetic fields up to 8 T. The magnetocaloric effect (MCE) is estimated by two methods: the direct method in cyclic magnetic fields and also indirect method from the data on the specific heat. An increase in the Mn atom concentration leads to a shift of the Curie temperature TC toward lower temperatures; in this case, the MCE magnitude is changed insignificantly.

Synergistic effect of thermoplastic phenolic resin and multiwalled carbon nanotubes on the crystallization of polyoxymethylene

AbstractTo reduce the crystallization rate of polyoxymethylene (POM) to meet the requirement of thick‐walled and large‐sized articles production, and maintain high crystallinity as well as obtain refined crystalline grains to ensure the strength and stiffness simultaneously, thermoplastic phenolic resin (PF) and multiwalled carbon nanotubes (MWCNTs) were used as crystal growth inhibitor and nucleating agent, respectively, and their effects on the crystallization of POM were studied in details. The results showed that PF is an effective inhibitor and MWCNTs exhibits excellent nucleation effect on POM. Based on the obtained results, their synergistic influences on the crystallization process of POM were investigated. It is found that the objective of decreasing the crystallization rate while maintaining high crystallinity and forming fine crystalline grains can be realized. The 97/3/1 wt% POM/PF/MWCNTs, compared with those of neat POM, The T c shifts by 3.3°C to a lower temperature, the crystallization enthalpy increases by 16.1 J/g and the full width at half‐maximum widens by 48.5%. The modulation effect of PF and MWCNTs on the crystallization is closely related to the PF content and dispersion, the distribution and dispersion of MWCNTs in the PF and POM phases.

Tunable M-type nano barium hexaferrite material by Zn2+/Zr4+ co-doping

The effect of Zn2+/Zr4+ dopant ions on the physical and magnetic properties of BaZnxZrxFe12−2xO19 (x = 0.00 to 0.80) synthesized using sol-gel auto combustion method has been investigated. FE-SEM micrographs reveal a decrease in average particle size from ≈350 nm to ≈115 nm with increase in Zn2+/Zr4+ dopant concentration. XRD reveals the formation of M-type hexaferrite (NBaM) pure phase and the absence of secondary impurities such as α/γ –Fe2O3 and BaCO3. The lattice parameters increase and the crystallite size has been found to decrease with increasing Zn2+/Zr4+ dopant concentration. The widening of Raman bands reveals local distortion of the crystal structure with increasing dopant ion concentration. Magnetic measurements reveal that the sample exhibit magnetic ordering and a gradual change in magnetic nature from hard to soft ferrite with increasing substitution of Zn2+/Zr4+ ions. The coercivity value decreases from 4686 Oe to 776 Oe with increasing Zn2+/Zr4+ dopant concentration. The saturation magnetization (Ms) value has been found to be maximum (61.53 emu g−1) in x = 0.40 sample. The shift in Tc from 711.9 K to 522.8 K with composition confirmed the weakening of super exchange interaction in these samples. These materials could be useful for magnetic recording applications.

Phonon-limited mobility of Dirac fermions in three-dimensional Dirac semimetal Cd3As2

A theoretical model is presented for the phonon-limited mobility of the Dirac fermion gas in three-dimensional (3D) Dirac semimetal Cd3As2, with consideration of the scattering from both the acoustic and optical phonons. Screening effects are taken into account and it is found that they lead to significant enhancement of the mobility. Simple analytical equations and power laws are obtained for both the Bloch–Grüneisen and equipartition regimes. The dependence of the mobility on the temperature T and electron density ne is investigated. The optical-phonon-limited mobility μop is found to dominate over the acoustic-phonon-limited mobility μap in the higher-temperature region. There is a crossover of μap and μop, and the crossover temperature Tc shifts to a higher value with increasing ne. Numerical calculations of the mobility are conducted for recently reported experimental samples and good agreement is obtained with the experimental results. A comparison is also made with the results for a conventional 3D electron gas in a degenerate semiconductor.

Tuning the superconducting transition of SrTiO3-based 2DEGs with light

The resistivity of the two dimensional electron gas that forms at the interface of strontium titanate with various oxides is sensitive to irradiation from visible light. In this letter, we present data on the interface between the bandgap insulators LaAlO3 (LAO) and SrTiO3 (STO). We operate a light emitting diode at temperatures below 1 K and utilize it to irradiate the LAO/STO interface at ultralow temperatures. On irradiation, the resistance of this system is lowered continuously by a factor of five and the resistance change is persistent at low temperatures as long as the sample is kept in the dark. This makes the characterization of transport properties in different resistive states over extended time periods possible. Our pristine sample gets superconductive below 265 mK. The transition temperature Tc shifts downward on the persistent photo-induced lowering of the resistance. The persistent photoconductance can be completely reverted by heating the structure above 10 K, in which case, Tc takes on its original value. Thus, very similar to field-effect induced changes of the electron density, irradiation at low temperatures offers a versatile tuning knob for the superconducting state of STO-based interfaces which in addition has the advantage to be nonvolatile.

Structure, morphology, infra-red and dielectric investigations in Er+3 doped nano phased lithium-barium-strontium-sodium niobates

Synthesis of nano phased Er+3 doped (x) Li optimized Barium Strontium Sodium Niobates (BSNN), viz.,Ba1.6-(3x/2)Sr2.4Na1.4Li0.6ErxNb10O30 by co-precipitation method reported. XRD confirmed orthorhombic structure. Lattice parameter ‘b’ exhibited anomaly for x ≥ 0.8. Orthorhombic Cmm2 and Ccm21 symmetries predict Ferro or Piezo and Pyro response. (401) for x = 0.9; and (401) and (801) for x = 1 reflection and split infer Ccm21 symmetry and lower ordered Cmm2 structure along diad axis in [001] direction. Absence of additional (hol) reflections for h-even and l-odd are addressed. Crystallite size D(x) exhibited anomaly for x around 0.9. Particle size and Porosity P(x) by FESEM and Density ρ(x) inferred systematic growth of ferroelectric(FE) phase. Anomaly in D is attributed to distinct dopant characteristic nucleation. Cluster population of 10–20 nm size prevail over entire dopant range. Concurrent behavior of D and P with x is observed. Low frequency (100 KHz) Dielectric permittivity inferred shift of Curie temperature Tc for ambiance. Large permittivity (K) is observed for 0 ≤ x ≤ 0.2. Decrease of K with P confirms growth of nano phase from bulk morphology. Decrease in K with P infers morphology influence field response. Trends of K with x infers shift of Tc (x) with an anomaly for x equal to 0.9. influence of D and P analyzed. FE transition progressively gets diffused with increasing x or decrease in D (∼30–20 nm) and P (for 45–25 nm). Critical analysis of nano phased Er doped Li BSNNs reveals growth of localized stress contributing to excess free volume of grain boundaries. Incongruent behavior of Tc discussed. Trends of axial ratio (AR), dielectric permittivity and Tc are analyzed in comparison with the reports in other systems of relevance.

Impact of ocean acidification on thermal tolerance and acid–base regulation of Mytilus edulis from the White Sea

Ocean warming and acidification are two important environmental drivers affecting marine organisms. Organisms living at high latitudes might be especially threatened in near future, as current environmental changes are larger and occur faster. Therefore, we investigated the effect of hypercapnia on thermal tolerance and physiological performance of sub-Arctic Mytilus edulis from the White Sea. Mussels were exposed (2 weeks) to 390 µatm (control) and 1120 µatm CO2 (year 2100) before respiration rate (MO2), anaerobic metabolite (succinate) level, haemolymph acid–base status and intracellular pH (pHi) were determined during acute warming (10–28 °C, 3 °C over night). In normocapnic mussels, warming induced MO2 to rise exponentially until it levelled off beyond a breakpoint temperature of 20.5 °C. Concurrently, haemolymph PCO2 rose significantly > 19 °C followed by a decrease in PO2 indicating the pejus temperature (TP, onset of thermal limitation). Succinate started to accumulate at 28 °C under normocapnia defining the critical temperature (TC). pHi was maintained during warming until it dropped at 28 °C, in line with the concomitant transition to anaerobiosis. At acclimation temperature, CO2 had only a minor impact. During warming, MO2 was stimulated by CO2 resulting in an elevated breakpoint of 25.8 °C. Nevertheless, alterations in haemolymph gases (> 16 °C) and the concomitant changes of pHi and succinate level (25 °C) occurred at lower temperature under hypercapnia versus normocapnia indicating a downward shift of both thermal limits TP and TC by CO2. Compared to temperate conspecifics, sub-Arctic mussels showed an enhanced thermal sensitivity, exacerbated further by hypercapnia, indicating their potential vulnerability to environmental changes projected for 2100.

Ethylene-co-Vinyl Acetate/MWCNTs/Hectorite Elastomeric Nanocomposites: Characterization and Electrical Properties.

MWCNTs/hectorite 3D hybrid filler was prepared from the 1:1 combination by weight of constituent MWCNTs (1D) and hectorite (2D) through simple dry grinding method. The resulting hybrid filler was subsequently utilized for the preparation of Ethylene-co-vinyl acetate (EVA)/MWCNTs/hectorite nanocomposites by solution intercalation method. Transmission electron microscopy and X-ray diffraction studies of the nanocomposites confirm homogeneous dispersion of the fillers in the polymer matrix. Mechanical, thermal and dielectric properties of neat EVA are significantly enhanced with increase in filler content. The improvement in tensile properties observed at 4 wt.% filler content are: 243% in tensile strength (1.78 MPa to 6.11 MPa), 105% in elongation at break (366% to 750%) and 426% in toughness (3.36 MPa to 17.67 MPa) without significant change (1.12 MPa to 1.31 MPa) in Young's modulus. Differential scanning calorimetry confirms significant increase in crystallinity, crystallisation temperature (Tc) and crystallite melting temperature (Tm) of the nanocomposites vis-a-vis neat EVA. Observed shift in Tc by more than 10 °C (~87 to ~98 °C) and Tm by more than 30 °C (~118 to ~151 °C) are really significant confirming the improved thermal stability of the nanocomposites. Maximum improvements in dielectric constant and AC conductivity observed at 5 wt.% hybrid filler content are ~25% and ~50% respectively. Superior mechanical properties of the nanocomposites combined with enhanced thermal stability and dielectric properties make it a prospective material for flexible dielectric applications. Properties attained can be attributed to improved polymer-filler interaction on account of homogeneous dispersion.

Hybrid microwave sintering and shifting of Tc in lead-free ferroelectric composition x(Ba0.7Ca0.3TiO3)-(1-x)(BaZr0.2Ti0.8O3)

Lead free ceramic composition of x(Ba0.7Ca0.3)TiO3-(1-x)Ba(Zr0.2Ti0.8)O3 with variation in x as, 0.15, 0.3, 0.4 and 0.5 was synthesized by hydroxide coprecipitation method. Synthesized compositions are subjected to sintering by microwave method at 1200 °C. After being sintered composite samples are characterized by various techniques for their structural, microstructural analysis, temperature dependent dielectric measurements, ferroelectric property and Raman spectroscopic analysis. From x-ray diffraction pattern the coexistence of tetragonal and rhombohedral crystal structures is observed. Broadening of Raman peaks at room temperature and two dielectric anomaly i.e. at lower temperature (R-T) and at higher temperature (T-C) in the temperature dependent dielectric spectroscopy could be assigned to the mutual existence of rhombohedral and tetragonal phases. Microstructural images confirm the well-separated grains having an irregular shape with a higher density. The transition temperature (Tc) (ferroelectric-paraelectric) shifts from 95 °C to 140 °C as the increase of BCT in the composition. Broad nature in the dielectric relaxation spectroscopic study reveals the existence of diffuse phase transition nature for samples. The saturated ferroelectric loop was observed for MPB composition 0.5BCT-0.5BZT. With the comparable values of Tc, ferroelectric parameters, existence of triple point and diffuse phase transition this type of lead-free materials found a suitable candidate to replace the lead-based materials.

Unusual sulfur isotope effect and extremely high critical temperature in H3S superconductor

Recent experiments have set a new record for the transition temperature at which a material (hydrogen sulfide, H3S) becomes superconducting. Moreover, a pronounced isotope shift of TC in D3S is evidence of an existence of phonon-mediated pairing mechanism of superconductivity that is consistent with the well established Bardeen-Cooper-Schrieffer scenario. Herein, we reported a theoretical studies of the influence of the substitution of 32S atoms by the heavier isotopes 33S, 34S and 36S on the electronic properties, lattice dynamics and superconducting critical temperature of H3S. There are two equally fundamental results presented in this paper. The first one is an anomalous sulfur-derived superconducting isotope effect, which, if observed experimentally, will be subsequent argument that proves to the classical electron-phonon interaction. The second one is fact that critical temperature rise to extremely high value of 242 K for H336S at 155 GPa. This result brings us closer to the room temperature superconductivity.

Effect of Bi substitution on the magnetic and magnetocaloric properties of Ni50Mn35In15-xBix Heusler alloys

The structural, magnetic, magnetocaloric, and transport properties of Ni50Mn35In15-xBix (x = 0, 0.25, 0.5, 1, 1.5) compounds has been studied through X-ray diffraction (XRD), differential scanning calorimetry, and magnetization measurements. A mixture of high temperature austenite phase (AP) and low temperature martensitic phase (MP) was observed from the XRD at room temperature. The saturation magnetization MS at 10 K was found to decrease with increasing Bi content. A shift in the martensitic transition temperature (TM) relative to the parent compound was observed with a maximum shift of ∼ 36 K for x = 1.5. Abnormal shifts in TC and TM to higher temperatures were observed at high field for x ≥ 0.5. Large magnetic entropy changes (ΔSM) of about 40 J/kg K (x = 0) and 34 J/kg K (x = 0.25) were observed at TM with H = 5 T, which reduced significantly for higher Bi concentrations. The doping of small amounts of Bi in the In sites increased the peak width of the ΔSM curves at the second order transition, leading to larger values of relative cooling power. A significant magnetoresistance (-30%) was observed near TM with ΔH = 5T for x = 0.5.

Bose–Einstein condensation of triplons with a weakly broken U(1) symmetry

The low-temperature properties of certain quantum magnets can be described in terms of a Bose–Einstein condensation (BEC) of magnetic quasiparticles (triplons). Some mean-field approaches (MFA) to describe these systems, based on the standard grand canonical ensemble, do not take the anomalous density into account and leads to an internal inconsistency, as it has been shown by Hohenberg and Martin, and may therefore produce unphysical results. Moreover, an explicit breaking of the U(1) symmetry as observed, for example, in TlCuCl3 makes the application of MFA more complicated. In the present work, we develop a self-consistent MFA approach, similar to the Hartree–Fock–Bogolyubov approximation in the notion of representative statistical ensembles, including the effect of a weakly broken U(1) symmetry. We apply our results on experimental data of the quantum magnet TlCuCl3 and show that magnetization curves and the energy dispersion can be well described within this approximation assuming that the BEC scenario is still valid. We predict that the shift of the critical temperature Tc due to a finite exchange anisotropy is rather substantial even when the anisotropy parameter γ is small, e.g., of Tc in H = 6 T and for .

Specific heat, thermal diffusion, thermal conductivity and magnetocaloric effect in Pr0.6Sr0.4Mn1−xFexO3 manganites

The results of investigation of specific heat, thermal diffusion, thermal conductivity, and magnetocaloric effect in manganites Pr0.6Sr0.4Mn1−xFexO3 (x=0 and 0.1) in the temperature range 80–350K and in magnetic fields of 0 and 18kOe are presented. When the atoms of Mn are replaced by Fe (x=0.1) atoms, TC shifts toward low temperatures at about 167K, while the MCE varies insignificantly with ΔSm=2.05J/kg K and 2.31J/kg K for x=0 and 0.10, respectively, which implies the possibility of regulating the area with a maximum MCE. The phonon mechanism of heat transfer is dominant in the samples studied.

Effect of magnetic ion (Mn) doping on structural, ferroelectric and magnetic properties of Ba0.90Mn0.10TiO3

Synthesis, formation, structural phase transition, dielectric and magnetic properties of polycrystalline Ba0.90Mn0.10TiO3 (BMTO) ceramic have been investigated to study the effect of magnetic ion (Mn) doping at Ba-site in the well-known ferroelectric (FE) material BaTiO3 (BTO). The polycrystalline ceramic BMTO has been prepared via solid state reaction route. The Wyckoff-site occupancy of the substituted Mn has been determined through x-ray diffraction (XRD) and soft X-ray absorption measurement via confirmation of its oxidation state to be 2+ and that of Ti to be 4+. The temperature dependent XRD, dielectric and pyroelectric studies show all the known transitions similar to the pristine BTO. However, the transition temperatures (Tc) were found shifted towards lower temperatures as compare to the pristine BTO. The shifts in Tc corresponding to cubic-tetragonal and tetragonal-orthorhombic transitions are significant as compared to that of orthorhombic-rhombohedral transition. Frequency dependent dielectric measurements show the space-charge related dielectric relaxation. The dc-magnetization measurements show antiferromagnetic interactions in BMTO. Our results also address the role of Ba site on the ferroelectric properties of BTO.

Effect of Sm substitution on magnetic and magnetocaloric properties of La0.7-xSmxBa0.3MnO3 (0 ≤ x ≤ 0.2) compounds

In the present paper we have studied the effect of Sm doping on the magnetic and magnetocaloric properties of La0.7-xSmxBa0.3MnO3 (0≤x≤0.2) compounds. These sample have been synthesized by conventional solid state reaction method. The analysis of synthesized samples by X-ray diffraction showed that the formation of single phase compositions and doping of La3+ by Sm3+ ion converted the chemical structure form rhombohedral (R-3C) to orthorhombic (Imma). The magnetic study measurement specified that the ferromagnetic double exchange interaction is weakened with increasing Sm content as a consequence of the curie temperature (Tc) shift in lower temperature from 340K for x=0, 290K for x=0.1 and 225K for x=0.2 compounds. Using the Banerjee’s criterion plots, it is found that the phase transition for all samples in the second-order. All reported compounds exhibit a maximum and large magneto-caloric effect near the Curie temperature (Tc). The magnitude of the maximum magnetic entropy change is found to be decrease with increasing of Sm doping content i.e. 4.39J/kgK for x=0, 4.22J/kgK for x=0.1 and 2.48J/kgK for x=0.2 in applied field change of 5T. The trend of large entropy change and the convenient adjustment of the Curie temperature make these oxides useful for magnetic refrigeration in an extended high and low temperature even at near room temperature.

Controlled Modification of Superconductivity in Epitaxial Atomic Layer-Organic Molecule Heterostructures.

Self-assembled organic molecules can potentially be an excellent source of charge and spin for two-dimensional (2D) atomic-layer superconductors. Here we investigate 2D heterostructures based on In atomic layers epitaxially grown on Si and highly ordered metal-phthalocyanine (MPc, M = Mn, Cu) through a variety of techniques: scanning tunneling microscopy, electron transport measurements, angle-resolved photoemission spectroscopy, X-ray magnetic circular dichroism, and ab initio calculations. We demonstrate that the superconducting transition temperature (Tc) of the heterostructures can be modified in a controllable manner. Particularly, the substitution of the coordinated metal atoms from Mn to Cu is found to reverse the Tc shift from negative to positive directions. This distinctive behavior is attributed to a competition of charge and spin effects, the latter of which is governed by the directionality of the relevant d-orbitals. The present study shows the effectiveness of molecule-induced surface doping and the significance of microscopic understanding of the molecular states in these 2D heterostructures.

The effect of post-annealing on the structure and magnetotransport properties of Pr0.5Sr0.5MnO3 thin film

Pr0.5Sr0.5MnO3 (PSMO) thin film is epitaxially grown on (001)-oriented LaAlO3 single-crystal substrate using pulsed laser deposition (PLD). It is found that the as-grown PSMO film shows compressive strain in plane and tensile strain out of the plane. Upon annealing at 900°C in the air, the strain is significantly relaxed. The paramagnetic to ferromagnetic phase transition temperature TC shifts from 200 K to 220 K, and the antiferromagnetic insulating phase is suppressed in the phase separated state at low temperature. In addition, the magnetoresistance (MR) is found to increase around the ferromagnetic transition temperature, whereas it decreases from 99% to 60% at low temperature of 20 K.

Enhanced superconductivity in TiO epitaxial thin films

Titanium oxides have many fascinating optical and electrical properties, such as the superconductivity at 2 K in cubic titanium monoxide (TiO) polycrystalline bulk. However, the lack of TiO single crystals or epitaxial films has prevented systematic investigations on its superconductivity. Here, we report the basic superconductivity characterizations of cubic TiO films epitaxially grown on (0001)-oriented α-Al2O3 substrates. The magnetic and electronic transport measurements confirmed that TiO is a type-II superconductor and the recorded high Tc is about 7.4 K. The lower critical field (Hc1) at 1.9 K, the extrapolated upper critical field Hc2(0), and coherence length are about 18 Oe, 13.7 T, and 4.9 nm, respectively. With increasing pressure, the value of Tc shifts to lower temperature while the normal state resistivity increases. Our results on the superconducting TiO films confirm the strategy to achieve higher Tc in the epitaxial films, which may be helpful for finding more superconducting materials in various related systems.

Isotope shift of the ferromagnetic transition temperature in itinerant ferromagnets

We present a theory of the isotope effect of the Curie temperature Tc in itinerant ferromagnets. The isotope effect in ferromagnets occurs via the electron–phonon vertex correction and the effective attractive interaction mediated by the electron–phonon interaction. The decrease of the Debye frequency increases the relative strength of the Coulomb interaction, which results in a positive isotope shift of Tc when the mass M of an atom increases. Following this picture, we evaluate the isotope effect of Tc by using the Stoner theory and a spin-fluctuation theory. When Tc is large enough as large as or more than 100 K, the isotope effect on Tc can be measurable. Recently, precise measurements on the oxygen isotope effect on Tc have been performed for itinerant ferromagnet SrRuO3 with Tc∼160 K. A clear isotope effect has been observed with the positive shift of Tc∼1 K by isotope substitution (O16→O18). This experimental result is consistent with our theory.

Microstructure and Dielectric Behavior of Ytterbium Doped BaZr0.1Ti0.9O3 Ceramics

BaZr0.1Ti0.9O3 ceramics of perovskite structure are prepared by solid state reaction method with addition of x Yb2O3 (x= 0.0, 0.1, 0.3, 0.5 and 0.7 mol%), respectively, and their dielectric properties are investigated. Room temperature X-ray diffraction study suggested that the compositions have a single phase with a perovskite structure. A small decrease in the grain size is observed with an increase in the Yb content. The bulk density increased with increasing the Yb content for the samples sintered at 1250 0C, and the bulk densities show the maximal value for the samples sintered at 1280 0C, 1300 0C and 1330 0C. The dielectric constant can reach to 4500 or more for the sample sintered at 1300 0C, and the variation of dielectric loss with temperature is more stable for Yb2O3-doped BZT10 ceramics, which proves that Yb2O3 doping has a great influence on the dielectric properties in BZT10 ceramics. With the increase of Yb2O3 content, Tc rises at first, and when x exceeds 0.1 mol%, Tc shifts towards to lower temperature with the Yb2O3 concentration increasing.