Control of Both Superconducting Critical Temperature and Critical Current by Means of Electric-Field-Induced Reconfigurable Strain

The controlled modification of superconductivity by any means is a long-standing issue in low-temperature physics. In this work, we present data on the control of the superconducting properties of conventional low critical-temperature (T_C) Nb thin films with thickness (d_Nb) =?15 and 20 nm under application of reconfigurable strain, S induced by an external electric field, and E_ex to a piezoelectric (PE) single crystal, namely (1 ? x)Pb(Mg_1/3Nb_2/3)O₃ ? x PbTiO₃ (PMN-PT) with x =?0.30–0.31. The experimental results (reduction of T_C and critical current (J_C) on the order of 6% and 90–100%, respectively) are nicely reproduced with a phenomenological model that incorporates the constitutive relation S(E_ex) that describes the electro-mechanical response of the PE crystal to well-established formulas that describe T_C and J_C of the SC thin films.     

Photoluminescence and electrical properties of Eu3+-doped Na0.5Bi4.5Ti4O15-based ferroelectrics under blue light excitation

Na_0.5Bi_4.5?x Eu _x Ti₄O₁₅ (NBT- _x Eu³⁺) ceramics with x = 0, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30 and 0.40 were prepared by conventional ceramics processing. NBT-0.25Eu³⁺ ceramics show the strongest red and orange emissions corresponding to the ⁵D₀ → ⁷F₂ (617 nm) and ⁵D₀ → ⁷F₁ (596 nm) transitions, respectively. The strongest excitation band around 465 nm matches well with the emission wavelength of commercial InGaN-based blue LED chip, indicating that Eu³⁺-doped NBT ceramics may be used as potential environmental friendly red-orange phosphor for W-LEDs application. As an inherent ferroelectric and piezoelectric material, the electrical properties of this potentially multifunctional electro-optical material have been also studied. The introduction of Eu³⁺ distinctly increased the Curie temperature (T _C ) of NBT- _x Eu³⁺ ceramics from 640°C to 711°C as x ranges from 0 to 0.40. For higher temperature applications, the electrical conductivity was also investigated. The conduction of charge carriers in high-temperature range originates from the conducting electrons from the ionization of oxygen vacancies. High T _C and low tanδ makes Eu³⁺-doped NBTceramic also suitable for high temperature piezoelectric sensor applications and electro-optical integration.     

Structure of CN<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> films (0 ≤ <Emphasis Type="Italic">x</Emphasis> ≤ 0.5) deposited by arc sputtering of graphite

The structure of a-C and CN _x coatings 1–3 μ thick deposited on metal substrates by arc-pulse sputtering of graphite target in nitrogen background at P = (10^?2 ?5) Pa was studied by transmission electron microscopy and scanning electron microscopy. Concentration and depth distribution of nitrogen in CN _x depend on progressive saturation of graphite target. Nitrogen-free coatings (x = 0) consist of amorphous matrix with multiwalled nanotubes. Structure of carbon nitride coatings depends on nitrogen concentration. Over the range of C_N/C_C < 0.15 and C_N/C_C > 0.4 CN _x coatings are amorphous. The structure of CN_0.15<x<0.4 is a mixture of two types of amorphous domains, one of which is decorated by microcrystalline inclusions. The same structured inclusions were found on graphite target modified by electric arc. The obtained results allow understanding the dependence of the CN_0≤x≤0.5 coatings properties on nitrogen concentration.     

Vortex Structure and Anisotropic Superconducting Gaps in Ba[Fe(Ni)]<Subscript>2</Subscript>As<Subscript>2</Subscript>

We studied nearly optimally Ni-substituted BaFe_2?x Ni _x As₂ (BFNA) single crystals with T _C ≈ 18.5 K. In irreversible magnetization measurements, we determined the field dependence of the critical current density and discuss the nature of observed strong bulk pinning. Using intrinsic multiple Andreev reflections effect (IMARE) spectroscopy, we directly determine two distinct superconducting gaps and resolve their moderate anisotropy in the momentum space. The BCS-ratio for the large gap 2Δ _L /k _B T _C > 4.1 evidences for a strong coupling in the Δ _L -bands.     

Magnetic and Mössbauer Studies of Ba(Fe<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ni<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)<Subscript>2</Subscript>As<Subscript>2</Subscript> Single Crystals

Magnetization and ⁵⁷Fe Mössbauer effect spectroscopy (MS) studies of Ba(Fe_1?x Ni _x )₂As₂ single crystals (x=0 to 0.054) at temperatures (5 K to 300 K) have been performed. Magnetic measurements show that for BaFe₂As₂ the magnetic moment decreases below T _N=136 K. T _N is suppressed monotonically by Ni doping. On the other hand, for higher x values the magnetic moment increases below T _N. Unexpectedly for x=0.024 (T _N=67 K), the virgin zero-field-cooled (ZFC) curve is higher than that of field-cooled (FC) one below 48 K. The magnetic MS spectra of this sample are composed of a superposition of two subspectra, corresponding to commensurate and incommensurate field distributions. The average magnetic hyperfine field H _eff decreases with T and becomes zero at 80 K. For higher x values, the samples become superconducting at T _C=15.5 and 19 K for x=0.046 and 0.054, respectively. For both samples below T _C, the FC curves are positive (the paramagnetic Meissner effect) up to applied field of H～15 Oe and the susceptibility is inversely proportional to H. The MS spectra below and above T _C are almost identical, indicating that the MS parameters are not sensitive enough to detect the superconducting state. The peculiar phenomena observed are attributed to disorder induced by the presence of Ni atoms in the Fe sublattice.     

Correlation between the electrical and luminescent properties of high-purity gallium arsenide

Epitaxial n-GaAs layers with a background impurity concentration of N_D-N_A<10¹⁵ cm^?3, grown by chloride vapor phase epitaxy in an open system, exhibit correlation between the electrical properties and the long persistence of the edge photoluminescence lines D⁰x and D⁰h related to the hole trapping centers. An increase in the concentration of such trapping centers in n-GaAs leads to a decrease in the mobility of free charge carriers.     

Effect of annealing on exchange bias in Mg<Subscript>0.2</Subscript>Fe<Subscript>x</Subscript>Cr<Subscript>1.8−x</Subscript>O<Subscript>3</Subscript> nano oxides

Magnesium-iron chromium oxides (Mg_0.2Cr_1.8?x Fe _x O₃ with x varying from 0.3 to 0.9) produced by hydrothermal process in a stirred pressure reactor from pure metal chlorides have been annealed at 700 °C. Single phase corundum structure and nanophase structure of the as-synthesized samples were confirmed by X-ray diffraction (XRD). Instead of the correlation between H _EB and D _XRD observed at T _A = 600 °C, we find significant changes. The H _EB increases with decreasing particle size reaches a maximum at ~43 nm (x = 0.5) then decreases.     

Insight into Phase Transition,Electronic, Magnetic,Mechanical, and Thermodynamic Properties of TbTe: a DFT Investigation

Theoretical investigation on TbTe for its structural, electronic, magnetic, and thermodynamic stuffs has been carried within density functional theory (DFT) as implemented in WIEN2K code. TbTe was found stable in ferromagnetic phase. The calculated ground-state parameters were found in a good agreement with the experimental data. The compound was found to have a structural stability in cubic B1 (NaCl-type structure) phase, but under the application of high pressure (at 27 GPa), it undergone to B2 (CsCl-type structure) phase of pressure. The second-order elastic constants and mechanical properties like Young’s modulus, Shear modulus, Poisson ratio, Cauchy pressure (C₁₂–C₄₄), and Pugh’s ratio (B/G) were calculated. The present calculations confirmed the ductile nature of TbTe. Further, the thermodynamic investigations have been carried using quasi-harmonic Debye approximation. We have calculated the pressure and temperature dependence of Debye temperature (??_D), bulk modulus (B), thermal expansion (α), heat capacities (C_V), and entropy (S) in the temperature range of 0 to 1000 K and pressure range of 0 to 25 GPa.     

Magnetic Behaviors of Ferromagnetic Semiconductor Zn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Cr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te Grown by MBE

The magnetic and structural properties of MBE-grown films of Zn_1?xCr _x Te were investigated. The magnetization versus magnetic field (M–H) measurement of Zn_1?xCr _x Te (x = 0.01–0.17) showed clear hysteresis loop at low temperatures. The ferromagnetic transition temperature (T_C) estimated from the Arrott-plot analysis increased almost linearly with the Cr composition (x) up to 275 K at x = 0.17. However, in the magnetization versus temperature (M–T) measurement, the irreversibility between the zero-field-cooled (ZFC) and field-cooled (FC) processes was observed. This is typically observed in the magnetic random system such as spin-glass or superparamagnetic phase. In the high resolution transmission microscopy (HRTEM) observations, structural defects such as stacking faults and polycrystalline-like structure were observed at high Cr compositions, whereas any apparent precipitates of different phases were not seen in all the range of Cr compositions examined. The correlation of the observed magnetic randomness with the local structural defects was discussed.     

Sintering behavior,phase structure and electric properties of KNNTS-BKNZ ceramics with excessive alkali metals

We investigated the effect of excessive alkali metals on sintering character, phase structure, dielectric, piezoelectric and ferroelectric properties in KNN-based ceramics thoroughly. The 0.96(K_0.48Na_0.52)_1+xNb_0.93Sb_0.02Ta_0.05O₃–0.04Bi_0.5(K_0.12Na_0.88)_0.5ZrO₃ [(KN) _x NTS-BKNZ] lead-free piezoelectric ceramics were prepared by conventional solid-state reaction method. The sintering temperature of (KN) _x NTS-BKNZ ceramics increases gradually while the sintering temperature zone becomes narrow with the increasing x value. For the composition of x?>?0.03, the grains could not grow in all sintering temperature. The coexistence system of rhombohedral and tetragonal phases has been found in (KN) _x NTS-BKNZ ceramics and the T_R?T, T_C, d₃₃, and ε_r show a upward tendency with the increasing x when 0?≤?x?≤?0.03. The maximal piezoelectric coefficient d₃₃ of 415 pC/N was obtained at x?=?0.03. In addition, these ceramics presented good temperature stability. This study exhibits that excessive alkali metals influence sintering character and electrical properties on KNN ceramics deeply. It was believed that this work would be helpful for further understanding the effect of excessive alkali metals on sintering behavior and electric properties in KNN-based ceramics.     

Fullerene-containing C<Subscript>60</Subscript>-CdTe(CdSe) composite nanostructures

We have developed a technology for obtaining homogeneous films based on fullerene-containing C₆₀-CdTe and C₆₀-CdSe composites. The surface morphology of the initial films is characterized by an average lateral roughness size of about 150 nm. Annealing in a vacuum of 10^?5 Torr for 3 h showed that C₆₀-CdSe films are stable at temperatures up to T=180°C, while the same treatment of films of the (C₆₀)_1?x(CdTe)_x system with x<0.5 leads to the appearance of surface clusters of the semiconductor component with an average size of about 500 nm. The cluster density and size increase with the content of CdTe in the initial composite powder. The photoluminescence spectrum of a (C₆₀)_1?x(CdTe)_x film with x=0.5 upon annealing displays a dominating peak at 730 nm, which is indicative of a significant modification of the film structure as a result of this post-growth treatment. It is demonstrated that fullerene-containing composite network nanostructures with a lateral resolution up to 250 nm can be created by direct electron-beam lithography.     

Effects of Ge<Superscript>4+</Superscript> acceptor dopant on sintering and electrical properties of (K<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>)NbO<Subscript>3</Subscript> lead-free piezoceramics

Lead-free (K_0.5Na_0.5)(Nb_1-xGe _x )O₃ (KNN-xGe, where x = 0-0.01) piezoelectric ceramics were prepared by conventional ceramic processing. The effects of Ge⁴⁺ cation doping on the phase compositions, microstructure and electrical properties of KNN ceramics were studied. SEM images show that Ge⁴⁺ cation doping improved the sintering and promoted the grain growth of the KNN ceramics. Dielectric and ferroelectric measurements proved that Ge⁴⁺ cations substituted Nb⁵⁺ ions as acceptors, and the Curie temperature (T_C) shows an almost linear decrease with increasing the Ge⁴⁺ content. Combining this result with microstructure observations and electrical measurements, it is concluded that the optimal sintering temperature for KNN-xGe ceramics was 1020°C. Ge⁴⁺ doping less than 0.4 mol.%can improve the compositional homogeneity and piezoelectric properties of KNN ceramics. The KNN-xGe ceramics with x = 0.2% exhibited the best piezoelectric properties: piezoelectric constant d₃₃ = 120 pC/N, planar electromechanical coupling coefficient k_p = 34.7%, mechanical quality factor Q_m = 130, and tanδ = 3.6%.     

First-Principle Study of Electronic and Half-Metallic Ferromagnetic Properties of Vanadium (V)-Doped Cubic BP and InP

In this study, we use the first-principle calculations of density functional theory with gradient generalized approximation of Wu–Cohen to investigate the doping effect of vanadium impurity on structural, electronic and magnetic properties of In_1?x V _x P and B_1?x V _x P alloys at various concentrations x = 0.0625, 0.125 and 0.25. Owing to the metallic nature of majority spin and semiconducting minority spin, the In_1?x V _x P compounds exhibit a half-metallic character with total magnetic moments of 2 μ _B, while the B_1?x V _x P has metallic nature for all concentrations. The results of exchange parameters revealed that exchange coupling between vanadium atoms and the conduction band is ferromagnetic, confirming the magnetic feature of In_1?x V _x P and B_1?x V _x P. From our findings, we have predicted that the In_1?x V _x P alloys seem to be potential materials for spintronics.     

Dual relaxation behaviors and large electrostrictive properties of Bi<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>TiO<Subscript>3</Subscript>–Sr<Subscript>0.85</Subscript>Bi<Subscript>0.1</Subscript>TiO<Subscript>3</Subscript> ceramics

Lead-free ceramics (1???x)Bi_0.5Na_0.5TiO₃–xSr_0.85Bi_0.1TiO₃ (BNT–xSBT, x?=?0.4, 0.5, 0.6 and 0.7) were prepared by a solid-state reaction process. Coexistence of ferroelectric relaxation at low temperature and Maxwell–Wagner dielectric relaxation at high temperature was revealed for the first time in this system. Meanwhile, hysteresis-free P–E loops combined with a very high piezoelectric strain coefficient (d₃₃) of 1658 pC/N concurrently with large electrostrictive coefficient Q?=?0.287 m⁴C^?2 were achieved. The ferroelectric relaxor behavior and large electrostrictive strain might be linked to easy reorientation and reversal of ergodic PNRs and the combined effect of Bi off-center position and lone pair electrons.     

Coexistence of Superconductivity and Spin Density Wave (SDW) in Ferropnictide Ba<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>K<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Fe<Subscript>2</Subscript>As<Subscript>2</Subscript>

This work focuses on the theoretical investigation of the coexistence of superconductivity and spin density wave (SDW) in ferropnictide Ba_1?x K _x Fe₂As₂. By developing a model Hamiltonian for the system and by using quantum field theory Green’s function formalism, we have obtained mathematical expressions for superconducting transition temperature (T _C), spin density wave transition temperature (T _sdw), superconductivity order parameter (Δ_Sc), and spin density wave order parameter (Δ_sdw). By employing the experimental and theoretical values of the parameters in the obtained expressions, phase diagrams of superconducting transition temperature (T _C) versus superconducting order parameter (Δ_Sc) and spin density wave transition temperature (T _sdw), versus spin density wave order parameter (Δ_sdw) have been plotted. By combining the two phase diagrams, we have demonstrated the possible coexistence of superconductivity and spin density wave (SDW) in ferropnictide Ba_1?x K _x Fe₂As₂.     

Mechanochemical synthesis of Co-C materials

Using mechanochemical processing, we prepared nonequilibrium Co-C solid solutions supersaturated with carbon to above 7 at %. X-ray diffraction characterization showed that the formation of fcc Co_{1 ? x} C _x solid solutions was accompanied by an increase in the probability of deformation stacking faults. In Co + C mixtures containing more than 10 at % carbon, the fcc solid solution converted to the metastable cobalt carbide Co₃C with an orthorhombic structure. We assessed the thermal stability of the nonequilibrium Co_{1 ? x} C _x solid solutions and the heat effect of the decomposition of the cobalt carbide Co₃C: ?ΔH = 23 kJ/mol.     

Luminescent properties of europium-activated yttrium gadolinium phosphates

Nanocrystalline Y_{1 ? x ? y} Gd _x Eu _y PO₄ phosphors have been prepared via precipitation from aqueous solutions. From their luminescence and excitation spectra, the intensity ratio I ₆₁₅/I ₅₉₄ of the Eu³⁺ luminescence bands corresponding to electric dipole and magnetic dipole transitions has been determined as a function of Gd³⁺ content. The critical concentration and effective energy transfer radius in Y_{1 ? x ? y} Gd _x Eu _y PO₄ have been evaluated. Excitation of Gd³⁺⁸ S _J ?⁶ D _J and ⁸ S _J ?⁶ J _J transitions to Eu³⁺ luminescence excitation levels in Y_{0.99 ? x} Gd _x Eu_0.01PO₄ involves efficient energy transfer. Under 250-nm excitation, the Eu³⁺ luminescence yield in Y_{0.99 ? x} Gd _x Eu_0.01PO₄ is a factor of 2.5–3 higher than that in Y_0.99Eu_0.01PO₄.     

Production of double-layer epitaxial structures based on the solid solution of the Cd-Hg-Te system using a combination of LPE and MOCVD techniques

The technique of growing CdTe and Cd _x Hg_{1 ? x} Te layers utilizing chemical metalloorganic compound vapor deposition (MOCVD) onto a CdZnTe(111) substrate with a preliminarily deposited Cd _y Hg_{1 ? y} Te layer using liquid phase epitaxy (LPE) has been developed. No noticeable changes in the electrophysical parameters and composition of the Cd _y Hg_{1 ? y} Te layer take place under such conditions during MOCVD deposition.     

Effect of Al Substitution in Structural and Magnetic Properties of MnCr<Subscript>2</Subscript>O<Subscript>4</Subscript>

Single-phase samples of Mn(Cr_1?x Al _x )₂O₄ (x = 0 – 0.30) with cubic spinel structure were prepared and the lattice constant is found to decrease from a = 8.4396 Å for x = 0 to a = 8.3801 Å for x = 0.30. The substitution of Al at Cr site is confirmed from the blue shift of Raman modes. Magnetization measurements and analysis show all the prepared samples exhibit ferrimagnetic transition with transition temperature in the range of 46 K for x = 0 to 33 K for x = 0.30. The saturation magnetization (M _s ) and the estimated anisotropy constant (K) show an anomalous behavior up to x = 0.10 and beyond that they decrease monotonously. They are explained by considering different site preferences of Al ³⁺ ions as the doping concentration is increased. The theoretical and experimental effective magnetic moment of the samples is found to be comparable and it decreases with increase in Al concentration.     

Crystal Structure of Nb<Subscript>5</Subscript>Si<Subscript>3 − <Emphasis Type="Italic">x</Emphasis></Subscript>P<Subscript>0.5 + <Emphasis Type="Italic">x</Emphasis></Subscript> (<Emphasis Type="Italic">x</Emphasis> = 0–0.5)

The structure of the ternary compound Nb₅Si_{3 ? x}P_{0.5 + x} (x = 0–0.5) is determined by single-crystal x-ray diffraction: sp. gr. P6₃mc, a = 1.32350(5) nm, c = 0.52954(1) nm; R _F = 0.028, R_w = 0.081 for 547 reflections with F _hkl > 4.0σ(F _hkl ). Relations between the structure of Nb₅Si_{3 ? x}P_{5 + x}, the Mn₅Si₃ structure type, and its derivatives are discussed.