Effect of Ge4+ and Mg2+ doping on superconductivity,fluctuation induced conductivity and interplanar coupling of TlSr2CaCu2O7−δ superconductors

Substitution of Ge⁴⁺ in place of Cu in Tl_0.85Cr_0.15Sr₂CaCu_2?xGe_xO_7?δ (x = 0–0.6) showed initial increase in zero critical temperature value, T_{c zero} from 98 K (x = 0) to 100 K (x = 0.1) and in the range of 85–86 K for x = 0.2–0.3. The slow decrease in T_{c zero} is unexpected as tetravalent Ge⁴⁺ substitution is expected to strongly reduce hole concentration in the samples and suppress T_{c zero}. Excess conductivity analyses of resistance versus temperature data based on Asmalazov–Larkin (AL) theory revealed that the substitution induced 2D-to-3D transition of fluctuation induced conductivity with the highest transition temperature, T_2D–3D observed at x = 0.1. FTIR spectroscopy analysis indicates Ge⁴⁺ substitution cause reduction in CuO₂/GeO₂ interplanar distance while our calculation based on Lawrence–Doniach model revealed highest superconducting coherence length, ξ_c(0) and interplanar coupling, J at x = 0.3. On the other hand, substitution of divalent Mg²⁺ for Ca²⁺ in (Tl_0.5Pb_0.5)(Sr_1.8Yb_0.2)(Ca_1?yMg_y)Cu₂O₇ (y = 0–1.0), which is not expected to directly vary hole concentration, surprisingly caused T_{c zero} to increase from 89.6 K (y = 0) to an optimum value of 95.9 K (y = 0.6) before decreasing with further increase in y. Excess conductivity analyses showed 2D-to-3D transition of fluctuation induced conductivity for all samples where the highest T_2D–3D was at y = 0.4. Similar calculation revealed highest values of ξ_c(0) and J also at y = 0.4. FTIR analysis of the samples indicates inequivalent Cu(1)O(2)Pb/Tl lengths and possible tilting of CuO₂ plane as a result of Mg²⁺ substitution. The increased ξ_c(0) and J as a result of the Ge⁴⁺ and Mg²⁺ substitutions are suggested to contributed to sustenance of superconductivity above 80 K in the samples.     

Time dependence of isothermal magnetization in single-phase B1 MoCy encapsulated in multiwall carbon nanocages

Isothermal magnetization M(t) in nanocrystalline single-phase B1 MoC_y encapsulated in multiwall carbon nanocages is studied within the time window of 100 < t < 5000 s. The current density J exhibits a linear logarithmic time decay. The effective activation energy U_eff increases linearly with increasing temperature T, and decreases linearly with increasing J. The behaviors of J(t), U_eff(T), and U_eff(J) can be described by the Anderson–Kim flux-creep model for thermally activated motion of uncorrelated vortices or vortex bundles over a net potential barrier U_eff. The slower relaxation of current density above the broad peak field in the isothermal magnetization curves suggests that the peak is a result of vortex dynamics.     

Vortex structure in Sr2RuO4

The vortex structure in p-wave superconductors is investigated by the Bogoliubov–de Gennes theory on a tight-binding model. We calculate the temperature dependence of the electronic state at each site in the vortex lattice state, and show the difference between sin p_x+i sin p_y-wave and sin p_x−i sin p_y-wave superconducting state. Furthermore the relation of the electronic structure and the site-dependence of the nuclear magnetic relaxation time is also discussed.     

Synthesis and characterization of layered Li1−x(Ni0.5Co0.5)1−yFeyO2(0 ≤ (1 − x) ≤ 1 and 0 ≤ y ≤ 0.2) cathodes

Layered Li(Ni_0.5Co_0.5)_1−yFe_yO₂ cathodes with 0 ≤ y ≤ 0.2 have been synthesized by firing the coprecipitated hydroxides of the transition metals and lithium hydroxide at 700 °C and characterized as cathode materials for lithium ion batteries to various cutoff charge voltages (up to 4.5 V). While the y = 0.05 sample shows an improvement in capacity, cyclability, and rate capability, those with y = 0.1 and 0.2 exhibit a decline in electrochemical performance compared to the y = 0 sample. Structural characterization of the chemically delithiated Li_1−x(Ni_0.5Co_0.5)_1−yFe_yO₂ samples indicates that the initial O3 structure is maintained down to a lithium content (1 − x) ≈ 0.3. For (1 − x) < 0.3, while a P3 type phase is formed for the y = 0 sample, an O1 type phase is formed for the y = 0.05, 0.1 and 0.2 samples. Monitoring the average oxidation state of the transition metal ions with lithium contents (1 − x) reveals that the system is chemically more stable down to a lower lithium content (1 − x) ≈ 0.3 compared to the Li_1−xCoO₂ system. The improved structural and chemical stabilities appear to lead to better cyclability to higher cutoff charge voltages compared to that found before with the LiCoO₂ system.     

Dynamics of vortices in type-II superconductors with periodic square columnar pins

The dynamics of a two-dimensional vortex system with strong periodic square columnar pins is investigated. For the case vortex number matching pinning number, we find that the vortex liquid is frozen into square lattice via a continuous transition, and the freezing (melting) temperature T_m is the same as the thermal depinning temperature of vortices, which are different from the first-order phase transition at weak pinning. The zero-temperature critical depinning force F_c0 is exactly the same as the maximum pinning force, and the depinning property at T = 0 can be expressed by scaling v ～ (F ? F_c0)^β with the exponent β close to 0.5. The v–F curves at temperatures below T_m show that vortices are pinned at small driving force.     

Pinning,thermally activated depinning and their importance for tuning the nanoprecipitate size and density in high Jc YBa2Cu3O7−x films

YBa₂Cu₃O_7?x (Y123) films with quantitatively controlled artificial nanoprecipitate pinning centers were grown by pulsed laser deposition (PLD) and characterized by transport over wide temperature (T) and magnetic field (H) ranges and by transmission electron microscopy (TEM). The critical current density J_c was found to be determined by the interplay of strong vortex pinning and thermally activated depinning (TAD), which together produced a non-monotonic dependence of J_c on c-axis pin spacing d_c. At low T and H, J_c increased with decreasing d_c, reaching the very high J_c ～ 48 MA/cm² ～20% of the depairing current density J_d at 10 K, self-field and d_c ～ 10 nm, but at higher T and H when TAD effects become significant, J_c was optimized at larger d_c because longer vortex segments confined between nanoprecipitates are less prone to thermal fluctuations. We conclude that precipitates should extend at least several coherence lengths along vortices in order to produce irreversibility fields H_irr(77 K) greater than 7 T and maximum bulk pinning forces F_p,max(77 K) greater than 7–8 GN/m³ (values appropriate for H parallel to the c-axis). Our results show that there is no universal pin array that optimizes J_c at all T and H.     

Relationship between structural phase transition and carrier concentration in La2−x−ySrxRyCuO4 (R = Ce or Tb)

We have studied the relationship between the crystal structure and the carrier concentration in La_2?x?ySr_xCe_yCuO₄ by low-temperature X-ray diffraction method. The analysis for the [1 1 0]_t peak of the tetragonal index confirms that the high-temperature tetragonal phase changes to the low-temperature orthorhombic one in both La_1.89Sr_0.11CuO₄ and La_1.88Sr_0.11Ce_0.01CuO₄. We have also examined the effects of Tb substitution for La-site on the superconductivity and the structure in La_2?x?ySr_xTb_yCuO₄. A dip of the critical temperature T_c(x) due to the 1/8 anomaly and a maximum of T_c(x) at the optimum carrier concentration do not depend on the Tb concentration. This result suggests the possibility that Tb is introduced as the trivalent ion for x = 0.07–0.18.     

Composition and lattice mismatch dependent dielectric constants and optical phonon modes of InAs1−x−ySbxPy quaternary alloys

Based on the pseudopotential formalism under the virtual crystal approximation, the dielectric and lattice vibration properties of zinc-blende InAs_1−x−ySb_xP_y quaternary system under conditions of lattice matching and lattice mismatching to InAs substrates have been investigated. Generally, a good agreement is noticed between our results and the available experimental and theoretical data reported in the literature. The variation of all features of interest versus either the composition parameter x or the lattice mismatch percentage is found to be monotonic and almost linear. The present study provides more opportunities to get diverse high-frequency and static dielectric constants, longitudinal and transversal optical phonon modes and phonon frequency splitting by a proper choice of the composition parameters x and y (0 ⩽ x ⩽ 0.30, 0 ⩽ y ⩽ 0.69) and/or the lattice mismatch percentage.     

Influence of the orthorhombic distortion on the van Hove singularities in the DOS and ARPES spectra of layered cuprates

Four atom states Cu3d_{x² −  y²}, Cu4s, O_a2p_xare involved in a tight-binding model for the superconducting CuO₂plane. The orthorhombic distortion is taken into account by the differences of Cu–O hopping amplitudes and single-site oxygen energies ε_aand ε_bof two oxygen positions in the elementary cell as well. An effective ‘oxygen’ Hamiltonian including only the electron amplitudes at the oxygen ions is derived. Simple expressions for the constant energy contours and the Fermi surface are obtained and they qualitatively describe the photoemission spectra. Extended saddle points nearp = (π,0) andp = (0,π) are observed in qualitative agreement with the ARPES data. The van Hove singularities of the density of states (DOS) related to the extended saddle points are calculated by a Monte Carlo method. It is found that the splitting of the singularity of the DOS at the bottom of the conduction band is created by the energy difference ε_a −  ε_b = 2δ.     

Luminescent properties of red-emitting LiSr4B3O(9−3x/2)Nx:Eu2+ phosphor for white-LEDs

An Eu²⁺-activated oxynitride LiSr_(4?y)B₃O_(9?3x/2)N_x:yEu²⁺ red-emitting phosphor was synthesized by solid-state reactions. The synthesized phosphor crystallized in a cubic system with space group Ia–3d. The LiSr₄B₃O_(9?3x/2)N_x:Eu²⁺ phosphors exhibited a broad red emission band with a peak at 610 nm and a full width at half maximum of 106 nm under 410 nm excitation, which is ascribed to the 4f⁶5d¹→4f⁷ transition of Eu²⁺. The optimal doped nitrogen concentration was observed to be x=0.75. The average decay times of two different emission centers were estimated to be 568 and 489 ns in the LiSr_3.99B₃O_8.25N_0.5:0.01Eu²⁺ phosphors, respectively. Concentration quenching of Eu²⁺ ions occurred at y=0.07, and the critical distance was determined as 17.86 Å. The non-radiative transitions via dipole–dipole interactions resulted in the concentration quenching of Eu²⁺-site emission centers in the LiSr₄B₃O₉ host. These results indicate LiSr₄B₃O_(9?3x/2)N_x:Eu²⁺ phosphor is promising for application in white near-UV LEDs.     

Dielectric and piezoelectric properties of (Bi1−x−yNdxNa1−y)0.5BayTiO3 lead-free ceramics

New lead-free (Bi_1−x−yNd_xNa_1−y)_0.5Ba_yTiO₃ ceramics were prepared by a conventional ceramic technique and their dielectric and piezoelectric properties were studied. X-ray diffraction studies reveal that Nd³⁺ and Ba²⁺ diffuse into the Bi_0.5Na_0.5TiO₃ lattices to form a new solid solution with a pure perovskite structure, and a morphotropic phase boundary (MPB) of rhombohedral and tetragonal phases is formed at 0.04 < y < 0.10. The partial substitutions of Nd³⁺ and Ba²⁺ decrease effectively the coercive field E_c and increase significantly the remanent polarization P_r. Because of lower E_c, larger P_r and the formation of the MPB, the piezoelectric properties of the ceramics are significantly enhanced at x/y = 0.02/0.06: d₃₃ = 150 pC/N and k_p = 30.5%. The ceramics exhibit relaxor characteristic, which is probably resulted from the cation disordering in the 12-fold coordination sites. The depolarization temperature T_d shows a strong compositional dependence and reaches a minimum value at the MPB. The temperature dependences of the ferroelectric and dielectric properties suggest that the ceramics may contain both the polar and non-polar regions near the depolarization temperature T_d, which cause the polarization hysteresis loop become deformed near/above T_d.     

Dynamic evolution of Riemann–Silberstein vortices for Gaussian vortex beams

The dynamic evolution of Riemann–Silberstein (RS) vortices for Gaussian vortex beams with topological charges m = ± 1 in free space is studied. It is shown that for Gaussian on-axis vortex beams there exist both RS vortex with m = + 2 and circular edge dislocation. For Gaussian off-axis vortex beams the circular edge dislocation splits into two RS vortices with opposite topological charges m = ± 1 and the RS vortex with m = + 2 decays into two vortices with same topological charges m = + 1. The motion of RS vortices takes place by varying the propagation distance, waist width, off-axis parameter, or topological charge. RS vortices for Gaussian vortex-free beams can be treated as a special case. The results are illustrated analytically and numerically.     

Influence of growth morphology on the Néel temperature of CrRu thin films and heterostructures

Dimensionality effects on epitaxial and polycrystalline Cr_1?xRu_x alloy thin films and in Cr/Cr–Ru heterostructures are reported. X-ray analysis on Cr_0.9965Ru_0.0035 epitaxial films indicates an increase in the coherence length in growth directions (1 0 0) and (1 1 0) with increasing thickness (d), in the range 20≤d≤300 nm. Atomic force microscopy studies on these films shows pronounced vertical growth for d>50 nm, resulting in the formation of columnar structures. The Néel temperatures (T_N) of the Cr_0.9965Ru_0.0035 films show anomalous behaviour as a function of d at thickness d≈50 nm. It is interesting to note that this thickness corresponds to that for which a change in film morphology occurs. Experiments on epitaxial Cr_1?xRu_x thin films, with 0≤x≤0.013 and d=50 nm, give T_N–x curves that correspond well with that of bulk Cr_1?xRu_x alloys. Studies on Cr/Cr_0.9965Ru_0.0035 superlattices prepared on MgO(1 0 0), with the Cr layer thickness varied between 10 and 50 nm, keeping the Cr_0.9965Ru_0.0035 thickness constant at 10 nm, indicate a sharp decrease in T_N as the Cr separation layers reaches a thickness of 30 nm; ascribed to spin density wave pinning in the Cr layers for d<30 nm by the adjacent CrRu layers.     

The effect of SixNy interlayer on the quality of GaN epitaxial layers grown on Si(1 1 1) substrates by MOCVD

In the present paper, the effects of nitridation on the quality of GaN epitaxial films grown on Si(1 1 1) substrates by metal–organic chemical vapor phase deposition (MOCVD) are discussed. A series of GaN layers were grown on Si(1 1 1) under various conditions and characterized by Nomarski microscopy (NM), atomic force microscopy (AFM), high resolution X-ray diffraction (HRXRD), and room temperature (RT) photoluminescence (PL) measurements. Firstly, we optimized LT-AlN/HT-AlN/Si(1 1 1) templates and graded AlGaN intermediate layers thicknesses. In order to prevent stress relaxation, step-graded AlGaN layers were introduced along with a crack-free GaN layer of thickness exceeding 2.2 μm. Secondly, the effect of in situ substrate nitridation and the insertion of an Si_xN_y intermediate layer on the GaN crystalline quality was investigated. Our measurements show that the nitridation position greatly influences the surface morphology and PL and XRD spectra of GaN grown atop the Si_xN_y layer. The X-ray diffraction and PL measurements results confirmed that the single-crystalline wurtzite GaN was successfully grown in samples A (without Si_xN_y layer) and B (with Si_xN_y layer on Si(1 1 1)). The resulting GaN film surfaces were flat, mirror-like, and crack-free. The full-width-at-half maximum (FWHM) of the X-ray rocking curve for (0 0 0 2) diffraction from the GaN epilayer of the sample B in ω-scan was 492 arcsec. The PL spectrum at room temperature showed that the GaN epilayer had a light emission at a wavelength of 365 nm with a FWHM of 6.6 nm (33.2 meV). In sample B, the insertion of a Si_xN_y intermediate layer significantly improved the optical and structural properties. In sample C (with Si_xN_y layer on Al_0.11Ga_0.89N interlayer). The in situ depositing of the, however, we did not obtain any improvements in the optical or structural properties.     

Effect of W-addition on pinning property of MgB2

The 5d transition metal W was added into the MgB₂ superconductor. The Mg, B and W were sintered at 1173 K for 30 min under H₂/Ar atmosphere in the electric furnace. The W_x(MgB₂)_1?x samples were prepared in the W concentration range of 0 ? x ? 0.05. Temperature and field dependences of magnetization were measured by the SQUID magneto-meter. The field and x dependences of J_c at 20 K were analyzed by the extended critical state model. The enhancement of J_c became maximum for the x = 0.02 sample.     

Novel BTlGaN semiconducting materials for infrared opto-electronic devices

BTlGaN quaternary alloys are proposed as new semiconductor materials for infrared opto-electronic applications. The structural and opto-electronic properties of zinc blende B_xTl_yGa_1−x−yN alloys lattice matched to GaN with (0 ⩽ x and y ⩽ 0.187) are studied using density functional theory (DFT) within full-potential linearized augmented plane wave (FP-LAPW) method. The calculated structural parameters such as lattice constant a₀ and bulk modulus B₀ are found to be in good agreement with experimental data using the new form of generalized gradient approximation (GGA-WC). The band gaps of the compounds are also found very close to the experimental results using the recently developed Tran–Blaha-modified Becke–Johnson (TB-mBJ) exchange potential. A quaternary B_xTl_yGa_1−x−yN is expected to be lattice matched to the GaN substrate with concentrations x = 0.125 and y = 0.187 allows to produce high interface layers quality. It has been found that B incorporation into BTlGaN does not significantly affect the band gap, while the addition of dilute Tl content leads to induce a strong reduction of the band gap, which in turn increases the emission wavelengths to the infrared region. The refractivity, reflectivity and absorption coefficient of these alloys were investigated. BTlGaN/GaN is an interesting new material to be used as active layer/barriers in quantum wells suitable for realizing advanced Laser Diodes and Light-Emitting Diodes as new sources of light emitting in the infrared spectrum region.     

Ionic and electronic conductivities,stability and thermal expansion of La10 − x(Si,Al)6O26 ± δ solid electrolytes

Apatite-type La_10 − xSi_6 − yAl_yO_{27 − 3x/2 − y/2} (x = 0–0.33; y = 0.5–1.5) exhibit predominant oxygen ionic conductivity in a wide range of oxygen partial pressures. The conductivity of silicates containing 26.50–26.75 oxygen atoms per formula unit is comparable to that of gadolinia-doped ceria at 770–870 K. The average thermal expansion coefficients are (8.7–10.8) × 10^− 6 K^− 1 at 373–1273 K. At temperatures above 1100 K, silicon oxide volatilization from the surface layers of apatite ceramics and a moderate degradation of the ionic transport with time are observed under reducing conditions, thus limiting the operation temperature of Si-containing solid electrolytes.     

On the origin of blue light emission from Ge-nanocrystals containing a-SiOxfilms

We have studied the photoluminescence of a-Si_xGe_yO_{1 − x − y}films with average Ge-nanocrystal sizes ranging from over 100 nm down to 2 nm. No systematic peak shift of the luminescence bands at 3.0 eV and 2.0 eV with the diameter of the nanocrystals is observed. Comparision with a simplified confinement model shows that quantum size effects cannot explain the blue luminescence. We propose the Ge²₀defect as a likely source for this band, based on considerations about the crystallization process.     

A ceria layer as diffusion barrier between LAMOX and lanthanum strontium cobalt ferrite along with the impedance analysis

A thin interlayer of samarium doped ceria (SDC) is applied as diffusion barrier between La_1 ? xSr_xCo_yFe_1 ? yO₃ x = 0.1–0.4, y = 0.2–0.8 (LSCF) cathode and La_1.8Dy_0.2Mo_1.6W_0.4O₉ (LDMW82) electrolyte to obstruct Mo–Sr diffusion and solid state reaction in the intermediate temperature range of SOFC. We demonstrate the effectiveness of the diffusion barrier through contrasting the clearly defined interfaces of LSCF/SDC/LDMW82 against a rugged growing product layer of LSCF/LDMW82 in 800 °C thermal annealing, and analyze the product composition and the probable new phase. In addition, the measured polarization resistance is considerably lower for the half-cell with a diffusion barrier. Therefore, the electrochemical performance of the LSCF cathode is investigated on the SDC-protected LDMW82. The cell with LSCF (x = 0.4) persistently outperforms the one with x = 0.2 in polarization resistance because of its small low-frequency contribution. The activation energy of polarization resistance is also lower for La_0.6Sr_0.4Co_yFe_1 ? yO₃ (112–135 kJ/mol), than that for La_0.8Sr_0.2Co_yFe_1 ? yO₃ (156–164 kJ/mol). La_0.6Sr_0.4Co_yFe_1 ? yO₃ y = 0.4–0.8 is the proper composition for the cathode interfaced to SDC/LDMW82.