Structure of multilayer ZrO<Subscript>2</Subscript>/SrTiO<Subscript>3</Subscript>

Multilayered oxide heteroepitaxial systems, including that of a 1-nm-thick Y₂O₃-stabilised ZrO₂ (YSZ) sandwiched between layers of SrTiO₃ (STO) [1], have been a subject of much interest lately due to their significantly enhanced ionic conductivities as compared to the bulk materials. We aim to provide the foundation for understanding this increase in conductivity by considering the atomic configurations at the interfaces of such systems, specifically a ZrO₂/STO multilayer system. Possible stable lattice structures of pure ZrO₂ in the system are explored using a genetic algorithm in which the interatomic interactions are modelled by simple pair potentials. The energies of several of the more stable of these structures are then evaluated more accurately within density functional theory (DFT). We find that the fluorite ZrO₂ phase is unstable as a coherently strained epitaxial layer in the multilayer system. Instead, anatase-, columbite-, rutile-, and pyrite-like ZrO₂ epitaxies are found to be more stable, with the anatase-like epitaxy being the most stable structure over a wide range of chemical potential of the components. We also find a high energy metastable structure resembling the tetragonal fluorite structure which is predicted by DFT to be stabilised by SrO-terminated STO but not by TiO₂-terminated STO.     

First principles study of three-component SrTiO<Subscript>3</Subscript>/BaTiO<Subscript>3</Subscript>/PbTiO<Subscript>3</Subscript> ferroelectric superlattices

The geometrical, chemical and ferroelectric properties of a new nanoscale short-period three-component SrTiO₃/BaTiO₃/PbTiO₃ perovskite superlattice are investigated using a first principles density functional approach. The study focuses on varying the thickness of each component in the superlattice and determining the resulting lattice distortion and total polarization. Thicknesses of up to three unit cells in a single component are considered and the in-plane lattice constants normal to the [001] stacking direction are fixed to the bulk SrTiO₃ values to simulate a rigid substrate. It is found that the PbTiO₃ layers play a key role in strain and polarization enhancement. By increasing the amount of PbTiO₃ in the superlattices the strain in the other components increases significantly resulting in an enhanced total polarization of the superlattice relative to bulk BaTiO₃. Increasing the number of BaTiO₃ layers also improves the overall polarization. All the SrTiO₃ layers in each superlattice are found to be highly polarized. Many of the calculated features are similar to those found previously by others for the SrTiO₃/BaTiO₃/CaTiO₃ superlattice, although in the present study significantly greater enhancement factors and polarization values are found. The predicted enhancement of the polarization is mostly attributed to lattice strain due to mismatch of the in-plane lattice constant of the three-component materials.     

Diffuse ferroelectric phase transition in SrTiO<Subscript>3</Subscript>-BiScO<Subscript>3</Subscript> system ceramics

Ceramic samples of the (1−x)SrTiO₃-(x)BiScO₃ system with x = 0, 0.05, 0.1, 0.2, 0.3, 0.4, and 0.5 have been synthesized for the first time. X-ray diffraction data showed that the samples with x = 0.2, 0.3, and 0.4 at room temperature comprise the mixture of two phases: (i) cubic nonpolar phase with Pm3m structure and (ii) tetragonal polar phase with P4mm structure. The temperature dependences of permittivity and dielectric loss tangent of these samples exhibit anomalies characteristic of ferroelectrics with diffuse phase transitions.     

Structural and Magnetic Properties of La<Subscript>2/3</Subscript>Sr<Subscript>1/3</Subscript>MnO<Subscript>3</Subscript>/SrTiO<Subscript>3</Subscript>/CoFe<Subscript>2</Subscript> Hard-Soft Magnetic Systems

We report on the growth and magnetic properties of La_2/3Sr_1/3MnO₃/SrTiO₃/CoFe₂ hard-soft magnetic systems prepared by pulsed laser deposition on SrTiO₃(001) substrates. In situ reflection high-energy electron diffraction along the [100]SrTiO₃ substrate azimuth and atomic force microscopy measurements reveal that La_2/3Sr_1/3MnO₃ and SrTiO₃ grow both in a three dimensional mode and that the roughness of the lower and upper magnetic/non-magnetic interfaces ranges between 2 and 4 Å. Cross-section transmission electron microscopy observations show that the layers are continuous, with an homogeneous thickness, and that the interfaces are mostly sharp and correlated. The magnetization curves show a two step reversal of the magnetization, with very distinct coercive fields. A small anisotropy is observed for the CoFe₂ layer with an in plane easy magnetization axis along the [110]SrTiO₃ direction. Minor magnetization loops indicate that the coupling between the magnetic layers is negligible.     

Interfacial capacitance in epitaxial heterostructures La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript>/SrTiO<Subscript>3</Subscript>/La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO

Epitaxial trilayer heterostructures of the type La_0.67Ca_0.33MnO₃/SrTiO₃/La_0.67Ca_0.33MnO₃ were grown by laser ablation on (001)[(LaAlO₃)_0.3+(Sr₂AlTaO₆)_0.7] substrates. The real part of the dielectric permittivity ε and the loss factor tan δ of a 1100-nm-thick SrTiO₃ interlayer were studied in the temperature interval T=4.2–300 K in a nonbiased state and at a bias voltage of ±2.5 V applied to the manganite electrodes. Using the temperature dependence ε(T) measured for the SrTiO₃ layer grown between the manganite electrodes, we have estimated the capacitance of La_0.67Ca_0.33MnO₃/SrTiO₃ interfaces (C₁≈2 μF/cm²) related to the electric field penetrating from the interlayer into La_0.67Ca_0.33MnO₃.     

Synthesis and microstructure of SrTiO<Subscript>3</Subscript> and BaTiO<Subscript>3</Subscript> ceramics by a reaction-sintering process

Strontium titanate and barium titanate ceramics prepared by a reaction-sintering process were investigated. The mixture of raw materials of stoichiometric SrTiO₃ and BaTiO₃ was pressed and sintered into ceramics without any calcination stage involved. A density 4.99 g/cm³ (97.5% of the theoretic value) was found in SrTiO₃ after 6 h sintering at 1,370 °C. Grains less than 1.5 μm were formed at 1,300–1,330 °C and became 2.2–3.3 μm at 1,350–1,370 °C SrTiO₃. A density 5.89 g/cm³ (97.9% of the theoretic value) was found in BaTiO₃ after 6 h sintering at 1,400 °C. Merged grains were observed in BaTiO₃ and were less than 10 μm after sintered at 1,400 °C.     

Valence-band offsets of InGaZnO<Subscript>4</Subscript>, LaAlO<Subscript>3</Subscript>, and SrTiO<Subscript>3</Subscript> heterostructures explained by interface-induced gap states

The intrinsic interface-induced gap states (IFIGS) which derive from the virtual gap states of the complex band structure are the fundamental mechanism that determines the band-structure lineup at semiconductor interfaces. The valence-band offsets of heterostructures are composed of a zero-charge-transfer term and an electrostatic-dipole contribution which are given by the difference of the p-type branch-point energies of the IFIGS and of the electronegativities, respectively, of the two semiconductors involved. The valence-band offsets of InGaZnO₄, LaAlO₃, and SrTiO₃ heterostructures are quantitatively and consistently explained by the IFIGS-and-electronegativity concept. The analysis of the experimental InGaZnO₄, LaAlO₃, and SrTiO₃ data yields the p-type branch-point energies as 2.37?±?0.18 eV, 2.59?±?0.13 eV, and 2.86?±?0.14 eV, respectively.     

Superconductivity in Ca<Subscript>0.5</Subscript>La<Subscript>0.5</Subscript>FBiSe<Subscript>2</Subscript>

Through the measurement of resistivity, magnetic susceptibility, and Hall effect, we discovered a novel BiSe₂-based superconductor Ca_0.5La_0.5FBiSe₂ with T _c of 3.9 K. A strong diamagnetic signal below T _c in susceptibility χ(T) is observed indicating the bulk superconductivity. The negative Hall coefficient throughout the whole temperature regime implies the dominant electron-type carriers in the sample. Different to most of BiS₂-based compounds where superconductivity develops from a semiconducting-like normal state, its resistivity in the present compound exhibits a metallic behavior down to T _c . Together with the enhanced T _c , the metallic character of the normal state implies that the electronic structure of Ca_0.5La_0.5FBiSe₂ may be different to those in the other BiS₂-based compounds.     

Paramagnetism and Superconductivity in Eu<Subscript>0.7</Subscript>Sm<Subscript>0.3</Subscript>Ba<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−δ</Subscript>

Magnetic properties of sintered Eu_0.7Sm_0.3Ba₂Cu₃O_7?δ were investigated both in dc and ac magnetic fields. The dc response reflects the interplay between the rare earth ion paramagnetic and the superconducting charge carrier subsystems, respectively. The harmonic susceptibilities exhibit special features: the second harmonic is anomalously high and the third harmonic in zero dc-field has reversed temperature dependence with respect to the theoretical models. The magnetic relaxation at low fields is monotonous and occurs as a two-stage relaxation, each stage obeying logarithmical time dependence with different rates. At high fields, the relaxation is nonmonotonous with a peak at intermediate time suggesting a temporary re-entrance of irreversibility when the flux-line density increases in the center of the sample because of the redistribution of the vortices toward that region.     

Influence of Nd doping on microwave dielectric properties of SrTiO<Subscript>3</Subscript> ceramics

Sr_1?x Nd _x TiO₃ (x?=?0.08–0.14) ceramics were prepared by conventional solid-state methods. The analysis of crystal structure suggested Sr_1?x Nd _x TiO₃ ceramics appeared to form tetragonal perovskite structure. The relationship between charge compensation mechanism, microstructure feature and microwave dielectric properties were investigated. Trivalent Nd³⁺ substituting Sr²⁺ could effectively decrease oxygen vacancies. This reduction and relative density were critical to improve Q?×?f values of Sr_1?x Nd _x TiO₃ ceramics. For ε _r values, incorporation of Nd could restrain the rattling of Ti⁴⁺ cations and led to the reduction of dielectric constant. The τ _f values were strongly influenced by tilting of oxygen octahedral. The τ _f values decreased from 883 to 650 ppm/°C with x increasing from 0.08 to 0.14. A better microwave dielectric property was achieved for composition Sr_0.92Nd_0.08TiO₃ at 1460 °C: ε _r ?=?160, Q?×?f?=?6602 GHz, τ _f ?=?883 ppm/°C.     

Liquid-phase epitaxy of NdAl<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> and Yb-doped YAl<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript>

Epitaxial layers of NaAl₃(BO₃)₄ (NAB) and YAl₃(BO₃)₄〈Yb〉 (YAB〈Yb〉) containing up to 10 at % Yb have been grown by liquid-phase epitaxy on YAB substrates. Their growth kinetics have been studied at relative supersaturations of the high-temperature solution from 2 × 10^?2 to 16 × 10^?2. The ytterbium concentration in YAB〈Yb〉 has been shown to vary little during the epitaxial process. Near the edges of the substrate, the surface morphology of the layers is complicated by vicinals, which have a spiral form in the case of YAB〈Yb〉. On \(\{ 10\overline 1 1\} \) YAB substrates, homogeneous single-crystal NAB films have been grown.     

Superconductivity at 5.5 K in Nb<Subscript>2</Subscript>PdSe<Subscript>5</Subscript> Compound

We report superconductivity in as-synthesized Nb₂PdSe₅, which is similar to a recently discovered Nb₂PdS₅ compound having very high upper critical field, clearly above the Pauli paramagnetic limit Zhang et al. (Sci. Rep. 3:1446, 2013). A bulk polycrystalline Nb₂PdSe₅ sample is synthesized by a solid-state reaction route in a phase-pure structure. The structural characterization has been done by X-ray diffraction, followed by Rietveld refinements, which revealed that the Nb₂PdSe₅ sample is crystallized in a monoclinic structure within the space group C2/m. Structural analysis revealed the formation of sharing of one-dimensional PdSe₂ chains. Electrical and magnetic measurements confirmed the superconductivity in Nb₂PdSe₅ compound at 5.5 K. Detailed magneto-resistance results exhibited the value of upper critical field to be around 8.2 T. The estimated H _c2(0) is within the Pauli paramagnetic limit, which is unlike the Nb₂PdS₅.     

Ginzburg-Landau Analysis on the Superconductivity in TlNi<Subscript>2</Subscript>Se<Subscript>2</Subscript>

Based on a two-band isotropic Ginzburg-Landau theory, we study the magnetic properties of the recently observed superconducting crystal TlNi₂Se₂. Our exact solution of upper critical field reproduces the experimental data in a broad temperature range. It directly underlies the multi-gap superconductivity in this crystal. We also show that the effective mass of the electron for one band is about 20m _e while that of the other band is only 0.6m _e. This semi-heavy-fermion feature can qualitatively explain the experimental value of the Kadowaki-Woods ratio in TlNi₂Se₂.     

Two-step growth of SrTiO<Subscript>3</Subscript> films on Sr-modulated Si(001) substrates

Perovskite-type SrTiO₃ (STO) films have been grown on the Sr-modulated Si(001) substrates using two-step growth method with a molecular beam epitaxy technique. About 6 Å Sr was deposited on the surface of chemically formed SiO₂/Si at 800 C, which resulted in the formation of Si(001)-Sr(2 × 1) surface. About 250-Å-thick STO films have been deposited on the Si(001)-Sr(2 × 1) surface at 350 C in the O₂ partial pressure of approximately 3 × 10^–7 Torr (first step), followed by an annealing at 600 C in a pressure of 3 × 10^–9 Torr (second step). Reflection high-energy electron diffraction patterns of grown films become elongated streaky features after the annealing, suggesting the solid-phase epitaxial growth of STO. The presence of strong STO(001) and STO(002) diffraction peaks in X-ray diffraction measurements suggests that the high-quality crystalline STO films grow on the Sr-modulated Si(001) substrates by the two-step growth method. Atomic force microscopy image shows the smooth surface with root mean square roughness of about 18 Å.     

Enhancement of Superconductivity in FeSe<Subscript>0.5</Subscript>Te<Subscript>0.5</Subscript> by Furnace Cooling

It has been revealed that the superconductivity of the polycrystalline FeSe_0.5Te_0.5 synthesized by using the melting technique depends on the way of cooling. To systematically study the effects of furnace cooling on the superconductivity, we prepared the polycrystalline FeSe_0.5Te_0.5 by the melting technique with the furnace cooling. Furthermore, quenching at different temperatures was also carried out for comparison. According to our results of magnetization measurements, the furnace cooling contributes to an enhancement of the superconductivity; however, the quenched samples only held degraded superconductivity. Moreover, the superconductivity of the quenched samples could also be enhanced when additional heat treatments with the furnace cooling were employed. Our results indicate that the melting technique with the furnace cooling and the resulting high-quality polycrystalline FeSe_0.5Te_0.5 are promising in applications.     

Crystal structure of [CH<Subscript>3</Subscript>NH<Subscript>3</Subscript>][(UO<Subscript>2</Subscript>)(H<Subscript>2</Subscript>AsO<Subscript>4</Subscript>)<Subscript>3</Subscript>]

The crystal structure of a previously unknown compound [CH₃NH₃][(UO₂)(H₂AsO₄)₃] was solved by direct methods and refined to R ₁ = 0.038 for 3041 reflections with |F _hkl | >-4σ |F _hkl |. The compound crystallizes in the monoclinic system, space group P2₁/c, a = 8.980(1), b = 21.767(2), c = 7.867(1) Å, β = 115.919(5)°, V = 1383.1(3) ų, Z = 4. In the structure of the compound, pentagonal bipyramids of uranyl ions, sharing bridging atoms with tetrahedral [H₂AsO₄]^? anions, form strongly corrugated layered complexes [(UO₂)(H₂AsO₄)₃]^? arranged parallel to the (100) plane. The protonated methylamine molecules [CH₃NH₃]⁺ form unidimensional tapelike packings parallel to the c axis and linked by hydrophilic-hydro-phobic interactions. The topology of the layered uranyl arsenate complex [(UO₂)(H₂AsO₄)₃]^? is unusual for uranyl compounds and was not observed previously. A specific feature of this topology is the presence of monodentate arsenate “branches” arranged within the layer.     

Superconductivity in Hydrogen-rich Material: GeH<Subscript>4</Subscript>

The electronic properties, lattice dynamics, and electron–phonon coupling of the Cmmm phase of GeH₄ have been studied by first-principle calculations using density functional perturbation theory. The electronic band structure shows the Cmmm phase metallic nature. It is found strong electron phonon interaction, and the superconducting critical temperature, predicted by Allen–Dynes modified McMillan equation, is about 40 K at 20 GPa.