Image force effects on trapezoidal barrier parameters in metal-insulator-metal tunnel junctions

A one-dimensional trapezoidal barrier potential U₀(x) with and without the image potential U_i(x) was used to model three tunnel junctions: Al-Al₂O₃-Au (-Ag, -Cu) and calculate the junction I-V curves. As the effective trapezoidal barrier parameters are determined by fitting the calculated I-V curves to the experimental ones, it is found that if the I-V curves are calculated based on the barrier potential U₀(x)+U_i(x), the parameters of the actual barrier U₀(x)−U_i(x) are obtained. Thus, the parameters of the actual trapezoidal barrier U₀(x)+U_i(x) can be obtained by extrapolating. The results show that the area of the actual U₀(x)+U_i(x) is smaller than that of the U₀(x) and for the Au-, Ag- and Cu-junction the area reduction is − 6.9, −5.4 and −4.2%, respectively. The variation of the barrier area reduction with the electrodes can be ascribed to the diffusion and oxidation reaction in the region of the junction interfaces, which results in smeared and indistinct interfaces and an increase in the actual distance between the metal mirrors of the two electrodes.     

Falkner–Skan flows past moving boundaries: an exactly solvable case

The Falkner–Skan flows past stretching boundaries are revisited in this paper. The usual assumption U _w (x) = λ U(x), i.e. the proportionality of the stretching velocity U _w (x) and the free stream velocity U(x) is adopted. For the special case of a converging channel (wedge nozzle), U(x) ~ ? 1/x, exact analytical solutions in terms of elementary hyperbolic functions are reported. In the range ? 2 < λ < + 1 dual solutions describing either opposing (λ < 0) or aiding (λ > 0) flow regimes were found. In the range λ > 1 unique solutions occur, while below the critical value λ _c  = ? 2 no solutions exist at all. The mechanical features of these solutions are discussed in some detail.     

Spin Fluctuations in A-like Superfluid Phase of 3He in Aerogel

The frequency spectrum of the spin fluctuations δ S(t) superimposed on the coherently precessing spin modes in the A-like superfluid phase in aerogel is analysed. It is shown that the low amplitude spin fluctuations could be most easily observed in the case of an uni-axially deformed aerogel. It is demonstrated, in particular, that for axially stretched (radially squeezed) aerogel described by the U(1)LIM model the fourth order harmonic in δ S _z (t) is erased, in contrast with what is expected for the long range Leggett orbital configuration in the ³He-A (ABM state).     

Pressure-induced rotation-libration transition in molecular solid hydrogen. A comparison between interaction potentials

The pressure-induced rotation-libration transition in solid molecular para-H₂ and ortho-D₂ is calculated using anisotropic nearest neighbor interaction potentials of the formV _ij=Σ _m C _m (r _ij )Y_2m (Ω _i )Y_2?m (Ω _j). The contribution of variousm terms is investigated. It is found that, for a trial wave function of Jastrow type, the expectation of them=1 term vanishes, while the ratio between the expectations of them=0 andm=2 terms is a rational numerical constant. The trial wave function incorporates two variational parameters and the variational integrals are evaluated by a Monte Carlo procedure. The potentials of Ree and Bender, and of Van Kranendonk and Nakamura were compared to each other. The Ree and Bender potential seems to be better suited for describing high-density properties, although the transition pressure turned out to be well above the experimental value.     

Magnetoresistance Study of Y3Ba5Cu8O18 Superconducting Phase Substituted by Nd3+ and Ca2+ Ions

Superconducting samples of type Y_3?x Nd _x Ba_5?x Ca _x Cu₈O₁₈ with 0.0 ≤ x ≤ 0.4 have been prepared via the solid-state reaction technique. The prepared samples were characterized using X-ray powder diffraction (XRD) technique for phase analysis. The elemental content of the prepared samples was determined using particle-induced X-ray emission (PIXE). In addition, the oxygen content of these samples was obtained using non-Rutherford backscattering spectroscopy (RBS) at 3 MeV proton beam. The results indicate that these substitutions do not affect the orthorhombic structure, while they decrease the oxygen content of Y-358 phase. The electrical resistivity of the prepared samples was measured by the conventional four-probe technique from room temperature down to the zero superconducting transition temperature (T ₀). A slight change in the superconducting transition temperature (T _c) is observed for 0 ≤ x ≤ 0.1, and then it decreases linearly with further increase in x. The linear decrease in T _c is attributed mainly to the partial substitution of Ba²⁺ ions by Ca²⁺ ions rather than the partial substitution of Y³⁺ ions by Nd³⁺ ions. The effect of magnetic fields up to 4.44 kG on the electrical resistivity has been studied to investigate the vortex dynamics for the prepared samples. The experimental data, in the second stage of superconducting transition, fit well with the thermally activated flux creep (TAFC) model, and the activation energy U(B) shows a power law dependence on magnetic field as B ^?β . Also, the transition width is related to the magnetic field according to the relation ΔT α B ⁿ . The values of β and n are strongly dependent on the Nd³⁺ and Ca²⁺ ion substitution. The magnetic field and temperature dependence of the activation energy U(B, T) is found to be U(B, T)? ΔT B ^?η , where η = β + n. Furthermore, the critical current density at zero temperature, J _c(0), as a function of the applied field was calculated for all the prepared samples. The result shows an enhancement in J _c(0) of Y-358 phase at x equals 0.4 at different applied fields.     

New solid ionic conductors

A new class of solid Li⁺ ionic conductors has been found, related to the compound LiZr₂(PO₄)₃. Solid solutions of the type Li_1?xZr_2?xTa_x(PO₄)₃ and Li_1?xHf_2?xTa_x(PO₄)₃ have conductivities of about 10^?3ohm^?1 cm^?1 at 200°C, which are relatively independent of composition. These systems are compared with the recently discovered class of Na⁺ conductors Na_1+xZr₂P_3?xSi_xO₁₂. Other solid solutions are also discussed.     

Dependence of Superconductivity and Its Weakly Linked Behavior in Bulk LaO1?x F x FeAs on F Doping

Samples of oxypnictide compound LaO_1?x F _x FeAs, with x=0.15 and 0.2 corresponding to over- and highly over-doped compositions, respectively, were prepared by solid-state reaction. We present their characterization by XRD and HRTEM, as well as resistivity ??(T), magnetization M(B) and microwave modulated absorption (MMA) response between 4.2?C300?K and applied fields B=0?C8?T. With change in?x, both the superconducting and magnetic behavior of the samples shows an interesting pattern. The ??magnetic anomaly?? at T??130?K, observed in M(T) for x=0, instead of getting totally suppressed shows a tendency to reappear in x=0.2 sample. Both samples typically show ??(300?K)>2.8×10^?3????cm and critical current density J _c(5?K, 1?T)<2×10⁷?A/m². The superconducting transitions as measured by ??(T) at B=0 are found broad for both x=0.15 and 0.2 samples with transition widths ??2.5 and 6?K, respectively. The slope |dB _c2/dT| (where B _c2 is upper critical field) determined by resistive onsets, for the x=0.15 and 0.2 samples, has values ??7.5 and 3.5?T/K, respectively. The superconducting state characteristics as reflected by ??(T,B), M(T), magnetic J _c(B) and MMA response are typical of the presence of weakly linked inter-grain regions in both the samples. Our HRTEM images of the x=0.15 sample show the presence of high angle (??43^°) grain boundaries, which are well known to limit the J _c in cuprate-based high T _c bulk materials.     

A Quantum Algorithm Detecting Concentrated Maps

We consider an arbitrary mapping f: {0, …, N − 1} → {0, …, N − 1} for N = 2ⁿ, n some number of quantum bits. Using N calls to a classical oracle evaluating f(x) and an N-bit memory, it is possible to determine whether f(x) is one-to-one. For some radian angle 0 ≤ θ ≤ π/2, we say f(x) is θ − concentrated if and only if e^2πif(x)/N ? e^{i[ψ₀?θ,ψ₀+θ]} for some given ψ₀ and any 0 ≤ x ≤ N − 1. We present a quantum algorithm that distinguishes a θ-concentrated f(x) from a one-to-one f(x) in O(1) calls to a quantum oracle function U_f with high probability. For 0 < θ < 0.3301 rad, the quantum algorithm outperforms random (classical) evaluation of the function testing for dispersed values (on average). Maximal outperformance occurs at θ=12sin−11π≈0.1620 rad.     

New data on rubidium vanadium phosphate: Structure determination of the disordered (VV–PV) compound Rb6(V2O3)2(VO)2(PO4)4(HP2−xVxO7) with x∼0.6

Brownish crystals of Rb₆(V₂O₃)₂(VO)₂(PO₄)₄(HP_2−xV_xO₇) were prepared hydrothermally. The structure was solved from single crystal X-ray diffraction data in the non centrosymmetric orthorhombic space group Pmn2₁ (No. 31) a=13.5505(2) Å, b=7.1407(2) Å, c=14.7040(2) Å. (R₁(Fo)=0.048, wR₂(Fo²)=0.111). The structure of Rb₆(V₂O₃)₂(VO)₂(PO₄)₄(HP_2−xV_xO₇) is closely related to the previously reported M₃(V₂O₃)(VO)(PO₄)₂(HPO₄) (M=K⁺, Tl⁺) with a disordered occupation of the tetrahedral voids by V^V and P^V atoms.     

Self-consistent approximations in the theory of triplet pairing superfluidity in3He

Spin-lattice relaxation times of bcc solid ³ He with impurity concentrationsx of ⁴ He, 1.4×10 ^?3 <x<7 × 10 ^?3 , have been studied by NMR pulse techniques in the temperature range 0.27 K<T<1.2 K. In the lower part of the temperature range a temperature-independent relaxation process governed by the impurities is observed and its characteristic timeT _1D varies as a simple function ofx:T _1D> ∞x ^?3; the features of this process seem to be consistent with a diffusion mechanism to some clusters of ⁴ He impurities. In this plateau region a long relaxation time is also measured and found to be insensitive tox and strongly dependent on the temperature. A phenomenological model is suggested.     

Effect of Zr substitution on microwave dielectric properties of Zn2SnO4 ceramics

The microwave dielectric properties of Zn₂(Sn_(1?x)Zr_x)O₄ ceramics were examined with a view to their exploitation for mobile communication. The Zn₂(Sn_(1?x)Zr_x)O₄ ceramics were prepared by the conventional solid-state method with various amount of Zr substitution. The diffraction peaks of Zn₂(Sn_(1?x)Zr_x)O₄ ceramics did not vary significantly as x varied. A density of 6.10 g/cm³ was obtained for Zn₂(Sn_0.99Zr_0.01)O₄ ceramic, sintered at 1,225 °C for 4 h. Dielectric constant (? _r ) of 9.6, quality factor (Q × f) of 87,000 GHz, and temperature coefficient of resonant frequency (τ _f ) of ?51 ppm/°C were obtained for Zn₂(Sn_0.99Zr_0.01)O₄ ceramics that were sintered at 1,225 °C for 4 h.     

Magnetic Transport Properties in GdBa2Cu3?x Ru x O7?δ Superconducting Phase

Bulk superconducting samples of type GdBa₂Cu_3?x Ru _x O_7?δ phase, Gd-123, with?x ranging from 0.0 to 0.15 were prepared by the conventional solid-state reaction technique. X-ray powder diffraction (XRD) and the electrical resistivity measurements were performed in order to investigate the effect of Ru⁴⁺ ions substitution on Gd-123 phase. Enhancement of the phase formation and the superconducting transition temperature T _c for GdBa₂Cu_3?x Ru _x O_7?δ phase up to x=0.05 was observed. The effect of magnetic field up to 4.4?kG on the electrical resistivity behavior of the prepared samples was studied to investigate the flux motion of this phase. The derived flux pinning energy?U, based on the thermally activated flux creep TAFC model, decreased with increasing the magnetic field?B. The flux pinning energy followed the exponent behavior as U(B)～B ^?β . The superconducting transition width ΔT increased as the magnetic field increased, showing the scaling relation as ΔT～B ⁿ . Using Ambegaokar and Halperin AH theory, the magnetic field and temperature dependence of U was found to be U(B,T)～ΔTB ^?η , η=β+n. The critical current density J _c (0) enhanced up to x=0.05, beyond which it decreased with further increase in Ru-content.     

Visible-light-induced hydrogen evolution reaction with WS<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se<Subscript>2−<Emphasis Type="Italic">x</Emphasis></Subscript>

WS₂ is a promising catalyst for the hydrogen evolution reaction. We have explored photocatalytic properties of ternary sulphoselenides of tungsten (WS _x Se _2?x ) by the dye-sensitized hydrogen evolution. WS _x Se _2?x solid solutions are found to exhibit high activity reaching 2339 μmol h^?1 g^?1 for WSSe, which is three times higher than that of WS₂ alone (866 μmol h^?1 g^?1 ). The turnover frequency is also high (0.7 h^?1 ). Such synergistic effect of selenium substitution in WS₂ is noteworthy.     

The influence of low-energy ion bombardment on the microstructure development and mechanical properties of TiBx coatings

The influence of the deposition parameters on the growth, structure and mechanical properties of the TiB_x coatings is studied. The TiB_x coatings represent a nanocomposite system, in which random or oriented TiB₂ nanocrystallites are embedded in an amorphous matrix as is revealed by cross-sectional transmission electron microscopy. We show that low-energy ion bombardment (E_i ) of growing TiB_x coating, influences the preferred orientation of TiB₂ crystallites. The increase of ion current density (i_s) by means of negative substrate bias voltage (U_s) leads to change from random to the (0001) preferred crystal orientation whereby the electrical biasing promotes crystal growth in the coating and the (0001) texture appears gradually during the film growth. Together with the (0001) preferred orientation selection the composition B/Ti ratio was changed from 2.9 (floating potential, E_i = 8 eV) to 2.4 (E_i = 94 eV). The highest amount of oriented (0001) crystallites and highest hardness H = 53 GPa exhibit TiB_x coatings deposited at E_i = 94 eV and i_s = 2.69 mA.cm⁻².     

Control of particle size in Si3N4 powders prepared by high-pressure carbothermal nitridation

The grain size variation in “unseeded” Si₃N₄ powders, prepared by high-pressure carbothermal nitridation of SiO₂ (in stoichiometric 1∶2 proportions with C), has been studied by means of scanning electron microscopy (SEM) and “Sedigraph” measurements. The size is a function of process parameters, of which the reactant surface area was found to be the most important. Specifically, with an SiO₂ areaA(SiO₂) ≈ 50 m² g^?1 in the reaction mixture, the resulting mean Si₃N₄ particle diameter,d(Si₃N₄), is very sensitive to the carbon surface area,A(C), such that the minimumd(Si₃N₄) ≈ 1 μm was obtained withA(C)=30m²g^?1 and the maximumd(Si₃N₄) ≈ 7μm withA(C)=115m²g^?1. Using mixtures withA(SiO₂)=50m²g^?1 andA(C)=115m²g^?1, a slight dependence ofd(Si₃N₄) on the furnace heating rate was also observed; larger grains (≈ 7 μm) were obtained with 20deg min^?1 than with 2deg min^?1 (≈5μm). The grain size was found to be virtually independent of nitrogen pressure (in the range 0.3–6.5 MPa), annealing temperature (1470–1830°C) and gas flow rate (2–20 l_(stp) min^?1).     

On the Structure of the Superfluid State and Quasiparticle Spectrum in a Bose Liquid with a Suppressed Bose–Einstein Condensate

A self-consistent model of the superfluid (SF) state of a Bose liquid with strong interaction between bosons and a weak single-particle Bose–Einstein condensate (BEC) is considered. The ratio of the BEC density n ₀ to the total particle density n of the Bose liquid is used as a small parameter of the model, n ₀/n?1, unlike in the Bogolyubov theory of a quasi-ideal Bose gas, in which the small parameter is the ratio of the number of supracondensate excitations to the number of particles in an intensive BEC, (n?n ₀)/n ₀?1. A closed system of nonlinear integral equations for the normal ~Σ₁₁(p, ω) and anomalous ~Σ₁₂(p, ω) self-energy parts is obtained with account for terms of first order in the BEC density. A renormalized perturbation theory is used, which is built on combined hydrodynamic (at p→0) and field (at p≠0) variables with analytic functions ~Σ _ij (p, ε) at pε0 and ε→0 and a nonzero SF order parameter ~Σ₁₂(0, 0)≠0, proportional to the density ρ _s of the SF component. Various pair interaction potentials U(r) with inflection points in the radial dependence and with an oscillating sign-changing momentum dependence of the Fourier component V(p) are considered. Collective many-body effects of renormalization (“screening”) of the initial interaction, which are described by the bosonic polarization operator Π(p, ω), lead to a suppression of the repulsion [V(p)<0] and an enhancement of the effective attraction [V(p)<0] in the respective domains of nonzero momentum transfer, due to the negative sign of the real part of Π(p, ω) on the “mass shell” ω=E(p). In the framework of the “soft spheres” model with the single fitting parameter—the value of the repulsion potential at r=0—the quasiparticle spectrum E(p) is calculated, which is in good accordance with the experimental spectrum E _exp(p) of elementary excitations in superfluid ⁴He. It is shown that the roton minimum in the quasiparticle spectrum is directly associated with the first negative minimum of the Fourier component of the renormalized (“screened”) potential of pair interaction between bosons.     

Divergence of the Grüneisen Parameter and Magnetocaloric Effect at Heavy Fermion Quantum Critical Points

At any pressure sensitive quantum critical point (QCP) the thermal expansion is more singular than the specific heat leading to a divergence of the Grüneisen parameter. For a magnetic field sensitive QCP, the complementary property is the magnetic Grüneisen ratio which equals the magnetocaloric effect. Here we use both properties to investigate magnetic QCPs in different heavy fermion (HF) metals starting from CeNi₂Ge₂. The influence of dimensionality on quantum criticality is addressed by the comparison of cubic CeIn_3?x Sn _x with layered CeMIn_5?x Sn _x (M = Co, Rh) systems, in which Sn doping both acts as tuning parameter and introduces slight disorder. Near the field-tuned QCP in undoped CeCoIn₅ a crossover scale T ^? is discovered which separates 2D (at T>T ^?) from 3D (at T<T ^?) quantum criticality. Disorder, introduced by Sn-doping, is found to increase T ^?, stabilizing 3D behavior. We also compare the magnetic Grüneisen ratio in the approach of the field-tuned QCP in YbRh₂Si₂ with zero-field Grüneisen parameter data on YbRh₂(Si_1?x Ge _x )₂ (x=0, x=0.05). Both properties indicate quantum criticality incompatible with the predictions of the itinerant theory.     

Improvement microwave dielectric properties of Zn2SnO4 ceramics by substituting Sn4+ with Si4+

The microwave dielectric properties of Zn₂(Sn_(1?x)Si_x)O₄ ceramics were examined with a view to their exploitation for mobile communication. The Zn₂(Sn_(1?x)Si_x)O₄ ceramics were prepared by the conventional solid-state method with various sintering temperatures. The X-ray diffraction patterns of the Zn₂(Sn_(1?x)Si_x)O₄ ceramics revealed no significant variation of phase with sintering temperatures. A maximum density of 6.24 g/cm³ was obtained for Zn₂(Sn_0.93Si_0.07)O₄ ceramic, sintered at 1,175 °C for 4 h. Dielectric constant (? _r ) of 8.12, quality factor (Q × f) of 55,500 GHz, and temperature coefficient of resonant frequency (τ _f ) of ?119.3 ppm/°C were obtained for Zn₂(Sn_0.93Si_0.07)O₄ ceramics that were sintered at 1,175 °C for 4 h.     

Urania-yttria solid solution electrodes for high-temperature electrochemical applications

Measurements of total electrical conductivity on fluorite-type U₃O₈-Y₂O₃ (Sc₂O₃) solid solutions have been made as a function of temperature and U/Y(Sc) ratio. The following compositions were studied: (U_0.7Y_0.3)O_2+x , (U_0.6Y_0.4)O_2+x , (U_0.5Y_0.5)O_2+x , (U_0.45Y_0.55)O_2+x , (U_0.4Y_0.6)O_2+x , (U_0.35Y_0.65)O_2+x , (U_0.3Y_0.7)O_2−x , (U_0.5Sc_0.5)O_2+x and (U_0.38Sc_0.62)O_2+x . Preliminary measurements on the latter two compositions were carried out for comparison purposes. The maximum conductivity value occurred for the U₃O₈-Sc₂O₃ solid solutions, and for (U_0.7Y_0.3)O_2+x in the U₃O₈-Y₂O₃ system. The conductivity in these fluorite-type solid solutions is mainly electronic, the conduction mechanism being hopping-type. The energy of activation lay between 25 and 40 kJ mol⁻¹. The (U_0.3Y_0.7)O_2−x composition appeared to be an ionic conductor with an activation energy of ∼110 kJ mol⁻¹ below 800 to 850° C. The diffusion of cations of U₃O₈-Y₂O₃ into ZrO₂-Y₂O₃ was studied during passage of current: no observable diffusion occurred over the period of current passage (384 h). Attempts were made to determine the anionic contribution to the total conductivity in U₃O₈-Y₂O₃ solid solutions using the blocking electrode technique. Results indicated that complete isolation of the specimen-blocking electrode (YSZ) interface from the ambient gases is necessary if such measurements are to be reliable. The diffusion coefficients calculated from the conductivity data using the Nernst-Einstein relation were two orders of magnitude higher than those obtained by a direct method.     

Current-voltage characteristics of Si/Si1 − x Ge x heterodiodes fabricated by direct bonding

We have studied the current-voltage (I–U) characteristics of Si/Si_{1 ? x} Ge _x (0.02 < x < 0.15) heterodiodes fabricated by direct bonding of (111)-oriented n-type single crystal silicon wafers with p-type Si_{1 ? x} Ge _x wafers of the same orientation containing 2–15 at % Ge. An increase in the germanium concentration N _Ge in Si_{1 ? x} Ge _x crystals is accompanied by a growth in the density of crystal lattice defects, which leads to a decrease in the minority carrier lifetime in the base of the heterodiode and an increase in the recombination component of the forward current and in the differential resistance (slope) of the I–U curve. However, for all samples with N _Ge ≤ 15 at %, the I–U curves of Si/Si_{1 ? x} Ge _x heterodiodes are satisfactory in the entire range of current densities (1 mA/cm²–200 A/cm²). This result shows good prospects for using direct bonding technology in the fabrication of Si/Si_{1 ? x} Ge _x heterostructures.