Superconductivity in amorphous Cr films

Superconducting transition has been observed for the first time in amorphous Cr films prepared by co-sputtering of Cr and BN. Superconducting transition temperatures (Tc) up to 1.14 K have been observed with an orbital contribution (i.e.(dH_c2⊥/dT)_Tc) as large as 49 kOe K^-1. The electronic specific heat coefficient (γ) has been estimated to be about 1.6 times as large as that of amorphous Mo-BN (or Mo-metalloid) alloys. This finding, i.e. the low Tc with the large γ for amorphous Cr is inconsistent with data reported for amorphous alloys of 4d and 5d transition elements. This inconsistency implies a formation of local magnetic moments or spin fluctuation in the amorphous Cr film.     

Analysis of the lattice,strong coupling series for g0φ4 field theory in d dimensions

We analyze the renormalized strong coupling series for lattice g₀φ⁴ field theory which is a double series in x = M⁴a⁴/g₀a_4?dandy = 1/M²a², where M is the renormalized mass, a the lattice spacing, g₀ the bare coupling constant and d the dimension of space-time. We extrapolate to large y for fixed x by using a Padé-like extrapolation technique. We study the dimensionless renormalized coupling constant G/M^4?d and find that as we approach the continuum(x → 0, y → ∞) the entire spectrum of g₀ from zero to infinity can be studied. Our results for d = 1,2,3,4 based on a series in y up to y⁵ and in x up to x³ show that for fixed lattice spacing a, G/M^4?d is a monotonic function of g₀ ranging from zero at g₀ = 0 to a maximum at g₀ = ∞. Using the high temperature expansion results, we have also derived 9 terms in y on 8 lattices of dimension 1,2,3 and 4 for the linear term in x, and studied this series to see if one can see a breakdown in this monotonic behavior of G for large y. The analysis of this latter series is inconclusive.     

Mössbauer studies in disordered (NiFe)1−xCrx alloys

The variation of hyperfine field, isomer shift and Curie temperature with chromium concentration has been studied for the alloys (NiFe)_1?xCr_x for x = 0, 5, 10 and 15 at .%. The decrease in hyperfine field and isomer shift suggests an emptying of the majority spin d-band with increasing Cr concentration (reducing number of average 3d + 4s electrons per atom). The Curie temperature is also found to reduce rapidly with Cr concentration in agreement with magnetisation data. But the Curie temperatures as determined from Mössbauer spectra are considerably lower than those given by magnetisation studies and for the 10 and 15 at .% Cr concentrations the magnetic transition is spread out over a range of temperatures suggesting a mictomagnetic behaviour.     

Studies of EPR g factors of the isoelectronic 3d series Cr, Mn and Fe in SrTiO3 crystals

The g-shifts Δg(=g−g_s, where g_s≈2.0023 is the free-ion value) of the isoelectronic 3d³ series Cr³⁺, Mn⁴⁺ and Fe⁵⁺ in SrTiO₃ crystals are calculated from the high-order perturbation formula based on the cluster approach for 3d³ ion in cubic octahedral site. The formula includes not only the contribution from the crystal-field (CF) mechanism, but also that from the charge-transfer (CT) mechanism (which is omitted in the CF theory). From the calculations, it is found that the contribution Δg_CT from the CT mechanism in sign is contrary to the corresponding Δg_CF from the CF mechanism and the relative importance of CT mechanism (characterized by |Δg_CT/Δg_CF|) increases with the increasing valence state (and hence the atomic number) of 3d³ ion. The positive g-shift Δg of SrTiO₃:Fe⁵⁺ is due mainly to the contribution of CT mechanism. So, for the explanations of g factors of the high valence state 3dⁿ ions (e.g. Mn⁴⁺ and Fe⁵⁺) in crystals, the contributions from both CF and CT mechanisms should be taken into account.     

Effects of alloying on the band structure of gold: Piezoreflectance measurements on some AuCo and AuV alloys

Piezoreflectance measurements have been made on a series of gold cobalt and gold vanadium alloys with impurity concentrations of up to 4% using a strain amplitude of 4 × 10^?4 at a frequency of 68 kHz. The spectra show that the main interband transitions occur at 2.4, 3.5 and 4.5 eV. The deformation potentials with impurity concentration have been found for these transitions. An additional interband transition was found to occur at an energy of 1.8eV and this was enhanced significantly with impurity concentration. This may be due to the impurity causing a broadening of the d bands and hence a smearing of the interband threshold or alternatively it may be caused by a d band to Fermi level transition along the Δ direction close to the X point.     

Trends in the density of states of hydrogen chemisorbed on the transition metal series

Extensive many-body calculations of the ground state properties of hydrogen chemisorbed on all 3d, 4d and 5d transition metal surfaces are performed. The trends of the location E_p of the discrete level under the bottom of the d-band are presented. It is found that, on going to the right of the series, E_p approaches the bottom of the d-band, eventually merges in it and detaches again from it for the last elements of the series.     

Increase in the magnetostrictive susceptibility of Tb0.3Dy0.67Ho0.03Fe2−x Co x alloys upon substitution of cobalt for iron

This paper reports on the results of investigations into the concentration dependences of the magnetostrictive susceptibility, the magnetostriction, the magnetization, and the Curie temperature for Tb_0.3Dy_0.67Ho_0.03Fe_2?x Co _x alloys upon substitution of cobalt for iron. It is revealed that the temperature of the spin-reorientation transition shifts toward room temperature with an increase in the cobalt content in the range 0 ≤ x ≤ 1.3. Substitution of cobalt for iron in the alloys leads to a decrease in the contribution of the 3d transition metal sublattice to the magnetic anisotropy owing to the opposite signs of the single-ion anisotropy constants for iron and cobalt. The decrease observed in the magnetocrystalline anisotropy compensated in both rare-earth and 3d transition metal sublattices is responsible for the high magnetostrictive susceptibility of the studied compounds at a high cobalt content (x = 1.3) in the room-temperature range.     

On the mechanical stability of bcc transition elements and alloys

A volume independent and a volume dependent lattice energy function involving short-range interatomic potentials were able to be fitted to the elastic constants, cohesive energy, lattice parameter and for the latter function to the vacancy formation energy and bcc-fcc lattice stability energy, as well, for fcc metals and bcc alkali metals, but not to the cohesive energy and C' elastic constant of bcc transition metals. The assumption that directional, but partial, covalent bonds exist between nearest-neighbors in the bcc transition elements provides an explanation for the latter results and in addition explains the identical dependence of C' and the bcc-fcc lattice stability upon N_d, where N_d is the average number of d electrons, for the bcc transition metals and alloys. Both the mechanical and thermodynamic stability of the bcc structure for transition metals and all transition metal alloys disappears for 5 ? N_d > 2 and <?1.     

Theoretical investigation of EPR and optical spectra of Mo(V) in [Mo6O19][N(C4H9)4]3 salt

The electron paramagnetic resonance (EPR) spectral data (the g factors and hyperfine structure constants) and d–d transition spectra for the tetragonal Mo⁵⁺ centre in [Mo₆O₁₉][N(C₄H₉)₄]₃ salt are theoretically investigated from the complete diagonalization method (CDM) for a 4d¹ ion in tetragonally compressed octahedron. The theoretical results are in good agreement with the experimental data. The dependency of the g factors of the ground state on the R_∥(MoO bond length) has been studied. It is shown that the g factors varied with the R_∥ approximately in a linear way.     

Gammaübergangswahrscheinlichkeit und Quasiteilchenapproximation

The transition probability of the isometricM4-transitions 1g _9/2-2p _1/2, 1h _11/2-2d _3/2 and 1i _13/2-2f _5/2 in near magic nuclei is calculated on the basis of the theory of finite Fermi systems. It is shown, that the constantg ₀ for the spin-spin interaction of quasiparticles, depends strongly on the special kind of representation for the single particle states. Using oscillator functions the valueg ₀=3.3 has to be taken, in order to get hindrance factors, which are approximately equal to one, whereas in the case of square well functions the constantg ₀=1.4 is necessary, in order to get agreement with the experimental transition probabilities.     

BaAl12O19:Mn green emitting nanophosphor for PDP application synthesized by solution combustion method and its Vacuum Ultra-Violet Photoluminescence Characteristics

Nanostructured BaAl₁₂O₁₉:Mn²⁺ phosphor particles of nano-rod morphology with diameter 40-100 nm and length up to 200-600 nm has been synthesized by solution combustion method and its photoluminescence characteristics have been studied by Vacuum Ultra-Violet Photoluminescence spectrometer (VUVPL) under 147 nm excitation. The crystallographic phase purity of BaAl₁₂O₁₉:Mn²⁺ nanostructured phosphor particle synthesized by solution combustion approach is confirmed by X-ray diffraction (XRD). The broadening of XRD diffraction peaks indicates nanocrystalline nature of particles present in powder. The emission spectrum of BaAl₁₂O₁₉:Mn²⁺ nanophosphor on 147 nm excitation consists of a wide green band with a peak at about 515 nm, which is due to a 3d⁵ (⁴T_1g)-3d⁵ (⁶A_1g) transition corresponds of Mn²⁺ ions. It is found that the concentration quenching is obtained when Mn²⁺ content (x) is 0.05 in BaAl₁₂O₁₉:xMn²⁺ nanophosphor on 147 nm excitation. The decay time of 3d⁵ (⁴T_1 g)-3d⁵ (⁶A_1 g) transition of Mn²⁺ ions at 147 nm excitation is about 23 ms for BaAl₁₂O₁₉:Mn²⁺ nanophosphor. This nanostructured green emitting BaAl₁₂O₁₉:Mn²⁺ phosphor can find potential application in Plasma Display Panels (PDPs) and mercury-free fluorescent lamps.     

Numerical Study on the Coagulation and Breakage of Nanoparticles in the Two-Phase Flow around Cylinders

The Reynolds averaged N-S equation and dynamic equation for nanoparticles are numerically solved in the two-phase flow around cylinders, and the distributions of the concentration M₀ and geometric mean diameter d_g of particles are given. Some of the results are validated by comparing with previous results. The effects of particle coagulation and breakage and the initial particle concentration m₀₀ and size d₀ on the particle distribution are analyzed. The results show that for the flow around a single cylinder, M₀ is reduced along the flow direction. Placing a cylinder in a uniform flow will promote particle breakage. For the flow around multiple cylinders, the values of M₀ behind the cylinders oscillate along the spanwise direction, and the wake region in the flow direction is shorter than that for the flow around a single cylinder. For the initial monodisperse particles, the values of d_g increase along the flow direction and the effect of particle coagulation is larger than that of particle breakage. The values of d_g fluctuate along the spanwise direction; the closer to the cylinders, the more frequent the fluctuations of d_g values. For the initial polydisperse particles with d₀ = 98 nm and geometric standard deviation σ = 1.65, the variations of d_g values along the flow and spanwise directions show the same trend as for the initial monodisperse particles, although the differences are that the values of d_g are almost the same for the cases with and without considering particle breakage, while the distribution of d_g along the spanwise direction is flatter in the case with initial polydisperse particles.     

Compositional effects on the optical and thermal properties of sodium borophosphate glasses

A series of sodium borophosphate glasses of the composition (1−x)NaPO₃−xB₂O₃ have been synthesised from Na₂CO₃, B₂O₃ and P₂O₅ and their optical and thermal properties investigated. The results show that refractive index (n) and glass transition temperature (T_g) show a maximum at about B/(B+P)=0.6 while thermal expansion coefficient (α) and thermo-optic coefficient (dn/dT) change monotonically with the B/(B+P) ratio. These observations can be interpreted based on the incorporation of BO₃ and BO₄ units into the glass structural network.     

Structural instability and superconductivity in niobium-titanium alloys

Niobium-titanium is the most widely used technical superconductor. Titaniumrich transition metal alloys, quenched from high temperatures, can generally be retained in the bccβ phase. This phase is metastable and the instability is relieved by a variety of low temperature structural transformations. This aspect has been investigated using x-ray, TEM, low temperature resistivity,T _c and dH _c2/dT studies, in a series of Nb-Ti alloys. The instability has been characterized by the normal state resistivityρ _n and dρ/dT. The commercially used Nb-Ti alloys are Ti rich per atom-wise. This stems basically from the anomalous increase in the normal state resistivityρ _n as the Ti concentration is increased. This is a consequence of a dynamical process through which theβ phase instability tends to be relieved leading to athermal ω precipitation. The resulting anomalous resistivity behaviour can be understood in terms of a ‘two-level system’ model generally invoked for amorphous materials. It has also been possible to induce instability towards athermal ω precipitation in a system spontaneously undergoing a martensitic transformation to become stable. Thus in an alloy of Nb-83 at % Ti, addition of 1% nitrogen has suppressed the martensitic transformation, giving a three-fold increase inρ _n (about 150µΘ cm), the highest known in Nb-Ti so far. The increase in the normal state resistivity has beneficial effects on the upper critical field. From studies on several Nb-Ti alloys, it is inferred that a peak inH _c2(0) occurs at 17–18 tesla at aρ _n value of 100µΘ cm. It is pointed out that in the present commercial alloys, the sequence of thermo-mechanical treatments given to optimizeJ _c, restrictsρ _n, perhaps owing to the partial relieving of the metastability of theβ phase. They are therefore non-optimized with respect toH _c2.     

An interpretation of the g factors for the tetragonally-compressed Cr centers in YVO4 and YPO4 crystals

The g factors g_∥ and g_⊥ for the tetragonally-compressed (CrO₄)³⁻ clusters in YMO₄ (M=V, P) crystals are calculated from the high-order perturbation formulas based on the two-mechanism model for the compressed d¹ tetrahedra with the ground state |d_z²〉. From the calculated values and by considering a small admixture of the first excited state |d_x²−y²〉 to the ground state |d_z²〉 due to the vibrational motion of ligands (which leads a twinkling compressed tetrahedron to become an elongated one), the observed g_∥ and g_⊥ for Cr⁵⁺ centers in YMO₄ crystals are explained reasonably. The difficulty of the large deviations of g_∥ from g_e (≈2.0023) in the two systems is therefore removed and the above dynamic effect may be the cause which results in the large deviation of g_∥ from g_e for some (CrO₄)³⁻ clusters in crystals.     

X-ray magnetic circular dichroism at IrL2,3 edges in Fe100−x Ir x and Co100−x Ir x alloys: Magnetism of 5d electronic states

The formation of induced 5d magnetic moment on Ir in Fe_100−x Ir _x (x=3, 10 and 17) and Co_100−x Ir _x (x=5, 17, 25 and 32) alloys has been investigated by X-ray magnetic circular dichroism (XMCD) at Ir L_2,3 absorption edges. Sum rule analysis of the XMCD data show that the orbital moment of Ir is in the range of −0.071(2)μB to −0.030(1)μB in Fe-Ir alloys and −0.067(2)μB to 0.024(1)μB in Co-Ir alloys. We find that the total moment of Ir in Fe-Ir alloys is approximately 1/5 of the total 3d moment on Fe at all the three compositions. In contrast, the total moment on Ir in Co-Ir alloys varies between 1/6 to 1/16 of the 3d moment on cobalt. The observed trends of Ir moments and the role of interatomic exchange interactions in 5d moment formation are discussed.     

Quantum theory of radiation-pressure induced electrical currents: I. Free electrons

The maximum superconducting transition temperatures,T _c , of noncrystalline (Ti, Zr and Hf)-(3d metal) alloys are reported. The alloys have been produced by low temperature implantation of the 3d metals. The systematic behavior ofT _c as a function of 3d element content is discussed in comparison with metallic glasses and vapor-quenched films. A correlation between the recently observedd-band splitting in metallic glasses and superconducting behavior is pointed out.     

Renormalization group functions of the φ<Superscript>4</Superscript> theory in the strong coupling limit: Analytical results

The previous attempts of reconstructing the Gell-Mann-Low function β(g) of the φ⁴ theory by summing perturbation series give the asymptotic behavior β(g) = β_∞ g in the limit g→∞, where α = 1 for the space dimensions d = 2, 3, 4. It can be hypothesized that the asymptotic behavior is β(g) ～ g for all d values. The consideration of the zero-dimensional case supports this hypothesis and reveals the mechanism of its appearance: it is associated with vanishing of one of the functional integrals. The generalization of the analysis confirms the asymptotic behavior β(g) ～ g in the general d-dimensional case. The asymptotic behaviors of other renormalization group functions are constant. The connection with the zero-charge problem and triviality of the φ⁴ theory is discussed.     

Effect of pressure on superconductivity in transition metal alloys

Measurements on twelve alloys of the series Zr-Nb-Mo show a close correlation ofdT _c /dp with thed-band structure of these alloys and suggest that thed-band is virtually rigid with respect to pressure. The results are not compatible with an empirical observation of McMillan thatT _c is governed only by a phonon factor Mω ².     

Experimental valence-band study of Ti(NiCu) alloys with different compositions and crystal structures

The density of valence-band electronic states of Ti(NiCu) alloys with different crystal structures and elemental compositions has been studied by X-ray photoelectron spectroscopy. It has been established that the change in the crystal state initiated by a martensitic transformation or a transition from the amorphous state to the crystal state does not affect the valence-band electronic state density distribution of the Ti₅₀Ni₅₀ and Ti₅₀Ni₂₅Cu₂₅ alloys. It has been shown that a change in the elemental composition leads to a noticeable redistribution of the electronic density in alloys of the Ti₅₀Ni_{50 ? x} Cu _x system (x = 0, 10, 15, 25, 30, 38, 50 at. %). As the copper concentration in the Ti(NiCu) alloys increases, the contribution of the Ni d states in the vicinity of the Fermi level decreases, with the d band of nickel shifting toward higher binding energies, and that of copper, toward lower binding energies.