The superconducting properties of YBa2(Cu1−xMx)3O7 for M=Zn and Ni

Transverse and zero-field μSR measurements were made on YBa₂(Cu_1−xNi_x)₃O_7−y withx=0.1 and 0.2, and YBa₂(Cu_1−x Zn _x )₃O_7−y withx=0.03, 0.06, 0.1, and 0.16, wherey≈0.1. Since doping may lead to magnetic ordering this was searched for with both zero and transverse field μSR, but no evidence was found over the temperature range studied: 10–100 K. However, depolarization rates as functions of temperature were obtained, and the low temperature values of these are σ=3.2 μs⁻¹.1.6μs⁻¹, and 1 μs⁻¹ forx=0.01, and 0.2 Ni, respectively, and σ=0.8 μs⁻¹, 0.75 μs⁻¹, 0.65 μs⁻¹, and 0.4 μs⁻¹ forx=0.03, 0.06, 0.1, and 0.16 Zn, respectively. Estimates for the effect of decreasing electron concentration for Zn are made, but these alone do not account for the drop in σ. Estimates for the effect of scattering on λ and hence σ are made. The reduction in σ for Ni dopant is in surprisingly good agreement with these estimates. For Zn the order of magnitude is correct, but the relative lack of further change in σ after the effect of the first 0.03 addition seems to imply a saturation of the effect of scattering.     

Near-IR laser inelastic electronic light scattering spectroscopy on transitions between ground and excited states of acceptor centers in GaAs and InP crystals

We report the development of a method for recording the low-temperature (T=6 K) near-IR inelastic light scattering spectra and the observation of electronic scattering on the transitions 1s _3/2(Γ₈) → 2s _3/2(Γ₈) between the ground and excited states of different shallow acceptor centers in a n-type semi-insulating crystal si-GaAs (n=1.0 × 10⁸ cm⁻³) and in a doped p-InP crystal (p=3.6×10¹⁷ cm⁻³). Moreover, a new line, associated with the transition 1s _3/2(Γ ₈) → 2p _3/2(Γ₈) and due to a dielectric local mode, recorded for the first time in the spectra of narrow-gap semiconductors, was found in the residual-frequency band in the p-InP spectrum between TO(Γ) and LO (Γ) phonons. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 5, 334–339 (10 March 1998)     

Pseudovector heavy quarkonia in e + e −2 collisions

It is shown that BR(χ _b1(1 P)→e ⁺ e ⁻)≃3.3· 10⁻⁷ and BR(χ _c1(1 P)→e ⁺ e ⁻)≃10⁻⁸. This gives realistic possibilities for searching for the production of χ _b1(1 P) and ξ _c1(1 P) states in e ⁺ e ⁻ collisions, even on the present-day colliders, to say nothing of b and c-τ factories. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 8, 569–574 (25 April 1996)     

Temperature dependences of the high-field magnetization of dilute frustrated ferrimagnetic spinels

Results are presented of an investigation of the magnetic properties of dilute frustrated ferrimagnetic spinels Li_0.5Fe_2.5−x Ga_xO₄ (x=0.8–1.2), which characterize the main parameters of the ferrimagnetic state and provide evidence of local violation of collinear spin ordering and frustrations. In particular, measurements were made of the concentration dependences of the magnetic moment n ₀(x) and the Curie point T _c (x), the magnetization isotherms σ _T (H) at T=4.2 K and H⩽10 kOe, and also the low-and high-field magnetization polytherms σ _H (T). It was established that for x⩾0.8 in fields exceeding the technical saturation field H _s ∼2 kOe, the temperature dependences of the high-field magnetization σ _H (T) between 4.2 and 230 K cannot be described by the Bloch T ^3/2 law whereas this law is satisfied for undiluted Li spinel (x=0). Over the entire temperature range (4.2–230 K) the experimental curves σ _H (T) may be approximated by σ _H (T)=σ ₀(1−AT ^3/2 − BT ^5/2) for x=0.8–1.0 and σ _H (T)=σ ₀[1−CT ^3/2exp(μ(H−H ₀)/k _B T)] for x=1.1, 1.2, where μH ₀∼15 K is the internal field produced by competition between exchange interactions and frustrations. Fiz. Tverd. Tela (St. Petersburg) 40, 1075–1079 (June 1998)     

Tails in harnesses

We consider space- and time-uniformd-dimensional random processes with linear local interaction, which we call harnesses and which may be used as discrete mathematical models of random interfaces. Their components are rea random variablesa _s ^t , wheres ∈ Z ^d andt=0, 1, 2.,... At every time step two events occur: first, every component turns into a linear combination of itsN neighbors, and second, a symmetric random i.i.d. “noise”v is added to every component. For any σ ∈Z _d ⁺ define Δ_σ a′ _s as follows. If σ=(0,...,0), σ=(0,...,0), Δ_σ a _s ^t =a _s ^t . Then by induction, wheree _i is thed-dimensional vector, whoseith component is one and other components are zeros. Denote |σ| the sum of components of σ. Call a real random variable ϕ symmetric if it is distributed as −ϕ. For any symmetric random variable ϕpower decay or P-decay is defined as the supremum of thoser for which therth absolute moment of ϕ is finite. Convergence a.s., in probability and in law whent→∞ is examined in terms of P-decay(v): Ifd=1, σ=0 ord=2, σ=(0,0), Δ_σ a _s ^t diverges. In all the other cases: If P-decay(v)<(d+2)/(d+|σ|), Δ_σ a _s ^t diverges; if P-decay(v)>(d+2)/(d+|σ|), Δ_σ a _s ^t , converges and P-decay(ν) For any symmetric random variable ϕexponential decay or E-decay is defined as the supremum of thoser for which the expectation of exp(|x|^r) is finite. Let E-decay(v)>0. Whenever Δ_σ a _s ^t converges (that is, ifd>2 or |σ|>0: Ifd>2, E-decay(lima _s ^t )=min(E-decay(v),d+2/2); if |σ|=1, E-decay (lim Δ_σ a _s ^t )=min(E-decay(ν),d+2); if |σ| ⩾, E-decay (lim Δ_σ a _s ^t )=E-decay(ν).     

Reciprocity relations for the effective electrical conductivity of randomly inhomogeneous media in the fractal regime

An exact relation for the realization-averaged effective conductivities in the fractal region is found for two-dimensional randomly inhomogeneous media. It has the form {σ _e (τ,L)~× {1/σ _e (−τ,L)~⁻¹=σ _e ² (τ=0, L≫ξ), where ξ is the correlation length (the self-averaging scale), L is the size of the system, τ=(p-p _c )/p _c , and p _c is the percolation threshold. For L≫ ξ, the system is self-averaged, and the relation transforms into the Dykhne reciprocity relation, A. M. Dykhne, Zh. éksp. Teor. Fiz. 59, 110 (1970) [Sov. Phys. JETP 32, 63 (1971)] σ _e (τ)σ _e (−τ])=σ _e ² (τ=0)= σ ₁ σ ₂. A similar relation is obtained for media with an exponentially broad distribution of local conductivities, as well as for individual realizations of some deterministic structures. Zh. éksp. Teor. Fiz. 113, 1484–1490 (April 1998)     

Progress in pion-decay channelling: Refractory bcc metals at high and low temperatures

By means of π⁺/μ⁺ channelling, positive pions (π⁺) implanted intoTa, Mo, andW are investigated up to high temperatures. A striking observation is that the channelling effect disappears in a rather narrow temperature interval centred at 0.26 (Ta) to 0.51 (W) of the melting temperature. From studies of π⁺ trapping by oxygen atoms inTa estimates for the low-temperature π⁺ diffusivity inTa [D _π(23K)=1.4·10^−10±0.3 m²s⁻¹,D _π(47K)=5.7·10^−10±0.3 m²s⁻¹] as well as for the binding enthalpy of π⁺ to 0 atoms (H _B=7·10⁻² eV) have been obtained. The diffusion data are in reasonable agreement with the theory of phonon-assisted tunnelling.     

Nature of conduction via impurities in uncompensated silicon

The static conductivity σ(E) and photoconductivity (PC) at radiation frequencies ħω=10 and 15 meV in Si doped with shallow impurities (density N=10¹⁶−6×10¹⁶ cm⁻³, ionization energy ε₁≃45 meV) with compensation K=10⁻⁴−10⁻⁵ in electric fields E=10–250 V/cm are measured at liquid-helium temperatures T. Special measures are taken to prevent the high-frequency part of the background radiation (ħω>16 meV) from striking the sample. It is found that the conductivity σ(E) is due to carrier motion along the D ⁻ band, which is filled with carriers under the influence of the field E. In fields E<E _q (E _q ≃100–200 V/cm) the carrier motion consists of hops along localized D ⁻ states in a 10–15 meV energy band below the bottom of the free band (energy ε=ε₁); for E>E _q carriers drift along localized D ⁻ states with energy ε∞ε₁−10 meV. An explanation is proposed for the threshold behavior of the field dependence of the photo-and static conductivities. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 4, 232–236 (25 August 1997)     

The hyperfine splittings in heavy-light mesons and quarkonia

Hyperfine splittings (HFS) are calculated within the Field Correlator Method, taking into account relativistic corrections. The HFS in bottomonium and the B _q (q = n, s) mesons are shown to be in full agreement with experiment if a universal coupling α _HF = 0.310 is taken in perturbative spinspin potential. It gives M(B*) −M(B) = 45.7(3) MeV, M(B _s ^* ) − M(B _s ) = 46.7(3) MeV (n _f = 4), while in bottomonium Δ_HF(b $ \bar b $ \bar b ) = M(Υ(9460)) − M(η _b (1S)) = 63.4 MeV for n _f = 4 and 71.1 MeV for n _f = 5 are obtained; just the latter agrees with recent BaBar data. For unobserved excited states we predict M(Υ(2S))−M(η _b (2S)) = 36(2)MeV,M(Υ(3S))−M(η _b (3S)) = 28(2)MeV, and also M(B _c ^*) = 6334(4) MeV, M(B _c (2S)) = 6868(4) MeV, M(B _c ^* (2S)) = 6905(4) MeV. The mass splittings between D(2³ S ₁) − D(2¹ S ₀), D _s (2³ S ₁) − D _s (2¹ S ₀) are predicted to be ∼75 MeV, which are significantly smaller than in several other studies but agree with the mass splitting between recently observed D(2533) and D*(2610).     

Observational constraint on generalized Chaplygin gas model

We investigate observational constraints on the generalized Chaplygin gas (GCG) model as the unification of dark matter and dark energy from the latest observational data: the Union SNe Ia data, the observational Hubble data, the SDSS baryon acoustic peak and the five-year WMAP shift parameter. The result is obtained that the best-fit values of the GCG model parameters with their confidence level are A _s=0.73_−0.06^+0.06 (1σ) _−0.09^+0.09 (2σ), α=−0.09_−0.12^+0.15 (1σ) _−0.19^+0.26 (2σ). Furthermore, in this model, we can see that the evolution of equation of state (EOS) for dark energy is similar to quiessence, and its current best-fit value is w _0de=−0.96 with the 1σ confidence level −0.91≥w _0de≥−1.00.     

Possible molecular states of D*s[`(D)]*sD^{*}_{s}\bar{D}^{*}_{s} system and Y(4140)

The interpretation of Y(4140) as a D^*_s[`(D)]<sup>*</sup><sub>s</sub>D^{*}_{s}\bar{D}^{*}_{s} molecule is studied dynamically in the one boson exchange approach, where σ, η and φ exchange are included. Ten allowed D<sup>*</sup><sub>s</sub>[`(D)]^*_sD^{*}_{s}\bar{D}^{*}_{s} states with low spin parity are considered, and we find that the J ^PC =0⁺⁺, 1⁺⁻, 0⁻⁺, 2⁺⁺ and 1⁻⁻ D^*_s[`(D)]<sup>*</sup><sub>s</sub>D^{*}_{s}\bar{D}^{*}_{s} configurations are most tightly bound. We suggest that the most favorable quantum numbers are J <sup>PC</sup> =0<sup>++</sup> for Y(4140) as a D<sup>*</sup><sub>s</sub>[`(D)]^*_sD^{*}_{s}\bar{D}^{*}_{s} molecule; however, J ^PC =0⁻⁺ and 2⁺⁺ cannot be excluded. We propose to search for the 1⁺⁻ and 1⁻⁻ partners in the J/ψ η and J/ψ η′ final states, which is an important test of the molecular hypothesis of Y(4140) and the reasonability of our model. The 0⁺⁺ B^*_s[`(B)]^*_sB^{*}_{s}\bar{B}^{*}_{s} molecule should be deeply bound; experimental search in the ϒ(1S)φ channel at Tevatron and LHC is suggested.     

Evaluated cross sections of the σ(γ, nX) and σ(γ, 2nX) reactions on 112, 114, 116, 117, 118, 119, 120, 122, 124Sn isotopes

A combined analysis of experimental data on total and partial photoneutron reaction cross sections, obtained using bremsstrahlung γ-radiation and quasi-monoenergetic annihilation photon beams, was performed for nine Sn isotopes. The partial reactions σ^eval(γ, nX) and σ^eval(γ, 2nX) cross sections were evaluated using an approach free of the shortcomings of experimental neutron multiplicity sorting methods. This approach involves calculations within the photonuclear reaction model, based on Fermi gas densities and considering the effects of nucleus deformation, the isospin splitting of its giant dipole resonance (GDR), and experimental data on the total photoneutron cross sections σ^exp(γ, xn) = σ^exp(γ, nX) + 2σ^exp(γ, 2nX) = σ^exp(γ, n) + σ^exp(γ, np) + … + 2σ^exp(γ, 2n) + 2σ^exp(γ, 2np) + …. The evaluated σ^eval(γ, nX) and σ^eval(γ, 2nX) reactions cross sections were obtained using the introduced transition multiplicity functions F ^theor = σ^theor(γ, 2nX)/σ^theor(γ, xn) = σ^theor(γ, 2nX)/[σ^theor(γ, nX) + 2σ^theor(γ, 2nX) + …]; and σ^eval.(γ, 2nX) = F ^theor.σ^exp.(γ, xn) = σ^eval(γ, nX) = (1 − 2F ^theor)σ^exp(γ, xn). The evaluated partial reaction cross sections were used to assess the total photoneutron reaction cross sections σ^eval(γ, sn) = σ^eval(γ, nX) + σ^eval(γ, 2nX) + … as functions of the mass number A. The GDR features of ^{112, 114, 116, 117, 118, 119, 120, 122, 124}Sn isotopes were studied and are discussed here.     

Zeeman effect in the luminescence spectra of terbium-yttrium-aluminum garnet

The Zeeman effect is investigated in the green luminescence band corresponding to the 4f→4f radiative transition ⁵D₄ → ⁷F₅ of Tb ³⁺ ions in Tb _0.2 Y _2.8 A _l5 O ₁₂ terbium-yttrium-aluminum garnet at the temperature T = 85 K. It is demonstrated that an external field causes not only the Zeeman shifts of the resonant frequencies of radiative transitions in the luminescence spectra but also changes in the emission line intensities caused by modification of the optical transition probabilities in the σ₋ and σ₊ opposite circular polarization states. Special features of field dependences of the Zeeman effect for the emission lines are qualitatively explained based on the results of numerical energy spectrum calculations for the ⁵D₄ and ⁷F₅ multiplets of the Tb ³⁺ ion in yttrium-aluminum garnet in the geometry of the longitudinal and transverse Zeeman effects. The possibility of nonequilibrium population of sublevels of the (Γ₁, Γ₄) quasi-doublet state of the ⁵D₄ multiplet with energy of ∼20585 cm⁻¹ is demonstrated in an external magnetic field. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 38–45, June, 2008.     

Transport properties of hexanoyl chitosan-based gel electrolyte

Films of hexanoyl chitosan-based polymer electrolyte were prepared by the technique of solution casting. The effect of plasticizer on the transport properties of hexanoyl chitosan:lithium trifluoromethanesulfonate (LiCF₃SO₃) electrolytes have been investigated. The plasticizer used was ethylene carbonate (EC). The highest room temperature conductivity achieved in the EC-plasticized hexanoyl chitosan-based electrolytes is 2.75×10⁻⁵ S cm⁻¹. The Rice and Roth model was used to explain the variations in the dc conductivity observed. The exponent, s, in Jonscher’s universal power law equation σ(ω)=σ _dc+Aω ^s , was analyzed as a function of temperature for the sample containing 30 wt% of EC. The analysis suggests that the conduction mechanism follows that proposed by the overlapping large polaron tunneling model.     

Overall values of conventional discrepancy indices for crystals with heavy atoms. Part II. Results for monoclinic and orthorhombic crystals when the heavy-atom part alone constitutes the model

Theoretical expressions of 〈y _N〉, 〈|y _N − σ₁ y _P ^c |〉 and 〈|y _N ² −σ ₁ ² (y _P ^c )²|〉 (wherey _N andy _P ^c are the normalized structure amplitudes of the structure and the model respectively) are derived in terms of the heavy atom contributionσ ₁ ² for monoclinic and orthorhombic crystals containing a few (i.e., 1 or 2) heavy atoms of the same kind per asymmetric unit by taking the heavy atom part alone as the model. Results are obtained for both the related and unrelated cases. The local values of 〈y _N〉 and 〈|y _N ⁿ − σ ₁ ⁿ (y _P ^c ) ⁿ |〉, (n=1, 2) calculated from these expressions can be used to calculate the overall values of the conventionalR-indicesR(F) andR(I) for the related and unrelated cases. These overall values could be used to check the correctness of heavy atoms located in the structure. Contribution No. 550     

Nonadiabatic effects in the phonon spectra of superconductors

The nonadiabatic corrections to the self-energy part Σ_s(q, ω) of the phonon Green’s function are studied for various values of the phonon vectors q resulting from electron-phonon interactions. It is shown that the long-range electron-electron Coulomb interaction has no direct influence on these effects, aside from a possible renormalization of the corresponding constants. The electronic response functions and Σ_s(q, ω) are calculated for arbitrary vectors qand energy ω in the BCS approximation. The results obtained for q=0 agree with previously obtained results. It is shown that for large wave numbers q, vertex corrections are negligible and Σ_s(q, ω) possesses a logarithmic singularity at ω=2Δ, where Δ is the superconducting gap. It is also shown that in systems with nesting, Σ_s(Q, ω) (where Q is the nesting vector) possesses a square-root singularity at ω=2Δ, i.e., exactly of the same type as at q=0. The results are used to explain the recently published experimental data on phonon anomalies, observed in nickel borocarbides in the superconducting state, at large q. It is shown, specifically, that in these systems nesting must be taken into account in order to account for the emergence of a narrow additional line in the phonon spectral function S(q, ω)≈−π ⁻¹ Im D _s (q, ω), where D _s (q, ω) is the phonon Green’s function, at temperatures T<T _c . Zh. éksp. Teor. Fiz. 115, 1799–1817 (May 1999)     

Superconductivity and magnetic ordering in YBa2(Cu1−x Fe x )3O7

Transverse-and zero-field μSR measurements have been made for YBa₂(Cu_1−x Fe _x )₃O₇ withx=0.04, 0.08 and 0.12. The temperature range studied was from approximately 7.5 K to 100 K. The onset of magnetic ordering commences at about 7.5 K forx=0.04, 10 K forx=0.08 and 20 K forx=0.12. The Gaussian depolarization parameter, σ ofG _x (t) = exp(−σ² t ²/2), is depressed by a factor of about 0.6 forx=0.04, but for thex=0.08 sample σ is depressed by a factor of 10 and increasing suppression is seen as the temperature is lowered below 45 K. This decrease in σ is interpreted in terms of decreasing electronic mean free paths.     

Absolute cross sections of electron attachment to molecular clusters. Part II: Formation of (H2O) N? , (N2O) N? , and (N2) N?

Absolute cross sections σ⁻(E, N) of electron attachment to clusters (H₂O) _N , (N₂O) _N , and (N₂) _N for varying electron energy E and cluster size N are measured by using crossed electron and cluster beams in a vacuum. Continua of σ⁻(E) are found that correlate well with the functions of electron impact excitation of molecules’ internal degrees of freedom. The electron is attached through its solvation in a cluster. In the formation of (H₂O) _N ⁻ , (N₂O) _N ⁻ , and (N₂) _N ⁻ , the curves σ⁻(N) have a well-defined threshold because of a rise in the electron thermalization and solvation probability with N. For (H₂O)₉₀₀, (N₂O)₃₅₀, and (N₂)₂₆₀ clusters at E = 0.2 eV, the energy losses by the slow electron in the cluster are estimated as 3.0 × 10⁷, 2.7 × 10⁷, and 6.0 × 10⁵ eV/m, respectively. It is found that the growth of σ⁻ with N is the fastest for (H₂O) _N and (N₂) _N clusters at E → 0 as a result of polarization capture of the s-electron. Specifically, at E = 0.1 eV and N = 260, σ⁻ = 3.0 × 10⁻¹³ cm² for H₂O clusters, 8.0 × 10⁻¹⁴ cm² for N₂O clusters, and 1.4 × 10⁻¹⁵ cm² for N₂ clusters; at E = 11 eV, σ⁻ = 9.0 × 10⁻¹⁶ cm² for (H₂O)₂₀₀ clusters, 2.4 × 10⁻¹⁴ cm² for (N₂O)₃₅₀ clusters, and 5.0 × 10⁻¹⁷ cm² for (N₂)₂₆₀ clusters; finally, at E = 30 eV, σ⁻ = 3.6 × 10⁻¹⁷ cm² for (N₂O)₁₀ clusters and 3.0 × 10⁻¹⁷ cm² for (N₂)₁₂₅ clusters. Original Russian Text ? A.A. Vostrikov, D.Yu. Dubov, 2006, published in Zhurnal Tekhnicheskoĭ Fiziki, 2006, Vol. 76, No. 12, pp. 1–15.     

Autoionization cross section of lithium atoms excited by electron impact

We measure the full autoionization cross section of lithium atoms excited by electrons in the energy range from the first autoionization threshold at 56.39 to 600 eV. Data are obtained by determining the total intensity of electron spectrum of autoionization states 1sn ₁ l ₁ n ₂ l ₂ detected at the “magic” observation angle of 54.7°. The cross section behavior is characterized by a sharp increase to a maximum value of 1.7 × 10⁻¹⁸ cm² in the energy interval of 56.4–60 eV and a subsequent monotonic drop to a value of 10⁻¹⁸ cm² at 600 eV. We have discovered a “thin” cross section structure that reflects the presence of strong resonances of Li⁻ ions in the near-threshold area of excitation of the lowest energy autoionization states (1s2s2)2S, (1s2s2p)⁴ P, 1s(2s2p ³ P)² P), and 1s(2s2p ¹ P)² P. We have established that the contribution of autoionization to the absolute cross section of single ionization of lithium atoms does not exceed 4%. We perform a comparative analysis of the data with analogous data for potassium and cesium atoms.     

Absence of Magnetism in Continuous-Spin Systems with Long-Range Antialigning Forces

We consider continuous-spin models on the d-dimensional hypercubic lattice with the spins σ _x a priori uniformly distributed over the unit sphere in ℝ ⁿ (with n≥2) and the interaction energy having two parts: a short-range part, represented by a potential Φ, and a long-range antiferromagnetic part λ|x−y|^−s σ _x ⋅σ _y for some exponent s>d and λ≥0. We assume that Φ is twice continuously differentiable, finite range and invariant under rigid rotations of all spins. For d≥1, s∈(d,d+2] and any λ>0, we then show that the expectation of each σ _x vanishes in all translation-invariant Gibbs states. In particular, the spontaneous magnetization is zero and block-spin averages vanish in all (translation invariant or not) Gibbs states. This contrasts the situation of λ=0 where the ferromagnetic nearest-neighbor systems in d≥3 exhibit strong magnetic order at sufficiently low temperatures. Our theorem extends an earlier result of A. van Enter ruling out magnetized states with uniformly positive two-point correlation functions.