McMillan-Rowell tunneling spectroscopy in the Nb-Zr system

We report the first full application of tunneling spectroscopy to a superconducting transition metal alloy: Nb_1?xZr_x at x = 0.25, corresponding to the maximum T_c in the Nb-Zr system. The spectral function α²F(ω) and related parameters, when compared to those for the Nb, confirm that the increase in T_c from 9.22 K (x = 0) to 10.8 K(x = 0.25) arises largely by softening of the effective phonon spectrum.     

The ratio of the charm structure functions F k c (k = 2, L) at low x in deep inelastic scattering with respect to the expansion method

We study the expansion method for the gluon distribution function at low x values and calculate the charm structure functions in the LO and NLO analysis. Our results provide a compact formula for the ratio R ^c = F _L ^c /F ₂ ^c , which is approximately independent of x and the details of the parton distribution function at low x values. This ratio could be a good probe of the charm structure function F ₂ ^c in the proton deduced from the reduced charm cross sections at DESY HERA. These results show that the charm structure functions obtained are in agreement with HERA experimental data and other theoretical models.     

X-ray determination of the linear compressibilities of (SN)x

The linear compressibilities of (SN)_x have been determined up to 22 kbar using a hydrostatic-pressure X-ray cell: at 1 bar, k_a = 5.5;k_b = 2.2; k_c = 5.0; k_[201] = 3.8 —; all in units of 10^-12cm²/dyne. The results are compared with other chain-like solids and confirm the anisotropic three-dimensional bonding character of (SN)_x. Pressure dependence of the electrical conductivity is discussed.     

Renormalization of Hierarchically Interacting Isotropic Diffusions

We study a renormalization transformation arising in an infinite system of interacting diffusions. The components of the system are labeled by the N-dimensional hierarchical lattice (N≥2) and take values in the closure of a compact convex set $\bar D \subset \mathbb{R}^d (d \geqslant 1)$ . Each component starts at some θ ∈ D and is subject to two motions: (1) an isotropic diffusion according to a local diffusion rate g: $\bar D \to [0,\infty ]$ chosen from an appropriate class; (2) a linear drift toward an average of the surrounding components weighted according to their hierarchical distance. In the local mean-field limit N→∞, block averages of diffusions within a hierarchical distance k, on an appropriate time scale, are expected to perform a diffusion with local diffusion rate F ^(k) g, where $F^{(k)} g = (F_{c_k } \circ ... \circ F_{c_1 } )$ g is the kth iterate of renormalization transformations F _c (c>0) applied to g. Here the c _k measure the strength of the interaction at hierarchical distance k. We identify F _c and study its orbit (F ^(k) g) _k≥0. We show that there exists a “fixed shape” g* such that lim _k→∞ σ_k F ^(k) g = g* for all g, where the σ _k are normalizing constants. In terms of the infinite system, this property means that there is complete universal behavior on large space-time scales. Our results extend earlier work for d = 1 and $\bar D = [0,1]$ , resp. [0, ∞). The renormalization transformation F _c is defined in terms of the ergodic measure of a d-dimensional diffusion. In d = 1 this diffusion allows a Yamada–Watanabe-type coupling, its ergodic measure is reversible, and the renormalization transformation F _c is given by an explicit formula. All this breaks down in d≥2, which complicates the analysis considerably and forces us to new methods. Part of our results depend on a certain martingale problem being well-posed.     

Anionic conductivity and thermal stability of single crystals of solid solutions based on calcium fluoride

The temperature and composition dependencies of the anionic conductivity in the temperature range from ambient to 1073 K were studied for single crystals of Ca_1−xGd_xF_2+x (x=1×10⁻⁴, 1×10⁻³, 1×10⁻² and 1×10⁻¹) and of Ca_0.8R_0.2F_2.2 (R=La, Ce, Pr, Nd, Gd, Dy, Er, Tm, Yb, Lu and Y), having the fluorite structure. The conductivity plots for the concentrated Ca_0.8R_0.2F_2.2 solid solutions display the low-temperature and high-temperature linear (Arrhenius) regions with the knee temperature T_k∼770 K. The values of the conductivity activation enthalpy obey the relation ΔH_HT(T>T_k)>ΔH_LT(T<T_k). The conductivity mechanism in heavily doped Ca_1−xR_xF_2+x crystals is associated with the clusters of the point defects which decrease the potential barriers for fluoride anions moving by hops over the structural sites of the anion sub-lattice. We studied the effect of the dimensional factor (doped cation radii) on the anionic transport in these crystals. A correlation between anionic transport and atomic structure of the studied crystals is discussed.     

23Na nuclear magnetic resonance investigation of the lattice distortion in α-NaxV2O5

The full temperature dependence of the electric field gradient tensor at the Na sites has been determined by nuclear magnetic resonance (NMR) in the temperature range 8–330 K in α-Na_{x 2}O₅ (x = 0.996). Above the spin-Peierls transition (T _c = 34.7 K), only a single Na site is observed in agreement with the Pmmn space group proposed to describe this compound as the first example of a 1/4-filled ladder system. Below T_c, eight distinct quadrupolar²³Na sites are observed according to the distortion wave vector k_c = (1/2, 1/2, 1/4) previously reported. In addition, the opening of a spin gap is evidenced by a rapid drop of the magnetic hyperfine shift²³K at T_c. The results are discussed in the context of a charge-order-driven spin-Peierls transition.     

Searching for key parameter for determining Tc in Fe-based superconductors: Study of P/As substitution in RFe(P,As)(O,F) [R=La and Nd]

We have investigated the relation between the temperature-dependence of resistivity and superconducting transition temperature T_c in RFeP_1−xAs_xO_0.90F_0.10 (x=0-1.0) (R=La and Nd). In contrast to the linear change of the crystal structure with increasing x, the temperature dependence of resistivity and T_c show non-monotonous x-dependence. When the As concentration x is increased, the temperature-dependence of resistivity changes from T² to T-linear, and T_c distinctly increases in all the La compounds and the Nd ones with x<0.60. The results indicate that the substitution of As for P induces the spin fluctuation and resultantly enhances T_c. On the other hand, we could not find any relation between the temperature-dependence of resistivity and T_c in the Nd samples with x>0.60. This may suggest the existence of other parameters for determining T_c besides the antiferromagnetic correlation in this system.     

Anisotropic transferred hyperfine fields acting on 119Sn in gadolinium metal and alloys

Mossbauer studies of 0.1% ¹¹⁹Sn in gadolinium alloys at 4.1 K were performed. The spectra were well reproduced by a superposition of hyperfine fields according to the RKKY model with k_F = 1.27 ± 0.04 A^-1. In Gd_0.95Tm_0.05 where the magnetization is along the c-axis, the transferred field per Gd ion is 4 ± 1% lower than in Gd_0.97Tb_0.03 where the magnetization is perpendicular to the caxis.     

As-NQR study of superconductivity in LaRAsO1−xFx (R=Fe and Ni)

We report ⁷⁵As nuclear quadrupole resonance (NQR) studies on oxypnictide superconductors LaFeAsO_1−xF_x (x=0.08, 0.15) and LaNiAsO_1−xF_x (x=0, 0.06, 0.10, 0.12). In LaFeAsO_0.92F_0.08 (T_c=23 K), nuclear spin-lattice relaxation rate 1/T₁ shows no coherence peak just below T_c and decreases with decreasing temperature accompanied by a hump structure at T∼0.4T_c, which is a characteristic of the multigap superconductivity. In the normal state, the quantity 1/T₁T increases with decreasing temperature to T_c, indicating that the existence of antiferromagnetic correlation originating from its multiple electronic band structure. On the other hand, LaNiAsO_1−xF_x shows a clear Hebel-Slichter (coherence) peak just below T_c, evidencing that the LaNiAsO_1−xF_x is a BCS superconductor. In the normal state, 1/T₁T is constant in the temperature range for all LaNiAsO_1−xF_x, which indicates electron correlations are weak. We suggest that the contrasting behavior of both superconductivity and electron correlations in LaFeAsO_0.92F_0.08 and LaNiAsO_1−xF_x between them relate to the difference of electronic band structure configuration. We also provide a possible interpretation for the pseudogap-like behavior in the normal state observed in both compounds.     

Single-spin asymmetry of inclusive neutral-pion production in pp ↑ interactions at 70 GeV in the region −0.4 < x F < −0.1

For the kinematical region specified by the inequalities ?0.4 < x _F < ?0.1 and 0.9 < p _T < 2.5 GeV/c, the results are presented that were obtained by experimentally determining the single-spin asymmetry of inclusive neutral-pion production in the reaction p + p _↑ → π ⁰ + X at 70 GeV. According to these results, the asymmetry is close to zero in the region ?0.2 < x _F < ?0.1 and grows in magnitude with decreasing x _F, amounting to (?10.6 ± 3.2)% for ?0.4 <x _F < ?0.2.     

Ionic conductivity properties and 19F NMR investigation of Pb1−xCrxF2+x solid solutions with fluorite-type structure

The F⁻ ion transport and diffusion properties inside the solid solution Pb_1−xCr_xF_2+x (0≤x≤0.12) with fluorite-type structure have been determined by impedance spectroscopy and ¹⁹F NMR investigation. The various electrical parameters of Pb_1−xCr_xF_2+x are compared with those of Pb_1−xAl_xF_2+x (0≤x≤0.12) whose structure derives also from the fluorite-type. The Cr³⁺ substitutional cation is of a slightly larger size than the Al³⁺ cation but both cations offer the same usual coordination (CN=6) in ternary fluorides. Comparison of ¹⁹F NMR spectra of some compositions Pb_1−xM_x′F_2+x (M′=Al, Cr) at different temperatures has been realized in order to discover whether the clustering process proposed in Pb_1−xAl_xF_2+x can be extended to Pb_1−xCr_xF_2+x.     

Anomalous phonon intensities in the Raman spectrum of disordered CsMg1−xCoxCl3

The five Raman-active $\text{k}\text{= 0}$ phonons have been measured at low temperatures and for a range of x in the disordered lattice CsMg_1-xCo_xCl₃. While the E_2g^a, E_2g^b, E_2g^c and A_1g modes at 55.0, 132.0, 189.0 and 255.0 cm^?1 for x = 0 exhibit normal one-mode behaviour, the intensity of the E_1g phonon at 127.5 cm^?1 has a most unusual concentration dependence which requires a new theory.     

M-Zentrenbildung in röntgenbestrahlten KCl-Kristallen

The production of F- and M-centres in KCl by X-irradiation has been studied at temperatures between ?20 °C and 50 °C. The optical absorption measurements could be conducted without interrupting the X-irradiation. The results can be summarized as follows: 1. In Harshaw KCl crystals the number of F-centres created by the so-called fast coloration process was proportional to the height of the absorption band at 204 mμ prior to the irradiation. 2. The F-centres formed by the fast process did not contribute to the formation of M-centres. 3. In crystals with a strong absorption band at 204 mμ unstable M-centres were observed, which decayed rapidly after the cessation of the X-irradiation. Their concentration was found to be independent of the F-centre concentration. 4. At temperatures below 0 °C the relation between the concentration of the stable M-centres and the F-centre concentration could not any longer be represented by [M]=k ₁₂·[F₁]·[F₂]+k ₂₂·[F₂]², F₁ and F₂ referring to the F-centres created respectively by the fast and the slow coloration process. Except at very low F₂-centre concentrations however the relationship [M]=k ₀+k ₂·[F₂]² represented the experimental data at all temperatures between ?20 °C and 50 °C. At constant temperaturek ₂ varied withL, the X-ray energy absorbed per unit time and unit volume, according to 1/k ₂=a+bL+cL ². The temperature dependence ofa ^?1 b ^?1 andc ^?1 could be approximated by Boltzmann factors. The corresponding activation energies wereE _a=0.12 eV,E _b=0.53 eV,E _c=0.97 eV.     

Quark masses and specific scaling in deep-inelastic processes

Within an operator-product expansion, the problem of the quark-mass dependence of the heavy-flavor contribution to the structure function F ₂ in the current-fragmentation region is considered at high values of the momentum transfer squared Q ². A linear combination of the structure functions F ₂ ^c and F ₂ is found that possesses a specific scaling property for Q ² → ∞ at small fixed values of the Bjorken variable x. A lower bound on the ratio F ₂ ^c /F ₂ is calculated and is compared with data obtained at the HERA collider.     

Critical Behavior for Maximal Flows on the Cubic Lattice

Let F ₀ and F _m be the top and bottom faces of the box [0, k]×[0, l]×[0, m] in Z ³. To each edge e in the box, we assign an i.i.d. nonnegative random variable t(e) representing the flow capacity of e. Denote by Φ _k, l, m the maximal flow from F ₀ to F _m in the box. Let p _c denote the critical value for bond percolation on Z ³. It is known that Φ _k, l, m is asymptotically proportional to the area of F ₀ as m, k, l→∞, when the probability that t(e)>0 exceeds p _c , but is of lower order if the probability is strictly less than p _c . Here we consider the critical case where the probability that t(e)>0 is exactly equal to p _c , and prove that $$\mathop {{\text{lim}}}\limits_{k,l,m \to \infty } \frac{1}{{kl}}\Phi _{k,l,m} = 0{\text{ a}}{\text{.s and in }}L_1 $$ The limiting behavior of related to surfaces on Z ³ are also considered in this paper.     

Point-contact Andreev-reflection spectroscopy in ReFeAsO1−xFx (Re = La,Sm): Possible evidence for two nodeless gaps

A deep understanding of the character of superconductivity in the recently discovered Fe-based oxypnictides ReFeAsO_1?xF_x (Re = rare-earth) necessarily requires the determination of the number of the gaps and their symmetry in k space, which are fundamental ingredients of any model for the pairing mechanism in these new superconductors. In the present paper, we show that point-contact Andreev-reflection spectroscopy experiments performed on LaFeAsO_1?xF_x (La-1111) polycrystals with T_c ～ 27 K and SmFeAsO_0.8F_0.2 (Sm-1111) polycrystals with T_c ～ 53 K gave differential conductance curves exhibiting two peaks at low bias and two additional structures (peaks or shoulders) at higher bias voltages, an experimental situation quite similar to that observed by the same technique in pure and doped MgB₂. The single-band Blonder–Tinkham–Klapwijk model is totally unable to properly fit the conductance curves, while the two-gap one accounts remarkably well for the shape of the whole experimental dI/dV vs. V curves. These results give direct evidence of two nodeless gaps in the superconducting state of ReFeAsO_1?xF_x (Re = La, Sm): a small gap, Δ₁, smaller than the BCS value (2Δ₁/k_BT_c ～ 2.2–3.2) and a much larger gap Δ₂ which gives a ratio 2Δ₂/k_BT_c ～ 6.5–9.0. In Sm-1111 both gaps close at the same temperature, very similar to the bulk T_c, and follow a BCS-like behaviour, while in La-1111 the situation is more complex, the temperature dependence of the gaps showing remarkable deviations from the BCS behaviour at T close to T_c.The normal-state conductance reproducibly shows an unusual, but different, shape in La-1111 and Sm-1111 with a depression or a hump at zero bias, respectively. These structures survive in the normal state up to T¹ ～ 140 K, close to the temperatures at which structural and magnetic transitions occur in the parent, undoped compound.     

Production of γ by 194 GeV/c negative pions on tungsten

On the basis of 2000 ?, ?′ and ?″ events obtained in π^?-W interactions at 194 GeV/c we extract a value for the cross section times the branching ratio of B_μμσ = (0.96±0.04±0.17) pb nucleon^?1. The $\text{(?′ + ?″)}\text{?}$ ratio is found to be 0.53 ± 0.19. The observed differential x_F and P_T distributions are compared with those of the Drell-Yan continuum, and the cross section and the x_F distribution are compared with theoretical predictions.     

Electron-electron interaction and instabilities in one-dimensional metals

The possible instabilities of a 1-dimensional itinerant electron gas are discussed, assuming electron-electron interaction to play the dominant role. As is well known, in the RPA, a 1-dimensional metal is prone to spin density wave (SDW), charge density wave (CDW) and Cooper pair (CP) instabilities. The spin channel decomposition of the irreducible scattering amplitude I is made and the spin channel projections are evaluated in terms of the matrix elements of bare electron-electron interactionV(x) for momenta of interest. It is found that if the bare electron interactionV(x) is repulsive and decreases monotonically with separation, only the SDW instability will occur. If the small separation (x?(2k _F )^?1) part of the interaction is greatly reduced or is made attractive,V(x) is non-monotonic,V _q (q?2k _F ) is negative, and a CDW instability is preferred. A CP instability is possible if the electron interaction is attractive,i.e., if [V _q (0<q<k _F )+V _q (q?2k _F )]<0. The above RPA results serve only as rough indicators, since in general there are important two-electron configurations with two-electron momentum close to zero and with electron hole momentum close to 2k _F , an example being the near Fermi energy configurationk ₁?k _F ,k ₂??k _F ,k ₃??k _F k ₄?k _F . Therefore as pointed out first by Bychkov, Gorkov and Dzhyaloshinskii (BGD), cross channel coupling is especially significant. It is shown that the cross channel coupling is constructive is some cases,eg., exchange of CD fluctuations leads to an effective electron-electron spin singlet attraction and vice-versa. A formalism for studying such effects is set up, and the particular example mentioned above is discussed. An RPA-like approximation is made for the form of the reducible singlet electron hole scattering amplitudeγ _s ^d and the resulting induced Cooper pair attraction is calculated to be $$\begin{gathered} [I_s ^e ]_{ind.} \rho _{{}^\varepsilon F} = [ln(\lambda \beta \omega _c )]^{ - 1} ln\{ [1 + 2\pi ^{ - 1} ln(\lambda \beta \omega _c )^2 ]/ \hfill \\ 1 + [8\pi ^{ - 1} \gamma _s ^d (q = 2k_F )^{ - 1} )^2 ]\} \hfill \\ \end{gathered} $$ where λ=1.14,β=(k _B T)^?1 andω ₀ is an electronic energy cut-off ～ε _F . The induced electron hole attraction due to the exchange of virtual Cooper pairs has a similar expression, but with a factor of (1/4) and withγ _s ^e (q=0) replacingγ _s ^d (q=2k _F ). The induced Cooper pair attraction is seen to be quite large over a broad range of temperatures close to but aboveT _CDW [i.e., aboveT such thatγ _s ^d (q=2k _F )^?1=0]. There is no requirement thatγ _s ^d (q=2k _F ) andγ _s ^e (q=0) become singular at the same temperature, as found by BGD. The BGD prediction is seen to arise from the neglect of real particle hole and particle-particle excitations while calculatingγ _s ^d andγ _s ^e . The effect of impurities, of electron-phonon coupling, of interchain coupling and of interaction between thermal order parameter fluctuations is discussed. The results are then applied to a discussion of the properties of TTF-TCNQ, where it is suggested that a CDW instability occurs becauseV _q (q=2k _F )<0,i.e., because the small separation electron repulsion is strongly reduced by the highly polarizable TTF. Because of substantial interchain coupling, the bulk CDW instability occurs close to the RPA instability temperature. The giant conductivity observed by Colemanet al is attributed to superconductive fluctuations in a 1-dimensional system with large mean field superconductive transition temperatureT _CP ^MF of order 300°K. Such a largeT _CP ^MF is shown to result from the induced Cooper pair attraction due to CD fluctuation exchange.