A metastable de-excitation spectroscopy (MDS) study on oxygen adsorption on a polycrystalline zirconium surface

The adsorption of oxygen on a polycrystalline zirconium surface at room temperature has been studied by metastable de-excitation spectroscopy (MDS) in conjunction with UPS and AES. From the analysis of the measured spectra, we have shown the following. (1) At the initial stage of oxygen adsorption (exposure <1.2 L), the surface density of states (SDOS) of zirconium changes little at around the Fermi level (E_F), while it decreases appreciably at 1–2 eV below E_F (E_B=1–2 eV) by oxygen adsorption. (2) The SDOS at E_B=0–2 eV decreases with increasing oxygen exposure at >1.2 L and disappears at >8 L. (3) The oxygen 2p states (E_B=5–8 eV) are localized at the subsurface region at oxygen exposure 0–2 L. (4) The ZrO₂ phase appears at the outermost zirconium surface at around 2 L, then grows with increasing exposure, and finally dominates at >8 L. It is suggested that two different phases (ZrO₂ phase and that in which oxygen occupies subsurface sites) coexist at the outermost surface at 2–8 L.     

An empirical study of the Mn partial density of states in icosahedral Al70Pd20Mn10

Using the line-shape analysis of the Mn 2p_3/23d3d Auger transitions and by observing the Mn 3p→3d transitions in electron energy-loss spectroscopy, we have studied the local density of electronic states of the crystalline cubic and the quasicrystalline icosahedral phases of Al₇₀Pd₂₀Mn₁₀. We find that, in the cubic phase the Mn 3d states coincide with the Fermi level E_F, but in the quasicrystalline state the Mn 3d band is fully occupied and shifted to lower binding energies causing a significant reduction in the density of states at E_F.     

Minijet activity in high ET jet events at the tevatron

Gluon bremsstrahlung in scattering events with high transverse momentum jets is expected to increase markedly with the hardness (∑E_T) of the primary event. Within perturbative QCD we estimate a probability of order unity to see additional minijets with E_T 15 GeV in “dijet” events with ∑E_T > 400 GeV. The veto of such minijets is a promising background rejection tool for the Higgs search at the LHC.     

Electronic structure and Fermi surface topology of the infinite-layered superconductor Sr1−xCaxCuO2

Superconductivity reported at 110 K, and recently at 150–170 K, in the infinite-layered (Sr_xCa_1−x)CuO₂ system is investigated by means of the full potential linear muffin-tin orbital (FLMTO) method. As in other high-T_c cuprates, the electronic structure of the parent compounds, CaCuO₂ and SrCuO₂, and of the separately calculated composition Sr_0.7Ca_0.3CuO₂ using the experimental lattice parameters, displays strong 2D features including a low density of states at E_F (lower than in the other cuprates) and a simple 2D Fermi surface (rounded square) with strong nesting due to a single hybridized Cu d-O p band. As in La₂CuO₄, a major van Hove saddle-point singularity exists near E_F. The drastic changes of the Fermi surface when Ca/Sr vacancies shift the Fermi energy to the van Hove singularity may have a strong influence on the superconducting properties of the compounds and indicate the need for Sr/Ca vacancies in inducing the high T_c.     

A relation between the atomic charge of carbon and the C1s core electron binding energy estimated from a comparison of gas phase and solid state data and the molar polarizabilities of the investigated molecules

A method for internal calibration of ESCA (XPS ) spectra is described that permits the use of a simple relation between binding energy (E_b) and atomic charge (q) : E_b = kq + E_b0. This relation has been shown to hold for a large number of elements. So far the relation for carbon has, however, not been very well established. A method is now described that allows the use of carbon charges determined from E_b, vs. q correlations for other elements and has also made it possible to establish a linear relation for carbon. The method does not use carbon charges determined directly from quantum chemical calculations.

As an essential part of this procedure we investigated empirically the possible relation between the difference between the gas phase calibration standard and that for the same element in the solid state, “a”, and the molar polarizability of the substance, P. It was found that “a” decreases with increasing polarizability, tentatively as a=4.15+8.30 P^−1/3. Therefore, knowledge of the polarizability of carbon-containing molecules from the literature made it possible to determine “a” and then the charge of all other atoms in the molecule. In this way a set of E_b and qc date was obtained. These data were complemented by another set using theoretical q_c snd solid state E_b of phenyl compounds previously made.

The results of the investigation show a linear relation between E_b(Cls) and q_c valid for the solid state: E_b,(Cls) = 4.68 q_c + 286.2 eV with a correlation coefficient of 0.945. This brings the previously obtained k value for carbon, which seemed anomalously high, down to a value of similar magnitude as for other light elements.     

Completeness of “good” Bethe ansatz solutions of a quantum-group-invariant Heisenberg model

The sl_q(2) quantum-group-invariant Heisenberg model with open boundary conditions is investigated by means of the Bethe ansatz. As is well known, quantum groups for q equal to a root of unity possess a finite number of “good” representations with non-zero q-dimension and “bad” ones with vanishing q-dimension. Correspondingly, the state space of an invariant Heisenberg chain decomposes into “good” and “bad” states. A “good” state may be described by a path of only “good” representations. It is shown that the “good” states are given by all “good” Bethe ansatz solutions with roots restricted to the first periodicity strip, i.e. only positive-parity strings (in the language of Takahashi) are allowed. Applying Bethe's string-counting technique completeness of the “good” Bethe states is proven, i.e. the same number of states is found as the number of all restricted paths on the sl_q(2) Bratteli diagram. It is the first time that a “completeness” proof for an anisotropic quantum-invariant reduced Heisenberg model is performed.     

Factorization and the soft overlap contribution to heavy-to-light form factors

Using the formalism of soft-collinear effective theory, a complete separation of short- and long-distance contributions to heavy-to-light transition form factors at large recoil is performed. The universal functions ζ_M(E) parameterizing the “soft overlap” contributions to the form factors are defined in terms of matrix elements in the effective theory. Endpoint configurations corresponding to kinematic situations where one of the valence partons in the external mesons carries very small momentum are accounted for in terms of operators involving soft-collinear messenger fields. They contribute at leading order in Λ_QCD/E and spoil factorization. An analysis of operator mixing and renormalization-group evolution in the effective theory reveals that the intermediate scale is without significance to the soft functions ζ_M(E), and that the soft overlap contribution does not receive a significant perturbative (Sudakov) suppression.     

A study of the Ca(d, p)Ca reaction (I). The single-particle states

The reaction ⁴⁰Ca(d, p)⁴¹Ca has been studied with a resolution 30 keV. Excitation functions for the first three strong states have been measured in the energy interval 9.80 to 12.12 MeV. Cross section fluctuations are found to be entirely within statistics, i.e. < 5 %, and it is concluded that σ_CN(θ)/(2J_f+1) < 5 μb/sr. Differential cross sections have been measured for elastic scattering, and the (d, p) transitions to the ground state and the strong single-particle states at E^* = 1.949, 2.471, 3.623 and 3.954 MeV at E_d = 12.00 MeV and, over a limited angular range, at E_d = 11.00 MeV. The angular distributions have been analysed by the DWBA method and spectroscopic factors have been determined. It is suggested that the 3.623 MeV state may be the third state predicted by Gerace and Green rather than as assigned by earlier studies.     

The ground state of the two-leg Hubbard ladder a density-matrix renormalization group study

We present density-matrix renormalization group results for the ground state properties of two-leg Hubbard ladders. The half-filled Hubbard ladder is an insulating spin-gapped system, exhibiting a crossover from a spin liquid to a band insulator as a function of the interchain hopping matrix element. When the system is doped, there is a parameter range in which the spin gap remains. In this phase, the doped holes from singlet pairs and the pair field and the “4k_F” density correlations associated with pair-density fluctuations decay as power laws, while the “2k_F” charge density wave correlations decay exponentially. We discuss the behavior of the exponents of the pairing and density correlations within this spin-gapped phase. Additional one-band Luttinger liquid phases which occur in the large interband hopping regime are also discussed.     

Ti掺杂NbSe2电子结构的第一性原理研究

采用基于密度泛函理论框架下的第一性原理平面波超软赝势方法, 计算了理想2H-NbSe₂和Ti掺杂2H-NbSe₂晶体的几何结构及电子结构; 对掺杂前后超胞的能带图、态密度及分波态密度图进行了分析. 结果表明, 掺杂后费米能级附近能量区域的电子态密度出现了较高的峰值, 且费米能级位置发生了改变. 理论上可以认为Ti的掺杂会使得NbSe₂的导电性增强, 有利于开发新型的电接触复合材料.     

Flux creep through the surface barrier and columnar defects in HTSC

The rate of flux creep over the surface barrier in clean HTSC samples is found. This rate proves to be significantly different for the cases of vortex entry and escape (relaxation “in” and “out”, respectively). Since a flat surface is topologically similar to the artificial columnar defects, their comparison is useful. The vortex creep over the surface is equivalent to that over the cylinder defect of radius r > λ, whereas the radii of real defects obtained so far are of order . We discuss the effect of the smallness of defect radius on the creep process, which results in very small activation energies U at large critical current J_c. The importance of the increase of the defect radius is predicted.     

The Bi(d,γ)Po reaction

The yield of the ²⁰⁹Bi(d, γ)^211g.s.Po and ^211mPo (T_1/2 = 25.2s) reaction was measured for deuteron energies E_d = 8–11.5 MeV. The reaction was identified by the -activities of the Po isotope. At E_d = 10.43 MeV, the (d, γ) cross section for the population of the ground state of ²¹¹Po is σ_g.s. = 16 ± 3 μb, the ratio relative to the cross section for the metastable state is σ_g.s./σ_m = 25.4 ± 0.9. These values and the yield curves were compared with calculations using a simple model for the population of the two states. In the excitation region E* = 15–19 MeV, the branching ratio of γ- to particle emission is nearly constant and has a value of about 0.4 × 10⁻⁴.     

Optical absorption, disorder and the Anderson transition

The optical gap, E_g, of amorphous indium-oxide films is measured as a function of static disorder near the metal-insulator transition. On the insulating side of the transition the optical gap obeys a scaling relation, ΔE_g = -E_*Δg where E_* is of the order of the Fermi energy of the given sample and g≡K_Fl. These results are ascribed to the continuous shift of the mobility-edge in the conduction band with disorder.     

Spontaneous breaking of scale invariance in a supersymmetric model

The phase structure of a large N, O(N) supersymmetric model in three dimensions is studied. Of special interest is the spontaneous breaking of scale invariance which occurs at a fixed value of the coupling constant, λ₀=λ_c=4π. In this phase the bosons and fermions acquire a mass while a Goldstone boson (dilaton) and Goldstone fermion (“dilatino”) are dynamically generated as massless bound states. The absence of renormalization of the dimensionless coupling constant λ₀ leaves these Goldstone particles massless.     

Elastic and magnetoelastic effects in invar alloys

A review of the elastic behaviour of Invar alloys is given. This includes (a) ferromagnetic alloys (Fe-Ni, Fe-Pt, stainless Invar); (b) antiferromagnetic alloys (Fe-Mn, Mn- and Cr-based alloys); (c) very weak ferromagnets (ZrZn₂, Ni₃Al, Ni-Pt); (d) amorphous alloys (Fe-B). It is shown that elastic anomalies (ΔE effects) can but need not occur simultaneously with a large thermal expansion anomaly. Various magnetoelastic contributions exist (ΔE_λ, ΔE_ω, ΔE_m) which are discussed in terms of existing theoretical models. The effect associated with a volume distortion, i.e. the bulk modulus anomaly, is shown to fit into the framework of the itinerant electron model.     

On the electronic and geometric structure of the Si(113) surface

The Si(113) surface of a p-type sample was studied by AES, LEED and ARUPS. On the clean surface, a (3 × 2) and a (3 × 1) LEED pattern coexist for a large range of annealing temperatures. Annealing to 900 K results in (3 × 1), while temperatures higher than 1050 K favour the (3 × 2) superstructure. ARUPS reveals two weakly dispersing surface resonances around 0.9 and 2.6 eV below E_F which are connected with the (3 × 2) and (3 × 1) structures, respectively. The work function was determined as φ = 4.81 eV and the photoionization threshold as ξ = 5.36 eV. The bands are bent downwards by 0.43 eV at room temperature.     

The pion velocity at chiral restoration and the vector manifestation

We study the effects of Lorentz non-invariance on the physical pion velocity at the critical temperature T_c in an effective theory of hidden local symmetry (HLS) with the “vector manifestation” fixed point. We match at a “matching scale” Λ_M the axial-vector current correlator in the HLS with the one in the operator product expansion for QCD, and present the matching condition to determine the bare pion velocity. We find that the physical pion velocity, which is found to be one at T=T_c when starting from the Lorentz invariant bare HLS, remains close to one with the Lorentz non-invariance, v_π(T_c)=0.83–0.99. This result is quite similar to the pion velocity in dense matter.     

Yb (Ba, Sr)2Cu4O8: a “124” phase superconductor with a reduced unit cell volume

Yb(Ba_1−xSr_x)₂Cu₄O₈ (0.1x0.3) superconductors of the YBa₂Cu₄O₈(“124”) structure were successfully synthesized using an O₂-hot-isostaticc-pressing (HIP) technique. The samples were characterized with respect to the crystal structure and superconducting properties. The lattice parameters of the samples decreased as the substitution of Sr for Ba proceeded. The superconducting transition temperatures of all the Yb-“124” samples were more or less the same, being around 80 K.     

Mass spectrum and elastic scattering in the massive SU(2)f Schwinger model on the lattice

We calculate numerically scattering phases for elastic meson-meson scattering processes in the strongly coupled massive Schwinger model with an SU(2) flavor symmetry. These calculations are based on Lüscher's method in which finite size effects in two-particle energies are exploited. The results from Monte Carlo simulations with staggered fermions for the lightest meson (“pion”) are in good agreement with the analytical strong-coupling prediction. Furthermore, the mass spectrum of low-lying mesonic states is investigated numerically. We find a surprisingly rich spectrum in the mass region [m_π, 4m_π].