Monte Carlo calculation of thermal desorption spectra: Application to CO/Ru(001)

The thermal desorption spectrum and phase diagram for the triangular lattice gas model with strong repulsive 1NN and weak attractive 2NN interaction are calculated using Monte Carlo method and the transition state rate theory. Starting from an extreme case of the hard hexagon model, the magnitudes of the interactions are varied to reach a good coincidence with the experimental desorption spectrum of CO/Ru(001). The calculated spectrum describes well the characteristic zero-order (-like) behavior and asymmetric sharp peak observed in experiment, which are known to arise from the critical point in the model. Also, calculation of the phase diagram confirms that the tricritical temperature appears in the same magnitude with the 2NN interaction, kT_t ≈ ¦ε₂¦. The determined 1NN and 2NN interaction energies, ε₁ ≈ 8 to 10 kJ/mol, ε₂ ≈ −1.6 to −1.2 kJ/mol, are about t than those estimated previously by the isosteric Arrhenius analysis of the experimental desorption spectrum.     

Effects of redox thermal treatments on the transverse resistance peak at the edge of the superconducting transition in Bi2Sr2CaCu2O8+x

The growth of the transverse resistivity just above the transition in Bi₂Sr₂CaCu₂O_8+x (BSCCO 2212) expitaxial films is studied in the framework of the recent fluctuation model proposed by Ioffe, Larkin, Varlamov and Yu Lu. The effects of different redox thermal treatments on the transverse resistive peak is investigated. The fit of the experimental curves with the theory is good and allows one to estimate the values of some physical parameters included in the theory itself (namely the in-plane Fermi velocity ν_F, the carrier lifetime τ and the hopping integral w). It is shown that the oxidizing thermal treatment gives rise to a strong decrease in the transverse resistance peak just above T_c. Such effect is ascribed to the increase in ν_F due to the growth of the carrier concentration induced by the oxidizing treatment. Effects on the carrier lifetime and the hopping integral are hindered by the large variation of ν_F.     

Structural study and magnetoresistance effect of epitaxial La0.67Sr0.33MnO3–δ and Pr0.7Ca0.3MnO3−δ multilayer films

La_0.67Sr_0.33MnO_3−δ (LSMO) and Pr_0.7Ca_0.3MnO_3−δ (PCMO) multilayer epitaxial films, which were fabricated with different LSMO and PCMO layer thickness on LaAlO₃ single crystal substrates of (0 0 1) orientation by a direct current magnetron sputtering technique, were studied further, after the structure, magnetoresistance effect and magnetic properties of LSMO/PCMO/LSMO (LPL) trilayer epitaxial films were systemically studied. The superlattice structures of multilayer films were observed according to the diffraction peaks of X-ray diffraction patterns at small angles. The metal–insulator transition temperature (T_P) and peak resistivity (ρ_max) obviously changed when we altered the thickness of PCMO middle layer and the intra-field related with the thickness of those layers and their interaction. Considering the effect of the distribution of electrical field and current, and the interaction among the layers of LSMO and PCMO, an effective fact n* was introduced to replace n (the number of layer). All the calculated values of ρ (the resistivity of multilayer films) accorded with the experimental values.     

Pionic K X-rays in liquid He

Energies and lorentzian widths of pionic K X-ray transitions have been measured in liquid ³He with improved accuracy. The strong interaction on the π³He 1s level is found to be attractive and to produce an increase in the K transition energies of 34 ± 4 eV; the measured lorentzian width is 36 ±7 eV. Measured values are also presented for K X-ray energies in liquid π⁴He and μ⁴He, the lorentzian width of the 1s level in π⁴He, and relative intensities of K X-ray transitions in π³He, π⁴He and μ⁴He. The measurements are compared with those of others and with recent theoretical calculations.     

Chiral symmetry and the A1 contribution to the nucleon-nucleon interaction

The contribution of A₁ exchange to the nucleon-nucleon potential is studied in a broken chiral symmetric model. The A₁ is treated as a finite-width resonance in the πρ s-wave. Connections between pseudoscalar and pseudovector pion-nucleon coupling in the underlying model lagrangian are studied in detail. It is found that large terms in the NN interaction arising from πρ exchange with pseudoscalar coupling are suppressed by interference with a₁ exchange. With pseudovector coupling there is a suppression of the A₁ exchange by the so-called “seagull” terms in πρ exchange which arise from gauge invariance. The suppression becomes an exact cancellation in the limit of infinite ρ and a₁ masses and exact chiral symmetry. We found that inclusion of the a₁ decay into the πρ state strongly modifies the a₁] exchange potential, suppressing the tensor part but leaving the spin-spin part almost unchanged.     

Electrical transport properties of dense bulk YBa2Cu4O8 produced by hot isostatic pressing

Highly dense sintered YBa₂Cu₄O₈ has been produced by hot isostatic pressing (HIP). The electrical resistivity of this material has been measured as a function of temperature T and pressure in the range 40–650 K and 0–0.7 GPa. Both the temperature dependence and the pressure dependence of are found to be well described by a model based on the standard Bloch-Grüneisen theory. It is pointed out that is liner in T only under isobaric conditions, while is strongly nonlinear in all high-T_c superconductors under isochoric (constant volume) conditions. The critical current density of the material is 900 A/cm² at 4 K, while the resistivity is 630 μΩ cm at 294 K.     

Effect of Pr and Ca substitution on superconductivity in Y(1−x−yPrxCay)Ba2Cu3O7−δ

Pr substituted at constant Ca concentration for Y in (Y_1−x−yPr_xCa_y)Ba₂Cu₃O_7−δ superconductors have been prepared under identical conditions and the temperature dependence of the electrical resistivity of these samples are measured. The resistively determined values of T_c decrease linearly with increasing x (0 ≤ x ≤ 0.2) for constant y = 0.10 and 0.15 which provides convincing evidence that the suppression of superconductivity by Pr is mainly due to magnetic pair breaking. The suppression of superconductivity can also be correlated to the observed changes in oxygen content determined by iodometric analysis and to the average Cu-valences. However, it is found that the observed suppression of T_c cannot be compensated by appropriate hole doping with Ca.     

Phase separation in YBa2Cu3O7−δ single crystals near δ = 0

X-ray diffraction, resistivity, AC and DC magnetization data on high-quality single crystals of YBa₂Cu₃O_7−δ are presented. We demonstrate that for δ<0.08, the (0 0 l) diffraction lines are split into two, indicating that at these high oxygen concentrations the crystals are no longer single phase but actually consist of two (or more) different phases with slightly different c-axis parameters. In the two-phase region, the electrical resistivity and the AC and DC magnetic susceptibilities show a broadening of the superconducting transition. This broadening is thought to be due to the proximity effect or strains in two finely dispersed phases with slightly different transition temperatures.     

The reactions pp→pΛK and pp→pΣK near their thresholds

The reactions pp→pΛK⁺ and pp→pΣ⁰K⁺ are studied near their thresholds. The strangeness production process is described by the π- and K exchange mechanisms. Effects from the final state interaction in the hyperon-nucleon system are taken into account rigorously. The Λ production turns out to be dominated by K exchange whereas K- as well as π exchange play an important role for the Σ⁰ case. It is shown that the experimentally observed strong suppression of Σ⁰ production compared to Λ production at the same excess energy can be explained by a destructive interference between π and K exchange in the reaction pp→pΣ⁰K⁺. Implications of such an interference on the reaction pp→nΣ⁺K⁺ are pointed out.     

Carbon solubility and superconductivity in MgB2

Successful replacement of B by C in the series MgB_2−xC_x for values of x upto 0.3 is reported. Resistivity and ac susceptibility measurements have been carried out in the samples. Solubility of carbon, inferred from the observed change in the lattice parameter with carbon content indicates that carbon substitutes upto x=0.30 into the MgB₂ lattice. The superconducting transition temperature, T_c measured both by zero resistivity and the onset of the diamagnetic signal shows a systematic decrease with increase in carbon content upto x=0.30, beyond which the volume fraction decreases drastically. The temperature dependence of resistivity in the normal state fits to the Bloch–Gruneisen formula for all the carbon compositions studied. The Debye temperature, θ_D, extracted from the fit, is seen to decrease with carbon content from 900 to 525 K, whereas the electron–phonon interaction parameter, λ, obtained from the McMillan equation using the measured T_c and θ_D, is seen to increase monotonically from 0.8 in MgB₂ to 0.9 in the x=0.50 sample. The ratio of the resistivities between 300 and 40 K versus T_c is seen to follow the Testardi correlation for the C substituted samples. The decrease in T_c is argued to mainly arise due to large decrease in θ_D with C concentration and a decrease in the hole density of states at N(E_F).     

Superconductivity and normal state resistivity of Ba0.6K0.4BiO3: an optical phonon approach

We discuss the nature of the pairing mechanism and the physical properties associated with the normal as well as the superconducting state of cubic perovskites Ba_0.6K_0.4BiO₃using the strong coupling theory. An interaction potential which includes the Coulomb, electron–optical phonon and electron–plasmon interactions is developed to elucidate the superconducting state. A model dielectric function is constructed with these interactions fulfilling thef-sum rule. The screening parameter (μ^* = 0.26) infers the poor screening of charge carriers. The electron–optical phonon strength (λ) estimated as 0.98 is consistent with an attractive electron–electron interaction and supports the moderate to strong coupling theory. The superconducting transition temperature of Ba_0.6K_0.4BiO₃is then estimated as 32 K. Ziman's formula of resistivity is employed to analyse and compare this with the temperature-dependent resistivity of a single crystal. The estimated contribution from the electron–optical phonon together with the residual resistivity clearly infers a difference when a comparison is made with experimental data. The subtracted data infer a quadratic temperature dependence in the temperature domain (30 ≤ T ≤ 200 K). The quadratic temperature dependence of ρ [ = ρ_exp − (ρ₀ + ρ_e–ph)] is understood in terms of 3D electron–electron inelastic scattering. The presence of these el–el and el–ph interactions allows a coherent interpretation of the physical properties. Analysis reveals that a moderate to strong coupling exists in the Ba_0.6K_0.4BiO₃system and the coupling of electrons with the high-energy optical phonons of the oxygen breathing mode will be a reason for superconductivity. The implications of the above analysis are discussed.     

Chiral symmetry amplitudes in the S-wave isoscalar and isovector channels and the σ, f0(980), a0(980) scalar mesons

We use a non-perturbative approach which combines coupled channel Lippmann-Schwinger equations with meson-meson potentials provided by the lowest order chiral Lagrangian. By means of one parameter, a cut off in the momentum of the loop integrals, which results of the order of 1 GeV, we obtain singularities in the S-wave amplitudes corresponding to the σ, f₀ and a₀ resonances. The ππ → ππ, phase shifts and inelasticities in the T = 0 scalar channel are well reproduced as well as the π⁰η and mass distributions in the T = 1 channel. Furthermore, the total and partial strong decay widths of the f₀ and a₀ resonances are properly reproduced. The results seem to indicate that chiral symmetry constraints at low energy and unitarity in coupled channels is the basic information contained in the meson-meson interaction below GeV.     

Plane-chain coupling in YBa2Cu3O7: temperature dependence of the penetration depth

We have studied the penetration depth for a model of YBa₂Cu₃O₇ involving pairing, both in the CuO₂ planes and in the CuO chains. In this model pairing in the planes is due to an attractive interaction, while Coulomb repulsion induces in the chains an order parameter with opposite sign. Due to the anti-crossing produced by hybridization between planes and chains, one obtains a d-wave like order parameter which changes sign on a single sheet of the Fermi surface and has nodes in the gap. We find that our model accounts quite well for the anisotropy of the penetration depth and for the absolute values. We reproduce fairly well the whole temperature dependence for both the a- and the b-directions, including the linear dependence at low temperature. We use a set of parameters which are all quite reasonable physically. Our results for the c-direction are also satisfactory, although the situation is less clear both experimentally and theoretically.     

Vortices and phase transitions in two-dimensional non-abelian spin models

We modify O(n) models (n>2) in two dimensions so as to comparedifferent thoeries with identical local properties and different global ones. Our O′ (3) model with a particular interaction has vortex-like configuration (π₁(P₂) = Z₂) though it is locally equivalent to an O (3) model (π₁(S₂) = 0). Our results have been obtained by means of strong coupling methods. The Padé extrapolants show a critical value x_c = 11.8.     

High field magnetoresistance of CeAl2 at low temperatures

We have measured the magnetoresistance ρ(H,T₀) of the magnetic Kondo lattice CeAl₂ at temperatures T₀ in the range 0.035 – 1.3 K, well below the Néel temperature T_N=3.8 K, in magnetic fields H up to 145 k0e. The ρ vs H curve exhibits a rapid decrease between H=45–65 kOe corresponding to the metamagnetic transition at H=H_M. The resistivity then levels out to a value which depends only weakly on field and temperature. For HM a small positive magnetoresistance was observed with the derivative dρ/dH increasing as the temperature is lowered. The present results are compared with the ρ(H,T₀) data obtained for the CePb₃ magnetic Kondo lattice, where the decrease in the resistivity at HH_M was considered earlier to be evidence of field induced superconductivity.     

Temperature dependence of the electrical resistivity of amorphous Co80−xErxB20 alloys

The temperature dependence of the electrical resistivity of amorphous Co_80−xEr_xB₂₀ alloys with x=0, 3.9, 7.5 and 8.6 prepared by melt spinning in pure argon atmosphere was studied. All amorphous alloys investigated here are found to exhibit a resistivity minimum at low temperature. The electrical resistivity exhibits logarithmic temperature dependence below the temperature of resistivity minimum T_min. In addition, the resistivity shows quadratic temperature behavior in the interval T_min<T<77 K. At high temperature, the electrical resistivity was discussed by the extended Ziman theory. For the whole series of alloys, the composition dependence of the temperature coefficient of electrical resistivity shows a change in structural short range occurring in the composition range 8–9 at%.     

CP violation in multi-Higgs supersymmetric models

We consider supersymmetric extensions of the standard model with two pairs of Higgs doublets. We study the possibility of spontaneous CP violation in these scenarios and present a model where the origin of CP violation is soft, with all the complex phases in the Lagrangian derived from complex masses and vacuum expectation values (VEVs) of the Higgs fields. The main ingredient of the model is an approximate global symmetry, which determines the order of magnitude of Yukawa couplings and scalar VEVs. We assume that the terms violating this symmetry are suppressed by powers of the small parameter ε_PQ = O(m_b/m_t). The tree-level flavor-changing interactions are small due to a combination of this global symmetry and a flavor symmetry, but they can be the dominant source of CP violation. All CP-violating effects occur at order ε_PQ² as the result of exchange of almost decoupled extra Higgs bosons and/or through the usual mechanisms with an almost real CKM matrix. On dimensional grounds, the model gives ε_K ≈ ε_PQ² and predicts for the neutron electric dipole moment (and possibly also for ε_K¹) a suppression of order ε_PQ² with respect to the values obtained in standard and minimal supersymmetric scenarios. The predicted CP asymmetries in B decays are generically too small to be seen in the near future. The mass of the lightest neutral scalar, the strong CP problem, and possible contributions to the Z decay into b quarks (the R_b puzzle) are also briefly addressed in the framework of this model.     

Upper critical fields of ferromagnet/superconductor layered structures

The temperature dependence of the upper critical fields, both perpendicular H_c2 and parallel H_c2 to layer planes of ferromagnet/superconductor bi- and multilayers, is theoretically investigated. The secular equation of the superconducting order parameter for determining the phase diagram (H, T) is obtained by solving exactly the linearized Usadel equations in the multimode method taking into account the material parameter values. For the bilayers system, the influence of the boundary resistivity on the critical fields, and the dimensional crossover behavior of H_c2(T) are studied in details. For the multilayered structure, the effects of the π-phase state on both the superconducting transition temperature T_c and the upper critical fields (H_c2, and H_c2) are also considered. The nonmonotonic T_c behaviors are predicted. The interplay between 0- and π-phases leading to the strong oscillations of T_c as well as the temperature dependence of the zero temperature critical fields on the ferromagnetic layer thickness are investigated theoretically.     

Temperature dependence of the resistivity and its anisotropy in n-type Nd1.85Ce0.15CuO4 single crystal

Measurements of the in-plane and c-axis normal-state resistivity in a superconducting Nd_1.85Ce_0.15CuO_4−δ single crystal are reported. The resistivity anisotropy of this n-type material is ≤250, much smaller than BiSCCO and comparable to YBaCuO. Both, _ab(T) and _c(T) displays a metallic-like positive temperature coefficient of resistivity with a basic T² dependence. We discuss some possible origins of this peculiar temperature dependence.