Effect of lateral energy transport on ablation pressure scaling in laser irradiated planar foil targets

Experimental investigations on ablatively accelerated thin plastic foil targets irradiated by a 6J, 5 nsec Nd: glass laser pulse, were conducted using shadowgraphy technique. A 2 nsec, 0.53 μm probe pulse, derived from the main laser was used for recording the foil motion. It was observed that 6 μm plastic foils could be accelerated to a velocity of about 3 × 10⁶ cm/sec for an incident laser intensity of 5 × 10¹³ W/cm² and the corresponding ablation pressure was 0.4 Mbar. Ablation pressure (P) scaling against absorbed laser intensity (I _a ) was slower (P ∝I _a ^0.4 ) for a smaller laser focal spot (30 μm) as compared to the scaling (P ∝I _a ^0.7 ) for a larger focal spot (500 μm). This result has been explained considering the loss due to lateral energy transport from the laser plasma interaction region.     

NMR and NQR studies of spin dynamics and superconductivity in High-T c oxides

The magnetic and superconducting properties in the high-T _c cuprates have been investigated over a wide hole doping range by⁶³Cu,¹⁷O and²⁰⁵Tl NMR and NQR in the lightly-doped La_2?xSr_xCuO₄ (LSCO), the heavily-doped Tl₂Ba₂CuO_6+y (TBCO) and the Zn-doped YBa₂Cu₃O₇ (YBCO₇). In low doping region, the large antiferromagnetic (AF) spin correlation around the zone boundary (q=Q) causes the Curie-Weiss behavior of⁶³(1/T ₁ T) associated with that of the staggered susceptibility χ_O(T) in LSCO. In the vicinity of the hole content whereT _c has a peak, the AF spin correlation still survives, although the magnetic coherence length ξ_M is considerably short being presumably (ξ_M/a) ～ 1. The further doping destroys progressively the AF spin correlation, which is no longer present is non-superconducting TBCO compounds. These NMR evidences signify that there is an intimate relation between the presence of the AF spin correlation and the onset of the superconductivity. The local collapse of AF spin correlation is a primary cause for the unexpected strong reduction ofT _c in case of the substitution of Zn impurities into the CuO₂ plane. The superconducting properties clarified by NMR experiments cannot be accounted for by the conventional BCS model or other isotropic s-wave models. A d-wave model is applicable in interpreting consistently most of the NMR results, if the finite density of states at the Fermi level is taken into consideration and is associated with the pair breaking effect. There are increasing evidences that the magnetic mechanism for the superconductivity is promising in high-T _c cuprates.     

On the spectrum of particles created in a Robertson-Walker universe

Created particle spectra are calculated in Robertson-Walker universes with scale factora∝t ^α(0 <α≦ 1) anda∝e ^t, and discussed with a special emphasis on their dependence upon the initial and final times at which a WKB-like positive frequency condition should be imposed. It is shown that the obtained spectra are very sensitive to these times if the WKB approximation for the field equation is not valid in their neighborhood. It is also shown that the total number density of created particles remains finite if the final time is set to be finite.     

Second- and third-order elastic moduli of Co I

Elastic moduli of Co with A ₁-type structure are calculated using models in which the total energy is presented in the form of the energy of clusters consisting of pairs, triplets, and quadruplets of identical atoms. The irreducible energies of the clusters are assumed to contain two terms: E ∝ ?τ^?6, corresponding to the mutual attraction of the particles, and E ∝ ?τ^?6, corresponding to their mutual repulsion. Here, τ stands for the half-edge of an elementary cell. All 27 types of models expected in accordance with symmetry theory are analyzed. Of these, the 13 that yielded elastic modulus values satisfying the structural stability criteria are selected. The potentials of the selected models are used to predict the rigidity moduli of Co films with an A2-type structure.     

Magnetoresistance of a highly correlated electron liquid

The behavior in a magnetic field of a highly correlated electron liquid approaching the fermion condensation quantum phase transition from the disordered phase is considered. We show that, at sufficiently high temperatures T≥T*(x), the effective mass starts to depend on T, M* ∝T^?1/2. This T^?1/2 dependence of the effective mass at elevated temperatures leads to the non-Fermi liquid behavior of the resistivity, σ(T) ∝ T and at higher temperatures σ(T) ∝ T^3/2. The application of a magnetic field B restores the common T² behavior of the resistivity. The effective mass depends on the magnetic field, M*(B) ∝ B^?2/3, being approximately independent of the temperature at T≤T*(B) ∝ B^4/3. At T≥T*(B), the T^?1/2 dependence of the effective mass is reestablished. We demonstrate that this B-T phase diagram has a strong impact on the magnetoresistance (MR) of the highly correlated electron liquid. The MR as a function of the temperature exhibits a transition from negative values of MR at T→0 to positive values at T ∝ B^4/3. Thus, at T≥T*(B), MR as a function of the temperature possesses a node at T ∝ B^4/3.     

RSOS revisited

We investigate the issues of unitarity and reality of the spectrum for the imaginary coupled affine Toda field theories based on a₁⁽¹⁾ and a₂⁽²⁾ and the perturbed minimal models that arise from their various RSOS restrictions. We show that while all theories based on a₁⁽¹⁾ have real spectra in finite volume, the spectra of a₂⁽²⁾ models is in general complex, with some exceptions. We also correct the S matrices conjectured earlier for the φ₁₅ perturbations of minimal models and give evidence for a conjecture that the RSOS spectra can be obtained as suitable projections of the folded ATFTs in finite volume.     

Conditions for T<Superscript>2</Superscript> resistivity from electron-electron scattering

Many complex oxides (including titanates, nickelates and cuprates) show a regime in which resistivity follows a power law in temperature (ρ ∝ T ²). By analogy to a similar phenomenon observed in some metals at low temperature, this has often been attributed to electron-electron (Baber) scattering. We show that Baber scattering results in a T ² power law only under several crucial assumptions which may not hold for complex oxides. We illustrate this with sodium metal (ρ _el?el ∝ T ²) and strontium titanate (ρ _el?el \(\hbox{$\not\propto$}\) T ²). We conclude that an observation of ρ ∝T ² is not always sufficient evidence for electron-electron scattering.     

Improved continuum limit lattice action for QCD with wilson fermions

Two possible ways of extending Symanzik's improvement programme to lattice fermions namely improvement to first and second order in the lattice sppacing a are discussed. The corresponding lattice actions for fermions are constructed and tree-level improvement conditions are derived by considering “classical” improvement. The concept of “on-shell” improvement is generalized to the lattice fermions studied here and the free parameters are determined for O(a) and O(a²) on-shell improved actions to all orders of perturbation theory. No evidence is found that the complicated structure of the O(a²) on-shell improved action, especially thearising fermion contact terms can be removed beyond tree level. The effect of terms in the action that explicitly break chiral symmetry and therefore remove the phenomenon of species doubling are investigated by considering the energy-momentum relations of the arising tree-level improved actions. Our main result is that the O(a) improved action is a slightly modified Wilson fermion action can still be written with only nearest-neighbour fermion interactions.     

Influence of polarization and iron content on the transport properties of praseodymium–barium manganite

Polarization and iron effects on the electrical properties of Pr_0.67Ba_0.33Mn_1−xFe_xO₃ have been studied using impedance measurements. When iron is introduced, the insulator–metal transition (MI), observed in free compound, disappears and destroying such transition needs an iron concentration less than 5%. We also found that electrical conductance decreases when increasing Fe content. Such results are attributed to the decrease of Mn³⁺/Mn⁴⁺ ratio. Also, they are ascribed to the high probability of encountering Fe³⁺–O–Fe³⁺ and Mn³⁺–O–Fe³⁺ interactions, which greatly weakens the influence of Mn³⁺–O–Mn⁴⁺ interactions. The AC conductivity studies indicate that different types of hopping are involved. The contribution of hopping mechanism is confirmed by the temperature dependence of the frequency exponent ‘s’. Conductivity analysis shows that small polaron hopping (SPH) and variable range hopping (VRH) models are present in the conduction process. For small iron concentrations (x<0.1), we found that activation energy (E_a) does not changes significantly. Such result is in good agreement with the literature. But, for high iron concentrations (x>0.1), we found that E_a depend strongly in Fe content. We also found in this work that DC-bias does not affect the conduction process but proves its thermal activation. The variation of the conductance with polarization is a proof of an electro-resistance effect.     

Investigation of lithium forward scattering for the analysis of carbon and oxygen in human amniotic fluid

Lithium forward elastic scattering is investigated as an additional method for Z<11 multielemental analysis in human amniotic fluid (AF). Concentrations of C and O are obtained from the analysis of an AF sample that is diluted with distilled water, deposited on a formvar backing and dried naturally. Solid residues of this diluted sample are bombarded with 13 MeV ^6,7Li beams and the elastically scattered beam is detected at 16.45^o, 20.45^o and 28.0^o simultaneously. The quality of elastic spectra improves with sample dilution. The content of C and O in the backing is subtracted. Carbon and oxygen concentrations of the non-diluted AF sample are determined by assuming that elemental concentration varies linearly with dilution.     

The mechanism and kinetics of water formation on Pt(111)

We use modulated molecular beam relaxation spectroscopy (MMBRS) to identify the sequence of elementary reaction steps and to quantify the rate parameters for catalytic water (D₂O) formation on Pt(111). This investigation is restricted to surface temperatures in the range 373 ⩽ T_S ⩽ 723 K and oxygen and deuterium pressures on the order of 10⁻⁵ mbar, where coverages of oxygen-containing species are low (ϑ ⩽ 0.04 monolayer), and rate parameters can be assumed coverage-independent. Under these conditions, we find that adsorbed hydroxyl (OD_a) formation is rapid and (nearly) irreversible; the rate-limiting, majority pathway for water production is D_a + OD_a → D₂O (E_a = 16 kcal/mol); and water production by OD_a disproportionation. i.e. 2 OD_a → D₂O + O_a, contributes as a minority pathway (E_a = 18 kcal/mol). From the results we construct potential energy diagrams that account for the energetics of nearly all elementary steps in these reactions.     

An empirical study on the importance of a speed-dependent Voigt line shape model for tropospheric water vapor profile remote sensing

We use high quality ground-based solar absorption spectra measured in close coincidence with Vaisala RS92 radiosonde in situ water vapor profiles to demonstrate that a Voigt line shape model yields systematic errors in the remotely sensed tropospheric water vapor profiles. We analyse absorption signatures of 4 H₂¹⁶O and 2 HD¹⁶O bands situated between 790 and 4710 cm⁻¹. We find that applying a speed-dependent Voigt line shape model instead of a Voigt line shape model significantly improves the agreement between the water vapor profiles obtained by the radiosondes and by infrared remote-sensing in the different bands. An optimal agreement is obtained for a Γ₂ (relaxation rate for speed-dependence) of 6-21% of Γ₀ (Voigt relaxation rate), which is consistent to the values derived from laboratory experiments. Our study suggests that further extensive laboratory investigations of line shape models are a key for improving the quality of modern water vapor remote sensing products.     

Adiabatic theorem in axiomatic quantum field theory

It is shown that Møller matricesS _± and scattering matrixS in axiomatic field theory can be expressed through their adiabatic analogs. In particular, it is proved under certain conditions that \(S_ - = \mathop {s\lim }\limits_{\alpha \to 0} S_\alpha (0,\infty )W_\alpha \) whereW _α is a trivial phase factor [i.e. a unitary operator of the form exp i / α ∝r(k)a ⁺ (k)a(k)dk]. Corresponding results in Hamiltonian approach are discussed.     

An infinite lie group of symmetry of one-dimensional gas flow,for a class of entropy distributions

We have shown in earlier works the existence of three previously unknown symmetries of the equations of one-dimensional gas dynamics, with arbitrary entropy distribution and arbitrary polytropic index γ. These symmetries are seen here to form a group whenever the equation of state is of the form P = ?³(a₀ + a₁M + a₂M²)^?2 where M = ∝?dr is the Lagrangian mass coordinate.Introducing the remaining symmetry of space-translation enlarges the group into a Lie group of symmetry of infinite order, from which an infinite number of conservation laws can be deduced by application of Noether's theorem. The Lie group has a finite sub-algebra of order eight, which has SU3 structure; the list of associated conservation laws includes each of the six ones that are derivable from general physical principles, namely: the energy, momentum and the center-of-mass integrals, two integrals expressing scale invariance, and one associated with the virial theorem; the remaining two integrals of the octet are of a new type.Such a situation reminds us of the case of the Korteweg-de Vries equation in the soliton problem, where the symmetries and infinite number of conservation laws arise as a result of the possibility to linearize through the inverse-scattering method. Thus the question is raised of whether the inverse-scattering method also applies to gas-dynamical equations (with the above equation of state), or else whether another method of linearization may be found.     

Ideal hydrodynamics outside and inside a black hole: Hamiltonian description in Painlevé-Gullstrand coordinates

We show that when the Painlevé-Gullstrand coordinates are used in their Cartesian version, the Hamiltonian of relativistic ideal hydrodynamics in the vicinity of a nonrotating black hole differs by only one simple term from the corresponding Hamiltonian in a flat spacetime. The interior region of the black hole is also described in a unified way, because there is no singularity on the event horizon in Painlevé-Gullstrand coordinates. We present the exact solution describing the steady accretion of extremely hard matter (? ∝ n ²) onto a moving black hole up to the central singularity. In the local induction approximation, we derive the equation of motion for a thin vortex filament against the background of such an accretion flow. We explicitly calculate the Hamiltonian for a fluid with an ultrarelativistic equation of state, ? ∝ n ^4/3, and solve the problem of a centrally symmetric steady flow of such matter.     

Nuclear dissipation,fission probability and neutron multiplicity prior to fission

We discuss the effects of nuclear dissipation on fission probabilities that are characteristic of a diffusion model of the fission process. Reproducing the experimental fission probabilities at low excitation energies fixes the ratioa _f/a_n of the level density parameters for a given strength of the reduced dissipation coefficientβ. These low energy constraints ona _f/a_n andβ balance the effects of transients on neutron multiplicities prior to fission at higher excitation energies. For the competitive decay of¹⁵⁸Er formed in the reaction¹⁶O+¹⁴²Nd at 207 MeV we show that dueto transients only the multiplicity of pre-fission neutrons is enhanced with respect to the prediction of the statistical model in a manner consistent with our earlier general analysis.     

Electron mass enhancement and magnetic phase separation near the Mott transition in double-layer ruthenates

We present a detailed investigation of the specific heat of Ca₃(Ru_1-xM_x)₂O₇ (M = Ti, Fe, Mn) single crystals. Depending on the dopant and doping level, three distinct regions are present: a quasitwo-dimensional metallic state with antiferromagnetic (AFM) order formed by ferromagnetic bilayers (AFM-b), a Mott insulating state with G-type AFM order (G-AFM), and a localized state with a mixed AFM-b and G-AFM phase. Our specific heat data provide deep insights into the Mott transitions induced by Ti and Mn doping. We observed not only an anomalous large mass enhancement, but also an additional term in the specific heat, i.e., C ∝ T², in the localized region. The C ∝ T² term is most likely due to long-wavelength excitations with both FM and AFM components. A decrease in the Debye temperature is observed in the G-type AFM region, indicating lattice softening associated with the Mott transition.     

A method for the determination of lattice parameters on single crystals

The ratio method, designed originally for powder materials, is modified for single crystals. A relatively simple goniometer with photographic registration is used. Discussion of random and systematic errors shows that an accuracyΔa/a = 10^?4, or better, can be achieved. An application for single crystals of Fe and Fe-Si alloys is demonstrated. For pure iron the valuea= = (2·8664±00001)×10^?10 m is found.     

A study of 225 000 τ+ decays

In this article, results of the analysis of 225 000 τ⁺ decays in flight recorded at the CERN-PS with an electronic detector are presented. A quadratic fit of the matrix element |M|² ∝ 1 + aY + bX² + cY² gives the following values for the parameters: a = 02814 ± 0.0082; b = ?0.099 ± 0.019; c = ?0.001 ± 0.023.