Electrical properties and conduction mechanisms in La2/3Ca1/3MnO3 thin films prepared by pulsed laser deposition on different substrates

Perovskite manganite La_2/3Ca_1/3MnO₃ thin films were directly grown on MgO(100), Si(100) and glass substrates by pulsed laser deposition. From the XRD patterns, the films are found to be polycrystalline, single-phase orthorhombic. The metal–insulator transition temperature is 209 K for LCMO/MgO, 266 K for LCMO/Si and 231 K for film deposited on the glass substrate. The conduction mechanism in these films is investigated in different temperature regimes. Low-temperature resistivity data below the phase transition temperature (T _P) have been fitted with the relation \( \rho = \rho_{0} + \rho_{2} T^{2} + \rho_{4.5} T^{4.5} \) , indicating that the electron–electron scattering affects the conduction of these materials. The high-temperature resistivity data (T > T _P) were explained using variable-range hopping (VRH) and small-polaron hopping (SPH) models. Debye temperature values are 548 K for LCMO/Cg, 568 K for LCMO/Si and 508 K for LCMO/MgO thin films. In all thin films, the best fitting in the range of VRH is found for 3D dimension. The density of states near the Fermi level N (E _F) for LCMO/MgO is lower due to the prominent role of the grain boundary in LCMO/MgO and increase in bending of Mn–O–Mn bond angle, which decreases the double exchange coupling of Mn³⁺–O²–Mn⁴⁺ and in turn makes the LCMO/MgO sample less conducting as compared to the other films.     

Crystalline ground states for classical particles

Pair interactions whose Fourier transform is non-negative and vanishes above a wave number K(0) are shown to give rise to periodic and aperiodic infinite volume ground state configurations (GSCs) in any dimension d. A typical three-dimensional example is an interaction of asymptotic form cosK(0)r/r(4). The result is obtained for densities rho > or = rho(d), where rho(1) = K(0)/2(pi), rho(2) = (sq.rt(3)/8)(K(0)/pi)(2), and rho(3) = (1/8sq.rt(2)) x (K(0)/pi)(3). At rho(d) there is a unique periodic GSC which is the uniform chain, the triangular lattice, and the bcc lattice for d = 1,2,3, respectively. For rho > rho(d), the GSC is nonunique and the degeneracy is continuous: Any periodic configuration of density rho with all reciprocal lattice vectors not smaller than K(0), and any union of such configurations, is a GSC. The fcc lattice is a GSC only for rho > or = (1/6 sq.rt(3)) x (K(0)/pi)(3).     

Precise simulation of near-critical fluid coexistence

We present a novel method to derive liquid-gas coexisting densities, rho(+/-)(T), from grand canonical simulations (without knowledge of T(c) or criticality class). The minima of Q(L) identical with (2)(L)/(L) in an LxLxL box with m=rho-(L) are used to generate recursively an unbiased universal finite-size scaling function. Monte Carlo data for a hard-core square-well fluid and for the restricted primitive model electrolyte yield rho(+/-) to +/-1%-2% of rho(c) down to 1 part in 10(4)-10(3) of T(c) (and confirm well Ising character). Pressure mixing in the scaling fields is unequivocally revealed and indicates Yang-Yang ratios R(mu)=-0.04(4) and 0.2(6) for the two models, respectively.     

Finite-size scaling and universality of the thermal resistivity of liquid 4He near Tlambda

We present measurements of the thermal resistivity rho(t,P,L) near the superfluid transition of 4He at saturated vapor pressure and confined in cylindrical geometries with radii L=0.5 and 1.0 microm [t identical with T/T(lambda)(P)-1]. For L=1.0 microm measurements at six pressures P are presented. At and above T(lambda) the data are consistent with a universal scaling function F(X)=(L/xi(0))(x/nu)(rho/rho(0)), X=(L/xi(0))(1/nu)t valid for all P (rho(0) and x are the pressure-dependent amplitude and effective exponent of the bulk resistivity rho, and xi=xi(0)t(-nu) is the correlation length). Indications of breakdown of scaling and universality are observed below T(lambda).     

Chiral approach to nuclear matter: Role of explicit short-range <Emphasis Type="Italic">NN</Emphasis>-terms

We extend a recent chiral approach to nuclear matter by including the most general (momentum-independent) NN-contact interaction. Iterating this two-parameter contact vertex with itself and with one-pion exchange the emerging energy per particle exhausts all terms possible up to and including fourth order in the small momentum expansion. Two (isospin-dependent) cut-offs are introduced to regularize the (linear) divergences of some three-loop in-medium diagrams. The equation of state of pure neutron matter, , can be reproduced very well up to quite high neutron densities of by adjusting the strength of a repulsive nn-contact interaction. Binding and saturation of isospin-symmetric nuclear matter is a generic feature of our perturbative calculation. Fixing the maximum binding energy per particle to MeV we find that any possible equilibrium density lies below . The additional constraint from the neutron matter equation of state leads however to a somewhat too low saturation density of . We also investigate the effects of the NN-contact interaction on the complex single-particle potential U(p,k _f ) + i W(p,k _f ). We find that the effective nucleon mass at the Fermi surface is bounded from below by . This property keeps the critical temperature of the liquid-gas phase transition at somewhat too high values MeV. The downward bending of the asymmetry energy A(k _f ) above nuclear-matter saturation density is a generic feature of theapproximation to fourth order. We furthermore investigate the effects of the NN-contact interaction on the -term in the nuclear energy density functional . Altogether, there is within this complete fourth-order calculation no magic set of adjustable short-range parameters with which one could reproduce simultaneously and accurately all semi-empirical properties of nuclear matter. In particular, the conditions for a good neutron matter equation of state and for good single-particle properties are mutually exclusive.Received: 28 October 2003, Revised: 9 December 2003, Published online: 23 July 2004PACS: 12.38.Bx Perturbative calculations - 21.65. + f Nuclear matter - 24.10.Cn Many-body theory - 31.15.Ew Density-functional theory     

Q2 evolution of the neutron spin structure moments using a 3He target

We have measured the spin structure functions g(1) and g(2) of 3He in a double-spin experiment by inclusively scattering polarized electrons at energies ranging from 0.862 to 5.058 GeV off a polarized 3He target at a 15.5 degrees scattering angle. Excitation energies covered the resonance and the onset of the deep inelastic regions. We have determined for the first time the Q2 evolution of Gamma(1)(Q2)= integral (1)(0)g(1)(x,Q2)dx, Gamma(2)(Q2)= integral (1)(0)g(2)(x,Q2)dx, and d(2)(Q2)= integral (1)(0)x(2)[2g(1)(x,Q2)+3g(2)(x,Q2)]dx for the neutron in the range 0.1< or =Q2< or =0.9 GeV2 with good precision. Gamma(1)(Q2) displays a smooth variation from high to low Q2. The Burkhardt-Cottingham sum rule holds within uncertainties and d(2) is nonzero over the measured range.     

Well-Posedness Theory for Aggregation Sheets

In this paper, we consider distribution solutions to the aggregation equation ${\rho_{t} + \mathrm{div}(\rho \mathbf{u} ) = 0, \; \mathbf{u} = -\nabla V * \rho}$ in ${\mathbb{R}^{d}}$ , where the density ρ concentrates on a co-dimension one manifold. We show that an evolution equation for the manifold itself completely determines the dynamics of such solutions. We refer to such solutions aggregation sheets. When the equation for the sheet is linearly well-posed, we show that the fully non-linear evolution is also well-posed locally in time for the class of bi-Lipschitz surfaces. Moreover, we show that if the initial sheet is C ¹ then the solution itself remains C ¹ as long as it remains Lipschitz. Lastly, we provide conditions on the kernel ${g(s) = -\frac{\mathrm{d}V}{\mathrm{d}s}}$ that guarantee the solution remains a bi-Lipschitz surface globally in time, and construct explicit solutions that either collapse or blow up in finite time when these conditions fail.     

Electrical resistivity in the ferromagnetic metallic state of La-Ca-MnO\mathsf{_{3}}: Role of electron-phonon interaction

The temperature-dependent resistivity of the perovskite manganites La_1-x Ca _x MnO₃, with x = 0.33, is theoretically analysed within the framework of the classical electron-phonon model of resistivity, i.e., the Bloch-Gruneisen model. Due to inherent acoustic (low-frequency) phonons ( as well as high-frequency optical phonons ( , the contributions to the resistivity have first been estimated. The acoustic phonons of the oxygen-breathing mode yield a relatively larger contribution to the resistivity compared to the contribution of optical phonons. Furthermore, the nature of phonons changes around T = 167 K exhibiting a crossover from an acoustic to optical phonon regime with elevated temperature. The contribution to resistivity estimated by considering both phonons, i.e. and , when subtracted from thin film data, infers a power temperature dependence over most of the temperature range. The quadratic temperature dependence of is understood in terms of electron-electron scattering. Moreover, in the higher temperature limit, the difference can be varies linearly with T ^4.5 in accordance with the electron-magnon scattering in the double exchange process. Within the proposed scheme, the present numerical analysis of temperature dependent resistivity shows similar results as those revealed by experiment.Received: 8 April 2004, Published online: 12 August 2004PACS: 5.47.Gk Colossal magnetoresistance - 72.15.-v Electronic conduction in metals and alloys - 74.25.Kc Phonons - 75.30.Ds Spin wavesD. Varshney: dvboson.sop@dauniv.ac.in     

Suitable resolution of EOS tables for neutron star investigations

Inasmuch as the hydrostatic structure of the interior of neutron stars uniquely depends on the equation of state(EOS), the inverse constraints on EOS from astrophysical observations have been an important method for revealing the properties of high density matter. Currently, most EOS for neutron star matter are given in tabular form,but these numerical tables can have quite different resolution. To guarantee both the accuracy and efficiency in computing the Tolman-Oppenheimer-Volkoff equations, a concise standard for generating EOS tables with suitable resolution is investigated. It is shown that EOS tables with 50 points logarithmic-uniformly distributed in the supra-nuclear density segment [ρ_0, 10_(ρ0)], where ρ_0 is the nuclear saturation density, correspond to the interpolation induced errors of ～0.02% for the gravitational mass M and ～0.2% for the tidal deformability ∧.     

Precision spectroscopy of pionic 1s states of Sn nuclei and evidence for partial restoration of chiral symmetry in the nuclear medium

Deeply bound 1s states of pi(-) in (115,119,123)Sn were preferentially observed using the Sn(d,3He) pion-transfer reaction under the recoil-free condition. The 1s binding energies and widths were precisely determined and were used to deduce the isovector parameter of the s-wave pion-nucleus potential to be b1=-(0.115+/-0.007)m(-1)(pi). The observed enhancement of |b(1)| over the free piN value (b(free)1/b1=0.78+/-0.05) indicates a reduction of the chiral order parameter, f*pi(rho)2/f2pi approximately 0.64, at the normal nuclear density, rho=rho(0).     

Improved Measurement of the Cabibbo-Kobayashi-Maskawa angle alpha using B0(B) --> rho+rho- decays

We present results from an analysis of B(0)B(0)--> rho(+)rho(-) using 232 x 10(6) Gamma (4S) --> BB decays collected with the BABAR detector at the PEP-II asymmetric-energy B factory at SLAC. We measure the longitudinal polarization fraction f(L) = 0.978 +/- 0.014(stat) + 0.021 / -0.029(syst) and the CP-violating parameters S(L)= -0.33 +/- 0.24(stat) + 0.08 / -0.14(syst) and C(L)= -0.03 +/- 0.18(stat) +/- 0.09(syst). Using an isospin analysis of B --> rhorho decays, we determine the unitarity triangle parameter alpha. The solution compatible with the standard model is alpha = (100 +/- 13) degrees.     

The “dark side” of gravitational experiments

Theoretical speculations about the quantum nature of the gravitational interaction have motivated many recent experiments. But perhaps the most profound and puzzling questions that these investigations address surround the observed cosmic acceleration, or Dark Energy. This mysterious substance comprises roughly two-thirds of the energy density of the universe. Current gravitational experiments may soon have the sensitivity to detect subtle clues that will reveal the mechanism behind the cosmic acceleration. On the laboratory scale, short-range tests of the Newtonian inverse-square law (ISL) provide the most sensitive measurements of gravity at the Dark Energy length scale, where is the observed Dark Energy density. This length scale may also have fundamental significance that could be related to the “size” of the graviton. At the University of Washington, we are conducting the world’s most sensitive, short-range test of the Newtonian ISL. Fourth Award in the 2006 Essay Competition of the Gravity Research Foundation.     

Search for the radiative decays B-->rhogamma and B0-->omegagamma

A search of the exclusive radiative decays B-->rho(770)gamma and B0-->omega(782)gamma is performed on a sample of about 84x10(6) BBmacr; events collected by the BABAR detector at the SLAC PEP-II asymmetric-energy e+e- storage ring. No significant signal is seen in any of the channels. We set upper limits on the branching fractions B of B(B0-->rho(0)gamma)<1.2 x 10(-6), B(B+-->rho+gamma)<2.1 x 10(-6), and B(B0-->omegagamma)<1.0 x 10(-6) at 90% confidence level (C.L.). Using the assumption that Gamma(B-->rhogamma)=Gamma(B+-->rho(+)gamma)=2 x Gamma(B0-->rho(0)gamma), we find the combined limit B(B-->rhogamma)<1.9 x 10(-6), corresponding to B(B-->rhogamma)/B(B-->K*gamma)<0.047 at 90% C.L.     

Sodium Bose-Einstein condensates in the F = 2 state in a large-volume optical trap

We have investigated the properties of Bose-Einstein condensates of sodium atoms in the upper hyperfine ground state. Condensates in the high-field seeking [F=2, m(F)=-2> state were created in a large volume optical trap from initially prepared [F=1, m(F)=-1> condensates using a microwave transition at 1.77 GHz. We found condensates in the stretched state [F=2, m(F)=-2> to be stable for several seconds at densities in the range of 10(14) atoms/cm(3). In addition, we studied the clock transition [F=1, m(F)=0> --> [F=2, m(F)=0> in a sodium Bose-Einstein condensate and determined a density-dependent frequency shift of (2.44+/-0.25+/-0.5) x 10(-12) Hz cm(3).     

Superfluid density in a highly underdoped YBa_{2}Cu_{3}O_{6+y} superconductor

The superfluid density rho_{s}(T) identical with1/lambda;{2}(T) has been measured at 2.64 GHz in highly underdoped YBa_{2}Cu_{3}O_{6+y}, at 37 dopings with T_{c} between 3 and 17 K. Within limits set by the transition width DeltaT_{c} approximately 0.4 K, rho_{s}(T) shows no evidence of critical fluctuations as T-->T_{c}, with a mean-field-like transition and no indication of vortex unbinding. Instead, we propose that rho_{s} displays the behavior expected for a quantum phase transition in the (3+1)-dimensional XY universality class, with rho_{s0} proportional, variant(p-p_{c}), T_{c} proportional, variant(p-p_{c});{1/2}, and rho_{s}(T) proportional, variant(T_{c}-T);{1} as T-->T_{c}.     

Thermal unparticles: a new form of energy density in the universe

An unparticle with scaling dimension has peculiar thermal properties due to its unique phase space structure. We find that the equation of state parameter , the ratio of pressure to energy density, is given by providing a new form of energy in our universe. In an expanding universe, the unparticle energy density evolves dramatically differently from that for photons. For , even if at a high decoupling temperature T _D is very small, it is possible to have a large relic density at present photon temperature T _γ ⁰, large enough to play the role of dark matter. We calculate T _D and using photon–unparticle interactions for illustration.     

Universality class of criticality in the restricted primitive model electrolyte

The 1:1 equisized hard-sphere electrolyte or restricted primitive model has been simulated via grand-canonical fine-discretization Monte Carlo. Newly devised unbiased finite-size extrapolation methods using loci in the temperature-density or (T,rho) plane of isothermal rho(2-k) vs pressure inflections, of Q identical with(2)/ maxima, and of canonical and C(V) criticality, yield estimates of (T(c),rho(c)) to +/-(0.04,3)%. Extrapolated exponents and Q ratio are (gamma,nu,Q(c)) = [1.24(3), 0.63(3); 0.624(2)], which support Ising (n = 1) behavior with (1.23(9), 0.630(3); 0.623(6)), but exclude classical, XY (n = 2), self-avoiding walk (n = 0), and n = 1 criticality with potentials varphi(r)>Phi/r(4.9) when r-->infinity.     

Dynamics of liquid 4He in vycor

Using inelastic neutron scattering, we have observed well-defined phonon-roton ( p-r) excitations in superfluid 4He in Vycor over a wide wave-vector range, 0.3相似文献   

Long range bond-bond correlations in dense polymer solutions

The scaling of the bond-bond correlation function P1(s) along linear polymer chains is investigated with respect to the curvilinear distance s along the flexible chain and the monomer density rho via Monte Carlo and molecular dynamics simulations. Surprisingly, the correlations in dense three-dimensional solutions are found to decay with a power law P1(s) approximately s(-omega) with omega=3/2 and the exponential behavior commonly assumed is clearly ruled out for long chains. In semidilute solutions, the density dependent scaling of P1(s) approximately g(-omega(0))(s/g)(-omega) with omega(0)=2-2nu=0.824 (nu=0.588 being Flory's exponent) is set by the number of monomers g(rho) in an excluded volume blob. Our computational findings compare well with simple scaling arguments and perturbation calculation. The power-law behavior is due to self-interactions of chains caused by the chain connectivity and the incompressibility of the melt.     

BFKL resummation effects in γ*γ* → ρρ

We calculate the leading order BFKL amplitude for the exclusive diffractive process in the forward direction, which can be studied in future high energy e ⁺ e ^- linear colliders. The resummation effects are very large compared to the fixed-order calculation. We also estimate the next-to-leading logarithmic corrections to the amplitude by using a specific resummation of higher order effects and find a substantial growth with energy, but smaller than in the leading logarithmic approximation. CPHT Unité mixte 8627 du CNRS LPT Unité mixte 8627 du CNRS An erratum to this article is available at .