21,722 research outputs found

    Thermodynamics of the frustrated J1J_1-J2J_2 Heisenberg ferromagnet on the body-centered cubic lattice with arbitrary spin

    Full text link
    We use the spin-rotation-invariant Green's function method as well as the high-temperature expansion to discuss the thermodynamic properties of the frustrated spin-SS J1J_{1}-J2J_{2} Heisenberg magnet on the body-centered cubic lattice. We consider ferromagnetic nearest-neighbor bonds J1<0J_1 < 0 and antiferromagnetic next-nearest-neighbor bonds J2≥0J_2 \ge 0 and arbitrary spin SS. We find that the transition point J2cJ_2^c between the ferromagnetic ground state and the antiferromagnetic one is nearly independent of the spin SS, i.e., it is very close to the classical transition point J2c,clas=23∣J1∣J_2^{c,{\rm clas}}= \frac{2}{3}|J_1|. At finite temperatures we focus on the parameter regime J2<J2cJ_2<J_2^c with a ferromagnetic ground-state. We calculate the Curie temperature TC(S,J2)T_{C}(S,J_{2}) and derive an empirical formula describing the influence of the frustration parameter J2J_{2} and spin SS on TCT_C. We find that the Curie temperature monotonically decreases with increasing frustration J2J_2, where very close to J2c,clasJ_2^{c,{\rm clas}} the TC(J2)T_C(J_2)-curve exhibits a fast decay which is well described by a logarithmic term 1/log(23∣J1∣−J2)1/\textrm{log}(\frac{2}{3}|J_1|-J_{2}). To characterize the magnetic ordering below and above TCT_C, we calculate the spin-spin correlation functions ⟨S0SR⟩\langle {\bf S}_{\bf 0} {\bf S}_{\bf R} \rangle, the spontaneous magnetization, the uniform static susceptibility χ0\chi_0 as well as the correlation length ξ\xi. Moreover, we discuss the specific heat CVC_V and the temperature dependence of the excitation spectrum. As approaching the transition point J2cJ_2^c some unusual features were found, such as negative spin-spin correlations at temperatures above TCT_C even though the ground state is ferromagnetic or an increase of the spin stiffness with growing temperature.Comment: 19 pages, 10 figures, version as in EPJ

    A theoretical analysis of the current-voltage characteristics of solar cells

    Get PDF
    The current-voltage characteristics and efficiencies of solar cells are discussed. For one solar cell structure detailed curves are presented which include carrier densities, current densities, potential, and quasi-Fermi levels at different voltage levels both with and without optically generated carriers (AMO conditions). In addition some results are presented concerning the influence of various parameter variations such as lifetime, cell thickness, and high-low junction width on solar cell performance

    A theoretical analysis of the current-voltage characteristics of solar cells

    Get PDF
    Various mechanisms which limit the conversion efficiency of silicon solar cells were studied. The effects of changes in solar cell geometry such as layer thickness on performance were examined. The effects of various antireflecting layers were also examined. It was found that any single film antireflecting layer results in a significant surface loss of photons. The use of surface texturing techniques or low loss antireflecting layers can enhance by several percentage points the conversion efficiency of silicon cells. The basic differences between n(+)-p-p(+) and p(+)-n-n(+) cells are treated. A significant part of the study was devoted to the importance of surface region lifetime and heavy doping effects on efficiency. Heavy doping bandgap reduction effects are enhanced by low surface layer lifetimes, and conversely, the reduction in solar cell efficiency due to low surface layer lifetime is further enhanced by heavy doping effects. A series of computer studies is reported which seeks to determine the best cell structure and doping levels for maximum efficiency

    A theoretical study of heterojunction and graded band gap type solar cells

    Get PDF
    The work performed concentrated on including multisun effects, high temperature effects, and electron irradiation effects into the computer analysis program for heterojunction and graded bandgap solar cells. These objectives were accomplished and the program is now available for such calculations

    Solving the characteristic initial value problem for colliding plane gravitational and electromagnetic waves

    Get PDF
    A method is presented for solving the characteristic initial value problem for the collision and subsequent nonlinear interaction of plane gravitational or gravitational and electromagnetic waves in a Minkowski background. This method generalizes the monodromy transform approach to fields with nonanalytic behaviour on the characteristics inherent to waves with distinct wave fronts. The crux of the method is in a reformulation of the main nonlinear symmetry reduced field equations as linear integral equations whose solutions are determined by generalized (``dynamical'') monodromy data which evolve from data specified on the initial characteristics (the wavefronts).Comment: 4 pages, RevTe

    ARXPS-studies ofcˆ-axis textured YBa2Cu3Ox-films

    Get PDF
    YBa2Cu3Ox sputter deposited cold on MgO grows in O2 annealing epitaxially to a transparent, superconducting film with Tc 80K. The unscraped surfaces of these films are smooth showing XPS lines changing with photoelectron take-off angle. This enhanced data base allows to separate the different chemical compounds (hydroxide, peroxide, carbonate, carboxyle, cuprate, graphite ...) and to obtain their spatial distribution. This yields the compounds, their amount and distribution making up the cinder growing with O2-anneal at internal and external surfaces. The cinder stoichiometry gives insights in the chemistry going on in O2 annealing. Below the cinder the signature ofcˆ-axis oriented YBa2Cu3Ox is identified, showing that a Ba-oxide layer forms the stable surface. This coats insulating CuO2 and Y-oxide layers yielding so an intrinsic dead layer

    Child Custody for Disabled Adults: What Kentucky Families Need

    Get PDF
    • …
    corecore