A simple classical mapping of the spin-polarized quantum electron gas: distribution functions and local field corrections

We use the now well known spin-unpolarized exchange-correlation energy E_{xc} of the uniform electron gas as the basic ``many-body'' input to determine the temperature T_q of a classical Coulomb fluid having the same correlation energy as the quantum system. It is shown that the spin-polarized pair distribution functions (SPDFs) of the classical fluid at T_q, obtained using the hyper-netted chain (HNC) equation are in excellent agreement with those of the T=0 quantum fluid obtained by quantum Monte Carlo (QMC) simulations. These methods are computationally simple and easily applied to problems which are currently outside the scope of QMC. Results are presented for the SPDFs and the local-field corrections to the response functions of the electron fluid at zero and finite temperatures.Comment: 4 pags (Revtex), 3 posscript figure

Spin and Valley dependent analysis of the two-dimensional low-density electron system in Si-MOSFETS

The 2-D electron system (2DES) in Si metal-oxide field-effect transistors (MOSFETS) consists of two distinct electron fluids interacting with each other. We calculate the total energy as a function of the density $n$, and the spin polarization $\zeta$ in the strongly-correlated low-density regime, using a classical mapping to a hypernetted-chain (CHNC) equation inclusive of bridge terms. Here the ten distribution functions, arising from spin and valley indices, are self-consistently calculated to obtain the total free energy, the chemical potential, the compressibility and the spin susceptibility. The T=0 results are compared with the 2-valley Quantum Monte Carlo (QMC) data of Conti et al. (at T=0, $\zeta=0$) and found to be in excellent agreement. However, unlike in the one-valley 2DES, it is shown that {\it the unpolarized phase is always the stable phase in the 2-valley system}, right up to Wigner Crystallization at $r_s=42$. This leads to the insensitivity of $g^*$ to the spin polarization and to the density. The compressibility and the spin-susceptibility enhancement calculated from the free energy confirm the validity of a simple approach to the two-valley response based on coupled-mode formation. The three methods, QMC, CHNC, and Coupled-mode theory agree closely. Our results contain no {\it ad hoc} fit parameters. They agree with experiments and do not invoke impurity effects or metal-insulator transition phenomenology.Comment: 10 pages 4 figure

Quantum Brayton cycle with coupled systems as working substance

We explore the quantum version of Brayton cycle with a composite system as the working substance. The actual Brayton cycle consists of two adiabatic and two isobaric processes. Two pressures can be defined in our isobaric process, one corresponds to the external magnetic field (characterized by $F_x$) exerted on the system, while the other corresponds to the coupling constant between the subsystems (characterized by $F_y$). As a consequence, we can define two types of quantum Brayton cycle for the composite system. We find that the subsystem experiences a quantum Brayton cycle in one quantum Brayton cycle (characterized by $F_x$), whereas the subsystem's cycle is of quantum Otto in another Brayton cycle (characterized by $F_y$). The efficiency for the composite system equals to that for the subsystem in both cases, but the work done by the total system are usually larger than the sum of work done by the two subsystems. The other interesting finding is that for the cycle characterized by $F_y$, the subsystem can be a refrigerator while the total system is a heat engine. The result in the paper can be generalized to a quantum Brayton cycle with a general coupled system as the working substance.Comment: 7 pages, 3 figures, accepted by Phys. Rev.

Structure of the Local-field factor of the 2-D electron fluid. Possible evidence for correlated scattering of electron pairs

The static local-field factor (LFF) of the 2-D electron fluid is calculated {\it nonperturbatively} using a mapping to a classical Coulomb fluid $\lbrack$Phys. Rev. Lett., {\bf 87}, 206$\rbrack$. The LFF for the paramagnetic fluid {\it differs markedly} from perturbation theory where a maximum near 2$k_F$ is expected. Our LFF has a quasi-linear small-k region leading to a maximum close to 3$k_F$, in agreent with currently available quantum Monte Carlo data. The structure in the LFF and its dependence on the density and temperature are interpretted as a signature of correlated scattering of electron pairs of opposite spin.The lack of structure at $2k_F$ implies weakened Friedel oscillations, Kohn anomalies etc.Comment: 4 pages, 3 figures, version 2 of condmat/0304034, see http://nrcphy1.phy.nrc.ca/ims/qp/chandre/chnc/ Changs in the text, figure 2 and updated reference

Idealized Slab Plasma approach for the study of Warm Dense Matter

Recently, warm dense matter (WDM) has emerged as an interdisciplinary field that draws increasing interest in plasma physics, condensed matter physics, high pressure science, astrophysics, inertial confinement fusion, as well as materials science under extreme conditions. To allow the study of well-defined WDM states, we have introduced the concept of idealized-slab plasmas that can be realized in the laboratory via (i) the isochoric heating of a solid and (ii) the propagation of a shock wave in a solid. The application of this concept provides new means for probing the dynamic conductivity, equation of state, ionization and opacity. These approaches are presented here using results derived from first-principles (density-functional type) theory, Thomas-Fermi type theory, and numerical simulations.Comment: 37 pages, 21 figures, available, pdf file only. To appear in: Laser and Particle beams. To appear more or less in this form in Laser and Particle beam

L'élevage traditionnel des petits ruminants au Sénégal. II. Pratiques de conduite et d'exploitation des animaux chez les éleveurs de la région de Louga

Les auteurs présentent ici une partie des résultats du programme "pathologie et productivité des petits ruminants en milieu traditionnel" (ISRA/IEMVT-CIRAD), à partir desquels ils analysent les pratiques et les stratégies des éleveurs de petits ruminants dans la région de Louga. La méthodologie s'appuie sur des enquêtes ponctuelles sur le terrain et l'analyse statistique des données micro-économiques recueillies lors d'un suivi hebdomadaire de 194 troupeaux de concession pendant une durée de 3 ans. Les enquêtes ont orienté le traitement des données, qui, à son tour, a soulevé des questions et généré de nouvelles enquêtes auprès des acteurs concernés. Les auteurs décrivent ensuite les conditions de l'élevage des petits ruminants: milieux naturel (zone sahélienne), humain (deux ethnies - Peul et Wolof - se partagent la région), et systèmes de production, puis les pratiques de conduite et de gestion économique, sensiblement différentes entre Peul et Wolof. Deux modes d'élevage sont toujours associés: l'élevage de case (surtout pratiqué par les Wolof), qui concerne les béliers les mieux conformés, restant à l'attache dans la concession et nourris à l'auge de 6 à 18 mois, avant d'être vendus au meilleur prix; l'élevage pastoral, qui concerne les autres ovins et les caprins partant pâturer la journée sous la conduite d'un berger et revenant le soir dans la concession où ils reçoivent une complémentation à l'auge. Le rôle de l'élevage de petits ruminants (production laitière, abattages festifs, épargne mobilisable en cas de besoin) est illustré par les résultats du suivi. Ceux-ci montrent clairement que les stratégies commerciales (choix des animaux à vendre ou à abattre, époque de vente, prix de vente, etc.) des deux ethnies sont tout à fait différentes et reflètent leurs objectifs majeurs: production laitière chez les Peul, ventes d'animaux chez les Wolof. (Résumé d'auteur

The Equation of State and the Hugoniot of Laser Shock-Compressed Deuterium

The equation of state and the shock Hugoniot of deuterium are calculated using a first-principles approach, for the conditions of the recent shock experiments. We use density functional theory within a classical mapping of the quantum fluids [ Phys. Rev. Letters, {\bf 84}, 959 (2000) ]. The calculated Hugoniot is close to the Path-Integral Monte Carlo (PIMC) result. We also consider the {\it quasi-equilibrium} two-temperature case where the Deuterons are hotter than the electrons; the resulting quasi-equilibrium Hugoniot mimics the laser-shock data. The increased compressibility arises from hot $D^+-e$ pairs occuring close to the zero of the electron chemical potential.Comment: Four pages; One Revtex manuscript, two postscipt figures; submitted to PR