Survey of Biomass Resource Assessments and Assessment Capabilities in APEC Economies

This survey of biomass resource assessments and assessment capabilities in Asia-Pacific Economic Cooperation (APEC) economies considered various sources: academic and government publications, media reports, and personal communication with contacts in member economies

Future of Liquid Biofuels for APEC Economies

This project was initiated by APEC Energy Working Group (EWG) to maximize the energy sector's contribution to the region's economic and social well-being through activities in five areas of strategic importance including liquid biofuels production and development

Critical Dynamics of a Vortex Loop Model for the Superconducting Transition

We calculate analytically the dynamic critical exponent $z_{MC}$ measured in Monte Carlo simulations for a vortex loop model of the superconducting transition, and account for the simulation results. In the weak screening limit, where magnetic fluctuations are neglected, the dynamic exponent is found to be $z_{MC} = 3/2$. In the perfect screening limit, $z_{MC} = 5/2$. We relate $z_{MC}$ to the actual value of $z$ observable in experiments and find that $z \sim 2$, consistent with some experimental results

3D Lowest Landau Level Theory Applied to YBCO Magnetization and Specific Heat Data: Implications for the Critical Behavior in the H-T Plane

We study the applicability of magnetization and specific heat equations derived from a lowest-Landau-level (LLL) calculation, to the high-temperature superconducting (HTSC) materials of the YBa$_2$Cu$_3$O$_{7-\delta}$ (YBCO) family. We find that significant information about these materials can be obtained from this analysis, even though the three-dimensional LLL functions are not quite as successful in describing them as the corresponding two-dimensional functions are in describing data for the more anisotropic HTSC Bi- and Tl-based materials. The results discussed include scaling fits, an alternative explanation for data claimed as evidence for a second order flux lattice melting transition, and reasons why 3DXY scaling may have less significance than previously believed. We also demonstrate how 3DXY scaling does not describe the specific heat data of YBCO samples in the critical region. Throughout the paper, the importance of checking the actual scaling functions, not merely scaling behavior, is stressed.Comment: RevTeX; 10 double-columned pages with 7 figures embedded. (A total of 10 postscript files for the figures.) Submitted to Physical Review

Extreme Type-II Superconductors in a Magnetic Field: A Theory of Critical Fluctuations

A theory of critical fluctuations in extreme type-II superconductors subjected to a finite but weak external magnetic field is presented. It is shown that the standard Ginzburg-Landau representation of this problem can be recast, with help of a novel mapping, as a theory of a new "superconductor", in an effective magnetic field whose overall value is zero, consisting of the original uniform field and a set of neutralizing unit fluxes attached to $N_{\Phi}$ fluctuating vortex lines. The long distance behavior is related to the anisotropic gauge theory in which the original magnetic field plays the role of "charge". The consequences of this "gauge theory" scenario for the critical behavior in high temperature superconductors are explored in detail, with particular emphasis on questions of 3D XY vs. Landau level scaling, physical nature of the vortex "line liquid" and the true normal state, and fluctuation thermodynamics and transport. A "minimal" set of requirements for the theory of vortex-lattice melting in the critical region is also proposed and discussed.Comment: 28 RevTeX pages, 4 .ps figures; appendix A added, additional references, streamlined Secs. IV and V in response to referees' comment

Scaling in high-temperature superconductors

A Hartree approximation is used to study the interplay of two kinds of scaling which arise in high-temperature superconductors, namely critical-point scaling and that due to the confinement of electron pairs to their lowest Landau level in the presence of an applied magnetic field. In the neighbourhood of the zero-field critical point, thermodynamic functions scale with the scaling variable $(T-T_{c2}(B))/B^{1/2\nu}$, which differs from the variable $(T - T_c(0))/B^{1/2\nu}$ suggested by the gaussian approximation. Lowest-Landau-level (LLL) scaling occurs in a region of high field surrounding the upper critical field line but not in the vicinity of the zero-field transition. For YBaCuO in particular, a field of at least 10 T is needed to observe LLL scaling. These results are consistent with a range of recent experimental measurements of the magnetization, transport properties and, especially, the specific heat of high-$T_c$ materials.Comment: 22 pages + 1 figure appended as postscript fil

Nature of the Low Field Transition in the Mixed State of High Temperature Superconductors

We have numerically studied the statics and dynamics of a model three-dimensional vortex lattice at low magnetic fields. For the statics we use a frustrated 3D XY model on a stacked triangular lattice. We model the dynamics as a coupled network of overdamped resistively-shunted Josephson junctions with Langevin noise. At low fields, there is a weakly first-order phase transition, at which the vortex lattice melts into a line liquid. Phase coherence parallel to the field persists until a sharp crossover, conceivably a phase transition, near $T_{\ell} > T_m$ which develops at the same temperature as an infinite vortex tangle. The calculated flux flow resistivity in various geometries near $T=T_{\ell}$ closely resembles experiment. The local density of field induced vortices increases sharply near $T_\ell$, corresponding to the experimentally observed magnetization jump. We discuss the nature of a possible transition or crossover at $T_\ell$(B) which is distinct from flux lattice melting.Comment: Updated references. 46 pages including low quality 25 eps figures. Contact [email protected] or visit http://www.physics.ohio-state.edu:80/~ryu/ for better figures and additional movie files from simulations. To be published in Physical Review B1 01Jun9

Critical scaling of the a.c. conductivity for a superconductor above Tc

We consider the effects of critical superconducting fluctuations on the scaling of the linear a.c. conductivity, \sigma(\omega), of a bulk superconductor slightly above Tc in zero applied magnetic field. The dynamic renormalization- group method is applied to the relaxational time-dependent Ginzburg-Landau model of superconductivity, with \sigma(\omega) calculated via the Kubo formula to O(\epsilon^{2}) in the \epsilon = 4 - d expansion. The critical dynamics are governed by the relaxational XY-model renormalization-group fixed point. The scaling hypothesis \sigma(\omega) \sim \xi^{2-d+z} S(\omega \xi^{z}) proposed by Fisher, Fisher and Huse is explicitly verified, with the dynamic exponent z \approx 2.015, the value expected for the d=3 relaxational XY-model. The universal scaling function S(y) is computed and shown to deviate only slightly from its Gaussian form, calculated earlier. The present theory is compared with experimental measurements of the a.c. conductivity of YBCO near Tc, and the implications of this theory for such experiments is discussed.Comment: 16 pages, submitted to Phys. Rev.

Systematics of two-component superconductivity in YBa2Cu3O6.95YBa_{2}Cu_{3}O_{6.95} from microwave measurements of high quality single crystals

Systematic microwave surface impedance measurements of YBCO single crystals grown in $BaZrO_3$ crucibles reveal new properties that are not directly seen in similar measurements of other YBCO samples. Two key observations obtained from complex conductivity are: a new normal conductivity peak at around 80K and additional pairing below 65K. High pressure oxygenation of one of the crystals still yields the same results ruling out any effect of macroscopic segregation of O-deficient regions. A single complex order parameter cannot describe these data, and the results suggest at least two superconducting components. Comparisons with model calculations done for various decoupled two-component scenarios (i.e. s+d, d+d) are presented. Systematics of three single crystals show that the 80K quasiparticle peak is correlated with the normal state inelastic scattering rate. Close to Tc, the data follow a mean-field behavior. Overall, our results strongly suggest the presence of multiple pairing temperature and energy scales in $YBa_{2}Cu_{3}O_{6.95}$.Comment: 14 pages, 2-column, Revtex, 5 embedded postscript figures, uses graphicx. Postscript version also available at http://sagar.physics.neu.edu/preprints.htm