Interfering directed paths and the sign phase transition

We revisit the question of the "sign phase transition" for interfering directed paths with real amplitudes in a random medium. The sign of the total amplitude of the paths to a given point may be viewed as an Ising order parameter, so we suggest that a coarse-grained theory for system is a dynamic Ising model coupled to a Kardar-Parisi-Zhang (KPZ) model. It appears that when the KPZ model is in its strong-coupling ("pinned") phase, the Ising model does not have a stable ferromagnetic phase, so there is no sign phase transition. We investigate this numerically for the case of {\ss}1+1 dimensions, demonstrating the instability of the Ising ordered phase there.Comment: 4 pages, 4 figure

Negative Magnetoresistance in the Nearest-neighbor Hopping Conduction

We propose a size effect which leads to the negative magnetoresistance in granular metal-insulator materials in which the hopping between two nearest neighbor clusters is the main transport mechanism. We show that the hopping probability increases with magnetic field. This is originated from the level crossing in a few-electron cluster. Thus, the overlap of electronic states of two neighboring clusters increases, and the negative magnetoresistance is resulted.Comment: Latex file, no figur

Anomalous Hall Effect of Calcium-doped Lanthanum Cobaltite Films

The Hall resistivity, magnetoresistance, and magnetization of La_{1-x}Ca_{x}CoO_{3} epitaxial films with x between 0.25 and 0.4 grown on lanthanum aluminate were measured in fields up to 7 T. The x=1/3 film, shows a reentrant metal insulator transition. Below 100 K, the x=1/3 and 0.4 films have significant coercivity which increases with decreasing temperature. At low temperature the Hall resistivity remains large and essentially field independent in these films, except for a sign change at the coercive field that is more abrupt than the switching of the magnetization. A unique magnetoresistance behavior accompanies this effect. These results are discussed in terms of a percolation picture and the mixed spin state model for this system. We propose that the low-temperature Hall effect is caused by spin-polarized carriers scattering off of orbital disorder in the spin-ordered clusters.Comment: REVTeX 4, 3 pages with 4 encapsulated postscript graphics. Submitted to MMM 2002 conference proceedings (J. Appl. Phys.

The binding constraint on firms'growth in developing countries

Firms in developing countries face numerous and serious constraints on their growth, ranging from corruption to lack of infrastructure to inability to access finance. Countries lack the resources to remove all the constraints at once and so would be better off removing the most binding one first. This paper uses data from World Bank Enterprise Surveys in 2006-10 to identify the most binding constraints on firm operations in developing countries. While each country faces a different set of constraints, these constraints also vary by firm characteristics, especially firm size. Across all countries, access to finance is among the most binding constraints; other obstacles appear to matter much less. This result is robust for all regions. Smaller firms must rely more on their own funds to invest and would grow significantly faster if they had greater access to external funds. As a result, a low level of financial development skews the firm size distribution by increasing the relative share of small firms. The results suggest that financing constraints play a significant part in explaining the"missing middle"-- the failure of small firms in developing countries to grow into medium-size or large firms.Access to Finance,Environmental Economics&Policies,Microfinance,Debt Markets,Banks&Banking Reform