A microscopic approach to spin dynamics: about the meaning of spin relaxation times

We present an approach to spin dynamics by extending the optical Bloch equations for the driven two-level system to derive microscopic expressions for the transverse and longitudinal spin relaxation times. This is done for the 6-level system of electron and hole subband states in a semiconductor or a semiconductor quantum structure to account for the degrees-of-freedom of the carrier spin and the polarization of the exciting light and includes the scattering between carriers and lattice vibrations on a microscopic level. For the subsystem of the spin-split electron subbands we treat the electron-phonon interaction in second order and derive a set of equations of motion for the 2x2 spin-density matrix which describes the electron spin dynamics and contains microscopic expressions for the longitudinal (T_1) and the transverse (T_2) spin relaxation times. Their meaning will be discussed in relation to experimental investigations of these quantities.Comment: 9 pages, 3 figures, Replacement of cond-mat/0407358 due to substantial revisio

Optical Constants of Magnesium Oxide in the Far Ultraviolet

Optical constants of magnesium oxide in far ultraviole

Optical Constants of Lithium Fluoride in the Far Ultraviolet

Optical constants of lithium fluoride in far ultraviole

Methods of regularization for computing orbits in celestial mechanics

Numerical and analytical methods for orbit computation in celestial mechanics during and beyond collision by introduction of regularized coordinate

Imaging magnetoelectric subbands in ballistic constrictions

We perform scanning gate experiments on ballistic constrictions in the presence of small perpendicular magnetic fields. The constrictions form the entrance and exit of a circular gate-defined ballistic stadium. Close to constrictions we observe sets of regular fringes creating a checker board pattern. Inside the stadium conductance fluctuations governed by chaotic dynamics of electrons are visible. The checker board pattern allows us to determine the number of transmitted modes in the constrictions forming between the tip-induced potential and gate-defined geometry. Spatial investigation of the fringe pattern in a perpendicular magnetic field shows a transition from electrostatic to magnetic depopulation of magnetoelectric subbands. Classical and quantum simulations agree well with different aspects of our observations.Comment: 18 pages, 7 figure

Firm Formation with Complementarities: The Role of the Entrepreneur

We model the emergence of organization forms in a game between prospective entrepreneurs. Complementary roles arise endogenously in a way that admits a stable assignment of workers to firms. This contrasts with existing work on job matching, where stability typically requires workers to be substitutes. Our approach demonstrates that the labor market selection of entrepreneurs and their profit-maximizing choices lead to specific technologies in which certain workers are substitutes and others are complements. We give a simple characterization of equilibrium firm memberships and organizations. We show that payoffs in our non-cooperative solution lie in the core of the corresponding cooperative game, and can be obtained in a decentralized process that reduces information and planning requirements for the entrepreneur

Is More Entrepreneurship better?

We develop a new perspective on the boundary of the firm that is consistent with the empirical observation that the share of entrepreneurs first decreases and then increases in the course of economic development. Existing theory based on transaction costs is difficult to relate to these well-established dynamics. Our approach focuses on changing incentives to specialize and adapt, in order to access complementarities that arise from diverse abilities and access to wealth. We discuss why the efficient number of entrepreneurs is bounded and changes in the course of economic development

Entrepreneurship and Organization Design

We model entrepreneurship and the emergence of rms as an out- come of simultaneous bidding for labor services among heterogeneous agents. What distinguishes our approach from prior work is that oc- cupational choice and job matching are determined simultaneously, so that the opportunity costs of entrepreneurs are accounted for. Those who are relatively unmanageable, while possibly excellent managers themselves, become entrepreneurs. Entrepreneurs compete and create value by building e¢ cient organizations and o¤ering potentially well- paid jobs to others. While the entry of an additional entrepreneur typically reduces some individual wages, we show that it always raises the average wage and depresses the average income of incumbent en- trepreneurs. This result may help explain the empirically low returns to entrepreneurship