1,208 research outputs found
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
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