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

On the existence of self-similar spherically symmetric wave maps coupled to gravity

We present a detailed analytical study of spherically symmetric self-similar solutions in the SU(2) sigma model coupled to gravity. Using a shooting argument we prove that there is a countable family of solutions which are analytic inside the past self-similarity horizon. In addition, we show that for sufficiently small values of the coupling constant these solutions possess a regular future self-similarity horizon and thus are examples of naked singularities. One of the solutions constructed here has been recently found as the critical solution at the threshold of black hole formation.Comment: 15 pages, LaTe

Machine Learning Estimation of Heterogeneous Causal Effects: Empirical Monte Carlo Evidence

We investigate the finite sample performance of causal machine learning estimators for heterogeneous causal effects at different aggregation levels. We employ an Empirical Monte Carlo Study that relies on arguably realistic data generation processes (DGPs) based on actual data. We consider 24 different DGPs, eleven different causal machine learning estimators, and three aggregation levels of the estimated effects. In the main DGPs, we allow for selection into treatment based on a rich set of observable covariates. We provide evidence that the estimators can be categorized into three groups. The first group performs consistently well across all DGPs and aggregation levels. These estimators have multiple steps to account for the selection into the treatment and the outcome process. The second group shows competitive performance only for particular DGPs. The third group is clearly outperformed by the other estimators

ORIGINS OF OVERCONFIDENCE, CONTINGENT MANAGEMENT STYLE AND ENTREPRENEURIAL FIRM SURVIVAL AND PERFORMANCE

This research concerns the role of overconfidence (OC), dependent on its psychological origins on three dimensions of entrepreneurial management that are supposed to influence entrepreneurial firm survival and performance. OC, defined as an overestimation of one’s own ability to make accurate forecasts, or, alternatively, as an overestimation of one’s own ability relative to others (the ‘better-than-average’ effect), is an important issue in entrepreneurship. On one hand, it could be shown that OC is positively linked to entrepreneurship (firm creation) and that entrepreneurs are generally more over-confident than other individuals such as managers. On the other hand, it had been argued that OC might increase the mortality risk of firms. In this paper, we enrich the OC discussion by taking into account the origins of OC and the effects of sources of OC on an entrepreneur’s management style, which in turn influences the survival and performance of entrepreneurial firms

Make it work - The challenge to diversity in entrepreneurial teams: A configurational perspective

Teams and timing are considered decisive for firm survival. We investigate the impact on firm survival of entrepreneurial team composition, in terms of diversity, and the speed of entering markets. Unlike research analysing the effects of low or high diversity, our research understands new venture teams as configurations of multiple, concurrent dimensions of diversity by untangling it in variety, separation, and disparity. By identifying distinct survival and failure configurations, we demonstrate that team va- riety is functional for firm survival if challenged by separation or disparity

The Stability of the Focal Firm in the Business Network: The Effect of Competence Shifts

In this paper, we explore the role of competence transfer at the actors’ level as driver of change in strategic networks. We use categories deriving from two different ways to see networks: strategy and IMP. Intrigued by the phenomenon that an increasing number of cases emerged in which companies from traditional industrialized countries that have been holders of major brands, have been acquired by former suppliers based in new industrial countries, we question the implicit assumption of the stability of the position of the focal firm in its strategic network. Through a longitudinal case study analysis of the sports shoe industry, we develop theoretical propositions as to how competence transfer leads to changes both at the network challenging the position of the focal firm

EIT ground-state cooling of long ion strings

Electromagnetically-induced-transparency (EIT) cooling is a ground-state cooling technique for trapped particles. EIT offers a broader cooling range in frequency space compared to more established methods. In this work, we experimentally investigate EIT cooling in strings of trapped atomic ions. In strings of up to 18 ions, we demonstrate simultaneous ground state cooling of all radial modes in under 1 ms. This is a particularly important capability in view of emerging quantum simulation experiments with large numbers of trapped ions. Our analysis of the EIT cooling dynamics is based on a novel technique enabling single-shot measurements of phonon numbers, by rapid adiabatic passage on a vibrational sideband of a narrow transition

An operator expansion for integrable quantum field theories

A large class of quantum field theories on 1+1 dimensional Minkowski space, namely, certain integrable models, has recently been constructed rigorously by Lechner. However, the construction is very abstract and the concrete form of local observables in these models remains largely unknown. Aiming for more insight into their structure, we establish a series expansion for observables, similar but not identical to the well-known form factor expansion. This expansion will be the basis for a characterization and explicit construction of local observables, to be discussed elsewhere. Here, we establish the expansion independent of the localization aspect, and analyze its behavior under space-time symmetries. We also clarify relations with deformation methods in quantum field theory, specifically, with the warped convolution in the sense of Buchholz and Summers.Comment: minor corrections and clarifications, as published in J. Phys A; 24 page

Deformations of Fermionic Quantum Field Theories and Integrable Models

Considering the model of a scalar massive Fermion, it is shown that by means of deformation techniques it is possible to obtain all integrable quantum field theoretic models on two-dimensional Minkowski space which have factorizing S-matrices corresponding to two-particle scattering functions S_2 satisfying S_2(0) = -1. Among these models there is for example the Sinh-Gordon model. Our analysis provides a complement to recent developments regarding deformations of quantum field theories. The deformed model is investigated also in higher dimensions. In particular, locality and covariance properties are analyzed.Comment: 20 page