Controls, belief updating, and bias in medical RCTs

We develop a formal model of placebo e§ects. If subjects in seemingly-ideal single-stage RCTsupdate beliefs about breakthroughs based upon personal physiological responses, mental e§ectsdi§er across medications received, treatment versus control. Consequently, the average cross-arm health di§erence becomes a biased estimator. Constructively, we show: bias can be alteredthrough choice of control; higher-e¢ cacy controls mitigate upward bias; and e¢ cacy states canbe revealed through controls of intermediate e¢ cacy or controls that mimic a subset of e¢ cacystates. Consistent with experimental evidence, our theory implies outcomes within-arm andcross-arm di§erences can be non-monotone in treatment probability. Finally, we develop noveldi§erences-in-di§erences and triangle equality tests to detect RCT bias

Rational expectations and the paradox of policy-relevant natural experiments

Policy experiments using large microeconomic datasets have recently gained ground in macro- economics. Imposing rational expectations, we examine robustness of evidence derived from ideal natural experiments applied to atomistic agents in dynamic settings. Paradoxically, once experi-mental evidence is viewed as su¢ ciently clean to use, it then becomes contaminated byex post endo- geneity: Measured responses depend upon priors and the objective function into which evidence is fed. Moreover, agentsípolicy beliefs become endogenously correlated with their causal parameters, severely clouding inference, e.g. sign reversals and non-invertibility may obtain. Treatment-control di§erences are contaminated for non-quadratic adjustment costs. Constructively, we illustrate how inference can be corrected accounting for feedback and highlight factors mitigating contamination

Privatization and governance regulation in frontier emerging markets: The case of Romania

We investigate the link between the regulation of control transactions and the institutional and corporate features of public companies, by analyzing the massive delisting activity in the Romanian capital market. The peculiar ownership reforms involving a large number of listed companies offer a unique opportunity to test Bebchuk and Roe’s (2000) theory of path dependence. Over time, the Romanian authorities have undertaken wide-ranging institutional reforms, most of which favoring blockholders over small and dispersed shareholders. Our empirical approach, based on logit and duration models, allows us to analyze the evolution of public companies over this period and sheds light on the likely events causing the eclipse of frontier emerging markets. Our main findings reveal that delisting is more likely to occur when (i) the shareholdings acquired from the privatization authority by circumventing the capital market are high; (ii) the company experiences frequent takeover bids; and (iii) the stock liquidity is low.ou

Observation of inter-Landau-level quantum coherence in semiconductor quantum wells

Using three-pulse four-wave-mixing femtosecond spectroscopy, we excite a non-radiative coherence between the discrete Landau levels of an undoped quantum well and study its dynamics. We observe quantum beats that reflect the time evolution of the coherence between the two lowest Landau level magnetoexcitons. We interpret our observations using a many-body theory and find that the inter Landau level coherence decays with a new time constant, substantially longer than the corresponding interband magnetoexciton dephasing times. Our results indicate a new intraband excitation dynamics that cannot be described in terms of uncorrelated interband excitations.Comment: 5 pages, 5 figures, to appear in Phys. Rev. B Rapid Communication

Ultrafast dynamics of coherences in the quantum Hall system

Using three-pulse four-wave-mixing optical spectroscopy, we study the ultrafast dynamics of the quantum Hall system. We observe striking differences as compared to an undoped system, where the 2D electron gas is absent. In particular, we observe a large off-resonant signal with strong oscillations. Using a microscopic theory, we show that these are due to many-particle coherences created by interactions between photoexcited carriers and collective excitations of the 2D electron gas. We extract quantitative information about the dephasing and interference of these coherences.Comment: 4 pages, 4 figures, to be published in Phys. Rev. Let

Photon-energy dissipation caused by an external electric circuit in "virtual" photo-excitation processes

We consider generation of an electrical pulse by an optical pulse in the ``virtual excitation'' regime. The electronic system, which is any electro-optic material including a quantum well structure biased by a dc electric field, is assumed to be coupled to an external circuit. It is found that the photon frequency is subject to an extra red shift in addition to the usual self-phase modulation, whereas the photon number is conserved. The Joule energy consumed in the external circuit is supplied only from the extra red shift.Comment: 4 pages, 1 fugur

Parity forbidden excitations of Sr2CuO2Cl2 revealed by optical third-harmonic spectroscopy

We present the first study of nonlinear optical third harmonic generation in the strongly correlated charge-transfer insulator Sr2CuO2Cl2. For fundamental excitation in the near-infrared, the THG spectrum reveals a strongly resonant response for photon energies near 0.7 eV. Polarization analysis reveals this novel resonance to be only partially accounted for by three-photon excitation to the optical charge-transfer exciton, and indicates that an even-parity excitation at 2 eV, with a_1g symmetry, participates in the third harmonic susceptibility.Comment: Requires RevTeX v4.0beta

Photonic crystals of coated metallic spheres

It is shown that simple face-centered-cubic (fcc) structures of both metallic and coated metallic spheres are ideal candidates to achieve a tunable complete photonic bandgap (CPBG) for optical wavelengths using currently available experimental techniques. For coated microspheres with the coating width to plasma wavelength ratio $l_c/\lambda_p \leq 10$ and the coating and host refractive indices $n_c$ and $n_h$, respectively, between 1 and 1.47, one can always find a sphere radius $r_s$ such that the relative gap width $g_w$ (gap width to the midgap frequency ratio) is larger than 5% and, in some cases, $g_w$ can exceed 9%. Using different coatings and supporting liquids, the width and midgap frequency of a CPBG can be tuned considerably.Comment: 14 pages, plain latex, 3 ps figures, to appear in Europhys. Lett. For more info on this subject see http://www.amolf.nl/research/photonic_materials_theory/moroz/moroz.htm