The onset of exciton absorption in modulation doped GaAs quantum wells

We study the evolution of the absorption spectrum of a modulation doped GaAs/AlGaAs semiconductor quantum well with decreasing the carrier density. We find that there is a critical density which marks the transition from a Fermi edge singularity to a hydrogen-like behavior. At this density both the lineshape and the transitions energies of the excitons change. We study the density dependence of the singularity exponent $\alpha$ and show that disorder plays an important role in determining the energy scale over which it grows.Comment: 10 pages, 3 figure

Do soccer players play the mixed-strategy Nash equilibrium?

Mixed-strategy Nash equilibrium (MSNE) is a commonly-used solution concept in game-theoretic models in various fields in economics, management, and other disciplines, but the experimental results whether the MSNE predicts well actual play in games is mixed. Consequently, evidence for naturally-occurring games in which the MSNE predicts the outcome well is of great importance, as it can justify the vast use of MSNE in models. The game between the kicker and goalkeeper in soccer penalty kicks is a real-world game that can be used to examine the application of the MSNE concept or its accuracy because payoffs are a common knowledge, the players have huge incentives to play correctly, the game is simple enough to analyze, its Nash equilibrium is in mixed strategies, and players' actions can be observed. We collected and analyzed data on the direction of kicks and jumps in penalty kicks in various top leagues and tournaments. Our analysis suggests that the MSNE predictions are the closest to the actual sample data, even though some other prediction methods use information on the marginal distribution of kicks or jumps whereas the MSNE does not.Soccer; Football; MSNE; Mixed-strategies; Mixed-strategy Nash equilibrium; Sports; Penalty kicks

Combined High Power and High Frequency Operation of InGaAsP/InP Lasers at 1.3μm

A simultaneous operation of a semiconductor laser at high power and high speed was demonstrated in a buried crescent laser on a P-InP substrate. In a cavity length of 300μm, a maximum CW power of 130mW at room temperature was obtained in a junction-up mounting configuration. A 3dB bandwidth in excess of 12GHz at an output power of 52mW was observed

Anomalous Microfluidic Phonons Induced by the Interplay of Hydrodynamic Screening and Incompressibility

We investigate the acoustic normal modes ("phonons") of a 1D microfluidic droplet crystal at the crossover between 2D flow and confined 1D plug flow. The unusual phonon spectra of the crystal, which arise from long-range hydrodynamic interactions, change anomalously under confinement. The boundaries induce weakening and screening of the interactions, but when approaching the 1D limit we measure a marked increase in the crystal sound velocity, a sign of interaction strengthening. This non-monotonous behavior of the phonon spectra is explained theoretically by the interplay of screening and plug flow.Comment: http://link.aps.org/doi/10.1103/PhysRevLett.99.124502 http://www.weizmann.ac.il/complex/tlusty/papers/PhysRevLett2007.pd

The absorption spectrum around nu=1: evidence for a small size Skyrmion

We measure the absorption spectrum of a two-dimensional electron system (2DES) in a GaAs quantum well in the presence of a perpendicular magnetic field. We focus on the absorption spectrum into the lowest Landau Level around nu=1. We find that the spectrum consists of bound electron-hole complexes, trion and exciton like. We show that their oscillator strength is a powerful probe of the 2DES spatial correlations. We find that near nu=1 the 2DES ground state consists of Skyrmions of small size (a few magnetic lengths).Comment: To be published in Phys Rev Lett. To be presented in ICSP2004, Flagstaff, Arizona. 4 figures (1 of them in color). 5 page

Denaturation of Circular DNA: Supercoil Mechanism

The denaturation transition which takes place in circular DNA is analyzed by extending the Poland-Scheraga model to include the winding degrees of freedom. We consider the case of a homopolymer whereby the winding number of the double stranded helix, released by a loop denaturation, is absorbed by \emph{supercoils}. We find that as in the case of linear DNA, the order of the transition is determined by the loop exponent $c$. However the first order transition displayed by the PS model for $c>2$ in linear DNA is replaced by a continuous transition with arbitrarily high order as $c$ approaches 2, while the second-order transition found in the linear case in the regime $1<c\le2$ disappears. In addition, our analysis reveals that melting under fixed linking number is a \emph{condensation transition}, where the condensate is a macroscopic loop which appears above the critical temperature.Comment: 9 pages, 4 figure

Spontaneous Symmetry Breaking through Mixing

We discuss a model, in which the negative mass square needed in the Higgs mechanism is generated by mixing with a heavy scalar. We have two scalar doublets in the standard model. Phenomenological properties of the heavy new scalar are discussed. The heavy scalar can be detected by the LHC.Comment: 4 page

Ultralow threshold graded-index separate-confinement heterostructure single quantum well (Al,Ga)As lasers

Broad area graded‐index separate‐confinement heterostructure single quantum well lasers grown by molecular‐beam epitaxy (MBE) with threshold current density as low as 93 A/cm^2 (520 μm long) have been fabricated. Buried lasers formed from similarly structured MBE material with liquid phase epitaxy regrowth had threshold currents at submilliampere levels when high reflectivity coatings were applied to the end facets. A cw threshold current of 0.55 mA was obtained for a laser with facet reflectivities of ∼80%, a cavity length of 120 μm, and an active region stripe width of 1 μm. These devices driven directly with logic level signals have switch‐on delays <50 ps without any current prebias. Such lasers permit fully on–off switching while at the same time obviating the need for bias monitoring and feedback control