We present in this Letter the results from two high quality, low density GaAs quantum wells. In sample A of electron density n=5.0x10^10 cm^-2, anisotropic electronic transport behavior was observed at u=7/2 in the second Landau level. We believe th
at the anisotropy is due to the large Landau level mixing effect in this sample. In sample B of density 4.1x10^10 cm^-2, strong 8/3, 5/2, and 7/3 fractional quantum Hall states were observed. Furthermore, our energy gap data suggest that, similar to the 8/3 state, the 5/2 state may also be spin unpolarized in the low density limit. The results from both samples show that the strong electron-electron interactions and a large Landau level mixing effect play an import role in the competing ground states in the second landau level.
We have carried out tilt magnetic field (B) studies of the u=12/5 fractional quantum Hall state in an ultra-high quality GaAs quantum well specimen. Its diagonal magneto-resistance Rxx shows a non-monotonic dependence on tilt angle (theta). It first
increases sharply with increasing theta, reaches a maximal value of ~ 70 ohms at theta ~ 14^o, and then decreases at higher tilt angles. Correlated with this dependence of Rxx on theta, the 12/5 activation energy (Delta_{12/5}) also shows a non-monotonic tilt dependence. Delta_{12/5} first decreases with increasing theta. Around theta = 14^{o}, Delta_{12/5} disappears as Rxx becomes non-activated. With further increasing tilt angles, Delta_{12/5} reemerges and increases with theta. This tilt B dependence at u=12/5 is strikingly different from that of the well-documented 5/2 state and calls for more investigations on the nature of its ground state.
We compare the energy gap of the u=5/2 fractional quantum Hall effect state obtained in conventional high mobility modulation doped quantum well samples with those obtained in high quality GaAs transistors (heterojunction insulated gate field-effect
transistors). We are able to identify the different roles that long range and short range disorders play in the 5/2 state and observe that the long range potential fluctuations are more detrimental to the strength of the 5/2 state than short-range potential disorder.
How can we model influence between individuals in a social system, even when the network of interactions is unknown? In this article, we review the literature on the influence model, which utilizes independent time series to estimate how much the sta
te of one actor affects the state of another actor in the system. We extend this model to incorporate dynamical parameters that allow us to infer how influence changes over time, and we provide three examples of how this model can be applied to simulated and real data. The results show that the model can recover known estimates of influence, it generates results that are consistent with other measures of social networks, and it allows us to uncover important shifts in the way states may be transmitted between actors at different points in time.
The quantum Hall plateau transition was studied at temperatures down to 1 mK in a random alloy disordered high mobility two-dimensional electron gas. A perfect power-law scaling with kappa=0.42 was observed from 1.2K down to 12mK. This perfect scalin
g terminates sharply at a saturation temperature of T_s~10mK. The saturation is identified as a finite-size effect when the quantum phase coherence length (L_{phi} ~ T^{-p/2}) reaches the sample size (W) of millimeter scale. From a size dependent study, T_s propto W^{-1} was observed and p=2 was obtained. The exponent of the localization length, determined directly from the measured kappa and p, is u=2.38, and the dynamic critical exponent z = 1.
