37 research outputs found
Magnetotransport Study of the Canted Antiferromagnetic Phase in Bilayer Quantum Hall State
Magnetotransport properties are investigated in the bilayer quantum Hall
state at the total filling factor . We measured the activation energy
elaborately as a function of the total electron density and the density
difference between the two layers. Our experimental data demonstrate clearly
the emergence of the canted antiferromagnetic (CAF) phase between the
ferromagnetic phase and the spin-singlet phase. The stability of the CAF phase
is discussed by the comparison between experimental results and theoretical
calculations using a Hartree-Fock approximation and an exact diagonalization
study. The data reveal also an intrinsic structure of the CAF phase divided
into two regions according to the dominancy between the intralayer and
interlayer correlations.Comment: 6 pages, 7 figure
Simultaneous Excitation of Spins and Pseudospins in the Bilayer Quantum Hall State
The tilting angular dependence of the energy gap was measured in the bilayer
quantum Hall state at the Landau level filling by changing the density
imbalance between the two layers. The observed gap behavior shows a continuous
transformation from the bilayer balanced density state to the monolayer state.
Even a sample with 33 K tunneling gap shows the same activation energy anomaly
reported by Murphy {\it et al.}. We discuss a possible relation between our
experimental results and the quantum Hall ferromagnet of spins and pseudospins.Comment: 4 pages, 4 figure
MCMC implementation for Bayesian hidden semi-Markov models with illustrative applications
Copyright © Springer 2013. The final publication is available at Springer via http://dx.doi.org/10.1007/s11222-013-9399-zHidden Markov models (HMMs) are flexible, well established models useful in a diverse range of applications.
However, one potential limitation of such models lies in their inability to explicitly structure the holding times of each hidden state. Hidden semi-Markov models (HSMMs) are more useful in the latter respect as they incorporate additional temporal structure by explicit modelling of the holding times. However, HSMMs have generally received less attention in the literature, mainly due to their intensive computational requirements. Here a Bayesian implementation of HSMMs is presented. Recursive algorithms are proposed in conjunction with Metropolis-Hastings in such a way as to avoid sampling from the distribution of the hidden state sequence in the MCMC sampler. This provides a computationally tractable estimation framework for HSMMs avoiding the limitations associated with the conventional EM algorithm regarding model flexibility. Performance of the proposed implementation is demonstrated through simulation experiments as well as an illustrative application relating to recurrent failures in a network of underground water pipes where random effects are also included into the HSMM to allow for pipe heterogeneity
Bayesian lasso binary quantile regression
In this paper, a Bayesian hierarchical model for variable selection and estimation in the context of binary quantile regression is proposed. Existing approaches to variable selection in a binary classification context are sensitive to outliers, heteroskedasticity or other anomalies of the latent response. The method proposed in this study overcomes these problems in an attractive and straightforward way. A Laplace likelihood and Laplace priors for the regression parameters are proposed and estimated with Bayesian Markov Chain Monte Carlo. The resulting model is equivalent to the frequentist lasso procedure. A conceptional result is that by doing so, the binary regression model is moved from a Gaussian to a full Laplacian framework without sacrificing much computational efficiency. In addition, an efficient Gibbs sampler to estimate the model parameters is proposed that is superior to the Metropolis algorithm that is used in previous studies on Bayesian binary quantile regression. Both the simulation studies and the real data analysis indicate that the proposed method performs well in comparison to the other methods. Moreover, as the base model is binary quantile regression, a much more detailed insight in the effects of the covariates is provided by the approach. An implementation of the lasso procedure for binary quantile regression models is available in the R-package bayesQR
Irregularly Spaced AR and ARCH (ISAR-ARCH) Models
High frequency data in finance are time series which are often measured at unequally or irregularly spaced time intervals. This paper suggests a modeling approach by so-called AR response surfaces where the AR coefficients are declining functions in continuous lag time. The irregularly spaced AR-ARCH (ISAR-ARCH) models contain the usual AR-ARCH models as a special case if the time series is equally spaced. The time between observation arrivals is treated as a stochastic time varying process and modeled as a conditional Weibull distribution to capture the stylized fact of duration clustering. For the ISAR-ARCH process and the conditional Weibull duration (CWD) process, we show how to carry out an exact Bayesian analysis using a Markov chain Monte Carlo method. Model selection and forecasting are handled using the predictive density. Finally, we illustrate our methodology with two examples