Theory of Exciton Recombination from the Magnetically Induced Wigner Crystal

We study the theory of itinerant-hole photoluminescence of two-dimensional electron systems in the regime of the magnetically induced Wigner crystal. We show that the exciton recombination transition develops structure related to the presence of the Wigner crystal. The form of this structure depends strongly on the separation $d$ between the photo-excited hole and the plane of the two-dimensional electron gas. When $d$ is small compared to the magnetic length, additional peaks appear in the spectrum due to the recombination of exciton states with wavevectors equal to the reciprocal lattice vectors of the crystal. For $d$ larger than the magnetic length, the exciton becomes strongly confined to an interstitial site of the lattice, and the structure in the spectrum reflects the short-range correlations of the Wigner crystal. We derive expressions for the energies and the radiative lifetimes of the states contributing to photoluminescence, and discuss how the results of our analysis compare with experimental observations.Comment: 10 pages, no figures, uses Revtex and multicol.st

Theory of anyon excitons: Relation to excitons of nu=1/3 and nu=2/3 incompressible liquids

Elementary excitations of incompressible quantum liquids (IQL's) are anyons, i.e., quasiparticles carrying fractional charges and obeying fractional statistics. To find out how the properties of these quasiparticles manifest themselves in the optical spectra, we have developed the anyon exciton model (AEM) and compared the results with the finite-size data for excitons of nu=1/3 and nu=2/3 IQL's. The model considers an exciton as a neutral composite consisting of three quasielectrons and a single hole. The AEM works well when the separation between electron and hole confinement planes, h, is larger than the magnetic length l. In the framework of the AEM an exciton possesses momentum k and two internal quantum numbers, one of which can be chosen as the angular momentum, L, of the k=0 state. Existence of the internal degrees of freedom results in the multiple branch energy spectrum, crater-like electron density shape and 120 degrees density correlations for k=0 excitons, and the splitting of the electron shell into bunches for non-zero k excitons. For h larger than 2l the bottom states obey the superselection rule L=3m (m are integers starting from 2), all of them are hard core states. For h nearly 2l there is one-to-one correspondence between the low-energy spectra found for the AEM and the many- electron exciton spectra of the nu=2/3 IQL, whereas some states are absent from the many-electron spectra of the nu=1/3 IQL. We argue that this striking difference in the spectra originates from the different populational statistics of the quasielectrons of charge conjugate IQL's and show that the proper account of the statistical requirements eliminates excessive states from the spectrum. Apparently, this phenomenon is the first manifestation of the exclusion statistics in the anyon bound states.Comment: 26 pages with 9 figures, typos correcte

Theory of Photoluminescence of the ν=1\nu=1 Quantum Hall State: Excitons, Spin-Waves and Spin-Textures

We study the theory of intrinsic photoluminescence of two-dimensional electron systems in the vicinity of the $\nu=1$ quantum Hall state. We focus predominantly on the recombination of a band of initial ``excitonic states'' that are the low-lying energy states of our model at $\nu=1$. It is shown that the recombination of excitonic states can account for recent observations of the polarization-resolved spectra of a high-mobility GaAs quantum well. The asymmetric broadening of the spectral line in the $\sigma_-$ polarization is explained to be the result of the ``shake-up'' of spin-waves upon radiative recombination of excitonic states. We derive line shapes for the recombination of excitonic states in the presence of long-range disorder that compare favourably with the experimental observations. We also discuss the stabilities and recombination spectra of other (``charged'') initial states of our model. An additional high-energy line observed in experiment is shown to be consistent with the recombination of a positively-charged state. The recombination spectrum of a negatively-charged initial state, predicted by our model but not observed in the present experiments, is shown to provide a direct measure of the formation energy of the smallest ``charged spin-texture'' of the $\nu=1$ state.Comment: 23 pages, 7 postscript figures included. Revtex with epsf.tex and multicol.sty. The revised version contains slightly improved numerical results and a few additional discussions of the result

Energy spectra of fractional quantum Hall systems in the presence of a valence hole

The energy spectrum of a two-dimensional electron gas (2DEG) in the fractional quantum Hall regime interacting with an optically injected valence band hole is studied as a function of the filling factor $\nu$ and the separation $d$ between the electron and hole layers. The response of the 2DEG to the hole changes abruptly at $d$ of the order of the magnetic length $\lambda$. At $d<\lambda$, the hole binds electrons to form neutral ($X$) or charged ($X^-$) excitons, and the photoluminescence (PL) spectrum probes the lifetimes and binding energies of these states rather than the original correlations of the 2DEG. The ``dressed exciton'' picture (in which the interaction between an exciton and the 2DEG was proposed to merely enhance the exciton mass) is questioned. Instead, the low energy states are explained in terms of Laughlin correlations between the constituent fermions (electrons and $X^-$'s) and the formation of two-component incompressible fluid states in the electron--hole plasma. At $d>2\lambda$, the hole binds up to two Laughlin quasielectrons (QE) of the 2DEG to form fractionally charged excitons $h$QE$_n$. The previously found ``anyon exciton'' $h$QE$_3$ is shown to be unstable at any value of $d$. The critical dependence of the stability of different $h$QE$_n$ complexes on the presence of QE's in the 2DEG leads to the observed discontinuity of the PL spectrum at $\nu={1\over3}$ or ${2\over3}$.Comment: 16 pages, 14 figures, submitted to PR

Enterprise Education Competitions: A Theoretically Flawed Intervention?

The demand for including enterprise in the education system, at all levels and for all pupils is now a global phenomenon. Within this context, the use of competitions and competitive learning activities is presented as a popular and effective vehicle for learning. The purpose of this chapter is to illustrate how a realist method of enquiry – which utilises theory as the unit of analysis – can shed new light on the assumed and unintended outcomes of enterprise education competitions. The case developed here is that there are inherent flaws in assuming that competitions will ‘work’ in the ways set out in policy and guidance. Some of the most prevalent stated outcomes – that competitions will motivate and reward young people, that they will enable the development of entrepreneurial skills, and that learners will be inspired by their peers – are challenged by theory from psychology and education. The issue at stake is that the expansion of enterprise education policy into primary and secondary education increases the likelihood that more learners will be sheep dipped in competitions, and competitive activities, without a clear recognition of the potential unintended effects. In this chapter, we employ a realist-informed approach to critically evaluate the theoretical basis that underpins the use of competitions and competitive learning activities in school-based enterprise education. We believe that our findings and subsequent recommendations will provide those who promote and practice the use of competitions with a richer, more sophisticated picture of the potential flaws within such activities.Peer reviewedFinal Published versio

Laboratory Capacity Building in Asia for Infectious Disease Research: Experiences from the South East Asia Infectious Disease Clinical Research Network (SEAICRN)

Heiman Wertheim and colleagues discuss a network that aims to improve infectious disease management through integrated, collaborative clinical research in South East Asia

An Integrated TCGA Pan-Cancer Clinical Data Resource to Drive High-Quality Survival Outcome Analytics

For a decade, The Cancer Genome Atlas (TCGA) program collected clinicopathologic annotation data along with multi-platform molecular profiles of more than 11,000 human tumors across 33 different cancer types. TCGA clinical data contain key features representing the democratized nature of the data collection process. To ensure proper use of this large clinical dataset associated with genomic features, we developed a standardized dataset named the TCGA Pan-Cancer Clinical Data Resource (TCGA-CDR), which includes four major clinical outcome endpoints. In addition to detailing major challenges and statistical limitations encountered during the effort of integrating the acquired clinical data, we present a summary that includes endpoint usage recommendations for each cancer type. These TCGA-CDR findings appear to be consistent with cancer genomics studies independent of the TCGA effort and provide opportunities for investigating cancer biology using clinical correlates at an unprecedented scale. Analysis of clinicopathologic annotations for over 11,000 cancer patients in the TCGA program leads to the generation of TCGA Clinical Data Resource, which provides recommendations of clinical outcome endpoint usage for 33 cancer types

Intracellular Calcium Deficits in Drosophila Cholinergic Neurons Expressing Wild Type or FAD-Mutant Presenilin

Much of our current understanding about neurodegenerative diseases can be attributed to the study of inherited forms of these disorders. For example, mutations in the presenilin 1 and 2 genes have been linked to early onset familial forms of Alzheimer's disease (FAD). Using the Drosophila central nervous system as a model we have investigated the role of presenilin in one of the earliest cellular defects associated with Alzheimer's disease, intracellular calcium deregulation. We show that expression of either wild type or FAD-mutant presenilin in Drosophila CNS neurons has no impact on resting calcium levels but does give rise to deficits in intracellular calcium stores. Furthermore, we show that a loss-of-function mutation in calmodulin, a key regulator of intracellular calcium, can suppress presenilin-induced deficits in calcium stores. Our data support a model whereby presenilin plays a role in regulating intracellular calcium stores and demonstrate that Drosophila can be used to study the link between presenilin and calcium deregulation

Investigation of gene-environment interactions in relation to tic severity

Tourette syndrome (TS) is a neuropsychiatric disorder with involvement of genetic and environmental factors. We investigated genetic loci previously implicated in Tourette syndrome and associated disorders in interaction with pre- and perinatal adversity in relation to tic severity using a case-only (N = 518) design. We assessed 98 single-nucleotide polymorphisms (SNPs) selected from (I) top SNPs from genome-wide association studies (GWASs) of TS; (II) top SNPs from GWASs of obsessive–compulsive disorder (OCD), attention-deficit/hyperactivity disorder (ADHD), and autism spectrum disorder (ASD); (III) SNPs previously implicated in candidate-gene studies of TS; (IV) SNPs previously implicated in OCD or ASD; and (V) tagging SNPs in neurotransmitter-related candidate genes. Linear regression models were used to examine the main effects of the SNPs on tic severity, and the interaction effect of these SNPs with a cumulative pre- and perinatal adversity score. Replication was sought for SNPs that met the threshold of significance (after correcting for multiple testing) in a replication sample (N = 678). One SNP (rs7123010), previously implicated in a TS meta-analysis, was significantly related to higher tic severity. We found a gene–environment interaction for rs6539267, another top TS GWAS SNP. These findings were not independently replicated. Our study highlights the future potential of TS GWAS top hits in gene–environment studies