Band topology and quantum spin Hall effect in bilayer graphene

We consider bilayer graphene in the presence of spin orbit coupling, to assess its behavior as a topological insulator. The first Chern number $n$ for the energy bands of single and bilayer graphene is computed and compared. It is shown that for a given valley and spin, $n$ in a bilayer is doubled with respect to the monolayer. This implies that bilayer graphene will have twice as many edge states as single layer graphene, which we confirm with numerical calculations and analytically in the case of an armchair terminated surface. Bilayer graphene is a weak topological insulator, whose surface spectrum is susceptible to gap opening under spin-mixing perturbations. We also assess the stability of the associated topological bulk state of bilayer graphene under various perturbations. Finally, we consider an intermediate situation in which only one of the two layers has spin orbit coupling, and find that although individual valleys have non-trivial Chern numbers, the spectrum as a whole is not gapped, so that the system is not a topological insulator.Comment: 9 pages. 9 figures include

Electrically controllable magnetism in twisted bilayer graphene

Twisted graphene bilayers develop highly localised states around AA-stacked regions for small twist angles. We show that interaction effects may induce either an antiferromagnetic (AF) and a ferromagnetic (F) polarization of said regions, depending on the electrical bias between layers. Remarkably, F-polarised AA regions under bias develop spiral magnetic ordering, with a relative $120^\circ$ misalignment between neighbouring regions due to a frustrated antiferromagnetic exchange. This remarkable spiral magnetism emerges naturally without the need of spin-orbit coupling, and competes with the more conventional lattice-antiferromagnetic instability, which interestingly develops at smaller bias under weaker interactions than in monolayer graphene, due to Fermi velocity suppression. This rich and electrically controllable magnetism could turn twisted bilayer graphene into an ideal system to study frustrated magnetism in two dimensions, with interesting potential also for a range of applications.Comment: 7 pages, 3 figures. Minor correction

Modeling the distributional dynamics of attention and semantic interference in word production

In recent years, it has become clear that attention plays an important role in spoken word production. Some of this evidence comes from distributional analyses of reaction time (RT) in regular picture naming and picture-word interference. Yet we lack a mechanistic account of how the properties of RT distributions come to reflect attentional processes and how these processes may in turn modulate the amount of conflict between lexical representations. Here, we present a computational account according to which attentional lapses allow for existing conflict to build up unsupervised on a subset of trials, thus modulating the shape of the resulting RT distribution. Our process model resolves discrepancies between outcomes of previous studies on semantic interference. Moreover, the model's predictions were confirmed in a new experiment where participants' motivation to remain attentive determined the size and distributional locus of semantic interference in picture naming. We conclude that process modeling of RT distributions importantly improves our understanding of the interplay between attention and conflict in word production. Our model thus provides a framework for interpreting distributional analyses of RT data in picture naming tasks

Stacking boundaries and transport in bilayer graphene

Pristine bilayer graphene behaves in some instances as an insulator with a transport gap of a few meV. This behaviour has been interpreted as the result of an intrinsic electronic instability induced by many-body correlations. Intriguingly, however, some samples of similar mobility exhibit good metallic properties, with a minimal conductivity of the order of $2e^2/h$. Here we propose an explanation for this dichotomy, which is unrelated to electron interactions and based instead on the reversible formation of boundaries between stacking domains (`solitons'). We argue, using a numerical analysis, that the hallmark features of the previously inferred many-body insulating state can be explained by scattering on boundaries between domains with different stacking order (AB and BA). We furthermore present experimental evidence, reinforcing our interpretation, of reversible switching between a metallic and an insulating regime in suspended bilayers when subjected to thermal cycling or high current annealing.Comment: 13 pages, 15 figures. Published version (Nano Letters

Role Play As An Approach In Developing Students Communicative Competence

This study was conducted to examine the outcome of role play as an approach in developing the communicative competence of freshmen. Moreover, the focus of this study was on how the role-play as an approach was conducted; hence, necessary enhancement should be offered if needed. The information were gathered through Focused Group Discussion (FGD). Guide questions were prepared and validated; interviews were recorded, transcribed, interpreted and analyzed. Results revealed that role play as a type of communicative activity was a valuable classroom approach to assist students in developing their communicative competence and to provide the students an opportunity to practice the English language with fluency and accuracy. It was also discovered that role play was beneficial in developing students’ communicative competence provided that it was not done too long. It encouraged students to express themselves; provided self-confidence and opportunity for practicing the rules of social behavior; and allowed learners to use their learned language

Universal scaling of current fluctuations in disordered graphene

We analyze the full transport statistics of graphene with smooth disorder at low dopings. First we consider the case of 1D disorder for which the transmission probability distribution is given analytically in terms of the graphene-specific mean free path. All current cumulants are shown to scale with system parameters (doping, size, disorder strength and correlation length) in an identical fashion for large enough systems. In the case of 2D disorder, numerical evidence is given for the same kind of identical scaling of all current cumulants, so that the ratio of any two such cumulants is universal. Specific universal values are given for the Fano factor, which is smaller than the pseudodiffusive value of ballistic graphene (F=1/3) both for 1D (F=0.243) and 2D (F=0.295) disorder. On the other hand, conductivity in wide samples is shown to grow without saturation as \sqrt{L} and Log L with system length L in the 1D and 2D cases respectively.Comment: 9 pages, 7 figures. Published version, includes corrected figure for Fano facto

Noise reduction using wavelet cycle spinning: analysis of useful periodicities in the z-transform domain

Cycle spinning (CS) and a'trous algorithms are different implementations of the undecimated wavelet transform (UWT). Both algorithms can be used for UWT and even though the resulting wavelet coefficients are different, they keep a correspondence. This paper describes an analysis of the CS algorithm performed in the z-transform domain, showing the similarities and differences with the a'trous implementation. CS generates more wavelet coefficients than a'trous, but the number of significative and different coefficients is the same in both cases because of the occurrence of a periodic repetition in CS coefficients. Mathematical expressions for the relationship between CS and a'trous coefficients and for CS coefficient periodicities are provided in the z-transform domain. In some wavelet denoising applications, periodicities (present in the coefficients of the CS procedure) can also be found in the performance measure of the processed signals. In particular, in ultrasonic CS denoising applications, periodicities have been appreciated in the signal-to-noise ratio (SNR) of the ultrasonic denoised signals. These periodicities can be used to optimize the number of CS coefficients for an efficient implementation. Two examples showing the periodicities in the SNR are included. A selection of several reduced sets of CS wavelet coefficients has been utilized in the examples, and the SNRs resulting after denoising are analyzed.This work was partially supported by Spanish MCI Project DPI2011-22438 and MEC Project TIN2013-47272-C2-1-R. The translation of this paper was funded by the Universitat Politecnica de Valencia, Spain.Rodríguez-Hernández, MA.; San Emeterio, JL. (2016). Noise reduction using wavelet cycle spinning: analysis of useful periodicities in the z-transform domain. Signal, Image and Video Processing. 10(3):519-526. https://doi.org/10.1007/s11760-015-0762-8S519526103Daubechies, I.: Ten Lectures on Wavelets. 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Hall-effect and resistivity measurements in CdTe and ZnTe at high pressure: Electronic structure of impurities in the zincblende phase and the semi-metallic or metallic character of the high-pressure phases

We carried out high-pressure resistivity and Hall-effect measurements in single crystals of CdTe and ZnTe up to 12 GPa. Slight changes of transport parameters in the zincblende phase of CdTe are consitent with the shallow character of donor impurities. Drastic changes in all the transport parameters of CdTe were found around 4 GPa, i.e. close to the onset of the cinnabar to rock-salt transition. In particular, the carrier concentration increases by more than five orders of magnitude. Additionally, an abrupt decrease of the resistivity was detected around 10 GPa. These results are discussed in comparison with optical, thermoelectric, and x-ray diffraction experiments. The metallic character of the Cmcm phase of CdTe is confirmed and a semi-metallic character is determined for the rock-salt phase. In zincblende ZnTe, the increase of the hole concentration by more than two orders of magnitude is proposed to be due to a deep-to-shallow transformation of the acceptor levels. Between 9 and 11 GPa, transport parameters are consistent with the semiconducting character of cinnabar ZnTe. A two orders of magnitude decrease of the resistivity and a carrier-type inversion occurs at 11 GPa, in agreement with the onset of the transition to the Cmcm phase of ZnTe. A metallic character for this phase is deduced.Comment: 20 pages, 4 figure

Geographic Proximity Not a Prerequisite for Invasion: Hawaii Not the Source of California Invasion by Light Brown Apple Moth (Epiphyas postvittana)

BACKGROUND: The light brown apple moth (LBAM), Epiphyas postvittana (Walker), is native to Australia but invaded England, New Zealand, and Hawaii more than 100 years ago. In temperate climates, LBAM can be a major agricultural pest. In 2006 LBAM was discovered in California, instigating eradication efforts and quarantine against Hawaiian agriculture, the assumption being that Hawaii was the source of the California infestation. Genetic relationships among populations in Hawaii, California, and New Zealand are crucial to understanding LBAM invasion dynamics across the Pacific. METHODOLOGY/PRINCIPAL FINDINGS: We sequenced mitochondrial DNA (mtDNA) from 1293 LBAM individuals from California (695), Hawaii (448), New Zealand (147), and Australia (3) to examine haplotype diversity and structure among introduced populations, and evaluate the null hypothesis that invasive populations are from a single panmictic source. However, invasive populations in California and New Zealand harbor deep genetic diversity, whereas Hawaii shows low level, shallow diversity. CONCLUSIONS/SIGNIFICANCE: LBAM recently has established itself in California, but was in Hawaii and New Zealand for hundreds of generations, yet California and New Zealand show similar levels of genetic diversity relative to Hawaii. Thus, there is no clear relationship between duration of invasion and genetic structure. Demographic statistics suggest rapid expansion occurring in California and past expansions in New Zealand; multiple introductions of diverse, genetically fragmented lineages could contribute to these patterns. Hawaii and California share no haplotypes, therefore, Hawaii is not the source of the California introduction. Paradoxically, Hawaii and California share multiple haplotypes with New Zealand. New Zealand may be the source for the California and Hawaii infestations, but the introductions were independent, and Hawaii was invaded only once. This has significant implications for quarantine, and suggests that probability of invasion is not directly related to geographic distance. Surprisingly, Hawaiian LBAM populations have much lower genetic diversity than California, despite being older

Modest Declines in Proteome Quality Impair Hematopoietic Stem Cell Self-Renewal.

Low protein synthesis is a feature of somatic stem cells that promotes regeneration in multiple tissues. Modest increases in protein synthesis impair stem cell function, but the mechanisms by which this occurs are largely unknown. We determine that low protein synthesis within hematopoietic stem cells (HSCs) is associated with elevated proteome quality in vivo. HSCs contain less misfolded and unfolded proteins than myeloid progenitors. Increases in protein synthesis cause HSCs to accumulate misfolded and unfolded proteins. To test how proteome quality affects HSCs, we examine Aarssti/sti mice that harbor a tRNA editing defect that increases amino acid misincorporation. Aarssti/sti mice exhibit reduced HSC numbers, increased proliferation, and diminished serial reconstituting activity. Misfolded proteins overwhelm the proteasome within Aarssti/sti HSCs, which is associated with increased c-Myc abundance. Deletion of one Myc allele partially rescues serial reconstitution defects in Aarssti/sti HSCs. Thus, HSCs are dependent on low protein synthesis to maintain proteostasis, which promotes their self-renewal