Inverse flux quantum periodicity of magnetoresistance oscillations in two-dimensional short-period surface superlattices

Transport properties of the two-dimensional electron gas (2DEG) are considered in the presence of a perpendicular magnetic field $B$ and of a {\it weak} two-dimensional (2D) periodic potential modulation in the 2DEG plane. The symmetry of the latter is rectangular or hexagonal. The well-known solution of the corresponding tight-binding equation shows that each Landau level splits into several subbands when a rational number of flux quanta $h/e$ pierces the unit cell and that the corresponding gaps are exponentially small. Assuming the latter are closed due to disorder gives analytical wave functions and simplifies considerably the evaluation of the magnetoresistivity tensor $\rho_{\mu\nu}$. The relative phase of the oscillations in $\rho_{xx}$ and $\rho_{yy}$ depends on the modulation periods involved. For a 2D modulation with a {\bf short} period $\leq 100$ nm, in addition to the Weiss oscillations the collisional contribution to the conductivity and consequently the tensor $\rho_{\mu\nu}$ show {\it prominent peaks when one flux quantum $h/e$ passes through an integral number of unit cells} in good agreement with recent experiments. For periods $300- 400$ nm long used in early experiments, these peaks occur at fields 10-25 times smaller than those of the Weiss oscillations and are not resolved

Primary or secondary? A dichotomy of the strontium isotope anomalies in the Ediacaran carbonates of Saudi Arabia

Secular variation of 87Sr/86Sr in carbonate strata has been widely used in regional and global chemostratigraphic correlations. Typically, diagenesis results in higher 87Sr/86Sr signals relative to their primary composition due to the alteration by Rb-rich fluids and radiogenic decay of 87Rb to 87Sr. Surprisingly, 87Sr/86Sr values in the Ediacaran limestones from Saudi Arabia (from 0.7029 to 0.7059) are significantly lower than typical Ediacaran seawater values (mostly from 0.7080 to 0.7090) based on a global compilation. Understanding the origin of these anomalies is important insofar as early macrofossils are preserved in these strata. Two hypotheses have been independently evaluated in this study. The first hypothesis shows a low temperature scenario with isolated oceans or lakes in proximity to a mafic source. The second hypothesis is characterized by a high temperature scenario with profound overprints by juvenile hydrothermal fluids. Integrated Sr and Nd isotope data reveal that the 87Sr/86Sr anomalies are closely coupled with positive εNd(t = 560 Ma) values (up to +4.1). Clear covariations between 87Sr/86Sr, εNd, TOC, δ13Ccarb, δ13Corg, and δ18Ocarb were found. Based on multiple lines of petrographic, field, and geochemical evidence, the second hypothesis (i.e., hydrothermal alteration by juvenile fluids) is preferred in this study. Thermogenic breakdown of organic matter in host sediments may have caused the increasingly low TOC abundance and high δ13Corg signals of the residual organic matter. We argue that the concept that the Ediacaran biotic radiation took place in an isolated lake environment should be treated with caution. These remarkably low 87Sr/86Sr signals have neither temporal nor biogeochemical significance. Sr isotope chemostratigraphy in this particular region may not be a reliable tool for stratigraphic correlations

Baryons: What, When and Where?

We review the current state of empirical knowledge of the total budget of baryonic matter in the Universe as observed since the epoch of reionization. Our summary examines on three milestone redshifts since the reionization of H in the IGM, z = 3, 1, and 0, with emphasis on the endpoints. We review the observational techniques used to discover and characterize the phases of baryons. In the spirit of the meeting, the level is aimed at a diverse and non-expert audience and additional attention is given to describe how space missions expected to launch within the next decade will impact this scientific field.Comment: Proceedings Review for "Astrophysics in the Next Decade: JWST and Concurrent Facilities", ed. X. Tielens, 38 pages, 10 color figures. Revised to address comments from the communit

Nucleosomes in gene regulation: theoretical approaches

This work reviews current theoretical approaches of biophysics and bioinformatics for the description of nucleosome arrangements in chromatin and transcription factor binding to nucleosomal organized DNA. The role of nucleosomes in gene regulation is discussed from molecular-mechanistic and biological point of view. In addition to classical problems of this field, actual questions of epigenetic regulation are discussed. The authors selected for discussion what seem to be the most interesting concepts and hypotheses. Mathematical approaches are described in a simplified language to attract attention to the most important directions of this field