Uniform spin chain physics arising from NCN bridges in CuNCN: surprises on the way from copper oxides to their nitride analogs

We report on the unexpected uniform spin chain physics in CuNCN, the insulating nitride analog of copper oxides. Based on full-potential band structure calculations, we derive the relevant microscopic parameters, estimate individual exchange couplings, and establish a realistic spin model of this compound. The structure of CuNCN contains chains of edge-sharing CuN(4) squares. As a surprise, in contrast to analogous [CuO(2)] chains in "edge-sharing" cuprates, the leading magnetic interactions J ~ 2500 K run perpendicular to the structural [CuN(2)] chains via bridging NCN groups. The resulting spin model of a uniform chain is in agreement with the experimentally observed temperature-independent magnetic susceptibility below 300 K. The nearest-neighbor and next-nearest-neighbor interactions along the structural [CuN(2)] chains are J(1) ~ -500 K and J(2) ~ 100 K, respectively. Despite the frustrating nature of J(1) and J(2), we assign the anomaly at 70 K to long-range magnetic ordering, which is likely collinear with antiparallel and parallel arrangement of spins along the 'c' and 'a' directions, respectively. The pronounced one-dimensionality of the spin system should lead to a reduction in the ordered moment and to a suppression of the transition anomaly in the specific heat, thus impeding the experimental observation of the long-range ordering. Our results suggest CuNCN as a promising material for ballistic heat transport within spin chains, while the sizable bandwidth W ~ 3 eV may lead to a metal-insulator transition and other exotic properties under high pressure.Comment: 10 pages, 5 figures. Submitted to Phys. Rev.

Identifying Missing Data Mechanisms in (2 x 2)-Contingency Tables

Consider the sample of two binary variables X and Y with some missing structure within X or Y. The knowledge about the corresponding values of the observed covariate allows to play through all possible `originally' complete data sets. After defining the notation, including some theoretical work, a test for non--MCAR within the complete case table is presented. Simulating all possible tables enables some testing on non--MAR. A simulation experiment is used to illustrate this context

The Classical Linear Regression Model with one Incomplete Binary Variable

We present three different methods based on the conditional mean imputation when binary explanatory variables are incomplete. Apart from the single imputation and multiple imputation especially the so-called pi imputation is presented as a new procedure. Seven procedures are compared in a simulation experiment when missing data are confined to one independent binary variable: complete case analysis, zero order regression, categorical zero order regression, pi imputation, single imputation, multiple imputation, modified first order regression. After a brief theoretical description of the simulation experiment, MSE-ratio, variance and bias are used to illustrate differences within and between the approaches

Identification of a catalytic active but non-aggregating MDM2 RING domain variant

As a key regulator of the tumour suppressor protein p53, MDM2 is involved in various types of cancer and has thus been an attractive drug target. So far, small molecule design has primarily focussed on the N-terminal p53-binding domain although on-target toxicity effects have been reported. Targeting the catalytic RING domain of MDM2 resembles an alternative approach to drug MDM2 with the idea to prevent MDM2-mediated ubiquitination of p53 while retaining MDM2's ability to bind p53. The design of RING inhibitors has been limited by the extensive aggregation tendency of the RING domain, making it challenging to undertake co-crystallization attempts with potential inhibitors. Here we compare the purification profiles of the MDM2 RING domain from several species and show that the MDM2 RING domain of other species than human is much less prone to aggregate although the overall structure of the RING domain is conserved. Through sequence comparison and mutagenesis analyses, we identify a single point mutation, G443T, which greatly enhances the dimeric fraction of human MDM2 RING domain during purification. Neither does the mutation alter the structure of the RING domain, nor does it affect E2(UbcH5B)-Ub binding and activity. Hence, MDM2-G443T facilitates studies involving binding partners that would be hampered by the low solubility of the wild-type RING domain. Furthermore, it will be valuable for the development of MDM2 RING inhibitors

Statistische Methoden bei unvollständigen Daten

Dieser Artikel gibt einen überblick über die Problematik fehlender Daten im Rahmen der statistischen Datenanalyse. Im Prinzip sollte er auch Lesern mit geringem mathematischen und statistischen Wissen dienlich sein und sie mathematisch nicht überfordern.Gegebenenfalls kann über allzu theoretische Komponenten hinweggelesen werden

Object-guided Spatial Attention in Touch: Holding the Same Object with Both Hands Delays Attentional Selection

Abstract Previous research has shown that attention to a specific location on a uniform visual object spreads throughout the entire object. Here we demonstrate that, similar to the visual system, spatial attention in touch can be object guided. We measured event-related brain potentials to tactile stimuli arising from objects held by observers' hands, when the hands were placed either near each other or far apart, holding two separate objects, or when they were far apart but holding a common object. Observers covertly oriented their attention to the left, to the right, or to both hands, following bilaterally presented tactile cues indicating likely tactile target location(s). Attentional modulations for tactile stimuli at attended compared to unattended locations were present in the time range of early somatosensory components only when the hands were far apart, but not when they were near. This was found to reflect enhanced somatosensory processing at attended locations rather than suppressed processing at unattended locations. Crucially, holding a common object with both hands delayed attentional selection, similar to when the hands were near. This shows that the proprioceptive distance effect on tactile attentional selection arises when distant event locations can be treated as separate and unconnected sources of tactile stimulation, but not when they form part of the same object. These findings suggest that, similar to visual attention, both space- and object-based attentional mechanisms can operate when we select between tactile events on our body surface.</jats:p

Measurement of absolute transition frequencies of 87Rb to nS and nD Rydberg states by means of electromagnetically induced transparency

We report the measurement of absolute excitation frequencies of 87Rb to nS and nD Rydberg states. The Rydberg transition frequencies are obtained by observing electromagnetically induced transparency on a rubidium vapor cell. The accuracy of the measurement of each state is < 1 MHz, which is achieved by frequency stabilizing the two diode lasers employed for the spectroscopy to a frequency comb and a frequency comb calibrated wavelength meter, respectively. Based on the spectroscopic data we determine the quantum defects of 87Rb, and compare it with previous measurements on 85Rb. We determine the ionization frequency from the 5S1/2(F=1) ground state of 87Rb to 1010.0291646(3) THz, providing the binding energy of the ground state with an accuracy improved by two orders of magnitude

Walking Through the Method Zoo: Does Higher Education Really Meet Software Industry Demands?

Software engineering educators are continually challenged by rapidly evolving concepts, technologies, and industry demands. Due to the omnipresence of software in a digitalized society, higher education institutions (HEIs) have to educate the students such that they learn how to learn, and that they are equipped with a profound basic knowledge and with latest knowledge about modern software and system development. Since industry demands change constantly, HEIs are challenged in meeting such current and future demands in a timely manner. This paper analyzes the current state of practice in software engineering education. Specifically, we want to compare contemporary education with industrial practice to understand if frameworks, methods and practices for software and system development taught at HEIs reflect industrial practice. For this, we conducted an online survey and collected information about 67 software engineering courses. Our findings show that development approaches taught at HEIs quite closely reflect industrial practice. We also found that the choice of what process to teach is sometimes driven by the wish to make a course successful. Especially when this happens for project courses, it could be beneficial to put more emphasis on building learning sequences with other courses

Parametric and Nonparametric Regression with Missing X's - A Review

This paper gives a detailed overview of the problem of missing data in parametric and nonparametric regression. Theoretical basics, properties as well as simulation results may help the reader to get familiar with the common problem of incomplete data sets. Of course, not all occurences can be discussed so this paper could be seen as an introduction to missing data within regression analysis and as an extension to the early paper of Little (1992)

The type II poly(A)-binding protein PABP-2 genetically interacts with the let-7 miRNA and elicits heterochronic phenotypes in Caenorhabditis elegans

The type II poly(A)-binding protein PABP2/PABPN1 functions in general mRNA metabolism by promoting poly(A) tail formation in mammals and flies. It also participates in poly(A) tail shortening of specific mRNAs in flies, and snoRNA biogenesis in yeast. We have identified Caenorhabditis elegans pabp-2 as a genetic interaction partner of the let-7 miRNA, a widely conserved regulator of animal stem cell fates. Depletion of PABP-2 by RNAi suppresses loss of let-7 activity, and, in let-7 wild-type animals, leads to precocious differentiation of seam cells. This is not due to an effect on let-7 biogenesis and activity, which remain unaltered. Rather, PABP-2 levels are developmentally regulated in a let-7-dependent manner. Moreover, using RNAi PABP-2 can be depleted by >80% without significantly impairing larval viability, mRNA levels or global translation. Thus, it unexpectedly appears that the bulk of PABP-2 is dispensable for general mRNA metabolism in the larva and may instead have more restricted, developmental functions. This observation may be relevant to our understanding of why the phenotypes associated with human PABP2 mutation in oculopharyngeal muscular dystrophy (OPMD) seem to selectively affect only muscle cell