Cluster Perturbation Theory for Hubbard models

Cluster perturbation theory is a technique for calculating the spectral weight of Hubbard models of strongly correlated electrons, which combines exact diagonalizations on small clusters with strong-coupling perturbation theory at leading order. It is exact in both the strong- and weak-coupling limits and provides a good approximation to the spectral function at any wavevector. Following the paper by S\'en\'echal et al. (Phys. Rev. Lett. {\bf 84}, 522 (2000)), we provide a more complete description and derivation of the method. We illustrate some of its capabilities, in particular regarding the effect of doping, the calculation of ground state energy and double occupancy, the disappearance of the Fermi surface in the $t-t'$ Hubbard model, and so on. The method is applicable to any model with on-site repulsion only.Comment: 11 pages, 10 figures (RevTeX 4

Global DNA hypomethylation prevents consolidation of differentiation programs and allows reversion to the embryonic stem cell state.

DNA methylation patterns change dynamically during mammalian development and lineage specification, yet scarce information is available about how DNA methylation affects gene expression profiles upon differentiation. Here we determine genome-wide transcription profiles during undirected differentiation of severely hypomethylated (Dnmt1⁻/⁻) embryonic stem cells (ESCs) as well as ESCs completely devoid of DNA methylation (Dnmt1⁻/⁻;Dnmt3a⁻/⁻;Dnmt3b⁻/⁻ or TKO) and assay their potential to transit in and out of the ESC state. We find that the expression of only few genes mainly associated with germ line function and the X chromosome is affected in undifferentiated TKO ESCs. Upon initial differentiation as embryoid bodies (EBs) wild type, Dnmt1⁻/⁻ and TKO cells downregulate pluripotency associated genes and upregulate lineage specific genes, but their transcription profiles progressively diverge upon prolonged EB culture. While Oct4 protein levels are completely and homogeneously suppressed, transcription of Oct4 and Nanog is not completely silenced even at late stages in both Dnmt1⁻/⁻ and TKO EBs. Despite late wild type and Dnmt1⁻/⁻ EBs showing a much higher degree of concordant expression, after EB dissociation and replating under pluripotency promoting conditions both Dnmt1⁻/⁻ and TKO cells, but not wild type cells rapidly revert to expression profiles typical of undifferentiated ESCs. Thus, while DNA methylation seems not to be critical for initial activation of differentiation programs, it is crucial for permanent restriction of developmental fate during differentiation

Age-Differentiated Leadership and Healthy Aging at Work: Evidence from the Early Stages of the COVID-19 Pandemic

Little is known about the relative influence of age-differentiated leadership on healthy aging at work. Likewise, the age-conditional influence of age-differentiated leadership is understudied, and especially so in the context of the COVID-19 pandemic. Using a three-wave longitudinal study, we examined the role that age-differentiated leadership plays in the prediction of work ability, as measured three times over six months (n = 1130) during the early stages of the COVID-19 pandemic in Germany (i.e., December 2019, March 2020, and June 2020). The results suggest that although there were no systematic changes in work ability on average, there was notable within-person variability in work ability over time. Additionally, we find that a balanced approach to age-differentiated leadership that considers the needs of both older and younger employees matters most and complements the positive influence of leader–member exchange for predicting within-person variability in work ability. We also find that older employees’ work ability benefits from an approach to age-differentiated leadership that considers older employee’s needs, whereas younger employees’ work ability especially benefits from leader–member exchange and a balanced approach to age-differentiated leadership. Overall, these results provide initial support for the idea that an age-differentiated approach to leadership is important when considering healthy aging at work

Interrelation of Superconducting and Antiferromagnetic Gaps in High-Tc Compounds: a Test Case for a Microscopic Theory

Recent angle resolved photoemission (ARPES) data, which found evidence for a d-wave-like modulation of the antiferromagnetic gap, suggest an intimate interrelation between the antiferromagnetic insulator and the superconductor with its d-wave gap. This poses a new challenge to microscopic descriptions, which should account for this correlation between, at first sight, very different states of matter. Here, we propose a microscopic mechanism which provides a definite correlation between these two different gap structures: it is shown that a projected SO(5) theory, which aims at unifying antiferromagnetism and d-wave superconductivity via a common symmetry principle while explicitly taking the Mott-Hubbard gap into account, correctly describes the observed gap characteristics. Specifically, it accounts for both the dispersion and the order of magnitude difference between the antiferromagnetic gap modulation and the superconducting gap.Comment: 8 pages, 5 figure

Personal pack display and active smoking at outdoor café strips: assessing the impact of plain packaging 1 year postimplementation

Aims: We observed tobacco pack display and smoking at outdoor venues over three summers to assess changes in their prevalence following Australia's introduction of plain tobacco packaging with larger pictorial health warnings. Methods: Between January and April 2012 (preplain packaging (PP)), 2013 (early post-PP) and 2014 (1 year post-PP), we counted patrons, smokers and tobacco packs at cafés, restaurants and bars with outdoor seating. Pack type (branded, plain or unknown) and orientation were noted. Rates of active smoking, pack display and pack orientation were analysed using multilevel Poisson regression. Results: Prevalence of pack display among patrons declined from pre-PP (1 pack per 8.7 patrons) to early post-PP (1 pack per 10.4), and remained low 1 year post-PP (1 pack per 10.3). This appeared to be driven by a sustained decline in active smoking post-PP (pre-PP: 8.4% of patrons were smoking; early post-PP: 6.4%; 1 year post-PP: 6.8%). Notably, active smoking declined more in venues with children present than in those without. While early post-PP, plain packs were less often displayed face-up (74.0%) and more often concealed (8.9%) than branded packs pre-PP (face-up: 85.2%; concealed: 4.0%), this was not sustained 1 year post-PP (face-up: 85.7%; concealed: 4.4%). Also, external case use increased from pre-PP (1.2%) to early post-PP (3.5%), but returned to pre-PP levels 1 year post-PP (1.9%). Conclusions: This study demonstrated a sustained reduction in visibility of tobacco products and smoking in public, particularly in the presence of children, from pre-PP to 1 year post-PP. This effect is likely to reduce smoking-related social norms, thereby weakening an important influence on smoking uptake and better supporting quit attempts.Meghan Zacher, Megan Bayly, Emily Brennan, Joanne Dono, Caroline Miller, Sarah Durkin, Michelle Scollo, Melanie Wakefiel

A pulsed, mono-energetic and angular-selective UV photo-electron source for the commissioning of the KATRIN experiment

The KATRIN experiment aims to determine the neutrino mass scale with a sensitivity of 200 meV/c^2 (90% C.L.) by a precision measurement of the shape of the tritium $\beta$-spectrum in the endpoint region. The energy analysis of the decay electrons is achieved by a MAC-E filter spectrometer. To determine the transmission properties of the KATRIN main spectrometer, a mono-energetic and angular-selective electron source has been developed. In preparation for the second commissioning phase of the main spectrometer, a measurement phase was carried out at the KATRIN monitor spectrometer where the device was operated in a MAC-E filter setup for testing. The results of these measurements are compared with simulations using the particle-tracking software "Kassiopeia", which was developed in the KATRIN collaboration over recent years.Comment: 19 pages, 16 figures, submitted to European Physical Journal