Calculating critical temperatures for ferromagnetic order in two-dimensional materials

Magnetic order in two-dimensional (2D) materials is intimately coupled to magnetic anisotropy (MA) since the Mermin-Wagner theorem implies that rotational symmetry cannot be spontaneously broken at finite temperatures in 2D. Large MA thus comprises a key ingredient in the search for magnetic 2D materials that retains the magnetic order above room temperature. Magnetic interactions are typically modeled in terms of Heisenberg models and the temperature dependence on magnetic properties can be obtained with the Random Phase Approximation (RPA), which treats magnon interactions at the mean-field level. In the present work we show that large MA gives rise to strong magnon-magnon interactions that leads to a drastic failure of the RPA. We then demonstrate that classical Monte Carlo (MC) simulations correctly describe the critical temperatures in the large MA limit and agree with RPA when the MA becomes small. A fit of the MC results leads to a simple expression for the critical temperatures as a function of MA and exchange coupling constants, which significantly simplifies the theoretical search for new 2D magnetic materials with high critical temperatures. The expression is tested on a monolayer of CrI$_3$, which were recently observed to exhibit ferromagnetic order below 45 K and we find excellent agreement with the experimental value.Comment: 8 pages, 6 figure

An Empirical Analysis of 'Acting White'

There is a debate among social scientists regarding the existence of a peer externality commonly referred to as 'acting white.' Using a newly available data set (the National Longitudinal Study of Adolescent Health), which allows one to construct an objective measure of a student's popularity, we demonstrate that there are large racial differences in the relationship between popularity and academic achievement; our (albeit narrow) definition of 'acting white.' The effect is intensified among high achievers and in schools with more interracial contact, but non-existent among students in predominantly black schools or private schools. The patterns in the data appear most consistent with a two-audience signaling model in which investments in education are thought to be indicative of an individual's opportunity costs of peer group loyalty. Other models we consider, such as self-sabotage among black youth or the presence of an oppositional culture, all contradict the data in important ways.

Moon timber - myth or reality

Znanost ni mogla potrditi pozornost zbujajočih lastnosti "lunarnega" lesa, tj.lesa, posekanega ob "pravem času", kot ga "narekujejo" lunine mene, zodiaška znamenja in presenetljivi fiksni termini. Lunarni les naj bi imel posebne oz. za les nenavadne lastnosti: ne trohni, ne gnije, ne lotijo ga se insekti, ne gori, je bolj suh, se ne krči, ne poka, se ne veži in je zelo trd.Predstavljen je Gozdarsko-lesarski lunarni koledar za leto 2005. Ob dvomu o učinku Luninih "impulzov" na kvaliteto lesa, se iz povsem koledarskih razlogov zdijo problematični "fiksni" termini (npr. 1. marec) in vloga (premaknjenih) zodiaških znamenj. Lunarni les resda utegne imeti boljše lastnosti, vendar ne zaradi kozmičnih vplivov, ampak zaradi skrbnega izbora najboljših dreves (npr. "resonančna" starejša smrekovina, nem. Geigenbäume) inskrbnega ravnanja po poseku.Science has not been able to confirm the salient characteristics of moontimber, i.e. timber felled at "proper time", as "dictated" by moon phases,zodiac signs and (surprising) fixed dates. Moontimber is said to have some special and unusual characteristics: it does not rot, it is not attacked by insects, it does not burn, it is drier, it does not shrink, it does not crack, it does not bend, and it is very hard. In the present article, the Forestry Moon Calendar for the Year 2005 is presented. Most problematic apart from doubts about the impact of the lunar "impulses" on timber quality seem, for entirely calendar reasons, the "fixed" dates (e.g. March 1st) and the roleof (shifted) zodiac signs. Moontimber may indeed be of better quality, although not due to cosmic influences but owing to a wise selection of the best trees (e.g. "resonant" older spruce-wood) and careful handling after being felled

Davorjevo Brezno e il nodo idrografico tra Matarsko Podolje e Škocjanske Jame (Slovenia)

Viene descritta una nuova, grande e interessate grotta scoperta ed esplorata da un team di speleologi italiani e sloveni. La cavità si è rivelata essere in profondità un collettore di più inghiottitoi alimentati da acque provenienti da scorrimenti idrici sul sovrastante flysch. Per la sua particolare posizione, tra la Valsecca di Castelnuovo (Matarsko Podolje) e il Complesso di San Canziano (Škocjanske Jame) – Abisso dei Serpenti (Kaćna Jama), la grotta si sviluppa in un’area di elevato interesse idrografico; la profondità (280 metri) e il suo sviluppo complessivo (oltre i 2 km), unitamente alla bellezza dei vani ed alle caratteristiche morfologiche e sedimentologiche, fanno di questa grotta una delle principali e più interessanti, nonché delle più complesse, di tutta l’area.A new, large and interesting cave discovered and explored by a team of Italian and Slovenian speleologists is hereby described. The cavity was, in its depths, found to be a collector of several sinkholes fed by water coming from flows on the overlying flysch. Due to its particular position, located between Valsecca di Castelnuovo (Matarsko Podolje) and the San Canziano Complex, (Škocjanske Jame) – the Abisso dei Serpenti (Abyss of Snakes) (Kaćna Jama) is spread across an area of extreme hydro-geographical interest; its depth (280 metres) and overall length (over 2 km), together with the beauty of its shafts and its morphological and sedimentological characteristics, make this cave one of the important, interesting, and not to mention complex caves of the whole area

Covering of elliptic curves and the kernel of the Prym map

Motivated by a conjecture of Xiao, we study families of coverings of elliptic curves and their corresponding Prym map $\Phi$. More precisely, we describe the codifferential of the period map $P$ associated to $\Phi$ in terms of the residue of meromorphic $1$-forms and then we use it to give a characterization for the coverings for which the dimension of $\ker(dP)$ is the least possibile. This is useful in order to exclude the existence of non isotrivial fibrations with maximal relative irregularity and thus also in order to give counterexamples to the Xiao's conjecture mentioned above. The first counterexample to the original conjecture, due to Pirola, is then analysed in our framework.Comment: 21 pages, no figures. The seminal ideas at the base of this article were born in the framework of the PRAGMATIC project of year 201