Anomalous phase behavior of a soft-repulsive potential with a strictly monotonic force

We study the phase behavior of a classical system of particles interacting through a strictly convex soft-repulsive potential which, at variance with the pairwise softened repulsions considered so far in the literature, lacks a region of downward or zero curvature. Nonetheless, such interaction is characterized by two length scales, owing to the presence of a range of interparticle distances where the repulsive force increases, for decreasing distance, much more slowly than in the adjacent regions. We investigate, using extensive Monte Carlo simulations combined with accurate free-energy calculations, the phase diagram of the system under consideration. We find that the model exhibits a fluid-solid coexistence line with multiple re-entrant regions, an extremely rich solid polymorphism with solid-solid transitions, and water-like anomalies. In spite of the isotropic nature of the interparticle potential, we find that, among the crystal structures in which the system can exist, there are also a number of non-Bravais lattices, such as cI16 and diamond.Comment: 21 pages, 7 figures, in press on Phys. Rev.

Hexatic phase and water-like anomalies in a two-dimensional fluid of particles with a weakly softened core

We study a two-dimensional fluid of particles interacting through a spherically-symmetric and marginally soft two-body repulsion. This model can exist in three different crystal phases, one of them with square symmetry and the other two triangular. We show that, while the triangular solids first melt into a hexatic fluid, the square solid is directly transformed on heating into an isotropic fluid through a first-order transition, with no intermediate tetratic phase. In the low-pressure triangular and square crystals melting is reentrant provided the temperature is not too low, but without the necessity of two competing nearest-neighbor distances over a range of pressures. A whole spectrum of water-like fluid anomalies completes the picture for this model potential.Comment: 26 pages, 14 figures; printed article available at http://link.aip.org/link/?jcp/137/10450

Residual Multiparticle Entropy for a Fractal Fluid of Hard Spheres

The residual multiparticle entropy (RMPE) of a fluid is defined as the difference, $\Delta s$, between the excess entropy per particle (relative to an ideal gas with the same temperature and density), $s_\text{ex}$, and the pair-correlation contribution, $s_2$. Thus, the RMPE represents the net contribution to $s_\text{ex}$ due to spatial correlations involving three, four, or more particles. A heuristic `ordering' criterion identifies the vanishing of the RMPE as an underlying signature of an impending structural or thermodynamic transition of the system from a less ordered to a more spatially organized condition (freezing is a typical example). Regardless of this, the knowledge of the RMPE is important to assess the impact of non-pair multiparticle correlations on the entropy of the fluid. Recently, an accurate and simple proposal for the thermodynamic and structural properties of a hard-sphere fluid in fractional dimension $1<d<3$ has been proposed [Santos, A.; L\'opez de Haro, M. \emph{Phys. Rev. E} \textbf{2016}, \emph{93}, 062126]. The aim of this work is to use this approach to evaluate the RMPE as a function of both $d$ and the packing fraction $\phi$. It is observed that, for any given dimensionality $d$, the RMPE takes negative values for small densities, reaches a negative minimum $\Delta s_{\text{min}}$ at a packing fraction $\phi_{\text{min}}$, and then rapidly increases, becoming positive beyond a certain packing fraction $\phi_0$. Interestingly, while both $\phi_{\text{min}}$ and $\phi_0$ monotonically decrease as dimensionality increases, the value of $\Delta s_{\text{min}}$ exhibits a nonmonotonic behavior, reaching an absolute minimum at a fractional dimensionality $d\simeq 2.38$. A plot of the scaled RMPE $\Delta s/|\Delta s_{\text{min}}|$ shows a quasiuniversal behavior in the region $-0.14\lesssim\phi-\phi_0\lesssim 0.02$.Comment: 10 pages, 3 figures; v2: minor change

Buzzing in the pat : a new large-scale method for studying dung beetle functions

Lantakuoriaiset ovat tärkeitä maatalousekosysteemien toiminnan kannalta. Lantakuoriaisten keskuu-dessa useat erilaiset toiminnalliset ryhmät hajottavat laiduntavien eläinten lantaa ja kierrättävät lannan ravinteita takaisin maahan. Monet Suomen lantakuoriaiset ovat kuitenkin taantuneet elinympäristöjen vähenemisen ja muuttumisen myötä. Siksi on tärkeää selvittää, millaisia seurauksia eri lajien ja ryhmi-en taantumisella on ekosysteemien toiminnan kannalta. Monissa aihetta käsittelevissä tutkimuksissa tutkimusmenetelmät ovat kuitenkin perustuneet pienen mittakaavan kokeisiin, mikä saattaa vaikuttaa niissä havaittuihin tuloksiin. Tämän tutkielman tavoitteena on selvittää, voidaanko suurikokoisia aitauksia käyttää lantakuori-aisten toiminnan tutkimiseen Suomen olosuhteissa. Uuden menetelmän avulla halusin tutkia, miten eri lajeista kootut lantakuoriaisyhteisöt vaikuttavat lannan hajoamiseen ja laidunnurmen kasvuun. Lisäksi halusin selvittää laidunpaineen vaikutuksen lantakuoriaisten toimintaan. Kokeessa muodostin lantiais- ja sittiäislajeista erilaisia lantakuoriaisyhteisöjä, jotka sijoitin suu-riin, kahdesta vaihtoehtoisesta verkosta rakennettuihin aitauksiin. Pienempiverkkoisiin aitauksiin lisä-sin kaikki lantakuoriaiset ja suurempiverkkoisiin aitauksiin sittiäisiä pienikokoisemmat lantiaiset pääsi-vät itse verkon läpi. Testatakseni aitausten toimivuutta mittasin lantakuoriaisten määrää verkon ulko- ja sisäpuolella useaan otteeseen kokeen aikana. Aitauksien sisällä oli eri laidunpainetta nurmen pituudella kuvastavia ruutuja. Niillä seurasin lantaläjien hajoamista punnitsemalla läjiä kuudesti kokeen aikana. Lopuksi tutkin lantakuoriaisten vaikutusta laidunnurmen kasvuun punnitsemalla kasvibiomassaa läjien ympäriltä ja kasvattamalla rairuohoa läjien alta kaivetussa maassa. Suuret lantakuoriaishäkit näyttivät toimivan hyvin Suomen olosuhteissa. Eri lantakuoriaisyhtei-söjen vaikutuksesta lannan hajoamiseen sain vaihtelevia tuloksia eri aitaustyypeistä. Suurempiverkkoi-sissa aitauksissa sittiäiset osoittautuivat lantiaisia merkittävämmiksi lannan hajottajiksi, mutta pienem-piverkkoisissa aitauksissa eroja eri yhteisöjen toiminnallisesta tehokkuudesta ei löytynyt. Erot saattoi-vat johtua siitä, että toiseen aitaustyyppiin lisäsin lantakuoriaiset ja toiseen lantiaiset pääsivät itse ver-kon läpi. Tämä saattoi vaikuttaa lantakuoriaisten toimintaan. Laidunpaine vaikutti selvästi lannan hajoamisnopeuteen. Lantaläjät kevenivät huomattavasti nopeammin lyhyessä nurmessa kuin pitkässä. Lämpötilaerot erikorkuisten nurmien välillä saattoivat vaikuttaa lantakuoriaisten hakeutumiseen lyhyessä nurmessa olleisiin läjiin. Sen sijaan eri lanta-kuoriaisyhteisöillä ei ollut havaittavaa vaikutusta laidunnurmen kasvuun. Kaiken kaikkiaan tutkimuksestani selviää, että verkosta rakennettuja aitauksia voi käyttää tehok-kaasti lantakuoriaisten toiminnan tutkimiseen. Tutkimus vahvisti myös osittain aiempia havaintoja toiminnallisten ryhmien erilaisesta merkityksestä. Lisäksi laidunpaineen vaikutus lannan hajoamisno-peuteen osoitti, että laidunympäristöllä saattaa olla tärkeä merkitys lantakuoriaisten toimintaan.Dung beetles affect ecosystem functioning in agricultural landscapes. Among these beetles several functional groups decompose cattle dung and contribute to recycling its nutrients back to the soil. To-day many Finnish dung beetle species are threatened because of habitat change and fragmentation. Therefore, the impacts of changes in dung beetle community structure on ecosystem functioning needs to be addressed. However, many earlier studies of dung beetle functions have focussed only on small-scale manipulations with possible implications for interpretation of the results on larger scale. The aim of this study was to assess whether large-scale enclosures can be used to manipulate dung beetle communities in Finland. With this new method I wanted to examine how different dung beetle communities affected dung decomposition and nutrient cycling. I also wanted to examine wheth-er grazing pressure affected dung decomposition. I used two types of mesh enclosures to create different communities of Aphodius and Geotrupes species. In small mesh enclosures I added dung beetles manually, while in the large mesh enclosures the smaller Aphodius species could get through the mesh, and Geotrupes were then added manually to some of these enclosures. During the experiment I repeatedly measured the amount of dung beetles inside and outside of the mesh to assess the performance of the enclosures. Within enclosures I created plots that reflected different grazing pressure. In every plot I placed a dung pat which I then weighed six times during the summer. At the end of the experiment I studied the effect of dung beetles on nutri-ent cycling by weighing the biomass of grass. Overall, I found that the large enclosures are usable and work well in Finland. My results from the enclosures with the bigger mesh size showed that Geotrupes are more efficient decomposers of dung than are Aphodius. However, there were no significant differences between Aphodius and Geotrupes in enclosures made of smaller mesh. Thus, different enclosures may have affected the functioning of the dung beetles because I put the beetles to some enclosures and to some they could get through the mesh. Dung decomposition rates changed remarkably between the different grazing pressures. The end weight of pats was lighter in short grass compared to pats in long grass. It is possible that this reflects microhabitat selection by beetles choosing the warmer pats in the short grass. Dung beetle community structure did not affect rates of nutrient recycling, as measured by grass growth. In conclusion, the results of my study suggest that large-scale enclosures can be effectively used to study dung beetle functions in grassland habitats. This study also confirms earlier observations re-garding the significance of different functional groups of dung beetles, and shows that the grazing envi-ronment may have an important impact on the ecosystem functions provided by dung beetles