Instability of the Sb vacancy in GaSb

We demonstrate that the instability of the Sb vacancy in GaSb leads to a further increase in the acceptor-type defect concentration in proton irradiated undoped, p-type GaSb. Using positron annihilation spectroscopy in situ with 10 MeV proton irradiation at 35 K, we find that the irradiation produces both native vacancy defects in GaSb. However, the Sb vacancy is unstable above temperatures of 150 K and undergoes a transition resulting in a Ga vacancy and a Ga antisite. The activation energy of this transition is determined to be 0.6 eV +/- 0.1 eV. Our results are in line with the established amphoteric defect model and prove that the instability of the Sb vacancy in GaSb has a profound role on the native defect concentration in GaSb.Peer reviewe

Fracture of three-dimensional fuse networks with quenched disorder

We study a fracture on a quasistatic time scale in a three-dimensional (3D) fuse network model with “strong” and “weak” disorder. These two cases differ noticeably in the development of the fracture. For strong disorder the damage scaling is very close to volumelike [number of broken bonds Nb∼L3/(lnL)0.3] unlike for weak disorder [Nb∼L2.4/(lnL)0.3]. With strong disorder global load sharing is only approximately valid. The size distribution of “avalanches” of broken fuses in the failure follows roughly a power-law scaling. The power-law exponent τ has a value close to 2, close to but differing from the exponent −5/2 expected of global load sharing. For weak disorder τ is about 1.5 which means that the decay of the size distribution is much slower than expected. These exponent values that characterize the development of damage prior to catastrophic failure are comparable to experimental ones. For the final fracture surfaces we observe a roughness exponent ζ≈0.4 for weak disorder. For strong disorder, severe finite size effects are seen, but the exponent seems to converge to the same value as for weak disorder, which is close to the one for the 3D random bond Ising domain wall universality class.Peer reviewe

Direct observations of the vacancy and its annealing in germanium

Weakly n-type doped germanium has been irradiated with protons up to a fluence of 3×10 exp 14 cm exp −2 at 35 K and 100 K in a unique experimental setup. Positron annihilation measurements show a defect lifetime component of 272±4 ps at 35 K in in situ positron lifetime measurements after irradiation at 100 K. This is identified as the positron lifetime in a germanium monovacancy. Annealing experiments in the temperature interval 35–300 K reveal two annealing stages. The first at 100 K is tentatively associated with the annealing of the Frenkel pair, the second at 200 K with the annealing of the monovacancy. Above 200 K it is observed that mobile neutral monovacancies form divacancies, with a positron lifetime of 315 ps.Peer reviewe

Cloud-base vertical velocity statistics: a comparison between an atmospheric mesoscale model and remote sensing observations

The statistics of cloud-base vertical velocity simulated by the non-hydrostatic mesoscale model AROME are compared with Cloudnet remote sensing observations at two locations: the ARM SGP site in Central Oklahoma, and the DWD observatory at Lindenberg, Germany. The results show that, as expected, AROME significantly underestimates the variability of vertical velocity at cloud-base compared to observations at their nominal resolution; the standard deviation of vertical velocity in the model is typically 4-6 times smaller than observed, and even more during the winter at Lindenberg. Averaging the observations to the horizontal scale corresponding to the physical grid spacing of AROME (2.5 km) explains 70-80% of the underestimation by the model. Further averaging of the observations in the horizontal is required to match the model values for the standard deviation in vertical velocity. This indicates an effective horizontal resolution for the AROME model of at least 4 times the physically-defined grid spacing. The results illustrate the need for special treatment of sub-grid scale variability of vertical velocities in kilometer-scale atmospheric models, if processes such as aerosol-cloud interactions are to be included in the future

Kolmiulotteisen astian mallin tekeminen ja tulostus 3D-tekniikalla

Tiivistelmä. 3D-tulostus eli materiaalia lisäävä valmistus on moderni menetelmä valmistaa kolmiulotteisia kappaleita. 3D-tulostimelle tuodun CAD-mallin avulla tulostin valmistaa mallin mukaisen kappaleen kerros kerrokselta. Menetelmä on suhteellisen nopea, ja käytössä olevia valmistusmateriaaleja on useita. Kandidaatintyön tavoitteena oli tutustua 3D-tulostukseen ja sen antamiin mahdollisuuksiin prosessiteollisuudessa. Työ oli rajattu esittelemään muovien 3D-tulostusta. Työssä esiteltiin 3D-tulostamisen etuja, muovien tulostustekniikkaa sekä tulostusmateriaaleja. Teoreettiseen taustaan tutustumisen jälkeen tehtiin kokeellisena osana jo olemassa olevan vaahdotusastian mallintaminen sekä tulostus 3D-tekniikalla. Uuteen vaahdotusastiaan tehtiin muutamia muutoksia helpottamaan sen käyttöä ja tekemään siitä toimivampi. Valmistusmateriaaliksi valittiin ABS-muovi, joka sopi tilanteeseen parhaiten. Työn aikana käytiin läpi tarvittavat vaiheet, jotta ideasta saatiin aikaan valmis kappale 3D-tulostuksen avulla. Vaahdotusastian mallintaminen ja tulostaminen onnistuivat hyvin. Uudesta vaahdotusastiasta tuli sellainen kuin haluttiin ja laadun tarkkuus riitti käyttötarkoitukseen. 3D-tulostuksen käyttöä prosessiteollisuudessa tarkasteltiin ja muutamia käyttökohteita löytyi. Nämä eivät ole mitään teollisuutta mullistavaa, mutta kuitenkin elämää helpottavia sekä tarjoavat mahdollisuuksia kappaleiden uudenlaiseen muotoiluun

Growing season precipitation in Finland under recent and projected climate

v2010okKA

Failure of planar fiber networks

We study the failure of planar random fiber networks with computer simulations. The networks are grown by adding flexible fibers one by one on a growing deposit [K. J. Niskanen and M. J. Alava, Phys. Rev. Lett. 73, 3475 (1994)], a process yielding realistic three dimensional network structures. The network thus obtained is mapped to an electrical analogue of the elastic problem, namely to a random fuse network with separate bond elements for the fiber-to-fiber contacts. The conductivity of the contacts (corresponding to the efficiency of stress transfer between fibers) is adjustable. We construct a simple effective medium theory for the current distribution and conductivity of the networks as a function of intra-fiber current transfer efficiency. This analysis compares favorably with the computed conductivity and with the fracture properties of fiber networks with varying fiber flexibility and network thickness. The failure characteristics are shown to obey scaling behavior, as expected of a disordered brittlematerial, which is explained by the high current end of the current distribution saturating in thick enough networks. For bond breaking, fracture load and strain can be estimated with the effective medium theory. For fiber breaking, we find the counter-intuitive result that failure is more likely to nucleate far from surfaces, as the stress is transmitted more effectively to the fibers in the interior.Peer reviewe

Explicit representation of subgrid variability in cloud microphysics yields weaker aerosol indirect effect in the ECHAM5-HAM2 climate model

Peer reviewe

Micro-crystalline inclusions analysis by PIXE and RBS

A characteristic feature of the nuclear microprobe using a 3 MeV proton beam is the long range of particles (around 70 \mu m in light matrices). The PIXE method, with EDS analysis and using the multilayer approach for treating the X-ray spectrum allows the chemistry of an intra-crystalline inclusion to be measured, provided the inclusion roof and thickness at the impact point of the beam (Z and e, respectively) are known (the depth of the inclusion floor is Z + e). The parameter Z of an inclusion in a mineral can be measured with a precision of around 1 \mu m using a motorized microscope. However, this value may significantly depart from Z if the analyzed inclusion has a complex shape. The parameter e can hardly be measured optically. By using combined RBS and PIXE measurements, it is possible to obtain the geometrical information needed for quantitative elemental analysis. This paper will present measurements on synthetic samples to investigate the advantages of the technique, and also on natural solid and fluid inclusions in quartz. The influence of the geometrical parameters will be discussed with regard to the concentration determination by PIXE. In particular, accuracy of monazite micro-inclusion dating by coupled PIXE-RBS will be presented