HIGHLIGHTS OF RESEARCH

Formation of ion irradiation induced small-scale defects on graphite surfaces

Scanning tunneling microscopy (STM) experiments show that low-dose ion irradiation of (1000) graphite results in the formation of isolated defects comprising of a few tens of atoms. Since STM measurements of graphite surfaces show one protrusion in the observed electronic density per two surface atoms, it is difficult to deduce the atomic structure from experiment alone.

We have used molecular dynamics (MD) simulations of collision cascades to study the formation of the small-scale defects, and ab initio calculations to study the stability and electronic structure of these defects. A vacancy is, somewhat surprisingly, seen as a hillock in STM measurements due to enhancement of the partial charge densities in electron states close to the Fermi energy. We found that the predominant source of small-scale hillocks seen in STM measurements are vacancies in the surface layer, and that at most half of the defects may be formed by interstitial atom clusters.

We further identified a surface defect in which an extra atom had entered the surface layer forming a nearly equilateral triangular arrangement of carbon atoms. This "D3" defect structure was found to be stable both during high-temperature annealing in the classical MD simulations and within the framework of density-functional theory calculations. The D3 defect resembles triangular structures recently shown to be significant in amorphous carbon networks and during fullerene formation. The recognition of these three-atom rings has, once again, widened our knowledge of the large set of bonding structures carbon exhibits in nature.

K. Nordlund, J. Keinonen and T. Mattila, Formation of ion induced small-scale defects on graphite surfaces, Phys. Rev. Lett. 77 (1996) 699-702

Observations of new particle formation and growth at 'SMEAR II' forest station, Southern Finland

We have observed aerosol particle formation at a boreal forest site in Southern Finland (Hyytiälä, 61 51'N, 24 17'E, 170 m asl).Our measurement data show several occasions of aerosol growth from 3 to 40 nm during one day. This represents the first direct observation of new particle formation in the atmosphere. Anthropogenic pollution sources not being nearby, the origin of the new particles observed in this study may be most probably connected with biogenic activity.

See SMEAR homepage.

J.M. Mäkelä, P. Aalto, V. Jokinen, T. Pohja, A. Nissinen, S. Palmroth, T. Markkanen, K. Seitsonen, H. Lihavainen and M. Kulmala, Observations of ultrafine aerosol particle formation and growth in boreal forest, Geophys. Res. Lett., in press

Magnetic scattering at relativistic photon energies

The high energy beam line at ESRF has been used to study the interaction between the electron spin and an electromagnetic field. The interaction is weak, it increases with the photon energy and circularly polarized radiation is required to isolate the spin and charge contributions. All experiments done so far have been limited to energies below 100 keV, too low to make any conclusions about the interaction at relativistic energies.

In a joint project between the University of Helsinki, the University of Warwick (U.K.) and ESRF photon energies from 100 keV to 1 MeV were used to study the magnetic cross section in ferromagnetic iron. An external magnetic field was applied to flip the spin direction. Although synchrotron radiation is linearly polarized at the orbital plane, above or below that plane radiation is elliptically polarised. The magnetic signal was obtained by reversing the directions of the spins because charge scattering is unaffected by the spin direction. For the first time the energy dependence of magnetic scattering was measured at photon energies extending to about twice the rest energy of the electron. This offers a stringent test of the theory and also gives information about the optimal energy range for magnetic scattering studies.

J.E. McCarthy, M.J. Cooper, P.K. Lawson, D.N. Timms, S.O. Manninen, K. Hämäläinen and P. Suortti, High-Energy Magnetic Compton Scattering from Iron, J. Synchrotron Rad. 4 (1997) 102-109

Development of an ice detection sensor

We have developed a simple and robust ice detection sensor which initially was intended to be used on airplane wings. It has turned out that a large volume application is ice formation on wind turbine blades as even a thin layer of ice degrades the efficiency of the aerodynamics of the blades. A large fraction of wind turbines works in icy conditions and part of the generated electricity has to be fed back in order to melt the ice reducing the overall efficiency of electricity production. If the existence of ice can be reliably detected only the melting heating needs to be switched on.

M. Luukkala: Detecteur pour indiquer une formation de glace sur lÕaile dÕun aeronef, FR-Patent N:o 9310563, Detector for indicating ice formation on the wing of an aircraft, US-Patent N:o 5,467,944

Diamond coatings

In total hip prostheses, ultrahigh molecular weight polyethylene (UHMWPE) is commonly used as a cup material sliding against a CoCrMo or alumina ball. One of the main problems with UWMWPE is wear debris which is probably causing many harmful processes leading to prosthesis loosening. We have been able to reduce the wear of UHMWPE significantly by using an amorphous diamond (AD) coating on the counterpart material. The wear rate of UHMWPE disc was reduced by a factor of 35 and 600 by AD coating compared to uncoated CoCrMo or alumina ball, respectively. The tests were run in a pin-on-disc tester with 1 wt.% NaCl solution and runs lasted 3 milj. rounds. If these results can be reproduced in real implants, this is a major achievement in the development of implant materials. These tests are now proceeding. A diamond coated hip prosthesis is shown on the front cover.

R. Lappalainen and A. Anttila: Amorphous diamond coatings for medical implants, Acta Orthop. Scand., in press

Particle cosmology

The present structure of our universe was determined during the very first moments of its existence about 10 to 15 billion years ago. A particularly important role in this process was played by cosmological phase transitions through which the elementary particle matter went while the universe cooled.

In recent years, the particle cosmology group has concentrated on studying the so-called electroweak phase transition, which took place at temperatures of the order of 100 GeV. During this phase transition the elementary particles achieved their masses as a result of the so-called Higgs mechanism. It has been proposed that the observed matter-antimatter asymmetry in the universe was also generated at the same time. The details depend on the unknown Higgs particle mass.

In 1996 an important discovery was made. The electroweak phase transition was studied in an effective three dimensional field theory approach both by analytic and numerical methods. It was found that for large Higgs particle masses there actually is no first order transition, but only a continuous cross-over, in the Minimal Standard Model of elementary particle interactions. This has important consequences for particle cosmology since it implies that for the generation of the observed matter-antimatter asymmetry one needs some completely new physics.

K. Kajantie, M. Laine, K. Rummukainen and M. Shaposhnikov: Is there a hot electroweak phase transition at mH ³ mW ? Phys. Rev. Lett. 77 (1996) 2887-2890

Shedding light on condensates

The Atomic, Molecular and Optical Physics group has studied the interaction of light with Bose-Einstein condensates (BEC). It became experimentally possible to observe BEC in alkali gases in 1995, and since then the field has been a rapidly growing, exciting field of research.

An important aspect in the studies of BEC is how to observe and control the condensates with light. The theoretical results obtained by the local group in collaboration with researchers from Oxford and NIST (Gaithersburg, MD, USA) showed that the light-induced losses of atoms from the condensate can be reduced by selecting the frequency of the light appropriately. Later in 1996 the MIT group demonstrated non-destructive probing of Na condensate by laser light. Furthermore, the results indicate that one may be able do high resolution spectroscopy with condensed atoms.

K. Burnett, P.S. Julienne, and K.-A. Suominen, Laser-driven collisions between atoms in a Bose-Einstein condensed gas, Phys. Rev. Lett. 77 (1996) 1416-1419