The science of clusters: An emerging field

Abstract

A group ofatoms bound together by interatomic forces is called an atomic cluster (AC). There is no qualitative distinction between small ACs and molecules. However, as the number of atoms in the system increases, ACs acquire more and more specific properties making them unique physical objects different from both si,ngle molecules and from the solid state. In nature, there are many different types ofAC: van der Waals ACs, metallic ACs, fullerenes, molecular, semiconductor, mixed ACs, and their shapes can depart considerably from the common spherical form: arborescent, linear, spirals, etc. Usually, one can distinguish between different types ofACs by the nature ofthe forces between the atoms, or by the principles of spatial organization within the ACs. ACs can exist in all forms ofmatter: solid state, liquid, gases and plasmas. The novelty ofAC physics arises mostly from the fact that AC properties explain the transition from single atoms or molecules to the solid state. Modern experimental techniques have made it possible to study this transition. Byincreasing the AC size, one can observe the emergence ofthe physical features in the system, such as plasmon excitations, electron conduction band formation, superconductivityand superfluidity, phase transitions, fission and many more. Most of these many-body phenomena exist in solid state but are absent for single atoms. Below we briefly summarize various aspects of AC physics, which to our mind make it an attractive field of research: (i) ACs provide a small, self-contained 'laboratory' in which the major interactions and many-body effects present also in solids can be analysed and studied as a function of their size; (ii) ACs straddle the limit between microscopic and quasi-classical systems, so they can be used to probe the boundary between quantum mechanics and semi-classical systems; (iii) ACs are the appropriate physical objects for studying statistical and thermodynamic laws in nanoscale systems, both classical and quantum; (iv) Small ACs are tractable computationally by ab initio methods; (v) ACs can be made and observed in the laboratory by using modern beam or deposition techniques; (vi) ACs provide new examples of many-body forces in a regime which is different from those of atomic, nuclear or solid-state physics, but is related to all of them; (vii) ACs can serve as building blocks for new forms of matter, the formation ofAC-based molecules and new materials; (vii) ACs are of similar size to nanoscale devices, and so their physics is closely related to the physics ofvery small devices, in which quantum effects begin to appear, such as must occur when one wishes to make smaller and smaller chips for microcomputers. The science ofACs is a highly interdisciplinary field. ACs concern astrophysicists, atomic and molecular physicists, chemists, molecular biologists, solid-state physicists, nuclear physicists, plasma physicists, technologists all of whom see them as a branch oftheir subjects but AC physics is a new subject in its own right. This becomes clear after a briefstudy ofthe problems which