Structures

Abstract

What is ‘condensed matter’? Where does the name come from? What characterises this state? Answers are easy to give: Condensed matter is the general description for atomic and molecular substances in the liquid and solid state, and, as the name suggests, this is the state which forms from the condensation of a gas. Such a condensation process leads to an increase of several orders of magnitude in the density. Moreover, in contrast to the case of a gas, where densities can be arbitrarily small, the densities encountered in condensed phases are restricted in range and exhibit only small changes as a function of pressure and temperature. The transition from the gas phase results in a complete change in the motion of the atoms or molecules on a microscopic scale. In gases, motions occur largely unhindered, with the translational movement of the molecules only being altered by collisions. In condensed phases, attractive interactions become active because of the much higher densities. This causes any motion to be severely restricted and leads to the stabilisation of states, which ‘hold themselves together’. Two different fundamental states, solid and liquid, are to be distinguished. Solids have, on their own, a definite form, while liquids take up the form of their container. In between states also exist, for example, highly viscous liquids, which can maintain their form for a certain time, as well as very malleable solids, whose shape changes upon the application of only small external forces. How do liquids and solids differ in their microscopic structure and dynamics? First, it is to be stated that structure and local motion are closely coupled. In a crystalline solid, where there is order over macroscopic distances, motion is restricted to small oscillations of the atoms about their equilibrium positions in the lattice. In contrast, the much greater, for long times even unrestricted motion in a liquid allows only a minor local ordering, which is constantly changing. In a liquid, the inter-atomic interactions are strong enough such that the molecules are attracted together by the action of a commonly created molecular field, but they are not sufficient to force the molecules to occupy fixed positions.