Fundamentals of Condensed Matter Physics

Classification of solids

Condensed matter physics and solid state physics usually refer to the same area of physics, but in principle the former title is broader. Condensed matter is meant to include solids, liquids, liquid crystals, and some plasmas in or near solids. This is the largest branch of physics at this time, and it covers a broad scope of physical phenomena. Topics range from studies of the most fundamental aspects of physics to applied problems related to technology

The focus of this book will be primarily on the quantum theory of solids. To begin, it is useful to start with the concept of a solid and then describe the two commonly used models that form the basis for modern research in this area. The word “solid” evokes a familiar visual picture well described by the definition in the Oxford Dictionary: “Of stable shape, not liquid or fluid, having some rigidity.” It is the property of rigidity that is basic to the early studies of solids. These studies focused on the mechanical properties of solids. As a result, until the nineteenth century the most common classification of solids involved their rigidity or mechanical properties. The Mohs hardness scale (talc – 1; calcite – 3; quartz – 7; diamond – 10) is a typical example. This is a useful but limited approach for classifying solids.

The advent of atomic theory brought more microscopic concepts about solids. Solids were viewed as collections of more or less strongly interacting atoms. From the point of view of atomic theory, a gas is described in terms of a collection of almost independent atoms, while a liquid is formed by atoms that are weakly interacting. This picture leads to a description of the formation of solids, under pressure or by freezing, in which the distances between atoms are reduced and, in turn, this causes them to interact more strongly. Molecular solids are formed by condensing molecular gases.

Hence, the development of atomic physics and chemical analysis led to a more detailed classification of solids according to chemical composition. Although for most studies of solids it is necessary to establish the identity of the constituent atoms, such a scheme provides limited insight into many of the basic concepts of condensed matter physics.