Solid state physics
Solid state physics is the study of how atoms are arranged in solids and what properties those solids exhibit as a result of their atomic arrangement. This fundamental field of physics attempts to explain how the macroscopic properties of a solid arise from its microscopic components. Many modern technological advances, such as semiconductors, lasers, and magnetic materials, are based on the principles of solid state physics.
Structure of solids
One of the first things to understand about solids is their structure. When atoms come together to form solids, they often arrange themselves in an orderly pattern called a crystal lattice.
A crystal lattice can be thought of as a three-dimensional arrangement of atoms, ions, or molecules that occurs in a repeating pattern. The smallest repeating unit, often called a parallelepiped, is known as a unit cell. Some common types of unit cells are as follows:
- Cube: All sides of a cube are equal, and all angles are 90 degrees.
- Quadrilateral: Two sides are equal, and the angles are 90 degrees.
- Orthorhombic: All sides are unequal, but the angles are 90 degrees.
Binding forces in solids
The type of chemical bond that holds the lattice together can significantly affect the properties of a solid. There are several types of bond:
- Ionic bond: attraction between positive and negative ions. Example: sodium chloride (NaCl).
- Covalent bonding: shared pairs of electrons between atoms. Example: diamond.
- Metallic bond: Electrons are free to move around in a "sea of electrons." Example: copper.
- Van der Waals force: Weak force due to permanent or induced dipoles. Example: graphite.
Thermal properties of solids
Since temperature affects the vibration of atoms, it significantly affects the thermal expansion, heat capacity, and other thermal properties of solids.
The specific heat of a solid is related to the energy needed to raise its temperature. An important model for describing the specific heat of solids is the Dulong-Petit law. It approximates the molar heat capacity of many solids as 3R, where R
is the ideal gas constant.
Electrical and magnetic properties
Solids are classified into conductors, insulators, and semiconductors based on their electrical conductivity. This classification relies heavily on the electronic band structure, which describes the energy levels that electrons are allowed to occupy in a solid.
Ohm's law is important in understanding electrical conduction:
V = IR
This equation relates voltage V
, current I
, and resistance R
Band theory of solids
Band theory explains the nature of electrical conductors, insulators and semiconductors. According to this theory, the electron energy levels in a solid are not clearly defined like those in an atom but are spread over continuous bands.
- Conduction band: The higher energy band in which electrons are free to conduct electricity.
- Valence band: Energy band filled with valence electrons.
- Band gap: The difference in energy between the conduction band and the valence band. Large band gaps are characteristic of insulators, small or no gaps are characteristic of conductors, and intermediate band gaps are characteristic of semiconductors.
Semiconductors
Semiconductors have had a significant impact on modern technology. They have electrical conductivity between metals and insulators. Their conductivity can be controlled by adding impurities through a process called doping.
- N-type: Extra electrons are added to the structure. This is usually done by adding elements such as phosphorus or arsenic.
- p-type: holes (absence of electrons) are formed by adding elements such as boron or gallium.
Magnetic properties of solids
The magnetic properties of solids are determined by the magnetic moments of the atoms and the order of these moments. There are different types of magnetic order:
- Diamagnetism: All electron spins are paired, and the solid is weakly repelled by a magnetic field.
- Paramagnetism: A solid material is weakly attracted to a magnetic field due to some unpaired electron spins.
- Ferromagnetism: The spins are aligned parallel in a large domain, creating a strong net magnetic field. Example: Iron.
- Antiferromagnetism: adjacent spins are aligned antiparallel, cancelling each other out. Example: manganese oxide.
Superconductivity
Finally, superconductivity is a phenomenon in which a material exhibits zero electrical resistance below a certain critical temperature. This allows the free and unhindered flow of electrons.
Josephson effect: Quantum mechanical phenomenon in which current can flow between two superconductors separated by a thin insulating layer.