basics
Examples
By Bloch-Floquet theory the available energies of electron-excitations in crystals depends smoothly on the momentum/wave number $k$ of the excitation, taking values in the Brillouin torus, and otherwise only on a discrete index set labelling the remaining “quantum numbers?” (the electron‘s orbitals?, spin. etc.). Fixing the latter values (and jointly denoting them $\mathbf{n}$, say), the given function $k \mapsto E_{\mathbf{n}}(k)$ (or rather its graph) is called the $\mathbf{n}$th electronic band (or just $\mathbf{n}$th band, for short) of the material.
Often, the values of $E_{\mathbf{n}}(-)$ are closely spaced as $\mathbf{n}$ varies in certain subsets of its allowed values. For instance if $\mathbf{n}$ includes the spin of the electrons and if there is a (typically small) spin-orbit coupling and no sizeable external magnetic field, then the energies $E_{n,\uparrow}(-)$ and $E_{n, \downarrow}$ differ (only) slightly. In these cases the graphs of these values jointly look approximately like a single but thickened curve (as such shown in the following schematic graphics), which is where the name “band” originates from.
The band geometry around the electron chemical potential of a material controls its electrical conductivity:
metal/conductor | the electron chemical potential is inside the valence band |
insulator | the electron chemical potential is inside a large gap between (what is then) the valence- and conduction-band |
semi-conductor | the electron chemical potential is inside a small gap between valence and conduction band |
semi-metal | there is a large gap between valence and conduction band, except over a codimension$\geq 2$ locus, where the gap closes right at the chemical potential |
Review:
Account with focus on topological phases of matter (topological insulators, semimetals etc.):
David Vanderbilt, Berry Phases in Electronic Structure Theory – Electric Polarization, Orbital Magnetization and Topological Insulators, Cambridge University Press (2018) (doi:10.1017/9781316662205)
Jérôme Cayssol, Jean-Noël Fuchs, Topological and geometrical aspects of band theory, J. Phys. Mater. 4 (2021) 034007 (arXiv:2012.11941, doi:10.1088/2515-7639/abf0b5)
See also:
The electronic band structure of graphene was predicted in
Last revised on May 17, 2022 at 08:30:04. See the history of this page for a list of all contributions to it.