🔬Condensed Matter Physics Unit 2 – Electronic band theory

Fundamentals of Electronic Band Theory

• Describes the behavior of electrons in solid materials, particularly crystalline solids
• Explains the formation of energy bands and gaps due to the periodic potential of the crystal lattice
• Provides a framework for understanding electrical, optical, and thermal properties of materials
• Considers the wave-particle duality of electrons and their interaction with the periodic potential
• Incorporates concepts from quantum mechanics, such as wave functions and energy quantization
• Enables the classification of materials into metals, semiconductors, and insulators based on their band structure
• Forms the basis for understanding electronic devices (transistors, diodes) and their applications

Crystal Structure and Lattices

• Crystalline solids exhibit a regular, repeating arrangement of atoms or molecules in three dimensions
• The smallest repeating unit of a crystal structure is called the unit cell
• Lattices are mathematical abstractions representing the periodic arrangement of points in space
  • Examples include simple cubic, body-centered cubic (BCC), and face-centered cubic (FCC) lattices
• The lattice constant is the length of one edge of the unit cell and varies for different materials
• The reciprocal lattice is a mathematical construct used to describe the periodic structure in momentum space
  • It is essential for understanding the behavior of electrons in crystals
• The Brillouin zone is the primitive cell of the reciprocal lattice and plays a crucial role in band theory
• Crystal structures determine the symmetry and periodicity of the potential experienced by electrons

Bloch's Theorem and Wave Functions

• Bloch's theorem states that the wave function of an electron in a periodic potential can be expressed as the product of a plane wave and a periodic function
  • The plane wave represents the free-electron-like behavior, while the periodic function captures the influence of the crystal potential
• The wave function takes the form $\Psi_{n\mathbf{k}}(\mathbf{r}) = e^{i\mathbf{k} \cdot \mathbf{r}} u_{n\mathbf{k}}(\mathbf{r})$, where $\mathbf{k}$ is the wave vector and $u_{n\mathbf{k}}(\mathbf{r})$ is a periodic function
• The wave vector $\mathbf{k}$ represents the crystal momentum of the electron and is confined to the first Brillouin zone
• The periodic function $u_{n\mathbf{k}}(\mathbf{r})$ has the same periodicity as the crystal lattice
• Bloch's theorem simplifies the problem of solving the Schrödinger equation for electrons in a periodic potential
• The index $n$ in the wave function denotes the band index, which distinguishes different energy bands
• The wave functions and energy eigenvalues depend on the wave vector $\mathbf{k}$, leading to the formation of energy bands

Energy Bands and Band Gaps

• The periodic potential of the crystal lattice leads to the formation of energy bands and gaps
• Energy bands are continuous ranges of allowed energy states for electrons in a solid
• Band gaps are energy ranges where no allowed electronic states exist, separating the energy bands
• The valence band is the highest occupied energy band at absolute zero temperature
  • Electrons in the valence band are responsible for bonding and contribute to the material's properties
• The conduction band is the lowest unoccupied energy band at absolute zero temperature
  • Electrons in the conduction band are free to move and contribute to electrical conductivity
• The band gap is the energy difference between the top of the valence band and the bottom of the conduction band
• The size of the band gap determines the electrical properties of a material (metal, semiconductor, or insulator)
• The band structure, which shows the energy bands as a function of wave vector, is unique for each material

Metals, Semiconductors, and Insulators

• Materials can be classified into metals, semiconductors, and insulators based on their band structure and band gap
• Metals have overlapping valence and conduction bands, or the Fermi level lies within a partially filled band
  • This allows for a large number of free electrons, resulting in high electrical conductivity
  • Examples of metals include copper, aluminum, and gold
• Semiconductors have a small band gap (typically less than 3 eV) between the valence and conduction bands
  • At absolute zero, semiconductors behave as insulators, but at higher temperatures, electrons can be thermally excited across the band gap
  • Examples of semiconductors include silicon, germanium, and gallium arsenide
• Insulators have a large band gap (typically greater than 3 eV) between the valence and conduction bands
  • The large band gap prevents electrons from being easily excited into the conduction band, resulting in poor electrical conductivity
  • Examples of insulators include diamond, rubber, and glass
• The distinction between semiconductors and insulators is not always clear-cut and depends on the specific material and application
• Doping, which is the introduction of impurities into a semiconductor, can modify its electrical properties and create n-type or p-type semiconductors

Fermi Surfaces and Density of States

• The Fermi surface is the surface in reciprocal space that separates the occupied and unoccupied electronic states at absolute zero temperature
• It represents the set of all wave vectors corresponding to the highest occupied energy states in a material
• The shape of the Fermi surface depends on the band structure and can be complex, with multiple disconnected parts or pockets
• The Fermi surface is crucial for understanding the electronic, magnetic, and transport properties of materials
• The density of states (DOS) is the number of electronic states per unit energy range
  • It quantifies the number of available states for electrons to occupy at a given energy level
• The DOS is material-specific and depends on the band structure and dimensionality of the system
• The DOS near the Fermi level plays a significant role in determining the electronic and thermal properties of a material
• Van Hove singularities are points in the DOS where the function diverges or has a discontinuous derivative
  • They occur at critical points in the band structure and can lead to interesting physical phenomena
• Experimental techniques, such as photoemission spectroscopy and scanning tunneling spectroscopy, can probe the Fermi surface and DOS of materials

Band Structure Calculations

• Band structure calculations aim to determine the energy bands and electronic properties of materials from first principles
• The most common approach is density functional theory (DFT), which maps the many-body problem onto a single-particle problem
  • DFT relies on the Hohenberg-Kohn theorems and the Kohn-Sham equations to calculate the ground-state properties of a system
• The local density approximation (LDA) and generalized gradient approximation (GGA) are widely used exchange-correlation functionals in DFT
• Pseudopotentials are employed to simplify the treatment of core electrons and reduce computational cost
• The plane-wave basis set is commonly used to expand the wave functions, taking advantage of the periodicity of the crystal
• The Brillouin zone is sampled using a discrete set of k-points, with denser k-point grids leading to more accurate results
• Band structure calculations provide valuable insights into the electronic, optical, and magnetic properties of materials
• The results of band structure calculations can be compared with experimental data from techniques such as angle-resolved photoemission spectroscopy (ARPES)

Applications and Experimental Techniques

• Electronic band theory has numerous applications in condensed matter physics, materials science, and engineering
• Understanding the band structure is crucial for designing electronic devices, such as transistors, diodes, and solar cells
• Band engineering involves modifying the band structure to achieve desired properties, such as increased carrier mobility or reduced band gap
  • This can be achieved through techniques like strain engineering, quantum confinement, or alloying
• Photovoltaic devices, such as solar cells, rely on the band gap and band alignment to convert light into electrical energy
• Thermoelectric materials, which convert temperature gradients into electrical energy, depend on the band structure and density of states near the Fermi level
• Experimental techniques play a vital role in probing the electronic structure and validating band structure calculations
• Angle-resolved photoemission spectroscopy (ARPES) measures the energy and momentum of electrons emitted from a material, providing direct access to the band structure
• Scanning tunneling spectroscopy (STS) probes the local density of states and can map out the electronic structure with atomic resolution
• Optical spectroscopy techniques, such as absorption and photoluminescence, can determine the band gap and optical transitions in materials
• Electrical transport measurements, such as resistivity and Hall effect, provide information about the carrier concentration, mobility, and Fermi surface