Band structure

1. Band structure Dung Nguyen

2. Content • Lattice structure • Lattice symmetry • Reciprocal lattice • Brillouin zone • Schrodinger equation • Bloch theorem • Tight-binding method

3. Lattice structure • Solid state has lattice structure. • Lattice structure has translation symmetry, so that atomic structure (lattice structure) remains invariant under translation through combination of basic vectors: • Bravais lattice: all the points are constructed by basic translation: Where are integers.

4. Unit cell • The whole crystal is made of repetition of object called unit cell. • As we see the definition of unit cell is not unique.

5. Wigner-Seitz cell • If there is some center symmetry about a lattice point ( and hence all equivalent points) . We chose it as center of unit cell. • Draw the perpendicular bisector plane of the translation vector from center point to nearest equivalent lattice sites. • The volume inside the bisector planes is a unit cell called Wigner-Seitz cell

6. Reciprocal lattice • Consider periodic function in rectangular lattice: • Then we can Fourier transform function f(r) as: • In which the reciprocal vector: • We have:

7. With non-rectangular lattice: Where are integers. Then we have:

8. Brillouin zone • Wigner-Seitz cell of the reciprocal lattice is called Brillouin zone.

9. Schrodinger equation • The Hamiltonian of perfect crystal can be written as: • Obviously, the Hamiltonian is unsolvable, we need simplifications.

10. Firstly, we separate valence electrons and core electrons. Core electron are those in filled orbital. They are mostly localized around atoms and combine to ion core. • Now • The second approximation is Born-Oppenheimer or adiabatic approximation. -The ion cores are much heavier than valence electrons, so they move slowly. As a result, compared to electron the ions are essentially stationary. The Hamiltonian can be rewritten:

11. In calculation for band structure, we mainly interested in electron Hamiltonian: • The last approximation is mean-field approximation. We assume that electrons feel the same potential V(r): • Each eigenstate can have up to 2 electrons due to Pauli principle.

12. Bloch theorem • Consider periodic potential: • Schrodinger equation: • Hamiltonian under translation:

13. Consider eigen-state under translation by a lattice vector: • Translation invariant of Hamiltonian: • Eigen-state equation: • The equation is identical with: • So we have:

14. If eigen-state is non-degenerate, we have: • By normalized condition: • So that: • Similarly: • In general:

15. Define a vector in the form: • Then we have: • So for each eigen-wave function, there exist a vector such that: • For the case of degenerate eigen-state, we use the fact that: • In which • Thus can be diagonalized together.

16. Reduction to first BZ • By Bloch theorem, we can label every eigen-wave function by its wave vector • We know that free electron wave satises this form: • So we want to make eigen-wave functions as much as possible like free electron wave: • Such that

17. Then we have: • Then the wave-vector is not uniquely defined, it can only defined up to differences by reciprocal lattice vectors.

18. Example: Consider one-dimension lattice with lattice size a, the reciprocal lattice vector: • we may assign any wave number in the set: • Thus is only defined by modulo • So how we define unique wave-vector? We always choose for a value wave-number in first BZ in reciprocal lattice space.Any state may be characterized by its reduced wave vector.

19. There will be multiple states with the same reduced wave vector in first BZ with different energies. • Ex: Free electron energy: • In reduced zone:

20. If we turn on small potential, consider free electron eigen-state: • The perturbative energy at first order: • With periodic potential: • With is Fourier transform of potential. We only sum over reciprocal lattice vectors. • With degeneracy of energies:

21. We have energy gap: • Because:

22. Tight binding calculation: • There are some different methods to calculate band structure, of course with different order of uncertainty. • I just want to introduce a simple method is Tight binding or LCAO method. In which, we use atomic's valence electron orbitals as basic wave functions.

23. Toy example I • Consider one-dim solid composed by N atoms, each atom has one valence electron orbital. • Hamiltonian in valence electron orbitals basic:

24. In which we consider overlap of electron orbitals wave function of nearest atoms. • In order to determine band structure (eigen-value problem), we have to diagonalizeNxN matrix to fine eigen-energies at each wave vector k. • We have eigen-value equation: • Or: • In which we have N-component vector:

25. By consequence of Bloch theorem ( translation invariant), we consider ansatz: • Thus we have: • Therefore:

26. Toy example II • Consider one-dimension solid whose unit cell consists of two orbitals • We have Hamiltonian:

27. Eigen-value equation: • We rewrite eiegen-value equation: • And Hamiltonian:

28. We again solve the eigen-value by ansatz: • Thus we have: • We used:

29. General case • Consider particular unit cell n that has non-zero Hamiltonian coupling to its neighboring unit cells m by a matrix [Hnm] of size (bxb), where b being the number of basic functions per unit cell.

30. We can write the overall matrix equation: • We solve it by ansatz: • Band structure can be derived by diagonalizingbxb matrix:

31. Band structure of Graphene Carbon Sp2 orbital • 2s, 2px and 2py electrons hybridize to sp2 orbital:

32. Graphene lattice • sp2electrons combine together by sigma-bond to make hexagonal graphene lattice on x-y plane:

33. At low energy, we only consider the electron in pzorbitals. • We use nearest-neighbor tight binding: • We found egien-energy:

34. We have band structure and first Brillouin zone: • We have 6 Dirac points but only 2 independent points K and K’(-K), other can be derived with translation by reciprocal lattice vectors. Brillouin Zone Dirac Cone Dirac Point

35. Band structure of GaAs • Lattice structure and BZ:

36. We have each unit cell includes 2 atoms. Each atom has 5 valence electron orbitals , thus we have 10 orbitals basic per unit cell. • Tight binding Hamiltonian:

37. In which we have parameters:

38. We found band structure:

39. In order to modify tight binding method for counting spin orbit and also other magnetic effect, we have to write down Hamiltonian for spin up and spin down electrons in each orbital ( double number of basic wave function) • In the case of GaAs, we have to diagonalize 20x20 Hamiltonian. matrix.

40. Reference: • J.M. Ziman, Principles of the theory of solids. •  Peter Y.  Yu, Fundamental of semiconductor. • S. Datta, Quantum transport: Atom to Transitor.