Lecture 10 Notes: Waves in Periodic Potentials (Part I)

Lecture 10 Waves in Periodic Potentials Today: 1. Direct lattice and periodic potential as a convolution of a lattice and a basis. 2. The discrete translation operator: eigenvalues and eigenfunctions. 3. Conserved quantities in systems with discrete translational symmetry. 4. Bloch’s theorem. Questions you should be able to address after today’s lecture: 1. How to mathematically represent a crystal 2. What is the definition of an “inverse lattice”? 3. What is the Hamiltonian for a periodic system? 4. What is the definition and eigenvalues/eigenfunctions of the discrete translational operator (DTO)? 5. How is the DTO used to find the energy e-values/e-functions of a system with a periodic potential? 6. What is the conserved number associated with the discrete translational symmetry in a periodic system? 1 Many materials have crystalline structure and ions are arranged in a periodic lattice. As all the ions in the lattice exert Coulombic potentials on an electron, the overall potential experienced by the electron appears periodic. The periodicity of the potential defines the electronic and optical properties of the crystalline solids and is consequently “electrons in periodic potentials” is the most important problem of this course. I Hamiltonians for periodic potentials: For simplicity lets consider one-electron Hamiltonian of a crystal - a system with a periodic potential:  2  2  2 2  V r   Hˆ   V x 2m 2m x 2 where,    V r  R  V r V x  na  V x         Take a closer look at what makes a potential periodic – the repetition of a basic unit at regular spacings (crystal lattice)! II Eigenfunctions of infinite crystal: The effect of a periodic potential    One can find eigenfunctions for this operator: T̂ u  x    u  x  Let us define a discrete translation operator: T̂a x   x  a a By inspection the eigenfunctions of the discrete translational operator are of the form:  k  x   e ikx f  x     Where f(x) is a function that is periodic in a (a is the lattice period): f x  a  f x      ik xa Let’s verify our hypothesis: T̂a k x  e   f x  a  e ika e ikx f x  e ika k x      k  x Our objective is to identify the eigenfunctions of the Hamiltonian. We will use the fact that the discrete translational operator Tˆa commutes with the periodic potential Hamiltonian: 2  2 2   2 2 Tˆ , Ĥ  x  Tˆ     V x  x      V a  a  2 2  2m x   2m x  2    2 2 2      V x  a  x  a  V x  2   2m  x  a 2  2m x            x T̂a x          x  a     2 2   2 2     V x  x  a   V x  xa 0    2 2  2m x  2m x         Thus eigenfunctions, which are common to the Hamiltonian and the discrete translation operator can be found of the form:  k  x   e ikx f  x  , f  x  a   f  x  Bloch’s Theorem The eigenfunctions of the one-electron Hamiltonian with a periodic potential:      2  2  V r , V r  R V r , where R is the lattice vector, can be written in 2m      ik r  r   e f nk r , where fnk  r  has the periodicity of the underlying the form: n,k     Bravais lattice, f nk r  R  f nk r for all lattice vectors R .    Ĥ         Another way of stating Bloch’s Theorem is: The eigenfunctions of the one-electron Hamiltonian with a periodic potential have the     ik R form:  n,k r  R  e  n,k r    Two important comments: 1. The eigenvalues corresponding to the discrete translational operator depend only on k and not on the particular form of the periodic function. 2. The only distinct eigenvalues correspond to k in the first Brillouin Zone (BZ).    For 1D case with the lattice constant a, BZ is k   , :  a a ika  n,k x  a  e  n,k x     2  i k n a 2 k' k  n  e ik 'a  e  a   e ika e i 2  n  e ika a 3 How do we label our states? What is the conserved quantity in this case? k is the Bloch wave number (or wave vector, sometimes called the crystal momentum) and represents a conserved quantity associated with the discrete translation symmetry in the crystal. As we have demonstrated above, crystal momentum is related to the ika eigenvalue of the discrete translation operator eika : T̂a un,k x  e  n,k x .   n is associated with the energy eigenvalues. We can plot the dispersion relation or a band diagram: E vs. k. Is the Crystal momentum identical to the Momentum? Is the momentum conserved in crystals?  p̂ n,k x  i     n,k x  i e ikx f x  ke ikx f x  ie ikx f x  const   n,k x x x x       n,k  x  are not eigenfunctions of momentum, this means that momentum operator does not commute with a Hamiltonian – let’s check this:     p̂, Ĥ  x  pĤ ˆ  x  ˆ  x  Hp̂       2 2      2 2  x   V x  i  V x     i  x   2 2 x x  2m x     2m x         x V x i3 2  x i3 2  x  iV x  i  x   x  iV x  2m x 2 x x 2m x 2 x V x  i x x V x  p̂, Ĥ   i 0   x          Then according to the Ehrenfest theorem momentum is NOT conserved in crystals! k – Crystal momentum (Bloch wavevector) is conserved and is different from momentum! 4