# Introduction to Mathematical Physics/N body problem in quantum mechanics/Crystals

## Bloch's theorem

sectheobloch

Consider following spectral problem:

Problem:

Find ${\displaystyle \psi (r)}$  and ${\displaystyle \epsilon }$  such that

${\displaystyle [-{\frac {\hbar ^{2}}{2m}}\nabla ^{2}+V(r)]\psi (r)=\epsilon \psi (r)}$

where ${\displaystyle V(r)}$  is a periodical function.

Bloch's theorem [ma:equad:Dautray5], [ph:solid:Kittel67], [ph:physt:Diu89] allows\index{Bloch theorem} to look for eigenfunctions under a form that takes into account symmetries of considered problem.

Theorem:

theobloch

Bloch's theorem. If ${\displaystyle V(r)}$  is periodic then wave function ${\displaystyle \psi }$  solution of the spectral problem can bne written:
${\displaystyle \psi (r)=\psi _{k}(r)=e^{ikr}u_{k}(r)}$

with ${\displaystyle u_{k}(r)=U_{k}(r+R)}$  (function ${\displaystyle u_{k}}$  has the lattice's periodicity).

Proof:

Operator ${\displaystyle -{\frac {\hbar ^{2}}{2m}}\nabla ^{2}+V(r)}$  commutes with translations ${\displaystyle \tau _{j}}$  defined by ${\displaystyle \tau _{a}\psi (r)=\psi (r+a)}$ . Eigenfunctions of ${\displaystyle \tau _{a}}$  are such that: {IMP/label

${\displaystyle \tau _{a}\psi =\psi }$

Properties of Fourier transform\index{Fourier transform} allow to evaluate the eigenvalues of ${\displaystyle \tau _{j}}$ . Indeed, equation tra can be written:

${\displaystyle \delta (x-a)*\psi (r)=\psi (r)}$

where ${\displaystyle *}$  is the space convolution. Applying a Fourier transform to previous equation yields to:

${\displaystyle e^{-2i\pi ka}{\hat {\psi }}={\hat {\psi }}}$

That is the eigenvalue is ${\displaystyle \lambda =e^{-2i\pi k_{n}a}}$  with ${\displaystyle k_{n}=n/a}$  [1]. On another hand, eigenfunction can always be written:

${\displaystyle \psi _{k}(r)=e^{ikr}u_{k}(r).}$

Since ${\displaystyle u_{k}}$  is periodical[2] theorem is proved. }}

## Free electron model

Hamiltonian can be written ([ph:solid:Kittel67],[ph:solid:Callaway64]) here:

${\displaystyle H=-{\frac {\hbar ^{2}}{2m}}\nabla ^{2}+V(r)}$

where ${\displaystyle V(r)}$  is the potential of a periodical box of period ${\displaystyle a}$  (see figure figpotperioboit) figeneeleclib.

Eigenfunctions of ${\displaystyle H}$  are eigenfunctions of ${\displaystyle \nabla ^{2}}$  (translation invariance) that verify boundary conditions. Bloch's theorem implies that ${\displaystyle \phi }$  can be written:

${\displaystyle \psi _{k}=e^{ikr}u_{k}({\bar {r}})}$

where ${\displaystyle u_{k}({\bar {r}})}$  is a function that has crystal's symmetry\index{crystal}, that means it is translation invariant:

${\displaystyle u_{k}({\bar {r}}+{\bar {R}}_{i})=u_{k}({\bar {r}})}$

Here (see [ph:solid:Callaway64]), any function ${\displaystyle u_{k}}$  that can be written

${\displaystyle u_{k}({\bar {r}})=e^{iK_{n}r}}$

is valid. Injecting this last equation into Schr\"odinger equation yields to following energy expression:

${\displaystyle E_{k}={\frac {\hbar ^{2}}{2m}}|k+K_{n}|^{2}}$

where ${\displaystyle K_{n}}$  can take values ${\displaystyle {\frac {2n\pi }{a}}}$ , where ${\displaystyle a}$  is lattice's period and ${\displaystyle n}$  is an integer. Plot of ${\displaystyle E}$  as a function of ${\displaystyle k}$  is represented in figure figeneeleclib.

## Quasi-free electron model

Let us show that if the potential is no more the potential of a periodic box, degeneracy at ${\displaystyle k={\frac {K_{1}}{2}}}$  is erased. Consider for instance a potential ${\displaystyle V(x)}$  defined by the sum of the box periodic potential plus a periodic perturbation:

${\displaystyle V(x)=V_{box}+\epsilon e^{iK_{1}r}}$

In the free electron model functions

${\displaystyle {\begin{matrix}\psi _{1k}&=&e^{ikr}\\\psi _{2k}&=&e^{ikr}e^{iK_{1}r}\end{matrix}}}$

are degenerated. Diagonalization of Hamiltonian in this basis (perturbation method for solving spectral problems, see section chapresospec) shows that degeneracy is erased by the perturbation.

## Thigh binding model

Tight binding approximation [ph:solid:Ashcroft76] consists in approximating the state space by the space spanned by atomic orbitals centred at each node of the lattice. That is, each eigenfunction is assumed to be of the form:

${\displaystyle \psi (r)=\sum _{j}c_{j}\phi _{at}(r-R_{j})}$

Application of Bloch's theorem yields to look for ${\displaystyle \psi _{k}}$  such that it can be written:

${\displaystyle \psi _{k}(r)=e^{ikr}u_{k}(r)}$

Identifying ${\displaystyle u_{k}(r)}$  and ${\displaystyle u_{k}(r+R_{i})}$ , it can be shown that ${\displaystyle c_{l}=e^{ikK_{l}}}$ . Once more, symmetry considerations fully determine the eigenvectors. Energies are evaluated from the expression of the Hamiltonian. Please refer to [ph:solid:Ashcroft76] for more details.

1. So, each irreducible representation\index{irreducible representation} of the translation group is characterized by a vector ${\displaystyle k}$ . This representation is labelled ${\displaystyle \Gamma _{k}}$ .
2. Indeed, let us write in two ways the action of ${\displaystyle \tau _{a}}$  on ${\displaystyle \phi _{k}}$ :
${\displaystyle \tau _{a}\psi _{k}=e^{ika}\psi _{k}}$

and

${\displaystyle \tau _{a}\psi _{k}=e^{ik(r+a)}u_{k}(r+a)}$