1 Tight binding approximation Masatsugu Sei Suzuki Department of Physics, SUNY at Binghamton (Date: March 22, 2013) The tight-binding model is opposite limit to the nearly free electron model. The potential is so large that the electrons spend most of their lives near ionic cores, only occasionally shift to nearest core atom quantum mechanically. The nearly free electron method looks at the wave-functions outside the atomic cores, where they look very like plane waves. Within the cores they look like atomic orbitals. This suggests an entirely different scheme for the construction of electron wave-functions: we try to combine atomic orbitals, each localized on a particular atom, to represent a state running throughout the crystal. 1. One dimensional case: Bloch theorem Suppose that the electrons are tightly bound to the nuclei. Thewavefunction would coincide within the n cell, with an atomic eigenfunction a (such as 1s, 2s, 2p, ). Then the wavefunction of the system may be expressed by a linear combination of atomic orbital (LCAO) function, 1 1 () ( ) N k n a n x C x na N . x x
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1
Tight binding approximation
Masatsugu Sei Suzuki
Department of Physics, SUNY at Binghamton
(Date: March 22, 2013)
The tight-binding model is opposite limit to the nearly free electron model. The
potential is so large that the electrons spend most of their lives near ionic cores, only
occasionally shift to nearest core atom quantum mechanically.
The nearly free electron method looks at the wave-functions outside the atomic cores,
where they look very like plane waves. Within the cores they look like atomic orbitals.
This suggests an entirely different scheme for the construction of electron wave-functions:
we try to combine atomic orbitals, each localized on a particular atom, to represent a state
running throughout the crystal.
1. One dimensional case: Bloch theorem
Suppose that the electrons are tightly bound to the nuclei. Thewavefunction would
coincide within the n cell, with an atomic eigenfunction a (such as 1s, 2s, 2p, ). Then
the wavefunction of the system may be expressed by a linear combination of atomic orbital
(LCAO) function,
1
1( ) ( )
N
k n a
n
x C x naN
.
x
x
2
The coefficient Cn can be determined as
ikna
n eC ,
from the requirement that )(xk should be the Bloch wave function;
)()( xeax k
ika
k .
((Proof))
Since
1
1 1
1 1( ) ( ) ( )
N N
k n a j a
n j
x a C x a na C x jaN N
,
1
1( ) ( )
Nika ika
k j a
j
e x e C x jaN
,
we get the relation
j
ika
j CeC 1 .
When C1 = ikae , nC is obtained as
ikna
n eC .
Then we have the Bloch wave as
1
1( ) ( )
Nikna
k a
n
x e x naN
.
((Note)) It is also clear that this form of )(xk satisfies the Bloch theorem,
( )
1
1( ) ( ) ( )
Nikx ik x na ikx
k a k
n
x e e x na e u xN
,
where
( )
1
( ) ( )N
ik x na
k a
n
u x e x na
.
)(xuk is a periodic function with a lattice period a,
3
[ ( 1) ]
1
( ) [ ( 1) ] ( )N
ik x n a
k a k
n
u x a e x n a u x
.
It can be easily shown that )(xk possesses all the required properties of the Bloch waves,
1 1
( ) ( ) ( ) ( )N N
ikna iGna ikna
k G a a k
n n
x e e x na e x na x
.
((Note)) Validity of the choice of above wave function
From the Bloch theorem, we have
( ) ( )ikx
k kx e u x
We choose ( )ku x as
( ) ( )k a
n
u x x na
we note that ( )x na is a wave function localized at x na [like a Dirac delta function
( )x na ]. It is clear that ( )ku x is periodic such that
( ) ( ) [ ( 1) ] ( )k a a k
n n
u x a x a na x n a u x
We note that
( ) ( )
( )
( )
ikx
k k
ikx
a
n
ikna
a
n
x e u x
e x na
e x na
In general case (3D system)
( ) ( )
( )
( )l
i
i
a l
l
i
a l
l
e u
e
e
k r
k k
k r
k R
r r
r R
r R
4
2. Three dimensional case
According to the Bloch theorem, we consider the wavefunction given by
N
j
ja
i
kje
N 1
)(1
)( RrrRk ,
where we assume that there is one atom per unit cell. The number of atoms is N.
jllajad )()(*
RrRrr .
The wavefunction )(rk is normalized as follows.
1
1
)()(1
)()(
,
,
**
N
lj
jl
ii
N
lj
laja
ii
kk
lj
lj
eeN
deeN
d
RkRk
RkRk
RrRrrrrr
The Hamiltonian H is defined by
)(2
22
rVm
H ℏ
,
where V(r) is the periodic potential of the present system.
Rl-1 Rl Rl+1
VaHr-Rl L
VHrL
5
Fig. Potential in the tight binding approximation. The potential ( )a lV r R is the
potential from the isolated atom at Rl. 1
( ) ( )N
a j
j
V V
r r rɶ . V(r) is a periodic
potential of the system: V(r)< ( )V rɶ .
Here ( )a lV r R is the potential of the atom isolated at the position vector lR . The
wavefunction ( )a l r R satisfies the Schrödinger equation,
2
2 (0)[ ( )] ( ) ( )
2a l a l a a lV
m r R r R r R
ℏ,
where )0(
a is the energy eigenvalue of the isolated atom. The Hamiltonian of the system is
given by
)(2
22
rVm
H ℏ
where )(rV is the periodic potential of the system. It is different from the potential ( )V rɶ
which is defined by
1
( ) ( )N
a j
j
V V
r r rɶ
and
)()( rRr VV l , ( ) ( )lV V r R rɶ ɶ
We start with the eigenvalue problem with the Schrödinger equation for the Hamiltonian,
)()( rrkkk
H ,
Noting that
1
( ) ( ) ( ) ( )
( ) ( )
N
a l a j a l
j
a l a l
V V
V
r r R r r r R
r r r R
ɶ
we get
6
2 22 2
1
22
1
22
1
1
1[ ( )] ( ) [ ( )] ( )
2 2
1{ ( ) [ ( ) ( )]} ( )
2
1[ ( )] ( )
2
1[ ( ) ( )] ( )
l
l
l
l
Ni
a l
l
Ni
a l
l
Ni
a l a l
l
Ni
a l
l
V e Vm mN
e V V VmN
e VmN
e V VN
k R
k
k R
k R
k R
r r r r R
r r r r R
r R r R
r r r R
ℏ ℏ
ℏɶ ɶ
ℏ
ɶ
Thus we have
2
2 (0)
1
1
1
1[ ( )] ( ) ( )
2
1[ ( ) ( )] ( )
1( )
l
l
l
Ni
a a l
l
Ni
a l
l
Ni
a l
l
V em N
e V VN
eN
k R
k
k R
k R
k
r r r R
r r r R
r R
ℏ
ɶ
By multiplying both sides of this equation by )( '
*
la Rr and integrating over all the space,
we have
(0) *
'
1
*
'
1
( ) ( ) ( )
( )[ ( ) ( )] ( )
l
l
Ni
a a l a l
l
Ni
a l a l
l
e d
e d V V
k R
k
k R
r r R r R
r r R r r r Rɶ
We note that
','
*)()( lllalad RrRrr
(orthogonality of the wave function)
Thus we get
(0) *
, ' '
1 1
( ) ( )[ ( ) ( )] ( )l l
N Ni i
a l l a l a l
l l
e e d V V
k R k R
kr r R r r r Rɶ
or
7
'(0) *
'
1
( ) ( )[ ( ) ( )] ( )l l
Ni i
a a l a j
l
e e d V V
k R k R
kr r R r r r Rɶ
When we define the potential difference by
( ) ( ) ( )V V V r r rɶ
which is negative, we get
'( )(0) *
'
1
( ) ( ) ( )l l
Ni
a a l a l
l
e d V
k R R
kr r R r r R
Note that this integral remains unchanged when the variable of the integral is changed from
lRr to r, since
( ) ( )lV V r R r .
Here we put
hll RRR ' ,
Fig. Schematic form of the potential energy for tightly bound electrons, alng a given
direction in the crystal. (Rigamonti and Carretta, Structure of Matter, Springer,
2007).
Then we have
8
N
h
aha
i
a Vde h
1
*)0( )()()( rrRrrRk
k
When the matrix element ht is defined by
)()()(*
rrRrr ahah Vdt ,
the energy eigenvalue is approximated by
...
..
)0(
...
..
)0(
)0(
nn
i
nna
nn
i
nna
h
i
hak
h
h
h
e
et
et
Rk
Rk
Rk
.
The sum over hR includes only values for which Rl denotes a nearest neighbor of Rj. In
the present case, both and are positive since V is negative. Note that the n.n denotes
the nearest neighbor pairs.
3. 3D systems: scc, fcc, and bcc
(a) Simple cubic
9
For the simple cubic lattice with the lattice constant a,
)coscos(cos2)0(
akakak zyxak ,
10
Fig. ContourPlot for the constant energy surface in the reciprocal lattice space.
75.0coscoscos akakak zyx .
When 1akx , 1aky , and 1akz , we get
)(62222)0(
zyxak kkka .
(b) fcc
There are twelve nearest neighbor atoms in the fcc lattice;
)1,1,0(2
1
aa , )1,0,1(
22
aa , )0,1,1(
23
aa , )1,0,1(
24
aa , )1,1,0(
25
aa ,
)0,1,1(2
6
aa , )0,1,1(
27
aa , )1,1,0(
28
aa , )1,0,1(
29
aa , )0,1,1(
210
aa ,
)1,0,1(2
11 a
a , )1,1,0(2
12 a
a
11
]2
cos2
cos2
cos2
cos2
cos2
[cos4)0(
akakakakakak xzzyyx
ak
.
Fig. ContourPlot for the Fermi surface of Copper (M. Suzuki and I.S. Suzuki).
1. M. Suzuki and I.S. Suzuki, Lecture note on solid state Physics, Bloch theorem and
energy band. http://www2.binghamton.edu/physics/docs/note-energy-band.pdf 2. See Out of the Crystal Maze, Chapters from the history of solid state physics edited
by L. Hoddeson, E. Braun, J. Teichmann, and S. Wert (Oxford University Press, New York, 1992).
3. S.L. Altmann, Band Theory of Metals (Pergamon Press, Oxford 1970). S.L. Altmann, Band Theory of Solids An Introduction from the point of view of symmetry
(Clarendon Press, Oxford 1991). 4. J.M. Ziman, Principle of the Theory of Solids (Cambridge University Press 1964).
5. C. Kittel, Introduction to Solid State Physics, seventh edition (John Wiley & Sons, New York, 1996).
6. C. Kittel, Quantum Theory of Solids (John Wiley & Sons, 1963). 7. N.W. Ashcroft and N.D. Mermin, Solid State Physics (Holt, Rinheart and Winston,
New York, 1976). 8. E.M. Lifshitz and L.P. Pitaevskii, Statistical Physics Part2 Landau and Lifshitz
Course of Theoretical Physics volume 9 (Pergamon Press, Oxford 1980). 9. J. Callaway, Quantum Theory of the Solid State, second edition (Academic Press,
New York, 1991). 10. A.A. Abrikosov, Introduction to the Theory of Normal Metals (Academic Press,
New York, 1972). 11. M. Kuno, IntroductoryNanoscience Physical and Chemical Concepts
(Garland Science, Taylor & Francis Group, London and New York, 2012). 12. R.L. Liboff Introductory Quantum Mechanics (Addison-Wesley Publishing
Company, New York, 1980). 13. C. Kittel Introduction to Solid State Physics eighth edition (John Wiley &
Sons, New York, 2005). 14. A. Rigamonti and P. Caretta, Structure of Matter: An Introduction Course with