simulating the energy spectrum of quantum dots
DESCRIPTION
Simulating the Energy Spectrum of Quantum Dots. J. Planelles. PROBLEM 1 . Calculate the electron energy spectrum of a 1D GaAs/AlGaAs QD as a function of the size. Hint: consider GaAs effective mass all over the structure. L. AlGaAs. AlGaAs. 0.25 eV. GaAs. m* GaAs = 0.05 m 0. - PowerPoint PPT PresentationTRANSCRIPT
Simulating
the Energy Spectrum of
Quantum Dots
J. Planelles
PROBLEM 1. Calculate the electron energy spectrum of a 1D GaAs/AlGaAs QD as a function of the size.
Hint: consider GaAs effective mass all over the structure.
L
GaAs
AlGaAs AlGaAs0.25 eV
m*GaAs = 0.05 m0
)()()](*2
[2
22
xfExfxVdx
d
m
The single-band effective mass equation:
L
0.25 eV
m*GaAs = 0.05 m0
V(x)
)()()]()/*(2
1[
2
2
0
xfExfxVdx
d
mm
Let us use atomic units (ħ=m0=e=1)
LbLb
f(0)=0BC: f(Lt)=0
)()()](*2
1[
2
2
xfExfxVdx
d
m
h
fffxf iiii 2
)( 11''
h
fffxf iiii 2
)('1
'1''''
Numerical integration of the differential equation: finite differences
Discretization grid
x1 x2xn
How do we approximate the derivatives at each point?
i-1 i+1i
h h
fi
fi-1
fi+1
211 2
...h
fff iii
Step of the grid
f1 f2 ... fn
...
1. Define discretization grid
2. Discretize the equation:
FINITE DIFFERENCES METHOD
)()()](*2
1[
2
2
xfExfxVdx
d
m
0)(
0)0(:
tLf
fBCs
iiiiii fEfVfffhm
]2[*2
1112
i=1 i=ni=2
h0 Lt
iiiii fEfbfafb 11
1h
Ln t
iiii fEfVfm
''
*2
1
iiiii fEfhm
fVhm
fhm
12212 *2
1
*
1
*2
1
3. Group coefficients of fwd/center/bwd points
1
2
3
2
1
2
3
2
1
2
3
2
n
n
n
n
n
n
f
f
f
f
E
f
f
f
f
ab
bab
bab
ba
We now have a standard diagonalization problem (dim n-2):
Trivial eqs: f1 = 0, fn = 0.
Matriz (n-2) x (n-2) - sparse
Extreme eqs:
11121 nnnnn fEfbfafbni0
232212 fEfbfafbi 0
iiiii fEfbfafb 11
i=1 i=ni=2
The result should look like this:
n=1
n=2
n=3
n=4
finite wall
infinite wall
PROBLEM 1 – Additional questions
a) Compare the converged energies with those of the particle-in-the-box with infinite walls for the n=1,2,3 states.
b) Use the routine plotwf.m to visualize the 3 lowest eigenstates for L=15 nm, Lb=10 nm. What is different from the infinite wall eigenstates?
22
22
2n
mLEn
PROBLEM 2. Calculate the electron energy spectrum of two coupled QDs as a function of their separation S.
Plot the two lowest states for S=1 nm and S=10 nm.
L=10 nm
GaAs
AlGaAs
L=10 nm
GaAs
AlGaAsS
x
bonding antibonding bonding antibonding
The result should look like this:
PROBLEM 3. Calculate the electron energy spectrum of N=20 coupled QDs as a function of their separation S.
Plot the charge density of the n=1,2 and n=21,22 states for S=1 nm and L=5 nm.
The result should look like this (numerical instabilities aside):
n=1
n=2
n=21
n=22
PROBLEM 4. Write a code to calculate the energies of an electron in a 2D cylindrical quantum ring with inner radius Rin and outer radius Rout, subject to an axial magnetic field B.
Calculate the energies as a function of B=0-20 T for a structure with (Rin,Rout)=(0,30) nm –i.e. a quantum disk- and for (3,30) nm –a quantum ring-. Lb=10 nm. Discuss the role of the linear and quadratic magnetic terms in each case.
ρ
B
V(ρ,Φ)GaAs
AlGaAs
Hint 1: after integrating Φ, the Hamiltonian reads (atomic units):
)()()(8
1
*2
1 22
2
2
2
2
fEfVB
BMM
m zz
with Mz =0, ±1, ±2... the angular momentum z-projection. 1 atomic unit of magnetic field = 235054 Tesla.
GaAs
AlGaAs
Hint 2: describe the radial potential as
where ρ=0 is the center of the ring
ρ
Hint 3: use the following BC
f(Lt)=0 (i.e. fn=0)
If Mz=0, then f’(0)=0 (i.e. f1=f2)
If Mz≠0, then f(0)=0 (i.e. f1=0)
LbRout
Rin
The results should look like this:
Disk(Rin=0)
Ring(Rin=3 nm)