2. Theory
Consider the interface between a quasi-two-dimensional semiconductor and the surface of a TI, as shown in
Figure 1, where region I is the semiconductor and region II is the TI. The edges of the two materials touch. Without loss of generality, we will consider that only the lowest subband is occupied by electrons in region I. We also ignore any spin–orbit interaction in region I. If region I is made of a wide bandgap semiconductor, then the Rashba spin–orbit interaction is weak [
6] and the Dresselhaus spin–orbit interaction can also be weak, depending on the crystal structure. The semiconductor that we chose for our example later in
Section 5 is CdTe (a wide band semiconductor) which has a relatively small Dresselhaus spin–orbit constant of 11.75 eV-Å
3 [
7]. Therefore, we can neglect spin–orbit interaction in the semiconductor.
The Hamiltonian for the composite system can be written approximately as [
8,
9]
where,
,
v0 is the Dirac cone velocity,
σ-s are the Pauli spin matrices,
λ is the band-structure warping constant and
k+, k− are wavevectors in the two spin eigenstates in the TI [
8,
9]. The quantities
v0 and
λ are zero in region I and non-zero in region II, while
V(
x) =
V in region I and
V(
x) = 0 in region II.
Let us assume that, in region I, the effective mass of an electron is
and, in region II, it is
. Let us also call the wave vector components in region I
and in region II
. The energy dispersion relation of electrons in the surface of the TI (region II) can be obtained by diagonalizing the TI Hamiltonian, and can be written as [
9,
10].
where the two branches represent the dispersion relations in the two spin bands and
is the azimuthal angle subtended by the electron wave vector with the
x-axis. From
Figure 1, it is obvious that
is also the angle of refraction in region II. The energy dispersion relation in the quasi-2D semiconductor (region I), on the other hand, can be written as:
where
V is the (step) potential discontinuity at the interface.
The “spin refractive indices” (
,
) of the two regions for any incident electron energy will obey the relation [
5]:
where
kI and
kII are the magnitudes of the electron wave vectors for an arbitrary electron energy
E in the two regions.
Using Equation (4), we will now define a quantity
k as:
The angles of incidence
θi and refraction
θr are defined in
Figure 1. We note from this figure that
.
Since the Hamiltonian in Equation (1) is invariant in the
y-coordinate,
ky is a good quantum number. In other words, it is conserved across the interface, and hence
, which immediately yields the equivalent of Snell’s law from the last set of relations:
It is also easy to see from the conservation of the wave vector’s y-component that the angle of reflection is always equal to the angle of incidence in region I. This is, of course, identical to the situation in conventional optics.
We will now show that for any angle of incidence, there will be two different angles of refraction into the two spin eigenstates in the TI surface, and hence there will be two different refractive indices as well, as we had tacitly assumed. Since energy is conserved in the process of reflection/refraction (which are elastic events), we must have
, which yields from Equations (2) and (3):
This equation immediately shows that there will be generally two different solutions for
for the two spin eigenstates at any incident energy
E and hence, according to Equation (4), there will be two different refractive indices
for refracting into the two spin eigenstates in region II. Snell’s law (Equation (6)) will then ensure that, for a fixed angle of incidence, there will be two different refraction angles
which will obey the relation
The last equation is derived by using Equation (5) in (7) and can be recast (using Equation (6)) as:
Because there are two different refractive indices in region II, it is obvious that there can be none, one, or two different critical angles of incidence. If there are two, we can find them by setting
radians in Equation (9). This yields an equation for the critical angle associated with the first refracted beam (or first spin eigenstate in region II) as:
where we made use of the relation
.
Equation (10) is a quadratic equation that yields two solutions for the critical angle of incidence for a spin refracting into the first spin eigenstate:
The solution is negative and is hence an extraneous solution. Negative refraction is a well-known phenomenon in optics, but is not relevant here.
The critical angles of incidence for a spin refracting into the second spin eigenstate are found from the quadratic equation:
which again yields two solutions:
The solution is negative and hence extraneous.
In order to obtain simplified and tractable expressions for the two refraction angles for a given angle of incidence, we will next neglect the warping effects and set
λ = 0. Band warping has a strong effect on such phenomena as circular dichroism [
11], but not so much on spin properties that we study here. Then, from Equation (9), we obtain the following expressions for the two refraction angles as a function of the incident angle:
It is easy to see from Equation (14) that:
which tells us that
. This then also means that
and
. From Equations (11), (13) and (14), we can also recover the familiar relations (familiar from optics) that
.
3. Calculating the Refraction and Reflection Amplitudes
The wave function of an electron in region I (with arbitrary spin polarization) can be written as:
where the first term on the right-hand side is the incident wave and the last two terms constitute the reflected wave, with
r being the reflection amplitude into the incident spin eigenstate and
r’ the reflection amplitude into the orthogonal spin eigenstate, i.e., reflection with a spin flip. Note that
, where the asterisk denotes a complex conjugate. There can be a spin-flip at the interface for the reflected electron in the semiconductor quantum well (region I), and hence we will have to consider that possibility here.
Neglecting warping effects, the Hamiltonian in region II (see Equation (1)) can be written as:
Using the result
, we can find that the two eigenspinors of this Hamiltonian are:
Because , the two eigenspinors are not mutually orthogonal. They do not have to be since they are actually eigenspinors of two different Hamiltonians due to the fact that . Note that the spin orientations in these two eigenstates are perpendicular to the directions of the corresponding refracted beams, which is characteristic of spin-momentum locking. To see this, note that the spin polarization components in the two beams are . This immediately shows that in region II, in either refracted beam. Note also that, as a consequence of birefringence, which makes , the spins in the two beams are not mutually antiparallel, since and . This of course happens due to the fact that the two eigenspinors in Equation (18) are not orthogonal.
The wave function of the refracted electron can be written as:
where
is the transmission amplitude into the first spin eigenstate and
t_ is that into the second spin eigenstate in the refraction medium (TI). The
x-components of the wave vectors in the two refracted beams are, of course,
. For an electron of energy
incident from region I at an angle
on the interface, and these wavevector components in region II (TI) are given by the relations:
and in region I (semiconductor), they are given by the relations:
Since , the wave vector becomes imaginary when , while the wave vector becomes imaginary when indicating the well-known fact that the refracted wave will be evanescent when the angle of incidence exceeds the critical angle.
Enforcing the continuity of the wave function at the junction between the two regions (at
x = 0), we get that:
which can be re-written as:
Enforcing current continuity across the interface, we get that at
x = 0:
which yields:
and that can be recast as:
where:
Defining new matrices as
and
, we can re-write Equation (23) as:
and Equation (26) as:
From Equation (28), we get:
where the dagger denotes Hermitian conjugate. Note that the matrix
is unitary, and hence its inverse is its Hermitian conjugate matrix.
Substituting the last result in Equation (29), we get:
Let us now define a new 2×2 matrix
. Then, from Equation (31), we get the transmission amplitudes in the three different intervals
for a given incident angle
as
while the reflection amplitudes are found from Equation (31).
5. Numerical Examples
For numerical examples, we consider the semiconductor material to be CdTe and the topological insulator to be Bi
2Te
3. This means
and
, where
is the free electron mass. The band alignment of this heterostructure is shown in
Figure 2 [
12]. This means that
V = +1.12 eV. The conduction band offset at the interface causes a delta-function electric field which can give rise to a localized Rashba-type spin–orbit interaction at the interface which might cause a spin-flip at the interface as the electrons transmit into region II. We ignore that effect here.
We will assume that an electron is incident on the interface from region I (CdTe) with a kinetic energy
E −
V of 0.05 eV. If this is the Fermi energy, then this corresponds to a Fermi wave vector of 3.8 × 10
8 m
−1 and hence an electron concentration of ~2.29 × 10
16 m
−2 in the CdTe layer. This means
E = 1.12 + 0.05 = 1.17 eV in the TI. The Dirac cone velocity ν
0 in Bi
3Te
2 is about 4 × 10
5 m/s [
13]. From Equation (14), we get that for this energy,
= 7.3 and
= 9.3. Since both ratios are greater than unity, there are no real solutions for the critical angles
and
. In other words, an incident spin will transmit into the topological insulator, regardless of the angle of incidence.
In order to have the lower refractive index ratio
, which will permit a real solution for at least the critical angle , we will need the Fermi energy E − V in CdTe to be so high that it will require an impractical carrier concentration. Hence, for this material system, there are no critical angles, i.e., all incident angles will transmit and there will be no total internal reflection.
In
Figure 3, we plot the refraction angles
and
as functions of the incident angle
of the electron in region I (CdTe) obtained from Equation (14). We have verified that the two refraction angles satisfy the relation in Equation (15). In
Figure 4, we plot
and
as functions of the incident angle
obtained from Equation (20). We then calculate the squared magnitudes of the transmission amplitudes into the two spin eigenstates in the TI as well as the squared magnitudes of the reflection amplitudes with and without spin flip,
, and
, as functions of the incident angle of the electron in region I for two different spin polarizations:
x-polarized spin
and
z-polarized spin
. These results are obtained from Equations (26)–(32) and shown in
Figure 5 and
Figure 6.
We call these quantities “squared magnitudes of the amplitudes”, instead of transmission and reflection “probabilities” for a reason. A probability cannot exceed unity, but the squared magnitudes can, as is obvious from Equation (36). This is the reason why in quantum transport literature the transmission probability is usually defined as instead of as because the former quantity cannot exceed unity, but the latter can.
One interesting feature in
Figure 6 is that
and
for all incident angles for z-polarized spin, but not for x-polarized spin. This feature can be derived analytically from the results presented here.
6. Conclusions
In conclusion, we have derived the laws of reflection and refraction of a spin at the interface of a quasi-2D semiconductor region and a topological insulator, touching on their sides, neglecting band warping effects.
Note that the problem we explored in this paper is not a transport problem; rather, it is an “interface” problem. The sharp interface between the semiconductor and the TI is of zero physical extent, and hence no transport can occur through the interface. Only what happens at the interface matters. What happens to the spin before reaching the interface (i.e., whether it suffers scattering, etc.) and what happens to the spin after it passes through the interface, are of no consequence and do not affect the laws of reflection and refraction. In the case of optics or electromagnetics, the laws of reflection and refraction are determined by the continuity of the electric and magnetic field components at the interface only, and what scattering the electromagnetic wave or light wave experiences before reaching the interface or after passing through the interface, does not affect Snell’s law. The same is true here.