Inversion asymmetry effect on quantum oscillations in 3-D crystals with Cnv symmetry
The peculiarities of quantum oscillations in bulk semiconductors with Cnv symmetry caused by the lack of their symmetry centre are considered. A quasi-qubic model is used for finding the magnetic levels. The algorithm for numerical calculating the levels in the presence of the tilted magnetic field...
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| Опубліковано в: : | Semiconductor Physics Quantum Electronics & Optoelectronics |
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| Дата: | 2005 |
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| Мова: | English |
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
2005
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| Цитувати: | Inversion asymmetry effect on quantum oscillations in 3-D crystals with Cnv symmetry / V.V. Ivchenko, A.N. Sergeev, V.S. Elnik, G.P. Chuiko // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2005. — Т. 8, № 1. — С. 22-27. — Бібліогр.: 13 назв. — англ. |
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Ivchenko, V.V. Sergeev, A.N. Elnik, V.S. Chuiko, G.P. 2017-06-12T15:44:14Z 2017-06-12T15:44:14Z 2005 Inversion asymmetry effect on quantum oscillations in 3-D crystals with Cnv symmetry / V.V. Ivchenko, A.N. Sergeev, V.S. Elnik, G.P. Chuiko // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2005. — Т. 8, № 1. — С. 22-27. — Бібліогр.: 13 назв. — англ. 1560-8034 PACS: 71.28.+d, 71.18.+y https://nasplib.isofts.kiev.ua/handle/123456789/120658 The peculiarities of quantum oscillations in bulk semiconductors with Cnv symmetry caused by the lack of their symmetry centre are considered. A quasi-qubic model is used for finding the magnetic levels. The algorithm for numerical calculating the levels in the presence of the tilted magnetic field is suggested. Numerical estimations are performed for Сd₃As₂ and Cd₃₋xZnxAs₂. It is shown that the most suitable conditions for observing the beating effect in these compounds are the high values of the electron concentrations and small angles between the direction of the magnetic field and crystal symmetry axis. en Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України Semiconductor Physics Quantum Electronics & Optoelectronics Inversion asymmetry effect on quantum oscillations in 3-D crystals with Cnv symmetry Article published earlier |
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Inversion asymmetry effect on quantum oscillations in 3-D crystals with Cnv symmetry |
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Inversion asymmetry effect on quantum oscillations in 3-D crystals with Cnv symmetry Ivchenko, V.V. Sergeev, A.N. Elnik, V.S. Chuiko, G.P. |
| title_short |
Inversion asymmetry effect on quantum oscillations in 3-D crystals with Cnv symmetry |
| title_full |
Inversion asymmetry effect on quantum oscillations in 3-D crystals with Cnv symmetry |
| title_fullStr |
Inversion asymmetry effect on quantum oscillations in 3-D crystals with Cnv symmetry |
| title_full_unstemmed |
Inversion asymmetry effect on quantum oscillations in 3-D crystals with Cnv symmetry |
| title_sort |
inversion asymmetry effect on quantum oscillations in 3-d crystals with cnv symmetry |
| author |
Ivchenko, V.V. Sergeev, A.N. Elnik, V.S. Chuiko, G.P. |
| author_facet |
Ivchenko, V.V. Sergeev, A.N. Elnik, V.S. Chuiko, G.P. |
| publishDate |
2005 |
| language |
English |
| container_title |
Semiconductor Physics Quantum Electronics & Optoelectronics |
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| format |
Article |
| description |
The peculiarities of quantum oscillations in bulk semiconductors with Cnv symmetry caused by the lack of their symmetry centre are considered. A quasi-qubic model is used for finding the magnetic levels. The algorithm for numerical calculating the levels in the presence of the tilted magnetic field is suggested. Numerical estimations are performed for Сd₃As₂ and Cd₃₋xZnxAs₂. It is shown that the most suitable conditions for observing the beating effect in these compounds are the high values of the electron concentrations and small angles between the direction of the magnetic field and crystal symmetry axis.
|
| issn |
1560-8034 |
| url |
https://nasplib.isofts.kiev.ua/handle/123456789/120658 |
| citation_txt |
Inversion asymmetry effect on quantum oscillations in 3-D crystals with Cnv symmetry / V.V. Ivchenko, A.N. Sergeev, V.S. Elnik, G.P. Chuiko // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2005. — Т. 8, № 1. — С. 22-27. — Бібліогр.: 13 назв. — англ. |
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AT ivchenkovv inversionasymmetryeffectonquantumoscillationsin3dcrystalswithcnvsymmetry AT sergeevan inversionasymmetryeffectonquantumoscillationsin3dcrystalswithcnvsymmetry AT elnikvs inversionasymmetryeffectonquantumoscillationsin3dcrystalswithcnvsymmetry AT chuikogp inversionasymmetryeffectonquantumoscillationsin3dcrystalswithcnvsymmetry |
| first_indexed |
2025-11-26T06:25:15Z |
| last_indexed |
2025-11-26T06:25:15Z |
| _version_ |
1850615387296628736 |
| fulltext |
Semiconductor Physics, Quantum Electronics & Optoelectronics. 2005. V. 8, N 2. P. 22-27.
© 2005, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
22
PACS: 71.28.+d, 71.18.+y
Inversion asymmetry effect on quantum oscillations
in 3D crystals with Cnv symmetry
V.V. Ivchenko1, A.N. Sergeev1, V.S. Elnik1, G.P. Chuiko2
1Kherson State University, Chair of Physics, 27, 40 Rokiv Zhovtnya str., 73000 Kherson, Ukraine
Phone: +380-0552-326768, e-mail: ivchenko@ksu.kherson.ua
2Kherson State Technical University, Department of Cybernetics, 24, Beryslavske shosse, 73008 Kherson, Ukraine
Phone: +380-0552-326922, e-mail: chuiko@public.kherson.ua
Abstract. The peculiarities of quantum oscillations in bulk semiconductors with Cnv
symmetry caused by the lack of their symmetry centre are considered. A quasi-qubic
model is used for finding the magnetic levels. The algorithm for numerical calculating
the levels in the presence of the tilted magnetic field is suggested. Numerical estimations
are performed for Сd3As2 and Cd3-xZnxAs2. It is shown that the most suitable conditions
for observing the beating effect in these compounds are the high values of the electron
concentrations and small angles between the direction of the magnetic field and crystal
symmetry axis.
Keywords: spin splitting of bands, Fermi surface, quantum oscillations, beating effect,
node, cadmium arsenide.
Manuscript received 31.03.05, accepted for publications 18.05.05.
1. Introduction
In the crystals without the symmetry center while
straying from the Brillouin zone center, complete
removing the energy band degeneracy may occur. The
opposite signs of the electron spin projections onto the
quantization axis correspond to the splitted states. For
the crystals with υnC symmetry class, such a splitting is
linear in the wavevector k. The appropriate isoenergetic
surface is described by the following equation in the
cylindrical coordinate system [1]:
1
)(
2
2
2
2
0 =+
±⊥
b
k
a
kk z , (1)
where a , b are the semiaxes of the ellipse in which
axial rotation along zk produces the surface; 0k is the
nonzero (as a result of the symmetry center lack) value
of the ellipse center shift from the origin of coordinates
within the plane perpendicular to the crystal main axis.
Since 0,, kba depend on the band parameters as well
as on the electron energy ε, the alteration of the latter
one may lead to changes in the shape and topology of
isoenergetic surface. When ak >0 , the surface is
topologically equivalent to the torus while for ak <0 it
is the fourth order surface self-crossing in two conical
points (Fig. 1).
The possibility to manifest peculiarities of
isoenergetic surfaces (1) in quantum-oscillatory
phenomena (Shubnikov – de Haas (SdH) and de Haas –
van Alphen effects (dHvA)) was discussed in [2, 3].
However, these works dealt only with the cases when the
magnetic field was parallel or perpendicular to the zk -
axis and the simple one-band parabolic model was used.
In this paper, we present a more comprehensive study of
this problem. In particular, we take into account the
possible nonparabolicity of the energy spectrum and
using the effective mass method consider the conditions
for beating appearing in the case of different electron
concentrations and orientations of magnetic field. Our
theoretical analysis was accompanied by numerical
calculations performed for Cd3As2 and 2xx-3 AsZnCd
that have 12
4υC space group symmetry.
2. Theory and numerical calculations
It is known [4] that, in the quasi-classical approximation,
the period of quantum oscillations is defined by the
extreme in zk ′ ( zk ′ is a component of the wavevector
along the magnetic field B) cross-section area mS of the
Fermi surface (FS) normal to the external magnetic field
direction: T = 2πe/ћSm. For FS which has more than one
extreme cross-section area normal to a given magnetic
field direction, there will be oscillatory effects associated
with each such area.
Semiconductor Physics, Quantum Electronics & Optoelectronics. 2005. V. 8, N 2. P. 22-27.
© 2005, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
23
Fig. 1. Shape of the isoenergetic surfaces described by
equation (1): ak <0 (a), ak >0 (b).
For the majority of semiconductors, the characteristic
of the deviation of FS (1) from an ellipsoidal shape
r = k0/a << 1. The quasi-classical theory developed for
this case [2, 3] predicts the existence of regular “beats”
in quantum oscillations connected with the existence of
two close central cross-section areas of FS. It is easy to
derive the expression for finding the beating period in
the form
)(80 κπ ErTTB = (2)
where
F
baabeT F εεθθε =+= )sincos2()( 22222
0 h is
the oscillatory period obtained by ignoring the inversion
asymmetry; εF is the Fermi energy; )(κE is the complete
elliptic integral of the second kind with the parameter
θκ 222 cot)(11 ab+= ; θ is the angle between the
field direction and zk -axis. It should be noted, however,
that within the limits of quasi-classical approach the
interaction between the electron spins and magnetic field
(Zeeman’s term) is not taken into account. Very often
such an interaction complicates the beating pattern [5, 6].
So, to correctly describe this effect one should use the
quantum-mechanical approach.
It is known [4] that the harmonics amplitudes of the
SdH(dHvA)-signal contain the multiplicative factor
cos(jπν) (j is the harmonic number; cnn ωεεν h)( −+ −= ,
where ±
nε is the energy of the Landau magnetic subband
with the extreme value zk ′ , cω is the cyclotron
frequency). When the ν-factor magnetic field depen-
dence is available, the harmonics amplitudes may turn
into zero for certain values of the magnetic field – a
“node” appears in the oscillatory curve. As a rule, these
peculiarities manifest themselves in the range of the
magnetic fields where it is possible to detect only the
first harmonic. The condition of the node appearing for it
is written as 5.0−= iν , where i is the node number.
To determine the subband energies, we use the quasi-
cubic band model [7], which properly describes the
effects of spin splitting, anisotropy and nonparabolicity
of bands energy. In the eight-band effective mass
approximation, the electronic wavefunction can be
chosen in the following form:
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
+−+
+++
=⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
↓
↑
=
−+
+−
RRZS
RRZS
N 7542
86311
Ψ
Ψ
ϕϕϕϕ
ϕϕϕϕ
Ψ ,
where N is the number of a unit cell in crystal;
( ) 2iYXR ±=± and the symbols ↑ and ↓ mean the
spin-up and spin-down functions, respectively.
ZYXS ,,, are the periodic Bloch amplitudes transfor-
med like atomic s- and p-functions under the operations
of the tetrahedral group at Γ point. ϕ i are envelope
functions satisfying the set of eight coupled differential
equations, which in the case of quasi-cubic approxima-
tion can be represented in the following matrix form [8]:
.0
000000ˆ
00000ˆ0
00000ˆ
0000ˆ0
0000ˆ0
00000ˆ̂ˆ
0ˆ0ˆˆ00
ˆ0ˆ00ˆ0
8
7
6
5
4
3
2
1
31
31
12
12
1
1
=
⎟
⎟
⎟
⎟
⎟
⎟
⎟
⎟
⎟
⎟
⎟
⎠
⎞
⎜
⎜
⎜
⎜
⎜
⎜
⎜
⎜
⎜
⎜
⎜
⎝
⎛
⎟⎟
⎟
⎟
⎟
⎟
⎟
⎟
⎟
⎟
⎟
⎟
⎠
⎞
⎜⎜
⎜
⎜
⎜
⎜
⎜
⎜
⎜
⎜
⎜
⎜
⎝
⎛
−−
−−
∆−
∆
∆+−
∆+−
−+
+
−
+
+
−
−+
+−
ϕ
ϕ
ϕ
ϕ
ϕ
ϕ
ϕ
ϕ
ε
ε
ε
ε
εξ
εξ
ξε
ξε
kiP
kiP
kiP
kiP
kiP
kPi
kiPkiPkiP
kiPkiPkiP
z
z
z
z
(3)
Semiconductor Physics, Quantum Electronics & Optoelectronics. 2005. V. 8, N 2. P. 22-27.
© 2005, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
24
,0cosˆ)ˆˆ(sin)ˆˆ(2
sin)ˆˆ(2
cosˆ)ˆˆ(
2
1
2
||2
2
22
123
232
||2
2
22
1
=⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
⎟
⎟
⎟
⎟
⎠
⎞
⎜
⎜
⎜
⎜
⎝
⎛
−−+−++
+−−+−+−
⊥
⊥
ϕ
ϕ
θ
γθ
θθ
γ
l
f
kfkkf
l
fkikif
l
fkikif
l
f
kfkkf
zyxyx
yxzyx
(5)
Here εεε −= g1 , )3(2 ∆++−= δεε , =3ε
)32( ∆+−= ε , 321 ∆=∆ ; )ˆˆ)(21(ˆ
yx kikk ±=± ,
where zyx kkk ˆ,ˆ,ˆ are the components of the operator
A)(ˆ heik +∇−= , A is the vector-potential of field.
Hence the model along with the optical energy gap gε
needs to know the values of spin-orbit splitting of the
valence band ∆ and interband momentum matrix
element P, crystal field splitting the valence band at Γ
point δ , and ξ is the parameter defined by
”interaction” between S and Z states via the asymmetric
part of the crystal potential [8, 9].
Let us introduce the axis transformation
zxx θ−θ=′ sincos , yy =′ , zxz θθ cossin +=′ , and
choose the “Landau” gauge in the form
).0,0,( yB ′−=′A In this case, we have the next
commutation relations
[ ] ,cosˆ,ˆ
2 θ
l
ikk yx −= [ ] ,sinˆ,ˆ
2 θ
l
ikk zy −= [ ] 0ˆ,ˆ =zx kk , (4)
where eBl h= is the cyclotron radius. Let us express
83 ϕϕ K in terms of 1ϕ , 2ϕ and eliminate them by
substituting in the first two equations of the set (3).
Then using (4) we obtain
where
[ ])32()()()( ∆++∆+−= εδεεεεεεγ g ,
3)(2
|| ∆+= δεPf , 32 ∆=⊥ εPf ,
[ ])3()32()( 2
1 ∆++∆+= εδεεε Pf ,
)32()( 2
2 ∆+= εεε Pf , 3)(3 ∆= ξεε Pf .
From (5) it follows that 0,, kba may be expressed in
terms of polynomials ,γ if as
2
1
2
31
2 )( fffa += γ , 21
2
31
2 )( ffffb += γ , 130 ffk = .
It is convenient to perform a unitary transformation
1ˆ −UUH ,
⎟⎟
⎟
⎟
⎠
⎞
⎜⎜
⎜
⎜
⎝
⎛
−
=
2
cos
2
sin
2
sin
2
cos
ββ
ββ
U ,tan ||ff⊥=β
in order to make a new two-component quasi-
Hamiltonian to be a diagonal one up to the terms
proportional to 3f . Expand now the new components
1
~ϕ , 2
~ϕ into an infinite series of linear harmonic
oscillatory functions:
∑′′+′′=
n
nnzx azkxki φϕ ))(exp(~
1 ,
∑′′+′′=
n
nnzx bzkxki φϕ ))(exp(2 ,
and introduce the creation and annihilation operators
according to [10]:
)ˆˆ)(ˆ(
4
)ˆ(ˆ
4
†
zyx kCkBiikA
AB
laa ′+′−+′= , [ ] 1ˆ,ˆ † =aa ,
where
θθ 2
2
2
1 sincos ffA += , 1fB = ,
θθ
θθ
2
2
2
1
21
sincos
cossin)(
ff
ffC
+
−
= .
It is possible to express the components of the
wavevector operator via these operators as follows:
zx k
A
Caa
Al
ABk ′⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
−++= ˆcossin)ˆˆ(
2
cos4ˆ †
4
θθθ ,
)ˆˆ(
2
4ˆ †
4
aa
Bl
ABik y −−= ,
zz k
A
Caa
Al
ABk ′⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
+++−= ˆsincos)ˆˆ(
2
sin4ˆ †
4
θθθ .
Taking into account these remarks, it is easy to show
that the system (5) is reduced to the infinite system of
algebraical equations, a fragment of which has the form:
[ ]
[ ]
0
L0
LL
0L
1
1
1
1
1
T
1
1
T
1
=
⎟
⎟
⎟
⎟
⎟
⎟
⎟
⎟
⎠
⎞
⎜
⎜
⎜
⎜
⎜
⎜
⎜
⎜
⎝
⎛
⎟⎟
⎟
⎟
⎠
⎞
⎜⎜
⎜
⎜
⎝
⎛
Λ
−Λ
−Λ
+
+
−
−
++
+
−
n
n
n
n
n
n
nn
nnn
nn
b
a
b
a
b
a
i
ii
i
, (6)
where
⎟
⎟
⎟
⎟
⎠
⎞
⎜
⎜
⎜
⎜
⎝
⎛
−⎟
⎠
⎞
⎜
⎝
⎛ +−
+⎟
⎠
⎞
⎜
⎝
⎛ +−
=Λ
22
110
0
22
11
0
0
0
0
ν
ν
n
BT
n
BT
n ,
3
2 2 3 4
0 1 1 2 1
|| 1 0
|| 1 0
2L
(4 ( cos sin ))
sin ( )cos
( )cos sin
n
f l n
f f f f
f f f
f f f
ν θ θ
θ ν θ
ν θ θ
⊥
⊥
= ×
+
+⎛ ⎞
×⎜ ⎟− −⎝ ⎠
,
Semiconductor Physics, Quantum Electronics & Optoelectronics. 2005. V. 8, N 2. P. 22-27.
© 2005, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
25
F
fff
ff
F
εε
θθ
θθ
εν
=
⎟
⎟
⎟
⎠
⎞
⎜
⎜
⎜
⎝
⎛
+
+
= ⊥
)sincos(
)sincos(
)( 2
2
2
11
2222
||
0 is the value of
ν -factor in the limit of strong magnetic fields (in (6) we
put 0=′zk , because it corresponds to the extreme FS
cross-section).
It is possible to derive the analytical expression for
finding the energy levels from (6) only in the case when
0=ξθ . With 0=ξ ( 03 =f ), we have the result first
obtained in [11]. If 0=θ one gets
2
0
2
0
2
0 )1(8
2
11 ν+++= knlnBT , (7a)
2
0
2
0
2
0 )1()1(8
2
111 ν+++−+= klnnBT (7b)
(while obtaining the expressions (7), we performed
1+→ nn shift for the system of “–” levels, because
this numeration is more preferable from the physical
point of view).
Using the formulae (7) and applying the method for
ν -factor calculation suggested in [11] for this direction
of the magnetic field, one obtains
F
BTrBF
εε
νεν
=⎥⎦
⎤
⎢⎣
⎡ −++= 1)1()/4(),( 2
0
2
0 (8)
The dependence of electron ν -factor on the
magnetic field can be explained in this case by
complicated nature of the spin-orbit interaction and as a
result of it, the spin splitting of the Landau levels is no
longer linear dependent on the magnetic field. When
1>>i (i.e., for relatively weak fields), from (8) it
follows that rTBB ii 40
11
1 =− −−
+ . Hence, the nodes with
large numbers are periodically located along the scale
1−B with the period (2). As the magnetic field
increases, the location of the nodes turns to be irregular
(the Zeeman subbands splitting reveals itself). If we
take into account the fact that with the lowering the
node number the difference between the magnetic field
values corresponding to two neighbour nodes grows
rapidly, it becomes obvious that only some first nodes
can be observed in the fields where quantum
oscillations start to manifest themselves.
Let us perform the specific calculations for Cd3As2
and Cd3–xZnxAs2. For this purpose, we use the rank
values of their band parameters suggested in [8]. Since
the value of the parameter ξ is known only by its order
[8], all the calculations are made for several values of
this parameter.
Fig. 2 shows the concentration dependences of the
first nodes’ location calculated for three different values
of the parameter ξ . The characteristic feature of all
these curves is the increase (which is more or less
smooth depending on ξ value) of the field value
corresponding to the node when increasing the electron
concentration.
Let us now examine the behaviour of ν -factor in the
tilted magnetic field. The difficulty of this task is caused
by the fact that it is impossible to apply the standard
perturbation theory to the parameter 3f because the
combination nf3 being a part of (4.7) is not small
[12]. Besides, the additional difficulties in comparison
with 2D systems are connected with the fact that only
the magnetic field component normal to the 2D layer
effects on the orbital moment, while in the bulk crystal
such an interaction is determined as a whole by the
field.
For defining the energy spectrum when 0, ≠ξθ , let
us use the technique of folding the determinants of the
block matrixes. Applying the well-known Schur
formula it is possible to show that
,0)L)(LL)(Ldet( 1II
1
TT
1
1I
11 =Λ−Λ−Λ −
−+
−
++ nnn nnnn
where the matrixes ,I
1+Λ n and II
1−Λn are determined
using the following recurrent relations:
,L)(L T
2
1I
221
I
1 +
−
++++ Λ−Λ=Λ nnnnn
.L)(L 1
1II
2
T
11
II
1 −
−
−−−− Λ−Λ=Λ nnnnn
Fig. 2. Theoretical dependence of the first (a), second (b) and third (c) node location on the electron concentration for Cd3As2:
meV30=ξ (1); meV15=ξ (2); meV7=ξ (3).
Semiconductor Physics, Quantum Electronics & Optoelectronics. 2005. V. 8, N 2. P. 22-27.
© 2005, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
26
Fig. 3. Theoretical dependence of the maximal magnetic field corresponding to the nodes of oscillation on the azimuth angle for
Cd3-xZnxAs2: eV1.0−=gε (a-c), eV1.0=gε (d-f); 324 m103 −×=en (a),(d); 324 m10 −=en (b), (e); 323 m104 −×=en (c), (f);
meV30=ξ (1), meV15=ξ (2), meV7=ξ (3).
While performing the specific numerical calculations,
this algorithm is realized through the truncating the
infinite matrix on the both sides of the fixed block
0nΛ .
In our calculations, the convergence for the eigenvalues
was attained for the truncated matrix of 20×20 size.
Fig. 3 shows the results of numerical calculations for
the dependence of the maximal magnetic field
corresponding to the nodes on the azimuth angle. The
following features are clearly seen:
1. With the increasing θ the magnetic field
decreases monotonously.
2. Beating disappears in the case of large values θ ;
the critical angle ( o70~ ) at which the beating starts to
disappear has a weak dependence on the band
parameters and the carrier concentration.
3. With the decreasing ξ and rising gε , the values
of the magnetic field reduce and locate within the
narrower range.
4. Conclusions
The investigations carried out in this work show that the
beating patterns in crystals with υnC symmetry have
their own specific character. Particularly, for V
2
II
3 BA
compounds one should expect the decrease of the
maximal magnetic field corresponding to the nodes of
oscillation when the electron concentration decreases
and the field tilting about the symmetry axis increases.
Moreover, starting from a certain azimuth angle the
beatings should disappear. The mentioned peculiarities
are supposed to help in specifying the value of the
parameter ξ which defines the magnitude of zero spin
splitting of bands when studying the beating effect in
V
2
II
3 BA semiconductors (in which it has already been
observed experimentally [13]).
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