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This article is part of the series International Conference on Superlattices, Nanostructures, and Nanodevices (ICSNN 2012).

Open Access Nano Express

Inelastic light scattering by 2D electron system with SO interaction

Alexander V Chaplik12, Lev I Magarill12* and Ritta Z Vitlina1

Author Affiliations

1 Institute of Semiconductor Physics, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, 630090, Russia

2 Novosibirsk State University, Novosibirsk, 630090, Russia

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Nanoscale Research Letters 2012, 7:537  doi:10.1186/1556-276X-7-537

The electronic version of this article is the complete one and can be found online at: http://www.nanoscalereslett.com/content/7/1/537


Received:16 July 2012
Accepted:2 September 2012
Published:28 September 2012

© 2012 Chaplik et al.; licensee Springer.

This is an Open Access article distributed under the terms of the Creative Commons Attribution License ( http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Inelastic light scattering by electrons of a two-dimensional system taking into account the Rashba spin-orbit interaction (SOI) in the conduction band is theoretically investigated. The case of resonance scattering (frequencies of incident and scattered light are close to the effective distance between conduction and spin-split-off bands of the AIIIBV-type semiconductor) is considered. As opposed to the case of SOI absence, the plasmon peak in the scattering occurs even at strictly perpendicular polarizations of the incident and scattered waves. Under definite geometry, one can observe the spectrum features conditioned by only single-particle transitions. In the general case of elliptically polarized incident and scattered light, the amplitude of the plasmon peak turns out to be sensitive to the sign of the SOI coupling.

Keywords:
Two-dimensional system; Inelastic light scattering; Spin-orbit interaction; Rashba model

Background

It is well known that the spectrum of light scattering by a two-dimensional (2D) electron system is characterized by two contributions. One of them is determined by charge density excitations which is commonly called screened scattering. The shift of frequency equals the 2D plasmon frequency. The maximum of intensity of the corresponding peak in light scattering is reached when polarizations are parallel, and it is equal to 0 when polarizations are perpendicular.

The other contribution corresponds to single-particle excitations (SPE). The typical frequency shift is of the order qvF, where q is the wave vector transfer and vF is the Fermi velocity. The intensity of this peak is maximal for perpendicular polarizations of the incident and scattered waves. As to polarized scattering, the SPE contribution strongly depends on the resonance parameter (see [1]): if the incident frequency is close to the effective bandgap (including the Moss-Burstein shift), the SPE peak can be comparable with the plasmon one.

The SOI substantially changes the spectrum of inelastic light scattering. A new peak (of a nontrivial shape) appears with the frequency shift equal to the spin splitting at the Fermi momentum. The polarization dependences are changed qualitatively. The plasmon peak can occur even at crossed polarizations. Finally, the left to right symmetry of circularly polarized incident light is violated: the cross section is invariant under simultaneous change of signs of polarizations and the SOI constant. This allows, in principle, to determine the sign of the Rashba constant experimentally.

Methods

Expressions for the scattering cross section

In the random-phase approximation, the differential cross section for the scattering by a 2D system can be written as follows [2-4]:

<a onClick="popup('http://www.nanoscalereslett.com/content/7/1/537/mathml/M1','MathML',630,470);return false;" target="_blank" href="http://www.nanoscalereslett.com/content/7/1/537/mathml/M1">View MathML</a>

(1)

where L2,L1, and <a onClick="popup('http://www.nanoscalereslett.com/content/7/1/537/mathml/M2','MathML',630,470);return false;" target="_blank" href="http://www.nanoscalereslett.com/content/7/1/537/mathml/M2">View MathML</a> are respectively given by the expressions

<a onClick="popup('http://www.nanoscalereslett.com/content/7/1/537/mathml/M3','MathML',630,470);return false;" target="_blank" href="http://www.nanoscalereslett.com/content/7/1/537/mathml/M3">View MathML</a>

(2)

Here, ω1,2 are the incident and scattered light frequencies, respectively; q = q1 q2, q1,2 are the in-plane components of the incident and scattered light wave vectors, respectively; ω = ω1ω2 is the frequency shift in the inelastic light scattering; nω = 1/(eω/T−1) is the Bose distribution function; J(q) = eiqr, β is the set of quantum numbers characterizing an electron state in the conduction band; S is the normalization area; κ is the background dielectric constant; <a onClick="popup('http://www.nanoscalereslett.com/content/7/1/537/mathml/M4','MathML',630,470);return false;" target="_blank" href="http://www.nanoscalereslett.com/content/7/1/537/mathml/M4">View MathML</a> is the scattering operator; and = 1 is assumed throughout this paper. The longitudinal dielectric function of electrons in the conduction band ε has the form

<a onClick="popup('http://www.nanoscalereslett.com/content/7/1/537/mathml/M5','MathML',630,470);return false;" target="_blank" href="http://www.nanoscalereslett.com/content/7/1/537/mathml/M5">View MathML</a>

(3)

<a onClick="popup('http://www.nanoscalereslett.com/content/7/1/537/mathml/M6','MathML',630,470);return false;" target="_blank" href="http://www.nanoscalereslett.com/content/7/1/537/mathml/M6">View MathML</a>

(4)

where fβ f(εβ), f(ε) is the Fermi distribution function, εβ is the energy of an electron in the conduction band, and <a onClick="popup('http://www.nanoscalereslett.com/content/7/1/537/mathml/M7','MathML',630,470);return false;" target="_blank" href="http://www.nanoscalereslett.com/content/7/1/537/mathml/M7">View MathML</a>.

The resonant situation is considered when the frequencies of incident (scattered) wave ω1(ω2) are close to E0 + Δ0, i.e., resonance with the spin-orbit split-off band takes place (E0 and Δ0 are the band parameters of the bulk AIIIBV semiconductor). In this case, the operator of scattering <a onClick="popup('http://www.nanoscalereslett.com/content/7/1/537/mathml/M8','MathML',630,470);return false;" target="_blank" href="http://www.nanoscalereslett.com/content/7/1/537/mathml/M8">View MathML</a> reads

<a onClick="popup('http://www.nanoscalereslett.com/content/7/1/537/mathml/M9','MathML',630,470);return false;" target="_blank" href="http://www.nanoscalereslett.com/content/7/1/537/mathml/M9">View MathML</a>

(5)

where Eg is the effective bandgap width, <a onClick="popup('http://www.nanoscalereslett.com/content/7/1/537/mathml/M10','MathML',630,470);return false;" target="_blank" href="http://www.nanoscalereslett.com/content/7/1/537/mathml/M10">View MathML</a>, P pcv/m0 is the Kane parameter, e1,2 are the polarizations of incident and scattered photons, and σ are the Pauli matrices. We treat here the enhanced resonant factor A in Equation 5 just as a constant that is true for not extremely resonant regime: the denominator in Equation 5 is much larger than the Fermi energy of electrons. We do this in order to simplify calculations because our main goal in this paper is to demonstrate the qualitatively new features of the scattering process due to spin-orbit interaction.

The substitution of Equation 5 into Equation 1 yields an expression comprising four characteristic contributions to the scattering:

<a onClick="popup('http://www.nanoscalereslett.com/content/7/1/537/mathml/M11','MathML',630,470);return false;" target="_blank" href="http://www.nanoscalereslett.com/content/7/1/537/mathml/M11">View MathML</a>

(6)

where

<a onClick="popup('http://www.nanoscalereslett.com/content/7/1/537/mathml/M12','MathML',630,470);return false;" target="_blank" href="http://www.nanoscalereslett.com/content/7/1/537/mathml/M12">View MathML</a>

(7)

<a onClick="popup('http://www.nanoscalereslett.com/content/7/1/537/mathml/M13','MathML',630,470);return false;" target="_blank" href="http://www.nanoscalereslett.com/content/7/1/537/mathml/M13">View MathML</a>

(8)

<a onClick="popup('http://www.nanoscalereslett.com/content/7/1/537/mathml/M14','MathML',630,470);return false;" target="_blank" href="http://www.nanoscalereslett.com/content/7/1/537/mathml/M14">View MathML</a>

(9)

<a onClick="popup('http://www.nanoscalereslett.com/content/7/1/537/mathml/M15','MathML',630,470);return false;" target="_blank" href="http://www.nanoscalereslett.com/content/7/1/537/mathml/M15">View MathML</a>

(10)

The values of Z and <a onClick="popup('http://www.nanoscalereslett.com/content/7/1/537/mathml/M16','MathML',630,470);return false;" target="_blank" href="http://www.nanoscalereslett.com/content/7/1/537/mathml/M16">View MathML</a> are given by expressions for L1 and <a onClick="popup('http://www.nanoscalereslett.com/content/7/1/537/mathml/M17','MathML',630,470);return false;" target="_blank" href="http://www.nanoscalereslett.com/content/7/1/537/mathml/M17">View MathML</a> in Equation 2 with γ replaced by γ2.

The contribution R1 determines the scattering of light by fluctuations of charge density. The value R2 determines unscreened mechanism of scattering and corresponds to single-particle excitations. It can be shown that in the absence of SOI in the conduction band, the values of Z and <a onClick="popup('http://www.nanoscalereslett.com/content/7/1/537/mathml/M18','MathML',630,470);return false;" target="_blank" href="http://www.nanoscalereslett.com/content/7/1/537/mathml/M18">View MathML</a> and, respectively, R3 and R4 are equal to 0 identically.

Equations 7 to 10 are general. They are valid for any Hamiltonian, describing electron states in the conduction band. In this paper, we consider the light scattering for the so-called Rashba plane, namely 2D electron gas in the presence of SOI. Such a system is described by the Hamiltonian [5]

<a onClick="popup('http://www.nanoscalereslett.com/content/7/1/537/mathml/M19','MathML',630,470);return false;" target="_blank" href="http://www.nanoscalereslett.com/content/7/1/537/mathml/M19">View MathML</a>

(11)

Here, p is the 2D momentum of the electron, m is the effective mass, α is the Rashba parameter, and nis the unit vector normal to the plane of the system. The spectrum of this Hamiltonian has the form

<a onClick="popup('http://www.nanoscalereslett.com/content/7/1/537/mathml/M20','MathML',630,470);return false;" target="_blank" href="http://www.nanoscalereslett.com/content/7/1/537/mathml/M20">View MathML</a>

(12)

where β = (p,μ) and the parameter μ = ± 1 labels two branches of the spin-split spectrum. The wave functions of the Hamiltonian (Equation 11) are

<a onClick="popup('http://www.nanoscalereslett.com/content/7/1/537/mathml/M21','MathML',630,470);return false;" target="_blank" href="http://www.nanoscalereslett.com/content/7/1/537/mathml/M21">View MathML</a>

(13)

Results and discussion

Numerical calculations

Equations 7 to 10, 12, and 13 were used for numerical calculations of scattering cross section as a function of frequency shift ω. They were carried out for 2D electron gas at temperature T = 0 in the scattering geometry when incident and scattered beams make a right angle and lie in the same plane (Figure 1). The structure InAs/GaSb with α = 1.44 × 106 cm/s, m = 0.055 m0, and κ = 15.69 was considered at the areal concentration ns= 1011 cm−2. The contributions R1,R3, and R4 contain plasmon poles (zeros of ε). To get finite results, it is necessary to introduce a finite damping. We replace δ in Equation 4 by the relaxation frequency ν = e/ (μ is the mobility). For q and ν, we have chosen the following values: ℏq = 0.004 pF, ℏν = 0.001 εF.

thumbnailFigure 1. Scattering configuration for which numerical calculations are performed. The incident (Q1) and scattered (Q2) beams form a right angle; θ is the angle of incidence.

If the polarizations of incident and scattered waves are strictly parallel (e1||e2), the cross section is determined by only R1. The spectrum of scattering has two peaks: the plasmon peak <a onClick="popup('http://www.nanoscalereslett.com/content/7/1/537/mathml/M22','MathML',630,470);return false;" target="_blank" href="http://www.nanoscalereslett.com/content/7/1/537/mathml/M22">View MathML</a> and the SOI-induced peak at 2αpF. The dependence of the cross section on the frequency shift for both peaks is similar to the plasmon absorption and is given by Figure 2.

thumbnailFigure 2. Scattering cross section versus frequency shift fore1e2 .α = 0 (1, red line); α = 1.44 × 106 cm/s (2, blue line).

Let the geometry of scattering in such a way that incident and scattered waves are linearly polarized and, moreover, e1 e2 and ay = 0. It can be realized, e.g., if incident and scattered beams are perpendicular and one of them is polarized in the incidence plane but the other is perpendicular to it. In this case, the spectrum demonstrates peculiarities due to only single-particle transitions (contribution R2): one peak near the frequency qvF and another peak near the frequency 2αpF. This case is demonstrated by Figure 3.

thumbnailFigure 3. Scattering cross section versus frequency shift for α = 0 (1, red) and α = 1.44 ×106cm/s (2, blue). The angle of incidence is θ = Π/6.

Due to SOI, the plasmon peak in the light scattering spectrum can occur even at strictly perpendicular polarizations of incident and scattered waves. It occurs when vector a has a nonzero projection onto axis y (axes z and x were chosen along vectors n and q, respectively). The nonvanishing contribution to the cross section is due to the sum R2 + R3. This case is presented by Figure 4.

thumbnailFigure 4. Scattering cross section versus frequency shift for α = 0(1, red) and α = 1.44 ×106cm/s (2, blue).

For the existence of the contribution R4, polarization vectors e1 and e2 should be arbitrarily oriented with respect to each other (neither parallel nor perpendicular). Besides, at least one of the waves should not be linearly polarized. When these conditions are justified and R4 ≠ 0, all other contributions (R1,R2,R3) also exist. Herewith, due to the sensitivity of the contribution R4 to the sign of the effective Rashba SOI αand to polarization vector phases, the total cross section of scattering also depends on these parameters. Therefore, measurements of inelastic light scattering can be, in principle, used for the determination of the sign of the constant α. Figure 5 shows an example of the inelastic light scattering spectrum in the most interesting case when the incident wave has right or left circular polarization while the scattered one is linearly polarized at the angle Π/4 to the incidence plane. It is seen that at α > 0, the amplitude of the plasmon peak for left polarization is distinctly larger than that for right polarization. For α < 0, the curves should be interchanged.

thumbnailFigure 5. Dependence of scattering cross section on frequency when one of the waves has circular polarization.α = 0 (1, red), α = 1.44 × 106cm/s (2, blue and 3, green); incident wave with left polarization (2, blue), incident wave with right polarization (3, green).

Conclusions

Thus, allowing SOI essentially (qualitatively) changes the spectrum of inelastic light scattering by a 2D electron system. It should be especially noted that in the absence of external magnetic field, the symmetry between left and right polarizations is violated.

Competing interests

The authors declare that they have no competing interests.

Authors’ contributions

AVC, LIM, and RZV equally contributed in writing the manuscript and in performing the theoretical analysis. All authors read and approved the final manuscript.

Acknowledgements

This research has been supported in part by RFBR grant nos. 11-02-00730 and 11-02-12142.

References

  1. Das Sarma S, Wang DW: Resonant Raman scattering by elementary electronic excitations in semiconductor structures.

    PRL 1999, 83:816. Publisher Full Text OpenURL

  2. Blum FA: Inelastic light scattering from semiconductor plasmas in a magnetic field.

    Phys Rev B 1970, 1:1125. Publisher Full Text OpenURL

  3. Vasko FT, Raichev OE: Quantum Kinetic Theory and Applications: Electrons, Photons, Phonons. Springer, New York; 2005. OpenURL

  4. Ivchenko EL: Optical Spectroscopy of Semiconductor Nanostructures. Alpha Science, Harrow; 2005. OpenURL

  5. Bychkov YuA, Rashba EI: Properties of a 2D electron gas with lifted spectrum degeneracy.

    JETP Lett 1984, 39:78.

    [Pis’ma v ZhETF 1984, 39:66]

    OpenURL