[Paper Review] Spin-galvanic effect due to optical spin orientation

Sergey Ganichev, Petra Schneider+14 more|arXiv (Cornell University)|Mar 11, 2003

Magneto-Optical Properties and Applications3 citations

TL;DR

This paper demonstrates the spin-galvanic effect in n-type GaAs quantum wells under circularly polarized infrared excitation without an external magnetic field, using resonant inter-subband transitions via a free-electron laser. The key result is the observation of a helicity-dependent photocurrent along the [110] direction, driven by optical spin orientation and the interplay of Rashba and Dresselhaus spin-orbit coupling, with a microscopic theory confirming the experimental findings.

ABSTRACT

Under oblique incidence of circularly polarized infrared radiation the spin-galvanic effect has been unambiguously observed in (001)-grown $n$-type GaAs quantum well (QW) structures in the absence of any external magnetic field. Resonant inter-subband transitions have been obtained making use of the tunability of the free-electron laser FELIX. It is shown that a helicity dependent photocurrent along one of the $<110>$ axes is predominantly contributed by the spin-galvanic effect while that along the perpendicular in-plane axis is mainly due to the circular photogalvanic effect. This strong non-equivalence of the [110] and [1$\bar{1}$0] directions is determined by the interplay between bulk and structural inversion asymmetries. A microscopic theory of the spin-galvanic effect for direct inter-subband optical transitions has been developed being in good agreement with experimental findings.

Motivation & Objective

• To demonstrate the spin-galvanic effect in zero magnetic field using optical excitation.
• To investigate the role of spin-orbit coupling (Rashba and Dresselhaus) in generating spin-polarized currents.
• To distinguish the spin-galvanic effect from the circular photogalvanic effect in terms of symmetry and microscopic origin.
• To develop and validate a microscopic theory for the spin-galvanic effect in direct inter-subband transitions.

Proposed method

• Resonant inter-subband transitions were excited using a tunable free-electron laser (FELIX) at oblique incidence.
• Photocurrents were measured in unbiased GaAs/AlGaAs quantum wells with well widths of 7.6–8.6 nm at room temperature.
• Circularly polarized light (σ⁺ and σ⁻) was generated using a Fresnel rhomb to control helicity.
• Currents were measured along [110] and [1̄10] crystallographic directions to probe directional asymmetry.
• A microscopic theory based on spin-dependent scattering matrix elements and the Elliot-Yafet mechanism was developed.
• Theoretical current expressions were derived using the spin-orbit coupling parameters βxy and βyx, and the spin relaxation time τ′s.

Experimental results

Research questions

• RQ1 Can the spin-galvanic effect be observed in the absence of an external magnetic field using optical excitation?
• RQ2 What is the role of the interplay between bulk and structural inversion asymmetry in breaking [110] and [1̄10] direction symmetry?
• RQ3 How does the spin-galvanic effect differ microscopically from the circular photogalvanic effect despite similar phenomenological descriptions?
• RQ4 What is the dependence of the spin-galvanic current on spin polarization, absorption, and spin-orbit coupling parameters?

Key findings

• A helicity-dependent photocurrent was observed along the [110] direction, with a sign reversal at the absorption peak, indicating a dominant spin-galvanic effect.
• The current along [1̄10] followed the absorption spectrum without sign reversal, indicating a dominant circular photogalvanic effect.
• The spin-galvanic current was proportional to the spin polarization and the Rashba spin-splitting parameter βxy(1), with jSGE,y ∝ eβxy(1)/ħ × (τpτs/τ′s) × (η21I/ħω)Pcircξ.
• The spin-galvanic current decay time matched the total spin relaxation time τs, confirming its origin in spin relaxation processes.
• Theoretical modeling using the spin-dependent scattering matrix and Elliot-Yafet mechanism showed good agreement with experimental data.
• The ratio τp/τ′s was found to be independent of momentum relaxation time, indicating a fundamental link between spin relaxation and current generation.