Photogalvanic effects in topological insulators

Abstract

We discuss optical absorption in topological insulators and study possible photoelectric effects theoretically. We found that absorption of circularly polarized electromagnetic waves in twodimensional topological insulators results in electric current in the conducting 1D edge channels, the direction of the current being determined by the light polarization. We suggest two ways of inducing such a current: due to magnetic dipole electron transitions stimulated by irradiation of frequency below the bulk energy gap, and due to electric dipole transitions in the bulk at frequencies larger than the energy gap with subsequent capture of the photogenerated carriers on conducting edge states. PACS numbers: 73.25.+i,72.25.Fe, 72.40.+w The rich new physics and unusual properties of topological insulators (TI) attract much attention in recent years, for review see [1][2][3]. The spectrum of TI in the bulk has a finite energy gap E G , while there are topologically protected conductive surface states inside the gap. The states with somewhat similar properties were predicted also long ago at the surface of a conventional narrow-gap semiconductor [4] and at the interface of heterojunctions made of two insulators with inverted energy bands [5] without appealing to topological reasons. The chiral states studied in recent works on TI [1-3] and the chiral states considered in old papers [4,5] were derived using different theoretical models and the states have different spin structure. Derivation of the surface state of TI was essentially based on quadratic dispersion relation of the mass term in a Dirac-type Hamiltonian which is valid for materials like Bi 2 Se 3 [2, 3] or HgTe/CdTe semiconductor quantum wells [1,6], and the wave functions of the surface states are formed from the states of bulk bands with the same spin directions. Therefore, the electron surface states in TI can be characterized by spin oriented perpendicular to the momentum. While in Refs. [4,5] the mass dispersion is not needed to derive the surface states, and the surface states are formed from the bulk band states of the opposite spin directions.Experimental study of transport properties related to the surface states is typically complicated by residual electrons and holes in the bulk. In this respect it would be interesting to study the possibility to induce the current in the surface state by absorption of circularly polarized light. As the left/right circular polarization of the light is associated with the angular momentum projection ±1, the absorption of polarized light can vary the number of electrons with definite spin. Since the directions of the spin and momentum of electrons in the chiral states are related, variation of the spin distribution may affect the symmetry of the momentum distribution, and, hence, excite the current. As these effects are related only to the chiral surface state, the bulk contribution to the current should be absent.Below we set e,h and k B to unity, restoring dimensional units in final expressions when necessary.Semicon...

Optical properties of helical edge channels in zinc-blende-type topological insulators: selection rules, circular and linear dichroism, circular and linear photocurrents

“…The photocurrent is proportional to the degree of circular polarization and is reversed by switching the sign of the photon helicity. We note, that the magneto-dipole transitions also give a contribution to the circular photocurrent 17,18 . However, this contribution is a few orders of magnitude smaller than the contribution of the electro-dipole transitions.…”

“…where D 1 = eu 1 /|k y ω s −s | and D 2 = eu 2 /|k y ω s −s | are independent of k y . Equations (18) are written for the edge of arbitrary orientation. At θ = πn/2, they correspond to Eq.…”

Interplay between collective modes in hybrid electron-gas–superconductor structures

“…To find the dispersion law of the AV mode, we have to, first, find the conductivity σ s of single-particle excitations above the SC gap. In their original work [10], Artemenko and Volkov used the quasi-classical approach based on the kinetic equations. Later, the validity of their results was confirmed by the quantum field theory methods [20].…”

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Photogalvanic effect in the HgTe/CdTe topological insulator due to edge-bulk optical transitions

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