We report on the observation of photogalvanic effects in epitaxially grown Sb2Te3 and Bi2Te3 three-dimensional (3D) topological insulators (TI). We show that asymmetric scattering of Dirac fermions driven back and forth by the terahertz electric field results in a dc electric current. Because of the "symmetry filtration" the dc current is generated by the surface electrons only and provides an optoelectronic access to probe the electron transport in TI, surface domains orientation, and details of electron scattering in 3D TI even at room temperature.

Spin-orbit coupling provides a versatile tool to generate and to manipulate the spin degree of freedom in low-dimensional semiconductor structures. The spin Hall effect, where an electrical current drives a transverse spin current and causes a nonequilibrium spin accumulation observed near the sample boundary, the spin-galvanic effect, where a nonequilibrium spin polarization drives an electric current, or the reverse process, in which an electrical current generates a nonequilibrium spin polarization, are all consequences of spin-orbit coupling. In order to observe a spin Hall effect a bias driven current is an essential prerequisite. The spin separation is caused via spin-orbit coupling either by Mott scattering (extrinsic spin Hall effect) or by Rashba or Dresselhaus spin splitting of the band structure (intrinsic spin Hall effect). Here we provide evidence for an elementary effect causing spin separation which is fundamentally different from that of the spin Hall effect. In contrast to the spin Hall effect it does not require an electric current to flow: It is spin separation achieved by spin-dependent scattering of electrons in media with suitable symmetry. We show that by free carrier (Drude) absorption of terahertz radiation spin separation is achieved in a wide range of temperatures from liquid helium up to room temperature. Moreover the experimental results give evidence that simple electron gas heating by any means is already sufficient to yield spin separation due to spin-dependent energy relaxation processes of nonequilibrium carriers.Comment: 19 pages, 4 figures, 1 tabl

We demonstrate that the terahertz/infrared radiation induced photogalvanic effect, which is sensitive to the surface symmetry and scattering details, can be applied to study the high frequency conductivity of the surface states in (Bi1−xSbx)2Te3 based three dimensional (3D) topological insulators (TI). In particular, measuring the polarization dependence of the photogalvanic current and scanning with a micrometre sized beam spot across the sample, provides access to (i) topographical inhomogeneity's in the electronic properties of the surface states and (ii) the local domain orientation. An important advantage of the proposed method is that it can be applied to study TIs at room temperature and even in materials with a high electron density of bulk carriers.Electronic, optical and opto-electronic properties of topological insulators (TI) have attracted continuously growing attention yielding challenging fundamental concepts and being of potential interest for novel applications in the fields of spintronics and opto-electronics 1-5 . Hence, the fabrication of high quality topological insulators and their characterization yielding feedback to technologists is of particular importance. Until now a large variety of materials was proposed and confirmed to host topological protected surface states in three-dimensional (3D) TI and edge channels in two-dimensional (2D) TI. Particular examples are (Bi 1−x Sb x ) 2 Te 3 based 3D TI. Their fabrication in view of good insulating properties of the bulk at room temperature, homogeneity of a large area growth materials is still a challenging task. The former problem is caused by the high density of residual impurities serving parallel channels to the surface transport 6-8 . A promising way to overcome this problem serves the recent progress in growth of 3D TI applying molecular-beam-epitaxy (MBE) technique, see e.g., [9][10][11][12]. Owing to the progress in material growth, low temperature electron transport and magneto-transport studies becomes possible providing information on average electronic properties of Dirac fermions and carrier scattering mechanisms in 3D TIs 13-21 as well as to observe the quantum anomalous Hall effect reported for Cr or V doped (Bi 1−x Sb x ) 2 Te 3 based 3D TI 22-25 . An important issue for improvement of the material properties is their characterization allowing insights in the material properties and providing a feedback for technologists. For that a palette of methods has been developed and widely used. An insight into the band structure of the surface states of 3D TIs, especially proof for the single Dirac cone, is obtained by varios modifications of the angle resolved photoemission spectroscopy (ARPES) 26-32 , including spin-resolved and time-resolved ARPES, as well * permanent address: Ioffe Physical-Technical Institute, St. Petersburg, Russia † permanent address: Saint-Petersburg State Polytechnic University, St. Petersburg, Russia as by time-resolved two-photon photoelectron (2PPE) spectroscopy [33][34][35] , with which an enlightening pi...