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Register a new userRoom temperature high frequency transport of Dirac fermions in epitaxially grown Sb_2Te_3 based topological insulators
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We report on the observation of photogalvanic effects in epitaxially grown Sb_2Te_3 three-dimensional (3D) topological insulators (TI). We show that asymmetric scattering of Dirac electrons driven back and forth by the terahertz electric field results in a dc electric current. Due to the symmetry filtration the dc current is generated in the surface electrons only and provides an opto-electronic access to probe the electric transport in TI, surface domains orientation and details of electron scattering even in 3D TI at room temperature where conventional surface electron transport is usually hindered by the high carrier density in the bulk.
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Topological insulators are new states of quantum matter with surface states protected by the time-reversal symmetry. In this work, we perform first-principle electronic structure calculations for $Sb_2Te_3$, $Sb_2Se_3$, $Bi_2Te_3$ and $Bi_2Se_3$ crystals. Our calculations predict that $Sb_2Te_3$, $Bi_2Te_3$ and $Bi_2Se_3$ are topological insulators, while $Sb_2Se_3$ is not. In particular, $Bi_2Se_3$ has a topologically non-trivial energy gap of $0.3 eV$, suitable for room temperature applications. We present a simple and unified continuum model which captures the salient topological features of this class of materials. These topological insulators have robust surface states consisting of a single Dirac cone at the $Gamma$ point.
The transport length $l_textrm{tr}$ and the mean free path $l_textrm{e}$ are experimentally determined for bulk and surface states in a Bi$_2$Se$_3$ nanoribbon by quantum transport and transconductance measurements. We show that the anisotropic scattering of spin-helical Dirac fermions results in a strong enhancement of $l_textrm{tr}$, which confirms theoretical predictions cite{Culcer2010}. Despite strong disorder ($l_textrm{e}approx30$~nm), our result further points to the long-range nature of the scattering potential, giving a large ratio $l_textrm{tr}/l_textrm{e}approx8$ that is likely limited by a finite bulk/surface coupling. This suggests that the spin-flip length could reach the micron size in disordered 3D topological insulator nanostructures with a reduced bulk doping, even if due to charge compensation.
This study shows that a terahertz (THz) wave can be generated from the (001) surface of cleaved Bi$_{textrm{2}}$Se$_{textrm{3}}$ and Cu-doped Bi$_{textrm{2}}$Se$_{textrm{3}}$ single crystals using 800 nm femtosecond pulses. The generated THz power is strongly dependent on the carrier concentration of the crystals. An examination of the dependence reveals the two-channel free carrier absorption to which Dirac fermions are indispensable. Dirac fermions in Bi$_{textrm{2}}$Se$_{textrm{3}}$ are significantly better absorbers of THz radiation than bulk carriers at room temperature. Moreover, the characteristics of THz emission confirm the existence of a recently proposed surface phonon branch that is normalized by Dirac fermions.
Excitation of a topological insulator by a high-frequency electric field of a laser radiation leads to a dc electric current in the helical edge channel whose direction and magnitude are sensitive to the radiation polarization and depend on the physical properties of the edge. We present an overview of theoretical and experimental studies of such edge photoelectric effects in two-dimensional topological insulators based on semiconductor quantum wells. First, we give a phenomenological description of edge photocurrents, which may originate from the photogalvanic effects or the photon drag effects, for edges of all possible symmetry. Then, we discuss microscopic mechanisms of photocurrent generation for different types of optical transitions involving helical edge states. They include direct and indirect optical transitions within the edge channel and edge-to-bulk optical transitions.
The growth of single-layer MoS2 with chemical vapor deposition is an established method that can produce large-area and high quality samples. In this article, we investigate the geometrical and optical properties of hundreds of individual single-layer MoS2 crystallites grown on a highly-polished sapphire substrate. Most of the crystallites are oriented along the terraces of the sapphire substrate and have an area comprised between 10 {mu}m2 and 60 {mu}m2. Differential reflectance measurements performed on these crystallites show that the area of the MoS2 crystallites has an influence on the position and broadening of the B exciton while the orientation does not influence the A and B excitons of MoS2. These measurements demonstrate that differential reflectance measurements have the potential to be used to characterize the homogeneity of large area CVD grown samples.
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