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.
In layered semiconductors with spin-orbit interaction (SOI) a persistent spin helix (PSH) state with suppressed spin relaxation is expected if the strengths of the Rashba and Dresselhaus SOI terms, α and β, are equal. Here we demonstrate gate control and detection of the PSH in two-dimensional electron systems with strong SOI including terms cubic in momentum. We consider strain-free InGaAs/InAlAs quantum wells and first determine a ratio α/β 1 for nongated structures by measuring the spin-galvanic and circular photogalvanic effects. Upon gate tuning the Rashba SOI strength in a complementary magnetotransport experiment, we monitor the complete crossover from weak antilocalization via weak localization to weak antilocalization, where the emergence of weak localization reflects a PSH-type state. A corresponding numerical analysis reveals that such a PSH-type state indeed prevails even in presence of strong cubic SOI, however no longer at α = β. An electron moving in an electric field experiences, in its rest frame, an effective magnetic field pointing perpendicularly to its momentum. The coupling of the electron's spin to this magnetic field is known as spin-orbit interaction (SOI). The ability to control the corresponding magnetic field, and thereby spin states, all electrically in gated semiconductor heterostructures 1,2 is a major prerequisite and motivation for research towards future semiconductor spintronics. However, on the downside, the momentum changes of an electron moving through a semiconductor cause sudden changes in the magnetic field leading to spin randomization. Hence, suppression of spin relaxation in the presence of strong, tunable SOI is a major challenge of semiconductor spintronics.In III-V semiconductor heterostructures two different types of SOI exist: (i) Rashba SOI, 3 originating from structure inversion asymmetry (SIA), is linear in momentum k with a strength α that can be controlled by an electric gate.(ii) Dresselhaus SOI 4 is due to bulk inversion asymmetry (BIA), which gives rise to a band spin splitting, given by k-linear and k-cubic contributions. 5 The strength of the linear in k term β = γ k 2 z (where γ is a material parameter) can hardly be changed as it stems from crystal fields. These various spin-orbit terms in layered semiconductors are described by the Hamiltonian H SO = H R + H D with Rashba and Dresselhaus termswith σ x ,σ y the Pauli spin matrices. 7 If the k-cubic terms can be neglected, a special situation emerges if Rashba and Dresselhaus SOI are of equal strength: α = ±β.Then spin relaxation is suppressed. 8,9 A collinear alignment of Rashba and Dresselhaus effective magnetic fields gives rise to spin precession around a fixed axis, leading to spatially periodic modes referred to as persistent spin helix (PSH) and reflecting the underlying SU (2) symmetry in this case. 10The PSH is robust against all forms of spin-independent scattering. This favorable situation where spin relaxation is suppressed while the spin degree of freedom is still susceptible to electric...
We observe photocurrents induced in single-layer graphene samples by illumination of the graphene edges with circularly polarized terahertz radiation at normal incidence. The photocurrent flows along the sample edges and forms a vortex. Its winding direction reverses by switching the light helicity from left to right handed. We demonstrate that the photocurrent stems from the sample edges, which reduce the spatial symmetry and result in an asymmetric scattering of carriers driven by the radiation electric field. The developed theory based on Boltzmann's kinetic equation is in a good agreement with the experiment. We show that the edge photocurrents can be applied for determination of the conductivity type and the momentum scattering time of the charge carriers in the graphene edge vicinity.
A periodically driven system with spatial asymmetry can exhibit a directed motion facilitated by thermal or quantum fluctuations. This so-called ratchet effect has fascinating ramifications in engineering and natural sciences. Graphene is nominally a symmetric system. Driven by a periodic electric field, no directed electric current should flow. However, if the graphene has lost its spatial symmetry due to its substrate or adatoms, an electronic ratchet motion can arise. We report an experimental demonstration of such an electronic ratchet in graphene layers, proving the underlying spatial asymmetry. The orbital asymmetry of the Dirac fermions is induced by an in-plane magnetic field, whereas the periodic driving comes from terahertz radiation. The resulting magnetic quantum ratchet transforms the a.c. power into a d.c. current, extracting work from the out-of-equilibrium electrons driven by undirected periodic forces. The observation of ratchet transport in this purest possible two-dimensional system indicates that the orbital effects may appear and be substantial in other two-dimensional crystals such as boron nitride, molybdenum dichalcogenides and related heterostructures. The measurable orbital effects in the presence of an in-plane magnetic field provide strong evidence for the existence of structure inversion asymmetry in graphene.
Symmetry and spin dephasing in (110)-grown GaAs quantum wells (QWs) are investigated applying magnetic field induced photogalvanic effect and time-resolved Kerr rotation. We show that magnetic field induced photogalvanic effect provides a tool to probe the symmetry of (110)-grown quantum wells. The photocurrent is only observed for asymmetric structures but vanishes for symmetric QWs. Applying Kerr rotation we prove that in the latter case the spin relaxation time is maximal; therefore, these structures set the upper limit of spin dephasing in GaAs QWs. We also demonstrate that structure inversion asymmetry can be controllably tuned to zero by variation of delta-doping layer positions.
We report on the observation of cyclotron resonance induced photocurrents, excited by continuous wave terahertz radiation, in a 3D topological insulator (TI) based on an 80 nm strained HgTe film. The analysis of the photocurrent formation is supported by complimentary measurements of magneto-transport and radiation transmission. We demonstrate that the photocurrent is generated in the topologically protected surface states. Studying the resonance response in a gated sample we examined the behavior of the photocurrent, which enables us to extract the mobility and the cyclotron mass as a function of the Fermi energy. For high gate voltages we also detected cyclotron resonance (CR) of bulk carriers, with a mass about two times larger than that obtained for the surface states. The origin of the CR assisted photocurrent is discussed in terms of asymmetric scattering of TI surface carriers in the momentum space. Furthermore, we show that studying the photocurrent in gated samples provides a sensitive method to probe the effective masses and the mobility of 2D Dirac surface states, when the Fermi level lies in the bulk energy gap or even in the conduction band.
We report on the observation of the giant photocurrents in HgTe/HgCdTe quantum well (QW) of critical thickness at which a Dirac spectrum emerges. At an exciting QW of 6.6 nm width by terahertz (THz) radiation and sweeping magnetic field we detected a resonant photocurrent. Remarkably, the position of the resonance can be tuned from negative (−0.4 T) to positive (up to 1.2 T) magnetic fields by means of optical doping. The photocurrent data, accompanied by measurements of radiation transmission as well as Shubnikov-de Haas and quantum Hall effects, prove that the photocurrent is caused by cyclotron resonance in a Dirac fermion system, which allows us to obtain the effective electron velocity v ≈ 7.2 × 10 5 m/s. We develop a microscopic theory of the effect and show that the inherent spin-dependent asymmetry of light-matter coupling in the system of Dirac fermions causes the electric current to flow.
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