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Register a new userOptical orientation with linearly polarized light in transition metal dichalcogenides
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We study the optical properties of semiconducting transition metal dichalcogenide monolayers under the influence of strong out-of-plane magnetic fields, using the effective massive Dirac model. We pay attention to the role of spin-orbit coupling effects, doping level and electron-electron interactions, treated at the Hartree-Fock level. We find that optically-induced valley and spin imbalance, commonly attained with circularly polarized light, can also be obtained with linearly polarized light in the doped regime. Additionally, we explore an exchange-driven mechanism to enhance the spin-orbit splitting of the conduction band, in n-doped systems, controlling both the carrier density and the intensity of the applied magnetic field.
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Manipulating spin polarization of electrons in nonmagnetic semiconductors by means of electric fields or optical fields is an essential theme of the conceptual nonmagnetic semiconductor-based spintronics. Here we experimentally demonstrate a method of generating spin polarization in monolayer transition metal dichalcogenides (TMD) by the circularly polarized optical pumping. The fully spin-polarized photocurrent is achieved through the valley dependent optical selection rules and the spin-valley locking in monolayer WS2, and electrically detected by a lateral spin-valve structure with ferromagnetic contacts. The demonstrated long spin lifetime, the unique valley contrasted physics and the spin-valley locking make monolayer WS2 an unprecedented candidate for semiconductor based spintronics.
We explore proximity-induced ferromagnetism on transition metal dichalcogenide (TMD), focusing on molybdenum ditelluride (MoTe$_{2}$) ribbons with zigzag edges, deposited on ferromagnetic europium oxide (EuO). A three-orbital tight-binding model incorporates the exchange and Rashba fields induced by proximity to the EuO substrate. For in-gap Fermi levels, electronic modes in the nanoribbon are strongly spin-polarized and localized along the edges, acting as one-dimensional (1D) conducting channels with tunable spin-polarized currents. Hybrid structures such as the MoTe$_{2}$/EuO configuration can serve as building blocks for spintronic devices, and provide versatile platforms to further understand proximity effects in diverse materials systems.
Monolayer transition metal dichalcogenides are promising materials for valleytronic operations. They exhibit two inequivalent valleys in the Brillouin zone, and the valley populations can be directly controlled and determined using circularly polarized optical excitation and emission. The photoluminescence polarization reflects the ratio of the two valley populations. A wide range of values for the degree of circularly polarized emission, Pcirc, has been reported for monolayer WS2, although the reasons for the disparity are unclear. Here we optically populate one valley, and measure Pcirc to explore the valley population dynamics at room temperature in a large number of monolayer WS2 samples synthesized via chemical vapor deposition. Under resonant excitation, Pcirc ranges from 2% to 32%, and we observe a pronounced inverse relationship between photoluminescence (PL) intensity and Pcirc. High quality samples exhibiting strong PL and long exciton relaxation time exhibit a low degree of valley polarization, and vice versa. This behavior is also demonstrated in monolayer WSe2 samples and transferred WS2, indicating that this correlation may be more generally observed and account for the wide variations reported for Pcirc. Time resolved PL provides insight into the role of radiative and non-radiative contributions to the observed polarization. Short non-radiative lifetimes result in a higher measured polarization by limiting opportunity for depolarizing scattering events.
The investigation of 2D van der Waals (vdW) materials is a vibrant, fast moving and still growing interdisciplinary area of research. 2D vdW materials are truly 2D crystals with strong covalent in-plane bonds and weak van der Waals interaction between the layers with a variety of different electronic, optical and mechanical properties. A very prominent class of 2D materials are transition metal dichalcogenides (TMDs) and amongst them particularly the semiconducting subclass. Their properties include bandgaps in the near-infrared to the visible range, decent charge carrier mobility together with high (photo-)catalytic and mechanical stability and exotic many body phenomena. These characteristics make the materials highly attractive for both fundamental research as well as innovative device applications. Furthermore, the materials exhibit a strong light matter interaction providing a high sun light absorbance of up to 15% in the monolayer limit, strong scattering cross section in Raman experiments and access to excitonic phenomena in vdW heterostructures. This review focuses on the light matter interaction in MoS2, WS2, MoSe2, and WSe2 that is dictated by the materials complex dielectric functions and on the multiplicity of studying the first order phonon modes by Raman spectroscopy to gain access to several material properties such as doping, strain, defects and temperature. 2D materials provide an interesting platform to stack them into vdW heterostructures without the limitation of lattice mismatch resulting in novel devices for application but also to study exotic many body interaction phenomena such as interlayer excitons. Future perspectives of semiconducting TMDs and their heterostructures for applications in optoelectronic devices will be examined and routes to study emergent fundamental problems and many-body quantum phenomena under excitations with photons will be discussed.
Impurities play an important role during recombination processes in semiconductors. Their important role is sharpened in atomically-thin transition-metal dichalcogenides whose two-dimensional character renders electrons and holes highly susceptible to localization caused by remote charged impurities. We study a multitude of phenomena that arise from the interaction of localized electrons with excitonic complexes. Emphasis is given to the amplification of the phonon-assisted recombination of biexcitons when it is mediated by localized electrons, showing that this mechanism can explain recent photoluminescence experiments in ML-WSe$_2$. In addition, the magnetic-field dependence of this mechanism is analyzed. The results of this work point to (i) an intriguing coupling between the longitudinal-optical and homopolar phonon modes that can further elucidate various experimental results, (ii) the physics behind a series of localization-induced optical transitions in tungsten-based materials, and (iii) the importance of localization centers in facilitating the creation of biexcitons and exciton-exciton annihilation processes.
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