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Register a new userThermalization of photoexcited carriers in two-dimensional transition metal dichalcogenides and internal quantum efficiency of van der Waals heterostructures
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Van der Waals semiconductor heterostructures could be a platform to harness hot photoexcited carriers in the next generation of optoelectronic and photovoltaic devices. The internal quantum efficiency of hot-carrier devices is determined by the relation between photocarrier extraction and thermalization rates. Using textit{ab-initio} methods we show that the photocarrier thermalization time in single-layer transition metal dichalcogenides strongly depends on the peculiarities of the phonon spectrum and the electronic spin-orbit coupling. In detail, the lifted spin degeneracy in the valence band suppresses the hole scattering on acoustic phonons, slowing down the thermalization of holes by one order of magnitude as compared to electrons. Moreover, the hole thermalization time behaves differently in MoS$_2$ and WSe$_2$ because spin-orbit interactions differ in these seemingly similar materials. We predict that the internal quantum efficiency of a tunneling van der Waals semiconductor heterostructure depends qualitatively on whether MoS$_2$ or WSe$_2$ is used.
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We study direct and indirect magnetoexcitons in Rydberg states in monolayers and heterostructures of transition-metal dichalcogenices (TMDCs) in an external magnetic field, applied perpendicular to the monolayer or heterostructures. We calculate binding energies of magnetoexcitons for the Rydberg states 1$s$, 2$s$, 3$s$, and 4$s$ by numerical integration of the Schr{o}dinger equation using the Rytova-Keldysh potential for direct magnetoexcitons and both the Rytova-Keldysh and Coulomb potentials for indirect magnetoexcitons. Latter allows understanding the role of screening in TMDCs heterostructures. We report the magnetic field energy contribution to the binding energies and diamagnetic coefficients (DMCs) for direct and indirect magnetoexcitons. The tunability of the energy contribution of direct and indirect magnetoexcitons by the magnetic field is demonstrated. It is shown that binding energies and DMCs of indirect magnetoexcitons can be manipulated by the number of hBN layers. Therefore, our study raises the possibility of controlling the binding energies of direct and indirect magnetostrictions in TMDC monolayers, bilayers and heterostructures using magnetic field and opens an additional degree of freedom to tailor the binding energies and DMCs for heterostructures by varying the number of hBN sheets between TMDC layers. The calculations of the binding energies and DMCs of indirect magnetoexcitons in TMDC heterostructures are novel and can be compared with the experimental results when they will be available.
Van der Waals materials can be easily combined in lateral and vertical heterostructures, providing an outstanding platform to engineer elusive quantum states of matter. However, a critical problem in material science is to establish tangible links between real materials properties and terms that can be cooked up on the model Hamiltonian level to realize different exotic phenomena. Our review aims to do precisely this: we first discuss, in a way accessible to the materials community, what ingredients need to be included in the hybrid quantum materials recipe, and second, we elaborate on the specific materials that would possess the necessary qualities. We will review the well-established procedures for realizing 2D topological superconductors, quantum spin-liquids and flat bands systems, emphasizing the connection between well-known model Hamiltonians and real compounds. We will use the most recent experimental results to illustrate the power of the designer approach.
Van der Waals (vdW) heterobilayers formed by two-dimensional (2D) transition metal dichalcogenides (TMDCs) created a promising platform for various electronic and optical properties. ab initio band results indicate that the band offset of type-II band alignment in TMDCs vdW heterobilayer could be tuned by introducing Janus WSSe monolayer, instead of an external electric field. On the basis of symmetry analysis, the allowed interlayer hopping channels of TMDCs vdW heterobilayer were determined, and a four-level kp model was developed to obtain the interlayer hopping. Results indicate that the interlayer coupling strength could be tuned by interlayer electric polarization featured by various band offsets. Moreover, the difference in the formation mechanism of interlayer valley excitons in different TMDCs vdW heterobilayers with various interlayer hopping strength was also clarified.
We propose to engineer time-reversal-invariant topological insulators in two-dimensional (2D) crystals of transition metal dichalcogenides (TMDCs). We note that, at low doping, semiconducting TMDCs under shear strain will develop spin-polarized Landau levels residing in different valleys. We argue that gaps between Landau levels in the range of $10-100$ Kelvin are within experimental reach. In addition, we point out that a superlattice arising from a Moire pattern can lead to topologically non-trivial subbands. As a result, the edge transport becomes quantized, which can be probed in multi-terminal devices made using strained 2D crystals and/or heterostructures. The strong $d$ character of valence and conduction bands may also allow for the investigation of the effects of electron correlations on the topological phases.
The development of van der Waals (vdW) crystals and their heterostructures has created a fascinating platform for exploring optoelectronic properties in the two-dimensional (2D) limit. With the recent discovery of 2D magnets, the control of the spin degree of freedom can be integrated to realize 2D spin-optoelectronics with spontaneous time-reversal symmetry breaking. Here, we report spin photovoltaic effects in vdW heterostructures of atomically thin magnet chromium triiodide (CrI3) sandwiched by graphene contacts. In the absence of a magnetic field, the photocurrent displays a distinct dependence on light helicity, which can be tuned by varying the magnetic states and photon energy. Circular polarization-resolved absorption measurements reveal that these observations originate from magnetic-order-coupled and thus helicity-dependent charge-transfer exciton states. The photocurrent displays multiple plateaus as the magnetic field is swept, which are associated with different spin configurations enabled by the layered antiferromagnetism and spin-flip transitions in CrI3. Remarkably, giant photo-magnetocurrent is observed, which tends to infinity for a small applied bias. Our results pave the way to explore emergent photo-spintronics by engineering magnetic vdW heterostructures.
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