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Register a new userExciton diffusion in WSe2 monolayers embedded in a van der Waals heterostructure
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We have combined spatially-resolved steady-state micro-photoluminescence ($mu$PL) with time-resolved photoluminescence (TRPL) to investigate the exciton diffusion in a WSe$_2$ monolayer encapsulated with hexagonal boron nitride (hBN). At 300 K, we extract an exciton diffusion length $L_X= 0.36pm 0.02 ; mu$m and an exciton diffusion coefficient of $D_X=14.5 pm 2;mbox{cm}^2$/s. This represents a nearly 10-fold increase in the effective mobility of excitons with respect to several previously reported values on nonencapsulated samples. At cryogenic temperatures, the high optical quality of these samples has allowed us to discriminate the diffusion of the different exciton species : bright and dark neutral excitons, as well as charged excitons. The longer lifetime of dark neutral excitons yields a larger diffusion length of $L_{X^D}=1.5pm 0.02 ;mu$m.
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The designer approach has become a new paradigm in accessing novel quantum phases of matter. Moreover, the realization of exotic states such as topological insulators, superconductors and quantum spin liquids often poses challenging or even contradictory demands for any single material. For example, it is presently unclear if topological superconductivity, which has been suggested as a key ingredient for topological quantum computing, exists at all in any naturally occurring material . This problem can be circumvented by using designer heterostructures combining different materials, where the desired physics emerges from the engineered interactions between the different components. Here, we employ the designer approach to demonstrate two major breakthroughs - the fabrication of van der Waals (vdW) heterostructures combining 2D ferromagnetism with superconductivity and the observation of 2D topological superconductivity. We use molecular-beam epitaxy (MBE) to grow two-dimensional islands of ferromagnetic chromium tribromide (CrBr$_3$) on superconducting niobium diselenide (NbSe$_2$) and show the signatures of one-dimensional Majorana edge modes using low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS). The fabricated two-dimensional vdW heterostructure provides a high-quality controllable platform that can be integrated in device structures harnessing topological superconductivity. Finally, layered heterostructures can be readily accessed by a large variety of external stimuli potentially allowing external control of 2D topological superconductivity through electrical, mechanical, chemical, or optical means.
The properties of van der Waals (vdW) heterostructures are drastically altered by a tunable moire superlattice arising from periodic variations of atomic alignment between the layers. Exciton diffusion represents an important channel of energy transport in semiconducting transition metal dichalcogenides (TMDs). While early studies performed on TMD heterobilayers have suggested that carriers and excitons exhibit long diffusion lengths, a rich variety of scenarios can exist. In a moire crystal with a large supercell size and deep potential, interlayer excitons may be completely localized. As the moire period reduces at a larger twist angle, excitons can tunnel between supercells and diffuse over a longer lifetime. The diffusion length should be the longest in commensurate heterostructures where the moire superlattice is completely absent. In this study, we experimentally demonstrate that the moire potential impedes interlayer exciton diffusion by comparing a number of WSe2/MoSe2 heterostructures prepared with chemical vapor deposition and mechanical stacking with accurately controlled twist angles. Our results provide critical guidance to developing twistronic devices that explore the moire superlattice to engineer material properties.
An emerging class of semiconductor heterostructures involves stacking discrete monolayers such as the transition metal dichalcogenides (TMDs) to form van der Waals heterostructures. In these structures, it is possible to create interlayer excitons (ILEs), spatially indirect, bound electron-hole pairs with the electron in one TMD layer and the hole in an adjacent layer. We are able to clearly resolve two distinct emission peaks separated by 24 meV from an ILE in a MoSe2/WSe2 heterostructure fabricated using state-of-the-art preparation techniques. These peaks have nearly equal intensity, indicating they are of common character, and have opposite circular polarizations when excited with circularly polarized light. Ab initio calculations successfully account for these observations - they show that both emission features originate from excitonic transitions that are indirect in momentum space, are split by spin-orbit coupling, and that including interlayer hybridization is essential in correctly describing the ILE transition. Although well separated in momentum space, we find that in real space the electron has significant weight in both the MoSe2 and WSe2 layers, contrary to the commonly assumed model. This is a significant consideration for understanding the static and dynamic properties of TMD heterostructures.
The fabrication of van der Waals heterostructures, artificial materials assembled by individually stacking atomically thin (2D) materials, is one of the most promising directions in 2D materials research. Until now, the most widespread approach to stack 2D layers relies on deterministic placement methods which are cumbersome when fabricating multilayered stacks. Moreover, they tend to suffer from poor control over the lattice orientations and the presence of unwanted adsorbates between the stacked layers. Here, we present a different approach to fabricate ultrathin heterostructures by exfoliation of bulk franckeite which is a naturally occurring and air stable van der Waals heterostructure (composed of alternating SnS2-like and PbS-like layers stacked on top of each other). Presenting both an attractive narrow bandgap (<0.7 eV) and p-type doping, we find that the material can be exfoliated both mechanically and chemically down to few-layer thicknesses. We present extensive theoretical and experimental characterizations of the materials electronic properties and crystal structure, and explore applications for near-infrared photodetectors (exploiting its narrow bandgap) and for p-n junctions based on the stacking of MoS2 (n-doped) and franckeite (p-doped)
The investigation of excitons in van-der-Waals heterostructures has led to profound insights into the interplay of crystal symmetries and fundamental effects of light-matter coupling. In particular, the polarization selection rules in undistorted, slightly twisted heterostructures of MoSe$_2$/WSe$_2$ were found to be connected with the Moire superlattice. Here, we report the emergence of a significant degree of circular polarization of excitons in such a hetero-structure upon non-resonant driving with a linearly polarized laser. The effect is present at zero magnetic field, and sensibly reacts on perpendicularly applied magnetic field. The giant magnitude of polarization, which cannot be explained by conventional birefringence or optical activity of the twisted lattice, suggests a kinematic origin arising from an emergent pyromagnetic symmetry in our structure, which we exploit to gain insight into the microscopic processes of our device.
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