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Register a new userQuantum confined Stark effect in a MoS$_2$ monolayer van der Waals heterostructure
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Added by
Guillaume Froehlicher
Publication date
2017
fields
Physics
and research's language is
English
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The optics of dangling-bond-free van der Waals heterostructures containing transition metal dichalcogenides are dominated by excitons. A crucial property of a confined exciton is the quantum confined Stark effect (QCSE). Here, such a heterostructure is used to probe the QCSE by applying a uniform vertical electric field across a molybdenum disulfide (MoS$_2$) monolayer. The photoluminescence emission energies of the neutral and charged excitons shift quadratically with the applied electric field provided the electron density remains constant, demonstrating that the exciton can be polarized. Stark shifts corresponding to about half the homogeneous linewidth were achieved. Neutral and charged exciton polarizabilities of $(7.8~pm~1.0)times 10^{-10}~tr{D~m~V}^{-1}$ and $(6.4~pm~0.9)times 10^{-10}~tr{D~m~V}^{-1}$ at relatively low electron density ($8 times 10^{11}~tr{cm}^{-2}$) have been extracted, respectively. These values are one order of magnitude lower than the previously reported values, but in line with theoretical calculations. The methodology presented here is versatile and can be applied to other semiconducting layered materials as well.
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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 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)
Heavy fermion systems represent one of the paradigmatic strongly correlated states of matter. They have been used as a platform for investigating exotic behavior ranging from quantum criticality and non-Fermi liquid behavior to unconventional topological superconductivity. Heavy fermions arise from the exchange interaction between localized magnetic moments and conduction electrons that leads to the well-known Kondo effect. In a Kondo lattice, the interaction between the localized moments gives rise to a band with heavy effective mass. This intriguing phenomenology has so far only been realized in compounds containing rare-earth elements with 4f or 5f electrons. Here, we realize a designer van der Waals heterostructure where artificial heavy fermions emerge from the Kondo coupling between a lattice of localized magnetic moments and itinerant electrons in a 1T/1H-TaS$_2$ heterostructure. We study the heterostructure using scanning tunneling microscopy (STM) and spectroscopy (STS) and show that depending on the stacking order of the monolayers, we can either reveal the localized magnetic moments and the associated Kondo effect, or the conduction electrons with a heavy-fermion hybridization gap. Our experiments realize an ultimately tuneable platform for future experiments probing enhanced many-body correlations, dimensional tuning of quantum criticality, and unconventional superconductivity in two-dimensional artificial heavy-fermion systems.
When two superconductors are connected across a ferromagnet, the spin configuration of the transferred Cooper pairs can be modulated due to magnetic exchange interaction. The resulting supercurrent can reverse its sign across the Josephson junction (JJ) [1-4]. Here we demonstrate Josephson phase modulation in van der Waals heterostructures when Cooper pairs from superconducting NbSe$_2$ tunnel through atomically thin magnetic insulator (MI) Cr$_2$Ge$_2$Te$_6$. Employing a superconducting quantum interference device based on MI JJs, we probe a doubly degenerate non-trivial JJ phase ($phi$) originating from the magnetic barrier. This $phi$-phase JJ is formed by momentum conserving tunneling of Ising Cooper pairs [5] across magnetic domains in the Cr$_2$Ge$_2$Te$_6$ barrier. The doubly degenerate ground states in MI JJs provide a two-level quantum system that can be utilized as a new disipationless component for superconducting quantum devices, including phase batteries [6], memories [7,8], and quantum Ratchets [9,10].
Trions, quasi-particles consisting of two electrons combined with one hole or of two holes with one electron, have recently been observed in transition metal dichalcogenides (TMDCs) and drawn increasing attention due to potential applications of these materials in light-emitting diodes, valleytronic devices as well as for being a testbed for understanding many-body phenomena. Therefore, it is important to enhance the trion emission and its stability. In this study, we construct a MoSe2/FePS3 van der Waals heterostructure (vdWH) with type-I band alignment, which allows for carriers injection from FePS3 to MoSe2. At low temperatures, the neutral exciton (X0) emission in this vdWH is almost completely suppressed. The ITrion/Ix0 intensity ratio increases from 0.44 in a single MoSe2 monolayer to 20 in this heterostructure with the trion charging state changing from negative in the monolayer to positive in the heterostructure. The optical pumping with circularly polarized light shows a 14% polarization for the trion emission in MoSe2/FePS3. Moreover, forming such type-I vdWH also gives rise to a 20-fold enhancement of the room temperature photoluminescence from monolayer MoSe2. Our results demonstrate a novel approach to convert excitons to trions in monolayer 2D TMDCs via interlayer doping effect using type-I band alignment in vdWH.
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