Indirect excitons (IXs) are explored both for studying quantum Bose gases in semiconductor materials and for the development of excitonic devices. IXs were extensively studied in III–V and II–VI semiconductor heterostructures where IX range of existence has been limited to low temperatures. Here, we present the observation of IXs at room temperature in van der Waals transition metal dichalcogenide (TMD) heterostructures. This is achieved in TMD heterostructures based on monolayers of MoS2 separated by atomically thin hexagonal boron nitride. The IXs we realize in the TMD heterostructure have lifetimes orders of magnitude longer than lifetimes of direct excitons in single-layer TMD and their energy is gate controlled. The realization of IXs at room temperature establishes the TMD heterostructures as a material platform both for a field of high-temperature quantum Bose gases of IXs and for a field of high-temperature excitonic devices.

Indirect excitons (IX) in semiconductor heterostructures are bosons, which can cool below the temperature of quantum degeneracy and can be effectively controlled by voltage and light. IX quantum Bose gases and IX devices were explored in GaAs heterostructures where an IX range of existence is limited to low temperatures due to low IX binding energies. IXs in van der Waals transition-metal dichalcogenide (TMD) heterostructures are characterized by large binding energies giving the opportunity for exploring excitonic quantum gases and for creating excitonic devices at high temperatures. TMD heterostructures also offer a new platform for studying single-exciton phenomena and few-particle complexes. In this work, we present studies of IXs in MoSe 2 /WSe 2 heterostructures and report on two IX luminescence lines whose energy splitting and temperature dependence identify them as neutral and charged IXs. The 1 arXiv:1901.08664v2 [cond-mat.mes-hall] 20 Nov 2019 experimentally found binding energy of the indirect charged excitons, i.e. indirect trions, is close to the calculated binding energy of 28 meV for negative indirect trions in TMD heterostructures [Deilmann, Thygesen, Nano Lett. 18, 1460]. We also report on the realization of IXs with a luminescence linewidth reaching 4 meV at lowtemperatures. An enhancement of IX luminescence intensity and the narrow linewidth are observed in localized spots.

Optical control of exciton fluxes is realized for indirect excitons in a crossed-ramp excitonic device. The device demonstrates experimental proof of principle for all-optical excitonic transistors with a high ratio between the excitonic signal at the optical drain and the excitonic signal due to the optical gate. The device also demonstrates experimental proof of principle for all-optical excitonic routers.

We report an experimental study of excitons in a double quantum well van der Waals heterostructure made of atomically thin layers of MoS2 and hexagonal boron nitride (hBN). The emission of neutral and charged excitons is controlled by gate voltage, temperature, and both the helicity and the power of optical excitation. Van der Waals heterostructures composed of ultrathin layers of transition metal dichalcogenides (TMD), such as MoS 2 , WSe 2 , etc., offer an opportunity to realize artificial materials with designable properties, forming a new platform for studying basic phenomena and developing optoelectronic devices [1]. In TMD structures, excitons have high binding energies and are prominent in the optical response. The energy, intensity, and polarization of exciton emission gives information about electronic, spin, and valley properties of TMD materials . * ecalman@gmail.com Exciton phenomena are expected to become even richer in structures that contain two 2D layers. The energy-band diagram of such a coupled quantum well (CQW) structure is shown schematically in Figure 1b. Previous studies of GaAs [24], AlAs [25], and InGaAs [26] CQWs showed that excitons in these structures can be effectively controlled by voltage and light. Two types of excitons are possible in a CQW structure. Spatially direct excitons (DXs) are composed of electrons and holes in the same layer, while indirect excitons (IXs) are bound states of electrons and holes in the different layers separated by a distance d, Figure 1b. IXs can form quantum degenerate Bose gases [27,28]. The realization and control of quantum IX gases was demonstrated [29,30] in GaAs CQW structures at temperatures T below a few degrees Kelvin. In a recent theoretical work [31] it was predicted that the large exciton binding energies in TMD CQW structures may bring the domain of these phenomena to high temperatures. On the other hand, DXs in TMD CQW structures have a high oscillator strength making these structures good emitters . CQW structures allow control of the exciton emission by voltage. These properties make CQW structures an interesting new system for studying exciton phenomena in TMD materials.The DX binding energy E DX is larger [31] than that E IX of the IXs, so in the absence of an external field the DXs are lower in energy. The electric field F normal to the layers induces the energy shift eF d of IXs. The transition between the direct regime where DXs are lower in energy to the indirect regime where IXs are lower in energy occurs when eF d > E DX − E IX [26]. Both direct and indirect regimes show interesting exciton phenomena. The indirect regime was considered in earlier studies of GaAs [24], AlAs [25], InGaAs [26], and TMD [18,21] CQW structures. The direct regime in TMD CQW structures is considered in this work. Exploring the direct regime is essential for understanding both the universal properties of complex exciton systems in CQW structures and the specific properties of direct excitons in TMD layers. We found that the exciton spectra in the direc...

We report on spatially-and time-resolved emission measurements and observation of transport of indirect excitons in ZnO/MgZnO wide single quantum wells.An indirect exciton (IX) in a semiconductor quantum well (QW) structure is composed of an electron and a hole confined to spatially separated QW layers. IXs were realized in wide single quantum wells (WSQW) [1][2][3][4] and in coupled quantum wells (CQW) [5][6][7][8] [2-4, 6, 7]. Their long lifetimes allow IXs to travel over large distances before recombination, providing the opportunity to study exciton transport by optical imaging [9][10][11][12][13][14][15] and explore excitonic circuit devices based on exciton transport, see [16] and references therein.Materials with a high IX binding energy allow extending the operation of the excitonic devices to high temperatures [17][18][19]. Furthermore, such materials can allow the realization of high-temperature coherent states of IXs [19]. These properties make materials with robust IXs particularly interesting. However, so far, studies of IX transport mainly concerned GaAs-based CQW. In this paper, we probe transport of IXs in ZnO/MgZnO WSQW structures. IXs in these structures are much more robust than in GaAs structures: their binding energy ∼ 30 meV [4] is considerably higher than that in GaAs/AlGaAs and GaAs/AlAs CQW (∼ 4 and ∼ 10 meV, respectively [7,20]). The binding energy of IXs is smaller than that of excitons in bulk ZnO (∼ 60 meV), however it is large enough to make the IXs stable at room temperature. At the same time, the measurements reported in this work show that transport lengths of IXs in WSQW ZnO structures reach ∼ 4 µm. In comparison, for excitons in bulk ZnO and direct excitons in ZnO structures, transport lengths are within ∼ 0.2 µm [21,22].In this work, we study polar and semipolar ZnO/MgZnO QW structures. The samples were grown by molecular beam epitaxy as in Refs. [4,23] Fig. 1a. The charges on the interfaces between ZnO and MgZnO result in a built-in electric field in the structure, which is stronger for polar samples [4,23]. The built-in electric field pulls the electron and the hole toward opposite borders of the QW, resulting in the spatial separation required for an IX.