No abstract

Excitons in semiconductors, bound pairs of excited electrons and holes, can form the basis for new classes of quantum optoelectronic devices. A van der Waals heterostructure built from atomically thin semiconducting transition metal dichalcogenides (TMDs) enables the formation of excitons from electrons and holes in distinct layers, producing interlayer excitons with large binding energy and a long lifetime. Employing heterostructures of monolayer TMDs, we realize optical and electrical generation of long-lived neutral and charged interlayer excitons. We demonstrate the transport of neutral interlayer excitons across the whole sample that can be controlled by excitation power and gate electrodes. We also realize the drift motion of charged interlayer excitons using Ohmic-contacted devices. The electrical generation and control of excitons provides a new route for realizing quantum manipulation of bosonic composite particles with complete electrical tunability.As bosonic composite particles, long-lived excitons can be potentially utilized for the realization of coherent quantum many-body systems (1, 2) or as quantum information carriers (3,4). In conventional semiconductors, the exciton lifetime can be increased by constructing double quantum well (DQW) heterostructures, where spatially separated electrons and holes form interlayer excitons (IEs) across the quantum wells (5-10). Strongly bound IEs can also be formed in atomically thin DQW. By stacking two

Summary Size specification of macromolecular assemblies in the cytoplasm is poorly understood [1]. In principle, assemblies could scale with cell size, or use intrinsic mechanisms to achieve fixed, but regulated, sizes. For the mitotic spindle, scaling with cell size is expected, since the function of this assembly is to physically move sister chromatids into the center of nascent daughter cells. Anecdotally, spindle length does scale with cell length, but it is not clear if this scaling mechanism could operate at very large cell lengths. Eggs of Xenopus laevis are among the largest cells known that cleave completely during cell division. Cell length in this organism changes by two orders of magnitude (~1200 µm to ~12 µm) while it develops from a fertilized egg into a tadpole [2]. We wondered if, and how, mitotic spindle length and morphology adapt to function at these different length scales. Here, we show that spindle length increases with cell length in small cells, but in very large cells spindle length approaches an upper limit of ~60 µm. To transport the DNA into the center of the daughter cells, the relatively small spindle length is compensated by an enormous anaphase B-like movement. Further evidence for an upper limit to spindle length comes from an embryonic extract system that recapitulates mitotic spindle assembly in a test tube. We conclude that early mitotic spindle length in Xenopus laevis is uncoupled from cell length, reaching an upper bound determined by mechanisms that are intrinsic to the spindle.