While photons in free space barely interact, matter can mediate interactions between them resulting in optical nonlinearities. Such interactions at the single-quantum level result in an on-site photon repulsion [1, 2], crucial for photon-based quantum information processing and for realizing strongly interacting many-body states of light [3][4][5][6][7]. Here, we report repulsive dipole-dipole interactions between electric field tuneable, localized interlayer excitons in MoSe 2 /WSe 2 heterobilayer. The presence of a single, localized exciton with an out-of-plane, non-oscillating dipole moment increases the energy of the second excitation by ∼ 2 meV -an order of magnitude larger than the emission linewidth and corresponding to an inter-dipole distance of ∼ 5 nm. At higher excitation power, multi-exciton complexes appear at systematically higher energies. The magnetic field dependence of the emission polarization is consistent with spin-valley singlet nature of the dipolar molecular state. Our finding is an important step towards the creation of excitonic few-and many-body states such as dipolar crystals with spin-valley spinor in van der Waals (vdW) heterostructures.Optical response in atomically thin layered semiconductors is determined by excitons and other excitonic complexes such as trions and biexcitons which are strongly bound due to increased Coulomb interactions in truly 2D limit [5,9]. In addition, due to the type-II band alignment in heterobilayer of MoSe 2 /WSe 2 , an interlayer exciton comprising of an electron in the MoSe 2 layer and hole in the WSe 2 layer is found to be stable and long-lived [4,10,11,13]. As shown in Fig. 1a, due to the spatial separation of electron and hole, the interlayer exciton carries a static, out-of-plane electric dipole moment which allows for the tuning of its energy by an external electric field (E). The orientation of this dipole is fixed by the ordering of MoSe 2 and WSe 2 layers and hence leads to a repulsive interaction between interlayer excitons. arXiv:1910.08139v1 [cond-mat.mes-hall] 17 Oct 2019 10 µm WSe2 MoSe2 a b c U d-d on-site E x E x -+ Mo W Se ℎ + − Electric field Figure 1: Interlayer exciton dipoles in WSe 2 /MoSe 2 heterostructure. a, A schematic showing the interlayer exciton in WSe 2 -MoSe 2 heterobilayer under an external electric field E. Due to the type-II band alignment, electron and hole are separated in MoSe 2 and WSe 2 , respectively, forming a permanent out-ofplane dipole. The dipole energy red-shifts (blue-shifts) when E is parallel (anti-parallel) to the direction of dipole. b, Energy diagram of localized interlayer exciton and biexciton in a potential well. The energy of biexciton is raised up by on-site dipole-dipole interaction U on−site dd . c, An optical image of WSe 2 /MoSe 2 heterobilayer with graphite bottom gate. Monolayer WSe 2 (MoSe 2 ) is depicted in orange (yellow) dashed line. The final device has graphite bottom and top gates with h-BN as dielectric on both sides.This dipolar interaction is potentially interesting for inducing e...

Control and manipulation of single charges and their internal degrees of freedom, such as spins, is a fundamental goal of nanoscience with promising technological applications. Recently, atomically thin semiconductors such as WSe 2 have emerged as a platform for valleytronics, offering rich possibilities for optical, magnetic and electrical control of the valley index [1, 2]. While progress has been made in controlling valley index of ensemble of charge carriers [3-5], valley control of individual charges, crucial for valleytronics, remains unexplored. Here, we provide unambiguous evidence for localized holes with net spin in optically active WSe 2 quantum dots (QDs) and control their spin-valley state with the helicity of the excitation laser under small magnetic field. We estimate a lower bound on the valley lifetime of a single charge in QD from recombination time to be ∼ nanoseconds. Remarkably, neutral QDs do not exhibit such a control, demonstrating the role of excess charge in prolonging the valley lifetime. Our work extends the field of 2D valleytronics to the level of single spin-valley, relevant for quantum information and sensing applications.Localized single spins in solid-state have been widely studied for quantum information technology, spintronics and quantum sensing [6,7], in addition to serving as a versatile playground for exploring many-body physics [8]. With the rise of semiconducting van der Waals (vdW) materials having direct band gap such as group VI-B transition metal dichalcogenides (TMDs), a arXiv:1810.01887v1 [cond-mat.mes-hall]

Charge carriers in two-dimensional transition metal dichalcogenides (TMDs), such as WSe 2 , have their spin and valley-pseudospin locked into an optically-addressable index that is proposed as a basis for future information processing. The manipulation of this spin-valley index requires tuning its energy, typically through external magnetic field (B), which is cumbersome. Thus, other efficient routes like all-optical control of spin-valley index are desirable. Here, we show that many-body interactions amongst interlayer excitons in WSe 2 /MoSe 2 heterobilayer induce a steady-state valley Zeeman splitting corresponding to B ∼6 Tesla. This anomalous splitting, present at incident powers as low as µWs, increases with power and enhances, suppresses or even flips the sign of a B-induced splitting. Moreover, the g-factor of valley Zeeman splitting can be tuned by ∼30% with incident power. In addition to valleytronics, our results are relevant for achieving optical non-reciprocity using two-dimensional materials.

We utilize variable-temperature, variable-frequency magneto-optical transverse magnetic susceptibility technique to study the static and dynamical magnetic properties of a thin-film CoO/Permalloy bilayer. Our measurements demonstrate that in the studied system, the directional asymmetry of the hysteresis loop is associated mainly with the difference in the reversal mechanisms between the two reversed states of magnetization stabilized by the exchange-induced uniaxial anisotropy. The latter is found to be much larger than the exchange-induced unidirectional anisotropy of the ferromagnet. We also observe an abrupt variation of the frequency-dependent imaginary part of ac susceptibility near the exchange bias blocking temperature, consistent with the magnetic freezing transition inferred from the previous time-domain studies of magnetic aging in similar systems. The developed measurement approach enables precise characterization of the dynamical and static characteristics of thin-film magnetic heterostructures that can find applications in reconfigurable magnonic and neuromorphic circuits.Recent measurements of the time-dependence of the magnetization state in thin-film F/AF bilayers revealed slow power-law magnetic aging at low temperatures, which appears to be universal for such systems [13][14][15]. The dependence of aging on the magnetic history was inconsistent with the Arrhenius-type activation, instead indicating cooperative behaviors akin to the avalanche dynamics in glassy systems [16]. Since aging was observed only for thin films, this glassy dynamics was tentatively attributed to the emergence of HDS. The relationship between HDS and glasses was further elucidated arXiv:1807.05990v2 [cond-mat.mtrl-sci]