In 1954, Schafroth proposed a mechanism for superconductivity that is physically possible, but ended up not being the explanation of the well known BCS superconductors. The proposal argued correctly that a Bose condensate of charged bosons should also be a superconductor. In 1996, V.I. Yudson proposed a way to produce a charged boson by attaching two free charges to an exciton in a semiconductor, to make a "quaternion". While that state was never seen in III-V semiconductors, our calculations show that it is predicted to be stable in structures made with monolayers of transition metal dichalcogenide (TMD) materials. We present experimental spectroscopic measurements that agree with this theory, which indicate that we have observed this charged-boson state in this type of structure. This opens up a new path for pursuing room temperature superconductivity. [Read More]
Interlayer excitons (IXs) possess a much longer lifetime than intralayer excitons due to the spatial separation of the electrons and holes; hence, they have been pursued to create exciton condensates for decades. The recent emergence of two-dimensional (2D) materials, such as transition metal dichalcogenides (TMDs), and of their van der Waals heterostructures (HSs), in which two different 2D materials are layered together, has created new opportunities to study IXs. Here we present the observation of IX gases within two stacked structures consisting of hBN/WSe2/hBN/p: WSe2/hBN. The IX energy of the two different structures differed by 82 meV due to the different thickness of the hBN spacer layer between the TMD layers. [Read More]
We demonstrate that the exciton energy of a monolayer of tungsten diselenide (WSe2) on SiO2/Si substrate can be tuned by an applied in-plane electric eld for two samples with different dielectric capping materials. The exciton energy can be either red-shifted or blue-shifted up to 10 meV based on the polarity of the applied electric eld. We argue that there is a large internal electric field created through a piezoelectric effect which is either partially aligned or partially anti-aligned with our externally applied electric eld. Additionally, as we cycle the external electric eld, we see a hysteresis loop, leading us to conclude that there are trapped charges.[Read More]
Large scale growth of monolayer transition metal dichalcogenides (TMDs) via chemical vapor deposition.
Single atomic layers of two-dimensional (2D) transition metal dichalcogenides (TMDs) are promising candidates for the integration of optical and electronic circuits due to their extraordinary optical oscillator strength and large exciton binding energy. Customizing the exciton energy of the TMDs is a direct way to control the light-matter interaction. Here we demonstrate that the electronic bandgap of tungsten diselenide WSe2 can be tuned continuously with the application of the uniaxial tensile strain. [Read More]