Strongly Interacting Electrons and Holes in Ultrathin MoTe2 Heterostructures

ABSTRACT OF THE DISSERTATIONStrongly Interacting Electrons and Holes in Ultrathin MoTe2 HeterostructuresbyDennis PleskotDoctor of Philosophy, Graduate Program in Materials Science and EngineeringUniversity of California, Riverside, March 2019Dr. Nathaniel Gabor, ChairpersonHeterostructures composed of atomic layer materials (ALMs) bonded through van der Waals (vdW) interactions have demonstrated great potential for use in next generation optoelectronic devices. However, the role of strong charge carrier interactions in the interlayer photocurrent in such devices is not fully understood. One such ALM, molybdenum ditelluride (MoTe2), has a band gap energy near 1.0 eV, suggesting that MoTe2 is a promising material for ultrasensitive infrared optical photodetectors and next-generation solar photocells. Like other ALMs, the photoresponse in MoTe2 should give experimental access to strong many-body phenomena. Here, we report on the advanced fabrication and dynamic photoresponse of graphene-MoTe2-graphene vdW heterostructure photocells. We find that the power dependence of the interlayer photocurrent is well described by a single power law at low power, where the power law exponent parameterizes the nonlinearity of the photoresponse. We develop a detailed 2-particle Auger recombination model that accounts for careful time integration of the dynamics, resulting in an analytic solution that reproduces the non-linear power dependence. Additionally, we observed sharp suppression of conventional power law behavior above a critical power threshold, accompanied with a critical phase transition in the spatial expansion of the suppressed photocurrent region. We attribute this transition to the emergence of an electron-hole liquid phase that is remarkably stable at room temperature and moderate operating conditions. Additional measurements of the dynamic photoresponse support the existence of this phase, allowing us to fully explore the evolution of electron-hole pairs with increasing density in MoTe2

Search for dark matter produced in association with bottom or top quarks in √s = 13 TeV pp collisions with the ATLAS detector

A search for weakly interacting massive particle dark matter produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and miss- ing transverse momentum are considered. The analysis uses 36.1 fb−1 of proton–proton collision data recorded by the ATLAS experiment at √s = 13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are in- terpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour- neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross- section of 300 times the predicted rate for mediators with masses between 10 and 50 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour- charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements