Trion formation dynamics in monolayer transition metal dichalcogenides

We report charged exciton (trion) formation dynamics in doped monolayer transition metal dichalcogenides, specifically molybdenum diselenide (MoSe2), using resonant two-color pump-probe spectroscopy. When resonantly pumping the exciton transition, trions are generated on a picosecond time scale through exciton-electron interaction. As the pump energy is tuned from the high energy to low energy side of the inhomogeneously broadened exciton resonance, the trion formation time increases by ∼50%. This feature can be explained by the existence of both localized and delocalized excitons in a disordered potential and suggests the existence of an exciton mobility edge in transition metal dichalcogenides

Spin waves in CsVBr{sub 3}

Inelastic neutron scattering has been used to measure spin wave excitations in the quasi-one dimensional S = 3/2 magnetic material CsVBr{sub 3}. Dispersion relations were determined using standard triple-axis methods. Fits to linear spin wave theory yield model Hamiltonian parameters describing magnetic interactions in the system

Slow light in a 2D semiconductor plasmonic structure

Spectrally narrow optical resonances can be used to generate slow light, i.e., a large reduction in the group velocity. In a previous work, we developed hybrid 2D semiconductor plasmonic structures, which consist of propagating optical frequency surface-plasmon polaritons interacting with excitons in a semiconductor monolayer. Here, we use coupled exciton-surface plasmon polaritons (E-SPPs) in monolayer WSe2 to demonstrate slow light with a 1300 fold decrease of the SPP group velocity. Specifically, we use a high resolution two-color laser technique where the nonlinear E-SPP response gives rise to ultra-narrow coherent population oscillation (CPO) resonances, resulting in a group velocity on order of 105 m/s. Our work paves the way toward on-chip actively switched delay lines and optical buffers that utilize 2D semiconductors as active elements. © 2022, The Author(s).Open access journalThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Complex magnetic phases in LuFe2O4

DC magnetization and AC susceptibility measurements on LuFe2O4 single crystals reveal a ferrimagnetic transition at 240 K followed by additional magnetic transitions at 225 K and 170 K, separating cluster glass phases, and a kinetically arrested state below 55 K. The origin of giant magnetic coercivity is attributed to the collective freezing of ferrimagnetic clusters and enhanced domain wall pinning associated with a structural transition at 170 K. Magnetocaloric effect measurements provide additional vital information about the multiple magnetic transitions and the glassy states. Our results lead to the emergence of a complex magnetic phase diagram in LuFe2O4. (C) 2009 Elsevier Ltd. All rights reserved

Single-exciton trapping in an electrostatically defined two-dimensional semiconductor quantum dot

Interlayer excitons (IXs) in two-dimensional semiconductors have long lifetimes and spin-valley coupled physics, with a long-standing goal of single-exciton trapping for valleytronic applications. In this work, we use a nanopatterned graphene gate to create an electrostatic IX trap. We measure a unique power-dependent blueshift of IX energy, where narrow linewidth emission exhibits discrete energy jumps. We attribute these jumps to quantized increases of the number occupancy of IXs within the trap and compare to a theoretical model to assign the lowest energy emission line to single-IX recombination. © 2022 American Physical Society.Immediate accessThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Direct STM measurements of R-type and H-type twisted MoSe2/WSe2

When semiconducting transition metal dichalcogenide heterostructures are stacked, the twist angle and lattice mismatch lead to a periodic moiré potential. As the angle between the layers changes, so do the electronic properties. As the angle approaches 0° or 60°, interesting characteristics and properties, such as modulations in the band edges, flat bands, and confinement, are predicted to occur. Here, we report scanning tunneling microscopy and spectroscopy measurements on the bandgaps and band modulations in MoSe2/WSe2 heterostructures with near 0° rotation (R-type) and near 60° rotation (H-type). We find a modulation of the bandgap for both stacking configurations with a larger modulation for R-type than for H-type as predicted by theory. Furthermore, local density of states images show that electrons are localized differently at the valence band and conduction band edges. © 2022 Author(s).Open access journalThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Localized interlayer excitons in MoSe2-WSe2 heterostructures without a moiré potential

Interlayer excitons (IXs) in MoSe2-WSe2 heterobilayers have generated interest as highly tunable light emitters in transition metal dichalcogenide (TMD) heterostructures. Previous reports of spectrally narrow (<1 meV) photoluminescence (PL) emission lines at low temperature have been attributed to IXs localized by the moiré potential between the TMD layers. We show that spectrally narrow IX PL lines are present even when the moiré potential is suppressed by inserting a bilayer hexagonal boron nitride (hBN) spacer between the TMD layers. We compare the doping, electric field, magnetic field, and temperature dependence of IXs in a directly contacted MoSe2-WSe2 region to those in a region separated by bilayer hBN. The doping, electric field, and temperature dependence of the narrow IX lines are similar for both regions, but their excitonic g-factors have opposite signs, indicating that the origin of narrow IX PL is not the moiré potential. © 2022. The Author(s).Open access journalThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Temperature dependent moiré trapping of interlayer excitons in MoSe2-WSe2 heterostructures

MoSe2–WSe2 heterostructures host strongly bound interlayer excitons (IXs), which exhibit bright photoluminescence (PL) when the twist angle is near 0° or 60°. Over the past several years, there have been numerous reports on the optical response of these heterostructures but no unifying model to understand the dynamics of IXs and their temperature dependence. Here we perform a comprehensive study of the temperature, excitation power, and time-dependent PL of IXs. We observe a significant decrease in PL intensity above a transition temperature that we attribute to a transition from localized to delocalized IXs. Astoundingly, we find a simple inverse relationship between the IX PL energy and the transition temperature, which exhibits opposite power-dependent behaviors for near 0° and 60° samples. We conclude that this temperature dependence is a result of IX–IX exchange interactions, whose effect is suppressed by the moiré potential trapping IXs at low temperature. © 2021, The Author(s).Open access journalThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]