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The delafossites are a class of layered metal oxides that are notable for being able to exhibit optical transparency alongside an in-plane electrical conductivity, making them promising platforms for the development of transparent conductive oxides. Pressure-induced polymorphism offers a direct method for altering the electrical and optical properties in this class, and although the copper delafossites have been studied extensively under pressure, the silver delafossites remain only partially studied. We report two new high-pressure polymorphs of silver ferrite delafossite, AgFeO2, that are stabilized above ∼6 and ∼14 GPa. In situ X-ray diffraction and vibrational spectroscopy measurements are used to examine the structural changes across the two phase transitions. The high-pressure structure between 6 and 14 GPa is assigned as a monoclinic C2/c structure that is analogous to the high-pressure phase reported for AgGaO2. Nuclear resonant forward scattering reveals no change in the spin state or valence state at the Fe3+ site up to 15.3(5) GPa. 

We perform resonant Raman spectroscopy on 8◦ twisted bilayer graphene placed in an out-of-plane magnetic field. The high-quality device has narrow Landau level linewidth of less than 5 meV that enables detection of features from both electronic Raman scattering and magnetophonon resonance involving electronic transitions between the low energy Landau levels. Two magnetophonon resonances are observed, one at 4.6T in the strong coupling regime, and the other at 2.6T in the weak coupling regime. Using the measured Landau level transition energy, we analyze the renormalization of effective band velocity, whose dependence on magnetic field points to a 20% enhancement of dielectric constant due to the presence of an adjacent graphene layer, a quite prominent screening effect from a monolayer of carbon atoms in proximity. Both the Landau level transition electronic Raman and the magnetophonon resonance are gate tunable. Harnessing angular momentum conservation, we demonstrate charge tuning of electron phonon coupling strength for left and right circularly polarized G band phonons separately. 

Monolayer transition metal dichalcogenide semiconductors are promising valleytronic materials. Among various quasi-particle excitations hosted by the system, the valley polarized holes are particularly interesting due to their long valley lifetime preserved by the large spin–orbit splitting and spin–valley locking in the valence band. Here we report that in the absence of any magnetic field a surprising valley splitting of exciton polarons can be induced by such valley polarized holes in monolayer WSe2. The size of the splitting is comparable to that of the Zeeman effect in a magnetic field as high as 7 T and offers a quantitative approach to extract the hole density imbalance between the two valleys. We find that the density difference can easily achieve more than 1011 per cm2, and it is tunable by gate voltage as well as optical excitation power. Our study highlights the response of exciton polarons to optical pumping and advances understanding of valley dependent phenomena in monolayer transition metal dichalcogenide. 

Monolayer molybdenum di-selenide (1L-MoSe 2 ) stands out in the transition metal dichalcogenide family of materials as an outlier where optical generation of valley polarization is inefficient. Here we show that using charge doping in conjunction with an external magnetic field, the valley polarization of 1L-MoSe 2 can be controlled effectively. Most remarkably, the valley polarization can be tuned to negative values, where the higher energy Zeeman mode emission is more intense than the lower energy one. Our experimental observations are interpreted with valley-selective exciton-charge dressing that manifests when gate induced doping populates predominantly one valley in the presence of Zeeman splitting.