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SEMICONDUCTORS AND NANOSTRUCTURES 2009Oscillatory magneto-absorption under pressure in Indium SelenideIndium selenide is a layered semiconductor widely investigatedin the last decades for its potential optoelectronic applicationscharacterized by an anisotropic anomalous bandstructuredue to the balance between covalent and van derWaals bonds. Various experiments have revealed a puzzlingbehavior for the electron effective mass.Modern ab-initio band structure calculations by NAO-DFTcombined with optical measurements under high pressurehave explained this anomaly and have also unveiled newinteresting features such as the onset of a ring-shaped valenceband maximum above 2 GPa. In fact, a specific k.pmodel has recently been proposed for this class of layeredsemiconductors [Segura et al., Phys. Stat. Solidi B 235(2),267276 (2003)].Oscillatory magneto-absorption spectroscopy measurementsunder high pressure yield a very deep insight intothe unusual electronic properties of this compound. We obtainedfor the first time clear signature of the Landau quantizationin a wide energy range above the fundamental gap,giving access to the conduction band non-parabolicity (figures59 and 60). In addition, the high field regime for theexcitonic feature has been studied.The pressure dependence of the magneto-fingerprints unveilan increase of the reduced effective mass similar withthe evolution of the second gap transition, which confirmsthe main hypothesis of the specific k.p model. Moreover,the reentrant behavior of the pressure-quenched excitonpeak under high magnetic field (figure 61) unveils anunusual ring-shaped valence band maximum with a smallhole effective mass as suggested by previous transport experimentsunder pressure and ab-initio calculations.Figure 59: Oscillatory magneto-absorption spectroscopy colorplot of InSe measured at 4 K up to 53 T. The excitonic groundstate feature is clearly distinguishable around 1.34 eV as well asinterband Landau level transition which energy increases linearlywith the applied magnetic field.Using a toy model taking into account a linear Landau gapopening and the reported pressure dependence of the valenceband maxima and conduction band minima form previousoptical measurements under pressure we can estimatethe hole mass on the secondary maximum to be ∼ 0.03 m 0 .Figure 60: Interband Landau fan chart recorded in the diamondanvil cell.Figure 61: Reentrant behavior of the pressure-quenched excitonpeak under high magnetic field. This unveils an unusualring-shaped valence band maximum with a small hole effectivemass.M. Millot, S. George and J.M. BrotoA. Segura (ICMUV, Valencia, Spain)46