Optical Spectroscopy of Novel Semiconductors in High Magnetic Fields

Understanding new quantum phenomena and properties of new materials is the foundation of condensed matter physics. One can mention celebrated examples of integer and fractional quantum Hall effect, Aharonov-Bohm quantum interference effects, inventions of heterostructures and superlattices, and a recent discover of Dirac-like quasiparticles in atomically thin 2D crystals. Here we employ optical spectroscopy combined with large magnetic field and low temperatures to probe the electronic structure of several novel semiconductor materials. The discovery of graphene has opened the door to the study of other 2D materials. Here we focus on a new family of semiconducting layered 2D materials known as transition metal dichalcogenides (TMDs), which have recently emerged as a new class of direct bandgap 2D semiconductors with two degenerate, but non-equality valleys at the ±K points in the Brillouin zone. Due to the broken inversion symmetry in monolayer TMDs, this valley degree of freedom can be selectively addressed by optical helicity, opening the possibility for valleytronic and optoelectronic applications. By performing valley selective photoluminescence measurements on TMDs we demonstrate the lifting of the valley degeneracy and valley polarization in an applied perpendicular magnetic field. One of the most remarkable properties of graphene is its linear dispersion. Once relegated only to the realm of theoretical exploration, the past ten years has seen an explosion in the realization of new Dirac-like materials in condensed matter systems. One of the most important of these new Dirac-like materials is HgTe quantum wells (QWs). Here, we report on Landau level spectroscopy studies of a series of HgTe QWs grown near or at the critical well thickness, where the band gap vanishes. We observe a square root B dependence for the energy of the dominant cyclotron resonance (CR) transition over the broad range of magnetic fields, characteristic of Dirac fermions. While not in the same family of Dirac-like or 2D materials, metamorphic InAs₁₋xSbx alloys are a promising narrow gap semiconductors with bandgaps as low as 0.1 eV. New growth techniques now allow for the realization of bulk unstrained, unrelaxed InAs₁₋xSbx layers. Here, we report on the systematic study of the electronic properties of a series of InAs₁₋xSbx alloys over a broad range of Sb concentrations by infrared magneto-absorption and magneto-transport.