' s personal copy Excitons in Magnetic Fields

When a photon of energy greater than the band gap is absorbed by a semiconductor, a negatively charged electron is excited from the valence band into the conduction band, leaving behind a positively charged hole. The electron can be attracted to the hole via the Coulomb interaction, lowering the energy of the electron-hole (e-h) pair by a characteristic binding energy, Eb. The bound e-h pair is referred to as an exciton, and it is analogous to the hydrogen atom, but with a larger Bohr radius and a smaller binding energy, ranging from 1 to 100 meV, due to the small reduced mass of the exciton and screening of the Coulomb interaction by the dielectric environment. Like the hydrogen atom, there exists a series of excitonic bound states, which modify the near-band-edge optical response of semiconductors, especially when the binding energy is greater than the thermal energy and any relevant scattering rates. When the e-h pair has an energy greater than the binding energy, the electron and hole are no longer bound to one another (ionized), although they are still correlated. The nature of the optical transitions for both excitons and unbound e-h pairs depends on the dimensionality of the e-h system. Furthermore, an exciton is a composite boson having integer spin that obeys Bose-Einstein statistics rather than fermions that obey Fermi-Dirac statistics as in the case of either the electrons or holes by themselves. One of the most interesting consequences of the bosonic nature of excitons is the possibility of having many that occupy the same quantum state as opposed to fermions, which obey the Pauli exclusion principle. An applied magnetic field, B ! , will quantize the energies and angular momenta of charged particles and thus change the electronic state of an exciton. In general, the excitonic wavefunction shrinks in the direction perpendicular to B ! , and the exciton binding energy increases with B1⁄4 jB !j. How the energy of each exciton state varies with B depends on basic exciton parameters, and, therefore, the study of excitons in B provides a tool to characterize various exciton properties, such as the reduced mass, binding energy, Bohr radius, and g-factor. Importantly, the nature of the dependence of exciton energy on B depends on the strength of B. For example, at low B, where the effect of the Coulomb interaction is larger than that of B and thus B is treated as a perturbation, excitons retain a hydrogenic nature: diamagnetic shifts and Zeeman splittings modify the energies of excitonic states. Alternatively, in the high B limit where the effect of B dominates and the Coulomb interaction is treated as a perturbation, the energies of exciton states show Landau-level-like, linear-B dependence, similar to free e-h pairs. For the hydrogen atom, the binding energy, 13.6 eV, is much higher than those for excitons in semiconductors, and an extremely high B (over 2 10 T) is required to enter the high B regime. The much smaller binding energies of excitons in semiconductors allow one to enter the high-B regime readily, which, together with the bosonic nature of excitons, makes excitons in high B attractive for the investigation of many-body phenomena in condensed matter in a highly controllable manner. A large body of theoretical work has thus been devoted in the last several decades to e-h systems in strong B, and a fascinating array of possible physical phenomena/states has been predicted – the excitonic insulator phase, a gas-liquid-type phase transition, Bose-Einstein condensation of magnetoexcitons, and quantum chaos. Recently, attention has been paid to spatially separated two-dimensional (2D) electrons and holes in layered structures, since the pioneering work of Lozovik and co-workers, who considered the pairing of electrons and holes across an interface of two-media, and the superfluidity of such pairs. A very interesting but complicated many-particle situation arises, in the presence of strong perpendicular B. In such a situation, spatially separated 2D electrons and holes, both Landau quantized, are present, and there is a Coulomb attraction between them as well as electron-electron (e-e) and hole-hole (h-h) correlations. However, all these interactions are expected to cancel each other in the high-B limit, where the “hidden symmetry” of 2D magnetoexcitons protects them from ionization, leading to the absence of an excitonic Mott transition. Furthermore, excitonic gain enhancement occurs in a high-density e-h system in high B, which leads to cooperative and coherent light emission (known as superfluorescence) from the highest-occupied magnetoexciton state. Here, we will review various observations related to excitons in magnetic fields. First, we present the theory of excitons in magnetic fields and show how the exciton energy's magnetic field dependence relates to fundamental exciton parameters. We will explore how different dimensionalities of excitons and the strength of magnetic fields affect the exciton properties. Then, interband and intraband magneto-optical absorption observations are discussed in different semiconductor systems. Finally, some exotic phenomena unique to high-density excitons in high magnetic fields are presented.