Cyclotron Resonance and Quasiparticles

This introductory paper contains personal perspectives about the importance of cyclotron resonance in forming our modern view of solids. The papers following this one will discuss the discovery, refinements, and some of the latest developments. Although I will touch on some of these subjects, I leave the details to the other authors and in the main focus on the conceptual impact of the work. I propose that it was experiments based on cyclotron resonance which established the quasiparticle concept. In a mature field like condensed matter physics, a large effort is devoted to testing the newest ideas. Although one is aware that traditional models and concepts are being used to think about the newest ideas, it isn’t common to ask about the foundations, conceptions, or assumptions about our field until something doesn’t fit. We just use these concepts. For example, electronic band structures and effective masses of electrons and holes in solids are standard tools or concepts for interpreting experiments. Another case is the quasiparticle picture which assumes that electrons can act as “dressed” particles. Examples of quasiparticles include the polaron and the concept of a hole. Both are very convenient and useful ways of interpreting experimental data. Many younger researchers who use these ideas daily may feel that the concepts above are “obvious and natural” and have been here forever. However, when one views a material as a collection of interacting atoms, it’s almost a leap of faith to embrace the elementary excitation picture of a solid with its collective excitations and quasiparticles. It’s easy to state or demonstrate mathematically that lattice vibrations can be viewed as phonons and collective electronic motion can be represented by plasmons. Sometimes we lose touch with the fact that these and other boson-like excitations such as magnons are mental constructions. Similarly for the fermion-like excitations, one cannot make a beam of holes propagating across an empty tube, but to many of us, holes exist. So it is important to measure response functions such as dielectric functions and magnetic susceptibilities and show experimentally that they can be interpreted for the most part in terms of boson and fermion excitations such as the collective excitations and quasiparticles commonly listed in descriptions of the elementary excitation model of solids. One can then argue that since they describe physical behavior, they exist. Similarly, the concept of energy bands can be established from an interacting atom’s picture where atomic energy levels are perturbed and bands of allowed states are formed; however the calculated dispersion of the bands and the concept that individual particle-like excitations will occur near band edges are not obvious. In a sense some of this can be included with the general idea of a quasiparticle concept. The effective mass, in principle, can contain both band structure effects and electron mass renormalization arising from interactions of the electron with other elementary excitations. The measurement of effective masses using cyclotron resonance gives information about the interaction of an electron with the periodic potential and also about the formation of electronic quasiparticles such as polarons. In addition, band structure effects such as anisotropy, spin-orbit splittings, and band dispersions can be measured to test the calculations and the underlying conceptual models.