Exploring heavy fermions from macroscopic to microscopic length scales
Strong electronic correlations in solids may result in fascinating phenomena like unconventional superconductivity and quantum criticality in heavy fermion metals. The latter provide a particularly promising playground to explore the coexistence and competition of different phases as much as the emergence of novel phases. As such, research on heavy fermion materials continues to thrive at the forefront of solid state physics while challenging our understanding.
This issue becomes immediately obvious upon comparing the phase diagrams of such heavy fermion materials that exhibit superconductivity around a so-called quantum critical point with such highly discussed superconductors like the cuprates, iron-based or organic superconductors which share a number of commonalities. Hence, albeit heavy-fermion research is certainly mostly fundamental in nature, it may help paving the way for important applications by providing clues for future materials design for, e.g., exploiting superconductivity or thermoelectrics.
The present review gives some introductory perspective into the development of the field of strongly correlated electron systems over the past four decades. Special emphasis is laid on the Kondo effect (see figure) in competition with magnetic order which may give rise to quantum critical phenomena. Improvements to several experimental methods provided new insight into the physics of heavy fermion systems in recent years (including magnetic quantum oscillations and scanning tunneling microscopy) which, along with established electronic and thermal transport measurements, allow to draw a picture from macroscopic to microscopic length scales. On the other hand, progress on the theoretical understanding of such systems helped developing a more generalized phase diagram by recognizing the impact of quantum fluctuations. The recent advance of heavy fermion semiconductors is also highlighted.