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This special issue of physica status solidi (b) is devoted to the vibrant research on quantum criticality and novel phases. This research field is at the heart of current interest in condensed matter physics and continues to be highly topical.

It started in the 1990s when it was first realized that intermetallic materials at a magnetic instability, i.e., at a continuous transition from a magnetically ordered to a non-ordered ground state at absolute zero temperature, do no longer follow the predictions for normal metals. Instead, they exhibit unusual low-temperature properties due to the inherent quantum nature of the relevant critical spin fluctuations. Such transitions are therefore referred to as quantum critical points. In some materials even new phases, e.g., unconventional superconductivity, appear in the vicinity of quantum critical points. Quantum criticality and research on concomitant phenomena has seen considerable experimental as well as theoretical progress during the past years and is characterized by a strong mutual interplay of theoretical and experimental work.

In this special issue some of the most recent developments are highlighted. The presented research covers a broad spectrum of the different aspects of quantum criticality and includes related phenomena in quantum spin systems, itinerant d-electron systems as well as lanthanide- and actinide-based heavy fermion compounds. In addition, novel phases in the vicinity of quantum critical points, such as unconventional superconductivity or spin glass behavior, are intensively studied.

We would like to thank all authors for their well-prepared contributions to this special issue and for keeping the time despite the quite tight publication schedule. We trust that the papers in this special issue represent an excellent overview of the various experimental and theoretical activities in the field of quantum criticality and novel phases, and will therefore prove very useful for a broad readership.

Stefan Kirchner, Oliver Stockert, Steffen Wirth

(Guest editors)