Research

Quantum phase transitions

One of the most promising routes towards novel states of matter and functional materials with new properties is the investigation of compounds where several phases (e.g. magnetic and nonmagnetic, conducting and insulating) compete with each other. This competition is manifest near a quantum phase transition, where the system switches from one ground state to another.

Quantum phase transitions (QPTs) are phase transitions that occur in a strict sense only at temperature T=0. As opposed to classical phase transitions at finite temperature, quantum effects play a decisive role, as the fluctuations driving the transition follow quantum instead of classical statistical mechanics. Having been considered for a long time as a mere curiosity of theoretical physics, the impact of this field on physics is now emerging with steadily increasing pace.

 

Schematic phase diagram near a quantum phase transition

Schematic phase diagram near a quantum phase transition, as function of temperature T and control parameter δ. QCP denotes the quantum phase transition point, the regimes I and II correspond to the stable phases on either side of the quantum phase transition. At elevated temperatures, a quantum critical regime emerges, which is often associated with non-Fermi liquid behavior (NFL).

Quantum critical points (QCPs) which control continuous QPTs are at the heart of violent quantum fluctuations, a direct consequence of Heisenberg's uncertainty principle. They affect the finite-temperature behavior of condensed matter as well, with a multiplicity of new and unexpected phenomena. For instance, the standard model of metals with electronic interactions, the Landau Fermi-liquid picture, breaks down in the vicinity of a magnetic QPT. In addition, since QPTs occur between nearly degenerate phases whose characteristic energies or energy differences are driven to zero, new small energy scales may become important which would be otherwise masked by the primary energy scales. This has the prospect of finding novel phases around QCP. In an even broader sense, research on quantum phase transitions may play an important role also for the development of new concepts in theoretical particle physics where systems often are at essentially zero temperature.

Research focus

In the DFG Research Unit we will focus on quantum phase transitions in electronic systems, i.e. systems where the electronic, magnetic, and lattice properties are governed by the occurrence of a QCP, with vanishing magnetic order induced by a loss of magnetic moments due to electronic interactions or competing frustrated interactions. In a collaborative effort of experiment and theory, we aim at studying several classes of matter at QCPs: intermetallic f-electron compounds, weak itinerant transition-metal magnets, and transition metal oxides and sulfides. While the range of materials is deliberately rather wide, the issue at stake is extremely well defined and, at the same time experimentally and theoretically challenging. Thus the wide range of materials will help to separate materials-related issues from fundamentally new physics.