Quantum criticality and new phases at ultralow temperatures

Excitations freeze out as we cool down solids to low temperatures. At the absolute zero except for zero-point fluctuations deemed undetectable in macroscopic solids everything is totally silent, with one exception: quantum critical points where the material undergoes strong collective quantum fluctuations. Quantum criticality describes these fluctuations and their extension to finite temperature. Quantum critical behavior has been reported in various systems including the cuprate and pnictide superconductors, low-dimensional and frustrated quantum magnets, organic conductors, multilayer 3He films, and heavy fermion compounds. The latter have emerged as prototypal systems in the past decade. A wealth of studies has provided a good understanding of some aspects, but many important questions remain open. For instance, universal scaling curves over decades in temperature the cornerstone of classical criticality are absent for most quantum critical materials. Just as importantly, it is unclear whether the presence of unconventional superconductivity or other new phases near quantum critical points is universal or distinguishes between different classes of quantum critical points. In the present project we set out to answer these and related questions by extending the accessible temperature range by more than two orders of magnitude beyond state-of-the-art, to the microkelvin temperature range. We will do so by using a nuclear demagnetization cryostat operated by our group. With our recent breakthrough in electrical transport measurements to below 100 microkelvin we now have the unique capability to significantly advance the field. Some of the most interesting quantum critical materials, both made in house and provided by international collaborators, shall be studied. Intense interaction is planned also with renowned theorists. We expect our studies to answer key open questions in the fields of heavy fermion systems and quantum criticality but also to reveal entirely new phenomena, with ramifications for other fields, including quantum criticality in other materials classes, spintronics, quantum information, and high-energy physics.

Quantum criticality and new phases at ultralow temperatures Excitations freeze out as we cool down solids to low temperatures. At the absolute zero - except for zero-point fluctuations deemed undetectable in macroscopic solids - everything is totally silent, with one exception: at a quantum critical point matter undergoes strong collective quantum fluctuations. Quantum criticality describes these fluctuations and their extension to finite temperature. Quantum critical behavior has been reported in various systems including the cuprate and pnictide superconductors, low-dimensional and frustrated quantum magnets, organic conductors, multilayer 3He films, and heavy fermion compounds. The latter have emerged as prototypal systems in the past decade. A wealth of studies has provided a good understanding of some aspects, but many important questions remain open. For instance, do robust quantum critical scaling laws exist? Which kind of quantum critical behavior can lead to unconventional superconductivity? With the present project we set out to answer these and related questions by extending the accessible temperature range by more than two orders of magnitude beyond state-of-the-art, into the microkelvin temperature range. Using a nuclear demagnetization cryostat we realized high-resolution electrical transport measurements on a prototypal heavy fermion material to temperatures well below 1 milikelvin. As one of our key results we discovered unconventional superconductivity condensing out of an extreme "strange metal" state - where electrical resistivity is strictly linear in temperature over 3.5 orders of magnitude. We propose that the Cooper pairing is realized by the unusual fermionic quantum critical fluctuations associated with the underlying "Kondo destruction" quantum critical point (where the Kondo effect breaks up), a mechanism that may well be pertinent also to other materials classes. We expect our results on an ultralow temperature superconductor to boost progress also in other materials classes, including the high-temperature cuprate superconductors.