Ultra-low Temperature SQUID NMR of Strongly Correlated Electron Systems

Quantum materials reveal their nature at low temperatures, where thermal fluctuations vanish and the system falls into its lowest energy state. However, this state is far from static.

Quantum fluctuations, arising from the uncertainty principle and the exclusion rules of quantum mechanics, give rise to unexpected phenomena, such as the mediated pairing of electrons, leading to unconventional superconductivity. Prototypical compounds to study these phenomena are heavy-fermion (HF) superconductors.

In these materials superconductivity arises near magnetic quantum critical points -- the points at T = 0 where magnetically ordered phases are suppressed and quantum fluctuations are particularly strong. There exists growing evidence that pairing is mediated by magnetic critical fluctuations.

One of the best example is the HF superconductor YbRh2Si2 (Tc < 10 mK) in which strong ferromagnetic (FM) as well as antiferromagnetic (AFM) fluctuations were observed.

To understand the nature and role of FM and AFM fluctuations in this compound and generally in strongly correlated electron systems (SCES) and to explain their impact on quantum criticality and superconductivity - e.g. the parity of the Cooper pairs - we propose to build a broadband nuclear magnetic resonance (NMR) spectrometer.

NMR is a non-invasive and low-dissipation technique, well suited for probing in the stringent ultra-low temperature conditions under which these phenomena occur.

Utilizing a DC SQUID sensor, coupled to the pickup of the NMR spectrometer by a flux transformer, is a way of drastically improving signal-to-noise ratio and enable operation at very low static fields, in contrast to standard NMR techniques. This is essential for studying superconductivity.

Understanding the relationship between quantum fluctuations and unconventional superconductivity is crucial for developing new materials with higher superconducting transition temperatures and for potential applications in quantum devices.