A quantum critical point (QCP) develops in a material at the absolute zero in temperature when an order parameter vanishes continuously by variation of a non-thermal tuning parameter such as pressure, magnetic field, or chemical composition. QCPs are of great interest because of their singular ability to influence the finite temperature properties of materials, be it by creating unconventional excitations or by stabilizing novel emergent phases. Because of their low and competing energy scales, heavy fermion systems are highly tunable and continue to play a key role as model systems of various forms of quantum criticality.

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Fig.1: Sketch of a temperature (T)– magnetic field (B) phase diagram around an unconventional quantum critical point

Our current investigations focus on:

• The cubic compound Ce₃Pd₂₀Si₆ with a quartet ground state: Having both spin and orbital degrees of freedom leads not only to antiferromagnetic and antiferroquadrupolar order, but also to a sequence of two Kondo destruction QCPs.
• The canonical system YbRh₂Si₂: The Néel temperature of only 70 mK of this heavy-fermion antiferromagnet is continuously suppressed by application of small magnetic fields. Measurements of the Hall effect have indicated a collapse of the Fermi surface at the QCP, calling for entirely new theoretical descriptions. The dynamical critical scaling we have recently observed in the THz conductivity shows that charge degrees of freedom are an integral part of the criticality, providing a natural explanation for the Hall effect jump.

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Fig.2: Temperature-magnetic field phase diagram of Ce₃Pd₂₀Si₆, featuring a sequence of two Kondo destruction quantum critical points (square and star), where spin and orbital degrees of freedom selectively disentangle from the conduction electrons [from Martelli et al., Proc. Natl. Acad. Sci. USA 116, 08101R (2019)]

• Very recently, we have observed indications of quantum criticality in Ce₃Bi₄Pd₃ under high magnetic field.
• We constantly expand our spectrum of synthesis and characterization methods. Recent achievements are the setting up of electrical resistivity measurements at ultralow temperatures (to below 1 mK) and the epitaxial growth of heavy fermion compounds by molecular beam epitaxy.

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Fig.3: Dilution refrigerator for the investigation of low-temperature properties of quantum critical compounds

For details, see the list of publications or contact us.