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. 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.
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.
Sketch of a temperature (T)– magnetic field (B) phase diagram around an unconventional
quantum critical point Dilution refrigerator for the investigation of low-temperature properties of quantum critical
compounds 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)]
