A quantum critical point (QCP) develops in a material at the absolute zero in temperature when an order parameter is continuously suppressed to zero by variation of a non-thermal tuning parameter such as pressure, magnetic field or chemical composition. QCPs are of great current interest because of their singular ability to influence the finite temperature properties of materials. Recently, heavy-fermion metals have played a key role in the study of antiferromagnetic QCPs.

Our recent investigations have focussed on several systems:·

• YbRh₂Si₂ (image): 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 indicate a collaps of the Fermi surface at the QCP, calling for entirely new theoretical descriptions. In collaboration with MPI CPfS Dresden.
• Yb₂Pd₂(In_1-xSn_x): The phase diagram of this series of compounds is characterized by the observation that both end members do not show long range magnetic order. Magnetic instabilities, however, develop in a narrow concentration range for compounds rich in Sn. This particular feature enables the occurrence of two QCP’s within a single series of compounds. Such a feature was never found before in cerium or ytterbium based heavy fermion systems and allow to study non-Fermi liquid phenomena on two distinct different sites of a magnetically ordered regime. In co-operation with University of Genova.
• CeNi₉Ge₄ shows unusual non-Fermi liquid behaviour of the specific heat over at least two decades in temperature with a record value of the electronic specific heat coefficient γ at low temperature: γ ~ 6 J/molK² at 80mK in this system without any sign of magnetic order. The ground state properties are currently studied by neutron and muon spectroscopy and single crystal investigations.
• LaCo₉Si₄ and YCo₉Si₄ exhibit itinerant electron metamagnetism and weak itinerant ferromagnetism, respectively, both being very close to a ferromagnetic quantum critical point. Current focus is on single crystal growth and high pressure studies.

Contact: E. Bauer, S. Bühler-Paschen, H. Michor

Contact - Theory: K. Held