|
Partner Group for Nonmagnetic Kondo Effect
Research on nonmagnetic Kondo effect arising from structural two-level systems is a main objective of Low Temperature Laboratory at the  Institute of Low Temperature and Structure Research, Polish Academy of Sciences (ILT&SR PAS) in Wrcoław. This interdisciplinary scientific project linking Solid-State Chemistry and Metal Physics is carried out in the frame of a Partner Group of the Max Planck Institute for Chemical Physics of Solids (MPI CPfS) in Dresden.
Commonly, the Kondo effect covers a number of unusual low-temperature transport and thermodynamic properties in metals containing a small concentration of paramagnetic impurities. It is named after Jun Kondo who, in 1964, was able to explain the microscopic origin of a low-temperature minimum in the temperature dependence of the electrical resistivity in metals known since the early 1930s. Though the Kondo effect was originally devoted to a specific feature in condensed matter, it subsequently turned out to represent a quite general phenomenon. The latter concerns interaction processes of conduction electrons by scattering centers with dynamical internal degrees of freedom such as spins. Thus, for instance, the transport and magnetic properties of quantum dots, which play an important role in nanotechnology, are governed by the Kondo effect.
Metallic systems containing localized structural defects changing positions (two-level systems) also belong to the class of general Kondo materials. This was pointed out first by J. Kondo himself in 1976 and subsequently became an area of intensive theoretical research. According to the predictions of these efforts, an anomalous increase of the electrical resistivity upon cooling arises. The underlying process leaves of the conduction-electron spin unaltered. This is described by a two-channel “non-magnetic” Kondo effect, in which charge transport is taking place in two independent channels that differ by the orientation of the conduction-electron spin.
Work is dedicated to chemically homogenous, but nevertheless structurally disordered, single crystals of the tetragonal M-As-Se (M = Zr, Hf, Th) phases which are synthesized and characterized at the MPI CPfS. Diamagnetic M-As-Se metals display an unusual low-temperature dependence of the electrical resistivity ρ(T). At temperatures about 16 K, all investigated specimens show a shallow minimum in ρ(T) that is followed by its square-root temperature increase upon cooling. Irrelevance of an applied field up to B = 14 T on the –AT1/2 amplitude clearly highlights a non-magnetic origin of the effect observed. A universality of the magnetic-field-independent –AT1/2 term in the resistivity of M-As-Se phases strongly suggests a crucial role of the disorder in the As-Se sublattice. This points at a realization of non-magnetic Kondo effect derived from dynamical disorder.
Experiments at the ILT&SR PAS are carried out using thermodynamic (semi-adiabatic calorimetry), magnetic (high-resolution ac-susceptibility) and magnetotransport techniques over wide ranges of temperature (down to 35 mK) and magnetic fields (up to 14 T).
|