More about the programme
This ESF Programme was established in the fields of Physical and Engineering Sciences (PEN (formerly PESC)). The FERLIN programme offerred different types of grants for both young and senior scientists to foster exchange and joint experiments, and organised workshops relevant to the programme. The Steering Committee which had guided the programme was composed of expert scientists nominated by ESF Member Organisations in 9 countries (Austria, Belgium, France, Germany, Slovak Republic, Slovenia, Spain, Switzerland and United Kingdom) which contributed to the programme. FERLIN brochure (PDF 121 KB)
Scientific Background
The standard model for metals: Fermi liquids
The standard model of conduction electrons in metals presumes that the interaction among electrons can be described by a few parameters such as the effective mass which differs from the three-electron mass. This renormalisation leads to effectively independent quasiparticles that can be treated within the framework of Fermi-liquid theory. Even heavy-fermion systems - metals with a high concentration of rare-earth or actinide elements such as Ce, Yb, or U - can be regarded as Fermi liquids. The strong interactions between electrons in these materials lead to a very large effective mass m* derived from the huge linear specific-heat coefficient g = C/T and a correspondingly large Pauli susceptibility c , with C/Ta c = const at sufficiently low temperatures T.
Fermi-liquid Instabilities
Recently, striking deviations from the Fermi-liquid (FL) behavior have been found in several heavy-fermion systems, e.g. C/T a -1n(T/T0 ). In some cases, a linear T dependence of the electrical resistivity is found instead of the T2 Fermi-liquid dependence. This non-Fermi-liquid (NFL) behavior may have different microscopic origins such as the single-ion quadrupolar Kondo effect, a collective effect caused by the incipient antiferromagnetic order, or simply a distribution of Kondo temperatures arising from disorder. The scenario of incipient magnetic order is exemplified in CeCu6-xAux where concentration and pressure tuning are employed to reach the quantum critical point. In other cases such as CePd2Si2 or CeIn3 magnetism gives way to superconductivity upon approaching the critical point. 3d itinerant-electron systems such as MnSi and ZrZn2, as well as some further heavy-fermion systems, follow a behavior that can be explained in terms of spin-fluctuations. U systems, on the other hand are often described within single-ion models.
Scientific programme
The aim of the project was to delineate the similarities and differences of the different classes of metals with strong electronic correlations which exhibit a FL instability by a variety of macroscopic (thermodynamic and transport) and microscopic (neutron scattering, µSR) measurements. This has allowed us to make a definite assignment of the non-Fermi-liquid behavior in a given system to one of the above scenarios and to shed light on the microscopic origin, in particular on the type of excitations that are responsible for NFL behavior at the critical point of the incipient magnetic order.
Specific materials had been prepared - mostly in single-crystalline form - and investigated with the various techniques available by a collaborative effort of leading European laboratories. An important point investigated was the difference in the behavior at a quantum critical point of HFS vs. 3d itinerant magnets. Microscopic probes such as neutron scattering are necessary in addition to detailed macroscopic measurements in order to identify the relevant magnetic fluctuations causing NFL behavior. An interesting comparison between Ce compounds (one 4f electron) and Yb compounds (one 4f hole) is possible with pressure tuning. Both provide examples for FL instabilities. However, in Ce compounds a transition from magnetic to non-magnetic state occurs with increasing pressure, while for Yb compounds this transition occurs with decreasing pressure.
Another issue of considerable importance is the influence of disorder on the NFL properties. Disorder may play a role at two different levels: First in a disordered alloy, a distribution of Kondo temperatures is possible due to different local environments. The incoherent superposition of local Fermi liquids may lead to NFL behavior. Second, disorder is known to influence the critical behavior of classical phase transitions. Its influence on quantum phase transition is yet to be determined. We have addressed this problem by a comparison of NFL behavior of stoichiometric compounds and alloys with substitution on magnetic and nonmagnetic sites.
The intriguing observation that magnetism gives way to superconductivity approaching the critical point where it is suppressed may suggest that superconductivity is mediated by spinfluctuations instead of lattice vibrations in classical superconductors. A detailed study of the properties around the magnetic instability was required to lead to a microscopic understanding.
