Calorimetry at Ultra Low Temperatures to study NFL Behaviour

Fermi liquid (FL) theory has been very successful in describing complex metals with strongly correlated electrons where 3d, 4f or 5f electrons are involved giving rise to a strong electron mass enhancement of about up to three orders of magnitudes. Therefore these compounds are referred to as Heavy Fermion (HF) systems. Under certain conditions, however, this model fails to explain the observed thermodynamic and transport properties of HF compounds: Fermi-liquid instabilities may occur, when the temperature of a second order phase transition (as e.g. magnetic to non-magnetic transition) is shifted towards 0K. This leads to Non-Fermi Liquid (NFL) properties in the vicinity of a Quantum Critical Point (QCP). There are several scenarios which may lead to NFL behaviour and a unified theory to predict the thermodynamic and transport properties is not yet available. In order to prove and/or falsify the different scenarios it is necessary investigate new systems and to conduct the experiments down to ultralow temperatures. Therefore the set up and extension of susceptibility- resistivity- and heat capacity measurements from the Kelvin range down to the mK range in the available 3He/4He dilution refrigerator with external fields up to 17T is one of the experimental aims of this project with respect to measuring techniques and new development of instrumentation. Three types of intermetallic compounds are chosen to tune a quantum phase transition either by composition, or by hydrostatic pressure and/or by external fields to zero temperature from which we expect valuable new information in order to contribute to a deeper understanding of the ground state of these materials and to the origin of non- Fermi-liquid properties: The first category contains a non-magnetic HF compound as YbCu5 where by proper alloying a magnetic ground state can be achieved. From the tuning of these systems through the magnetic instability with Cu substitutions by different transition metals in YbCu5-xTx (T = Ag, Au, Al, Ga, Sn) we expect contributions to the understanding NFL properties and the influence of disorder. It is not yet clear how disorder modifies or whether it even produces NFL behaviour. Preliminary specific heat measurements of (GdxY1-x)3Co show in the vicinity of the critical concentration for the onset of magnetism a logarithmic divergence of Cp/T at low temperatures which might be related with a quantum critical behaviour. If this speculation can be proved (GdxY1-x)3Co would be the first system where the breakdown of long range order with stable and strongly localized 4f moments causes quantum critical behaviour. The second category contains an antiferromagnetic HF compund where hydrostatic pressure or an external magnetic field is used to tune these systems through their magnetic instability. CeNi9Ge4 is a novel intermetallic compound with extremely large quasiparticle mass where preliminary susceptibility measurements indicate that the magnetic instability occurs within the homogeneity range of this ordered compound. This offers the possibility to study the origin of NFL properties at ambient pressure in an ordered compound without disorder which is of great importance for fundamental solid state physics.

Fermi liquid theory has been very successful in describing complex metals with strongly correlated electrons where 3d, 4f, or 5f electrons are involved giving rise to a strong electron mass enhancement of about up to three orders of magnitudes. Therefore these compounds are referred to as Heavy Fermion (HF) systems. Under certain conditions, however, this model fails to explain the observed thermodynamic and transport properties of HF compounds: Fermi-liquid instabilities may occur when the temperature of a second order phase transition (as e. g. magnetic to non magnetic transition) is shifted to zero temperature by an external parameter (e.g external magnetic field or pressure). This leads to quantum fluctuations giving rise to significant deviations from Fermi liquid behaviour (i.e. Non-Fermi-Liquid (NFL) properties) in the vicinity of a Quantum Critical Point (QCP). We found novel intermetallic compounds with the stoichiometry RT 9 X4 (R = rare earth, T = Co, Ni and X = Ge, Si) crystallizing in the LaFe9Si4 structure type. While CeNi9 X4 (X = Si, Ge) exhibit heavy Fermion and Non- Fermi-Liquid properties, respectively, YCo 9 Si 4 is a weak itinerant ferromagnet. LaCo9 Si 4 is a spinfluctuation system which exhibits an itinerant metamagnetic transition at about 4T that is the lowest metamagnetic transition found in rare earth -3d intermetallics. The observation of weak itinerant ferromagnetism in LaCo 13-x Si x where the ordering temperature Tc approaches zero temperature approximately at the stoichiometric composition 1-9-4 suggests that LaCo9 Si 4 may be in the vicinity of a ferromagnetic QCP. We could show that CeNi9 Si 4 is a Kondo lattice system and CeNi9 Ge4 exhibits non-Fermi liquid properties characterized by a logarithmic increase of the specific heat divided by temperature (C/T). The latter serves as a measure of the electron mass enhancement which is supposed to diverge at the QCP. The value of C/T at about 80 mK is the largest value recorded so far and corresponds to an electron-mass enhancement of about 6000. Coexistence of antiferromagnetism and superconductivity was found in the novel non-centrosymmetric CePt 3 Si compound which is the first heavy Fermion superconductor without an inversion symmetry. This has important consequences for the pairing mechanism to form Cooper pairs and appears to be a challenge for new theoretical developments. The appearance of superconductivity below antiferromagnetic order which is also observed in the system CePt 3 (Si,Ge) is in accordance with the generic phase diagram, where quantum-criticality plays an important role and the pairing mechanism might arise from magnetic fluctuations.