| Summary: | When a strongly correlated electron system is at the border of magnetic order close to absolute zero, the normal description, in terms of quasiparticle excitations described by Fermi-liquid theory, is predicted to break down. The cross-over from magnetic order to non-magnetic order at vanishing temperature has been the focus of this investigation. Such phase transitions, induced by changing some parameter other than temperature in the limit of absolute zero, is called a quantum phase transition. In this study the parameter is hydrostatic pressure which non-intrusively reduces the cell volume and, if the compound is already close to the boundary, can suppress it. In this regime novel states such as superconductivity may arise. The compounds studied were three heavy-fermion materials which reside close to this boundary, antiferromagnetic CeRh<SUB>2</SUB>Si<SUB>2</SUB>, non-ordered CeNi<SUB>2</SUB>Ge<SUB>2</SUB>, and ferromagnetic CeIr<SUB>2</SUB>B<SUB>2</SUB>. The experiments involved their synthesis and the subsequent measurement of resistance and susceptibility up to pressures of 25 kbar and down to temperatures of 50 mK. The most remarkable result in this study was the emergence of a new superconducting transition below 1 K in CeNi<SUB>2</SUB>Ge<SUB>2</SUB> above 10 kbar, which may be associated with a second low-temperature phase transition arising in a similar pressure regime. At ambient pressure CeNi<SUB>2</SUB>Ge<SUB>2</SUB> appears to be right at the brink of magnetic order, and exhibits similar characteristics to the isoelectronic compound CePd<SUB>2</SUB>Si<SUB>2</SUB> at the pressure where its magnetic order is critically suppressed. These characteristics include non-Fermi liquid like properties such as quasi-linear resistance and even ambient-pressure superconductivity. It is therefore unusual that further superconductivity emerges which is not associated with suppressed magnetic order.
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