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We report the discovery of a new phase diagram in the magnetic fieldtemperature plane for single crystals o£ La2Cu04 that show no sign of antiferromagnetic order. The phase boundary is defined by magnetoresistance and sucseptibility measurements. We discuss several possibilities that may lead to the phase diagram observed.

We present results of specific heat, electrical resistance, and magnetic susceptibility measurements on the heavy fermion system CePtSi and compare it to CeRuSi. Both compounds show a low-temperature maximum in cp/T, usually associated with the formation of a coherent heavy fermion ground state. An additional feature in cp/T is observed for CePtSi and is attributed to cooperative magnetism, an interpretation confirmed by magnetic measurements. A comparison with other heavy fermion systems, especially stoichiometric and nonstoichiometric CeCu2Si2, is performed.