Heavy fermion systems are a class of strongly correlated electron systems in which charge carrier can have masses as large 1000 times the free electron mass. This large effective mass often derives from f orbitals of rare earth or actinide atoms. The ground state of heavy fermions is easily tuned and controlled with an external thermodynamic parameter. For instance, magnetic phases can be suppressed at a quantum phase transition with pressure or magnetic field.
Above we show the remarkable phenomenon of re-entrant superconductivity that occurs around a field-induced spin-rotation transition in the field-temperature phase diagram of URhGe.