Novel states of matter in ferromagnetic quantum metals

Tuning a material to the border line between competing ground states has become a successful guiding principle for discovering novel states of matter. Prototype examples are the high-temperature copper oxide superconductors. The suppression of the antiferromagnetic state in cuprates gives way to unconventional superconductivity. Similar behavior is found in heavy-fermion materials, iron-based superconductors or organic metals. At zero temperature this border line ends to what is called quantum phase transition (between the ground states) and in case of a continuous phase transition this point is called quantum critical point (QCP). During the last four decades an incredibly large number of systems where investigated in the region of the phase diagram close to their QCP. And with surprisingly spectacular results: Even ferromagnets could be tuned into a superconducting state, although the conventional picture (based on the BCS theory) would not allow it.

The most common way to tune the ground state of a material is applying pressure. This is

because with this method no disorder is induced in the material, which would be the case with chemical substitution or doping. Another “clean” way of modifying the ground state is by

applying an external static magnetic field. One textbook example is the Ising-like ferromagnet in transversal field. By applying a magnetic field perpendicular to the magnetic moments of a Ising-like ferromagnet the ferromagnetic state can be suppressed at a certain critical field. Here, we have a QCP where the fluctuations of the order parameter (the magnetization) become strongest and we might have the chance to observe new phases of matter.

In our Institute we have already grown and characterized low-temperature Ising-like ferromagnetic metallic single crystals like, e.g., CeRuPO. The goal of the applicant would be to prepare samples out of the large crystals for resistivity measurements and to carry out such measurements in a standard low-temperature device equipped with a static magnetic field.