Interplay of charge and magnetic ordering in rare-earth nickel dicarbides.

• Applicant: Dr. Sofia Michaela Souliou
• Subject Area: Experimental Condensed Matter Physics
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 441231589

The focus of the current proposal is the interplay between charge density wave (CDW) and magnetic ordering in rare-earth nickel dicarbides. CDW ordering is observed almost universally in the RNiC2 (R – rare earth) family, with onset temperatures scaling linearly with the unit cell volume and with periodicities both commensurate and incommensurate to the crystal lattice. The 4f rare-earth magnetic moments order at temperatures lower than the onset of the CDW, forming ferromagnetic and antiferromagnetic states with matching or distinct ordering vectors to the ones of the pre-existing charge modulations. Although there is evidence for a strong interplay between the two types of ordering, this has a non-uniform character upon varying the rare-earth element, depending in an unclear manner on the details of the CDW and magnetic states. Ambiguous remain also the microscopic mechanism of the CDW formation, the parameters controlling the CDW characteristics (e.g. the periodicity) and the overall significance of the Fermi surface nesting for the CDW. In this framework, a central question concerns the role played by the lattice dynamics. Despite indirect indications for the presence of soft phonons, the lattice dynamics and their potential contribution in driving the CDW and mediating its interplay with magnetic ordering remain fully unexplored in the entire family.We propose a work program which addresses these open issues, combining sample growth, laboratory and synchrotron-based diffraction and phonon spectroscopy with density functional theory calculations. More specifically, we will grow and characterize single crystals of several selected members of the RNiC2 series, exploiting its rich CDW and magnetic landscape and targeting a thorough and comparative survey. The characteristics of the CDW order, their development across the magnetic transitions and the connection to the Fermi surface nesting properties will be studied experimentally by x-ray diffraction and diffuse scattering and will be compared to the results of electronic structure calculations. Raman spectroscopy will be employed for the study of the zone center phonons, CDW gap and modes, as well as of the magnetic excitations and their potential coupling to the lattice and CDW modes. The presence and features of soft phonons across the CDW and magnetic ordering wavevectors will be explored using inelastic x-ray scattering and will be complemented by calculations of the lattice dynamics and the electron-phonon coupling.Our project will contribute to the ongoing scientific effort to understand the interplay between electronic orders of different origin in condensed matter physics and will offer an overview of the phonons’ involvement in monitoring it. Finally, it will allow to directly test the limits of the classic nesting driven CDW formation picture in a real system with quasi-1D structural characteristics.

• DFG Programme: Research Grants