Triangular Quantum Materials under Strain

• Applicant: Professorin Dr. Anna E. Böhmer
• Subject Area: Experimental Condensed Matter Physics
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 568914227

Phase transitions that single out a specific direction in the crystal lattice have received particular attention recently, notably with the ‘nematic’ phases in iron-superconductors. Concomitantly, anisotropic stress/strain as a control parameter has been established widely, since it is the conjugate variable to the rotational-symmetry breaking of a nematic transition. Most studies to date have, however, focused on square-lattice (tetragonal) materials, where anisotropic strain singles out one of two equivalent directions. Materials with triangular lattices are fundamentally different in that uniaxial strain only partially lifts the degeneracy: the strain singles out one of three equivalent directions, which leaves the other two degenerate. Here, we propose to exploit anisotropic strain as a probe and tuning parameter for the charge-density wave (CDW) phases and hidden orders of quantum materials with triangular lattices. Two complementary material platforms have been chosen: (i) the metallic transition-metal dichalcogenides NbSe2, TaSe2 and TiSe2 with a well-known 3Q-CDW phase and multiple indications for (strain- induced) states with additional rotational symmetry breaking, and (ii) the Kagome-lattice compound FeGe which features a recently discovered CDW that is coupled to antiferromagnetic order and a minute monoclinic distortion. With the use of pertinent and complementary experimental techniques for nematicity and charge density waves, that range from transport and thermodynamics to Raman spectroscopy and second harmonic generation spectroscopy, we aim to reveal genuine nematic instabilities with broken C3/C6 rotational symmetry and explore the landscape of intertwined phases in the potentially very rich temperature-strain phase diagrams. Our work will expand the understanding of broken-symmetry phases and strain tuning in important charge-density-wave platform materials, as well as in materials with triangular lattices in general.

• DFG Programme: Research Grants
• International Connection: France
• Cooperation Partner: Professor Dr. Yann Gallais