The project

General scientific objectives of the project:
1. Growing new materials, mainly as high-quality single crystals, and explore their properties.
2. Characterizing the Fermi Surfaces of these systems using direct (ARPES, quantum oscillations) and indirect probes.
3. Modeling these systems by complementary ab-initio-based calculations and model Hamiltonian analyses.
4. Exploring the physics of these systems with a focus on the emerging new states of electronic matter (unconventional orders).
High-quality crystals constitute an essential basis of our project. Various 4f-electron materials will be synthesized by the teams of C. Geibel (MPI-CPfS Dresden), C. Krellner (U Frankfurt) and P. Lejay (Institut Néel Grenoble) but complementary samples, including organics, will be provided by different experimental partners involved in the project. We aim for Ce, Y band U compounds showing very unusual magnetic and/or electronic properties. We are specifically interested in systems presenting evidence for magnetic field or pressure induced Lifshitz transitions. Our strategy for the materials will follow two complementary approaches: synthesizing high-quality crystals of known materials, and investigating new systems.
Experimental techniques
There are two main experimental techniques for the direct determination of Fermi Surface properties: angle resolved photoemission spectroscopy (ARPES) and magnetic quantum-oscillation effects. Both complementary techniques will be used within the project.
ARPES measurements will be performed by the CSNSM-Orsay and the TU-Dresden teams. The group at CSNSM is installing at present a laboratory-based low-temperature ultra-high-resolution ARPES setup which will be ideally suited to perform experiments on different materials proposed in this project. Additionally, the groups in Orsay and in Dresden will perform measurements at the 13 beamline at BESSY. The synchrotron measurements are an ideal complement to the planned lab-based measurements: many different samples can be tested in the lab setup without time restrictions, while photon-energy and photon-polarization dependent experiments, crucial to unravel the bulk states and to understand the orbital character and symmetries of the quasiparticle bands, can be performed during the access to synchrotron beamtime.
Quantum-oscillation experiments will be carried out in Dresden at MPI-CPfS and at HZDR and in Grenoble at LNCMI. Thereby, mainly magnetic-torque magnetometry or the modulation-field technique is used for de Haas-van Alphen (dHvA) effect measurements. DHvA measurements will also be performed in pulsed magnetic fields to beyond 70 T and temperatures down to 0.5 K at the HZDR. Finally, the teams will set up quantum oscillation measurements under pressure, which is an important tuning parameter in some of the projects described in this proposal.
Raman-scattering experiments realized at Institut Néel in Grenoble will complement the studies of the Fermi Surfaces with emphasis on the anisotropy of the Kondo physics, thanks to its ability to probe electronic excitations at a particular momentum space of the Brillouin zone. Our team can perform polarized electronic Raman scattering down to 2 K and under magnetic fields up to 10 T, as well as under high pressure up to 20 GPa down to 3 K, all able to probe low-energy excitations down to 1 meV. Thus, large areas of complex phase diagrams can be explored.
Theory and modelling
Theory and modeling teams in Bordeaux, at TU-Braunschweig, and in Grenoble, will join their expertises. One central focus of the Fermi-NESt project is to understand the evolution of the low-energy excitations in 4f and 5f systems with magnetic field and temperature. Of particular interest is the identification of modulated phases (CDW, SDW, FFLO, etc) and unconventional orders as well as the characterization of quantum critical points. For 4f systems the Fermi surfaces and f-spectralfunctions will be determined by the renormalized band method combining a fully self-consistent ab-initio treatmentof the weakly correlated conduction electrons with many-body calculationsfor the strongly correlated f states. The results can be directly compared with transport properties, measurements of magnetic quantum oscillations (SdH, dHvA) as well as ARPES data. For 5f systems the emphasis will be on the role of intra-atomic Hund’s rule-type correlations and their consequences.
At a more phenomenological level, we will study model Hamiltonians, investigating the effect of various microscopic mechanisms which may be involved in Fermi surface instabilities: local versus intersite Kondo coupling, possible valence fluctuations, intersite magnetic coupling, spin-liquid correlations, geometry of the lattice structure, or crystal-field splitting effects. The models we will study are thus generalizations of Kondo lattice, Anderson lattice and multi-orbital Hubbard models on regular or frustrated lattices, with extra Heisenberg or Dzyaloshinsky-Moriya interaction terms. Phase diagrams will be determined by tuning microscopic parameters and resulting Fermi surfaces will be characterized as well as the main physical properties.