A new theoretical approach to the strain dependence of magnetic crystal-field anisotropy

We report on the derivation of analytical equations for ab-initio calculations of the strain dependence of crystal-electric-field (CEF) parameters for arbitrary deformations. The calculation is based on the fundamental assumption that the charge distribution deforms in the same way as the crystal. Based on this deformed-charge model, simple formulas for the practical usage are given for various site symmetries of cubic lattices under uniform strain. These formulas can be used to predict the change of the magnetic crystal-field anisotropy under strain, which is important for the design of magnetic materials and devices. As an example for the power of the method, we present a calculation of the magnetic contribution to the thermal expansion in some rare-earth-based materials.

PyCrystalField: software for calculation, analysis and fitting of crystal electric field Hamiltonians

PyCrystalField is a Python software package for calculating single-ion crystal electric field (CEF) Hamiltonians. This software can calculate a CEF Hamiltonian ab initio from a point charge model for any transition or rare earth ion in either the J basis or the LS basis, perform symmetry analysis to identify nonzero CEF parameters, calculate the energy spectrum and observables such as neutron spectrum and magnetization, and fit CEF Hamiltonians to any experimental data. The theory, implementation and examples of its use are discussed.

Unexpected differences between surface and bulk spectroscopic and implied Kondo properties of heavy fermion CeRh2Si2

Ultra-violet angle-resolved photoemission spectroscopy (UV-ARPES) was used to explore the temperature dependence of the Ce-4f spectral responses for surface and bulk in the antiferromagnetic Kondo lattice CeRh2Si2. Spectra were taken from Ce- and Si-terminated surfaces in a wide temperature range, and reveal characteristic 4f patterns for weakly (surface) and strongly (bulk) hybridized Ce, respectively. The temperature dependence of the Fermi level peak differs strongly for both cases implying that the effective Kondo temperature at the surface and bulk can be rather distinct. The greatly reduced crystal–electric-field (CEF) splitting at the surface gives reason to believe that the surface may exhibit a larger effective Kondo temperature because of a higher local-moment effective degeneracy. Further, the hybridization processes could strongly affect the 4f peak intensity at the Fermi level. We derived the k-resolved dispersion of the Kondo peak which is also found to be distinct due to different sets of itinerant bands to which the 4f states of surface and bulk Ce are coupled. Overall our study brings into reach the ultimate goal of quantitatively testing many-body theories that link spectroscopy and transport properties, for both the bulk and the surface, separately. It also allows for a direct insight into the broader problem of Kondo lattices with two different local-moment sublattices, providing some understanding of why the cross-talking between the two Kondo effects is weak.

Long-range magnetic order in the ${\tilde S}=1/2$ triangular lattice antiferromagnet KCeS$_2$

Recently, several putative quantum spin liquid (QSL) states were discovered in {\tilde S} = 1/2S̃=1/2 rare-earth based triangular-lattice antiferromagnets (TLAF) with the delafossite structure. In order to elucidate the conditions for a QSL to arise, we report here the discovery of a long-range magnetic order in the Ce-based TLAF KCeS_22 below T_{\mathrm N} = 0.38TN=0.38 K, despite the same delafossite structure. Finally, combining various experimental and computational methods, we characterize the crystal electric field scheme, the magnetic anisotropy and the magnetic ground state of KCeS_22.