Dr Benjamin Huddart

Abstract:

We present the results of microscopic investigations of antiferromagnetic CeNiGe3 using neutron powder diffraction (NPD), inelastic neutron scattering (INS), and muon spin relaxation (𝜇⁢SR) measurements. CeNiGe3 crystallizes in a centrosymmetric orthorhombic crystal structure (space group 𝐶⁢𝑚⁢𝑚⁢𝑚) and undergoes antiferromagnetic (AFM) ordering. The occurrence of long-range AFM ordering at 𝑇N≃5.2K is confirmed by magnetic susceptibility, heat capacity, neutron diffraction, and 𝜇⁢SR measurements. The NPD data characterize the AFM state with an incommensurate helical magnetic structure having a propagation vector k = (0, 0.41, 1/2). In addition, INS measurements at 10 K identified two crystal electric field (CEF) excitations at 9.17 meV and 18.42 meV. We analyzed the INS data using a CEF model for an orthorhombic environment of Ce3+ (𝐽=5/2) and determined the CEF parameters and ground state wave functions of CeNiGe3. Moreover, zero-field 𝜇⁢SR data for CeNiGe3 at 𝑇<𝑇N show long-range AFM ordering with three distinct oscillation frequencies corresponding to three different internal fields at the muon sites. The internal fields at the muon-stopping sites have been further investigated using density functional theory calculations.

Abstract:

An ideal Weyl semimetal is characterized by a dispersion in which only Weyl cones intersect the Fermi level, with low-energy behavior being governed by Weyl fermions. Although ideal Weyl semimetals have long been anticipated, only a few are realized in nonmagnetic materials. In this study, we confirm the presence of Weyl-fermion excitations in the ideal Weyl semimetal CuTlSe₂ via a combination of magnetoresistance, Hall-effect, magnetic-susceptibility, nuclear magnetic resonance (NMR), and muon-spin relaxation (µ⁢SR) experiments. Magnetoresistance measurements reveal a negative longitudinal magnetoresistance (LMR), which scales as 𝐵2, while Hall-effect results indicate a predominant contribution from Weyl fermions with a hole-type charge. Magnetic susceptibility and µ⁢SR measurements indicate the lack of any intrinsic spontaneous magnetic moments down to base temperature. Finally, the NMR results can be modeled by a two-component effective Hamiltonian, which reproduces well the temperature-dependent ⁶³Cu NMR (𝑇_1⁢𝑇)⁻¹ factor, shown to scale as 𝑇² below 100 K and as 𝑇¹ above 100 K. Overall, we find that the extremely low concentration (10¹⁷cm⁻³) of carriers in CuTlSe₂ originates from an ideal nonmagnetic Weyl semimetallic state, persisting up to a thermal excitation energy of 9 meV (100 K), above which trivial electronic bands close to 𝐸_{F take over. Our findings highlight CuTlSe² as a new member of the intriguing class of Weyl semimetals.}