Muons and magnets

Abstract:

General relativity is a field theory that describes gravity. It engages profoundly with the nature of space and time and is based on simple ideas from the physics of fields. It can be summarised by the Einstein equation which relates a geometrical quantity, the curvature of space and time that follows from the metric field., to a physical quantity that reflects a field that describes the matter content of the Universe. We begin in Part I with an introduction to the geometry of flat spacetime, reviewing special relativity and setting up the mathematics of the metric. Part II introduces the mathematics of curvature and sets up the physics of general relativity and finishes with the Einstein field equation. Part III applies these ideas to the Universe and studies various models used in cosmology. Part IV turns to smaller structures inside the Universe: stars, black holes and their orbits. Part V contains a more formal treatment of geometry which may be of more interest to those with more mathematical inclinations. Part VI considers general relativity as a type of field theory and examines how one might link the ideas in our best theory of gravitation to our most successful theories of quantum fields.

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.