| Summary: | <p>Notions of topology are of considerable interest currently in physics, especially in the way topology influences the electronic properties of crystalline solids. This thesis concerns the experimental investigation of four different magnetic materials, which have been predicted to display a strong interplay between magnetism and the topology of the electronic band structure.</p>
<p>I demonstrate that EuCd<sub>2</sub>Sb<sub>2</sub>, below T<sub>N</sub>= 7.4 K, displays an A-type anti-ferromagnetic order on the Eu sub-lattice with a magnetic propagation vector of k=(0,0,1/2). I also establish that the C3 rotational symmetry along the crystal c axis (essential for the stabilisation of band crossings called Dirac nodes) is broken by the in-plane orientation of the Eu moments. </p>
<p>Following the initial discoveries of crystalline solids which can host topologically-protected band crossings called Weyl nodes, there is now a need for better material realisations, ideally comprising a single pair of nodes located at or very close to the Fermi level and in an energy window free from other overlapping bands. I propose that EuCd<sub>2</sub>As<sub>2</sub>, in a magnetic field of B >1.6 T along the c axis, to be such a system. This material is of general interest as it represents the simplest possible Weyl semimetal, and is therefore a model system for fundamental investigations of Weyl physics. </p>
<p>I refute the proposition that semimetallic YbMnBi<sub>2</sub> hosts Weyl nodes induced by the time-reversal symmetry breaking mechanism. Furthermore, the full magnetic excitation spectrum of YbMnBi<sub>2</sub>, which has been mapped for the first time, demonstrates that the magnetic order of the Mn sub-lattice is weakly coupled to the charge carriers in the Bi square net. </p>
<p>Finally, the complex magnetic order of the Mn sub-lattice in Mn<sub>3</sub>Ge has been elucidated for the first time. This will shed light on the origin of the large anomalous Hall effect in Mn<sub>3</sub>Ge, which is unusual for an antiferromagnet.</p>
|
|---|