Spin physics from topological insulators and spin split antiferromagnets*

The interplay of magnetism and topology has led to exciting novel phenomena such as the quantum anomalous Hall effect, axion insulators, and the parity anomaly state. However, these states have been only observed at very low temperatures due to the presence of disorder in the devices studied to date. In the first part of this abstract, we demonstrate the creation of topological insulator (BiSbTeSe₂)/magnet (Cr₂Ge₂Te₆) structures, with pristine interfaces by exfoliation of van der Waals layer and mechanical assembly within a glove box, which leads to a strong proximity coupling between the topological surface states (TSS) in BiSbTeSe₂ and magnetism in Cr₂Ge₂Te₆. These structures demonstrate that when the Fermi level is within the exchange gap, the anomalous Hall conductance is close to half-quantized (e²/2h), even at the high temperature of 9 K. This is a much higher temperature scale than any other reported quantum anomalous Hall system.

Secondly, we will discuss experiments on thermally generated spin currents by topological insulator Bi₂Se₃. We find that Bis₂Se₃ generates substantial thermally driven spin currents with the spin Nernst ratio is the largest among all the materials studied up to date. Strong thermally generated spin currents in Bi₂Se₃ can be understood via Mott relations to be due to an overall large spin Hall conductivity and its dependence on electron energy.

Finally, we will discuss experimental demonstration of tilted spin current generated by collinear antiferromagnet RuO₂. We demonstrate that RuO₂ can generate spin currents with the spin polarization aligned roughly to its Néel vector, consistent with the theoretical predictions of spin split bands. If the Néel vector is tilted relative to the sample plane, the polarization of the generated spin current has a strong component perpendicular to the sample plane which is useful for magnetic memory applications.