Magnetic Topological Insulators
In collaboration with Dr. Genda Gu – BNL
Surface states of topological insulators (TIs) are expected to exhibit many valuable new electronic phenomena when a 'mass gap' is opened in their Dirac spectrum by ferromagnetism (FM). Such ferromagnetic topological insulators (FMTI) should exhibit phenomena including the Quantum Anomalous Hall Effect (QAHE), the Jackiw-Rebbi Solitons (JRS), and Emergent Axionic Electrodynamics. The QAHE has indeed been observed but, mysteriously, it is only detected at mK temperatures.
To explore the intriguing physics of FMTI, we recently developed the first visualization technique for the Dirac mass of FMTI surface states. We found that the Dirac mass m(r) is extremely disordered and correlates with the local density of the magnetic dopant atoms generating FM state. This chaotic Dirac-mass landscape m(r) poses far more questions on FMTI than it answers.
FIGURE 3A. Intensity of tunneling conductance g(q,E) into the Dirac spectrum of surface-states of Cr0.08(Bi0.1Sb0.9)1.92Te3; the ferromagnetism opens a gap ΔFM~20meV around the Dirac point where the conductance reaches zero. B. Tunneling conductance at a single atomic location; again ΔFM~20meV where the conductance reaches zero. C. Typical spatial map of ΔFM(r) in Cr0.08(Bi0.1Sb0.9)1.92Te3.
a) In general, ferromagnets exhibit both FM domains and magnetic hysteresis, and FMTI are no different. But these phenomena should, in theory, have a profound influence on JRS and QAHE. We plan to measure the atomic-scale electronic structure throughout the hysteresis loops of Cr(Bi,Sb)Te3 and V(Bi,Sb)Te3 and thus to visualize the evolution of FM domains and the network of JR states that should exist between regions of opposite magnetization.
b) The QAHE only stabilizes at temperatures T<<1K. This likely means that nanoscale disorder (Fig. 3C) somehow shorts out the chiral edge currents, allowing them to pass through the centre of the sample so that the conductance is not quantized. Precisely how this happens is unknown. We plan to image topological surface states of FMTI approaching QAHE with falling temperature, to visualize how the bulk currents are destroyed and the QAHE edge current stabilized.
c) The interplay of electric field E and magnetic field B at the surface of FMTI should be analogous to that predicted theoretically for axions. We plan to pursue proposals for how to observe this effect by generating axionic phenomena with an STM tip and observing the nanoscale B-field response.