Topological kink plasmons on magnetic-domain boundaries

Abstract

Two-dimensional topological materials bearing time reversal-breaking magnetic fields support protected one-way edge modes. Normally, these edge modes adhere to physical edges where material properties change abruptly. However, even in homogeneous materials, topology still permits a unique form of edge modes – kink modes – residing at the domain boundaries of magnetic fields within the materials. This scenario, despite being predicted in theory, has rarely been demonstrated experimentally. Here, we report our observation of topologically-protected high-frequency kink modes – kink magnetoplasmons (KMPs) – in a GaAs/AlGaAs two-dimensional electron gas (2DEG) system. These KMPs arise at a domain boundary projected from an externally-patterned magnetic field onto a uniform 2DEG. They propagate unidirectionally along the boundary, protected by a difference of gap Chern numbers ($$\pm1$$ ± 1 ) in the two domains. They exhibit large tunability under an applied magnetic field or gate voltage, and clear signatures of nonreciprocity even under weak-coupling to evanescent photons.

Topological spin-plasma waves

“…The appearance of topologically protected gapless surface and edge states is a direct consequence of topological electron states in the bulk. In another frontier, the notion of bulk band topology has been extended to include nonelectron systems such as photonic systems [18][19][20][21][22], polaritons [23,24], phonons [25][26][27][28][29], magnons [30][31][32][33][34][35][36], magnetoelastics [37][38][39][40][41][42][43], and recently plasmons [44,45]. In all of these systems, which are described by bosonic collective modes, the band topology emanates from the nontrivial Berry curvature of the underlying Bloch wave description of bulk modes, which upon integration over the momentum space leads to an integer topological index.…”

Nonreciprocal and Topological Plasmonics

“…Furthermore, the boundary between two 2DEGs that are magnetized along opposite directions has a gap Chern number difference equal to 2 ± and supports two unidirectional edge modes (Figure 8). The presence of these two modes, as predicted by the bulk-edge correspondence principle, was recently experimentally validated in a GaAs-AlGaAs heterostructure [78].…”

“…Recently, these properties have also been connected to topological wave physics. Specifically, it was shown that the presence of an orthogonal magnetic bias not only breaks reciprocity, but also opens a non-trivial bandgap in the bulk-mode dispersion around zero frequency, with a gap Chern number equal to 1 ± [21,78], as shown in Figure 8. The unidirectional edge modes of the magnetized 2DEG are then tied to the topological properties of the bulk-mode bandgap through the bulk-edge correspondence, and are therefore topologically protected against defects that do not close this bandgap.…”

Twisted magnetic topological insulators

“…This is because the neighboring ferromagnetic domains with the opposite magnetic orders perpendicular to the surface have opposite Chern numbers [1]. This domain chiral edge mode has been realized experimentally on the surface of magnetic topological insulators [39,40], and with a more controlled method with the strong permanent magnet placed atop the GaAs/AlGaAs quantum Hall heterojunction [41]. The spin modulation of the spin spiral state with one spiral period can be approximately viewed as one domain wall, and there will naturally be a chiral edge mode associated with it.…”