Local optical control of ferromagnetism and chemical potential in a topological insulator

Abstract

Many proposed experiments involving topological insulators (TIs) require spatial control over time-reversal symmetry and chemical potential. We demonstrate reconfigurable micron-scale optical control of both magnetization (which breaks time-reversal symmetry) and chemical potential in ferromagnetic thin films of Cr-(Bi,Sb) 2 Te 3 grown on SrTiO 3 . By optically modulating the coercivity of the films, we write and erase arbitrary patterns in their remanent magnetization, which we then image with Kerr microscopy. Additionally, by optically manipulating a space charge layer in the underlying SrTiO 3 substrates, we control the local chemical potential of the films. This optical gating effect allows us to write and erase p-n junctions in the films, which we study with photocurrent microscopy. Both effects are persistent and may be patterned and imaged independently on a few-micron scale. Dynamic optical control over both magnetization and chemical potential of a TI may be useful in efforts to understand and control the edge states predicted at magnetic domain walls in quantum anomalous Hall insulators.topological insulators | ferromagnetism | Kerr microscopy | magneto-optical recording | photocurrent microscopy T he unusual topology of electronic bands in some materials can produce quantum states that are stabilized by symmetry. Topological insulators (TIs) have attracted particular attention for their symmetry-protected surface and edge states, which may hold promise for applications in spintronics and quantum computing (1, 2). Ferromagnetic TIs combine a topologically nontrivial band structure with ferromagnetism, which intrinsically breaks time-reversal symmetry (TRS) and therefore may produce unusual electromagnetic phenomena such as the topological magnetoelectric effect (3, 4), quantized Kerr and Faraday rotation (5, 6), and quantization of the anomalous Hall effect (QAHE) (7,8). However, while the quantum states in an ideal TI are protected by symmetry, most topologically nontrivial materials have proven difficult to grow and engineer. In particular, residual bulk conductivity, and the tendency of TI materials to degrade during semiconductor processing, have impeded many experimental efforts (9-11). Harnessing the unique physics of these materials will require better understanding and control of their magnetism and disorder, as well as methods to engineer TI devices without degrading the properties that make these materials unique.A major challenge in TI research is to identify systems in which the energies and symmetries of TI physics can be reliably controlled in the laboratory. For instance, a criterion for the QAH insulator state is for the chemical potential of electrons µ to lie within an energy gap ∆ in the surface state dispersion, where ∆ ∝ | M ·n|, M is the local magnetization andn is the surface normal (1, 2). At the edges of a uniformly magnetized film, the top and bottom surfaces meet and therefore M ·n changes sign, causing ∆ to vanish. In sufficiently thin films, the vanishing gap produces 1D co...

Confinement and fermion doubling problem in Dirac-like Hamiltonians

“…This is a consequence of the constant Fermi velocity of the linear dispersion, which allows perfect matching of the injected and transmitted waves. Consequently, to attain confinement, one needs to either open a gap by breaking a symmetry in the outer region [21,[33][34][35][36][37][38], or invoke finite-size effects [39].…”

Observation of superparamagnetism in coexistence with quantum anomalous Hall C = ±1 and C = 0 Chern states

“…2,13,16 Indeed, a number of studies using global magnetization measurements 14,17,18 and magnetic force microscopy 19,20 have observed FM-like response in magnetically doped TI thin films. Surprisingly, however, recent scanning SQUID magnetic imaging 21 and transport studies, 22,23 as well as analysis of Kerr microscopy imaging, 24 have suggested the presence of a superparamagnetic (SPM) state in Cr-doped (Bi, Sb) 2 Te 3 films in which instead of a long-range FM order the magnetism is comprised of single-domain magnetic islands. These islands are of characteristic size of few tens of nanometers and are only weakly coupled to each other.…”

“…19,20 Similarly, the SPM was demonstrated locally in films that do show QAH but at elevated temperatures at which no quantization was present. 21,24 Here we report scanning SQUID-on-tip (SOT) magnetic imaging of modulation-doped (Bi,Sb) 2 Te 3 films performed concurrently with transport measurements that show full QAH quantization. We observe unambiguous SPM behavior both in the magnetization reversal process in the plateau transition regions near H c , as well as within the macroscopically quantized state.…”

Quantized chiral edge conduction on domain walls of a magnetic topological insulator

“…1, A and B) (1,2). Notably, the position of the CES, in contrast to those at the sample edge (3,4,(9)(10)(11)(12)(13)(14) or at the local gate boundary (5-7), can potentially be manipulated via domain control by external fields such as local magnetic field, current-induced spinorbit torque (16,17), or optical illumination (18). Thus, a localized electronic channel based on the CES can be constructed in a reconfigurable manner.…”