Magnetism for Intelligent Devices (MIND)

Abstract:

Magnetically doped topological insulators (TIs) are key to realizing the quantum anomalous Hall (QAH) effect, with the prospect of enabling dissipationless electronic devices in the future. Doping of the well-established three-dimensional TIs of the (Bi,Sb)2(Se,Te)3 family with the transition metals Cr and V is now an established approach for observing the QAH state at very low temperatures. While the magnetic transition temperatures of these materials is on the order of 10’s of K, full quantization of the QAH state is achieved below ∼100 mK, governed by the size of the magnetic gap and thus the out-of-plane magnetic moment. In an attempt to raise the size of the magnetic moment and transition temperature, we carried out a structural and magnetic investigation of codoped (V,Cr):Sb2Te3 thin films. Starting from singly doped Cr:Sb2Te3 films, free of secondary phases and with a transition temperature of ∼72 K, we introduced increasing fractions of V and found a doubling of the transition temperature, while the magnetic moment decreases. In order to separate the properties and contributions of the two transition metals in the complex doping scenario independently, we employed spectroscopic x-ray techniques. Surprisingly, already small amounts of V lead to the formation of the secondary phase Cr2Te3. No V was detectable in the Sb2Te3 matrix. Instead, it acts as a surfactant and can be found in the near-surface layers at the end of the growth. Our study highlights the importance of x-ray-based studies for the doping of van der Waals systems, for which the optimization of magnetic moment or transition temperature alone is not necessarily a good strategy.

Abstract:

Strong spin-momentum coupling in topological insulators give rise to topological surface states, protected against disorder scattering by time reversal symmetry. The study of these exotic quantum states not only provides an opportunity to explore fundamental phenomenon in condensed matter physics such as the spin hall effect, but also lays the foundation for applications in quantum computing to spintronics. Conventional electrical measurements suffer from substantial bulk interference, making it difficult to clearly identify topological surface state from the bulk. We use terahertz time-domain spectroscopy to study the temperature-dependent optical behavior of a 23-quintuple-thick film of bismuth selenide (Bi2Se3) allowing the deconvolution of the surface state response from the bulk. The signatures of the topological surface state at low temperatures (< 30 K) with the optical conductance of Bi2Se3 exhibiting a metallic behavior are observed. Measurement of carrier dynamics, obtain an optical mobility, exceeding 2000 cm2/V•s at 4 K, indicative of a surface-dominated response. A scattering lifetime of ~0.18 ps and a carrier density of 6×1012 cm-2 at 4 K were obtained from the terahertz time-domain spectroscopy measurement. The terahertz conductance spectra reveal characteristic features at ~1.9 THz, attributed to the optical phonon mode, which becomes less prominent with falling temperature. The electrical transport measurements reveal weak antilocalization behavior in our Bi2Se3 sample, consistent with the presence of a topological surface state.