Shifei Qi

High-Temperature Quantum Anomalous Hall Effect in n-p Codoped Topological Insulators.

The quantum anomalous Hall effect (QAHE) is a fundamental quantum transport phenomenon that manifests as a quantized transverse conductance in response to a longitudinally applied electric field in the absence of an external magnetic field, and it promises to have immense application potential in future dissipationless quantum electronics. Here, we present a novel kinetic pathway to realize the QAHE at high temperatures by n-p codoping of three-dimensional topological insulators. We provide a proof-of-principle numerical demonstration of this approach using vanadium-iodine (V-I) codoped Sb_{2}Te_{3} and demonstrate that, strikingly, even at low concentrations of ∼2%  V and ∼1% I, the system exhibits a quantized Hall conductance, the telltale hallmark of QAHE, at temperatures of at least ∼50  K, which is 3 orders of magnitude higher than the typical temperatures at which it has been realized to date. The underlying physical factor enabling this dramatic improvement is tied to the largely preserved intrinsic band gap of the host system upon compensated n-p codoping. The proposed approach is conceptually general and may shed new light in experimental realization of high-temperature QAHE.

High-temperature quantum spin Hall insulator in compensated n-p codoped graphene

The quantum spin Hall effect (QSH) has been experimentally observed in some quantum wells. However, an ultralow temperature due to an extremely small band gap severely limits its potential application in dissipation-less quantum electronics. Using first-principles calculations, we present a novel kinetic pathway for realizing a high-temperature QSH insulator by n-p codoping of graphene. Our results show that a large and intrinsic band gap of about 26.9 meV, making it viable near room-temperature application, can be achieved via compensated n-p codoping, e.g. doping both thallium (Tl) and tetrafluorotetracyanoquinodimethane (F4-TCNQ) into graphene. In addition, the band gap of the Tl/graphene/F4-TCNQ system is tunable using the Tl doping concentration. This study provides a new solution for experimental studies and practical applications of the high-temperature QSH effect.

Strain-modulated ferromagnetism and band gap of Mn doped Bi2Se3

The quantized anomalous Hall effect (QAHE) have been theoretically predicted and experimentally confirmed in magnetic topological insulators (TI), but dissipative channels resulted by small-size band gap and weak ferromagnetism make QAHE be measured only at extremely low temperature (<0.1 K). Through density functional theory calculations, we systemically study of the magnetic properties and electronic structures of Mn doped Bi2Se3 with in-plane and out-of-plane strains. It is found that out-of-plane tensile strain not only improve ferromagnetism, but also enlarge Dirac-mass gap (up to 65.6 meV under 6% strain, which is higher than the thermal motion energy at room temperature ~26 meV) in the Mn doped Bi2Se3. Furthermore, the underlying mechanisms of these tunable properties are also discussed. This work provides a new route to realize high-temperature QAHE and paves the way towards novel quantum electronic device applications.

Realization of quantum anomalous Hall effect in graphene from n−p codoping-induced stable atomic adsorption

Using first-principles calculation methods, we study the possibility of realizing quantum anomalous Hall effect in graphene from stable 3\textit{d}-atomic adsorption via charge-compensated \textit{n}-\textit{p} codoping scheme. As concrete examples, we show that long-range ferromagnetism can be established by codoping 3\textit{d} transition metal and boron atoms, but only the Ni codopants can open up a global bulk gap to harbour the quantum anomalous Hall effect. Our estimated ferromagnetic Curie transition temperature can reach over 10 Kelvin for various codoping concentrations.