Recently topics:
Topological Insulators: The surface of a three-dimensional topological insulator (TI) hosts a unique type of 2D electron system, which is promising for developing topological quantum computation and novel types of spintronic devices. We have been working on the electron transport properties of 3D TI thin films since 2009. We demonstrated effective gate-voltage tuning of the chemical potential in Bi2Se3 into topological transport regime
(J. Chen et al., PRL 105, 176602, 2010), and utilized the weak antilocalization effect to detect the surface transport and its coupling to the bulk states
(J. Chen et al., PRB 83, 241304 (R), 2011). We proposed that the magnetoresistance in parallel magnetic fields can be used as a sensitive probe to detect the surface-surface coupling and the surface-bulk coupling in 3D TIs
(C. J. Lin et al., PRB 88, 041307 (R), 2013). We also observed a crossover from weak antilocalization to Anderson localization in ultrathin TI films
(J. Liao et al., PRL 114, 216601, 2015). Making use of the weak antilocalization effect, we also investigated electron dephasing in 3D TIs, and found that the dephasing rate of the surface states has a sublinear power law dependence on temperature, and proposed a dephasing mechanism related to inelastic processes in the bulk charge puddles to account for this anomalous dephasing behavior
(J. Liao et al., Nat. Commun. 8, 16071, 2017). In the aforementioned studies, electron transport measurements were mostly carried out on thin films (e.g. Bi2Se3, (Bi,Sb)2Te3) prepared with molecular beam epitaxy (MBE) by our collaborators (Kehui Wu group at IOP, and Qi-Kun Xue/Ke He group at Tsinghua University). More recently, we have developed techiques to fabricate dual-gated TI devices based on nanoflakes either exfoliated from (Bi,Sb)2(Te,Se)3 single crystals or synthesized with chemical vapor deposition, in addition to setting up our own MBE facility for the growth of TI thin films.
Topological insulator/magnetic insulator heterostructures: The interfacial interaction between a TI and a magnetic insulator (MI) with perpendicular anisotropy can open a Dirac gap in the surface states, laying a foundation for realization of the quantum anomalous Hall effect, magnetic monopoles, and Majorana interferometers. Moreover, the spin-momentum locked surface states can efficiently convert electrical current to spin polarization, thus providing a spin torque that can switch the magnetization of an adjacent magnetic layer. This offers an appealing approach to develop high performance nonvolatile random-access memory that may vastly improve the performance of modern computers. To achieve this, enormous experimental efforts are, however, required to discover more ideal TI materials, to realize high quality TI/MI interfaces, and to understand the interfacial interactions. In the past several years, we have worked on TI/MI heterostructures based on ferromagnetic (ferrimagnetic) insulators BaFe12O19 (W. M. Yang et al., APL 105, 092411, 2014), YIG, TIG, and antiferromagnetic insulator MnSe. Magnetoresistance and Hall effect measurements have been carried out to detect the interfacial interactions. The work is still in progress.
Novel magnetic systems: Novel magnetic systems, such as magnetic Weyl semimetals and high-TC magnetic semiconductors, not only offer possible routes for overcoming difficulties encountered in spintronic research, but also provide fertile ground for discovering exciting new physics. In the past several years, HgCr2Se4 has been extensively studied in our group. This material had been treated as a magnetic semiconductor for decades, before it was predicted to a candidate of Weyl semimetals in 2011. By using Andreev reflection spectroscopy, we showed that the ground state of n-type HgCr2Se4 is half-metallic (i.e. fully spin polarized)
(T. Guan et al., PRL 115, 087002, 2015). When the temperature is raised, it undergoes a metal-insulator transition driven near the Curie temperature. Colossal magnetic resistances (CMR) up to five orders of magnitude have been observed in n-HgCr2Se4, and our measurements suggested that spin correlations play an important role in the formation of magnetic polarons, which is responsible for the CMR effect
(C. J. Lin et al., PRB 94, 224404, 2016). We also observed unusually large quantum corrections to the anomalous Hall conductivity in HgCr2Se4 at low temperatures, challenging existing theory of the anomalous Hall effect
(S. Yang et al., PRL 123, 096601, 2019). Recently, we investigated other candidates for magnetic Weyl semimetals, such as Co3Sn2S2 and MnSb2Te4, as well as diluted magnetic semiconductors (e.g. (Ba,K)(Zn,Mn)2As2). Many-body effects manifested in ultralow temperature electron transport will be of our particular interest.
Quantum Hall Physics: The discoveries of the integer and fractional quantum Hall effects in early 1980s opened a new era for condensed matter physics. The interplay between Landau quantization, disorder, electron-electron and spin interactions gives rise to a large variety of fascinating electronic states. One characteristic of the quantum Hall research is that many electronic states are so fragile that they require the samples possessing extremely high carrier motilities and being cooled to the millikelvin temperatures. As a result, experimental techniques available to probe these states are quite limited, and there are still many open questions in this field despite a long history of research. We plan to tackle some of these questions by devising new types of samples based on ultrahigh mobility 2-dimensional electron systems and developing novel experimental techniques at ultralow temperatures and in high magnetic fields. In the past several years, we have not studied the quantum Hall physics as much as we originally hoped. We plan to resume the work on this subject in the near future. Our interest will not be limited to the quantum Hall phenomena in GaAs-based 2D electron systems alone.