Wen-Kai Lou

Quantum Hall effect in black phosphorus two-dimensional electron system

The development of new, high-quality functional materials has been at the forefront of condensed-matter research. The recent advent of two-dimensional black phosphorus has greatly enriched the materials base of two-dimensional electron systems (2DESs). Here, we report the observation of the integer quantum Hall effect in a high-quality black phosphorus 2DES. The high quality is achieved by embedding the black phosphorus 2DES in a van der Waals heterostructure close to a graphite back gate; the graphite gate screens the impurity potential in the 2DES and brings the carrier Hall mobility up to 6,000 cm(2) V(-1) s(-1). The exceptional mobility enabled us to observe the quantum Hall effect and to gain important information on the energetics of the spin-split Landau levels in black phosphorus. Our results set the stage for further study on quantum transport and device application in the ultrahigh mobility regime.

Polarization-Driven Topological Insulator Transition in a GaN/InN/GaN Quantum Well

Topological insulator (TI) states have been demonstrated in materials with a narrow gap and large spin-orbit interactions (SOI). Here we demonstrate that nanoscale engineering can also give rise to a TI state, even in conventional semiconductors with a sizable gap and small SOI. Based on advanced first-principles calculations combined with an effective low-energy k · p Hamiltonian, we show that the intrinsic polarization of materials can be utilized to simultaneously reduce the energy gap and enhance the SOI, driving the system to a TI state. The proposed system consists of ultrathin InN layers embedded into GaN, a layer structure that is experimentally achievable.

Interface-induced topological insulator transition in GaAs/Ge/GaAs quantum wells.

We demonstrate theoretically that interface engineering can drive germanium, one of the most commonly used semiconductors, into a topological insulating phase. Utilizing giant electric fields generated by charge accumulation at GaAs/Ge/GaAs opposite semiconductor interfaces and band folding, the new design can reduce the sizable gap in Ge and induce large spin-orbit interaction, which leads to a topological insulator transition. Our work provides a new method to realize topological insulators in commonly used semiconductors and suggests a promising approach to integrate it in well-developed semiconductor electronic devices.

Helical quantum states in HgTe quantum dots with inverted band structures.

We investigate theoretically the electron states in HgTe quantum dots (QDs) with inverted band structures. In sharp contrast to conventional semiconductor quantum dots, the quantum states in the gap of the HgTe QD are fully spin-polarized and show ringlike density distributions near the boundary of the QD and spin-angular momentum locking. The persistent charge currents and magnetic moments, i.e., the Aharonov-Bohm effect, can be observed in such a QD structure. This feature offers us a practical way to detect these exotic ringlike edge states by using the SQUID technique.

Landau levels and magneto-transport property of monolayer phosphorene

We investigate theoretically the Landau levels (LLs) and magneto-transport properties of phosphorene under a perpendicular magnetic field within the framework of the effective k·p Hamiltonian and tight-binding (TB) model. At low field regime, we find that the LLs linearly depend both on the LL index n and magnetic field B, which is similar with that of conventional semiconductor two-dimensional electron gas. The Landau splittings of conduction and valence band are different and the wavefunctions corresponding to the LLs are strongly anisotropic due to the different anisotropic effective masses. An analytical expression for the LLs in low energy regime is obtained via solving the decoupled Hamiltonian, which agrees well with the numerical calculations. At high magnetic regime, a self-similar Hofstadter butterfly (HB) spectrum is obtained by using the TB model. The HB spectrum is consistent with the LL fan calculated from the effective k·p theory in a wide regime of magnetic fields. We find the LLs of phosphorene nanoribbon depend strongly on the ribbon orientation due to the anisotropic hopping parameters. The Hall and the longitudinal conductances (resistances) clearly reveal the structure of LLs.

Excellent photothermal conversion of core/shell CdSe/Bi2Se3 quantum dots

AbstractWater-dispersed CdSe/Bi2Se3 core/shell QDs with a photothermal conversion coefficient of 27.09% have been synthesized by a cation exchange reaction. The microstructure and crystal structure of the QDs, which were confirmed by TEM and XRD, showed that partial cation exchange occurred inside the CdSe QDs. Two main mechanisms are responsible for the excellent photothermal conversion: inhibition of radiative recombination of carriers due to the formation of type-II semiconductor heterostructures, and the large surface-to-volume ratio of the QDs. Photothermal conversion experiments indicated that the CdSe/Bi2Se3 QDs showed high photothermal conversion efficiency and excellent NIR photostability.

Surface States of Bi2Se3 Nanowires in the Presence of Perpendicular Magnetic Fields

We study the surface states of Bi2Se3 nanowires (NWs) in the presence of perpendicular magnetic fields. It is found that the minigap of Bi2Se3, arising from the quantized surface states around the circumference of NWs can be closed by perpendicular magnetic fields. With increasing magnetic fields, the Landau levels and edge states appear and localize at the center and edge of NWs, respectively. More interestingly, magnetic fields split the electron surface subbands with opposite tangential momenta, leading to specific edge states with low group velocity.

The persistent charge and spin currents in topological insulator Bi2Se3 nanowires

We investigate theoretically the surface states of three-dimensional topological insulator cylinder nanowires analytically and numerically. In contrast to the conventional semiconductor cylinder nanowires, these surface states exhibit unique massless Dirac dispersion and interesting transport properties. We find that the persistent charge current and persistent spin current, i.e., the Aharonov-Bohm oscillation, can be induced by the driven magnetic flux. The amplitude of persistent charge current shows an oscillating behavior with increasing the electron density.

Quantum Hall Effect in Electron-Doped Black Phosphorus Field-Effect Transistors

The advent of black phosphorus field-effect transistors (FETs) has brought new possibilities in the study of two-dimensional (2D) electron systems. In a black phosphorus FET, the gate induces highly anisotropic 2D electron and hole gases. Although the 2D hole gas in black phosphorus has reached high carrier mobilities that led to the observation of the integer quantum Hall effect, the improvement in the sample quality of the 2D electron gas (2DEG) has however been only moderate; quantum Hall effect remained elusive. Here, we obtain high quality black phosphorus 2DEG by defining the 2DEG region with a prepatterned graphite local gate. The graphite local gate screens the impurity potential in the 2DEG. More importantly, it electrostatically defines the edge of the 2DEG, which facilitates the formation of well-defined edge channels in the quantum Hall regime. The improvements enable us to observe precisely quantized Hall plateaus in electron-doped black phosphorus FET. Magneto-transport measurements under high magnetic fields further revealed a large effective mass and an enhanced Landé g-factor, which points to strong electron-electron interaction in black phosphorus 2DEG. Such strong interaction may lead to exotic many-body quantum states in the fractional quantum Hall regime.

Artificial Gauge Field and Topological Phase in a Conventional Two-dimensional Electron Gas with Antidot Lattices

Based on the Born-Oppemheimer approximation, we divide the total electron Hamiltonian in a spin-orbit coupled system into the slow orbital motion and the fast interband transition processes. We find that the fast motion induces a gauge field on the slow orbital motion, perpendicular to the electron momentum, inducing a topological phase. From this general designing principle, we present a theory for generating artificial gauge field and topological phase in a conventional two-dimensional electron gas embedded in parabolically graded GaAs/InxGa1−xAs/GaAs quantum wells with antidot lattices. By tuning the etching depth and period of the antidot lattices, the band folding caused by the antidot potential leads to the formation of minibands and band inversions between neighboring subbands. The intersubband spin-orbit interaction opens considerably large nontrivial minigaps and leads to many pairs of helical edge states in these gaps.Based on the Born-Oppemheimer approximation, we divide total electron Hamiltonian in a spinorbit coupled system into slow orbital motion and fast interband transition process. We find that the fast motion induces a gauge field on slow orbital motion, perpendicular to electron momentum, inducing a topological phase. From this general designing principle, we present a theory for generating artificial gauge field and topological phase in a conventional two-dimensional electron gas embedded in parabolically graded GaAs/In