Jun-Wei Luo

Self-assembled quantum dots in a nanowire system for quantum photonics

Quantum dots embedded within nanowires represent one of the most promising technologies for applications in quantum photonics. Whereas the top-down fabrication of such structures remains a technological challenge, their bottom-up fabrication through self-assembly is a potentially more powerful strategy. However, present approaches often yield quantum dots with large optical linewidths, making reproducibility of their physical properties difficult. We present a versatile quantum-dot-in-nanowire system that reproducibly self-assembles in core-shell GaAs/AlGaAs nanowires. The quantum dots form at the apex of a GaAs/AlGaAs interface, are highly stable, and can be positioned with nanometre precision relative to the nanowire centre. Unusually, their emission is blue-shifted relative to the lowest energy continuum states of the GaAs core. Large-scale electronic structure calculations show that the origin of the optical transitions lies in quantum confinement due to Al-rich barriers. By emitting in the red and self-assembling on silicon substrates, these quantum dots could therefore become building blocks for solid-state lighting devices and third-generation solar cells.

Hidden spin polarization in inversion-symmetric bulk crystals

Spin polarization due to spin–orbit coupling requires broken inversion symmetry. Now, calculations show that the effect arises from local site-asymmetry rather than global space-group asymmetry, and that a hitherto overlooked form of spin polarization should also exist in centrosymmetric structures.

False-positive and false-negative assignments of topological insulators in density functional theory and hybrids

Density-functional theory (DFT) approaches have been used recently to judge the topological order of various materials despite DFT’s well-known band-gap underestimation. Use of the more accurate quasi-particle GW approach reveals few cases where DFT identifications are false positive, which can possibly misguide experimental searches for materials that are topological insulators (TIs) in DFT but not expected to be TIs in reality. We also present the case of false positives due to the incorrect choice of crystal structures and address the relevance of choice of crystal structure with respect to the ground-state one and thermodynamical instability with respect to binary competing phases. We conclude that it is necessary to consider both the correct ground-state crystal structure and the correct Hamiltonian in order to predict new TIs.

Mapping the orbital wavefunction of the surface states in three-dimensional topological insulators

Topological insulators are novel macroscopic quantum-mechanical phase of matter, which hold promise for realizing some of the most exotic particles in physics as well as application towards spintronics and quantum computation. In all the known topological insulators, strong spin-orbit coupling is critical for the generation of the protected massless surface states. Consequently, a complete description of the Dirac state should include both the spin and orbital (spatial) parts of the wavefunction. For the family of materials with a single Dirac cone, theories and experiments agree qualitatively, showing the topological state has a chiral spin texture that changes handedness across the Dirac point (DP), but they differ quantitatively on how the spin is polarized. Limited existing theoretical ideas predict chiral local orbital angular momentum on the two sides of the DP. However, there have been neither direct measurements nor calculations identifying the global symmetry of the spatial wavefunction. Here we present the first results from angle-resolved photoemission experiment and first-principles calculation that both show, counter to current predictions, the in-plane orbital wavefunctions for the surface states of Bi2Se3 are asymmetric relative to the DP, switching from being tangential to the k-space constant energy surfaces above DP, to being radial to them below the DP. Because the orbital texture switch occurs exactly at the DP this effect should be intrinsic to the topological physics, constituting an essential yet missing aspect in the description of the topological Dirac state. Our results also indicate that the spin texture may be more complex than previously reported, helping to reconcile earlier conflicting spin resolved measurements.

Matrix-embedded silicon quantum dots for photovoltaic applications: a theoretical study of critical factors

Si Quantum dots (QDs) are offering the possibilities for improving the efficiency and lowering the cost of solar cells. In this paper we study the PV-related critical factors that may affect design of Si QDs solar cell by performing atomistic calculation including many-body interaction. First, we find that the weak absorption in bulk Si is significantly enhanced in Si QDs, specially in small dot size, due to quantum-confinement induced mixing of Γ-character into the X-like conduction band states. We demonstrate that the atomic symmetry of Si QD also plays an important role on its bandgap and absorption spectrum. Second, quantum confinement has a detrimental effect on another PV property – it significantly enhances the exciton binding energy in Si QDs, leading to difficulty in charge separation. We observe universal linear dependence of exciton binding energy versus excitonic gap for all Si QDs. Knowledge of this universal linear function will be helpful to obtain experimentally the exciton binding energy by just measuring the optical gap without requiring knowledge on dot shape, size, and surface treatment. Third, we evaluate the possibility of resonant charge transport in an array of Si QDs via miniband channels created by dot-dot coupling. We show that for such charge transport the Si QDs embedded into a matrix should have tight size tolerances and be very closely spaced. Fourth, we find that the loss of quantum confinement effect induced by dot-dot coupling is negligible – smaller than 70 meV even for two dots at intimate contact.

Split Dirac cones in HgTe/CdTe quantum wells due to symmetry-enforced level anticrossing at interfaces

HgTeisaband-invertedcompoundwhichformsatwo-dimensionaltopologicalinsulatorifsandwichedbetween CdTe barriers for a HgTe layer thickness above the critical value. We describe the fine structure of Dirac states in the HgTe/CdTe quantum wells of critical and close-to-critical thicknesses and show that the necessary creation of interfaces brings in another important physical effect: the opening of a significant anticrossing gap between the tips of the Dirac cones. The level repulsion driven by the natural interface inversion asymmetry of zinc-blende heterostructures considerably modifies the electron states and dispersion but preserves the topological transition at the critical thickness. By combining symmetry analysis, atomistic calculations, and extended k · p theory with interface terms, we obtain a quantitative description of the energy spectrum and extract the interface mixing coefficient. Wediscusshowthefingerprintsofthepredictedzero-magnetic-field splittingoftheDiraccones could be detected experimentally by studying magnetotransport phenomena, cyclotron resonance, Raman scattering, and THz radiation absorption.

Quasi-Direct Optical Transitions in Silicon Nanocrystals with Intensity Exceeding the Bulk

Comparison of the measured absolute absorption cross section on a per Si atom basis of plasma-synthesized Si nanocrystals (NCs) with the absorption of bulk crystalline Si shows that while near the band edge the NC absorption is weaker than the bulk, yet above ∼ 2.2 eV the NC absorbs up to 5 times more than the bulk. Using atomistic screened pseudopotential calculations we show that this enhancement arises from interface-induced scattering that enhances the quasi-direct, zero-phonon transitions by mixing direct Γ-like wave function character into the indirect X-like conduction band states, as well as from space confinement that broadens the distribution of wave functions in k-space. The absorption enhancement factor increases exponentially with decreasing NC size and is correlated with the exponentially increasing direct Γ-like wave function character mixed into the NC conduction states. This observation and its theoretical understanding could lead to engineering of Si and other indirect band gap NC materials for optical and optoelectronic applications.

Revealing the Origin of Fast Electron Transfer in TiO2-Based Dye-Sensitized Solar Cells

In dye-sensitized solar cells (DSCs), the electron transfer from photoexcited dye molecules to semiconductor substrates remains a major bottleneck. Replacing TiO2 with ZnO is expected to enhance the efficiency of DSCs, owing to the latter possesses a much larger electron mobility, but similar bandgap and band positions as TiO2 remain. However, the record efficiency of ZnO-based DSCs is only 7% compared with 13% of TiO2-based DSCs due to the even slower electron-transfer rate in ZnO-based DSCs, which becomes a long-standing puzzle. Here, we computationally investigate the electron transfer from the dye molecule into ZnO and TiO2, respectively, by performing the first-principles calculations within the frame of the Marcus theory. The predicted electron-transfer rate in the TiO2-based DSC is about 1.15 × 10(9) s(-1), a factor of 15 faster than that of the ZnO-based DSC, which is in good agreement with experimental data. We find that the much larger density of states of the TiO2 compared with ZnO near the conduction band edge is the dominant factor, which is responsible for the faster electron-transfer rate in TiO2-based DSCs. These denser states provide additional efficient channels for the electron transfer. We also provide design principles to boost the efficiency of DSCs through surface engineering of high mobility photoanode semiconductors.

Genetic design of enhanced valley splitting towards a spin qubit in silicon

The long spin coherence time and microelectronics compatibility of Si makes it an attractive material for realizing solid-state qubits. Unfortunately, the orbital (valley) degeneracy of the conduction band of bulk Si makes it difficult to isolate individual two-level spin-1/2 states, limiting their development. This degeneracy is lifted within Si quantum wells clad between Ge-Si alloy barrier layers, but the magnitude of the valley splittings achieved so far is small—of the order of 1 meV or less—degrading the fidelity of information stored within such a qubit. Here we combine an atomistic pseudopotential theory with a genetic search algorithm to optimize the structure of layered-Ge/Si-clad Si quantum wells to improve this splitting. We identify an optimal sequence of multiple Ge/Si barrier layers that more effectively isolates the electron ground state of a Si quantum well and increases the valley splitting by an order of magnitude, to ∼9 meV.

Discovery of a Novel Linear-in-k Spin Splitting for Holes in the 2D GaAs/AlAs System

The spin-orbit interaction generally leads to spin splitting (SS) of electron and hole energy states in solids, a splitting that is characterized by a scaling with the wave vector k. Whereas for 3D bulk zinc blende solids the electron (heavy-hole) SS exhibits a cubic (linear) scaling with k, in 2D quantum wells, the electron (heavy-hole) SS is currently believed to have a mostly linear (cubic) scaling. Such expectations are based on using a small 3D envelope function basis set to describe 2D physics. By treating instead the 2D system explicitly as a system in its own right, we discover a large linear scaling of hole states in 2D. This scaling emerges from coupling of hole bands that would be unsuspected by the standard model that judges coupling by energy proximity. This discovery of a linear Dresselhaus k scaling for holes in 2D implies a different understanding of hole physics in low dimensions.