Junfeng Dai

Optical signature of symmetry variations and spin-valley coupling in atomically thin tungsten dichalcogenides

We report systematic optical studies of WS2 and WSe2 monolayers and multilayers. The efficiency of second harmonic generation shows a dramatic even-odd oscillation with the number of layers, consistent with the presence (absence) of inversion symmetry in even-layer (odd-layer). Photoluminescence (PL) measurements show the crossover from an indirect band gap semiconductor at multilayers to a direct-gap one at monolayers. A hot luminescence peak (B) is observed at ~0.4 eV above the prominent band edge peak (A) in all samples. The magnitude of A-B splitting is independent of the number of layers and coincides with the spin-valley coupling strength in monolayers. Ab initio calculations show that this thickness independent splitting pattern is a direct consequence of the giant spin-valley coupling which fully suppresses interlayer hopping at valence band edge near K points because of the sign change of the spin-valley coupling from layer to layer in the 2H stacking order.

Anomalously robust valley polarization and valley coherence in bilayer WS2.

Significance Coherence of electronic states is crucial for quantum manipulation through light–matter interactions. To achieve coherence in conventional solid-state systems, extreme conditions such as cryogenic temperatures are required, which is a long-term challenge for practical applications. The emerging atomically thin transition metal dichalcogenides provide an unprecedented platform to explore the interplay of quantum states of spin and valley. In this paper, we demonstrate room-temperature valley coherence and valley polarization in bilayer WS2 with polarization-resolved photoluminescence measurements. The robustness of the valley coherence and valley polarization is understood as the consequence of the coupling of spin, layer, and valley degrees of freedom in bilayer WS2. It inspires new perspectives on quantum manipulations in 2D solid-state systems. We report the observation of anomalously robust valley polarization and valley coherence in bilayer WS2. The polarization of the photoluminescence from bilayer WS2 follows that of the excitation source with both circular and linear polarization, and remains even at room temperature. The near-unity circular polarization of the luminescence reveals the coupling of spin, layer, and valley degree of freedom in bilayer system, and the linearly polarized photoluminescence manifests quantum coherence between the two inequivalent band extrema in momentum space, namely, the valley quantum coherence in atomically thin bilayer WS2. This observation provides insight into quantum manipulation in atomically thin semiconductors.

Measurements on quantum capacitance of individual single walled carbon nanotubes

We report a measurement on quantum capacitance of individual semiconducting and small band gap SWNTs. The observed quantum capacitance is remarkably smaller than that originating from density of states and it implies a strong electron correlation in SWNTs.

Magnetoelectric photocurrent generated by direct interband transitions in InGaAs/InAlAs two-dimensional electron gas

We report the observation of magnetoelectric photocurrent generated via direct interband transitions in an InGaAs/InAlAs two-dimensional electron gas by a linearly polarized incident light. The electric current is proportional to the in-plane magnetic field, which unbalances the velocities of the photoexcited carriers with opposite spins and consequently generates the electric current from a hidden spin photocurrent. The spin photocurrent can be evaluated from the measured electric current, and the conversion coefficient of spin photocurrent to electric current is self-consistently estimated to be 10(-3)-10(-2) per Tesla. The observed light-polarization dependence of the electric current is well explained by a theoretical model which reveals the wave vector angle dependence of the photoexcited carrier density.

Determination of the sign of g factors for conduction electrons using time-resolved Kerr rotation

The knowledge of electron g factor is essential for spin manipulation in the field of spintronics and quantum computing. While there exist technical difficulties in determining the sign of g factor in semiconductors by the established magneto-optical spectroscopic methods. We develop a technique to precisely measure the sign and the amplitude of electron g factor in semiconductors using time resolved Kerr rotation.