Anomalously Robust Valley Polarization and Valley Coherence in Bilayer WS2

Coherence is a crucial requirement to realize quantum manipulation through light-matter interactions. Here we report the observation of anomalously robust valley polarization and valley coherence in bilayer WS2. The polarization of the photoluminescence from bilayer WS2 inherits that of the excitation source with both circularly and linearly polarized and retains 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, while the linear 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 opens new perspectives for quantum manipulation in atomically thin semiconductors.

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

Motivated by the triumph and limitation of graphene for electronic applications, atomically thin layers of group VI transition metal dichalcogenides are attracting extensive interest as a class of graphene-like semiconductors with a desired band-gap in the visible frequency range. The monolayers feature a valence band spin splitting with opposite sign in the two valleys located at corners of 1st Brillouin zone. This spin-valley coupling, particularly pronounced in tungsten dichalcogenides, can benefit potential spintronics and valleytronics with the important consequences of spin-valley interplay and the suppression of spin and valley relaxations. Here we report the first 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 mutilayers to a direct-gap one at monolayers. The PL spectra and first-principle calculations consistently reveal a spin-valley coupling of 0.4 eV which suppresses interlayer hopping and manifests as a thickness independent splitting pattern at valence band edge near K points. This giant spin-valley coupling, together with the valley dependent physical properties, may lead to rich possibilities for manipulating spin and valley degrees of freedom in these atomically thin 2D materials.

Valley polarization in MoS2 monolayers by optical pumping

We report experimental evidences on selective occupation of the degenerate valleys in MoS2 monolayers by circularly polarized optical pumping. Over 30% valley polarization has been observed at K and K valley via the polarization resolved luminescence spectra on pristine MoS2 monolayers. It demonstrates one viable way to generate and detect valley polarization towards the conceptual valleytronics applications with information carried by the valley index.

Magneto-electric photocurrent generated by direct inter-band transitions in InGaAs/InAlAs two-dimensional electron gas

We report observation of magneto-electric photocurrent generated via direct inter-band transitions in an InGaAs/InAlAs two-dimensional electron gas excited 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 electric current from a spin photocurrent. The observed light polarization dependence of the electric current is explained microscopically by taking into account of the anisotropy of the photoexcited carrier density in wave vector space. The spin photocurrent can be extracted from the measured current and the conversion coefficient of spin photocurrent to electric current is estimated to be $10^{-3}$$sim$$10^{-2}$ per Tesla.

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 time resolved Kerr rotation technique to precisely measure the sign and the amplitude of electron g factor in semiconductors.

Observation of 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.