Yong-Tao Cui

Direct observation of the transition from indirect to direct bandgap in atomically thin epitaxial MoSe2

Quantum systems in confined geometries are host to novel physical phenomena. Examples include quantum Hall systems in semiconductors and Dirac electrons in graphene. Interest in such systems has also been intensified by the recent discovery of a large enhancement in photoluminescence quantum efficiency and a potential route to valleytronics in atomically thin layers of transition metal dichalcogenides, MX2 (M = Mo, W; X = S, Se, Te), which are closely related to the indirect-to-direct bandgap transition in monolayers. Here, we report the first direct observation of the transition from indirect to direct bandgap in monolayer samples by using angle-resolved photoemission spectroscopy on high-quality thin films of MoSe2 with variable thickness, grown by molecular beam epitaxy. The band structure measured experimentally indicates a stronger tendency of monolayer MoSe2 towards a direct bandgap, as well as a larger gap size, than theoretically predicted. Moreover, our finding of a significant spin-splitting of ∼ 180 meV at the valence band maximum of a monolayer MoSe2 film could expand its possible application to spintronic devices.

Measurement of the spin-transfer-torque vector in magnetic tunnel junctions

The transfer of spin angular momentum from a spin-polarized current to a ferromagnet can generate sufficient torque to reorient the magnet’s moment. This torque could enable the development of efficient electrically actuated magnetic memories and nanoscale microwave oscillators. Yet difficulties in making quantitative measurements of the spin-torque vector have hampered understanding. Here we present direct measurements of both the magnitude and direction of the spin torque in magnetic tunnel junctions, the type of device of primary interest for applications. At low bias V, the differential torque dτ/dV lies in the plane defined by the electrode magnetizations, and its magnitude is in excellent agreement with recent predictions for near-perfect spin-polarized tunnelling. We find that the strength of the in-plane differential torque remains almost constant with increasing bias, despite a substantial decrease in the device magnetoresistance, and that with bias the torque vector also rotates out of the plane.

Observation of Temperature-Induced Crossover to an Orbital-Selective Mott Phase in AxFe2-ySe2 (A=K, Rb) Superconductors

Using angle-resolved photoemission spectroscopy, we observe the low-temperature state of the A(x)Fe(2-y)Se(2) (A=K, Rb) superconductors to exhibit an orbital-dependent renormalization of the bands near the Fermi level-the d(xy) bands heavily renormalized compared to the d(xz)/d(yz) bands. Upon raising the temperature to above 150 K, the system evolves into a state in which the d(xy) bands have depleted spectral weight while the d(xz)/d(yz) bands remain metallic. Combined with theoretical calculations, our observations can be consistently understood as a temperature-induced crossover from a metallic state at low temperatures to an orbital-selective Mott phase at high temperatures. Moreover, the fact that the superconducting state of A(x)Fe(2-y)Se(2) is near the boundary of such an orbital-selective Mott phase constrains the system to have sufficiently strong on-site Coulomb interactions and Hunds coupling, highlighting the nontrivial role of electron correlation in this family of iron-based superconductors.

Bias and Angular Dependence of Spin-Transfer Torque in Magnetic Tunnel Junctions

We use spin-transfer-driven ferromagnetic resonance (ST-FMR) to measure the spin-transfer torque vector

Time-resolved measurement of spin-transfer-driven ferromagnetic resonance and spin torque in magnetic tunnel junctions

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Strong linewidth variation for spin-torque nano-oscillators as a function of in-plane magnetic field angle

in MgO-based magnetic tunnel junctions as a function of the offset angle between the magnetic moments of the electrodes and as a function of bias,

Sensitivity of spin-torque diodes for frequency-tunable resonant microwave detection

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Mobile metallic domain walls in an all-in-all-out magnetic insulator

. We explain the conflicting conclusions of two previous experiments by accounting for additional terms that contribute to the ST-FMR signal at large

Single-Shot Time-Domain Studies of Spin-Torque-Driven Switching in Magnetic Tunnel Junctions

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Resonant spin-transfer-driven switching of magnetic devices assisted by microwave current pulses

. Including the additional terms gives us improved precision in the determination of