David MacNeill

Breaking of Valley Degeneracy by Magnetic Field in Monolayer MoSe 2

Using polarization-resolved photoluminescence spectroscopy, we investigate the breaking of valley degeneracy by an out-of-plane magnetic field in back-gated monolayer MoSe2 devices. We observe a linear splitting of -0.22  meV/T between luminescence peak energies in σ+ and σ- emission for both neutral and charged excitons. The optical selection rules of monolayer MoSe2 couple the photon handedness to the exciton valley degree of freedom; so this splitting demonstrates valley degeneracy breaking. In addition, we find that the luminescence handedness can be controlled with a magnetic field to a degree that depends on the back-gate voltage. An applied magnetic field, therefore, provides effective strategies for control over the valley degree of freedom.

Pauli Blocking Effect on Efimov States near a Feshbach Resonance

In this Letter we study the effect of Pauli blocking on Efimov states in a quantum Fermi gas and illustrate that the universal Efimov potential is altered at large distances. We obtain the universal spectrum flow of Efimov trimers when the Fermi density is varied and further consider the effect of scattering of trimers by the Fermi sea. We argue that the universal flow is robust against fluctuating particle-hole pairs that result in an infrared catastrophe in impurity problems.

Thickness dependence of spin-orbit torques generated by WTe2

We study current-induced torques in WTe 2 /permalloy bilayers as a function of WTe 2 thickness. We measure the torques using both second-harmonic Hall and spin-torque ferromagnetic resonance techniques for samples with WTe 2 thicknesses that span from 16 nm down to a single monolayer. We confirm the existence of an out-of-plane antidamping torque, and we show directly that the sign of this torque component is reversed across a monolayer step in the WTe 2 . The magnitude of the out-of-plane antidamping torque depends only weakly on WTe 2 thickness, such that even a single-monolayer WTe 2 device provides a strong torque that is comparable to much thicker samples. In contrast, the out-of-plane fieldlike torque has a significant dependence on the WTe 2 thickness. We demonstrate that this fieldlike component originates predominantly from the Oersted field, thereby correcting a previous inference drawn by our group based on a more limited set of samples.

Spin–Orbit Torques in NbSe2/Permalloy Bilayers

We present measurements of current-induced spin-orbit torques generated by NbSe2, a fully metallic transition-metal dichalcogenide material, made using the spin-torque ferromagnetic resonance (ST-FMR) technique with NbSe2/Permalloy bilayers. In addition to the out-of-plane Oersted torque expected from current flow in the metallic NbSe2 layer, we also observe an in-plane antidamping torque with torque conductivity σS ≈ 103 (ℏ/2e)(Ωm)-1 and indications of a weak field-like contribution to the out-of-plane torque oriented opposite to the Oersted torque. Furthermore, in some samples we also measure an in-plane field-like torque with the form m̂ × ẑ, where m̂ is the Permalloy magnetization direction and ẑ is perpendicular to the sample plane. The size of this component varies strongly between samples and is not correlated with the NbSe2 thickness. A torque of this form is not allowed by the bulk symmetries of NbSe2 but is consistent with symmetry breaking by a uniaxial strain that might result during device fabrication.