Session L49: Valley, Spin and Topological Physics

2D Ferromagnetic Semiconductor and Heterostructure

Since the discovery of graphene, a wide range of 2D materials with different physical properties have been created. However, 2D magnets are still lacking. In this talk, I will discuss our recent progress on 2D ferromagnetic semiconductors, including magneto-optical Kerr effect and photoluminescence of excitons as a function of layer thickness, magnetic field, and temperature. In addition, I will present the study of heterostructures formed by layered ferromagnetic semiconductor and monolayer non-magnetic semiconductor, which reveals strong magnetic proximity effect, spin-orientation dependent charge hopping, and magnetic domain dynamics.   

Spin Hall Effect in Monolayer and Bilayer WSe$_2$

Transition metal dichalcogenides (TMDs) are expected to possess a large spin Hall effect thanks to the strong spin-orbit coupling in these materials. The recent progress in realization of high mobility WSe$_2$ samples with Ohmic contacts [1] can facilitate the experimental access to the rich physics of the TMDs. Using non-local resistance measurements in multi-terminal, high mobility hole-doped dual-gated WSe$_2$ samples we extract the spin Hall conductivity as a function of carrier density and temperature. We find that WSe$_2$ possesses a spin Hall conductivity that is weakly dependent on the carrier density in the range $5\times10^{12}$ cm$^{-2} - 10\times10^{12}$ cm$^{-2}$, increases with reducing the temperature down to 1.5 K, and is significantly larger than $e^2/h$. We discuss the different mechanisms, namely intrinsic, side-jump, and skew scattering that contribute to the measured spin Hall conductivity. [1] B. Fallahazad $et$ $al.$, Phys. Rev. Lett. 116, 086601 (2016).   

Floquet-Bloch state of bulk WSe$_2$ by time-resolved angle-resolved photoemission spectroscopy

Light-matter interaction in novel materials has attracted much attention recently. Pumped by a strong laser field, a non-equilibrium electronic state, Floquet-Bloch (FB) state may occur in crystals [1]. As an extension of photon dressed states in gas molecules to crystals, FB states show the same band dispersion of the ground state bands with energy spacings equal to integer multiples of the pump photon energy. In this study, the FB states of the valence bands near the K point of bulk 2H-phase WSe$_2$ are observed by time- and angle-resolved photoemission spectroscopy (TRARPES). The probe pulse is generated by high harmonic generation (28 eV) and the pump pulse is delivered by a femtosecond laser with a wavelength of 800 nm. The replica bands at E + h$\nu$ and E - h$\nu$ for the two topmost valence bands are observed within 120 fs, which reflects the cross correlation between the pump and probe pulses. Laser-assisted photoemission (LAPE) is another possible mechanism to generate replica bands in TRARPES. By changing the polarization of the pump pulse from p- to s-polarization, the contribution from LAPE decreases to zero; thus, the replica bands observed with s-polarization are pure FB state. [1] Wang, Y. H., Steinberg, H., Jarillo-Herrero, P. and Gedik, Science 342, 453 (2013)   

Bloch-Siegert shift in monolayer WS2

Coherent light-matter interaction can be used to manipulate the energy levels of atoms, molecules and solids. Under off-resonance photon driving in such systems, repulsion between photon-dressed (Floquet) states can lead to a shift of energy resonance. The dominant effect is the optical Stark shift, but there is an additional contribution from the so-called Bloch-Siegert shift. Although it is common in atoms, the observation of Bloch-Siegert shift in solids has so far been limited only to artificial atoms since the shifts were tiny and inseparable from the optical Stark shift. Here we observe an exceptionally large Bloch-Siegert shift in monolayer WS2 by virtue of the strong light-matter interaction in this system. Moreover, the Bloch-Siegert shift can be separated entirely from the optical Stark shift by breaking time-reversal symmetry.   

Dirac semimetal state in black phosphorus

Two-dimensional (2D) semiconductors have emerged as a class of materials that may impact our future electronics technologies. A key issue is controlling their electronic band structure widely to overcome the limit of natural properties. In this talk, I will introduce our recent angle-resolved photoemission spectroscopy studies on the widely tunable band gap in a 2D semiconductor, black phosphorus. We found that the in situ deposition of alkali-metal atoms modulates the band gap in the surface phosphorene layers of bulk black phosphorus by the giant Stark effect, resulting in the transition from a narrow-gap semiconductor to a 2D Dirac semimetal with a pair of Dirac cones. At the critical point of this transition, black phosphorus is a zero-gap semimetal, whose band dispersion is highly anisotropic, linear in armchair and quadratic in zigzag directions.   

Valley pump and beam splitting by a potential well on a monolayer hexagonal crystal

We investigate the possibilities of realizing two distinct valleytronic functionalities in a single device by tuning an in‐situ controllable parameter, without changing the architecture of the system. We show that by means of a two‐dimenensional p‐n‐p junction or a lateral hetereojunction formed in a monolayer, two kinds of valleytronic functions can be realized by just changing the band alignment at the junction for both transition metal dichalcogenides and graphene. With shallow potential well, valley beam splitting is observed. Deepening the potential well by the applied gate voltages, we find highly polarized valley flow over a large range of out‐going angles. We further reveal that inter‐valley population transfer, whose importance can be adjusted by the gate potential, plays a crucial role in manifesting these interesting functionalities. These properties are maintained for armchair oriented junction interface as well as other chiral orientations.   

Generation and evolution of spin-, valley- and layer-polarized excited carriers in inversion-symmetric WSe$_2$

We present the spin-selective optical excitation of carriers in inversion-symmetric bulk samples of the layered transition metal dichalcogenide WSe$_2$. Employing time- and angle-resolved photoelectron spectroscopy (trARPES) and time-dependent density functional theory (TDDFT), we observe spin-, valley- and layer-polarized excited state populations upon excitation with circularly polarized laser pulses. We find that scattering of carriers towards the global minimum of the conduction band is very efficient and proceeds within less than 100 fs. TDDFT reveals the character of the conduction band, into which electrons are initially excited, to be two-dimensional and localized within individual layers, whereas at the global minimum of the conduction band, states have a three-dimensional character, facilitating interlayer charge-transfer.   

Charge and Spin-Valley Transfer in Transition Metal Dichalcogenides Heterostructure

Monolayer transition metal dichalcogenides (TMDC) offer new avenues to control valley and spin polarization based on their valley circular dichroism and spin-valley locking. In this context, interesting issues arise when two TMDC layers are stacked in a vertical heterostructure and interlayer charge transfer processes become possible. Here we address the question of whether the spin and valley characteristics of photoexcited carriers are persevered under interlayer charge transfer. We have observed experimentally the preservation of the spin-valley characteristics for MoS$_{\mathrm{2}}$/ WSe$_{\mathrm{2}}$ heterostructures. We excite the A exciton in WSe$_{\mathrm{2}}$ (\textasciitilde 780 nm) with circularly polarized ultrafast pump radiation. The spin-valley characteristics of the transferred charge are examined through the induced Kerr rotation for both the A (\textasciitilde 670 nm) and B excitonic (\textasciitilde 620 nm) transitions in MoS$_{\mathrm{2}}$. These signatures are not observed in either of the separated monolayers, verifying the role of transfer from one layer to another.   

Dynamics of valley pseudospin in single-layer WSe2. Inter-valley scattering mediated by electron-phonon interaction

In a time-dependent Kerr experiment a circularly polarized laser field is used to selectively populate the $K^{\pm}$ electronic valleys of single-layer WSe$_2$. This carrier population corresponds to a finite pseudospin polarization that dictates the valleytronic properties of WSe$_2$, but whose decay mechanism still remains largely debated. Time-dependent Kerr experiments provide an accurate way to visualize the pseudospin dynamics by measuring the rotation of a linearly polarized probe pulse applied after a circularly polarized and short pump pulse. We present here a clear, accurate and parameter-free description of the valley pseudospin dynamics in single-layer WSe$_2$. By using an ab-initio approach we solve unambiguously the long—standing debate about the dominant mechanism that drives the valley depolarization. Our results are in excellent agreement with recent time-dependent Kerr experiments. The decay dynamics and peculiar temperature dependence is explained in terms of electron—phonon mediated processes that induce spin-flip inter-valley transitions.   

Magneto-optical studies of transition-metal dichalcogenides using visible pump, mid-infrared probe measurements

We report systematic magneto-optical measurements on transition-metal dichalcogenides such as MoS2 and WS2. We perform polarization-sensitive photoluminescence measurements when populating different valleys as a function of temperature. By measuring the mid-infrared (110--230 meV) Faraday and Kerr signals while populating different valleys using polarized visible light at zero magnetic field, we test time-reversal symmetry breaking in these materials. This work is supported by NSF-DMR1410599.   

Electron density dependent Zeeman splitting in WS$_{2}$ single layer

We report an experimental and theoretical study of the magnetic field dependence of the circular polarization $P$ and the Zeeman splitting $\Delta E_{z} $ of the photoluminescence (PL) due to recombination of charged excitons (X$^{-})$ formed because excess electrons are present. Results from three WS$_{2\, \, }$monolayers are reported. For one sample the circular polarization at $B \quad =$ 0 T is finite while $P$ is zero for the other two samples. The slope $\frac{dP}{dB}$ in all samples is similar ($\approx $2.4 {\%} per Tesla). In contrast, the Zeeman splittings are: -0.13 meV/T, -0.30 meV/T and -0.47 meV/T. The experimental results are compared with those from the microscopic tight binding model to evaluate spin, valley and orbital contribution to the Zeeman splitting. The effect of carrier density on $\Delta E_{z} $ is included in a Hartee-Fock picture assuming inhomogeneous carrier distribution. The electron droplets present are divided into spin-valley polarized and spin-valley unpolarized. The spin-valley polarized droplets contribute to Zeeman splitting and are responsible for the carrier density and sample dependent Zeeman splitting in WS$_{2}$.