We investigate valley dynamics associated with trions in monolayer tungsten diselenide (WSe2) using polarization resolved two-color pump-probe spectroscopy. When tuning the pump and probe energy across the trion resonance, distinct trion valley polarization dynamics are observed as a function of energy and attributed to the intra-valley and inter-valley trions in monolayer WSe2. We observe no decay of a near-unity valley polarization associated with the intra-valley trions during ~ 25 ps, while the valley polarization of the inter-valley trions exhibits a fast decay of ~ 4 ps. Furthermore, we show that resonant excitation is a prerequisite for observing the long-lived valley polarization associated with the intra-valley trion. The exceptionally robust valley polarization associated with resonantly created intra-valley trions discovered here may be explored for future valleytronic applications such as valley Hall effects.Keywords: atomically thin semiconductors, valley, trions, ultrafast dynamics PACS:The valley degree of freedom (DoF) indexes the crystal momentum of a local energy minimum within the electronic band structure, and has been proposed as an alternative information carrier, analogous to charge and spin [1]. In atomically thin transition metal dichalcogenides (TMDs), fundamental optical excitations, excitons (electron-hole pairs) and trions (charged excitons), are formed at the hexagonal Brillouin zone boundaries at the ( ′) points. As such, they inherit the valley index which is locked with electron spins in TMDs. Thus, exciton and trion resonances allow optical access and manipulation of the valley DoF in TMDs using circularly polarized light [2][3][4][5][6]. The exceptionally large binding energies of these quasiparticles (i.e. 200-500 meV for excitons and an additional binding energy of 20-40 meV for trions) further promise room temperature valleytronic applications [2,3,[7][8][9][10][11][12][13].High efficiency valley initialization and a long lifetime of valley polarization are preferred in valleytronic applications [14][15][16][17]. Initial experiments based on steadystate photoluminescence have shown the possibility of creating a near-unity valley polarization in MoS2 and WSe2 via exciton resonances [4,18]. Time-resolved measurements soon revealed that exciton valley polarization is quickly lost (~ 1 ps) due to intrinsic electron-hole exchange interaction [19]. The large initial exciton valley polarization observed in the steady-state PL results from the competition between the valley depolarization time (~ 1 ps) and the exciton population relaxation time (~ 100-200 fs) [13,20,21]. On the other hand, trions offer an interesting alternative route for optical manipulation of the valley index for a number of reasons. First, in contrast to the ultrafast exciton population relaxation time, trions exhibit an extended population relaxation time of tens of picoseconds in monolayer TMDs [22][23][24][25][26][27][28][29][30]. Secondly, trions as charged quasiparticles influence both transport and optical...

Two-dimensional nanoelectronics, plasmonics, and emergent phases require clean and local charge control, calling for layered, crystalline acceptors or donors. Our Raman, photovoltage, and electrical conductance measurements combined with ab initio calculations establish the large work function and narrow bands of α-RuCl 3 enable modulation doping of exfoliated single and bilayer graphene, chemical vapor deposition grown graphene and WSe 2 , and molecular beam epitaxy grown EuS. We further demonstrate proof of principle photovoltage devices, control via twist angle, and charge transfer through hexagonal boron nitride. Short-ranged lateral doping (≤65 nm) and high homogeneity are achieved in proximate materials with a single layer of α-RuCl 3 . This leads to the best-reported monolayer graphene mobilities (4900 cm 2 /(V s)) at these high hole densities (3 × 10 13 cm −2 ) and yields larger charge transfer to bilayer graphene (6 × 10 13 cm −2 ).

We present initial Raman spectroscopy experiments on exfoliated flakes of α-RuCl 3 , from tens of nm thick down to single layers. Besides unexpectedly finding this material to be air stable, in the thinnest layers we observe the appearance with decreasing temperature of a symmetry-forbidden mode in crossed polarization, along with an anomalous broadening of a mode at 164 cm −1 that is known to couple to a continuum of magnetic excitations. This may be due to an enhancement of magnetic fluctuations and evidence for a distorted honeycomb lattice in single-and bi-layer samples. arXiv:1709.00431v1 [cond-mat.str-el] 1 Sep 2017 the spin couple along different bonds (see Figure 1a). This model can be realized in materials under the right conditions of crystal electric field, spinorbit coupling and on-site Coulomb repulsion that produce an insulator with J ef f = 1/2 moments. In systems where the honeycomb lattice is formed by placing the magnetic atom inside edge-sharing octahedra, one can realize the necessary bond-dependent exchange due to the impact of strong spin-orbit coupling on the hopping (see Figure 1) [3,4,5]. A key difficulty with this proposal is that additional interaction terms may arise and produce long range order [6,7,8,9]. While some of these terms are enabled simply by symmetry, they are strongly enhanced by lattice distortions that mix the J ef f = 1/2 and J ef f = 3/2 states, altering the hopping terms. Recently, α-RuCl 3 has emerged as a potential candidate to realize a Kitaev quantum spin liquid state [10,11,2,12,13,14,15,16,17,18,19,20].IR, Raman and photo-emission spectroscopy combined with DFT calculations strongly suggest the system is close to the J ef f = 1/2 limit, with octahedra that are nearly undistorted at low temperatures. Perhaps due to the smaller spin-orbit coupling expected in a 4d system, α-RuCl 3 reveals an extremely narrow spin-orbit exciton (2 meV wide) well separated from charge excitations [14]. Thus the low-energy model of α-RuCl 3 does not contain any charge fluctuations, unlike the 5d Ir systems where the spin-orbit and onsite d−d excitations are overlapped in energy [9]. Perhaps most promising is the observed continuum of magnetic excitations, where the Raman temperature dependence and the excitation dispersion seen by neutrons is consistent with fractional particles expected from the pure Kitaev model [2,11,21,20].Despite its importance to the formation of an ordered state, the structure of α-RuCl 3 remains controversial. In particular, the exact structure appears to be sensitive to atomic disorder and stacking faults, which are not uncommon in van der Waals crystals such as α-RuCl 3 [15,22,23,21]. Surprisingly, the addition of stacking faults leads to an enhanced onset of antiferromagnetic order (higher T N ) [15]. This rather counterintuitive observation may result from additional tunneling pathways opened by the stacking disorder that boost the Heisenberg terms. If correct, this suggests that exfoliating α-RuCl 3 down to single layers could suppress the long range ord...

Magnetic van der Waals (vdW) materials are the centerpiece of atomically thin devices with spintronic and optoelectronic functions. Exploring new chemistry paths to tune their magnetic and optical properties enables significant progress in fabricating heterostructures and ultracompact devices by mechanical exfoliation. The key parameter to sustain ferromagnetism in 2D is magnetic anisotropy-a tendency of spins to align in a certain crystallographic direction known as easy-axis. In layered materials, two limits of easy-axis are in-plane (XY) and out-of-plane (Ising). Light polarization and the helicity of topological states can couple to magnetic anisotropy with promising photoluminescence or spin-orbitronic functions. Here, a unique experiment is designed to control the easy-axis, the magnetic transition temperature, and the optical gap simultaneously in a series of CrCl Br crystals between CrCl with XY and CrBr with Ising anisotropy. The easy-axis is controlled between the two limits by varying spin-orbit coupling with the Br content in CrCl Br . The optical gap, magnetic transition temperature, and interlayer spacing are all tuned linearly with x. This is the first report of controlling exchange anisotropy in a layered crystal and the first unveiling of mixed halide chemistry as a powerful technique to produce functional materials for spintronic devices.