Strongly Interaction-Enhanced Valley Magnetic Response in Monolayer WSe2

Abstract

Coulomb interaction between electrons lies at the heart of magnetism in solids 1, 2 . In contrast to conventional two-dimensional (2D) systems, electrons in monolayer transition metal dichalcogenides (TMDs) possess coupled spin and valley degrees of freedom by the spin-orbit interaction 3, 4 . The electrons are also strongly interacting even in the high-density regime because of the weak dielectric screening in two dimensions and a large band mass 5,6 . The combination of these properties presents a unique platform for exploring spin and valley magnetism in 2D electron liquids. Here we report an observation by magneto-photoluminescence spectroscopy of a nonlinear valley Zeeman effect, correlated with an over fourfold enhancement in the exciton g-factor in monolayer WSe 2 . The effect occurs when the Fermi level crosses the spin-split upper conduction band, corresponding to a change of the spin-valley degeneracy from 2 to 4. The enhancement increases, shows no sign of saturation as the sample temperature decreases. Our result suggests the possibility of rich manybody ground states in monolayer TMDs with multiple internal degrees of freedom.Electrons in monolayer transition metal dichalcogenide (TMD) semiconductors with a honeycomb lattice structure possess a two-fold valley degree of freedom, corresponding to the K and K' point of the Brillouin zone [3][4][5] . Because of the strong spinorbit interaction, the bands are spin split with the valley and spin locked to satisfy the time reversal symmetry 3-5 (Fig. 1a). Similar to the spin, the valley carries a magnetic moment and has been proposed as a new type of information carriers 3,7 . Several new valley dependent phenomena, including the valley contrasting optical selection rules [8][9][10][11][12] , valley Zeeman effect [13][14][15][16][17][18] and valley Hall effect 19,20 , have emerged in the independentparticle picture and provided means to manipulate the valley polarization. In particular, the valley exciton splitting in an out-of-plane magnetic field has been shown to depend linearly on the field up to 65 Tesla 18 and an exciton g-factor between -2 and -4 has been reported for various monolayer TMDs 6, 13-18 . On the other hand, even in the relatively high-density regime (~ 5×10 !" cm -2 ), electrons in monolayer TMDs are strongly interacting with the Coulomb energy (~ 100's meV) dominating all other energy scales (Fermi energy, conduction band spin splitting at K/K' ~ 10's meV for WSe 2 ) 6 . The valley magnetic response of the independent-particle picture is thus expected to be modified by the strong electron-electron interaction and the system may even develop a magnetically ordered ground state [21][22][23][24] . A unique scenario emerges when the Fermi level crosses the spin-split upper conduction band (Fig. 1a), where the spin-valley degeneracy ! ! changes from 2 to 4 ( ! and ! stand for the spin and valley degeneracy, respectively) 6 . We observe a strongly enhanced valley magnetic response, which can be understood as a consequence of a step rise in ...

Landau-Quantized Excitonic Absorption and Luminescence in a Monolayer Valley Semiconductor

“…This exceeds the g-factor (1.5) predicted by a single-particle model. Recent experiments also report g-factor enhancement in the conduction valleys of bilayer MoS2 [37], monolayer and bilayer MoSe2 [34,80], and the valence valleys of monolayer WSe2 [39,81]. Here we further demonstrate that many-body interactions can enhance the g-factors in both the conduction and valence valleys of gated monolayer WSe2 [ Fig.…”

Emerging photoluminescence from the dark-exciton phonon replica in monolayer WSe2

“…Light-matter interactions are often dictated by optical selection rules which enable access to unique material properties such as valley-spin degrees of freedom in transition metal dichalcogenides (TMDCs) [1][2][3][4][5][6][7][8][9][10] . However, the selection rules based on electric dipole approximations also render many important processes optically "dark", prohibiting the extraction of critical material properties.…”

Enhanced Valley Zeeman Splitting in Fe-Doped Monolayer MoS2

“…8 Therefore, the possibility of electron doping leading to the increase in g eff factor is ruled out. 8,18,19,43 We also measured Zeeman-type spin splitting of defect emission in monolayer MoS 2 (see Supplementary Information Fig. S8c-f).…”