Magnetic control of valley properties in two dimensional transition metal dichalcogenides
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Two dimensional transition metal dichalcogenides are of great interests during the past decade. The broken inversion symmetry in monolayer TMDs leads to two degenerate but inequivalent valleys K and K [1, 2]. Together with strong spin-orbit coupling which split the band edge states, K and K’ have opposite spin characters that make it possible to selectively excite the carriers within a particular valley using circularly polarized light [3-5]. Lifting the valley degeneracy is very important, since it would allow for control of valley polarization by electric field for memory and logic application [6-9]. In this thesis, I demonstrate that by using an exchange field from ferromagnetic substrates, the valley splitting of TMDs such as WSe2 and WS2 can be strongly enhanced.In the present thesis, the first chapter will be on the synthesis of 2D TMDs. We first study the synthesis of high quality TMDs using a modified chemical vapor deposition (CVD) method. Different from the traditional CVD method, we pre-deposited MoO3 or WO3 onto sapphire substrates using electron beam (e-beam) deposition . Then, the substrates with the oxide layer and S or Se powder were loaded into different zones of a tube furnace with different temperature. A N2/H2 gas mixture (forming gas) were used as the carrier gas. H2 will help to accelerate the reaction rate leading to larger single crystal samples. By this way we can get 100 µm sized samples with high optical quality, as evidenced by strong excitonic emission with peak width comparable to that of exfoliated samples. We then investigate exchange field induced valley splitting in TMD/ferromagnetic heterostructures, using the ferromagnetic insulator EuS as the magnetic substrate. Since Eu2+ has large magnetic moment (Sz = 7µB) and large exchange coupling ( J = 10 meV), which will lead to a large magnetic exchange field (MEF) [11, 12]. We use magnetic reflectance measurement to detect the valley splitting of WSe2 on EuS and WS2 on EuS. Valley splitting for WSe2(WS2) A exciton on Si/SiO2 is ~1.5meV(1.5meV), while it is 4meV(-20meV) for samples on EuS substrate in 7T 7K. And the slope of valley splitting vs. magnetic field at low field is enhanced by more than an order of magnitude, suggesting strong exchange coupling between TMDCs and EuS. Moreover, opposite signs of valley splitting were observed for WSe2 and WS2. At the same time, we measured the B exciton valley splitting of WSe2 on EuS, which also shows a sign reversal from the splitting of A exciton. A theoretic model is established to explain how the bands are shifted under the exchange field of EuS. It is found that surface terminations and band alignment play critical roles in determining the magnitude and sign of valley splitting.