Interactions and Magnetotransport through Spin-Valley Coupled Landau Levels in Monolayer MoS2

Abstract

The strong spin-orbit coupling and the broken inversion symmetry in monolayer transition metal dichalcogenides (TMDs) results in spin-valley coupled band structures. Such a band structure leads to novel applications in the fields of electronics and optoelectronics. Density functional theory calculations as well as optical experiments have focused on spin-valley coupling in the valence band. Here we present magnetotransport experiments on high-quality n-type monolayer molybdenum disulphide (MoS2) samples, displaying highly resolved Shubnikov-de Haas oscillations at magnetic fields as low as 2 T. We find the effective mass 0.7 me, about twice as large as theoretically predicted and almost independent of magnetic field and carrier density. We further detect the occupation of the second spin-orbit split band at an energy of about 15 meV, i.e. about a factor 5 larger than predicted. In addition, we demonstrate an intricate Landau level spectrum arising from a complex interplay between a density-dependent Zeeman splitting and spin and valley-split Landau levels. These observations, enabled by the high electronic quality of our samples, testify to the importance of interaction effects in the conduction band of monolayer MoS2.Monolayer transition metal dichalcogenides (TMDs) such as MoS 2 , MoSe 2 , WS 2 and WSe 2 are twodimensional (2D) semiconductors with band extrema at the corners (K, K -points) of the first Brillouin zone [1]. Due to the strong spin-orbit coupling the spin degeneracy in the K and K valleys is lifted, with opposite spin polarization normal to the layer plane in opposite valleys (see Fig. 2, inset). This peculiar band structure with coupled spin and valley degrees of freedom results in an anomalous Landau level (LL) structure [2][3][4]. Theoretical proposals predict the formation of LLs under the influence of a perpendicular magnetic field that are arranged differently from those in conventional semiconductor quantum wells and graphene [3]. Magnetotransport measurements have recently been performed in monolayer WSe 2 , MoSe 2 and bilayer MoS 2 revealing two-fold degenerate LLs, large effective masses and carrier density dependent Zeeman splitting [5][6][7][8][9]. Previous works on thicker MoS 2 , MoSe 2 and WSe 2 devices have measured the electron LLs structure at the Q and Q conduction band minima, showing the thickness dependence of the band structure in 2D TMDs [10,11]. Here we focus on single layer MoS 2 where for low electron densities electrons clearly reside at the K-K minima of the bandstructure.Here we report transport measurements in high mobility dual-gated monolayer MoS 2 under a perpendicular magnetic field. Our devices show ohmic contacts at temperatures as low as T ≈ 100 mK allowing us to uncover signatures of so far not reported rich interplay of strong spin-orbit coupling and electron-electron interactions. Shubnikov-de Haas (SdH) oscillations appear already at magnetic fields B ≈ 2 T at a temperature of T ≈ 100 mK. From the temperature dependence of the SdH oscillations we measure an...

Coulomb Blockade Spectroscopy of a MoS2 Nanotube

“…From the experimental side, quantum dots have been defined not only in layers of MoS 2 , but also WSe 2 and WS 2 . The large electron effective mass, recently found to be for MoS 2 and significantly exceeding previous density functional theory (DFT) results, leads to a small level spacing. Consequently, so far, it has been a challenge to reach the regime of single quantum level transport in nanofabricated TMDC quantum dots.…”

Absence of Interlayer Tunnel Coupling of K -Valley Electrons in Bilayer MoS2

“…4). The electron density is calculated from the period of the SdH oscillations in 1/B considering valley degenerate Landau levels at the K and K conduction band minima [13]. At T = 1.5 K and n = 3.8 × 10 12 cm −2 we observe the sequence of odd filling factors ν = 21, 23, 25, ....…”

“…Ohmic behavior of these contacts is achieved by applying a sufficiently positive top gate voltage (V TG ). The heterostructure is assembled using a dry pick-up and transfer method [11][12][13]. We fabricated and measured three bilayer MoS 2 samples, labeled A, B, and C, which show the same behavior.…”

“…1 (d) we extract the m * of the K and K electrons localized in the top MoS 2 layer, thus effectively calculating the effective mass of a monolayer MoS 2 [7]. The effective masses we extract in our bilayer samples are systematically 10−20% lower compared to the ones measured in monolayer MoS 2 [13]. In bilayer MoS 2 the top MoS 2 layer is encapsulated between hBN and the bottom MoS 2 layer, which is devoid of electrons.…”

Probing and Tuning the Spin Textures of the K and Q Valleys in Few‐Layer MoS2

“…The spin texture at the K / K′ valleys of the valence and conduction band has been widely studied by optical measurements, thanks to the direct bandgap nature that enhances optical transition; whereas the transition from the K / K′ point of the valence band to the Q/Q′ point of the conduction band is indirect, which gives a much weaker contribution to the optical response. In contrast, quantum oscillations in magnetoresistance (MR) have been proved to be a useful tool to study the electronic properties of the conduction band extrema of both K and Q pockets . Nevertheless, the observation of quantum oscillations requires not only high carrier mobility but also high‐quality electrical contact.…”