Ambipolar Landau levels and strong band-selective carrier interactions in monolayer WSe2

Gustafsson, Martin V.; Yankowitz, Matthew; Forsythe, Carlos; Rhodes, Daniel; Watanabe, Kenji; Taniguchi, Takashi; Hone, James; Zhu, Xiaoyang; Dean, Cory

doi:10.1038/s41563-018-0036-2

Cited by 83 publications

(86 citation statements)

References 38 publications

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“…At electron densities > 4 ×10 12 cm −2 , corresponding to a Fermi energy > 15 meV, the upper spin-orbit split bands start to be populated and the complex LL structure of the different valley-spin polarized bands is observed. We give evidence of intricate physics beyond the single particle picture that was employed to explain the experimental results in previous works [5][6][7][8][9].…”

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confidence: 80%

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

et al. 2018

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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...

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

et al. 2018

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“…All devices made use of single crystal hexagonal boron nitride gate dielectrics [24] and graphite gates [25], which in combination are known to minimize extrinsic charge disor-der. Devices A1, S1, and A2/S2 were fabricated using WSe 2 crystals grown by a flux method [32], while device A3 was fabricated using WSe 2 from a commercial source (2Dsemiconductors.com). A mixture of CHF 3 and O 2 was used to dry etch the stacks to define the device area and create connections to the bilayer graphene and top and bottom gates.…”

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confidence: 99%

Spin–orbit-driven band inversion in bilayer graphene by the van der Waals proximity effect

Island

Cui

Lewandowski

et al. 2019

Nature

179

166

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Spin orbit coupling (SOC) is the key to realizing time-reversal invariant topological phases of matter [1, 2]. Famously, SOC was predicted by Kane and Mele[3] to stabilize a quantum spin Hall insulator; however, the weak intrinsic SOC in monolayer graphene [4][5][6][7] has precluded experimental observation. Here, we exploit a layer-selective proximity effect-achieved via van der Waals contact to a semiconducting transition metal dichalcogenide[8-21]-to engineer Kane-Mele SOC in ultra-clean bilayer graphene. Using high-resolution capacitance measurements to probe the bulk electronic compressibility, we find that SOC leads to the formation of a distinct incompressible, gapped phase at charge neutrality. The experimental data agrees quantitatively with a simple theoretical model in which the new phase results from SOC-driven band inversion. In contrast to Kane-Mele SOC in monolayer graphene, the inverted phase is not expected to be a time reversal invariant topological insulator, despite being separated from conventional band insulators by electric field tuned phase transitions where crystal symmetry mandates that the bulk gap must close [22]. Electrical transport measurements, conspicuously, reveal that the inverted phase has a conductivity ∼ e 2 /h, which is suppressed by exceptionally small in-plane magnetic fields. The high conductivity and anomalous magnetoresistance are consistent with theoretical models that predict helical edge states within the inversted phase, that are protected from backscattering by an emergent spin symmetry that remains robust even for large Rashba SOC. Our results pave the way for proximity engineering of strong topological insulators as well as correlated quantum phases in the strong spin-orbit regime in graphene heterostructures. arXiv:1901.01332v2 [cond-mat.mes-hall]

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“…We also set realistic estimates for the mass in a TMD monolayer, m = 0.5 m e [38,39], its degeneracy g s σ g s v = 2 (corresponding to hole doping), and the velocity in existing DSMs v d = 10 6 m/s.…”

Section: Analytic Calculation Of Tc Within the Acoustic Plasmon Apmentioning

confidence: 99%

Synthesizing Coulombic superconductivity in van der Waals bilayers

Fatemi

Ruhman

2018

Phys. Rev. B

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Synthesizing a polarizable environment surrounding a low-dimensional metal to generate superconductivity is a simple theoretical idea that still awaits a convincing experimental realization. The challenging requirements are satisfied in a metallic bilayer when the ratio between the Fermi velocities is small and both metals have a similar, low carrier density. In this case, the slower electron gas acts as a retarded polarizable medium (a "dielectric" environment) for the faster metal. Here we show that this concept is naturally optimized for the case of an atomically thin bilayer consisting of a Dirac semimetal (e.g., graphene) placed in atomic-scale proximity to a doped semiconducting transition metal dichalcogenide (e.g., WSe 2 ). The superconducting transition temperature that arises from the dynamically screened Coulomb repulsion is computed using the linearized Eliashberg equation. In the case of graphene on WSe 2 , we find that T c can exceed 100 mK, and it increases further when the Dirac valley degeneracy is reduced. Thus, we argue that suspended van der Waals bilayers are in a unique position to realize experimentally this long-anticipated theoretical concept.

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Ambipolar Landau levels and strong band-selective carrier interactions in monolayer WSe2

Cited by 83 publications

References 38 publications

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

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

Spin–orbit-driven band inversion in bilayer graphene by the van der Waals proximity effect

Synthesizing Coulombic superconductivity in van der Waals bilayers

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