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Even – odd layer-dependent magnetotransport of high-mobility Q-valley electrons in transition metal disulfides

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ABSTRACT

In few-layer transition metal dichalcogenides (TMDCs), the conduction bands along the ΓK directions shift downward energetically in the presence of interlayer interactions, forming six Q valleys related by threefold rotational symmetry and time reversal symmetry. In even layers, the extra inversion symmetry requires all states to be Kramers degenerate; whereas in odd layers, the intrinsic inversion asymmetry dictates the Q valleys to be spin-valley coupled. Here we report the transport characterization of prominent Shubnikov-de Hass (SdH) oscillations and the observation of the onset of quantum Hall plateaus for the Q-valley electrons in few-layer TMDCs. Universally in the SdH oscillations, we observe a valley Zeeman effect in all odd-layer TMDC devices and a spin Zeeman effect in all even-layer TMDC devices, which provide a crucial information for understanding the unique properties of multi-valley band structures of few-layer TMDCs.

No MeSH data available.


BN-TMDC-BN heterostructure device.(a) The sandwiched TMDC heterostructure. (b) The BN-TMDC-BN heterostructure for selective etching. The etching window is marked by arrows. (c,d) Optical (c) and schematic image (d) of a BN-TMDC-BN FE transistor device with a Hall bar configuration. Scale bar, 10 μm. (e,f) Two-terminal ISD−VSD characteristics of a representative MoS2 device at 300 K (e) and 2 K (f). Linear I–V behaviour is observed in both cases. (g) Four-terminal conductance in the WS2 device plotted as a function of the gate voltage at various temperatures. (h,i) FE mobilities and Hall mobilities of MoS2 (h) and WS2 (i) at Vg=60 V at various temperatures.
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f1: BN-TMDC-BN heterostructure device.(a) The sandwiched TMDC heterostructure. (b) The BN-TMDC-BN heterostructure for selective etching. The etching window is marked by arrows. (c,d) Optical (c) and schematic image (d) of a BN-TMDC-BN FE transistor device with a Hall bar configuration. Scale bar, 10 μm. (e,f) Two-terminal ISD−VSD characteristics of a representative MoS2 device at 300 K (e) and 2 K (f). Linear I–V behaviour is observed in both cases. (g) Four-terminal conductance in the WS2 device plotted as a function of the gate voltage at various temperatures. (h,i) FE mobilities and Hall mobilities of MoS2 (h) and WS2 (i) at Vg=60 V at various temperatures.

Mentions: As discussed in our previous work23, to eliminate any impurity effects induced during device fabrication, we employ a polymer-free dry transfer technique2526 in an inert environment of argon or nitrogen, as schematically demonstrated in Fig. 1a–d. Using the encapsulation of few-layer TMDCs in BN sheets and the selective etching process, we can achieve high-quality low-temperature ohmic contacts (0.15–0.5 kΩ·μm) and ultrahigh FE mobilities (10,500–19,600 cm2 V−1 s−1) in TMDC channels. For example, Fig. 1e,f show the ISD–VSD curve of a nine-layer (9L) MoS2 device, where VSD is the voltage source and ISD is the measured current. The linear characteristic of this device is observed at both 300 and 2 K. The contact resistivity at T=2 K is ∼0.25 kΩ·μm (see details in Supplementary Fig. 1a–c for another 6L WS2 device).


Even – odd layer-dependent magnetotransport of high-mobility Q-valley electrons in transition metal disulfides
BN-TMDC-BN heterostructure device.(a) The sandwiched TMDC heterostructure. (b) The BN-TMDC-BN heterostructure for selective etching. The etching window is marked by arrows. (c,d) Optical (c) and schematic image (d) of a BN-TMDC-BN FE transistor device with a Hall bar configuration. Scale bar, 10 μm. (e,f) Two-terminal ISD−VSD characteristics of a representative MoS2 device at 300 K (e) and 2 K (f). Linear I–V behaviour is observed in both cases. (g) Four-terminal conductance in the WS2 device plotted as a function of the gate voltage at various temperatures. (h,i) FE mobilities and Hall mobilities of MoS2 (h) and WS2 (i) at Vg=60 V at various temperatures.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5036047&req=5

f1: BN-TMDC-BN heterostructure device.(a) The sandwiched TMDC heterostructure. (b) The BN-TMDC-BN heterostructure for selective etching. The etching window is marked by arrows. (c,d) Optical (c) and schematic image (d) of a BN-TMDC-BN FE transistor device with a Hall bar configuration. Scale bar, 10 μm. (e,f) Two-terminal ISD−VSD characteristics of a representative MoS2 device at 300 K (e) and 2 K (f). Linear I–V behaviour is observed in both cases. (g) Four-terminal conductance in the WS2 device plotted as a function of the gate voltage at various temperatures. (h,i) FE mobilities and Hall mobilities of MoS2 (h) and WS2 (i) at Vg=60 V at various temperatures.
Mentions: As discussed in our previous work23, to eliminate any impurity effects induced during device fabrication, we employ a polymer-free dry transfer technique2526 in an inert environment of argon or nitrogen, as schematically demonstrated in Fig. 1a–d. Using the encapsulation of few-layer TMDCs in BN sheets and the selective etching process, we can achieve high-quality low-temperature ohmic contacts (0.15–0.5 kΩ·μm) and ultrahigh FE mobilities (10,500–19,600 cm2 V−1 s−1) in TMDC channels. For example, Fig. 1e,f show the ISD–VSD curve of a nine-layer (9L) MoS2 device, where VSD is the voltage source and ISD is the measured current. The linear characteristic of this device is observed at both 300 and 2 K. The contact resistivity at T=2 K is ∼0.25 kΩ·μm (see details in Supplementary Fig. 1a–c for another 6L WS2 device).

View Article: PubMed Central - PubMed

ABSTRACT

In few-layer transition metal dichalcogenides (TMDCs), the conduction bands along the ΓK directions shift downward energetically in the presence of interlayer interactions, forming six Q valleys related by threefold rotational symmetry and time reversal symmetry. In even layers, the extra inversion symmetry requires all states to be Kramers degenerate; whereas in odd layers, the intrinsic inversion asymmetry dictates the Q valleys to be spin-valley coupled. Here we report the transport characterization of prominent Shubnikov-de Hass (SdH) oscillations and the observation of the onset of quantum Hall plateaus for the Q-valley electrons in few-layer TMDCs. Universally in the SdH oscillations, we observe a valley Zeeman effect in all odd-layer TMDC devices and a spin Zeeman effect in all even-layer TMDC devices, which provide a crucial information for understanding the unique properties of multi-valley band structures of few-layer TMDCs.

No MeSH data available.