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Register a new userOdd-integer quantum Hall states and giant spin susceptibility in p-type few-layer WSe2
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We fabricate high-mobility p-type few-layer WSe2 field-effect transistors and surprisingly observe a series of quantum Hall (QH) states following an unconventional sequence predominated by odd-integer states under a moderate strength magnetic field. By tilting the magnetic field, we discover Landau level (LL) crossing effects at ultra-low coincident angles, revealing that the Zeeman energy is about three times as large as the cyclotron energy near the valence band top at {Gamma} valley. This result implies the significant roles played by the exchange interactions in p-type few-layer WSe2, in which itinerant or QH ferromagnetism likely occurs. Evidently, the {Gamma} valley of few-layer WSe2 offers a unique platform with unusually heavy hole-carriers and a substantially enhanced g-factor for exploring strongly correlated phenomena.
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Exciting phenomena may emerge in non-centrosymmetric two-dimensional (2D) electronic systems when spin-orbit coupling (SOC) interplays dynamically with Coulomb interactions, band topology, and external modulating forces, etc. Here, we report illuminating synergetic effects between SOC and Stark in centrosymmetric few-layer black arsenic (BAs), manifested as giant Rashba valley splitting and exotic quantum Hall states (QHS) reversibly controlled by electrostatic gating. The unusual finding is rooted in the puckering square lattice of BAs, in which heavy $4p$ orbitals form highly asymmetric $Gamma$ valley with the $p_{z}$ symmetry and $D$ valleys of the $p_{x}$ origin, located at the Brillouin zone (BZ) center and near the time reversal invariant momenta of $X$, respectively. When the structure inversion symmetry is broken by perpendicular electric field, giant Rashba SOC is activated for the $p_{x}$ bands to produce strong spin-polarized $D^{+}$ and $D^{-}$ valleys related by time-reversal symmetry, coexisting with weak $Gamma$ Rashba bands constrained by the $p_{z}$ symmetry. Intriguingly, strong Stark effect shows the same $p_{x}$-orbital selectiveness for $D$, collectively shifting the valence band maximum of $D^{pm}$ valleys to exceed the $Gamma$ pockets. Such an orchestrating effect between SOC and Stark allows us to realize gate-tunable spin valley manipulations for 2D hole gas, as revealed by unconventional magnetic field triggered even-to-odd transitions in QHS. For electron doping, the quantization of the $Gamma$ Rashba bands is characterized by peculiar density-dependent transitions in band topology from two parabolic valleys to a unique inner-outer helical structure when charge carrier concentrations increase.
Stanene was proposed to be a quantum spin hall insulator containing topological edges states and a time reversal invariant topological superconductor hosting helical Majorana edge mode. Recently, experimental evidences of existence of topological edge states have been found in monolayer stanene films and superconductivity has been observed in few-layer stanene films excluding single layer. An integrated system with both topological edge states and superconductivity are higly pursued as a possible platform to realize topological superconductivity. Few-layer stanene show great potential to meet this requirement and is highly desired in experiment. Here we successfully grow few-layer stanene on bismuth (111) substrate. Both topological edge states and superconducting gaps are observed by in-situ scanning tunneling microscopy/spectroscopy (STM/STS). Our results take a further step towards topological superconductivity by stanene films.
As a high mobility two-dimensional semiconductor with strong structural and electronic anisotropy, atomically thin black phosphorus (BP) provides a new playground for investigating the quantum Hall (QH) effect, including outstanding questions such as the functional dependence of Landau level (LL) gaps on magnetic field B, and possible anisotropic fractional QH states. Using encapsulating few-layer BP transistors with mobility up to 55,000 cm2/Vs, we extract LL gaps over an exceptionally wide range of B for QH states at filling factors { u}=-1 to -4, which are determined to be linear in B, thus resolving a controversy raised by its anisotropy. Furthermore, a fractional QH state at { u}~ -4/3 and an additional feature at -0.56+/- 0.1 are observed, underscoring BP as a tunable 2D platform for exploring electron interactions.
We realize p-p-p junctions in few-layer black phosphorus (BP) devices, and use magneto-transport measurements to study the equilibration and transmission of edge states at the interfaces of regions with different charge densities. We observe both full equilibration, where all edge channels equilibrate and are equally partitioned at the interfaces, and partial equilibration, where only equilibration only takes place among modes of the same spin polarization. Furthermore, the inner p-region with low-doping level in the junction can function as a filter for highly doped p-regions which demonstrates gate-tunable transmission of edge channels.
The quantum Hall effect, with a Berrys phase of $pi$ is demonstrated here on a single graphene layer grown on the C-face of 4H silicon carbide. The mobility is $sim$ 20,000 cm$^2$/V$cdot$s at 4 K and ~15,000 cm$^2$/V$cdot$s at 300 K despite contamination and substrate steps. This is comparable to the best exfoliated graphene flakes on SiO$_2$ and an order of magnitude larger than Si-face epitaxial graphene monolayers. These and other properties indicate that C-face epitaxial graphene is a viable platform for graphene-based electronics.
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