Rashba valleys and quantum Hall states in few-layer black arsenic.

Exciting phenomena may emerge in non-centrosymmetric two-dimensional electronic systems when spin-orbit coupling (SOC)1 interplays dynamically with Coulomb interactions2,3, band topology4,5 and external modulating forces6-8. Here we report synergetic effects between SOC and the Stark effect in centrosymmetric few-layer black arsenic, which manifest as particle-hole asymmetric Rashba valley formation and exotic quantum Hall states that are reversibly controlled by electrostatic gating. The unusual findings are rooted in the puckering square lattice of black arsenic, in which heavy 4p orbitals form a Brillouin zone-centred Γ valley with pz symmetry, coexisting with doubly degenerate D valleys of px origin near the time-reversal-invariant momenta of the X points. When a perpendicular electric field breaks the structure inversion symmetry, strong Rashba SOC is activated for the px bands, which produces spin-valley-flavoured D± valleys paired by time-reversal symmetry, whereas Rashba splitting of the Γ valley is constrained by the pz symmetry. Intriguingly, the giant Stark effect shows the same px-orbital selectiveness, collectively shifting the valence band maximum of the D± Rashba valleys to exceed the Γ Rashba top. Such an orchestrating effect allows us to realize gate-tunable Rashba valley manipulations for two-dimensional hole gases, hallmarked by unconventional even-to-odd transitions in quantum Hall states due to the formation of a flavour-dependent Landau level spectrum. For two-dimensional electron gases, the quantization of the Γ Rashba valley is characterized by peculiar density-dependent transitions in the band topology from trivial parabolic pockets to helical Dirac fermions.

Two-dimensional superconductivity at the surfaces of KTaO3 gated with ionic liquid.

The recent discovery of superconductivity at the interfaces between KTaO3 and EuO (or LaAlO3) gives birth to the second generation of oxide interface superconductors. This superconductivity exhibits a strong dependence on the surface plane of KTaO3, in contrast to the seminal LaAlO3/SrTiO3 interface, and the superconducting transition temperature Tc is enhanced by one order of magnitude. For understanding its nature, a crucial issue arises: Is the formation of oxide interfaces indispensable for the occurrence of superconductivity? Exploiting ionic liquid (IL) gating, we are successful in achieving superconductivity at KTaO3(111) and KTaO3(110) surfaces with Tc up to 2.0 and 1.0 K, respectively. This oxide-IL interface superconductivity provides a clear evidence that the essential physics of KTaO3 interface superconductivity lies in the KTaO3 surfaces doped with electrons. Moreover, the controllability with IL technique paves the way for studying the intrinsic superconductivity in KTaO3.

Superconductivity in tantalum self-intercalated 4Ha-Ta1.03Se2.

TaSe2 has several different polytypes and abundant physical properties, such as superconductivity and charge density waves (CDW), which have been investigated in the past few decades. However, there has been no report on the physical properties of the 4Ha polytype up to now. Here we report the crystal growth and discovery of superconductivity in the tantalum self-intercalated 4Ha-Ta1.03Se2 single crystal with a superconducting transition onset temperature of [Formula: see text] K, which is the first observation of superconductivity in the 4Ha polytype of TaSe2. A slightly suppressed CDW transition is found around 106 K. A large [Formula: see text] value of about 4.48 is found when a magnetic field is applied in the ab-plane, which probably results from the enhanced spin-orbit coupling. Special stacking faults are observed, which further enhance the anisotropy. Although the density of states at the Fermi level is lower than that of other polytypes, T c remains the same, indicating that the stack mode of the 4Ha polytype may be beneficial to superconductivity in TaSe2.