Jianming Lu

Evidence for two-dimensional Ising superconductivity in gated MoS2

Locking the spins in a superconductor In Cooper pairs—pairs of electrons responsible for the exotic properties of superconductors—the two electrons spins typically point in opposite directions. A strong-enough external magnetic field will destroy superconductivity by making the spins point in the same direction. Lu et al. observed a two-dimensional superconducting state in the material MoS2 that was surprisingly immune to a magnetic field applied in the plane of the sample (see the Perspective by Suderow). The band structure of MoS2 and its spin-orbit coupling conspired to create an effective magnetic field that reinforced the electron pairing, with spins aligned perpendicular to the sample. Science, this issue p. 1353; see also p. 1316 Transport measurements are used to reveal a superconducting state that is only weakly affected by an in-plane magnetic field. [Also see Perspective by Suderow] The Zeeman effect, which is usually detrimental to superconductivity, can be strongly protective when an effective Zeeman field from intrinsic spin-orbit coupling locks the spins of Cooper pairs in a direction orthogonal to an external magnetic field. We performed magnetotransport experiments with ionic-gated molybdenum disulfide transistors, in which gating prepared individual superconducting states with different carrier dopings, and measured an in-plane critical field Bc2 far beyond the Pauli paramagnetic limit, consistent with Zeeman-protected superconductivity. The gating-enhanced Bc2 is more than an order of magnitude larger than it is in the bulk superconducting phases, where the effective Zeeman field is weakened by interlayer coupling. Our study provides experimental evidence of an Ising superconductor, in which spins of the pairing electrons are strongly pinned by an effective Zeeman field.

Inducing and Manipulating Heteroelectronic States in a Single MoS2 Thin Flake

By dual gating a few-layer MoS_{2} flake, we induce spatially separated electronic states showing superconductivity and Shubnikov-de Haas (SdH) oscillations. While the highly confined superconductivity forms at the K/K^{} valleys of the topmost layer, the SdH oscillations are contributed by the electrons residing in the Q/Q^{} valleys of the rest of the bottom layers, which is confirmed by the extracted Landau level degeneracy of 3, electron effective mass of 0.6m_{e}, and carrier density of 5×10^{12}u2009u2009cm^{-2}. Mimicking conventional heterostructures, the interaction between the heteroelectronic states can be electrically manipulated, which enables bipolarlike superconducting transistor operation. The off-on-off switching pattern can be continuously accessed at low temperatures by a field effect depletion of carriers with a negative back gate bias and the proximity effect between the top superconducting layer and the bottom metallic layers that quenches the superconductivity at a positive back gate bias.

Full superconducting dome of strong Ising protection in gated monolayer WS2

Significance Compared with 3D superconductors, atomically thin superconductors are expected to be easier to engineer for electronic applications. Here, we use field effect gating to induce superconductivity in a monolayer semiconducting transition metal dichalcogenide, WS2, grown by chemical vapor deposition. The remarkable doping range allows access to a cascade of electronic phases from a band insulator, a superconductor, to a reentrant insulator at high doping. The large spin-orbit coupling of ∼30 meV makes the Ising paring in WS2 arguably the most strongly protected superconducting state against external magnetic field. The wide tunability revealed by spanning over a complete superconducting dome paves the way for the integration of monolayer superconductors to functional electronic devices exploiting the field effect control of quantum phases. Many recent studies show that superconductivity not only exists in atomically thin monolayers but can exhibit enhanced properties such as a higher transition temperature and a stronger critical field. Nevertheless, besides being unstable in air, the weak tunability in these intrinsically metallic monolayers has limited the exploration of monolayer superconductivity, hindering their potential in electronic applications (e.g., superconductor–semiconductor hybrid devices). Here we show that using field effect gating, we can induce superconductivity in monolayer WS2 grown by chemical vapor deposition, a typical ambient-stable semiconducting transition metal dichalcogenide (TMD), and we are able to access a complete set of competing electronic phases over an unprecedented doping range from band insulator, superconductor, to a reentrant insulator at high doping. Throughout the superconducting dome, the Cooper pair spin is pinned by a strong internal spin–orbit interaction, making this material arguably the most resilient superconductor in the external magnetic field. The reentrant insulating state at positive high gating voltages is attributed to localization induced by the characteristically weak screening of the monolayer, providing insight into many dome-like superconducting phases observed in field-induced quasi-2D superconductors.

Inducing ferromagnetism and Kondo effect in platinum by paramagnetic ionic gating

Platinum thin film becomes ferromagnetic when under a large electric field and in proximity to local magnetic moments. Electrically controllable magnetism, which requires the field-effect manipulation of both charge and spin degrees of freedom, has attracted growing interest since the emergence of spintronics. We report the reversible electrical switching of ferromagnetic (FM) states in platinum (Pt) thin films by introducing paramagnetic ionic liquid (PIL) as the gating media. The paramagnetic ionic gating controls the movement of ions with magnetic moments, which induces itinerant ferromagnetism on the surface of Pt films, with large coercivity and perpendicular anisotropy mimicking the ideal two-dimensional Ising-type FM state. The electrical transport of the induced FM state shows Kondo effect at low temperature, suggesting spatially separated coexistence of Kondo scattering beneath the FM interface. The tunable FM state indicates that paramagnetic ionic gating could serve as a versatile method to induce rich transport phenomena combining field effect and magnetism at PIL-gated interfaces.

Continuous Low‐Bias Switching of Superconductivity in a MoS2 Transistor

Engineering the properties of quantum electron systems, e.g., tuning the superconducting phase using low driving bias within an easily accessible temperature range, is of great interest for exploring exotic physical phenomena as well as achieving real applications. Here, the realization of continuous field-effect switching between superconducting and non-superconducting states in a few-layer MoS2 transistor is reported. Ionic-liquid gating induces the superconducting state close to the quantum critical point on the top surface of the MoS2 , and continuous switching between the super/non-superconducting states is achieved by HfO2 back gating. The superconducting transistor works effectively in the helium-4 temperature range and requires a gate bias as low as ≈10 V. The dual-gate device structure and strategy presented here can be easily generalized to other systems, opening new opportunities for designing high-performance 2D superconducting transistors.