Jianting Ye

Superconducting Dome in a Gate-Tuned Band Insulator

What Do You Know? A Dome The superconducting dome—the appearance of a maximum in the transition temperature as a function of a tuning parameter—has been observed in compounds such as cuprates, pnictides, and heavy fermion materials and is thought of as a signature of unconventional superconductivity. Ye et al. (p. 1193) used a liquid gating technique combined with back gating to finely tune the carrier density in the band insulator MoS2, which allowed them to observe the formation of a dome. The unexpected finding awaits theoretical explanation but may suggest that the appearance of an optimal carrier density may be a more common occurrence than was previously thought. Liquid gating tunes the carrier density in molybdenum disulfide, revealing unconventional superconductivity. A dome-shaped superconducting region appears in the phase diagrams of many unconventional superconductors. In doped band insulators, however, reaching optimal superconductivity by the fine-tuning of carriers has seldom been seen. We report the observation of a superconducting dome in the temperature–carrier density phase diagram of MoS2, an archetypal band insulator. By quasi-continuous electrostatic carrier doping achieved through a combination of liquid and solid gating, we revealed a large enhancement in the transition temperature Tc occurring at optimal doping in the chemically inaccessible low–carrier density regime. This observation indicates that the superconducting dome may arise even in doped band insulators.

Accessing the transport properties of graphene and its multilayers at high carrier density

We present a comparative study of high carrier density transport in mono-, bi-, and trilayer graphene using electric double-layer transistors to continuously tune the carrier density up to values exceeding 1014 cm-2. Whereas in monolayer the conductivity saturates, in bi- and trilayer filling of the higher-energy bands is observed to cause a nonmonotonic behavior of the conductivity and a large increase in the quantum capacitance. These systematic trends not only show how the intrinsic high-density transport properties of graphene can be accessed by field effect, but also demonstrate the robustness of ion-gated graphene, which is crucial for possible future applications.

Formation of a Stable p–n Junction in a Liquid-Gated MoS2 Ambipolar Transistor

Molybdenum disulfide (MoS2) has gained attention because of its high mobility and circular dichroism. As a crucial step to merge these advantages into a single device, we present a method that electronically controls and locates p-n junctions in liquid-gated ambipolar MoS2 transistors. A bias-independent p-n junction was formed, and it displayed rectifying I-V characteristics. This p-n diode could perform a crucial role in the development of optoelectronic valleytronic devices.

Superconductivity Series in Transition Metal Dichalcogenides by Ionic Gating

Functionalities of two-dimensional (2D) crystals based on semiconducting transition metal dichalcogenides (TMDs) have now stemmed from simple field effect transistors (FETs) to a variety of electronic and opto-valleytronic devices, and even to superconductivity. Among them, superconductivity is the least studied property in TMDs due to methodological difficulty accessing it in different TMD species. Here, we report the systematic study of superconductivity in MoSe2, MoTe2 and WS2 by ionic gating in different regimes. Electrostatic gating using ionic liquid was able to induce superconductivity in MoSe2 but not in MoTe2 because of inefficient electron accumulation limited by electronic band alignment. Alternative gating using KClO4/polyethylene glycol enabled a crossover from surface doping to bulk doping, which induced superconductivities in MoTe2 and WS2 electrochemically. These new varieties greatly enriched the TMD superconductor families and unveiled critical methodology to expand the capability of ionic gating to other materials.

Field-Induced Superconductivity in Electric Double Layer Transistors

Electric field tuning of superconductivity has been a long-standing issue in solid state physics since the invention of the field-effect transistor (FET) in 1960. Owing to limited available carrier density in conventional FET devices, electric-field-induced superconductivity was believed to be possible in principle but impossible in practice. However, in the past several years, this limitation has been overcome by the introduction of an electrochemical concept, and electric-field-induced superconductivity has been realized. In the electric double layer (EDL) formed at the electrochemical interfaces, an extremely high electric field is generated and hence high-density charge carriers sufficient to induce superconductivity exist and are collectively used as a charge accumulation device known as an EDL capacitor. Field-induced superconductivity has been used to establish the relationship between Tc and carrier density and can now be used to search for new superconductors. Here, we review electric-field-induc...

Metallic ground state in an ion-gated two- dimensional superconductor

A 3D approach to make 2D superconductors When the thickness of a superconducting film becomes comparable to the typical size of its electron pairs, its superconductivity enters a two-dimensional (2D) regime. Thinner films usually have higher amounts of disorder, making it difficult to isolate the 2D effects. To circumvent this limitation, Saito et al. induced charge carriers on the surface of the 3D insulator ZrNCl. This approach produced a clean superconducting layer thinner than the unit cell of the crystal. The superconducting state was extremely sensitive to the application of a perpendicular magnetic field, as expected for clean systems. Science, this issue p. 409 An electric-double-layer technique is used to induce clean two-dimensional superconductivity in the layered insulator ZrNCl. Recently emerging two-dimensional (2D) superconductors in atomically thin layers and at heterogeneous interfaces are attracting growing interest in condensed matter physics. Here, we report that an ion-gated zirconium nitride chloride surface, exhibiting a dome-shaped phase diagram with a maximum critical temperature of 14.8 kelvin, behaves as a superconductor persisting to the 2D limit. The superconducting thickness estimated from the upper critical fields is ≅ 1.8 nanometers, which is thinner than one unit-cell. The majority of the vortex phase diagram down to 2 kelvin is occupied by a metallic state with a finite resistance, owing to the quantum creep of vortices caused by extremely weak pinning and disorder. Our findings highlight the potential of electric-field–induced superconductivity, establishing a new platform for accessing quantum phases in clean 2D superconductors.

Controlling charge-density-wave states in nano-thick crystals of 1T-TaS2

Two-dimensional crystals, especially graphene and transition metal dichalcogenides (TMDs), are attracting growing interests because they provide an ideal platform for novel and unconventional electronic band structures derived by thinning. The thinning may also affect collective phenomena of electrons in interacting electron systems and can lead to exotic states beyond the simple band picture. Here, we report the systematic control of charge-density-wave (CDW) transitions by changing thickness, cooling rate and gate voltage in nano-thick crystals of 1T-type tantalum disulfide (1T-TaS2). Particularly the clear cooling rate dependence, which has never been observed in bulk crystals, revealed the nearly-commensurate CDW state in nano-thick crystals is a super-cooled state. The present results demonstrate that, in the two-dimensional crystals with nanometer thickness, the first-order phase transitions are susceptible to various perturbations, suggestive of potential functions of electronic phase control.

Optical characterizations of iodine molecular wires formed inside the one-dimensional channels of an AlPO4-5 single crystal

A uniform array of one-dimensional molecular iodine wires is fabricated by introducing iodine molecules into the channels of AFI zeolite crystals through the vapor phase. The only possible wire structures are formed by linking individual iodine molecules along intramolecular bond axis into a single line. Polarization dependence of Raman intensity at modes of iodine species indicates that the wires are aligned perfectly along the c axis of AFI crystal. Polarized optical absorption as well as resonant Raman scattering reveal two wire species inside the channels of AFI zeolite: In− and (I2)n wires.

Electroresistance effect in gold thin film induced by ionic-liquid-gated electric double layer

Electroresistance effect was detected in a metallic thin film using ionic-liquid-gated electric-double-layer transistors (EDLTs). We observed reversible modulation of the electric resistance of a Au thin film. In this system, we found that an electric double layer works as a nanogap capacitor with 27 (-25) MV cm-1 of electric field by applying only 1.7 V of positive (negative) gate voltage. The experimental results indicate that the ionic-liquid-gated EDLT technique can be used for controlling the surface electronic states on metallic systems.

Electrostatic and electrochemical tuning of superconductivity in two-dimensional NbSe2 crystals

We report modulation of the superconducting critical temperature (Tc) of ultrathin niobium diselenide (NbSe2) single crystals by gating an electric double-layer transistor. We realized reversible and irreversible changes of the Tc by adjusting the operating range of the voltage. The reversible and irreversible responses correspond to the electrostatic carrier doping and the electrochemical etching of the crystal, respectively. The results suggest that electric double-layer gating provides opportunities to control and functionalize collective electronic phenomena in two-dimensional crystals.