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Superior carrier tuning in ultrathin superconducting materials by electric-field gating

Abstract

Two-dimensional (2D) superconductors with excellent crystallinity down to the monolayer have many unusual properties absent in the three-dimensional (3D) bulk, such as continuous phase transition, quantum metallic state and localization of electrons. Preparation of 2D superconductors has been challenging, owing to poor quality and extreme sensitivity to air exposure at ultralow thickness, but exfoliation methods have recently been developed to achieve perfectly crystallized ultrathin superconductors. The electric-double-layer transistor and ionic field-effect transistor are powerful electric-field gating strategies for modulating the carrier concentration of ultrathin materials, which can be up to 100 times those of conventional techniques limited by high-temperature phase separation and low voltage windows. Therefore, electric-field gating of 2D superconductors has become an essential way to find new high-temperature superconductors and investigate new quantum phenomena. This Technical Review summarizes recent advances in electric-field-gated superconductivity in various ultrathin superconducting materials, including iron-based superconductors, transition-metal dichalcogenides, honeycomb bilayer superconductors and cuprates. We also offer a perspective on open challenges and future development paths in this field.

Key points

  • 2D superconductors exhibit properties absent in the 3D bulk, such as continuous phase transitions, quantum metallic states and localization of electrons. This has been an important means of studying superconductivity at the 2D limit and understanding superconducting mechanisms.

  • An electric-double-layer transistor can precisely modulate the interfacial carrier concentration of an ultrathin superconductor in a reversible way, while keeping its crystal structure unchanged.

  • Ionic field-effect transistors can generate high carrier densities in a layered superconductor, as the electrochemical doping drives ionic intercalation into the interlayer space.

  • These gating strategies demonstrate great advantages over conventional techniques, including no phase separation, precise carrier modulation and a larger voltage window.

  • Electric-field gating techniques can continuously tune carrier densities in a 2D superconductor sample and strongly manipulate associated superconducting properties, including superconducting dome, upper critical field and quantum phase transition.

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Fig. 1: Schematics of gating methods.
Fig. 2: Electric-field-gated FeSe.
Fig. 3: Dependence of gated FeSe on thickness, substrate and electrolyte.
Fig. 4: EDLT-gated MoS2.
Fig. 5: Electric-field-gated WS2 and TaS2.
Fig. 6: Electric-field-gated ZrNCl.
Fig. 7: Electric-field-gated cuprates.

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Acknowledgements

X.L.W. acknowledges the Australian Research Council (ARC) for funding support through the ARC Centre of Excellence in Future Low-Energy Electronics Technologies (CE170100039) and ARC Professorial Future Fellowship project (FT130100778).

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Glossary

Helmholtz layer

A narrow charge accumulation region at the semiconductor–electrolyte interface in an electric field.

Upper critical field

The magnetic flux density of a type-II superconductor that destroys superconductivity at 0 K.

Cooper pairs

In a superconducting state, a pair of electrons coupled by the electron–phonon interaction, which can flow without resistance.

2D-Tinkham superconductivity

Superconductivity linked with angular dependence of the upper critical field showing a cusp-like feature.

Berezinskii–Kosterlitz–Thouless type

A 2D superconductor with the Berezinskii–Kosterlitz–Thouless (BKT) transition feature reflected by power-law dependence in the current–voltage characteristics.

Quantum Griffiths state

A critical state in which quantum phase transition shows a typical feature of a divergent dynamical exponent.

Bardeen–Cooper–Schrieffer state

A physical state in which electrons are bound into Cooper pairs by attractive electron–phonon interactions.

Bose–Einstein condensation state

A state of matter in which the majority of bosons enter the lowest quantum state, and wavefunction interference is on a macroscopic scale.

Thomas–Fermi screening length

The inverse of the screening parameter that indicates the strength of the screening effect.

Hall number

Effective carrier density, which is the inverse of the product of Hall coefficient and the elementary charge.

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Liu, P., Lei, B., Chen, X. et al. Superior carrier tuning in ultrathin superconducting materials by electric-field gating. Nat Rev Phys 4, 336–352 (2022). https://doi.org/10.1038/s42254-022-00438-2

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