Modulations in Superconductors: Probes of Underlying Physics

Abstract

development of various modulations techniques, breakthroughs in quantum materials are achieved, such as the quantum anomalous Hall effect (AHE) in topological insulators, [1][2][3][4] gate-tuned roomtemperature ferromagnetism in 2D van der Waals materials, [5][6][7][8] and emergent magnetic monopoles in artificial spin ice. [9] Especially, as the quantum materials of particular interest with zero dissipation, superconductors have been intensively studied with advanced modulations, aiming to reveal a plethora of emergent phenomena in condensed matter physics, such as superconductor-to-insulator transitions (SIT), [10][11][12] intermediate bosonic metallic state, [13] Bose-Einstein condensation (BEC)-Bardeen-Cooper-Schrieffer (BCS) crossover, [14,15] and intertwined orders, [16][17][18] as well as to develop applications in quantum computing [19][20][21] and ultrasensitive detections. [22][23][24] The superconductivity composes of two ingredients, including the Cooper pairs, each formed by two electrons, and phase coherence among Cooper pairs. [25] Superconductors such as metals and alloys are in the weak-coupling regime, which is well understood by BCS theory. [26,27] However, the later discovered superconductors, including cuprates, iron-based superconductors, and heavy-fermion superconductors, are strongly correlated systems, where a widely accepted microscopic mechanism yet remains absent. Therefore, various modulations have been utilized for the ultimate understanding of these superconductors and routines for room temperature superconductivity. Traditional modulations, such as elemental substitution, annealing, and polarization-induced gating are usually accompanied by some severe problems, including discontinuity, disorders, and limited ranges. Recently, state-of-the-art techniques have been developed with improved convenience, increased capability, and unprecedented dimensionalities, enabling more thorough investigations of unconventional superconductors.As a manifestation, we take some examples discussed in this review in advance. The carrier density can be tuned more conveniently and continuously. The techniques of ionic field-effect transistor (iFET) and electric double-layer transistor (EDLT) surpass conventional methods limited by solid solubility or dielectric breakdown, which enables the more precise phase diagram and detailed scaling analysis at the quantum phaseThe importance of modulations is elevated to an unprecedented level, due to the delicate conditions required to bring out exotic phenomena in quantum materials, such as topological materials, magnetic materials, and superconductors. Recently, state-of-the-art modulation techniques in material science, such as electric-double-layer transistor, piezoelectric-based strain apparatus, angle twisting, and nanofabrication, have been utilized in superconductors. They not only efficiently increase the tuning capability to the broader ranges but also extend the tuning dimensionality to unprecedented degrees of freedom, including quantum fluctuations of...