Kai Chang Lab
Low-Dimensional Quantum Materials - BAQIS
Low-dimensional superconductivity
Superconductivity is a fascinating macroscopic quantum phenomenon where certain materials exhibit zero electrical resistance and perfect diamagnetism below critical temperatures. A two-dimensional (2D) superconductor is a superconducting system confined to a thin, 2D plane. 2D superconductivity is of particular interest due to its quantum confinement, directional anisotropy, high critical temperature, and potential for novel electronic devices. Our focus is on preparing high-quality crystalline 2D superconductors, including interface-enhanced superconductors, topological superconductors, and monolayer superconductors. We then manipulate their properties to gain further insight into their behavior and explore ways to apply them in next-generation electronics and quantum computing.
1, Interface-enhanced superconductivity refers to the phenomenon where the superconducting properties of a material are improved when it is brought into contact with another material at the interface. The interface between the materials can modify the electronic properties of the superconductor, leading to an increase in the critical temperature. This enhancement of superconductivity has potential applications in developing more efficient energy transmission and storage systems. Monolayer FeSe on SrTiO3 exhibits a superconducting transition temperature of >40 K, which is significantly higher than the bulk FeSe. It has a unique electronic structure with a highly two-dimensional nature, which makes it an interesting system for studying the effects of dimensionality on superconductivity. The interface between FeSe and SrTiO3 can induce significant changes in the electronic properties of FeSe, including the enhancement of superconductivity. By studying this system, researchers can gain insights into the mechanisms underlying high-temperature superconductivity and explore new ways to enhance the superconductivity of iron-based materials. Additionally, monolayer FeSe on SrTiO3 as well as other related systems (e.g. FeTe1-xSex and FeTe films) offers a platform for studying the interplay between superconductivity and other phenomena, such as magnetism, nematicity and topological surface state.
2, FeTe1-xSex is a material that has been shown to exhibit topological superconductivity. This material is a type of iron-based superconductor that can be tuned to exhibit topological properties. When the amount of Te is increased to a certain level, the material undergoes a transition from a conventional superconductor to a topological superconductor. This transition is characterized by the appearance of zero-energy Majorana bound states (MBS) at the vortex cores, which are protected by the topological properties of the material. MBS have unique properties that make them attractive for quantum computing, such as their non-Abelian statistics and their ability to encode quantum information in a topologically protected way. These properties make it possible to perform quantum operations on qubits without destroying their coherence, which is a major challenge in building practical quantum computers. Therefore, studying topological superconductivity and find establish the method to manipulate the MBS in FeTe1-xSex is a significant for the realization of topological quantum computing.
3, Disorder-enhanced superconductivity is rare and has been observed only in some alloys or granular states. Owing to the entanglement of various effects, the mechanism of enhancement is still under debate. We report a well-controlled disorder effect in the recently discovered monolayer NbSe2 superconductor. The superconducting transition temperatures of NbSe2 monolayers are substantially increased by disorder. Realistic theoretical modelling shows that the unusual enhancement possibly arises from the multifractality of electron wavefunctions. We provide experimental evidence of the multifractal superconducting state.
4, Bardeen-Cooper-Schrieffer (BCS) superconductivity and Bose-Einstein condensation (BEC) are two asymptotic limits of a fermionic superfluid. We report direct spectroscopic evidence of BCS-BEC crossover in real space in a FeSe monolayer thin film by using spatially resolved scanning tunneling spectra. The crossover is driven by the shift of band structure relative to the Fermi level.
5, As a two-dimensional superconductor, PdTe2 is a promising material candidate for exploring the novel phenomena of superconductivity in the two-dimensional limit. We observed a transition from the narrow-gap semiconducting phase in the monolayer to the metallic phase in the multilayer films. Importantly, all the multilayer films exhibited robust superconductivity, exhibiting high in-plane critical magnetic field, which is explained as type-II Ising superconductivity.
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