The Functional Quantum Crystals (QCrystal) group was established in 2021 at the Institute of Physics, Chinese Academy of Sciences. With an interdisciplinary research team at the institute and through partnerships with global collaborators, the group investigates the physics and functionalities of quantum materials, specifically correlated, superconducting, and topological quantum crystals.
The goal of the Functional Quantum Crystals group is to substantially improve the understanding of distinct quantum electronic and magnetic phases of matter, leading to novel functionalities and enhanced properties. Research in the QCrystal group focuses on state-of-the-art characterization of spin wave function in quantum crystals and prototype devices.
Our recent research:
1. Magnetism controlled electronic states in quantum computing candidate materials
One promising route to achieve error-free quantum computing involves braiding and controlling Majorana bound states with non-Abelian statistics. Topological superconductors, in which phase-coherent Cooper pairs on the surface are protected by the bulk electronic band inversion, give rise to a viable direction to realize superior quantum computing hard wares. At the current research frontier, identifying suitable materials and elucidating the underlying physics are the essential steps towards technology applications.
In our research, we focus on quantum crystalline materials that present intriguing characteristics of non-Abelian quasiparticles and investigate the topological superconductor candidates with strong electron correlations and large spin-orbital coupling. Our recent works discovered that magnetism (short-range patterns of electron magnetic moments) in one of the archetypal topological superconductor candidates, Fe1+yTe1-xSex, has a large impact on the electron wave functions. By tunning electron coherence and spin-orbital coupling, we established quantum and topological phase transitions, pointing to a generic chemical control required for use of Majorana bound states.
Useful references:
1. Li, Y. et al. Electronic properties of the bulk and surface states of Fe1+yTe1−xSex. Nature Materials 20, 1221–1227 (2021).
[News at AAAS EurekAlert, Phys.org, Science Daily, Brookhaven, etc.]
2. Castelvecchi, D. Quantum computers ready to leap out of the lab in 2017. Nature 541, 9 (2017).
3. Zhu, S. et al. Nearly quantized conductance plateau of vortex zero mode in an iron-based superconductor. Science 367, 189 (2020).
4. Zhang, P. et al. Observation of topological superconductivity on the surface of an iron-based superconductor. Science 360, 182 (2018).
2. New states and orders in high-temperature superconductors
One promising route to achieve error-free quantum computing involves braiding and controlling Majorana bound states with non-Abelian statistics. Topological superconductors, in which phase-coherent Cooper pairs on the surface are protected by the bulk electronic band inversion, give rise to a viable direction to realize superior quantum computing hard wares. At the current research frontier, identifying suitable materials and elucidating the underlying physics are the essential steps towards technology applications.
High-temperature (Tc) superconductors, firstly discovered more than 30 years ago and with additional materials joining the family every a few years, have regained the attention of physicists due to emerging of intertwined quantum orders and new states of matter. One example is the long-sought metallic state formed by Cooper pairs with Bosonic statistics.
In this project, we seek to understand the underlying superconductivity mechanism in copper-oxide materials with alternating areas of electric charge and magnetism. We discovered an unusual metallic state when attempting to turn superconductivity off and found a transition from three-dimensional to two-dimensional superconductivity. The Cooper pairs are observed to survive in one-dimensional charge stripes when superconducting coherence is suppressed by high magnetic fields.
Useful references:
1. Li, Y. et al. Tuning from failed superconductor to failed insulator with magnetic field, Science Advance 5, aav7686 (2019)
[News at AAAS EurekAlert, Phys.org, Science Daily, Brookhaven, NHMFL etc.]
2. Li, Y. et al. Hole-pocket-driven superconductivity and its universal features in the electron-doped cuprates, Science Advance 5, aap7349 (2019)
3. Tsvelik, A. M. Superconductor-metal transition in odd-frequency–paired superconductor in a magnetic field, Proceedings of the National Academy of Sciences 116, 12729 (2019)
4. Yang, C. et al. Intermediate bosonic metallic state in the superconductor-insulator transition, Science 366, 1505 (2019)
5. Liu, C. et al. Two-dimensional superconductivity and anisotropic transport at KTaO3 (111) interfaces, Science 371, 716 (2021)
3. 3D prototype devices of functional quantum crystals
Quantum crystals provide a novel platform for exploring and engineering technologically advanced functional devices. Unlike thin film materials, precise geometric control of bulk crystalline materials is challenging and opens up opportunities for complex three-dimensional device fabrication. In our lab, we are developing a new Focused Ion Beam technique, for which in-situ device fabrication and characterization allow modification and enhancement of quantum properties in crystalline materials at micro/nanometer scales. Our recent works have achieved surface superconductivity on topological insulators and semimetals. Our next step is to build fabrication protocols for functional prototype devices.
Useful references:
1. Li, Y. et al. Large surface conductance and superconductivity in topological insulator microstructures, Applied Physics Letters 115, 173507 (2019)
[Industrial report at Superconductor Week]
2. Li, Y. et al. Magnetic-field control of topological electronic response near room temperature in correlated Kagome magnets, Physical Review Letters 123, 196604 (2019)
3. Bachmann, M. D. et al. Spatial control of heavy-fermion superconductivity in CeIrIn5, Science 371, 716 (2019)
4. Moll, P. J. W. Focused Ion Beam Microstructuring of Quantum Matter, Annual Review of Condensed Matter Physics 9, 147 (2018)
