Discovery and Study of Novel Quantum Materials
Our research program encompasses a methodical search for new materials in single-crystal form, and a systematic effort to elucidate the underlying physics of these materials.
Materials Discovery and Synthesis: We search for new, interesting quantum materials and sythesize them in single-crystal form using flux and floating-zone techniques (see Section Facilities).
We have currently worked on the following groups of materials: Iridates, Ruthenates, Rhodates and 4d- and 5d-electron-based chalcogenides (see Section Quantum Materials).
Field-Editing Technology: The discovery of novel quantum materials requires innovative approaches. We have developed a game-changing Field-Editing Technology for quantum materials synthesis that “field-edits” (borrowing from the phrase “genome editing”) crystal structures via application of high magnetic field during crystal growth, as schematically shown below (see npj Quantum Mater. 5, 83 (2020); https://doi.org/10.1038/s41535-020-00286-2)
Materials Characterization: We utilize a wide suite of tools for experimental studies of structural, transport, magnetic, thermal and dielectric properties as functions of chemical composition, temperature, magnetic field, pressure and electrical current. Measurements are often carried out at extreme conditions, i.e., ultralow temperatures, high magnetic fields and high pressures (see Section Facilities)
A Common Feature Shared by All Materials We Are Interested: Quantum states of these materials are primarily dictated by a combined effect of both spin-orbit and Coulomb interactions and feature a high susceptibility to external stimuli, such as magnetic field B, electrical field E, pressure P, light hv or electrical current I. These external stimuli readily couple to the lattice and are control "knobs" we utilize to discover and study exotic states, as schematically shown below.
Highlights of Our Recently Published Work
I. Control of Quantum States
A. Electrical-current control
- "Topical Review: Towards Electrical-Current Control of Quantum States in Spin-Orbit-Coupled Matter", Gang Cao, Journal of Physics: Condensed Matter, 2020. https://doi.org/10.1088/1361-648X/ab9d47
- "Nonequilibrium Orbital Transitions via Applied Electrical Current in Calcium Ruthenates", Hengdi Zhao, Bing Hu, Feng Ye, Christina Hoffmann, Itamar Kimchi and Gang Cao; Phys. Rev. B 100, 241104(R) Rapid Comm (2019). DOI: 10.1103/PhysRevB.100.241104
Highlights: The Temperature-Current-Density Phase Diagram for 3% Mn dope Ca2RuO4 illustrates that the applied current drives the system from the native AFM state (purple) through the critical regime near 0.15 A/cm2 (gray) to the current-induced, nonequilibrium orbital state (blue).
- "Electrical Control of Structural and Physical Properties via Spin-Orbit Interactions in Sr2IrO4", G. Cao, J. Terzic, H. D. Zhao, H. Zheng, Peter Riseborough, L. E. DeLong, Phys. Rev. Lett. 120, 017201 (2018). DOI: https://doi.org/10.1103/PhysRevLett.120.017201; Editor’s Suggestion
Highlights: Diagrams for Sr2IrO4 illustrate that the current-induced lattice expansion (a) causes the unusual I-V curves and (b) increases Ir-O-Ir bond angle (red) and decreases magnetic canting (black arrows) with increasing current I. The reduced lattice distortions lead to enhanced electron mobility.
B. Pressure control
- "Observation of a pressure-induced transition from interlayer ferromagnetism to intralayer antiferromagnetism in Sr4Ru3O10", H . Zheng, W.H. Song, J. Terzic, H. D. Zhao, Y. Zhang, Y. F. Ni, L. E. DeLong, P. Schlottmann and G. Cao, Phys. Rev. B 98, 064418 (2018);DOI: 10.1103/PhysRevB.98.064418; Editor’s Suggestion
Highlights: Temperature-Pressure Phase Diagram for Sr4Ru3O10 indicates that application of modest pressure readily drives the ground state from ferromagnetism to antiferromagnetism.
- "Persistent Insulating State at Megabar Pressures in Strongly Spin-Orbit-Coupled Sr2IrO4", Chunhua Chen, Yonghui Zhou, Xuliang Chen, Tao Han, Chao An, Ying Zhou, Yifang Yuan, Bowen Zhang, Shuyang Wang, Ranran Zhang, Lili Zhang, Changjing Zhang, Zhaorong Yang, Lance E. DeLong and Gang Cao, Phys. Rev. B 101, 144102 (2020); DOI: 10.1103/PhysRevB.101.144102
Highlights: The temperature dependence of the basal-plane resistance R of Sr2IrO4 over pressures ranging from 0.6 GPa to 185 GPa. Note that Sr2IrO4 unexpectedly remains insulating up to 185 GPa and R shows a tendency of saturation at P > 124 GPa. Inset: A snapshot of the diamond anvil cell at 27 GPa.
II. Key Issues Review on Iridates
- "The Challenge of Spin-Orbit-Tuned Ground States in Iridates: A Key Issues Review", Gang Cao and Pedro Schlottmann, Reports on Progress in Physics 81 042502 (2018) DOI: https://doi.org/10.1088/1361-6633/aaa979
Highlights: Schematics of spin-orbit-driven electronic bands for Iridates: (a) The traditionally anticipated broad t2g band for 5d-electrons; (b) The splitting of the t2g band into Jeff=1/2 and Jeff=3/2 bands due to strong spin-orbit interactions; (c) Ir4+(5d5) ions provide five 5d-electrons, four of them fill the lower Jeff = 3/2 bands, and one electron partially fills the Jeff = 1/2 band where the Fermi level EF resides; and (d) For Ir5+(5d4) ions, four 5d-electrons fill the Jeff=3/2 bands, leading to a singlet Jeff = 0 state for the strong spin-orbit interaction limit.
With a few exceptions, almost all iridates are magnetically insulating and highly sensitive to even slight chemical doping. However, the insulating state can persist up to megabar pressures. The lattice symmetry and dynamics play a unique, crucial role in determing the ground states of spin-orbit coupled matter, which offers a perspective for understanding the discrepancies between recent theoretical proposals and experimental results in iridates, including the absence of superconductivity in Sr2IrO4.
III. Quantum Liquid in an Unfrustrated Lattice
- "Quantum liquid from strange frustration in the trimer magnet Ba4Ir3O10", G. Cao, H. D. Zhao, H. Zheng, Y. F. Ni, Christopher. A. Pocs, Y. Zhang, Feng, Ye, Christina Hoffmann, Xiaoping Wang, Minhyea Lee, Michael Hermele and Itamar Kimchi, npj Quantum Mater. 5, 26 (2020); https://doi.org/10.1038/s41535-020-0232-6
Highlights: New Type of Quantum Liquid in a Un-frustrated Lattice Ba4Ir3O10: Trimer units (ovals on top) recombine into c-axis 1D zigzag chains (solid lines) that couple via the remaining trimer-midpoint spins (dashed lines). The average frustration parameter is greater than 2000.
- "A Novel ground state in a new dimer iridate Ba13Ir6O30 with Ir6+(5d3) Ions", Hengdi Zhao, Feng Ye, Hao Zheng, Bing Hu, Yifei Ni, Yu Zhang, Itamar Kimchi and Gang Cao, Phys. Rev. B 100, 064418 (2019)