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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 worked on the following groups of materials: Iridates, Ruthenates, Rhodates and 4d- and 5d-electron-based chalcogenides (see Section Quantum Materials). 

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. Electric-current control

• , Yu Zhang, Yifei Ni, Hengdi Zhao, Sami Hakani, Feng Ye, Lance DeLong, Itamar Kimchi and Gang Cao, Nature 611, 467–472 (2022) 

Current-controlled chiral orbital currents in a colossal magnetoresistance material

• , Gang Cao, Journal of Physics: Condensed Matter,  2020. 
• , Hengdi Zhao, Bing Hu, Feng Ye, Christina Hoffmann, Itamar Kimchi and Gang Cao; Phys. Rev. B 100, 241104(R) (2019)

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).

• , G. Cao, J. Terzic, H. D. Zhao, H. Zheng, Peter Riseborough, L. E. DeLong, Phys. Rev. Lett. 120, 017201 (2018); Editors' 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; Editors' 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

• , Gang Cao and Pedro Schlottmann, Reports on Progress in Physics 81 042502 (2018) 

Highlights: Schematics of spin-orbit-driven electronic bands for Iridates: (a) The traditionally anticipated broad t_2g band for 5d-electrons; (b) The splitting of the t_2g band into J_eff=1/2 and J_eff=3/2 bands due to strong spin-orbit interactions; (c) Ir⁴⁺(5d⁵) ions provide five 5d-electrons, four of them fill the lower J_eff = 3/2 bands, and one electron partially fills the J_eff = 1/2 band where the Fermi level E_F resides; and (d) For Ir⁵⁺(5d⁴) ions, four 5d-electrons fill the J_eff=3/2 bands, leading to a singlet J_eff = 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  

• , 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)
• ,Y. Shen, J. Sears, G. Fabbris, A. Weichselbaum, W. Yin, H. Zhao, D. G. Mazzone, H. Miao, M. H. Upton, D. Casa, R. S. Acevedo-Esteves, C. Nelson, A. M. Barbour, C. Mazzoli, G. Cao, and M. P. M. Dean, Phys. Rev. Lett.129, 207201 (2022)

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.