春水堂视频

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听search for new, interesting quantum materials and sythesize them in single-crystal form using flux and听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).听

Materials Characterization: We utilize a wide suite听of tools for experimental studies of structural, transport, magnetic, thermal and dielectric properties as functions of chemical composition, temperature, magnetic field,听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听All Materials We Are Interested:听听Quantum states of these materials听are primarily听dictated by a combined effect of both spin-orbit听and听Coulomb interactions and听feature听a high susceptibility to external stimuli, such as magnetic field B, electrical field E, pressure P, light hv or electrical current I.听听These external stimuli readily couple to the lattice and are control "knobs" we utilize to discover and听study exotic states, as schematically shown below.听听 听

Highlights of Our Recently Published Work

I. Control of Quantum States听

A. Electric-current control

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

Current-controlled chiral orbital currents in a colossal magnetoresistance material

• , Gang Cao,听Journal of Physics: Condensed Matter,听听2020.听

• , Hengdi Zhao, Bing Hu, Feng Ye, Christina Hoffmann, Itamar Kimchi听and Gang Cao;听Phys. Rev. B听100, 241104(R)听(2019)

Highlights:听The Temperature-Current-Density Phase Diagram for 3% Mn dope Ca₂RuO₄听illustrates that the applied current drives the system from the native AFM state (purple) through the critical regime near 0.15 A/cm²听(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.听120, 017201 (2018);听Editors'听Suggestion

Highlights:听Diagrams for Sr₂IrO₄听illustrate that the current-induced lattice expansion听(a)听causes the unusual I-V curves and听(b)听increases Ir-O-Ir bond angle听(red) and decreases magnetic canting (black arrows) with increasing current I. The reduced lattice distortions lead to enhanced electron mobility.听

B. Pressure control听

• 听"Observation of a pressure-induced transition from interlayer ferromagnetism to intralayer antiferromagnetism in Sr₄Ru₃O₁₀",听H . Zheng,听W.H. Song, J. Terzic, H. D. Zhao, Y. Zhang, Y. F. Ni, L. E. DeLong, P. Schlottmann and G. Cao,听Phys. Rev. B听98,听064418听(2018);DOI: 10.1103/PhysRevB.98.064418;听Editors'听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,听Yonghui Zhou,听Xuliang Chen, Tao Han,听Chao An, Ying Zhou, Yifang Yuan,听Bowen Zhang,听Shuyang Wang, Ranran Zhang,听Lili Zhang, Changjing Zhang, Zhaorong Yang, Lance E. DeLong听and听Gang听Cao,听Phys. Rev. B听101, 144102 (2020);听DOI:10.1103/PhysRevB.101.144102

Highlights:听The temperature dependence of the basal-plane resistance R of Sr₂IrO₄ over pressures ranging from听0.6 GPa to听185 GPa. Note that听Sr₂IrO₄听unexpectedly听remains insulating up to听185 GPa听and R shows a tendency of saturation at P > 124 GPa. Inset:听A snapshot of the diamond anvil cell at 27 GPa.听

听

II. Key Issues Review on Iridates

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

Highlights: Schematics of spin-orbit-driven electronic听bands听for Iridates:听(a)听The traditionally anticipated broad t_2g听band for 5d-electrons;听(b)听The听splitting of听the听t_2g听band into J_eff=1/2 and J_eff=3/2 bands due to strong spin-orbit interactions;听(c)听Ir⁴⁺(5d⁵) ions provide five 5d-electrons, four of them fill the lower J_eff听= 3/2 bands, and one electron partially fills the J_eff听= 1/2 band where the Fermi level E_F听resides; and听(d)听For Ir⁵⁺(5d⁴) ions, four 5d-electrons fill the J_eff=3/2 bands, leading to a singlet J_eff听= 0 state for the strong spin-orbit interaction听limit.听听

With a few exceptions, almost all听iridates听are magnetically insulating and听highly sensitive to even slight chemical doping. However,听the insulating state can persist up to听megabar pressures. The lattice symmetry and dynamics play a unique, crucial role in determing the ground states of spin-orbit coupled matter, which听offers a perspective for understanding听the discrepancies between recent theoretical proposals and听experimental results in iridates, including the absence of听superconductivity in Sr₂IrO₄.听听

III. Quantum Liquid in an Unfrustrated Lattice 听

• , G. Cao, H. D. Zhao, H. Zheng, Y. F. Ni, Christopher. A. Pocs, Y. Zhang, Feng, Ye,听Christina Hoffmann, Xiaoping Wang,听Minhyea Lee, Michael Hermele and Itamar Kimchi, 听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,听Phys. Rev. Lett.129, 207201听(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.听