Dr. Weiguo Yin [Brookhaven National Laboratory, USA]

Title: Novel Mechanisms of Magnetic Anisotropy and Frustration in a Copper-Iridium Oxide

Time: 2:00 - 3:00 PM, Tuesday, July 19, 2016

Place: Auditorium Room 410, HPSTAR (Shanghai)

Host: Dr. Xiao-Jia Chen

Abstract:

High-temperature superconductivity in the 3d transition-metal compounds (TMCs), such as copper oxides and iron pnictides, and relativistic Mott insulating state in the 5d TMCs, such as iridium oxides, are among the most extensively studied phenomena in condensed matter physics and materials science.  We now begin to explore the novel physics in mixed 3d-5d TMCs not present in the pure 3d or pure 5d ones.  Its origin lies in the unusual exchange pathways opened by mixed spin-orbit coupling strengths.  Such materials could naturally take the form of the double-perovskite-like structure AB1-xB’xO3 or the ABO3/A’B’O3 heterostructure epitaxy films where, for example, B=3d and B’=5d ions.  In this talk, we shall use Sr3CuIrO6, the prototypical parent compound of a family of copper-iridium oxides, to illustrate novel mechanisms of magnetic anisotropy and frustration in mixed 3d-5d systems [1,2].  Sr3CuIrO6 is known as a spin one-half chain ferromagnet exhibiting three unexpected phenomena: (i) large spin excitation gap, (ii) small saturation magnetic moment, and (iii) antiferromagnetic Curie-Weiss behavior of the high-temperature magnetic susceptibility despite ferromagnetic exchange interaction.  We explain these puzzles using a first-principles derived effective Hamiltonian with alternating g-factors, namely g = 2 and –3 on the Cu and Ir ions, respectively.  Furthermore, we found an exotic magnetic-field driven critical point at which one half of the spins are frozen into a complete order and the other half are fully disordered at zero temperature.  The responses of this new state to external stimuli are also unique.

1.Wei-Guo Yin et al., “Ferromagnetic Anisotropy from Antiferromagnetic Superexchange in the Mixed 3d-5d Transition-Metal Compound Sr3CuIrO6,” Phys. Rev. Lett. 111, 057202 (2013).

2.Wei-Guo Yin, C. R. Roth, and A. M. Tsvelik, “Magnetic frustration and a ‘half fire, half ice’ critical point from nonuniform g factors,” arXiv:1510.00030 (2016).

Biography of the Speaker:

Wei-Guo Yin received his PhD degree from Nanjing University in 1998. He was named one of the Nation’s Outstanding PhD Dissertion Authors in 2000 for his work on high-Tc superconductivity. After extensive postdoctoral experiences in Fudan University, the Chinese University of Hong Kong, the University of Nebraska at Omaha, and Brookhaven National Laboratory, he joined the faculty of Brookhaven Lab in 2006. He has been focusing on fundamental problems in strongly correlated electron systems by developing combined first-principles and effective Hamiltonian approaches.  His work on cuprates helped determine the widely used extended t-J model. He was well recognized for his contributions to our understanding of the orbital effects in collossal magnetoresistance manganites and iron-based superconductors.  For iridates, he discovered the zigzag magnetic phase in the 213 family.  Recently, he has been interested in finding novel physics in mixed 3d and 5d transition-metal compounds and heterostructures.