Date/Time: 11.26 周二16:00 (玉泉教十二 201) 26 November 2019, 16:00 (Yuquan Blg 12, Room 201)

Speaker: Yejun Feng (Okinawa Institute of Science and Technology Graduate University, Okinawa, Japan) 

Title: Continuous all-in-all-out quantum phase transitions under pressure

Abstract: All-in-all-out (AIAO) spin order is often found in 5d pyrochlore compounds where the spin-orbit coupling is strong. As this unique magnetic order preserves all the lattice symmetry condition, and is often accompanied by a concurrent and continuous metal-insulator transition, there exist several intriguing scenarios ranging from strong-coupling physics to Weyl semimetal of broken time reversal symmetry, all near the AIAO quantum critical point. However, despite the fact that pressure is an effective tuning parameter, directly tracing the AIAO order under pressure poses a major experimental challenge.

Over the past decade, we have developed resonant x-ray magnetic diffraction techniques that is capable to directly explore the antiferromagnetic AIAO order under pressure tuning. Our experimental results revealed that there exist continuous AIAO quantum phase transitions in both Cd2Os2O7 and Sm2Ir2O7. However, their quantum critical points are largely different. InCd2Os2O7, while the AIAO order is suppressed at a very high critical pressure Pc = 35.8GPa, the lattice continuously evolves into a static breathing mode at high-pressure. Here the single quantum critical point anchors two second-order phase lines that divide the P-T phase space into three regions with different time-reversal and inversion symmetries, broadly placing it in the realm of deconfined quantum criticality. In Sm2Ir2O7, we found the pressure induced quantum critical point is dominated by spin fluctuations of Ising characteristics, in comparison to chemical series R2Ir2O7 where the spins lose the local axial anisotropy and their fluctuations become more Heisenberg like at the quantum critical point.

References:
1. Yishu Wang, T. F. Rosenbaum, A. Palmer, Y. Ren, J.-W. Kim, D. Mandrus, Yejun Feng, Strongly-coupled quantum critical point in an all-in-all-out antiferromagnet. Nature Communications 9, 2953 (2018).
2. Yishu Wang, T. F. Rosenbaum, D. Prabhakaran, A. T. Boothroyd, Yejun Feng, Approaching the quantum critical point in a highly-correlated all-in-all-out antiferromagnet. Under review.

Bio: Prof. Feng obtained his PhD (2003) from University of Washington. He then carried out postdoctoral work at University of Chicago, Argonne National Laboratory and California Institute of Technology. Since 2017, he is Principal investigator (Associate Professor) in the Electronic and Quantum Magnetism Unit, OIST. His work focuses on comprehensive experimental studies of correlated electrons, quantum magnets, and superconductors under pressure/magnetic field tuning.