FY2021 Annual Report

Electronic and Quantum Magnetism Unit
Associate Professor Yejun Feng


This is the fifth year of the Electronic and Quantum Magnetism Unit at OIST. 

1. Staff & Students

  • Magarita Dronova, Graduate Student 
  • Takatsugu Onish, Rotation Student 
  • Roman Mukhin, Intern 
  • Scott Cooper, Technical Staff 
  • Ardak Kussainova, Technician 
  • Yukiko Murabayashi, Administrative Assistant
  • Prof. Yoshitomo Karaki, Visiting Researcher, from Univ. of The Ryukyus

2. Collaborations

2.1 Continuous Metal-insulator transition driven by spin correlations

  • We discovered a continuous metal-insulator transition in Cd2Os2O7 through electric Hall coefficient measurement, which is induced by the spin correlation energy of the antiferromagnetism. However, the metal-insulation transition only happens when the antiferromagnetism is fully developed at low temperature, but not at the Neel temperature.  
  • Collaborators:
    • Dr. Yishu Wang, Institute for Quantum Matter, Johns Hopkins University
    • Professor David Mandrus, University of Tennessee 
    • Professor Patrick Lee, MIT
    • Professor Thomas Rosenbaum, Caltech

2.2 Optical Raman scattering under high pressure and low temperature

  • We used magnon Raman scattering to explore magnetic quantum phase transitions in Sr2IrO4.
  • Collaborators:
    • Dr. Xiang Li, Caltech
    • Professor David Hsieh, Caltech
    • Professor Thomas Rosenbaum, Caltech

2.3 Synchrotron x-ray magnetic diffraction under high pressure

  • We explore the all-in-all-out antiferromagnetic order in Eu2Ir2O7, using resonant x-ray magnetic diffraction under continous pressure tuning towards a quantum critical point. 
  • Collaborators:
    • Dr. Yishu Wang, Institute for Quantum Matter, Johns Hopkins University
    • Professor Satoru Nakatsuji, University of Tokyo

2.4 Single crystal growth of spinel magnets

  • We grow spinel magnets in the single crystal form and study the disorder issue in the material, through a series of techniques including x-ray element analysis, magnetic susceptibility, heat capacity, and neutron diffuse scattering and diffraction.
  • Collaborators:
    • Dr. Feng Ye, ORNL, USA
    • Dr. Christina Hoffmann, ORNL, USA
    • Prof. Yoshinori Okada, OIST, Japan
    • Prof. Daniel Khomskii, Univ. of Cologne, Germany

3. Activities and Findings

3.1 Magnetic driven metal-insulator transition. 

The metal-insulator transition in Cd2Os2O7 is finally resolved to some satisfaction. For the first time, the electric Hall coefficient in this family of materials is detected to a high precision and consistency. This sytem represents a rare situation that the bulk domain could become less conductive than the domain wall in between. This seemingly rather technical point turns out to be the key issue in properly measuring the Hall coefficient in this type of materials. The problem is resolved by utilizing two types of reciprocal relationships, the voltage-current reciprocity and Onsager's recirprocity. This technical issue of domain walls turns out to be the key issue in detecting the purely Slater type of metal-insulator transition, which is discovered here in Cd2Os2O7. 

3.2 Optical Raman scattering under high pressure

Visiting Postdoc Dr. Xiang Li published a paper on Physical Review B about the antiferromagnetic phase transitions under high pressure in Sr2IrO4Magnetic excitations in both ordered and disordered phase were explored by Optical Raman techniques at 5 K and over 20 GPa. 

3.3 Single crystal growth using high temperature solution technique

Margarita Dronova is leading the effort on single crystal growth of spinel magnets and taking effort on the subsequent charcterization process. Scott Cooper assists with electronic and mechanical instrument development for the growth apparatus. This work is experimentally collaborated with staff scientists at the Spallation Neutron Source at Oak Ridge National Laboratory. The theoretical collaboration is with Prof. Daniel Khomskii of University of Cologne in Germany. 


4. Publications

4.1 Journals

Yejun Feng, Yishu Wang, D. M. Silevitch, S. E. Cooper, D. Mandrus, P. A. Lee, T. F. Rosenbaum, A continuous metal-insulator transition driven by spin correlations. Nat. Commun. 12, 2779 (2021).

Xiang Li, S. E. Cooper, A. Krishnadas, A. de la Torre, R. S. Perry, F. Baumberger, D. M. Silevitch, D. Hsieh, T. F. Rosenbaum, Yejun FengMagnetic order, disorder, and excitations under pressure in the Mott insulator Sr2IrO4. Phys. Rev. B 104, L201111 (2021).

4.2 Books and other one-time publications

Nothing to report

4.3 Oral and Poster Presentations

Invited talks by the PI:

Nothing to report

Contributed talks with the PI listed:

Nothing to report

5. Intellectual Property Rights and Other Specific Achievements

Nothing to report

6. Meetings and Events

Nothing to report

7. Other

Our publication of the spin-correlation driven metal-insulator transition was highlighted by Caltech's News: