High-press. NMR　　　　　 Frustration　　　　 Mott-Insulator 　　　 Spin liquid　　　　 Orbital order

■ Competition between Peierls and Mott transition

In the holladite K2V8O16, application of hydrostatic pressure sensitively suppresses the metal-insulator transition, as generally seen in Mott transition. With increasing pressure further, the transition temperature turns to increase despite the decrease of electron correlation, and the first-order nature of the structural transition is enhanced. It indicates that electron-phonon coupling becomes large in the high-pressure insulating phase. We suggest the competition between Mott and Peierls nature in the metal-insulator transition [14].

■ Pressure induced antiferromagnetic correlation in pnictide

Pressure is a significant factor of superconducting transition in iron pnictide superconductors. In FeSe0.5Te0.5, we found that the antiferromagnetic correlation is developed with increasing Tc under pressure. The power n in 1/T1 = ATn increases under pressure, suggesting the pressure-induced change in the order parameter [15].

■ Development of high-pressure NMR

Pressure is an ideal tunable parameter of strongly correlated electron system without introducing disorders, which is crucial for NMR measurements sensitive to the local inhomogeneity. The previous NMR measurement has been limited below 5 GPa. We have succeeded to measure NMR using Bridgemann anvil cell under pressure up to 8.8 GPa down to low temperatures to 2 K. We revealed the magnetic pressure-temperature phase diagram and the site-dependent local spin susceptibility in the pressure-induced superconductor β-Na0.33V2O5 [16].

References

  • [14] Y. Shimizu, K. Okai, M. Itoh, M. Isobe, J. Yamaura, T. Yamauchi, Y. Ueda,
    "Metal-insulator transition in the hollandite K2V8O16 with a frustrated zigzag ladder probed by 51V NMR",
    Physical Review B, 83, 155111 (2011).

  • [15] Y. Shimizu, T. Yamada, T. Takami, S. Niitaka, H. Takagi, and M. Itoh,
    "Pressure induced antiferromagnetic fluctuations in the pnictide superconductor FeSe0.5Te0.5: 125Te NMR study",
    J. Phys. Soc. Jpn. 78, 123707 (2009).

  • [16] T. Suzuki, I. Yamauchi, Y. Shimizu, M. Itoh, N. Takeshita, C. Terakura, H. Takagi, Y. Tokura, T. Yamauchi, and Y. Ueda,
    "High-pressure 51V NMR study of the magnetic phase diagram and metal-insulator transition in the quasi-one-dimensional β-Na0.33V2O5",
    Physical Review B 79, 081101(R) (2009).


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