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Quantum spin liquid in honeycomb iridates H3LiIr2O6 : low temperature specific heat studies
日程 : 2017年10月2日(月) 13:30 - 14:30 場所 : 物性研究所本館6階 第5セミナー室 (A615) 講師 : Yosuke Matsumoto 所属 : Max Planck Institute for Solid State Research, Department of Quantum Materials (Takagi group) 世話人 : 榊原俊郎  (Ext:63245)

Recently, layered spin-orbit Mott insulators with two-dimensional (2D) honeycomb lattice attract a lot of attention because of possible quantum spin liquid (QSL) states. In particular, Kitaev model, which is a bond dependent Ising model on a honeycomb lattice provides an unique example of an exactly soluble 2D model with topological QSL ground state and fractionalized excitations [1]. Surprisingly, this rather artificial setting, i.e., the bond dependent Ising interactions, has been suggested to be realized in honeycomb iridates composed of edge-sharing IrO6 octahedra [2]. Here, the unique Jeff = 1/2 state arising from the strong spin-orbit interaction of Ir4+ ion is the key ingredient.

While all the candidates suggested so far, such as a-A2IrO3 (A = Li, Na) and b-Li2IrO3, magnetically order at finite temperatures [3-5], a recently developed new compound H3LiIr2O6 turned out to be a promising candidate of QSL on a honeycomb lattice [6]. Magnetic susceptibility and 7Li-, 1H-NMR measurements do not indicate any anomaly down to 0.5 K in spite of the large Weiss temperature qW ~ -90 K. NMR spectra are quite sharp with a width of about 0.001 mB and exhibit no broadening on cooling demonstrating the cleanest spin liquid ever observed. Both Knight shift and 1/(T1T) suggest the existence of gapless excitations. Especially, almost constant 1/(T1T) in weak magnetic fields below 20 K resembles Korringa law inherent to Fermi liquid.

Here I further present the results of the low temperature specific heat (C) measurements. No ordering have been found down to the lowest temperature of 30 mK. Interestingly, C/T at zero-magnetic field exhibits a weakly diverging behavior with 1/T0.5 temperature dependence. On the other hand, this weakly diverging behavior is suppressed under magnetic field and instead there appears a power law behavior with C ~ T2, which is consistent with 2D Dirac fermions. Furthermore, we found a T/B scaling with a form C/B0.5 ~ f(T/B) for all the data obtained in the temperature and field range below 1 K and 8 T. Interestingly, field induced suppression found in 1/T1 is fully consistent with the T/B scaling, if we assume that both C and 1/T1 reflect the density of states arising from the same fermionic excitations. In this presentation, I will further discuss the implication of these observations and possible scenarios for the QSL behaviors. This work has been done in collaboration with Tomohiro Takayama, Kentaro Kitagawa, Riku Takano, Robert Dinnebier, George Jackeli and Hidenori Takagi.

[1] A. Kitaev, Ann. Phys. (N.Y.) 321, 2 (2006).

[2] G. Jackeli and G. Khaliullin, Phys. Rev. Lett. 102, 017205 (2009).

[3] Y. Singh and P. Gegenwart, Phys. Rev. B 82, 064412 (2010).

[4] Y. Singh, S. Manni, J. Reuther, T. Berlijn, T. Thomale, W. Ku, S. Trebst and P. Gegenwart, Phys. Rev. Lett. 108, 127203 (2012).

[5] T. Takayama, A. Kato, R. Dinnebier, J. Nuss, H. Kono, L. S. Veiga, G. Fabbris, D. Haskel and H. Takagi, Phys. Rev. Lett. 114, 077202 (2015).

[6] K. Kitagawa, T. Takayama, Y. Matsumoto, A. Kato, R. Takano, Y. Kishimoto, R. Dinnebier, G. Jackeli and H. Takagi, preprint (2017).

(公開日: 2017年09月15日)