Magnetic Field Induced Insulator-to-Metal Mott Transition in λ-Type Organic Conductors
PI of Joint-use project: S. Fukuoka
Host lab: Kindo Group
Host lab: Kindo Group
The Mott transition is a key issue in condensed matter physics. Mott transitions give rise to novel physical phenomena, including unconventional superconductivity and quantum criticality. The interest in this research is the effects of the magnetic field on the Mott transition. To thoroughly investigate the effects of the magnetic field on the Mott transition, it is necessary to apply a magnetic field comparable to the energy scale of the Mott gap. However, the energy scale of the Mott gap is usually much larger than the practical limits of experimentally feasible magnetic fields. To avoid this problem, it is necessary to prepare materials near the Mott boundary region to reduce the Mott gap.
For studying the magnetic field effects on the Mott transition, we focused on λ-type organic conductors. Figure 1 shows the pressure-temperature (p - T) phase diagram of λ-type BETS salts λ-(BETS)2GaBrxCl4-x, where BETS is bis(ethylenedithio)tetraselenafulvalene. It has been reported that the increase in Br content works as a negative pressure effect and that the compound with x ~ 0.75 is located near the Mott boundary [1]. These indicate that by controlling the Br content x, we can access the Mott boundary region under ambient conditions, which allows us to study the magnetic field effects on the Mott transition over a wide temperature and magnetic field ranges using pulsed magnetic fields.
In this study, we performed magnetoresistance measurements using a 60 T pulse magnet at the International MegaGauss Science Laboratory. We synthesized λ-type organic conductors with x = 0.65, 0.75, and 0.8, which are located near the Mott boundary. Figures 2 (a) and 2(b) show the temperature dependence of the magnetoresistance for compounds with x = 0.75 and 0.65 [2]. We confirmed that in the compound with x = 0.75, a sharp drop with hysteresis in resistivity was observed at a certain magnetic field, suggesting that a first-order magnetic field induced insulator-to-metal Mott transition occurs. Interestingly, in the compound with x = 0.65, the suppression of the superconducting state at low magnetic fields and a Mott transition at high magnetic fields were observed, indicating a successive superconductor-to-insulator-to-metal transition. These results are summarized as a color plot of the field-temperature (H - T) phase diagram shown in Figs. 2 (c) and 2(d). We note that the magnetic field induced Mott transition observed previously in κ-type organic conductors is a transition from the metallic to the insulating state by applying a magnetic field, whereas the Mott transition observed in λ-type salts in this study is a transition from the insulating to the metallic state, that is, the opposite magnetic field response was observed [3].
We proposed that the difference in magnetic susceptibility between the insulating and metallic phases across the Mott boundary is key to explaining the opposite magnetic field effects on the Mott transition. The previous studies reported that the magnetic susceptibility of λ-(BETS)2GaBrxCl4-x decreases with increasing Br content at low temperatures [2]. This suggests that the magnetization of the metal phase is larger than that of the insulator phase. Since the spin state with larger magnetization is more stabilized under magnetic fields, the metallic state is stabilized in magnetic fields.
In this study, we have experimentally verified a magnetic-field-induced insulator-to-metal Mott transition and a successive superconductor-to-insulator-to-metal transition from magnetoresistance measurements for λ-type organic Mott insulators. These experimental results not only provide new insight into the magnetic field effects on the Mott transition but also highlight that the Mott transition can be induced in experimentally feasible magnetic fields at ambient pressure conditions through the control of chemical pressure in λ-type organic conductors, thereby paving the way for future microscopic investigations of the field-induced Mott transition.
References
- [1] H. Tanaka, A. Kobayashi, A. Sato, H. Akutsu, and H. Kobayashi, J. Am. Chem. Soc. 121, 760(1999).
- [2] S. Fukuoka, T. Oka, Y. Ihara, A. Kawamoto, S. Imajo, and K. Kindo, Phys. Rev. B 109, 195142 (2024).
- [3] F. Kagawa, T. Itou, K. Miyagawa, and K. Kanoda, Phys. Rev. Lett. 93, 127001 (2004).