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. 2017 Aug 11;3(8):e1601594.
doi: 10.1126/sciadv.1601594. eCollection 2017 Aug.

Slow dynamics of electrons at a metal-Mott insulator boundary in an organic system with disorder

Tetsuaki Itou  1   2 , Eri Watanabe  3 , Satoru Maegawa  3 , Akiko Tajima  4 , Naoya Tajima  4   5 , Kazuya Kubo  4   6 , Reizo Kato  4 , Kazushi Kanoda  7
Affiliations

Slow dynamics of electrons at a metal-Mott insulator boundary in an organic system with disorder

Tetsuaki Itou et al. Sci Adv. .

Abstract

The Mott transition-a metal-insulator transition caused by repulsive Coulomb interactions between electrons-is a central issue in condensed matter physics because it is the mother earth of various attractive phenomena. Outstanding examples are high-Tc (critical temperature) cuprates and manganites exhibiting colossal magnetoresistance. Furthermore, spin liquid states, which are quantum-fluctuation-driven disordered ground states in antiferromagnets, have recently been found in magnetic systems very near the Mott transition. To date, intensive studies on the Mott transition have been conducted and appear to have established a nearly complete framework for understanding the Mott transition. We found an unknown type of Mott transition in an organic spin liquid material with a slightly disordered lattice. Around the Mott transition region of this material under pressure, nuclear magnetic resonance experiments capture the emergence of slow electronic fluctuations of the order of kilohertz or lower, which is not expected in the conventional Mott transition that appears as a clear first-order transition at low temperatures. We suggest that they are due to the unconventional metal-insulator fluctuations emerging around the disordered Mott transition in analogy to the slowly fluctuating spin phase, or Griffiths phase, realized in Ising spin systems with disordered lattices.

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Figures

Fig. 1
Fig. 1. Structure and transport properties of EtMe3Sb[Pd(dmit)2]2.
(A) Side view of the layered structure of EtMe3Sb[Pd(dmit)2]2 (41). Mott-insulating Pd(dmit)2 layers are separated by closed-shell cation layers of EtMe3Sb. The cation layers have random orientations of the ethyl groups. (B) Temperature dependence of the in-plane resistivity of EtMe3Sb[Pd(dmit)2]2 under several pressures measured using the standard four-electrode method.
Fig. 2
Fig. 2. Temperature dependence of the 13C nuclear spin-lattice relaxation rate (T1−1) of EtMe3Sb[Pd(dmit)2]2 under several pressures, reflecting the amplitude of the fluctuations of the internal magnetic field on a megahertz time scale.
The dashed line shows a fit of the data at 15 kbar to the Korringa relation T1−1 ∝ T.
Fig. 3
Fig. 3. Profiles of the 13C nuclear spin-spin relaxation rate (T2−1) of EtMe3Sb[Pd(dmit)2]2 under several pressures.
(A) Temperature dependence of T2−1 of EtMe3Sb[Pd(dmit)2]2 under several pressures. For comparison, data for EtMe3P[Pd(dmit)2]2 at ambient pressure are also plotted as open black circles. The left axis shows the observed raw value of the whole spin-spin relaxation rate. The value of T2l−1 (the right axis) is estimated by subtracting 640 s−1 from T2−1 (see Materials and Methods and the Supplementary Materials). (B) The two components of T2l−1 of EtMe3Sb[Pd(dmit)2]2. The amplitude of the blue component shows a peak at 10 to 30 K. The amplitude of the blue component at low temperatures (where both components exist) is estimated by extrapolating the trend at higher temperatures. The amplitude of the red component is enhanced at low temperatures. This enhancement is particularly prominent at 7 kbar, which is near the Mott boundary.
Fig. 4
Fig. 4. Pressure-temperature phase diagram of the slow fluctuations in EtMe3Sb[Pd(dmit)2]2.
(A) Schematic pressure-temperature phase diagram of the contour plot of the slow fluctuations in EtMe3Sb[Pd(dmit)2]2. The magnitudes of the charge and spin fluctuations are represented in red and blue, respectively. The intensities are determined by the amplitudes of the blue and red components of T2l−1 in Fig. 3B. (B) Pressure-temperature phase diagram of the contour plot of the conductivity (the inverse of the resistivity shown in Fig. 1B), for comparison. Note that the horizontal axes in the two panels stand for the value of the pressure applied at room temperature, and the actual pressure in the temperature region discussed in these phase diagrams is 1.5 to 2 kbar lower than the value on the horizontal axes.
Fig. 5
Fig. 5. Schematic phase diagrams of Ising spin systems and Mott transition systems.
(A) Field-temperature phase diagram of a clean Ising spin system. (B) Pressure-temperature phase diagram of a clean Mott transition system. (C) Field-temperature phase diagram of an Ising spin system with randomness. (D) Our proposed pressure-temperature phase diagram of a Mott transition system with randomness.
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