Investigation oｆ spin liquid to Fermi liquid　Mott Boundary in κ-type dimer-Mott organic complexes

概要

In conventional band theory, energy levels of all the components of a crystal hybridize, creating electronic band structures. When the Fermi level is within a band, the material has electron conducting properties. On the other hand, if it lays in a band gap, it has insulating properties. However, strong electronic interactions can lead a material with a conducting band structure to have insulating properties. This phenomenon is called a Mott metal insulator transition. Such a transition has been seen in organic conductors made of charge transfer complexes. These materials have a variety of ground states near the Mott boundary, including novel quantum ground states such as the quantum spin liquid. The spin liquid state is a non-ordered fluctuating state of quantum spins. We are interested in the relations between these phases near that boundary. Somes of them have already been studied. Antiferromagnetic to spin liquid boundary has been studied with κ-EtMe3Sb[Pd(dmit)2]2 by heat capacity and thermal transport measurement. The Mott transition between a Fermi liquid and insulating states have been studied in several ways. The antiferromagnetic to Fermi liquid boundary has been widely studied via numerous materials such as κ-(BEDT-TTF)2Cu[N(CN)2]Br. This phase boundary is of 1st order. In 1991, κ-(BEDT-TTF)2Cu2(CN)3 has been first synthesized and gained strong interest due to its superconductive properties. In 2003, Shimizu observe that this material does not exhibit a long-range magnetic ordering down to extremely low temperatures. Combined with its triangularity creating strong spin frustration and its dimerized structure have made of it a candidate to the spin liquid. Under applied Pressure, this material goes from a spin liquid to a Fermi liquid state. However, heat capacity measurements under applied pressure are quite challenging and the superconducting properties break the symmetry, thus this boundary is not a genuine Mott boundary. This thesis focuses on investigating the spin liquid to Fermi liquid phase boundary.
To do so, we are measuring low temperature (1-10K) heat capacity by relaxation calorimetry and single crystal XRD on an alloying system of κ-[(BEDT-TTF)1-x(BEDT-STF)x]2Cu2(CN)3. In this system, when x=0, the ground state is spin liquid, and when x=1 it is a Fermi liquid. The alloying system works as chemical pressure instead of applying external pressure. It allows us simpler measurements, while avoiding the superconducting state. Thus, we can investigate the pure Mott transition. We observed the presence of low energy spin excitations proven by finite value of the Sommerfeld coefficient γ. We also observe large lattice contribution in the spin liquid region due to phonon softening of the lattice. Systematic changes in the heat capacity as well as wea changes in the crystalline structures seem to indicate a continuous transition around the quantum critical point. Via the heat capacity, it is possible to get the entropy. Entropy changes have been observed, indicating a Pomeranchuk-like transition. Finally, a quantum critical region rising from a competition between the spin liquid and the Fermi liquid state has been found in the low temperature region.
Previous research found a large dielectric increase around the boundary in the same alloying complex that they interpreted as evidence of phase separation. We tried to reproduce this by using a doped spin liquid, namely κ-(BEDT-TTF)4Hg2.89Br8. We measured the dielectric permittivity down to around 4K and compared it with a deuterated κ-(BEDT-TTF)2Cu[N(CN)2]Br, which shows phase separation. Very different behaviors have been observed between these 2 samples, indicating the absence of phase separation in κ-(BEDT-TTF)4Hg2.89Br8, but the existence of large nonlinear conductivity due to the mobile feature in the quantum spin liquid background. We conclude that the genuine phase transition between a quantum spin liquid and a Fermi liquid seems to be a continuous phase transition at the low energy levels. At the lower energy level, both states are similar, which may indicate a mixed state within the continuous quantum critical region.