Kazushi Kanoda, Spin liquid behavior in organic charge transfer salts

Kazushi presented his results for spin liquid behavior observed in a member of the two dimensional charge transfer salt family κ-(ET)₂X with counter ion X=Cu₂(CN)₃.

ET-molecules form a two dimensional pattern where two molecules form dimers and dimers form an anisotropic triangular lattice. The counter ions take one electron out of a dimer, leading to a half filled band. Kazushi stressed that in the organics, different ground states can be stabilized via changing the counter ions and tuning the pressure.

In distinction to the ambient pressure Mott insulator κ-(ET)2X with X=Cu2[N(CN)2]Cl,which undergoes antiferromagnetic long range order below 27K, the triangular lattice of the X=Cu2(CN)3 compound is almost isotropic. The key observation from NMR, µ-SR and susceptibility measurements is the absence of magnetic long range order down to 20mK (much smaller than the estimated size of the exchange interaction J~250K).

In the discussion it was pointed out that the Heisenberg model on a triangular lattice has an ordered ground state, but that a disordered ground state can be stabilized by ring exchange terms that are expected to be important close to a Mott transition. Indeed the material undergoes a pressure induced Mott transition from insulator to metal at p=0.3-0.4GPa.

Next to the existence of a Mott insulator without broken symmetry, the most spectacular observation of Kanoda and collaborators is the fact that the low T behavior of the system seems gapless. The heat capacity vanishes linearly with T and the susceptibility is finite. This is consistent with the observation that the entropy of the spin liquid is larger that the entropy of the pressure induced superconductor as deduced from the Clausius-Clapeiron relation.

While there seem to be no long range order at low T, ¹³C-NMR, thermal conductivity and heat capacity measurements show indications for the onset of some inhomogeneous state below T=5K. Also, the spin lattice relaxation rate decreases with T^3/2 below T=5K (it decreases as T^1/2 at higher T). A broadening of the ¹³C-NMR lines and stretched exponential relaxation is the key argument supporting a magnetic field induced inhomogeneous state.

Open problems in this context are:

-is the gapless nature of the ground state related to the anomaly at 5K and the onset on inhomogeneities?

-what is the nature of the superconducting state? Is it different from other organics? (In this context Kazushi mentioned that the Knight shift in the superconducting state does not seem to vanish)

-Why is the charge response of this system rather different from other organics?

Buttom line: This material seems the first gapless spin-liquid in a quasi two dimensional Mott insulator. Upon pressure it becomes a superconductor. So far no theory was able to account for the rich behavior of this system. It is clearly a very sharply defined outstanding challenge!

Relevant papers:

Y. Shimizu, K. Miyagawa, K. Kanoda, M. Maesato, and G. Saito, ¹H NMR and static susceptibility measurements have been performed in an organic Mott insulator with a nearly isotropic triangular lattice, κ-(BEDT-TTF)₂Cu₂(CN)₃, which is a model system of frustrated quantum spins. The static susceptibility is described by the spin S=1/2 antiferromagnetic triangular-lattice Heisenberg model with the exchange constant J∼250  K. Regardless of the large magnetic interactions, the ¹H NMR spectra show no indication of long-range magnetic ordering down to 32 mK, which is 4 orders of magnitude smaller than J. These results suggest that a quantum spin liquid state is realized in the close proximity of the superconducting state appearing under pressure.Phys. Rev. Lett. 91, 107001 (2003)

Y. Kurosaki, Y. Shimizu, K. Miyagawa, K. Kanoda, and G. Saito, Phys. Rev. Lett. 95, 177001 (2005)