Session H2: Spin Liquids in 2D Frustrated Quantum Magnets

Phase diagram of the 2D frustrated quantum magnet Cs$_2$CuCl$_4$ in applied field

We use neutron scattering, magnetization and specific heat to probe how the ground state of the 2D spin-1/2 anisotropic triangular lattice antiferromagnet Cs$_2$CuCl$_4$ evolves as a function of applied magnetic field. We observe that the zero- field incommensurate spin spiral is suppressed by relatively small in-plane fields giving way to a commensurate 2D antiferromagnetic pattern stable over most of the intermediate field region, with incommensurate order appearing again for a narrow field range just below ferromagnetic saturation. The commensurate phase is not captured by a classical mean-field approach, suggesting that quantum fluctuations may be important in stabilizing this type of order.   

Algebraic vortex liquid in spin-1/2 triangular antiferromagnets: Scenario for Cs$_2$CuCl$_4$

Motivated by inelastic neutron scattering data on Cs$_2$CuCl$_4$, we explore spin-1/2 triangular lattice antiferromagnets with both spatial and easy-plane exchange anisotropies, the latter due to an observed Dzyaloshinskii-Moriya interaction. Exploiting a duality mapping followed by a fermionization of the dual vortex degrees of freedom, we find a novel ``critical" spin-liquid phase described in terms of Dirac fermions with an emergent global SU(4) symmetry minimally coupled to a non-compact U(1) gauge field. This ``algebraic vortex liquid" supports gapless spin excitations and universal power-law correlations in the dynamical spin structure factor which are consistent with those observed in Cs$_2$CuCl$_4$. We suggest future neutron scattering experiments that should help distinguish between the algebraic vortex liquid and other spin liquids and quantum critical points previously proposed in the context of Cs$_2$CuCl$_4$.   

Spin liquid and Mott transition in organics with triangular lattice

The layered organics, k-(ET)2X, are model systems for the study of strongly correlated half-filled-band electrons. The Mott insulator k-(ET)2Cu2(CN)3 has a nearly isotropic triangular lattice and is a model system of frustrated quantum spins. The 1H and 13C NMR experiments show no indication of magnetic ordering down to 30 mK in spite of an exchange interaction of 250 K deduced from the susceptibility analysis. The spins are in the quantum liquid state. Under magnetic fields, an anomalous inhomogeneous spin state appears. Under pressure, it undergoes Mott transition to a Fermi liquid which shows superconductivity at low temperatures. The phase diagram and the nature of the superconductivity emerging from the spin liquid are also discussed.   

Spinon Fermi sea state as a candidate spin liquid in $\kappa$-(ET)$_2$Cu$_2$(CN)$_3$

Experimental studies of the quasi-two-dimensional organic compound $\kappa$-(ET)$_2$Cu$_2$(CN)$_3$ strongly suggest a spin liquid state in the insulating phase at ambient pressure. This material is modelled as a half-filled triangular lattice Hubbard system close to the Mott transition. By applying trial wave function approach to an effective spin Hamiltonian with significant ring exchanges, it is proposed that a spin liquid state with spinon Fermi surface is realized in this system. Properties of this state are reviewed and compared with experiments. Further experiments find that an inhomogeneous spin state is induced by strong magnetic field. It is argued that the origin of this anomalous response is magneto-orbital effects, whereby spinons see significant orbital field and have strong back action on the internal gauge field. The spin liquid ground state itself is readjusting in the magnetic field, possibly in a discontinuous manner in an ideal system; this persists in a broad temperature range and may be responsible for the inhomogenuous state in the real system.   

Spin excitations in frustrated Kagom\'{e} lattice systems

We present experimental results on the behavior of interacting spins on the two-dimensional frustrated Kagom\'{e} lattice. Basic concepts of geometrical frustration will be reviewed, and measurements on single crystal and powder samples of ideal Kagom\'{e} systems (both classical and quantum) will be described. For a system with S=5/2, our inelastic neutron scattering measurements of the spin wave excitations reveal a novel ``flat mode'' which is a direct consequence of the high degree of frustration. We observe interesting behavior related to spin chirality (both vector and scalar). Recently, we have performed measurements on a material with spin-1/2 ions in an ideal Kagom\'{e} lattice arrangement. We discuss our results in relation to the excitations proposed for novel quantum spin liquid states.