Field-induced staggered magnetic moment in the quasi-two-dimensional organic Mott insulator κ\kappa-(BEDT-TTF)2_{2}Cu[N(CN)2_{2}]Cl

We investigated the magnetism under a magnetic field in the quasi-two-dimensional organic Mott insulator $\kappa$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Cl through magnetization and $^{13}$C-NMR measurements. We found that in the nominally paramagnetic phase (i.e., above N\'eel temperature) the field-induced local moments have a staggered component perpendicular to the applied field. As a result, the antiferromagnetic transition well defined at a zero field becomes crossover under a finite field. This unconventional behavior is qualitatively reproduced by the molecular-field calculation for Hamiltonian including the exchange, Dzyaloshinsky-Moriya (DM), and Zeeman interactions. This calculation also explains other unconventional magnetic features in $\kappa$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Cl reported in the literature. The present results highlight the importance of the DM interaction in field-induced magnetism in a nominally paramagnetic phase, especially in low-dimensional spin systems.Comment: 11 pages, 12 figures, selected for Editors' Suggestion

Extended Hubbard Model with Unconventional Pairing in Two Dimensions

We rigorously prove that an extended Hubbard model with attraction in two dimensions has an unconventional pairing ground state for any electron filling. The anisotropic spin-0 or anisotropic spin-1 pairing symmetry is realized, depending on a phase parameter characterizing the type of local attractive interactions. In both cases the ground state is unique. It is also shown that in a special case, where there are no electron hopping terms, the ground state has Ising-type N\'eel order at half-filling, when on-site repulsion is furthermore added. Physical applications are mentioned.Comment: 5 pages, 2 figures, v2: title is changed, new results are added, minor change

Hole and electron dynamics in the triangular-lattice antiferromagnet -- interplay of frustration and spin fluctuations

Single-particle dynamics in the 120$^{\circ}$ ordered antiferromagnetic state of the triangular-lattice Hubbard model is studied using a physically transparent fluctuation approach in terms of multiple magnon emission and absorption processes within the noncrossing approximation. Hole and electron spectral features are evaluated at intermediate $U$, and analyzed in terms of a competition between the frustration-induced direct hopping and the virtual hopping terms. Finite $U$-induced competing interactions and frustration effects contributing through the magnon dispersion are also discussed. Finite contribution to self-energy correction from long-wavelength (Goldstone) modes, together with the high density of electron scattering states in the narrow, sharp peak in the upper band, result in strong fermion-magnon scattering leading to pronounced incoherent behaviour in the electron dynamics. The fluctuation-induced first-order metal-insulator transition due to vanishing band gap is also discussed.Comment: 10 pages, 13 figure

Magnetic-Field-Induced Mott Transition in a Quasi-Two-Dimensional Organic Conductor

We investigated the effect of magnetic field on the highly correlated metal near the Mott transition in the quasi-two-dimensional layered organic conductor, $\kappa$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Cl, by the resistance measurements under control of temperature, pressure, and magnetic field. It was demonstrated that the marginal metallic phase near the Mott transition is susceptible to the field-induced localization transition of the first order, as was predicted theoretically. The thermodynamic consideration of the present results gives a conceptual pressure-field phase diagram of the Mott transition at low temperatures.Comment: 4 pages, 4 figure

The origin of the phase separation in partially deuterated κ\kappa-(ET)2_2Cu[N(CN)2_2]Br studied by infrared magneto-optical imaging spectroscopy

The direct observation of the phase separation between the metallic and insulating states of 75 %-deuterated $\kappa$-(ET)$_2$Cu[N(CN)$_2$]Br ($d33$) using infrared magneto-optical imaging spectroscopy is reported, as well as the associated temperature, cooling rate, and magnetic field dependencies of the separation. The distribution of the center of spectral weight ($$) of $d33$ did not change under any of the conditions in which data were taken and was wider than that of the non-deuterated material. This result indicates that the inhomogenity of the sample itself is important as part of the origin of the metal - insulator phase separation.Comment: 4 pages, 3 figures, accepted for publication in Solid State Commu

Correlation effects on magnetic frustration in the triangular-lattice Hubbard model

Evolution of the magnetic response function in the triangular-lattice Hubbard model is studied with interaction strength within a systematic inverse-degeneracy expansion scheme which incorporates self-energy and vertex corrections and explicitly preserves the spin-rotation symmetry. It is shown that at half filling the response function goes through a nearly dispersionless regime around K for intermediate coupling strength, before undergoing an inversion at strong coupling, resulting in maximum response at the K point, consistent with the expected 120^o AF instability. Effects of finite hole/electron doping on the magnetic response function are also examined.Comment: 20 pages, 9 figure

Charge ordering in \theta-(BEDT-TTF)2RbZn(SCN)4: Cooperative effects of electron correlations and lattice distortions

Combined effects of electron correlations and lattice distortions are investigated on the charge ordering in \theta-(BEDT-TTF)2RbZn(SCN)4 theoretically in a two-dimensional 3/4-filled extended Hubbard model with electron-lattice couplings. It is known that this material undergoes a phase transition from a high-symmetry metallic state to a low-symmetry insulating state with a horizontal-stripe charge order (CO) by lowering temperature. By means of the exact-diagonalization method, we show that electron-phonon interactions are crucial to stabilize the horizontal-stripe CO and to realize the low-symmetry crystal structure.Comment: 7 peges, 7 figures, accepted for publication in Phys. Rev.