Frustration-Induced Insulating Chiral Spin State in Itinerant Triangular-Lattice Magnets

We study the double-exchange model at half-filling with competing superexchange interactions on a triangular lattice, combining exact diagonalization and Monte Carlo methods. We find that in between the expected itinerant ferromagnetic and 120° Yafet-Kittel phases a robust scalar-chiral, insulating spin state emerges. At finite temperatures the ferromagnet-scalar-chiral quantum critical point is characterized by anomalous bad-metal behavior in charge transport as observed in frustrated itinerant magnets $R_2{\mathrm{Mo}}_2{\mathrm{O}}_7$.

Figure 1

(a) Magnetization and (b) inverse magnetic susceptibility as a function of temperature for various values of superexchange coupling $J_S$. The inset in (b) shows the Curie-Weiss scale extracted from the ${\chi }^{-1}(T)$ as a function of $J_S$. (c) Spin-structure factor $S(\mathbf{q})$ at $T=0.001$ along the principal symmetry directions in the momentum space for different values of $J_S$. The inset shows the Brillouin zone for a triangular lattice and the path along which $S(\mathbf{q})$ is plotted.

Figure 2

Electronic density of states for varying temperatures with (a) $J_S=0.04$, (b) $J_S=0.08$, (c) $J_S=0.16$ and (d) $J_S=0.36$. A Lorentzian broadening $\gamma =0.04$ is used.

Figure 3

(a) Resistivity as a function of temperature for various values of $J_S$. (b) $J_S$ dependence of resistivity $\rho$ (filled symbols) and scaled DOS (open symbols) at Fermi level $E_F$ for low and high temperatures.

Figure 4

(a) Low-temperature spin-structure factor $S(\mathbf{q})$ as a function of superexchange coupling $J_S$ for various $\mathbf{q}$. The dashed line is the scalar chirality $\kappa$. (b) The phase diagram of the model in the temperature-$J_S$ plane. The spin directions within a unit cell are schematically shown for the 120° state, and are indicated as un-normalized ($S_x$, $S_y$, $S_z$) on each site for the noncoplanar SC state.