Photoinduced Frustration Modulation in $κ$-type Quantum Spin Liquid Candidates

Abstract

Geometric frustration is a key parameter controlling electronic and magnetic properties of quantum spin liquid systems, yet remains challenging to tune. Here, we coherently drive molecular vibrations with midinfrared pulses in two organic quantum spin liquid candidates, the insulating $κ$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$ and the metallic $κ$-(BEDT-TTF)$_4$Hg$_{2.89}$Br$_8$, and probe their electronic response through ultrafast reflectivity measurements. We observe a nonlinear coupling between local molecular vibrations and nonlocal phonons, which is expected to directly modulate the geometric frustration of their triangular lattice. Our findings establish a promising route to dynamically control frustration in nonbipartite quantum materials.

Figures (4)

• Figure 1: (a) Alternating cation and anion layers stacked along the a axis in the $\kappa$-type structure of ET-based organic charge-transfer salts. (b) In the cation layer, (ET)$_2^+$ dimers with effective charge $+e$ and spin $S = 1/2$ are arranged in a triangular lattice. (c)-(d) Equilibrium optical conductivity spectra of $\kappa$-CuCN and $\kappa$-HgBr measured at 100 K along the b-axis. The optical response depends critically on the structure of the anion layers. At $\omega\rightarrow 0$, $\kappa$-CuCN exhibits insulating behavior, whereas $\kappa$-HgBr manifests a Drude peak. At higher energy, the response is characterized by sharp molecular modes and charge-transfer excitations of the molecular dimers.
• Figure 2: (a) Sketch of the midinfrared (MIR) pump - near-infrared (NIR) probe experiment targeting the (ET)$_2^+$ dimers. (b) Pump and probe spectra (shaded areas) centered on the ethylene group vibrations and the intradimer absorption band of $\kappa$-CuCN and $\kappa$-HgBr, respectively. Solid curves are the sample reflectivities ( b-axis, 100 K) (c) Time-dependent reflectivity change of the intradimer resonance (symbols). Both compounds exhibit a transient electronic response (top) following a double exponential decay (black solid lines) together with a coherent oscillatory behavior, which we isolate by subtracting the exponential contribution (bottom). The oscillations are approximately centered at 1.2 THz (inset, Fourier-transform spectrum).
• Figure 3: Effective phonon photosusceptibility (symbols) and linear absorption $\alpha$ (dashed lines) as a function of pump photon energy for $\kappa$-CuCN (a) and $\kappa$-HgBr (b). The oscillatory response resonates with the ethylene molecular modes. The pump is polarized along the $c$- and $b$-axes, respectively. The probe polarization dependence of the effective phonon susceptibility (measured with 161 meV pump along $b$) is isotropic for $\kappa$-CuCN (c) and anisotropic for $\kappa$-HgBr (d), reflecting the distinct anion lattice symmetries.
• Figure 4: (a) Phonon-displaced (ET)$_2^+$ layers (S and C atoms only) for $\kappa$-CuCN (left) and $\kappa$-HgBr (right). (b) Sketch of the phonon-induced displacements of the (ET)$_2^+$ dimers, featuring intra-dimer distance changes in $\kappa$-CuCN and dimer rotation in $\kappa$-HgBr. The resulting structural distortions modify the Coulomb repulsion $U$ and the inequivalent hopping amplitudes $t$ and $t'$. (c) Phonon-induced modulation of geometrical frustration $t'/t$ (top) and effective electronic correlation $U/t$ (bottom) as a function of phonon displacement $Q_R$, obtained from ab initio band structure calculations supp.