Emergence of a Fluctuating Ground State in Y-kapellasite under Pressure

Abstract

Y-kapellasite (Y$_3$Cu$_9$(OH)$_{19}$Cl$_8$), which hosts an original anisotropic kagome sublattice, is a promising candidate for studying elusive and complex correlated physics. It exhibits a theoretically predicted in-plane $(1/3, 1/3)$ magnetic order [1] but its magnetic interaction values place it close to a phase boundary to a spin liquid state [2]. Our $μ$SR measurements under hydrostatic pressure demonstrate the complete suppression of static magnetism in favor of a fully dynamical ground state at $2.3$~GPa. Complementary high-pressure x-ray and optical phonon measurements reveal a gradual reduction of the kagome anisotropy, enhancing magnetic frustration without structural transitions. Our results establish Y-kapellasite as a rare clean kagome model in which long-range order is suppressed by pressure-tuned frustration, the first fingerprint for the realization of a quantum spin liquid without strong disorder.

Figures (4)

• Figure 1: Schematic view of a single kagome layer of Y-kapellasite inside the diamond anvil cell, highlighting the pressure-tuned magnetic "skeleton" made of two inequivalent Cu sites (Cu1 in orange, Cu2 in blue). The dominant exchange interaction $J_{\hexagon}$ along Cu1--Cu1 bonds (orange), the weaker Cu1--Cu2 interaction $J'$ (green), and the Cu2--Cu1 interaction $J$ (blue) are indicated. Gray arrows illustrate the effect of hydrostatic pressure, which pushes the Y ions (located at the centers of the hexagons) back toward the kagome plane.
• Figure 2: (a) Zero-field (ZF) asymmetries measured at $0.28\,\mathrm{K}$ under different pressures: ambient pressure, $1.14\,\mathrm{GPa}$, $1.8\,\mathrm{GPa}$, and $2.3\,\mathrm{GPa}$. The black dotted line indicates the contribution from the pressure cell. (b) Temperature evolution of the relaxation rate $\lambda$ at $P=2.3$ GPa. Inset: zero-field asymmetry spectra measured at $2.0\,\mathrm{K}$, $1.2\,\mathrm{K}$, $0.6\,\mathrm{K}$, and $0.25\,\mathrm{K}$ at $P=2.3$ GPa. (c) Time evolution of LF asymmetries at 0.28 K under three different fields: 0.01 T, 0.02 T, and 0.05 T at 2.3 GPa, along with the corresponding fits to the DKT model [see Eq. \ref{['eq:pressure_DKT']}] demonstrating a dynamical ground state.
• Figure 3: (a) Pressure dependence of the refined Cu-Cu bond lengths: Cu1--Cu1 (orange), Cu1-Cu2 (green), and Cu2-Cu1 (dark blue). (b) Corresponding pressure evolution of the Cu-O-Cu bond angles defining the superexchange pathways $J_{\hexagon}$ (Cu1-O-Cu1, orange), $J'$ (Cu1-O-Cu2, green), and $J$ (Cu2--O--Cu1, dark blue). (c) Classical phase diagram of the distorted kagome antiferromagnet as a function of the exchange interactions $J_{\hexagon}$, $J$, and $J'$Hering2022. The white region denotes the $(1/3,1/3)$ antiferromagnetic ordered phase, while the blue region corresponds to a spin liquid (SL) regime. The exchange parameters obtained from spin wave analysis of inelastic neutron scattering data are shown as a yellow star Chatterjee2023. Estimates of the pressure-dependent exchange couplings derived from Eq. (\ref{['eq:AF']}) are indicated by blue (low temperature) and red (high temperature) symbols, with the applied pressure color coded as indicated in the legend.
• Figure 4: Low temperature absorbance with increasing pressure at 100 K (a),(b) and at 14-18 K (c),(d). The spectra are shifted along the $y$-axis for clarity. The regions marked by * are excluded because of the contribution from the experimental setup. The dotted line corresponds to the ambient pressure results at 100 K and 20 K, respectively. Far (1,2) and midinfrared (3,4) regions are indicated by horizontal double arrows separating the regions with the dominant motion of different atoms in the phonon modes. The gray dashed lines are guides for the eyes to highlight the blueshifts and redshifts.