Signatures for spinons in the quantum spin liquid candidate Ca${}_{10}$Cr${}_7$O${}_{28}$

Abstract

We present new experimental low-temperature heat capacity and detailed dynamical spin-structure factor data for the quantum spin liquid candidate material Ca${}_{10}$Cr${}_7$O${}_{28}$. The measured heat capacity shows an almost perfect linear temperature dependence in the range $0.1$ K $\lesssim T\lesssim 0.5$ K, reminiscent of fermionic spinon degrees of freedom. The spin structure factor exhibits two energy regimes of strong signal which display rather different but solely diffuse scattering features. We theoretically describe these findings by an effective spinon hopping model which crucially relies on the existence of strong ferromagnetically coupled triangles in the system. Our spinon theory is shown to naturally reproduce the overall weight distribution of the measured spin structure factor. Particularly, we argue that various different observed characteristic properties of the spin structure factor and the heat capacity consistently indicate the existence of a spinon Fermi surface. A closer analysis of the heat capacity at the lowest accessible temperatures hints towards the presence of weak $f$-wave spinon pairing terms inducing a small partial gap along the Fermi surface (except for discrete nodal Dirac points) and suggesting an overall ${\mathbb{Z}}_2$ quantum spin liquid scenario for Ca${}_{10}$Cr${}_7$O${}_{28}$.

arXiv comment14 pages, 6 figures

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• Theory of Ca${}_{10}$Cr${}_7$O${}_{28}$ as a bilayer breathing-kagome magnet: Classical thermodynamics and semi-classical dynamics
• How many spin liquids are there in Ca${}_{10}$Cr${}_7$O${}_{28}$?
• Low energy structure of spiral spin liquids
• Spiral spin liquid in a frustrated honeycomb antiferromagnet: A single-crystal study of GdZnPO
• Spiral Spin Liquid Noise
• Two-stage evolution of magnetic correlations in spiral spin liquid material, Ca${}_{10}$Cr${}_7$O${}_{28}$
• Strong coupling of lattice and orbital excitations in quantum magnet Ca${}_{10}$Cr${}_7$O${}_{28}$: Anomalous temperature dependence of Raman phonons
• Origin of the intermediate-temperature magnetic specific heat capacity in the spin-liquid candidate Ca${}_{10}$Cr${}_7$O${}_{28}$