Theory of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>Ca</mml:mi><mml:mn>10</mml:mn></mml:msub><mml:msub><mml:mi>Cr</mml:mi><mml:mn>7</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>28</mml:mn></mml:msub></mml:mrow></mml:math>as a bilayer breathing-kagome magnet: Classical thermodynamics and semiclassical dynamics
Rico Pohle, Han Yan, Nic Shannon
Abstract
${\mathrm{Ca}}_{10}{\mathrm{Cr}}_{7}{\mathrm{O}}_{28}$ is a novel spin-$1/2$ magnet exhibiting spin liquid behavior, which sets it apart from any previously studied model or material. However, understanding ${\mathrm{Ca}}_{10}{\mathrm{Cr}}_{7}{\mathrm{O}}_{28}$ presents a significant challenge, because the low symmetry of the crystal structure leads to very complex interactions, with up to seven inequivalent coupling parameters in the unit cell. Here we explore the origin of the spin-liquid behavior in ${\mathrm{Ca}}_{10}{\mathrm{Cr}}_{7}{\mathrm{O}}_{28}$, starting from the simplest microscopic model consistent with experiment---a Heisenberg model on a single bilayer of the breathing-kagome (BBK) lattice. We use a combination of classical Monte Carlo (MC) simulation and (semi)classical molecular dynamics (MD) simulation to explore the thermodynamic and dynamic properties of this model and compare these with experimental results for ${\mathrm{Ca}}_{10}{\mathrm{Cr}}_{7}{\mathrm{O}}_{28}$. We uncover qualitatively different behaviors on different timescales, and argue that the ground state of ${\mathrm{Ca}}_{10}{\mathrm{Cr}}_{7}{\mathrm{O}}_{28}$ is born out of a slowly fluctuating ``spiral spin liquid'', while faster fluctuations echo the U(1) spin liquid found in the kagome antiferromagnet. We also identify key differences between longitudinal and transverse spin excitations in applied magnetic field, and argue that these are a distinguishing feature of the spin liquid in the BBK model.