Hidden Magnon Berry Curvature drives Vertical Magnon Transport

We predict an in-plane, or hidden, Berry curvature (BC) for magnons in electrically insulating quasi-2D magnets and demonstrate that the hidden magnon Berry curvature (HMBC) gives rise to a previously unrecognized form of vertical, out-of-plane, magnon transport. Combining a semiclassical framework with Boltzmann transport theory, we show that the vertical magnon transport (VMT) currents respond both linearly and nonlinearly to the in-plane gradients of magnetic field and temperature. The linear transport coefficients are tied to the total hidden magnon BC, while the nonlinear (second-order) coefficients for the magnetic field and temperature gradients are determined by the hidden magnon BC dipole and the hidden extended magnon BC dipole, respectively. Using linear spin-wave theory, we find that the hidden magnon BC over the Brillouin zone is given by the expectation value of a pseudo-${{\cal Z}}$ operator, representing vertical displacements, evaluated in the space of paraunitary matrices that diagonalize the magnon Hamiltonian. We estimate VMT in spin models of realistic magnets with ferro- and antiferromagnetic order, including the buckled honeycomb (BHC) lattice and bilayer Chromium trihalide (CrX$_3$; X = Cl, Br, I) systems. In BHC, both linear and nonlinear VMT arise when time-reversal symmetry is broken by Dzyaloshinskii-Moriya interactions. In CrX$_3$ systems, the nonlinear coefficients dominate, while the linear responses vanish due to time-reversal symmetry. Both systems exhibit distinctive features across a broad range of temperatures and parameters. Therefore, our prediction of VMT and its characteristic signatures is directly testable in present-day magnonic experiments, especially in atomically thin, few-layered van der Waals magnets.