We investigate anomalous planar transport phenomena in a recently identified class of collinear magnetic materials known as $d$-wave altermagnets (AMs). The anomalous planar effects manifest in a configuration when the applied electric field/temperature gradient, magnetic field, and the measured Hall voltage are all coplanar, but the planar magnetic field is instrumental in breaking ${\widehat{C}}_{4z}\widehat{\mathcal{T}}$ symmetry of the $d$-wave AM, where $\widehat{\mathcal{T}}$ is the time-reversal operator, resulting in a Zeeman gap at a shifted Dirac node and a nonzero Berry curvature monopole. We demonstrate that these systems exhibit nearly half-quantized anomalous planar Hall and planar thermal Hall effects at low temperatures that persist over a range of magnetic fields. The angular dependence of the planar transport reveals a $cos2\varphi$ dependence on the magnetic field direction, where $\varphi$ is the azimuthal angle made by the magnetic field. We also discuss the anomalous planar Nernst effect, or transverse thermopower, and demonstrate that the Nernst conductivity peaks when the chemical potential lies just outside the induced Zeeman gap and vanishes within the gap. We further explore the dependence of all three coefficients on the polar and the azimuthal angle of the magnetic field when it is rotated in the full three-dimensional space. Our results reveal the presence of an approximately half-quantized anomalous planar thermal Hall plateau for a range of in-plane magnetic fields without requiring topological superconductivity and conducting Majorana modes, which can be probed in experiments in $d$-wave AMs.

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• Altermagnetism
• Anomalous Hall effect
• Geometric & topological phases
• Nernst effect
• Thermal conductivity
• Topological phase transition
• Altermagnets
• Boltzmann theory