Title: Phase Diagram of $α$-RuCl$_3$ in an in-plane Magnetic Field,
Abstract: The low-temperature magnetic phases in the layered honeycomb lattice material
$\alpha$-RuCl$_3$ have been studied as a function of in-plane magnetic field.
In zero field this material orders magnetically below 7 K with so-called zigzag
order within the honeycomb planes. Neutron diffraction data show that a
relatively small applied field of 2 T is sufficient to suppress the population
of the magnetic domain in which the zigzag chains run along the field
direction. We found that the intensity of the magnetic peaks due to zigzag
order is continuously suppressed with increasing field until their
disappearance at $\mu_o$H$_c$=8 T. At still higher fields (above 8 T) the
zigzag order is destroyed, while bulk magnetization and heat capacity
measurements suggest that the material enters a state with gapped magnetic
excitations. We discuss the magnetic phase diagram obtained in our study in the
context of a quantum phase transition.