Comment on "Vortex Glass and Lattice Melting Transitions in a YNi2B2C Single Crystal"

In a recent Letter [1], Mun et al. claim to observe vortex glass and lattice melting transitions in superconducting YNi2B2C. A key observation, which they associate with vortex lattice melting, is the presence of a sharp transition in the dc resistivity in finite dc magnetic field. Mun et al. arrive at an HT phase diagram with a single phase boundary which separates the vortex liquid from the vortex solidyglass and a single Hc2sTd superconductor-normal (SN) boundary. In this Comment, we show that precision rf penetration depth measurements indicate a much more complex picture than that presented in Ref. [1]. Our experiments are carried out on samples obtained from the same source as in Ref. [1], using an rf (3 MHz) tunnel diode technique. Details of the measurement technique and particularly results on single crystal YNi2B2C are described elsewhere [2]. Typical lsHd vs H data observed for T , Tc are shown in the inset of Fig. 1. The data clearly show the presence of multiple transitions at characteristic fields, and these are accentuated in the dlydH also shown in the plot. We have studied these features over a wide range of temperatures and fields in the superconducting state. The HT phase diagram based on our results is shown in the main panel of Fig. 1 with HgsT d and HmsT d from Ref. [1] also included for comparison (filled squares and solid line). It is obvious that the phase diagram is quite complex [2], consisting of 3, possibly 4, transition lines. The vortex-glass transition identified by Ref. [1] lies in a region of the phase diagram where we have observed