Far-infrared investigations of η-Mo4O11: Using a magnetic field to open the gap

η-Mo4O11 has been of recent interest as both a charge density wave (CDW) and a quantum limit material. Indeed, previous low temperature far-infrared measurements have highlighted the unusual spectroscopic response, while transport studies have identified low-field quantum Hall behavior as well as a semimetal to semiconductor transition above 19 T. In order to explore the low-temperature electrodynamics in this prototype system, we have measured the unpolarized far-infrared response (30-400 cm-1) of η-Mo4O11 in an applied magnetic field (0-17 T). The reflectance ratio of η-Mo4O11 changes systematically at low fields, saturating above 7 T. Here, a small rise in ratio spectra is found near 120 cm-1, whereas the reflectance ratio drops below 70 cm-1. Combining this data with the low-temperature power reflectance, we have used a Kramers-Kronig analysis to calculate the frequency dependent conductivity in the magnetic field. The preliminary data shows that the main effect of the field is related to the CDW2 gap.Work at SUNY-Binghamton was supported by the Division of Materials Research, National Science Foundation under Grant No. DMR-9623221. The measurements at high magnetic field were performed at the NHMFL, in Tallahassee, Florida, which is supported by NSF Cooperative Agreement No. DMR-9527035 and by the State of Florida. We gratefully acknowledge C.C. Homes and A.W. McConnell for generous access to their polarized far-infrared data, and their helpful discussion.