Birefringence of hexagonal helium-4 at various densities Vos, J. E. ; Veenenga Kingma, R. ; Van der Gaag, F. J. ; Blaisse, B. S. Abstract Publication: Physics Letters A Pub Date: June 1967 DOI: 10.1016/0375-9601(67)90243-5 Bibcode: 1967PhLA...24..738V