We report the first direct observation of the influence of high magnetic
fields on the Jahn-Teller (JT) transition in TbVO4. Contrary to spectroscopic
and magnetic methods, X-ray diffraction directly measures the JT distortion;
the splitting between the (311)/(131) and (202)/(022) pairs of Bragg
reflections is proportional to the order parameter. Our experimental results
are compared to mean field calculations, taking into account all possible
orientations of the grains relative to the applied field, and qualitative
agreement is obtained.Comment: 11 pages, 4 figures, submitted to Phys. Rev. Let
