  Resonant soft x-ray Bragg diffraction at the Tb M4,5 edges and non resonant
Bragg diffraction have been used to investigate orbitals in TbB2C2. The Tb 4f
quadrupole moments are ordered in zero field below T_N and show a
ferroquadrupolar alignment dictated by the antiferromagnetic order. With
increasing applied field along [110] the Tb 4f magnetic dipole moments rotate
in a gradual manner toward the field. The quadrupole moment is rigidly coupled
to the magnetic moment and follows this field-induced rotation. The quadrupolar
pair interaction is found to depend on the specific orientation of the orbitals
as predicted theoretically and can be manipulated with an applied magnetic
field.
