Figure 2: Temperature dependence of the electric polarizations P//[010] and P⊥
for H//[010] at 18 T.
for H//[010] at 18 T.From: Ferroelectricity by Bose–Einstein condensation in a quantum magnet
Inset shows the spin structure in the field-induced phase of TlCuCl3 for H//[010], determined by the previous neutron diffraction measurements13. Grey shedding area shows a unit cell of TlCuCl3. Bold solid and dashed lines show the dimer bonds, which are connected each other by the two-hold helical 21 or the glide symmetry transformations of the P21/c, which is the space group of TlCuCl3. Arrows are spins of Cu2+ ion. The transverse components of the spins direct along the easy axis, which is inclined from [100] by 39° in (010) plane13. The spin ordering breaks the space inversion I and the 21 symmetries, whereas the glide plane parallel to (010) remains. Therefore, the electric polarization, induced by the magnetic field for H//[010], should lie in (010) plane. The experimental result that almost no P//[010] is observed, agrees with the symmetry of the spin structure.
