First-principles study of the lattice and electronic structures of TbMn${}_2$O${}_5$

Abstract

The structural, electronic and lattice dielectric properties of multiferroic TbMn${}_2$O${}_5$ are investigated using density functional theory within the generalized gradient approximation (GGA). We use collinear spin approximations and ignore the spin-orbit coupling. The calculated structural parameters are in excellent agreement with the experiments. We confirm that the ground state structure of TbMn${}_2$O${}_5$ is of space group $Pb{2}_{1}m$, allowing polarizations along the b-axis. The spontaneous electric polarization is calculated to be 1187 $nC\cdot$cm${}^{-2}$. The calculated zone-center optical phonons frequencies and the oscillator strengths of IR phonons agree very well with the experimental values. We then derive an effective Hamiltonian to explain the magnetically-induced ferroelectricity in this compound. Our results strongly suggest that the ferroelectricity in TbMn${}_2$O${}_5$ is driven by the magnetic ordering that breaks the the inversion symmetry, without invoking the spin-orbit coupling.

12 pages, 5 figures

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