Spin and charge density waves in the Lieb lattice

• We study the magnetic phase diagram of the Lieb lattice within the mean-field Hubbard model allowing for spin and charge density waves.
• We show that allowing charge and spin density modulation, the unit cell magnetization at half-filling saturates at the exact value 1/2 (Lieb's relation) for any value of U.
• Lieb׳s relation is verified approximately for large U, not only the ferromagnetic region of the phase diagram of the Lieb lattice, but also the adjacent spiral regions.
• The unit cell magnetization is simply the filling of the quasi-flat bands in the mean-field energy dispersion both for large and small U.

We study the mean-field phase diagram of the two-dimensional (2D) Hubbard model in the Lieb lattice allowing for spin and charge density waves. Previous studies of this diagram have shown that the mean-field magnetization surprisingly deviates from the value predicted by Lieb's theorem [1] as the on-site repulsive Coulomb interaction (U) becomes smaller [2]. Here, we show that in order for Lieb's theorem to be satisfied, a more complex mean-field approach should be followed in the case of bipartite lattices or other lattices whose unit cells contain more than two types of atoms. In the case of the Lieb lattice, we show that, by allowing the system to modulate the magnetization and charge density between sublattices, the difference in the absolute values of the magnetization of the sublattices, mLiebmLieb, at half-filling, saturates at the exact value 1/2 for any value of U  , as predicted by Lieb. Additionally, Lieb's relation, mLieb=1/2mLieb=1/2, is verified approximately for large U  , in the n∈[2/3,4/3]n∈[2/3,4/3] range. This range includes not only the ferromagnetic region of the phase diagram of the Lieb lattice (see Ref. [2]), but also the adjacent spiral regions. In fact, in this lattice, below or at half-filling, mLiebmLieb is simply the filling of the quasi-flat bands in the mean-field energy dispersion both for large and small U.