On the value of the Curie temperature in doped manganites

We have verified that the variational mean field theory approach suggested by Narimanov and Varma (preprint cond-mat/0002191) being applied to the realistic two-band model provides a good agreement with experimental data for the Curie temperature in doped manganites A$_{1-x}$B$_x$MnO$_3$ ($x\simeq{0.3}$). We have also considered the problem of an interplay between the ferromagnetic and antiferromagnetic interactions by using the same approach.Comment: ReVTeX, 4 pages, 2 figures. To appear in Solid State Com

Fermi Liquid Theory and Ferromagnetic Manganites at Low Temperatures

Fermi liquid characteristics for ferromagnetic ~manganites, A$_{1-x}$B$_x$MnO$_3$, are evaluated in the tight-binding approximation and compared with experimental data for the best studied region $x\simeq0.3$. The bandwidths change only slightly for different compositions. The Sommerfeld coefficient, $\gamma$, the $T^2$-term in resistivity and main scales in optical conductivity agree well with the two band model. The ``2.5'' - transition due to a ``neck'' forming at Fermi surface, is found at $x=0.3$. The mean free path may change from 3 to 80 interatomic distances in the materials, indicating that samples' quality remains a pressing issue for the better understanding of manganites.Comment: 4 pages, 2 figures. Submitted to Solid State Com

Symplectic large-

I present a theory of topological heavy-fermion semiconductors based on the large-N symplectic representation for the electron spin. The theory is exact in the limit when the number of spin flavors N = 2k is infinite. I find that both weak and strong topological insulating phases exist for k < 3. Furthermore, for k ≥ 3 the weak topological insulating state is fully suppressed while only strong topological and trivial insulator states survive. In addition, using the mean-field theory results, I consider the tunneling into topologically trivial and non-trivial phases of a generic heavy-fermion insulators by calculating the differential tunneling conductance. The implications of the presented results for the existing heavy-fermion semiconductors are discussed