1 1.5 2 q$_{x}$/ 0 1 2 3 4 5 6 7 S(q x ) q$_{y}$=0  q$_{y}$=  q$_{y}$=0  q$_{y}$=  q$_{y}$=0  q$_{y}$=  J=1.5 U$_{eff}$=-0.5  (a) 1 1.5 2 q$_{x}$/ 0 1 2 3 S(q x )  q$_{y}$=0   q$_{y}$=   q$_{y}$=0   q$_{y}$=  J=1.5 U$_{eff}$=-0.5  (b) 7 tendencies. Note that for the electron doped case, there is a small peak at q = (0 , π ) for densities ρ ≳ 2 . 2. Using ladders, we have studied analytically and nu- merically a two-orbital Hubbard model. Via the DMRG technique we were able to investigate the model defined on a two-leg ladder geometry for systems with linear sizes up to L = 24. Our spin structure factor data show that for the “undoped” density ρ = 2, a stripe-like AFM order is present, as observed in previous Exact Diagonalization studies. 12,13 We have also presented evidence for triplet pairing tendencies of added electrons/holes close to the density ρ = 2, in some range of couplings, in qualitative agreement with previous investigations using chains, 11 and with Exact Diagonalization calculations in a less ex- treme FM regime of models for pnictides. 12,13 More pre- cisely, we have found that pairing (and presumably su- perconductivity) and ferromagnetism co-exist for a large region of parameters in the regime U ′ < J . Even for U ′ comparable to J our results still indicate a (mild) ten- dency to pairing. Whether this range of couplings for U ′ and J is realized in real materials, such as heavy fermions or pnictides, is a matter to be decided via experiments, or with the help of ab-initio computer simulations. IV. CONCLUSION This research was supported by the Brazilian agen- cies FAPEMIG and CNPq, the National Science Foun- dation grant DMR-0706020, the Division of Materials Science and Engineering of the U.S. Department of En- ergy, and the Center for Nanophase Materials Sciences, sponsored by the Scientific User Facilities Division, Ba- sic Energy Sciences, U.S. Department of Energy, under contract with UT-Battelle. The authors are grateful to Maria Daghofer for providing us with Exact Diagonal- ization data to compare our DMRG results against and to Fernando Reboredo and Satoshi Okamoto for useful comments. q = ( π, 0) for densities close to ρ = 2. Note that these peaks have smaller intensity than those found for ρ = 2 in Fig. 4(a), for the system with size L = 16. We have also observed that the height of the peak at q = ( π, 0) increases with the system sizes for the densities close to ρ = 2. These results indicate that a stripe-like AFM magnetic order also exists for densities close to ρ = 2. As shown in Fig. 4(b), this order does not exist anymore for ρ ≳ 2 . 2 and ρ ≲ 1 . 7, at least within the precision of our calculations, and it is replaced by ferromagnetic Figure 4: (Color online). Spin-structure factor S ( q x ) vs. q x for the two-leg ladder system with size L = 16, J = 1 . 5, and U eff = − 0 . 5. (a) S ( q x ) for the densities ρ = 15 / 8, ρ = 33 / 16, and ρ = 17 / 8 (see legend). (b) S ( q x ) for the densities ρ = 7 / 4 and ρ = 9 / 4. Acknowledgments 1 N. D. Mathur, F. M. Grosche, S. R. Julian, I. R. Walker, D. M. Freye, R. K. W. Haselwimmer, and G. G. Lonzarich, Nature 394 , 39 (1998). 2 M. Jourdan, M. Huth, and H. Adrian, Nature 398 , 47 (1999). 3 M. Sigrist and K. Ueda, Rev. Mod. Phys. 63 , 239 (1991). 4 J. C. Xavier and E. Dagotto, Phys. Rev. Lett. 100 , 146403 (2008). 5 E. Dagotto, Rev. Mod. Phys, 66 , 763 (1994). 6 Y. Kamihara, T. Watanabe, M.Hirano M, and H. Hosono, 10 S. S. Saxena, K. Ahilan, P.Agarwal, F. M. Grosche, R. K. Haselwimmer, M. Steiner, E. Pugh, I. R. Walker, S. R. Julian, P. Monthoux, G. G. Lonzarich, A. Huxley, I. Sheikin, D. Braithweite and J. Flouquet, Nature 406 , 587 (2000). 9 L. Boeri, O. V. Dolgov, and A. A. Golubov, Phys. Rev. Lett. 101 , 026403 (2008). 8 T. Yildirim, Phys. Rev. Lett. 102 , 037003 (2009). 7 C. Wang and et al., Europhys. Lett. 83 , 67006 (2008). J. Am Chem. Soc. 130 , 3296 (2008).