Coexistence of Pairing Tendencies and Ferromagnetism in a Doped Two-Orbital Hubbard Model on Two-Leg Ladders arXiv:1001.3097v1 [cond-mat.str-el] 18 Jan 2010 J. C. Xavier, 1 G. Alvarez, 2 A. Moreo, 3 and E. Dagotto 3 $^{1}$Instituto de F´ısica, Universidade Federal de Uberlˆ andia, Caixa Postal 593, 38400-902 Uberlˆ andia, MG, Brazil $^{2}$Computer Science & Mathematics Division and Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA $^{3}$Department of Physics, University of Tennessee, Knoxville, TN 37996 and Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge,TN 37831 (Dated: June 17, 2018) Using the Density Matrix Renormalization Group and two-leg ladders, we investigate an electronic two-orbital Hubbard model including plaquette diagonal hopping amplitudes. Our goal is to search for regimes where charges added to the undoped state form pairs, presumably a precursor of a superconducting state. For the electronic density ρ = 2, i.e. the undoped limit, our investigations show a robust ( π, 0) antiferromagnetic ground state, as in previous investigations. Doping away from ρ = 2 and for large values of the Hund coupling J , a ferromagnetic region is found to be stable. Moreover, when the interorbital on-site Hubbard repulsion is smaller than the Hund coupling, i.e. for U ′ < J in the standard notation of multiorbital Hubbard models, our results indicate the coexistence of pairing tendencies and ferromagnetism close to ρ = 2. These results are compatible with previous investigations using one dimensional systems. Although further research is needed to clarify if the range of couplings used here is of relevance for real materials, such as superconducting heavy fermions or pnictides, our theoretical results address a possible mechanism for pairing that may be active in the presence of short-range ferromagnetic fluctuations. PACS numbers: 71.10.Fd,71.27.+a,74.20.-z It is widely believed that magnetism is a fundamen- tal ingredient to explain the origin of high-temperature superconductivity in several materials. In fact, there is experimental evidence that the superconductivity in many heavy fermion (HF) compounds is mediated by spin fluctuations. 1–4 Mechanisms for superconductivity based on antiferromagnetism have been extensively dis- cussed for the Cu-based high-temperature superconduc- tors as well. 5 Recently, considerable excitement has been generated by the discovery of high-temperature supercon- ductivity in the iron pnictides. 6 Except for the cuprates, the iron-based superconductors now have the highest su- perconducting (SC) critical temperature T$_{c}$ of any ma- terial (see for example Ref. 7). As in HF systems and cuprate superconductors, in the pnictides there is also evidence that the superconductivity is not mediated by the electron-phonon interaction. 8,9 Magnetism and superconductivity can appear in dif- ferent ways. In some HF compounds, superconductiv- ity and antiferromagnetic (AFM) order co-exist, 1 while for the cuprates the superconductivity emerges after the long-range AFM order is destroyed by doping. 5 In some HF systems, it is the superconductivity and ferromag- netism (FM) (as opposed to AFM order) that co-exist. 10 In this work, we will be interested in detecting a clear ev- idence of pairing of extra charges that are added to the undoped limit where magnetic order exists. Using a two- orbital model and two-leg ladders, it will be shown that a possible region for the robust coexistence of (spin triplet) pairing together with magnetic order is where ferromag- netism develops. This is in qualitative agreement with previous investigations carried out using one-dimensional systems. 11 Our effort should be considered simply as pro- viding the first steps in relating pairing and magnetism in a complex two-orbital model via computational tech- niques on ladder geometries. Antiferromagnetic order, as found in the pnictides, could also be favorable for pair- ing tendencies in the spin singlet channel, as discussed recently. 12,13 In principle, a theoretical investigation based on model Hamiltonians for strongly correlated materials starts with an effective tight-binding model, containing the minimum ingredients to describe the physics of the mate- rials under investigation. However, even if a well-defined reasonable model is used, it is still highly non-trivial to extract the ground state properties of these effective mod- els in two or three dimensions using unbiased numerical methods. In fact, at present there are no accurate tech- niques to study Hubbard-like models in dimensions two and three. Thus, in order to get at least some insight on the ground states properties of theses models, it is common practice to study the model Hamiltonians in quasi-one-dimensional geometries. In particular, a very popular route that has been used for several theoretical investigations is to study strongly correlated electronic systems using “ladder” geometries. 14 The N -leg ladders consist of N chains of length L coupled by some param- eter (as, for example, fermionic hopping terms). The I. INTRODUCTION