Electronic phase separation in the pyrochlore double-exchange model

Abstract

Electronic phase separation and related inhomogeneity is ubiquitously seen in strongly-correlated systems. A typical example is found between ferromagnetic metal and antiferromagnetic insulator in CMR manganese oxides. Here we demonstrate that the geometrical frustration brings distinctive aspects into the phase separation phenomena. From Monte Carlo simulation and a simple energy comparison for the pyrochlore double-exchange model, we show that such phase separation takes place between ferromagnetic and paramagnetic metals. We discuss the relevance of our results to a spin-glassy metallic phase found in Mo pyrochlore oxides under external pressure.PACS numbers: 71.30.+h, 71.27.+a, 71.20.Be, 71.10.Fd Electronic inhomogeneity is one of the key concepts in strongly correlated electron systems. It emerges from keen competition between different electronic phases, and gives rise to fascinating phenomena, such as nanometerscale domain structure and gigantic response to external perturbations. Many intriguing topics related to the electronic inhomogeneity have been studied intensively, such as granular superconductivity in cuprates [1], colossal magneto-resistance (CMR) in manganites [2], and relaxor response in dielectric compounds [3].One of the most interesting, well-known examples is found in CMR manganites. It was argued that the competition between ferromagnetic metal (FM) and antiferromagnetic insulator (AFI) results in an electronicallyinhomogeneous state in the presence of quenched disorder, and that the inhomogeneous state is highly sensitive to external magnetic field and gives rise to the CMR effect [4]. Theoretically, the double-exchange (DE) model and its extensions have been extensively studied, and it was clarified that the model has an instability toward the electronic phase separation (PS) [2]. The PS occurs with a discontinuous change in the electron density, e.g., between FM and AFI: The former appears in a wide range of density by the DE ferromagnetic interaction of kinetic origin [5] and the latter is stabilized at a commensurate electron density by the AF super-exchange (SE) interaction [6,7]. The energetically-competing phases lead to the electronic PS as well as bicritical behaviors, which are discussed as key players in the fascinating properties of CMR manganites.In this Letter, we elucidate a new type of electronic PS in the DE model, which emerges when the lattice structure is geometrically frustrated. Here we show that PS takes place between FM and a paramagnetic state for the model defined on the pyrochlore lattice which has a threedimensional network of corner-sharing tetrahedra [inset of Fig. 1(a)]. The paramagnetic state does not have a charge gap, in contrast to the AFI state in the unfrustrated cases. While the PS was observed in the previous study [8], the present work elucidates its ubiquitousness and origin. It should be noted that this PS with a discontinuous change of the density is non-trivial, since the transition from ferromagnet to paramagnet is continuous in general. Th...

Amorphous ferromagnetism and re-entrant magnetic glassiness in single-crystallineSm2Mo2O7

“…23 From a local point of view, bond-randomness is accepted to be the origin of glassiness in Y 2 Mo 2 O 7 , 24 the principal source of disorder arising from Y-Mo rather than Mo-Mo pairs. 25 However, more recent works 22,[26][27][28][29] suggest that a complicated interplay between spin and orbital degrees of freedom 30 on the basis of a Kugel-Khomskii mechanism 31 may be crucial in determining the overall magnetic properties of pyrochlore molybdates. In particular, on the basis of a multi-orbital Hubbard model, Shinaoka et al 29 conclude that Y 2 Mo 2 O 7 should be considered as the realization of a "spin-orbital frustrated Mott insulator " rather than a conventional frustrated magnet.…”

Magnetic and transport signatures of Rashba spin-orbit coupling on the ferromagnetic Kondo lattice model in two dimensions

“…For a large Hund coupling, it is known that a metallic ferromagnetic (FM) phase up to a filling ν ∼ 0.8, followed by a tendency towards an antiferromagnetic (AFM) state with semiconducting or insulating properties. 25,[27][28][29][30][31][32][33] Another issue that has been thoroughly discussed in the context of manganites is the presence of an instability towards phase separation.…”

Bond Orders in a Frustrated Itinerant System on a Kagome Lattice

“…However, due to frustrated behavior of the system, the transition toward antiferromagnetism is also suppressed. Then, it is expected that various instabilities in spin as well as charge sectors may occur, as in the case of the pyrochlore systems where magnetic phase competitions accompany metal-insulator as well as metal-metal transitions [3].…”

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