Doping and Field-Induced Insulator-Metal Transitions in Half-Doped Manganites

Abstract

We argue that many properties of the half-doped manganites may be understood in terms of a new two-(eg electron)-fluid description, which is energetically favorable at intermediate Jahn-Teller (JT) coupling. This emerges from a competition between canting of the core spins of Mn promoting mobile carriers and polaronic trapping of carriers by JT defects, in the presence of CE, orbital and charge order. We show that this explains several features of the doping and magnetic field induced insulator-metal transitions, as the particle-hole asymmetry and the smallness of the transition fields."Half-doped" manganites such as Re 1−x A x MnO 3 with x = 1/2 where Re is a 3+ rare-earth ion and A a 2+ alkaline earth ion have been the object of extensive studies for many years [1] ). The competition between the CE and A phases appears even in a simple one-orbital model [6] because of the interplay of ferromagnetic double-exchange and AF superexchange between the core t 2g spins of Mn (see also Fig. 1). The presence of charge and orbital order as proposed by Goodenough [3] is more difficult to establish. X-ray diffraction experiments do suggest the presence of large Jahn-Teller (JT) distortions [4,7] with two inequivalent Mn sites. In the CE phase, the alternating (3x 2 − r 2 )/(3y 2 − r 2 ) orbital order (consistent with the observed distortions) was shown to optimize the anisotropic hopping energy of the e g electrons in a more realistic two e g orbital model [8]. The origin of charge-order was attributed to on-site [8] or intersite Coulomb interactions [9,10], though the latter tends to favor a Wigner crystal [9] rather than the charge stacked order found experimentally [1]. The role of the JT coupling has been investigated using imposed JT distortions [11] as well as by extensive classical Monte-Carlo simulations that lead to the observed charge stacked ordering [12].However, several fundamental issues remain to be understood. One of them is the striking asymmetry with respect to the addition of electrons or holes. Experimentally, added electrons typically favor ferromagnetic metallic phases while added holes favor insulating phases [1]. In contrast, band structure arguments [8], and treatments including JT distortions adiabatically and classically [12] lead to metallic phases on both sides. Another puzzling feature, first seen in (Nd,Sm) 1/2 Sr 1/2 MnO 3 [13] and later seen to be ubiquitous [1], is that magnetic-fields ∼ 10−40 Tesla, which are extremely small compared with Néel or charge ordering temperatures ∼ 200 K, induce an insulator-metal transition. This can be viewed as another manifestation of the colossal magneto-resistance (CMR) in doped manganites [1]. An explanation is that this arises from the proximity of the CE phase to a ferromagnetic phase [6,10,14]; but it is difficult to understand why the parameters in so many systems should all be so finely tuned as to be near the phase boundary.Recently, starting from a large JT coupling picture, a two-fluid e g electron model, one polaronic and localized, and the ...

Jahn-Teller distortions and phase separation in doped manganites

“…The models of similar type were discussed in Refs. 26,27,30. However, the possibility of the phase separation was not considered in these papers.…”

“…In the first case, the value of m(T ) and the free energy of the system is found in the same way as for FM state from Eqs. (25)- (27). The only difference is that m = 1 at T = 0 in FM state (or ν = 0) while m(0) < 1 (or ν = 0) in the canted state.…”

“…Therefore, it is natural to assume that the e g electron can be either localized due to Jahn-Teller distortions with the energy gain ǫ JT (l electron) or to decrease its energy due to band broadening (b electron) 26,27,30 . So, there exists a competition between the localization and delocalization.…”

“…In particular, such distortions were taken into consideration in Refs. 26,27 in the analysis of the phase diagram of doped manganites. However, none of these papers dealt with the phase separation phenomena.…”

Colossal magnetoresistance in topological Kondo insulator

“…The aims of the paper is to consider an academic problem of the Kondo insulator in the aspect of a realization of colossal magnetic resistance in new compounds. In contrast to the theory of the colossal magnetic resistance in manganites 20 , we consider the realization of the colossal magnetic resistance as the result of a nontrivial topology of the complex system.…”