Experimental evidence for bipolaron condensation as a mechanism for the metal-insulator transition in rare-earth nickelates

Abstract

Many-body effects produce deviations from the predictions of conventional band theory in quantum materials, leading to strongly correlated phases with insulating or bad metallic behavior. One example is the rare-earth nickelates RNiO3, which undergo metal-to-insulator transitions (MITs) whose origin is debated. Here, we combine total neutron scattering and broadband dielectric spectroscopy experiments to study and compare carrier dynamics and local crystal structure in LaNiO3 and NdNiO3. We find that the local crystal structure of both materials is distorted in the metallic phase, with slow, thermally activated carrier dynamics at high temperature. We further observe a sharp change in conductivity across the MIT in NdNiO3, accompanied by slight differences in the carrier hopping time. These results suggest that changes in carrier concentration drive the MIT through a polaronic mechanism, where the (bi)polaron liquid freezes into the insulating phase across the MIT temperature.

High pO2 Floating Zone Crystal Growth of the Perovskite Nickelate PrNiO3

“…The transition temperature of the MIT depends on the size of the rare-earth ions, and correlates with the Ni-O-Ni bond angle [1][2][3]. The nature of the MIT has been discussed in the literature in terms of charge disproportionation [14,16,[31][32][33][34], negative charge transfer [5], and bipolaron condensation [4], but no consensus has been reached. In addition to the MIT, antiferromagnetic order at T N occurs concomitantly with MIT for R = Pr 3+ and Nd 3+ , and below T MIT for rare earth ions smaller than Nd.…”

Resonant inelastic x-ray scattering study of bond order and spin excitations in nickelate thin-film structures

“…Several independent findings point out the importance of BO fluctuations in the description of RNiO 3 , and in particular for R = La 21,31,67,68 . Recent experiments based on the pair distribution function (PDF) method have found evidence for two nonequivalent Ni sites in LNO even in the metallic phase 69,70 . Some studies even suggest that bond-length fluctuations are present in all RNiO 3 at high temperature, thereby classifying the metallic state in RNiO 3 as a polaronic liquid.…”

“…Some studies even suggest that bond-length fluctuations are present in all RNiO 3 at high temperature, thereby classifying the metallic state in RNiO 3 as a polaronic liquid. The MIT and the associated BO can then be explained in terms of stabilization/freezing of the pre-formed fluctuating rock-salt pattern of octahedra from the metallic state 69 . Additional evidence for charge/bond fluctuations can be found in the Fermi surface superstructure with wavevector Q BO = ( 1 /2, 1 /2, 1 /2) pc observed by angle resolved photo emission in metallic LNO 71 .…”

Probing Electronic and Magnetic Transitions of Short Periodic Nickelate Superlattices Using Synchrotron X-rays

“…Surprisingly, signatures of breathing distortion have been also found in the metallic phase of LaNiO3 [43]. On the other hand, there are also reports emphasizing the importance of structural transition [44,45] and polaron condensation [46]. Fermi surface nesting driven MIT has been also proposed to explain simultaneous electronic and magnetic transitions of NdNiO3 and PrNiO3 [47,48].…”