Spectroscopic evidence of a dimensionality-induced metal-to-insulator transition in the Ruddlesden−Popper Lan+1NinO3n+1 Series
Ruddlesden−Popper (R−P) phases are perovskite structures consisting of two-dimensional slabs interleaved with cations. The thickness of the slab is normally used to control the valence state of the metal ion, thereby tuning the electronic properties of the system. We have shown that the electrodynamic properties of nickelate R−Pseries behave very much alike that of ultrathin LaNiO3(LNO) thin films. Our measurements suggest that the dimensional transition is the driving mechanism underlying the metal to insulator transition (MIT) in both families.
Perovskite-based heterostructures have recently gained remarkable interest, thanks to atomic-scale precision engineering. These systems are very susceptible to small variations of control parameters, such as dimensionality, strain, lattice polarizability, and doping. Particularly, the rare-earth nickelate ReNiO3 and its heterostructures have represented in the past decade a very flourishing field of research. Nickelates are prototype compounds for strong correlations in which the change of the rare earth (Re) modifies the Ni−O−Ni bond angle, thus allowing the tuning of the Mott metal-to-insulator (MIT) temperature. By further increasing the angle, as in LaNiO3, the MIT disappears, leaving the field to a paramagnetic metal ground state. Being on the verge of a Mott transition makes it possible to achieve control of the nickelate’s ground state through the application of a small perturbation, thereby allowing many appealing technological applications. In this respect, a remarkable finding has been the discovery of a dimensionality-induced MIT in LaNiO3, as observed by Angle Resolved Photoemission Spectroscopy when the thickness of LaNiO3 films decreases below 2 unit cells.
The question then naturally arises whether the same confinement effects take place in films where the dimensionality is induced by a peculiar crystalline structure as in the Lan+1NinO3n+1 Ruddlesden−Popper series. The R−P series can be indeed visualized as a slab of n LaNiO3 layers, which are separated by two LaO blocking layers, and can therefore be seen as an alternative way to achieve dimensionality tuning.
By means of infrared spectroscopy we measured the reflectivity of five thin LaNiO3films and three R−P samples at the SISSI Material Science beamline at Elettra, from which we extracted their optical conductivity. The effective number of electrons contributing to transport can then be calculated by the optical spectral weight, i.e.the area underneath the optical conductivity up to a frequency cutoff Ω. In Figure 1 we compare the DC conductivity, obtained by transport measurements, with the effective number of carriers. Both quantities show the same abrupt drop with respect to the number of layers for either thin films (red dots) and R-P samples (green dots).
This shows that the Ruddlesden−Popper films, despite the difference in the electronic configuration of Ni ions with respect to ultrathin LaNiO3films, exhibit a similar behaviour of both dc and optical conductivity when the number of rock-salt blocks changes. The XAS measurements performed at the APE-HE beamline confirms indeed that the electronic configuration of Ni ions does not appear to substantially change for all the metallic members of the R−P series, thus questioning the validity of a simple ionic model, and underlying an unconventional origin of the MIT. Therefore the number of rock-salt layers plays the same role of the unit cells in LaNiO3thin films, even though the former results from a crystal growth procedure, without the need of “artificially” implementing a single-layer sample.
Figure 1. DC conductivities (left panel) calculated from resistivity data, compared to the effective number of carriers (right panel) as extracted from the spectral weight of the optical conductivities for all of the samples. The two graphs show an abrupt drop at the same number of unit cells equal to 2.
This research was conducted by the following research team:
Paola Di Pietro 1, Maryam Golalikhani 2, Kanishka Wijesekara 2, Sandeep Kumar Chaluvadi 3, Pasquale Orgiani 4, Xiaoxing Xi 2, Stefano Lupi 5 and Andrea Perucchi 1
1 Elettra – Sincrotrone Trieste, Italy
2 Physics Department, Temple University, Philadelphia, Pennsylvania, United States
3 CNR-IOM TASC Laboratory, Trieste, Italy
4 CNR-IOM TASC Laboratory, Trieste, Italy; CNR-SPIN, UOS Salerno, Italy
5 CNR-IOM and Dipartimento di Fisica, Università di Roma Sapienza, Roma, Italy
Contact persons:
