Metal-to-Insulator transitions in RNiO3 as seen by
Eike F. Schwier (Hiroshima Synchrotron Radiation Center, Hiroshima University, Japan,)

The family of nickel based rare earth perovskite oxides RNiO3, can be viewed as a model system to study temperature driven metal-to-insulator transitions (MIT). Apart from the presence of the MIT the nickelate phase diagram is host of additional magnetic as well as structural phases. At higher temperatures the nickelates are found to be in a nominally paramagnetic state, while an antiferromagnetic phase with a nontrivial spin arrangement emerges at low temperatures. By changing the rare earth atom the lattice distortion is increased in a similar way as by applying external pressure. The driving force behind the MIT as well as the nature of the antiferromagnetic ground state are still under debate and have been investigated for several decades [1].
Interestingly, the MIT temperature can be continuously tuned, by means of solid solutions between different rare earths, from more than 600 K down to 0 K [2]. The decrease of the MIT temperature leads to the phenomenon that for heavier rare earths, like SmNiO3, second-order-like magnetic and metal-toinsulator transitions are present at distinct temperatures, while both coincide as a combined first order transition for lighter rare earths like NdNiO3 and are finally completely suppressed in the case of LaNiO3. The nature of both phase transitions is still under debate and different mechanisms have been proposed to be responsible for their phase diagram.
Advances in thin films growth [3] reignited the interest in these materials through possible applications as temperature dependent switches in nano-electronics as well as the possibility to investigate single crystalline samples of sufficiently large size by photoemission and other techniques. Due to the absence of sufficiently large single crystals, epitaxial thin films are the closest means available to study the intrinsic properties of nickelates. In addition, thin films can be grown sufficiently thin in order to study the effect of epitaxial strain on their properties [4] as well as its role in the interplay between the electronic, magnetic and geometric degrees of freedom in general.
In this seminar I will give an introduction to the nickelate family of oxides, possible mechanisms for MITs, as well as the necessary basics of photoemission spectroscopy. I will then present results from temperature dependent measurements on RNiO3 (R=Sm,Nd,La) thin films across the MI and magnetic transitions. These results will be discussed in the context of different models explaining Metal-to-Insulator transition and a case for a possible coupling between the electronic and magnetic structure in these systems will be made.

[1] G. Catalan, R. M. Bowman, and J. M. Gregg, Physical Review B 62, 7892 (2000).
[2] J. B. TORRANCE, P. Lacorre, A. I. Nazzal, E. J. Ansaldo, and C. Niedermayer, Physical Review B 45, 8209 (1992).
[3] A. Tiwari, C. Jin, and J. Narayan, Applied Physics Letters 80, 4039 (2002).
[4] R. Scherwitzl, P. Zubko, I. G. Lezama, S. Ono, A. F. Morpurgo, G. Catalan, and J.-M. Triscone, Advanced Materials 22, 5517 (2010).