Charge distribution in an oxide heterostructure

Engineering new artificial materials represents the main avenue to the realization of novel electronic devices with desired functionalities. Since decades, heterostructures consisting of semiconducting materials have been proven to be key elements for the realization of compact and tunable laser sources, novel optoelectronic devices, high efficiency photovoltaic cells etc. The great potential of semiconducting heterostructures can be pushed even further by replacing the conventional semiconductors with strongly correlated electron systems. The interaction between conduction electrons makes the strongly correlated materials extremely susceptible to small variations of some control parameters, as temperature, magnetic fields, strain and photoexcitation. Thus the combination of the unique properties of strongly correlated materials with the design flexibility of heterostructures represents a very promising strategy to boost the performance of devices based on new concepts.

We address here heterostructure series consisting of two very unconventional building blocks. LaMnO₃ is the parent compound belonging to a class of materials known as colossal magnetoresistance manganites. Such systems display an electric resistance which can vary by a factor up to 10000 in the presence of a magnetic field, thus being very attractive for the design of read/write magnetic devices. The other constituent is LaNiO₃, a metal whose properties are strongly affected by electronic correlation so that small variation in its preparation (thickness, strain, etc.) can easily turn the material into a so-called Mott insulator. This means that the electronic repulsion energy opposes to the free motion of the electrons, thereby triggering an insulating behavior, contrary to the predictions of band structure calculations.
The (LaNiO₃)_n/( LaMnO₃)₂ superlattice series was built by stacking n LaNiO₃ sheets on top of 2 LaMnO₃ layers using molecular beam epitaxy at the Center for Nanoscale Materials, Argonne National Laboratory in the United States. The LaNiO₃/ LaMnO₃ superlattice displays a transition from metallic transport to insulating behavior as the thickness of the LaNiO₃ layer is decreased. The magnetization properties also vary with the LaNiO₃ layer number thus pointing out an intimate relationship between magnetism and charge transport. 

By employing infrared spectroscopy at the SISSI beamline of the Elettra synchrotron, we have evidenced the presence of significant excitations at mid-infrared frequencies in the LaNiO₃/LaMnO₃ superstructures (see Figure 1) not observed in LaNiO₃ or LaMnO₃ alone. We attribute these excitations to interfacial charge redistribution (see Figure 2). Our results are at variance with those obtained with LaNiO₃/LaAlO₃ superlattices and ultrathin LaNiO₃ films, where localization due to dimensional confinement and enhanced correlations is supposed to occur. By increasing the number of LaNiO₃ layers, it is possible to progressively enhance the coherent excitations, thereby tuning the degree of metallicity of the superlattice. The LaNiO₃/LaMnO₃ systems provide an exciting new platform to manipulate and control the interplay of electronic, magnetic and vibrational degrees of freedom in a disorder-free 2D oxide material. 

This research was conducted by the following research team:

1. INSTM UdR Trieste-ST and Elettra - Sincrotrone Trieste SCpA - Trieste, Italy
2. Argonne National Laboratory – Argonne, USA
3. CNR-IOM and Università “Sapienza” – Roma, Italy

Andrea Perucchi: