My research is motivated by the fascinating physical phenomena and functional properties of complex oxides at the nanoscale and has focused predominantly on the structural, electric and magnetic properties of ultrathin epitaxial films and artificially-layered heterostructures made up of perovskite oxides. Perovskite oxides boast an incredible variety of physical properties that include ferroelectricity, ferromagnetism, superconductivity, colossal magnetoresistance, metal-insulator transitions and many others. By exploiting epitaxial strain, dimensional confinement, electrostatic interactions and various interface phenomena these exceptional properties can be tuned, tailored or even replaced by completely new phenomena in epitaxial oxide heterostructures. Below are some more details about these and other research topic investigated in close collaboration with colleagues at the University of Geneva and University of Cambridge.

UCL undergraduates wishing to do a summer internship are welcome to get in touch.

Nanoscale ferroelectrics

Ferroelectrics—materials with a switchable spontaneous polarisation—are a technologically important class of crystals that are already exploited in many applications ranging from submarine sonars and medical ultrasound transducers to multilayer capacitors and ferroelectric random access memories. As the thickness of these materials is reduced to just a few nanometres, however, their properties change dramatically. One interesting an important aspect of nanoscale ferroelectrics is domain formation. The inability of ultrathin ferroelectrics to efficiently screen their spontaneous polarisation leads to the formation of dense regular nanodomains of opposite polarisation, which in turn dominate their functional properties. Oxide heterostructures, such as the superlattices below, composed of ferroelectric PbTiO3 and dielectric SrTiO3 layers, are ideal for studying the static and dynamic properties of such nanodomains using, for example, X-ray diffraction combined with in-situ dielectric impedance spectroscopy [1,2].

Transmission electron microscope image of a PbTiO3-SrTiO3 superlattice and its X-ray diffraction pattern. The weak in-plane satellites accompanying the intense superlattice peaks reveal the presence of ordered ferroelectric domains with a periodicity of a few nanometres.

Oxide interface physics

Interfaces between chemically different materials provide the ideal platform for the discovery and engineering of new physical phenomena. The breaking of translational symmetry at interfaces, as well as the possibility of charge transfer, novel couplings between structural degrees of freedom, and various types of electrostatic and magnetic interactions, can lead to the emergence of properties that are very different from those of the original constituents [3]. For example, in superlattices composed of alternating ferromagnetic LaMnO3 and paramagnetic LaNiO3 layers, interfacial charge transfer between Mn and Ni modifies the magnetic interaction between these cations and induces a new magnetic structure within the nominally paramagnetic LaNiO3 layers [4].

Some of the different mechanisms for generating new phenomena at oxide interfaces.

Metal-insulator transitions

Metal-insulator transitions (MITs) have fascinated physicists for over 70 years. One class of materials that exhibit spectacular metal-insulator transitions are the perovskite rare-earth nickelates. These MITs are accompanied by subtle structural changes and as-yet poorly understood charge ordering. In addition, their low-temperature ground state is magnetic with an unusual antiferromagnetic spin structure. The strong coupling between the structural and electronic degrees of freedom, typical of many perovskite oxides, offers many ways of tuning the electronic properties of nickelates, e.g. by exploiting epitaxial strain or electrostatic doping of ultrathin films using the field-effect technique [5].

Flexoelectricity

Flexoelectricity refers to the phenomenon whereby electrical polarisation is induced by inhomogeneous strain (or strain gradients). Unlike the piezoelectric effect, which describes polarisation induced by homogeneous strain in non-centrosymmetric materials, the flexoelectric effect is universal and has no symmetry restrictions. As shown below, this is because strain gradients break inversion symmetry and therefore can lead to a polarisation in any crystalline or even amorphous material.  In bulk, strain gradients are typically very small and flexoelectricity is not very important. At the nanoscale, however, strain gradients can be enormous and flexoelectricity can have dramatic effects on the properties of nanomaterials. For a short review on flexoelectricity in solids, its ubiquitous presence in many areas of nanoscience, and its potential applications see Ref. 6.

Cartoon illustration of the microscopic mechanism of flexoelectricity. Homogeneous strain (left) does not break inversion symmetry and therefore cannot lead to a polarisation in a material with a centrosymmetric unit cell. Inhomogeneous strain (e.g. due to bending), on the other hand, does break inversion symmetry and allows r a finite polarisation to develop, in this case caused by the displacement of the central ion.

References

[1] P. Zubko et al., X-ray Diffraction Studies of 180º Ferroelectric Domains in PbTiO3/SrTiO3 Superlattices under an Applied Electric Field, Physical Review Letters 104, 187601 (2010)

[2] P. Zubko et al., Electrostatic Coupling and Local Structural Distortions at Interfaces in Ferroelectric/Paraelectric Superlattices, Nano Letters 12, 2846 (2012)

[3] P. Zubko et al., Interface Physics in Complex Oxide Heterostructures, Annual Review of Condensed Matter Physics 2, 141 (2011)

[4] M. Gibert et al., Exchange bias in LaNiO3/LaMnO3 superlattices, Nature Materials 11, 195 (2012)

[5] R. Scherwitzl et al., Electric-field control of the metal-insulator transition in ultrathin NdNiO3 films, Advanced Materials 22, 5517 (2010)

[6] P. Zubko et al., Flexoelectric effect in solids, Annual Review of Materials Research 43, 387 (2013)