Dr Pavlo Zubko
Reader in Physics
London Centre for Nanotechnology
Faculty of Maths & Physical Sciences
- Joined UCL
- 1st Oct 2013
My research is motivated by the fascinating physical phenomena found in nanoscale complex oxides, and focuses predominantly on the structural, electric and magnetic properties of ultrathin epitaxial films and artificially-layered heterostructures of perovskite oxides. Perovskites boast an incredible variety of physical properties that include ferroelectricity, ferromagnetism, superconductivity, 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. Some of the topics investigated with our colleagues from around the world include:
Nanoscale ferroelectrics: Ferroelectrics—materials with a switchable spontaneous polarisation—are a technologically important class of crystals exploited in many applications, ranging from electromechanical sensors and actuators to multilayer capacitors and ferroelectric random access memories. However, as the dimensions of these materials are reduced to just a few nanometres, their properties change dramatically, in part due to the formation of nanoscale domains. Oxide heterostructures, such as superlattices, composed of alternating ferroelectric and dielectric layers, are ideal for studying the static and dynamic properties of such nanodomains using techniques such as X-ray diffraction and dielectric impedance spectroscopy. Among the fascinating consequences of domain formation is the peculiar effect that domains have on the dielectric response of the superlattices, with the ferroelectric layers behaving effectively as negative permittivity components. This negative permittivity (or negative capacitance) behaviour in ferroelectrics may prove to be useful in reducing the power consumption of field effect transistors.
Oxide interface physics: Interfaces between chemically different materials provide the ideal platform for the discovery of new physical phenomena. The breaking of 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. 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.
Metal-insulator transitions: Rare-earth nickelates exhibit spectacular metal-insulator transitions, accompanied by subtle structural changes and charge ordering. In addition, their low-temperature ground state has 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 field-effect methods.
Flexoelectricity: Flexoelectricity is a 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. This is because strain gradients break inversion symmetry and 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.