Correlated Electron Systems
The physics of materials with strong electronic correlations is remarkably rich and complex and cannot be understood within the conventional theories of metals and insulators. In correlated materials, charge, spin, orbital and lattice degrees of freedom result in competing interactions. These lead to phase transitions and the emergence of exotic phases including the pseudogap state in cuprates and manganites, high-temperature superconductivity, charge stripes in cuprates, even phase separation in some manganites and cuprates.
Multi-messenger nanoprobes of hidden magnetism in a strained manganite
McLeod et al. Nature Materials (2019). Article
The unique physical properties of correlated electron materials often arise from interactions among disparate degrees of freedom, such as charge, spin, and lattice distortions. Owing to these interactions, conductivity in thin films of the “colossal magnetoresistive” rare earth manganite La2/3Ca1/3MnO3 can be disabled by epitaxial strain, but curiously restored with ultrafast optical pulses. By combining cryogenic near-field infrared and magnetic force microscopies, we leverage multi-messenger nano-imaging to simultaneously interrogate several interacting orders, here metallic conductivity and magnetism, in individual domains of the photo-induced hidden phase. We reveal a light-induced ferromagnetic metal distinct from its strain-free counterpart, and “erasable” through delivery of nano-scale strain from the nano-imaging probe.
Coexisting first-and second-order electronic phase transitions in a correlated oxide NdNiO3
Post et al. Nature Physics (2018). Article
By harnessing a nanoscale optical probe of the local electronic conductivity, we reveal two physically distinct yet concurrent low-temperature phase transitions in the rare earth nickelate NdNiO3. Whereas the sample bulk exhibits a first-order transition between metal and insulator phases, we resolve anomalous nanoscale domain walls in the insulating state that undergo a distinctly continuous insulator–metal transition, with hallmarks of second-order behaviour. We ascribe these domain walls to boundaries between antiferromagnetically ordered domains within the charge ordered bulk. The close correspondence of these observations to predictions from a Landau theory of coupled charge and magnetic orders highlights the importance of coupled order parameters in driving the complex phase transition in correlated electron materials such as the rare earth nickelates.
Discovery of the electronic phase separation in a prototypical Mott insulator V2O3
McLeod et al. Nature Physics (2016). Article
Cooperative photoinduced metastable phase control in strained manganite films
(with R.A. Averitt) Zhang et al. Nature Materials 15, 956 (2016). Article .
Stripes and anisotropic electronic transport in VO2
Liu et al. Physical Review Letters 111, 096603 (2013) . Article .
The insulator–metal transition remains among the most studied phenomena in correlated electron physics. However, the spontaneous formation of spatial patterns amidst insulator–metal phase coexistence remains poorly explored at the meso and nanoscales. Here we present real-space evolution of the insulator–metal transition in a V2O3 thin film imaged at high spatial resolution by cryogenic near-field infrared microscopy. We resolve spontaneously nanotextured coexistence of metal and correlated Mott insulator phases near the insulator-to-metal transitio (160–180 K) associated with percolation and an underlying structural phase transition. A quantitative analysis of nano-infrared images acquired across the transition suggests decoupling of electronic
and structural transformations.
Mott physics near the insulator to metal transition in NdNiO3
Stewart et al. Physical Review Letters 107, 176401 (2011). Article .
An optical study of NdNiO3 ultrathin films with insulating and metallic ground states reveals new aspects of the insulator-to-metal transition that point to Mott physics as the driving force. In contrast with the behavior of charge-ordered systems, we find that the emergence of the Drude resonance across the transition is linked to a spectral weight transfer over an energy range of the order of the Coulomb repulsion U, as the energy gap is filled with states instead of closing continuously. The simple band picture based solely on charge-ordering fails to account for the salient spectral properties of NdNiO3 and Mott physics must therefore be taken into account to fully describe this correlated system.