Charge correlations indicating the presence of stripe order in a doped Mott insulator, measured with resonant x-ray scattering.

Stripe order in doped Mott insulators

 

The Mott insulator is the fundamental parent phase of most materials we refer to as ‘correlated electron systems’.  If carriers are doped into a Mott insulator (e.g. by removing a spin) there is a competition between their tendency to delocalize – to minimize kinetic energy – and the desire of the system to retain valence bond order.  One of our projects is to study the degree to which this competition tends to drive phase segregation, perhaps into charged magnetic domain lines, colloquially referred to as `stripes’.  This project is a close collaboration with E. Fradkin.

Superlattice reflections (above) from a heterostructure of LaMnO₃ and SrMnO₃ (below).  The presence of a reflection at L=3 indicates that the interfaces are electronically reconstructed.

Edge and interface states in transition metal oxide devices

 

The transition metal oxides, most of which are correlated electron systems (usually doped Mott insulators) exhibit many exotic phases.  Even more intricate behavior may be realized, however, near an edge or at the interface between two such systems, where translational symmetry is explicitly broken.  This might provide a route to new devices.  The purpose of this project is to explore what new phases exist in heterostructures and patterned arrays of such systems.  This project is a close collaboration with the Eckstein group, which grows the structures we use for e-beam patterning and for scattering experiments.

Diamond anvil cell used for x-ray scattering experiments to study the behavior of soft modes through a quantum phase transition.

Quantum phase transitions

 

It is possible for a system to undergo a change of state, even at zero temperature, as a function of some external parameter like pressure or applied magnetic field.  Such a change cannot be described in terms of a classical balance between energy and entropy because entropy is irrelevant at T=0.  The purpose of this project is to study how such phase transitions occur, particularly in materials that involve broken translational symmetry such as a charge density wave.  We are particularly interested in the behavior of soft modes, and whether in specific cases their dynamics can be tied to the concept of entanglement entropy.  This project is done in close collaboration with the Cooper group.

Image of the plasmon excitation in pyrolitic graphite, measured with inelastic x-ray scattering.  This excitation, which screens charge in this system, is necessary for the existence of Dirac points in graphene.

Collective excitations in interacting systems

 

A noninteracting system exhibits only single-particle excitations (i.e. electron-hole pairs).  If interactions are present, collective excitations may arise that do not necessarily obey Fermi statistics.   The simplest example is the “plasmon”, which is a spin 0 boson excitation of the interacting electron gas, which is responsible for screening in most real materials.  The purpose of this project is to use inelastic x-ray scattering to study collective electronic excitations in various (weakly or strongly) interacting systems.  We are particularly interested in applying phase retrieval algorithms to image such excitations in real space and time.  This project is a collaboration with the Cahill and Zuo groups in MatSE.