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’. 

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

Edge and interface states in transition metal oxide devices

In a metallic phase, the “stripes” in a doped Mott insulator are normally considered to be “fluctuating”, in the sense that they are well-defined, low energy excitations of the vacuum.  One can think of a fluctuating stripe state as an electron nematic which (by analogy with liquid crystals) has orientational but no translational order.  The purpose of this project is to determine whether, in a nematic state, stripe fluctuations might be pinned near the edge of a finite structure.  This phenomenon, if it occurs, might be thought of as the quantum mechanical analogue to layering of a liquid crystal near the walls of its container.

Array of transition metal oxide quantum wires, with a pitch of 150 nm, grown by e-beam lithography.  X-ray scattering studies of these wires, which scatter in a manner similar to a diffraction grating, reveal the spatial organization of electronic states in the wires.

Imaging dynamics in the attosecond regime

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” — 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. 

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, entropy being irrelevant at T=0.  The purpose of this project is to study whether the old picture of Shirane and Birgeneau — i.e. that a phase transition is associated with a “soft mode” whose temperature dependence above T_c is tied to the critical fluctuations — applies to a quantum phase transition.  We are currently studying the CDW order parameter in TiSe₂, which “melts” at zero temperature at a quantum critical point at P=3.5 GPa. 

Density disturbance induced in the prototypical insulator LiF by an idealized point source.  This image was reconstructed from inelastic x-ray scattering (IXS) data.  The sharp peaks are a real space depiction of Compton scattering. 

Diamond anvil cell used for scattering experiments at simultaneous high pressure (< 10 GPa) and low temperature (T>4.2K).

Department of Physics and

Frederick Seitz Materials Research Laboratory

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