CDWs    Epitaxial Graphene and Cu2O on Cu.

5d Electrons (Iridates)

Heavy atoms have enhanced spin orbit coupling and their 5d electrons have lower on-site Coulomb repulsion keeping them from tending towards localization. Both of these phenomena have led people to propose many novel ground states from high temperature superconductivity to topological insulators.We have begun investigating members of this family of compounds on the atomic scale to uncover their interesting phenomenology.

Structure of Sr2Ir)4

Structure of Sr2Ir2O4 showing the mirror plane of the Sr layer where we cleave the material

80K topography of Sr lattice on cleave

STM topographic image taken at 80K with the 4 Ålattice clearly visible. Since this material is a Mott insulator low temperature STM is difficult. But our ability to study materials at 80K there are enough thermally excited carriers to make detailed study possible.



Charge Density Waves.

One of the mechanisms materilas resolve competing effects is to change the periodicity of the material and open a gap. One simple and elegant mechanism for this to occus is the Peierls transition to a charge density wave (CDW). The mechanics and phenomenology of this state have been well studied for decades. Whether this process can lead to a CDW in quasi 2D materials like the 2H-transition metal dichalcogenides has been debated for decades. We have studied several materials in this family at various temperatures to compare their behavior to what is expected in the Peirels transition. We have found that lattice and orbital involvement is a large and important effect in these materials.

Ratio and topo

On the left the ratio of empty to filled states in TaSe2 (a commensurate CDW). On the right the concurent topography showing every Se atom and the 3x3 reconstruction. The nature of the CDW has never been seen in such detail.


Topographic image of NbSe2. Each Se atom is clear. Also the incommensurate nature of the CDW is evident in the variable nature of the 3x3 reconstruction. It is reminiscent of the Sierpinski Triangle


Local structure of superconductors

For the last couple of decades the underlying electronic structure leading to high temperature superconductivity in cuprates has remained elusive. Have investigated the interplay of the electronc states with local ordering tendencies by reconstructing the origins of real space responses. Unlike previous approaches this model required no prior knowledge of the electronic structure. In addition, to study the interplay of magnetic effects and superconductivity we have studied the local structure of Fe impurities placed in the CuO planes.

States that scatter

States that cause scattering at different dopings in Bi2Sr2CaCu2O8. This reconstruction shows the first Brillouin zone and the 4 fold symmetry is used for ease of presentation. In the upper right tight binding fermi surfaces are shown which show the expected progression of doping.

Fe State

The localized state in Bi2Sr2CaCu2O8 above an Iron atom imaged using spectroscopic imaging STM. The state that results from the magnetic impurity is quite localized but its effects on the superconductivity extend quite further. The image is ~12 Å



Growing and preparing surfaces

Under construction.

Cu on graphene

Graphene grown on copper. After exposure to oxygen at high temperatures red oxide grows on the uncovered surface. This is a much cheaper way to evaluate growth than a whole SEM

LEED of graphene on SiC

LEED pattern from graphene grown on SiC in our sample preparation chamber.