# Presentation: 642106

## Arthur R. Smith, Saw-Wai Hla, Nancy Sandler, and Sergio Ulloa,  Ohio University, Athens, OH-45701. 

**Growth, Manipulation, and Characterization at the Atomic Scale ****DMR-0304314****.**
**Arthur R. Smith, Saw-Wai Hla, Nancy Sandler, and Sergio Ulloa****,  Ohio University, Athens, OH-45701.**

- A STM image of a self assembled molecular layer of TBPPBr-Co on GaN (000-1) (left), a molecule model (middle), and a STM image showing preferential positioning of molecules (right).

**Self-Assembled Molecular Cluster of TBPPBr-Co on Gallium Nitride**
- Molecular self-assembly is the key process for the bottom-up approach of nanoscience. We have successfully formed a self-assembled molecular layer (SAM) of TBPPBr-Co on GaN(000-1) surface at low temperatures and probed its local electronic properties using tunneling spectroscopy. Since the TBPPBr-Co molecule includes a spin-active Co atom, this SAM formation allows a uniform 2-D spin lattice on a wide band gap semiconductor surface for the first time. This achievement may open novel applications in spintronic device development.

**Notes:**
	NIRT: Building Nanospintronic and Nanomagnetic Structures: Growth, Manipulation, and Characterization at the Atomic Scale; Arthur R. Smith, Saw-Wai Hla, Nancy Sandler, and Sergio Ulloa, Ohio University, Athens, OH, DMR-0304314.  
Aim of the Project
	Molecular self-assembly is the key process for the bottom-up approach of nanoscience. Electronic circuits having specific functions can be built by using SAM layers.  We seek to explore the possibilities for integrating magnetic SAM layers into gallium nitride surfaces to achieve novel spintronics effects.  This requires the preparation of atomically-clean GaN surfaces with atomic flatness followed by the formation of magnetic SAM layers.
Research Results
	We have succeeded with this experiment by growing high quality GaN surfaces in the molecular beam epitaxy chamber of Prof. Art Smith (NIRT director) and subsequently transporting the GaN sample to the Low T STM chamber of Prof. Saw-Wai Hla (NIRT co-PI).  Following this we have successfully imaged the GaN(000-1) surface at 6K, obtaining a plethora of interesting and first-ever results (not shown here).  Furthermore, we have successfully formed self-assembled molecular clusters of a porphyrin based molecule, 5, 10, 15, 20-Tetrakis-(4-bromophenyl)-porphyrin-Co (II) (Co-TBPPBr) on the GaN surface. Four bromine atoms are used to anchor the molecule to the surface. The molecules are preferentially positioned either parallel to each other or with 90 degree rotated positions. STM images and tunneling spectroscopy data are taken at 6 K.  Here we show first images of the self-assembled molecular layers on the GaN surface.
Significance of this Work
	This preferential self-alignment of a spin-active molecule, like CoTBPP-Br, on a GaN surface is a novel development for nitride spintronics.  For the first time, we have positioned a regular array of magnetic spins on a gallium nitride surface using molecular self-assembly.  Since the GaN is conductive only through the surface layer, not through the bulk (it is wide band gap), this unique system allows us to explore novel spin and Kondo physics.  It is quite different from the similar case for metal surfaces, where the substrate conductivity plays a major role in the spin interaction.  This novel magnetic molecule/GaN system not only allows an important investigation of our understanding of the Kondo effect, but it also provides a unique experimental playground for spin physics in general.
Future Plans
	In the next 2 years of the NIRT project, we intend to investigate how the spin physics of these molecule systems is affected by the properties of the GaN substrate, including magnetic doping and surface reconstruction.  As well, we intend to explore other unique nitride substrates where the resultant properties can be very different.  Finally, our results will be compared with predictions from theory performed by NIRT theory co-PI’s Sandler and Ulloa.

## 

- Surface and bulk variations in the electronic density of states result in different Kondo signatures, reflected in the spectral densities ...

- ...which in turn result in *drastically* distinct differential conductance signatures and Fano line shapes measured by STM probes

**NIRT: Building Nanospintronic and Nanomagnetic Structures: **
**Growth, Manipulation, and Characterization at the Atomic Scale ****DMR-0304314****.**
**Arthur R. Smith, Saw-Wai Hla, Nancy Sandler, and Sergio Ulloa****,  Ohio University, Athens, OH-45701.**

**Notes:**
NIRT: Building Nanospintronic and Nanomagnetic Structures: Growth, Manipulation, and Characterization at the Atomic Scale; Arthur R. Smith, Saw-Wai Hla, Nancy Sandler, and Sergio Ulloa, Ohio University, Athens, OH, DMR-0304314. 
1.      Aim of the projectIn the theoretical component of this NIRT, we have undertaken the development of a computer code to calculate the spectral density of the Kondo correlated system for the Kondo and Anderson Hamiltonians, using the Numerical Renormalization Group approach.  NRG is the only technique reliable in all the energy and parameter range to describe the Kondo physics of these Hamiltonians.  Our code is designed to compute not only the spectral function of the system, but the associated Green’s function that results in the STM conductance curves one is to expect from experiments.  In one word, our approach allows us to uniquely compare experimental features with theoretical parameters, and gives us insights into this fascinating physical system.

2.      Research results The NRG code results shown above as an example, indicate how the different features of the density of states (DOS) in the metal (associated with surface vs. bulk band features, possible surface confinement – e.g. corrals – or other defects) produce substantially different conductance curves in STM I-V experiments.  The four insets indicate different DOS models, while the left panel shows the resulting spectral density for such system in the Kondo regime (notice color coding). [The thin solid line is the symmetric regime for a square DOS: E_d = -U/2, U=0.5, and U/(Gamma*Pi)=5.066.]  As an additional DOS enhancement increases at the Fermi level of the surface (associated with the surface band in a metal), we observe a substantial change (broadening) in the spectral density, accompanied by a significantly broader Fano-like conductance curve.  Experiments would identify this with an increasing Kondo temperature of the system, while the spectral density is surprisingly without features at the Fermi level.  The Kondo temperature is *not* fully in agreement under close examination of the flow diagram (not shown here).

3.      Significance of this work 
This example and other related work demonstrates that the anticipated features at the Fermi level in the spectral density are not a necessary element for the system being in the Kondo regime, and that the connection with experimental conductance curves is not as straightforward as previously believed.  In other words, the community studying Kondo impurities on surfaces has a nearly unquestioning belief on the validity of Fano line shape fits to obtain the Kondo temperature, while we demonstrate that this is not always the case.  Our upcoming publication in this topic is expected to cause a significant reassessment of the way conductance curves are analyzed, and Kondo temperatures obtained from data.  Moreover, our calculations will motivate new experimental designs, where careful temperature dependence and other analyses are performed before it is established the system is truly in the Kondo regime.

4.      Future plans
Using our newly developed code, we can also calculate other features of the Kondo correlated system and will work on that in the next few months.  We will analyze the role of symmetry in multiple impurities, by starting with a set of two Kondo impurities.  We will also study the role of bonding environments on the conductance features measured by STM, and that would allow us to compare with recent experiments in the labs of our co-PI’s, Prof. Art Smith and Prof. Saw-Wai Hla.

## Arthur R. Smith, Saw-Wai Hla, Nancy Sandler, and Sergio Ulloa,  Ohio University, Athens, OH-45701. 

**K-12 School Visits****  **Sergio Ulloa, NIRT theorist, speaks to a group of SE Ohio high-school students about nanoscience, in one of several K-12 visits of the NIRT group this last year.  Of this visit, the teacher said “I have nothing but positive feedback from my students.”

**K-12 Nanoscience Essay Contest**  Seventh-grader – Mr. Isaac J. Smith, of Athens (blue shirt) won the nsnm nanoscience essay contest.  Here he is shown on a field trip to the nanoscience labs at Ohio University with his teacher (far right), his Dad (far left), and Dr. Sandler.

**Science Writers in Training**  Undergraduates *Suzanne Wilder* (left) and *Lauren Van Arsdale* (right) pose in front of their poster describing the nanoscience writing program, which they presented at the first Ohio Nanotechnology Summit.  The 2 science writers wrote articles about the Summit, which was held March 2-3, 2005 in Dayton, Ohio.

**NIRT: Building Nanospintronic and Nanomagnetic Structures: **
**Growth, Manipulation, and Characterization at the Atomic Scale ****DMR-0304314****.**
**Arthur R. Smith, Saw-Wai Hla, Nancy Sandler, and Sergio Ulloa****,  Ohio University, Athens, OH-45701.**