# News from our Group

## Coherent states in bilayer graphene

June 24, 2009, updated October 28, 2009We have a new publication,
M. J. Gilbert and J. Shumway,
“*Probing quantum coherent states in bilayer graphene,*”
J. Comput. Electron.
**8**, 51–59 (2009),
appearing in a special issue of the Journal of Computational Electronics
(pdf download, 10 pp. 3.3 MB).
In this work, we use path integral quantum Monte Carlo to study
condensation of electron-hole pairs in bilayer graphene. Within the assumptions
of our model, we find a transition temperature well above room
temperature. The article describes some background on the
bilayer graphene systems and explains how path integrals
reveal excitonic pairing and excitonic superfluidity.

## CLAS research apprenticeship program

June 24, 2009The College of Liberal Arts and Sciences will be launching a Research Apprenticeship Program (RAP) this fall semester. The RAP is an opportunity for undergraduates, including freshmen, to become part of a research team and interact with experienced faculty.

Our group is participating with a project: “Simulating Semiconductor Nanostructures on the nanoHUB” In our research group, we develop new ways to combine physics, mathematics, and high performance computation to predict properties of new materials, devices, and exotic states of matter. The RAP student or students will learn how to use computer simulations in research, to study semiconductor nanostructures that are grown at ASU and other universities. Students will learn some basic physics of semiconductor electronics and materials. An important part of this project is to develop tutorials, demos, and online simulations to share these research techniques with other students and scientists around the world. This project will be tailored to the math and physics background of each apprentice student. The work utilizes the Purdue nanoHUB and Google Codes for broad, worldwide impact.

## Dot charging paper published

February 27, 2009In this work with experimentalist Dr. Sutharsan Ketharanath
and Prof. Jeff Drucker, we studied the problem of how many
electrons sit in an n-doped Ge/Si quantum dot. The paper,
“*Electron charging in
epitaxial Ge quantum dots on Si(001),*”
J. Appl Phys. **105**, 044312 (2009),
describes Sutha's experiments, where he measured
C-V curves while depleting charge from Ge huts and domes in his
gated quantum dot sample. Sourabh provided quantum dot
modeling that shows that domes have much wider and deeper confinement
potentials than smaller pyramidal dots. In principle, PIMC should be able
to quantitatively predict these curves, and Sourabh is continuing
this theoretical application in his current research.

## Three-body cold atom physics

February 26, 2009We have a new preprint,
“*FFLO vs Bose-Fermi mixture in polarized 1D Fermi
gas on a Feshbach resonance: a 3-body study,*”
(arXiv:0902.4653)
with Stefan Baur and
Erich Mueller at Cornell University.
In this work, we study the issue of BEC-BCS crossover with a simple
1D model of three fermions. In this simple model, the only interaction is a vertex that
turns to opposite-spin fermions into a boson. A quasi-1D characteristic of the model
is that the composite boson does not directly interact with a fermion, in contrast to the
hard-core interaction required by Pauli exclusion in a strict 1D model. This simple
problem shows a transition from a symmetric to antisymmetric ground state as
the chemical potential of the boson is varied. I contributed a 1D PIMC
algorithm that explicitly converts bosons to fermion pairs and enforces a weakened
fixed-node condition for this quasi-1D situation.

# Archived news, from 2008

## Spin-coupled quantum dots

September 9th, 2008We have new PIMC results on the role of correlation in exchange coupling of spins in quantum dots and optical lattices. (arXiv:0809.0038 and an interactive tool on nanoHUB).

## Sabbatical at Cornell University

July 1st, 2008John Shumway is on sabbatical at Cornell Universty through June, 2009, as a visiting scientist in the Laboratory for Atomic and Solid State Physics (LASSP). This is a great chance to work in a new area, trapped ultra-cold atomic gases, with Prof. Erich Mueller (a former classmate from UIUC). Theoretically, there is a large overlap between quantum states of atoms in laser traps and quantum states of electrons in nanoscale devices. For example, paired fermionic atoms are theoretically very similar to exciton condensates or coherent states in graphene bilayers. Research work continues in PIMC and nanoscale electronics, strengthened with new ideas from trapped atom physics and the theorists at LASSP.

## Path integral Monte Carlo on nanoHUB

January 15, 2008PIMC tool “pi” is live on nanoHUB with wiki documentation.