Michael Stopa

1016091753b.jpg Department of Physics, Harvard University, 316 Laboratory for Integrated Science and Engineering (LISE), 11 Oxford St. Cambridge, MA 02138 phone: 617-496-6932 stopa@cns.fas.harvard.edu

 

 

 

 

 

 

Basic data

I am a computational physicist and material scientist specializing in transport in semiconductor heterostructures and the self-consistent electronic structure of quantum dots and nanowires. Since my Ph.D. from the University of Maryland, with Sankar Das Sarma, in 1990, I have worked for many years in Japan (NTT, Riken and the University of Tokyo) and for two years in Germany (Technical University of Munich – Walter Schottky Institute) before coming to Harvard in the Fall of 2004 to head the computation project of the National Nanotechnology Infrastructure Network (NNIN), both at Harvard and nationally. The NNIN project has recently been renewed by the National Science Foundation for five years, until 2013. My CV can be found here.

Research

I have over seventy publications with approximately 900 citations. My full list of publications can be found here. My recent research has focused on three areas:

 

1.      Surface Enhanced Raman Spectroscopy (SERS)

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In collaboration with the Aspuru-Guzik group in the Chemistry department I am investigating the properties of molecules adjacent to metal surfaces under a grant from DARPA on Fundamental S&T of SERS. We are particularly interested in the influences of the metal on the molecular analyte electronic structure. These effects include the image charge effect to stabilize the redox states (i.e. positive and negative ion states) of molecules as well as the quantum mechanical effects associated with tunneling of electrons between the molecule and the metal, or the so-called chemical enhancement effect. We are, among other approaches, employing my SETE code for density functional theory based electronic structure of semiconductor heterostructures in combination with the Octopus code for quantum chemistry to simulate Quantum Mechanics in a Complex Environment (QM/CE). Recently (Olivares et al., in preparation) we have demonstrated that the positively charged anion state of the benzene molecule, which is unstable in vacuum, becomes bound in the presence of a pair of metal surfaces (i.e. in a parallel plate capacitor).

2.      Inhomogeneous Overhauser Effect in Double Quantum Dots

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In collaboration with the Yacoby and Halperin groups in the Physics Department I have recently been investigating the physics of spin transfer from the electrons in a lateral semiconductor quantum double dot and the host nuclei via the hyperfine or Overhauser interaction. Specifically, since the pioneering work of Ono and Tarucha [Ono02] on spin blockade and the buildup of the Overhauser field through transfer of spin to the host nuclei, numerous experiments [Ono04, Reilly08, Yacoby09] have systematically investigated methods of pumping the gates, and thereby re-arranging the charge in quantum dot structures in order to inject spin into the nuclei. Our calculations have explored the spatial distribution of electron-nuclear hyperfine coupling and, for the case of the double dot, have shown that pumping sequences are characterized by a force which can either tend to equilibrate the spin of the nuclei between the two dots or else cause the nuclear spins of one dot to polarize while leaving the other dot unchanged (thereby causing the spin difference to tend to diverge). Our work [Stopa10] has been accepted for publication in Physical Review B Rapid Communications (in publication) and has appeared on the condensed matter archive.

3.      Fast sensing of double-dot charge arrangement with an rf sensor quantum dot

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In collaboration with the Marcus group and a visiting student from the Niels Bohr Institute in Copenhagen whom I have supervised, Morten Kjaergaard, I have been investigating the comparative sensitivity of quantum dots and quantum point contacts to the charge re-arrangement in a quantum dot qubit. Specifically, it is found experimentally a sensor quantum dot, SQD, operated in the Coulomb blockade regime and positioned adjacent to a double-dot qubit is more than an order of magnitude more sensitive to charge motion in the qubit than the traditional quantum point contact, QPC, (i.e. narrow 1D channel) sensor. Mr. Kjaergaard employed the SETE code to model the experimental device, which included both an SQD and a QPC, and the transport through these structures as a function of the placement of the electron charges in the nearby double dot. The results, which have been written up and will be submitted to Physical Review B [Barthel10], indicate a close agreement between experiment and calculation and show that the increased sensitivity of the SQD over the QPC is a result of two factors. The first factor expresses the geometric characteristics of the QPC valve in comparison to the more responsive lever arm of the Coulomb blockaded SQD. The second factor is a result of the reduced screening of the SQD from the adjacent electron gas due to its operation in the single electron regime.

[Ono02] K. Ono, D. G. Austing, Y. Tokura and S. Tarucha, Science 297, 1313 (2002).

[Ono04] K. Ono and S. Tarucha, Phys. Rev. Lett. 92, 256803 (2004).

[Reilly08] D. J. Reilly, J. M. Taylor, J. R. Petta, C. M. Marcus, M. P.

Hanson, and A. C. Gossard, Science 321, 817, (2008).

[Stopa10] M. Stopa, J. J. Krich and A. Yacoby, Phys. Rev. B Rapid Comm., in preparation (selected as an Editor’s Choice article).

[Barthel10] C. Barthel, M. Kjærgaard, J. Medford, M. Stopa, C. M. Marcus, M. P. Hanson, and A. C. Gossard, in preparation.

 

Applied Physics 298r presentation:

NanobyNumbers.ppt

SETE

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Sandia presentation:

Sandia1207.ppt