Welcome to the Stewart Research Group (dft4nano).

Note: This website captures the work of my Cornell research group (dft4nano) from 2004-2014 and several workshops I have hosted throughout the years.  The publication list is current for those interested in my recent work on different forms of non-volatile memory (MRAM, RRAM, PCM, etc) at Western Digital.

My research focuses on using density functional theory and transport approaches to understand thermal, ionic, electronic, and spin transport in materials and nanostructures.  Using these approaches, we have developed a first principles foundation for thermal transport, designed new nanostructured thermoelectrics for energy harvesting, and explored new materials for solid state batteries and optoelectronics.  

From 2004-2014, I directed the nanoscale simulation effort at the Cornell Nanoscale Facility and also serve as an adjunct professor in the Material Science and Engineering Department at Cornell.  I now work in the research division at Western Digital investigating new forms of non-volatile memory (MRAM, RRAM, etc).  Given that real-world applications are often messy, I am also very interested in simulation approaches that can handle complex materials and disorder.  If you are interested in learning more about our work, please check out the updated publication list.   Western Digital has programs to support academic collaborations and summer internships for graduate students.  If you are interested in collaborating, please let me know.             

Derek A. Stewart

dft4nano News and Events

Work on Wurtzite Indium Phosphide (InP) published in Phys. Rev. B!

posted Feb 22, 2014, 8:39 AM by Derek Stewart   [ updated Jun 6, 2014, 10:36 AM ]

III-V nanowires show great potential for applications ranging from field effect transistors to photo-detectors.  One of the interesting things about these systems is that for very small nanowires, the wurtzite crystal phase is preferred over the zinc-blende phase found in bulk crystals.  This change in crystal structure could have important implications in terms of band structure, dielectric constant, and phonon properties.  However, since the wurtzite phase can not be grown in bulk, it has been difficult to predict the lattice dynamics in this phase.  In our work recently published in Physical Review B (PRB, 89, 054302 (2014)), we provide the first ab-initio calculation for the phonon dispersion and Raman spectra for wurtzite InP.  We show that including the 4d In electrons as valence electrons is crucial to predict the correct frequencies of the optical branches.  We also find that the specific heat and group velocities of wurtzite and zincblende InP are very similar.  This means that any difference in the thermal conductivities in these two polytypes should be due to differences in phonon-phonon scattering (a topic of future work!).

Also, congrats to Saikat for his first (of many) publication with the group!

Keywords:  Lattice dynamics, Raman, Indium Phospide, Phonon Dispersion, Quantum Espresso

Phonon Spectrometer worked highlighted on Physics Today website

posted Jan 4, 2014, 11:07 AM by Derek Stewart

Our recent Nano Letters article probing phonon surface scattering in nanostructures using a microscale phonon spectrometer was recently featured on the Physics Today website!  You can find the article here.

Phonon Spectrometer work published in Nano Letters

posted Nov 24, 2013, 2:27 PM by Derek Stewart

Our joint work with Richard Robinson's group (MSE/Cornell) on the role of surface scattering on phonon transport in nanostructures has just been accepted to Nano Letters!  You can access the article online here.

Accurately characterizing the role of surface roughness of phonon transport and heat transfer is a difficult problem.  In typically thermal transport experiments, phonons from a wide range of frequencies contribute to heat transfer and it is nearly impossible to resolve surface interactions for a particular phonon frequency.  Richard Robinson's group has been working to address this issue by developing a microscale phonon spectrometer that can send phonons with specific frequencies through nanostructure devices.  By running experiments at low temperatures, it is possible to resolve to role of surface scattering on phonon transport.  In this joint publication, we used a combination of phonon spectrometer experiments and Monte Carlo simulations of phonon transmission to see how well common surface scattering models such as Ziman's specular parameter really do.  We find that the specularity parameter estimated based on measured surface roughness underestimates the thermal resistance due to surface scattering, a finding with implications for thermal management in nanoscale electronics and the design of nanostructured thermoelectrics

Saikat to present cubic-BN work at Fall MRS meeting

posted Nov 7, 2013, 12:27 PM by Derek Stewart

Saikat Mukhopadhyay (post-doc in the dft4nano group) will be giving a talk at the Fall MRS meeting on our recent work looking at the thermal conductivity in cubic boron nitride under pressure.  Although c-BN is similar to diamond in terms of crystal structure, lattice constant, and hardness, the presence of polar bonds leads to some important changes in the phonon dispersion that could affect thermal conductivity.

Ab-Initio Thermal Transport Chapter Published Online!

posted Nov 4, 2013, 2:51 PM by Derek Stewart

Our book chapter, "Ab-Initio Thermal Transport", in the book Length Scale Dependent Phonon Interactions is now available for download on the Springer webpage.  You can access it here.

Ab-Initio Thermal Transport Book Chapter in production

posted Oct 21, 2013, 5:32 AM by Derek Stewart

Our invited book chapter, "Ab-Initio Thermal Transport", is now in production for the upcoming Springer-Verlag Book, Length-Scale Dependent Phonon Interactions, edited by S. L. Shinde and G. P. Srivastava.  The book is slated to come out in November 2013.  This chapter provides an overview of the first principle techniques we have developed to investigate thermal transport in materials and nanostructures. The longer format also allows us room to delve into the technical details a bit more.

"Ab Initio Thermal Transport", N. Mingo, D. A. Stewart, D. A. Stewart, D. A. Broido, L. Lindsay, and W. Li, Length-Scale Dependent Phonon Interactions, eds. S. L. Shinde and G. P. Srivastava, Springer-Verlag, 2013

New article on thermal transport in Mg2Si, Mg2Sn, Mg2Si(x)Sn(1-x) alloys and nanowires

posted Nov 29, 2012, 10:34 AM by Derek Stewart

Physical Review B just published our recent work on thermal transport nanowires and alloys based on Mg2Si and Mg2Sn.  You can access the article here, "Thermal conductivity of bulk and nanowire Mg2SixSn1-x  alloys from first principles."  Thermoelectrics based on these alloys show great promise due to the fact that their base materials are non-toxic and relatively cheap.  Current thermoelectrics like PbTe rely on toxic and expensive materials.  However, in order for Mg2Si and Mg2Sn thermoelectrics to be competitive, their thermoelectric figure of merit still needs to be improved.  One route to do this is to reduce the thermal conductivity of the material through nanostructuring.  In this article, we use first principles calculations to examine how alloying and nanostructuring affect the thermal conductivity of these materials.  The initial results are encouraging and indicate that there is room for improvement in the figure of merit. 

Technical Note: Up to this point, much of our work predicting thermal conductivity in materials based on first principles, has been done using plane wave codes (i.e. Quantum Espresso).  In this study, we wanted to see if we could also get reliable results from localized orbital approaches like Siesta.  Overall, we found that both approaches gave good agreement for the phonon dispersions of these materials as well as thermal transport.  However, some care is needed with Siesta calculations to make sure you have a reasonable basis set and a good mesh cutoff. 

This work was done in collaboration with Natalio Mingo's group at CEA Grenoble, David Broido at Boston College, and Lucas Lindsay at the Naval Research Laboratory.  The research was funded by the NSF.

New Article published on the thermal conductivity of diamond nanowires in Phys Rev B

posted Jun 1, 2012, 7:44 AM by Derek Stewart

Our work on the thermal conductivity of diamond nanowires was recently published in Physical Review B.  You can access the article here, "Thermal conductivity of diamond nanowires from first principles".  This work was done in collaboration with Natalio Mingo's group at CEA-Grenoble, David Broido (Boston College) and Lucas Lindsay (Naval Research Laboratory).  One interesting aspect of this work is that the thermal conductivity in these nanowires depends strongly on the crystallographic orientation of the nanowire, even at room temperature.  In particular, the [001] growth direction always has the highest thermal conductivity.  Since experimental groups have recently grown diamond nanowires, it will be interesting to see if this prediction is correct.

PASI Energy Workshop highlighted at TACC

posted Jun 1, 2012, 7:34 AM by Derek Stewart

The Texas Advanced Computing Center has a news article discussing our recent PASI workshop in Santiago, Chile: Training Tomorrow's Energy Scientists.  You can also read it in Spanish here.  Participants in the workshop using the Ranger supercomputing for tutorial sessions.  Computing time on Ranger was made possible through a XSede educational grant.  TACC, thanks again for all your help!

Graphene Oxide Book Chapter published by Springer

posted Mar 19, 2012, 11:07 AM by Derek Stewart   [ updated Mar 19, 2012, 11:10 AM ]

About a year ago, Andre Mkhoyan (Univ. of Minnesota) and I put together a review chapter of graphene oxide research for the book, Graphene Nanoelectronics.  This review discuss the history of graphene oxide and touches on a wide range of theoretical and experimental work on this interesting cousin of graphene.  Springer just published the full book online yesterday!  If your institution has a contract with Springer, you should be able to view the book here. You can also download the pdf of our book chapter, "Graphene Oxide: Synthesis, Characterization, Electronic Structure, and Applications".  Hard copies of the book will be available in April 2012.

New article published on CrAl work in Phys Rev B

posted Mar 9, 2012, 9:14 AM by Derek Stewart

Our new paper on the intriguing X-phase of Cr3Al, "Antiferromagnetism in Cr3Al and relation to semiconducting behavior" has just been published in Physical Review B!  This work resulted from a strong collaboration between experiment and theory that involved researchers at Cornell, UC Berkeley, and ANSTO in Australia.  The theoretical work was done using the Akai-KKR code.

Post-doc position available in the dft4nano group

posted Feb 26, 2012, 8:59 PM by Derek Stewart

I currently have an opening for a post-doc in the area of first principles thermal transport.  You can find out more information on the position here.

New Article published in Computing in Science and Engineering

posted Feb 22, 2012, 11:23 AM by Derek Stewart

The paper, "A Cautionary Tale of Two Basis Sets and Graphene", was just published in the March/April 2012 issue of Computing in Science and Engineering.  This article resulted from some initial work I was doing for a graphene oxide project a few years back.  I noticed that the density of states calculated using a plane wave code (Quantum Espresso) and a localized orbital approach (Siesta) differed for some of the conduction bands.  The paper explains the reason for this difference and how it could affect interpretation of characterization techniques and electronic transport predictions.  A preprint of the article is also available on my publications page.

D. A. Stewart, "A Cautionary Tale of Two Basis Sets and Graphene", Computing in Science and Engineering, 14, 55 (2012).

(January 25, 2012) PASI Workshop featured in PUC: Thank you to all the organizers and participants that helped make the PASI workshop a success.  The Pontifica Universidad Catolica de Chile has just posted an article (in spanish) on the workshop here.

(September 9, 2011) New Paper! Our paper on thermal transport in SiGe alloys with embedded nanoparticles has just been published in Phys Rev B. here.

(September 6, 2011) 2012 PASI Workshop Receives NSF funding!:  I just received word that our NSF proposal for the Pan-American Advanced Studies Institute: Computational Materials Science for Energy Generation and Conversion has been funded.  This two-week workshop in Santiago, Chile is designed give young researchers (graduate students and post-docs) from across the Americas an introduction to essential simulation techniques for energy research.  Support will be provided for participants.  If you are interested in attending, please check out the workshop website, PASI 2012, for more details. 

(May 19, 2011) InAs Nanowire Thermal Conductivity: The paper "Thermal conductivity of indium arsenide nanowires with wurtzite and zincblende phases" has been published in Physical Review B.  This joint experiment/theory paper is the result of a collaboration with Li Shi's group at the University of Texas, Natalio Mingo at CEA-Grenoble, France, and Lars Samuelson's group at Lund University, Sweden.  The paper can be accessed here. 

(May 10, 2011) CiSE Preprint Available: The paper "A Cautionary Tale of Two Basis Sets and Graphene" has been accepted to Computing in Science and Engineering and is now available in preprint form on the CS Digital Library here.

(March 29, 2011) New NSF Grant!:  Derek Stewart will serve as PI for a new NSF grant CBET-01066406 "Ab-Initio Computation of Thermal Transport in Crystalline and Disordered Materials".  This grant will help fund research into first principles modeling of thermal conductivity in low thermal conductivity materials relevant for thermoelectric applications and thermal barrier coatings.  This project is in collaboration with joint research efforts at Boston College and CEA Grenoble.

(December 3, 2010) Our new paper examining the electronic structure of semiconducting CrAl alloy thin films was just published in Physical Review Letters.  This work combines density functional calculations with x-ray photoemission spectroscopy to provide insight into this unusual alloy.

Atomic structures at the Winter Olympics?! Last year, NBC came to Cornell to film a series of short videos on the Science behind the Vancouver Winter Olympics for NSF.  Melissa Hines (Director of the CCMR here at Cornell) does a great job explaining the role materials science plays in international sports in several of the videos.  The camera crew also came by my office and you can find a few of my screen shots of teflon and carbon nanotubes in this video on competition suits.

(December 17, 2009) My new paper examining a new type of spin filtering based on an reduced symmetry oxide (Mg3B2O6) has been published in Nano Letters ASAP.

(Sept. 16, 2009) Our new paper on predicting the thermal conductivity of diamond from first principles was just published in Physical Review B here.  The article was also selected as an Editor's Suggestion for the issue and it was also featured in a Cornell Chronicle article!

(June 16, 2009) Alistair Ward (now Dr. Ward!) successfully defended his dissertation on "First principles theory of the lattice thermal conductivity of semiconductors"at Boston College

(February 6, 2009) Our new paper on graphene oxide is now available online at Nano Letters!   This article combines electron microscopy, EELS data, and density functional calculations to examine oxygen bonding on graphene. (Article at Nano Letters ASAP)

(December 23, 2008) Derek Stewart was recently profiled in the Cornell Chronicle on Dec. 23, 2008 (Full Article)

In a paper published online in December 2008 in Nano Letters, we investigate the impact of isotope scattering on thermal transport in boron nitride nanotubes (Article at Nano Letters ASAP)

In a paper recently published in Physical Review Letters, we examine the role of phonon localization on thermal transport in nanotubes. (PRL 101 165502 (2008))