This page will provide updates on research projects, recent publications, upcoming workshops, and other news.
Our work on the thermal conductivity of cubic boron nitride under pressure is now available online at Physical Review Letters. Congrats to Saikat Mukhopadhyay for all his work on this project!
Saikat Mukhopadhyay and Derek Stewart, "Polar Effects on the Thermal Conductivity of Cubic Boron Nitride under Pressure", Physical Review Letters, 113
, 025901 (2014)
While the presence of polar bonds leads to LO-TO splitting in the phonon dispersion of materials, the impact of polar bonds on thermal conductivity has been unclear. Since optical branches do not participate directly in thermal transport, the presence of a LO-TO gap is typically assumed to have little influence on thermal transport. However, optical branches do play a key role in phonon-phonon scattering. Therefore, the best candidate for observing an effect due to polar bonds on the thermal conductivity would be a system with (1) a small number of phonon branches (i.e. small unit cell), (2) optical branches in close proximity to acoustic branches to ensure acoustic-optical interactions, and (3) a small phase space for phonon-phonon scattering. Cubic boron nitride (c-BN) satisfies all of these requirements and provides the perfect candidate for our study. In this work, we have used pressure to manipulate the phonon dispersion and LO-TO splitting of cubic boron nitride. Using a first principle Boltzmann transport approach, we have calculated the thermal conductivity of c-BN for a wide range of pressures and temperatures. We find that the change of thermal conductivity with pressure in c-BN goes through an interesting non-linear regime at low to intermediate pressures. This non-linear regime is something that has not been observed in similar non-polar materials like diamond. Through a careful analysis of phonon-phonon scattering and the changes in phonon dispersion with pressure, we relate this non-linear regime to the polar nature of c-BN. This work could also have importance implications for thermoelectric polar materials where there is significant LO-TO splitting at the Gamma point.
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
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
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 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.
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
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
Physical Review B just published our recent work on thermal transport nanowires and alloys based on Mg2
Si and Mg2
Sn. 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 Mg2
Si and Mg2
Sn 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.
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  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.
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!