**Unconventional thermal transport **

David Broido

Department of Physics

Boston College

According to conventional theories of heat conduction in semiconductors and insulators, only crystals composed of strongly-bonded light elements can have high lattice thermal conductivity, *k*_{L}, and intrinsic thermal resistance comes only from lowest-order anharmonic three-phonon interactions. In this talk, I will discuss aspects of a new paradigm for achieving high *k*_{L} that we proposed, in which the vibrational properties are tailored to reduce the phase space for three-phonon scattering [1]. * *Our* ab initio* calculations predicted that one candidate material, cubic Boron Arsenide (BAs), indeed had ultrahigh three-phonon limited *k*_{L} comparable to that of the best heat conductor, diamond, and significantly higher than any other semiconductor [1]. In BAs, three-phonon scattering can become so weak that four-phonon scattering also plays an important role in limiting *k*_{L} [2, 3]. Such unconventional transport behavior has been confirmed in recent experiments [3-5]. It gives rise to anomalous non-monotonic pressure dependence of *k*_{L} [6]. I will review the challenging material constraints, which must be overcome in order to achieve the unconventional high *k*_{L}. An interesting case is that of group V transition metal carbides, NbC, TaC and VC. These metals have ideal vibrational properties for the desired weak phonon-phonon scattering. But, their nested Fermi surfaces give rise to strong scattering between phonons and electrons, which results in an orders-of-magnitude lower *k*_{L} that is nearly temperature independent, contrary to the typical behavior.

[1] L. Lindsay, D. A. Broido, and T. L. Reinecke, Phys. Rev. Lett. 111, 025901 (2013).

[2] T. Feng, L. Lindsay, and X. Ruan, Phys. Rev. B, 96, 161201 (2017).

[3] F. Tian et al., Science 361, 582 (2018).

[4] J. S. Kang M. Li, H. Wu, H Nguyen, and Y. Hu., Science 361, 575 (2018).

[5] S. Li, Q. Zheng, Y. Lv, X. Liu, X. Wang, P. Y. Huang, D. G. Cahill, and B. Lv, Science 361, 579 (2018).

[6] N. K. Ravichandran and D. Broido, Nature Communications (2019).

[7] C. Li, N. K. Ravichandran, L. Lindsay, and D. Broido,Phys. Rev. Lett. 121, 175901 (2018).