【mTT2024】邀请报告人——Navaneetha Krishnan Ravichandrran

学术 2024-11-19 15:40 北京

mTT2024

会议时间：2024年11月23日-24日

会议地点：北京市·中国科技会堂

会议网站：http://mtt2024.csmnt.org.cn

Navaneetha Krishnan Ravichandrran

Dr. Navaneetha Krishnan is an assistant professor and an Infosys Young Investigator in the Mechanical Engineering department at the Indian Institute of Science (IISc), Bangalore, India. His research group develops new experimental and first-principles computational tools to probe the microscopic, quantum mechanical interactions that the energy carriers undergo in materials, that drive their macroscopic energy transport properties like electrical and thermal conductivities, and electronic mobility. He obtained his Dual Degree (B. Tech and M. Tech) in Mechanical Engineering from the Indian Institute of Technology (IIT) Madras, India. He obtained his Masters in Space Engineering and Ph.D. in Mechanical Engineering from the California Institute of Technology [Caltech], working with Prof. Austin Minnich. For his Ph.D., he worked on experimentally investigating boundary scattering of heat carriers (phonons) in thin silicon membranes using the transient grating experiment. Subsequently, he was postdoctoral research fellow at Boston College, where he worked with Prof. David Broido on developing a predictive first-principles computational framework to capture the thermal and electronic properties of materials at high temperatures and extreme environmental conditions.

报告摘要

As electronic devices continue to miniaturize, managing high heat flux becomes increasingly challenging. Consequently, there is growing interest in passive and energy-efficient solutions for the thermal management of these devices, particularly among the semiconductor industries worldwide. The latest advancements in enhancing heat dissipation are centered on the utilization of ultrahigh thermal conductivity (κ) materials that are capable of hosting exotic non-Fourier hydrodynamic and Poiseuille thermal transport regimes. In this context, the quantum mechanics-based first-principles method is crucial, as it facilitates the prediction of κ for any material without any fitting parameters [1-3]. This approach was instrumental in discovering new materials not previously found in nature, such as boron arsenide, whose κ exceeds that of silicon by about a factor of 10 [2]. However, for such ultrahigh-κ materials, the calculation of thermal transport properties is computationally challenging as it requires iteratively solving the governing transport equation for the transport of the energy carriers, as discussed below. As an alternative, here I will present an extensive study of a popular approximation to the complex governing transport equation for heat carriers – the Callaway approximation, which is regularly used for exploring thermal transport in several low to ultrahigh-κ materials, at a significantly lower computation cost [4].

In semiconductor materials, which are a major constituent in modern electronic devices, the heat is predominantly carried via the collective lattice vibrations called phonons. During the transport of heat energy, these phonons may scatter among themselves or with the impurities or boundaries. These scatterings can be dissipative scatterings, where the total momentum of the phonons involved is destroyed, or can be non-dissipative, where the total participating phonon momentum is conserved, and the strength of these scatterings is given by their scattering rates. Further, investigation of the scattering-driven phonon transport necessitates solving the governing Peierls-Boltzmann equation (PBE) for phonon transport, which is a non-linear, multi-dimensional, coupled set of the integral-differential equation, making it difficult to solve computationally even under its linearized form as the linearized Peierls-Boltzmann equation (LPBE).

To address these computational challenges, several approximate models for the LPBE are often employed. The most commonly used approximation is the relaxation time approximation (RTA), which assumes that all phonons scatterings will independently take the system to the local equilibrium, thus simplifying the solution of LPBE. However, the RTA is only applicable to low-κ materials like silicon, where the dissipative scattering rates exceed or are comparable to the non-dissipative scattering rates. In ultrahigh-κ materials, the scattering rates corresponding to non-dissipative scattering processes are larger than that for the dissipative scattering for a large fraction of the phonon spectrum; therefore, the RTA fails as it assumes both types of scattering lead the system to the same equilibrium. On the other hand, the Callaway approximation to the LPBE accurately differentiates these mechanisms, requiring that only dissipative scatterings take the system to local equilibrium, while non-dissipative scatterings result in a “drifting” distribution. This approximation also leads to a straightforward solution of LPBE like the RTA; but in contrast to the RTA, it is applicable to a broad range of low-κ (like silicon) to ultrahigh-κ (like diamond) materials.

Upon exhaustively testing the Callaway approximation for group IV and III-V semiconductors, we have found that it works well for all of these materials except for two ultrahigh-κ materials, namely boron arsenide (BAs) and boron antimonide (BSb). On investigating this exceptional failure of the Callaway model, we found that this anomaly is derived from the distinctive characteristics in the phonon dispersion of these materials, which limits the phonon scatterings by simultaneous activation of multiple scattering selection rules [4]. Our research establishes guidelines for easy identification of materials where Callaway approximation is applicable, without actually solving the full transport equation.

This work was supported by the Indian Science and Engineering Research Board's Core Research Grant No. CRG/2020/006166 and the Mathematical Research Impact Centric Support Grant No. MTR/2022/001043.

References

[1] N. K. Ravichandran and D. Broido, Physical Review B 98 (8), 085205, (2018).
[2] F. Tian, B. Song, X. Chen, N. K. Ravichandran et al., Science 361 (6402), 582-585, (2018).
[3] N. K. Ravichandran and D. Broido, Physical Review X 10, 021063, (2020).
[4] N. Malviya and N. K. Ravichandran, Physical Review B 108, 155201 (2023).

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