Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session M44: Heat Transport in Condensed Systems IIFocus
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Sponsoring Units: DCOMP Chair: Elif Ertekin, University of Illinois at Urbana-Champaign Room: 704 |
Wednesday, March 4, 2020 11:15AM - 11:27AM |
M44.00001: Coherent phonon manipulation in van der Waals Graphene-MoS2 Hetero-structure Shiqian Hu, Junichiro Shiomi Generally, the difference in the intrinsic lattice structures of the constituent materials inevitably generates interface disorder during the fabrication process, greatly limiting direct experimental observation of the coherent phonon transport. The flexible integration and atomistic interlayer smoothness of van der Waals hetero-structure provide an ideal platform for the coherent phonon transport manipulation. Thus, in this work, using the non-equilibrium molecular dynamics simulations, we investigate the coherent phonon transport in van der Waals graphene-MoS2 hetero-structure with different stacking order at room temperature. The histogram of the phonon transmissions in different disordered structure exhibits a log-normal distribution, which reveals the localization of the coherent phonons. Furthermore, the optimal stacking order of the graphene and MoS2 is efficiently identified from tens of thousands of candidates by machine learning. The significantly suppressed of the phonon transmission in the low frequency (<5THz) phonons of the optimized structure lead to a significant reduction (~95%) of the thermal conductance compared with the graphite. Finally, the effects of the temperature and strain effect on the graphite and optimized structures are also discussed. |
Wednesday, March 4, 2020 11:27AM - 11:39AM |
M44.00002: Amorphous-like Thermal Conductivity in Crystalline Solids Saikat Mukhopadhyay, Thomas Reinecke Thermoelectric materials (TEs) could play an important role in future energy management through environmentally sound cooling and power generation, e.g., converting waste heat into electricity. Efficient TEs inhibit the propagation of heat (low thermal conductivity, κ) but conduct electricity well (high power factor). Although κ in a given material can be reduced via alloying and nanostructuring, identifying materials with intrinsically low κ is still needed. Previously, it has been shown that soft phonon modes due to weakly bonded atoms and s2 lone-pair electrons are common to materials with low-κ. Here, we predict a series of new materials which are weakly bonded systems with same constituent elements but different stoichiometry either with s2 lone-pair electrons or high mass density. Due to resulting giant phonon anharmonicity and low phonon group velocities, they offer extremely low κ (0.3-0.6 W/mK) at 300K approaching those found in the amorphous/disordered regime. In addition to low-κ, high Seebeck coefficients and high electrical conductivities in these materials may provide a new opportunities for high-efficiency thermoelectrics at room temperature. |
Wednesday, March 4, 2020 11:39AM - 11:51AM |
M44.00003: Characterization of Phonon Dynamics and Thermal Environments in FinFET Architectures Morgan Henderson, Kyle Li, Sanghamitra Neogi Physical device scaling for traditional chip architectures is expected to end by 2021. As a result, alternatives are being aggressively pursued that can meet the increasing computing demands imposed by data-centric computing and sensor networks. Three-dimensional IC architectures are particularly promising due to high levels of integration and enhanced performance. However, increased integration leads to thermal bottlenecks that greatly reduce device performance. To understand the bottlenecks, we use atomistic modeling techniques to characterize the nanoscale phonon dynamics and map the thermal landscape of modern fin field-effect transistors (FinFET). We identify phonon modes of FinFET subcomponents that contribute strongly to thermal transport using a lattice dynamics (LD) framework based on a quasi-harmonic Green-Kubo approximation. In parallel, we generate subcomponent temperature profiles using molecular dynamics (MD) simulations. We establish a relationship between the MD temperature profiles and the corresponding populations of the LD-identified phonon modes using a machine learning algorithm. This relationship is used to predict the full phonon dynamics of the entire FinFET device. |
Wednesday, March 4, 2020 11:51AM - 12:27PM |
M44.00004: Acoustic Phonon Engineering Using Nanostructures Invited Speaker: Daniel LANZILLOTTI KIMURA Advances in material science and fabrication techniques enabled the fabrication of samples with nanometric dimensions where it is possible to control the propagation of photons (NIR range) and acoustic-phonons (GHz-THz frequencies) in a single device. In this presentation, I will describe the behavior of a plethora of devices able to control acoustic phonons and the interactions with light and charge at the nanoscale [1-3]. I will introduce some strategies to generate, manipulate and detect ultra-high frequency acoustic phonons both in the time and spectral domains [4-5]. |
Wednesday, March 4, 2020 12:27PM - 12:39PM |
M44.00005: Complex bond distortion behaviors of anharmonic thermal carriers in van der Waals bonded molecular crystal α-RDX Gaurav Kumar, Peter W. Chung Understanding of phonon properties in molecular crystals is critical due to their influence over, for instance, sensitivity of explosives, and charge separation in organic semiconductors. In particular, hot-spot formation in shocked energetic materials, are known to follow from bond scission events. However, the phonon-mediated mechanisms that promote bond breaking are still unclear. In this work, we examine how distortion of bonds carry heat in van der Waals bonded molecular crystal α-RDX. We rank order the phonons and their distortions to the lattice by their importance as carriers of heat. The motions of the atoms that constitute the bond distortions are determined using the phonon mode energy and mode shapes under the harmonic approximation. We also estimate the thermal conductivity of α-RDX using the Allen-Feldman (AF) harmonic theory. The AF model takes into account the contribution of highly anharmonic diffusive carriers which dominate thermal transport in many molecular crystals. Our preliminary results indicate that the the low frequency modes (< 4 THz) contribute ~90% to the total thermal conductivity as well as ~90% to the total bond strain, among which N-N and N-O bonds were found to exhibit the largest strains and rotations. |
Wednesday, March 4, 2020 12:39PM - 12:51PM |
M44.00006: Ab initio-based scanning Seebeck microscopy simulation of epitaxial graphene on 6H-SiC Euicheol Shin, Yong-Hyun Kim Heat, diffusive and incoherent phenomena, has not been considered as surface scanning means. In spite of those characteristics of heat, the local variation of charge carrier was shown by using coherent electron transport via the temperature difference between tip and sample which is so-called scanning thermoelectric microscopy (SThM). It successfully measured atomic-scale local majority carriers variation at room temperature. |
Wednesday, March 4, 2020 12:51PM - 1:03PM |
M44.00007: First-principles study of enhanced thermal conductivity in ordered AlGaO3 alloys Sai Mu, Hartwin Peelaers, Chris Van de Walle Monoclinic gallium oxide (Ga2O3) has promising applications in high-power and high-frequency electronics due to its wide band gap. However, practical device applications are hampered by its low thermal conductivity. To improve the thermal properties of Ga2O3 devices, we propose to alloy Ga2O3 with Al2O3, forming a low-energy ordered structure at the 50% concentration [1]. We investigate the lattice thermal conductivity of monoclinic Ga2O3 and of the ordered AlGaO3 alloy using the phonon Boltzmann transport equation, with the harmonic and third-order anharmonic force constants calculated from density functional theory. We find that the thermal conductivity of AlGaO3 is raised by more than 70% compared to Ga2O3. The enhancement is ascribed to (1) increased group velocities and (2) reduced anharmonic scattering rates due to the reduced weighted phase space. The findings offer an avenue towards improved heat dissipation from Ga2O3 devices. |
Wednesday, March 4, 2020 1:03PM - 1:15PM |
M44.00008: Disentangling mass effects from crystal chemistry in the thermal properties of III-V insulators Sabrina J Li, Ethan Ritz, Nicole A Benedek BAs, a III-V insulator, has a thermal conductivity comparable to diamond and a higher thermal conductivity than other cubic III-V boron compounds. The origin of this high thermal conductivity has been attributed to the mass ratio and unique chemical bonding character of BAs, both of which give rise to specific features in the phonon dispersion curve. We use first-principles density functional theory in combination with the Boltzmann transport equation to systematically explore and disentangle the effects of mass ratio and bonding and chemistry on the thermal conductivity of the entire column of cubic III-V boron compounds, from c-BN to BSb. Our preliminary results and previous work [J. Appl. Phys. 116 073503 (2014)] suggest that although the mass ratio of BAs may optimally maximize thermal conductivity, there may be a pathway to further increasing thermal conductivity through modifying bonding and chemistry. Our work provides hints regarding the chemical characteristics of high thermal conductivity materials. |
Wednesday, March 4, 2020 1:15PM - 1:27PM |
M44.00009: Precise yet Fast High-Throughput Search for Thermal Insulators Florian Knoop, Thomas A Purcell, Matthias Scheffler, Christian Carbogno We present a systematic and numerically precise computational search for thermal insulators in material space performed with the FHI-vibes high-throughput framework [1]. FHI-vibes employes a robust metric that quantifies the degree of anharmonicity in the nuclear dynamics via the statistical comparison of first-principles forces with those forces that would act in the harmonic approximation. |
Wednesday, March 4, 2020 1:27PM - 1:39PM |
M44.00010: Optimization of thermal conductivity at interfaces using learning algorithms Anne Chaka, Zexi Lu, Malachi Schram In material science, we are frequently interested in understanding the properties and design implication of material at interfaces. These interfaces can be manipulated to improve the desired characteristics of the bulk material. In this study, we are interested in understanding and optimize the impact of interfacial atomic defects on the thermal transport across a Cu/Si junction. To that end, we developed a reinforcement learning based framework to optimize over a potentially large parameter search. Using this technique allows us to accumulate knowledge of the system of a given type of atoms and store this information into a neural network. In this study, we present our results on optimizing the thermal transport by varying the fraction and length of the interfacial atomic defects using molecular dynamics (MD) simulations with normal mode analysis (NMA) to investigate thermal transport. |
Wednesday, March 4, 2020 1:39PM - 1:51PM |
M44.00011: Valley filtering effect of phonons in graphene with a grain boundary Xiaobin Chen, Yong Xu, Jian Wang, Hong Guo Due to their possibility to encode information and realize low-energy-consumption quantum devices, control and manipulation of the valley degree of freedom have been widely studied in electronic systems. In contrast, the phononic counterpart--valley phononics--has been largely unexplored, despite the importance in both fundamental science and practical applications. In this work, we demonstrate that the control of ''valleys" is also applicable for phonons in graphene by using a grain boundary. In particular, perfect valley filtering effect is observed at certain energy windows for flexural modes and found to be closely related to the anisotropy of phonon valley pockets. Moreover, valley filtering may be further improved using Fano-like resonance. Our findings reveal the possibility of valley phononics, paving the road towards purposeful phonon engineering and future valley phononics. |
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