Anomalously Low Thermal Conductivity in Undercoordinated Amorphous Silicon

Penghong Ci; Xiaolong Yang; Bengisu Yasar; Lin Zhang; Muhua Sun; Lei Jin; Houfu Song; Jiaman Liu; Manel Molina-Ruiz; Alpha T. N’Diaye; Yabin Chen; Lina Yang; Bo Sun; Zhongming Wei; Frances Hellman; Mary Scott; Wu Li; Junqiao Wu

doi:10.1103/m9hp-5h28

Anomalously Low Thermal Conductivity in Undercoordinated Amorphous Silicon

Penghong Ci ^1,*, Xiaolong Yang ^2,3,†, Bengisu Yasar^4,5, Lin Zhang ^2,3, Muhua Sun⁶, Lei Jin ⁴, Houfu Song⁷, Jiaman Liu ⁴, Manel Molina-Ruiz ^5,8 et al.

Alpha T. N’Diaye ⁹, Yabin Chen ⁴, Lina Yang¹⁰, Bo Sun ⁷, Zhongming Wei¹, Frances Hellman ^4,5,8, Mary Scott^4,5, Wu Li^11,‡, and Junqiao Wu ^4,§

¹State Key Laboratory of Semiconductor Physics and Chip Technologies, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China
²College of Physics and Center of Quantum Materials and Devices, Chongqing University, Chongqing 401331, China
³Chongqing Key Laboratory for Strongly Coupled Physics, Chongqing University, Chongqing 401331, China
⁴Department of Materials Science and Engineering, University of California, Berkeley, California 94720, USA
⁵Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
⁶Key Laboratory of Advanced Materials (MOE), School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China
⁷Tsinghua Shenzhen International Graduate School, Tsinghua University, Shenzhen 518055, China
⁸Department of Physics, University of California, Berkeley, California 94720, USA
⁹Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
¹⁰Department of Mechanical Engineering, University of California, Berkeley, California 94720, USA
¹¹Eastern Institute for Advanced Study, Eastern Institute of Technology, Ningbo 315200, China

^*Contact author: ci@semi.ac.cn
^†Contact author: yangxl@cqu.edu.cn
^‡Contact author: wu.li@eitech.edu.cn
^§Contact author: wuj@berkeley.edu

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Phys. Rev. Lett. 137, 196302 – Published 5 November, 2026

DOI: https://doi.org/10.1103/m9hp-5h28

Abstract

Ultralow thermal conductivity ( $κ$ ) underpins thermal insulation and nonequilibrium energy conversion at large temperature gradients. In compositionally pure amorphous silicon with predominantly tetrahedral coordination, a conventional benchmark for minimum thermal conductivity is provided by the Cahill-Watson-Pohl model, which constrains the phonon mean free path to half the wavelength. Here, we show an anomalously low $κ$ down to $0.6 7 W / mK$ at 300 K, about 33% below this conventional Cahill-Watson-Pohl benchmark, in amorphous Si obtained via high-pressure phase engineering. By combining the unified heat-transport framework with molecular dynamics simulations, we attribute the ultralow $κ$ to unusually undercoordinated Si sites generated by rapid mechanical quenching from high pressure. These sites generate inhomogeneous local force fields and seed rattlinglike lattice dynamics, leading to strong anharmonicity and vibrational-mode localization. Our results offer a microscopic blueprint for engineering solids with anomalously low $κ$ through undercoordination in amorphous networks, without introducing alloying or complex-unit-cell architectures.

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Anomalously Low Thermal Conductivity in Undercoordinated Amorphous Silicon

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Anomalously Low Thermal Conductivity in Undercoordinated Amorphous Silicon

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