Intrinsic Low Thermal Conductivity and Phonon Renormalization Due to Strong Anharmonicity of Single-Crystal Tin Selenide.Two-dimensional (2D) van der Waals material tin selenide (SnSe) has recently attracted intensive interest due to its exceptional thermoelectric performance.  However, the thermal properties and phonon transport mechanisms in its single-crystal form remain elusive.  Here, we measured high-quality SnSe single crystals using nanoscale thermometry based on ultrafast optical spectroscopy and found that its intrinsic thermal conductivity is highly anisotropic in different crystallographic directions.  To quantify phonon anharmonicity, we developed a new experimental approach combining picosecond ultrasonics and X-ray diffraction to enable direct measurement of temperature-dependent sound velocity, thermal expansion coefficient, and Grüneisen parameter.  Furthermore, we performed ab initio calculations using different methods: in comparison with self-consistent phonon theory, the harmonic and quasi-harmonic models that have been widely used in current phonon calculations fail to accurately predict these important thermophysical properties at room temperature and below.  This study represents an important research benchmark in characterizing high-performance thermal energy materials and provides fundamental insight into advancing modern calculation methods for phonon transport theory.
THERM