In most materials the electron–phonon (e-p) scattering is far weaker than phonon–phonon (p-p) scattering, and the e-p scattering is usually proportional to the e-p coupling strength. Here, we report strong e-p scattering but low e-p coupling strength in two-dimensional­(2D) Nb2C by first-principles calculations. Moreover, the intensity of e-p scattering is close to that of p-p scattering at 300 K in sharp contrast to normal cases. This abnormal e-p scattering is understood by a specific feature that the energy difference between occupied and empty electron states near the Fermi level is in the order of the characteristic phonon energy. By calculating the phonon transport property of 2D Nb2C, we show that this strong e-p scattering can result in great reduction in the lattice thermal conductivity.Our work also highlights a new way for searching novel 2D materials with low lattice thermal conductivity.

Crystalline solids with ultralow thermal conductivity are paramount for the development of thermoelectric materials and thermal barrier coatings for efficient thermal energy management. Here, by high-throughput ab initio calculations, we...

Two-dimensional Ti 2 CO 2 MXene is promising for applications in electronics and optoelectronics, where high intrinsic mobility is essential to achieving high performance. Therefore, accurate prediction of carrier mobility is important for these types of materials. Here, we show that full electron−phonon coupling (EPC) calculations can accurately predict the carrier mobility for polar materials like Ti 2 CO 2 MXene. Based on full EPC calculations and mode-by-mode analyses of the phonon-limited carrier transport in Ti 2 CO 2 MXene, we demonstrate that the EPC matrix of optical phonons is significantly higher than that of the acoustic modes, and the carrier scattering process is dominated by the longitudinal optical phonon (Froḧlich interaction). Consequently, the calculated carrier mobility of Ti 2 CO 2 at 300 K is 319.64 cm 2 /Vs for the hole and 16.69 cm 2 /Vs for the electron at a carrier concentration of n c = 1 × 10 12 cm −2 , which are over one order of magnitude lower than that predicted by the deformation potential theory method. The present work demonstrates the importance and necessity of considering the full EPC to accurately predict the carrier mobility of MXenes and other polar materials.

The band gap of most semiconductors decreases with temperature, but a few semiconductors exhibit an increase of the gap with temperature. This abnormal behavior is usually attributed to thermal expansion, band inversion, or d-states at the valence band maximum (VBM). However, the temperature-induced increase of the band gap in hexagonal (hex) crystalline Ge2Sb2Te5 cannot be understood by the current available mechanisms. Here, we propose a new mechanism, i.e., the interplay between antibonding states and phonon processes. In hex-Ge2Sb2Te5, the abnormal presence of antibonding-like states below the Fermi level induces weaker two-phonon processes than one-phonon processes at the VBM, resulting in a decrease in energy of the VBM with temperature. Moreover, p–d orbital hybridization leads to a comparable strength of two-phonon processes with one-phonon processes at the conduction band minimum (CBM), and thus this makes the CBM position almost temperature-independent. Furthermore, by considering the effects of the temperature-dependent electronic structure (TDES), the calculated electrical conductivity is in good agreement with the experiment, while the conventional rigid band approximation overestimates the values of the electrical conductivity over a wide range of temperature. Our work reveals the origin of the increase of the band gap with temperature in hex-Ge2Sb2Te5 and demonstrates the importance of TDES in electrical conductivity calculations.

A comprehensive light scattering study of an important polymer—polypropylene glycol, HO−[CH(CH3) −CH2O]n−H—having an average molecular weight of about 425 (PPG 425) was carried out. From the polarized spectra, the velocity and attenuation coefficient of the longitudinal hypersound as a function of temperature were determined. Measurements of the velocity and attenuation coefficient indicate the presence of molecular relaxations between 250−390°K. A maximum in the sound attenuation vs temperature curve was observed. The Landau−Placzek ratio over a wide range of temperatures was also measured and compared with the theory of Litovitz and co−workers on the relaxing molecular liquid over a wide temperature range. Depolarized scattering studies were also carried out. The molecular reorientation time and activation energy are determined from the spectral linewidth measurement as a function of temperature. Depolarization ratios (ρv and ρh) were determined. The Krishnan effect was observed at temperatures below 296°K.