The development of terahertz technology in the last few decades has made it possible to obtain a clear terahertz (THz) spectrum. THz vibrations clearly show the formation of weak bonds in crystals. The simultaneous progress in the code of first-principles calculations treating noncovalent interactions has established the position of THz spectroscopy as a powerful tool for detecting the weak bonding in crystals. In this review, we are going to introduce, briefly, the contribution of weak bonds in the construction of molecular crystals first, and then, we will review THz spectroscopy as a powerful tool for detecting the formation of weak bonds and will show the significant contribution of advanced computational codes in treating noncovalent interactions. From the second section, following the Introduction, to the seventh section, before the conclusions, we describe: (1) the crystal packing forces, the hydrogen-bonded networks and their contribution to the construction of organic crystals; (2) the THz vibrations observed in hydrogen-bonded molecules; (3) the computational methods for analyzing the THz vibrations of hydrogen-bonded molecules; (4) the dispersion correction and anharmonicity incorporated into the first-principles calculations and their effect on the peak assignment of the THz spectrum (5) the temperature dependence; and (6) the polarization dependence of the THz spectrum.

We measured the terahertz spectra of five powdered vitamins, nicotinamide, nicotinic acid, ascorbic acid, pyridoxine hydrochloride, and biotin, to investigate intermolecular hydrogen bonding. These vitamins have various weak to moderate hydrogen bonds between molecules and thus are good candidates for the investigation of various hydrogen bond stretching. Using dispersion-corrected density functional theory calculations on a periodic system, we assigned the observed THz peaks with excellent accuracy. The stretching vibrations of the intermolecular hydrogen bonds are mixed with the intermolecular translational and rotational modes. Depending on the strength of the hydrogen bond and the mass of the moiety linked via the hydrogen bond, the wavenumber of each type of hydrogen bond is located within a limited region. According to the present study, the stretching vibration of a weak hydrogen bond located at approximately 60–105 cm⁻¹ is summarized as having a sharp full width at half maximum of 1–2 cm⁻¹ and the molar extinction coefficient of 4–10 M⁻¹ cm⁻¹. The anharmonicity of the potential energy curve was found in modes containing weak intermolecular NH⋯N and NH⋯O stretching vibrations. Present research on the specific properties of hydrogen bond stretching vibrations observed in the THz region provides useful information for quickly estimating the existence of intermolecular hydrogen bonds.

We have performed high-resolution angle-resolved photoemission spectroscopy on Tl-doped PbTe. We observed a distinct energy shift of the valence band and core levels upon Tl doping, together with the evolution of a small hole pocket around the $\overline{X}$ point in the Brillouin zone, while no clear evidence for the localized states near the Fermi level is observed. These experimental results suggest that direct hole doping into the valence band and resultant emergence of a small Fermi surface are responsible for the metallic conductivity in Tl-doped PbTe.

We report the high-resolution temperature-dependent photoemission spectra of FeSi. We measure an extremely small gap of less than 5 meV in contrast to electrical-resistivity, magnetic-susceptibility, and optical-conductivity measurements, which show an activation energy of about 60 meV. The difference in the gap value is attributed to localized states near the Fermi level, which effectively increase the activation energy associated with a charge or spin excitation as observed in conductivity and susceptibility studies. The density of states at the Fermi level increase systematically on increasing temperature and result in a transition to a metallic phase at 300 K. The data suggest that localization effects play an important role in the ground-state properties of FeSi.