Infrared and Terahertz Spectroscopy of Strongly Correlated Electron Systems under Extreme Conditions

Abstract

Owing to its high brilliance, infrared and terahertz synchrotron radiation (IR/THz-SR) has emerged as a powerful tool for spectroscopy under extreme (i.e., technically more difficult) experimental conditions such as high pressure, high magnetic field, high spatial resolution, and a combination of these. The methodologies for pressure-and magneticfield-dependent spectroscopy and microscopy using IR/THz-SR have advanced rapidly worldwide. By applying them to strongly correlated electron systems (SCESs), many experimental studies have been performed on their electronic structures and phonon/molecular vibration modes under extreme conditions. Here, we review the recent progress of methodologies of IR/THz-SR spectroscopy and microscopy, and the experimental results on SCESs and other systems obtained under extreme conditions.

Optical evidence of the type-II Weyl semimetals MoTe2 and WTe2

“…As-grown surfaces were measured for the optical reflectivity [R(ω)] spectra. Nearnormal incident R(ω) spectra were acquired in a wide photon-energy range of 3 meV-30 eV to ensure accurate Kramers-Kronig analysis (KKA) 39 . Michelson-type and Martin-Puplett-type rapid-scan Fourier spectrometers were used at the photon energy ω regions of 8 meV-1.5 eV and 3-20 meV, respectively, with a feed-back positioning system to maintain the overall uncertainty level less than ±0.5 % in the temperature range of 10-300 K 40 .…”

Piston pressure cell for low-temperature infrared investigations

“…Due to the higher brilliance of the radiation and better focus of the beam, synchrotron light sources are the method of choice when diamond anvil cells are required to reach very high pressure. 16 Compared to transition metal oxides, where several hundreds of kilobar pressure are essential, materials based on organic molecules are rather soft, and consequently, their properties can be modified dramatically by much lower pressure. Only 300 bars are sufficient to cross the insulatorto-metal transition for κ-(BEDT-TTF) 2 Cu[N(CN) 2 ]Cl and achieve superconductivity at T c = 12.8 K, 17,18 for example; the first organic superconductor (TMTSF) 2 PF 6 needs 6.8 kilobars to exhibit superconductivity at 1.2 K. 19,20 With p = 9 kilobars, the neutral-ionic transition in TTF-CA can be reached at room temperatures, where T NI = 81 K is required at ambient pressure.…”

Optical Conductivity Study of f Electron States in YbCu2Ge2 at High Pressures to 20 GPa

“…(a)Reflectance spectra (R d ) of YbCu 2 Ge 2 measured with DAC at 295 K and at P =2,4,6,8,10,12,14, 16, and 20 GPa. The thick solid curves show the original data, where a spectral range around 0.27 eV is not indicated since this range could not be measured well due to strong absorption by the diamond.…”