Pressure-Tuning of thec–fHybridization in Yb Metal Detected by Infrared Spectroscopy up to 18 GPa

Abstract

It has been known that the elemental Yb, a divalent metal at ambient pressure, becomes a mixed-valent metal under external pressure, with its valence reaching ∼ 2.6 at 30 GPa. In this work, infrared spectroscopy has been used to probe the evolution of microscopic electronic states associated with the valence crossover in Yb at external pressures up to 18 GPa. The measured infrared reflectivity spectrum [R(ω)] of Yb has shown large variations with pressure. In particular, R(ω) develops a deep minimum in the mid-infrared, which shifts to lower energy with increasing pressure. The dip is attributed to optical absorption due to a conduction (c)-f electron hybridization state, similarly to those previously observed for heavy fermion compounds. The red shift of the dip indicates that the c-f hybridization decreases with pressure, which is consistent with the increase of valence.KEYWORDS: Yb metal, valence crossover, infrared synchrotron radiation, high pressure, diamond anvil cell Physical properties of strongly correlated "heavy fermion" compounds, most typically Ce-or Yb-based compounds containing partly filled 4f shell, have attracted much attention.1) In heavy fermion compounds, the hybridization between the conduction (c) electrons and the otherwise localized f electrons leads to many interesting phenomena, such as a crossover between itinerant and localized characters of the f electrons, and the formation of heavy fermion.Elemental Yb is a divalent metal at ambient pressure, hence the f electron configuration is 4f 14 with no local magnetic moment. However, it has been shown using Xray absorption spectroscopy (XAS) that the valence of Yb increases under external pressure, reaching ∼ 2.6 at 30 GPa.2, 3) Namely, Yb is a mixed-valent metal in this pressure range. Since the number of localized 4f holes increases with this valence crossover, it can be viewed from the 4f hole point of view as a crossover from an itinerant to a localized regime. Equivalently, it can be also viewed that the c-f hybridization becomes weaker as the applied pressure increases. Hence Yb can be regarded as an f electron system where an itinerant-localized crossover can be caused, and where the c-f hybridization can be tuned, by the external pressure. In addition, Yb has another interesting property: its resistivity (ρ) increases under pressure up to ∼ 4 GPa.4) It has been suggested by band structure calculation that the increase of ρ with pressure is due to a pseudogap formation at the Fermi level (E F ). 5)In this work, we have probed the interesting electronic structures of Yb metal under pressure using infrared (IR) reflectivity [R(ω)] measurement. IR spectroscopy has been quite successful in probing the electronic structures associated with the c-f hybridized state in mixedvalent heavy fermion compounds.6-9) We have used a diamond anvil cell (DAC) to produce high pressure. It * E-mail: okamura@kobe-u.ac.jp is technically challenging to do reflectivity experiment within the limited sample space in a DAC using long wavelength IR radiation....

Pressure suppression of unconventional charge-density-wave state in PrRu4P12studied by optical conductivity

“…σ(ω) of a sample at high pressure was obtained from the analysis of a reflectance spectrum, R(ω), measured with a diamond anvil cell (DAC). [16][17][18][19][20] A pair of diamond anvils with 0.6 mm culet diameter and a metal gasket (stainless steel 301) were used to seal the sample, pressure transmitting medium, and small ruby pieces used to monitor the pressure via its fluorescence. 17 A flat, asgrown surface of a single crystal sample 2 was closely attached on the culet surface of the anvil, which was of type IIa with low density of impurities.…”

A simple method for the Kramers-Kronig analysis of reflectance spectra measured with diamond anvil cell

“…These results show that, for any spectral change in R d (ω) (either temperature-or pressure-induced) below 1.5 eV, the corresponding σ(ω) can be obtained by the present method. In actual high pressure studies of strongly correlated materials with DAC [2][3][4][5][6][7][8][9][10][11][12][13], R d (ω) is usually measured below 1-2 eV. Hence, above the high energy limit of R d (ω) measurement, the R d (ω) expected from R 0 (ω) can be connected to the measured R d (ω), with the cutoff and extrapolation discussed above.…”

“…The IR spectroscopy technique has been also performed under high pressure using a diamond anvil cell (DAC) [2][3][4][5][6][7][8][9][10][11][12][13] since the strongly correlated materials show many interesting physical properties under high pressure. In a DAC, a pair of diamond anvils and a thin metal gasket are used to seal a sample and a pressure transmitting medium [6].…”

“…Infrared (IR) spectroscopy has been a powerful tool to study the microscopic carrier dynamics and electronic structures in strongly correlated electron materials, such as rare earth (f electron), transition metal (d electron), and organic (p electron) compounds [1]. The IR spectroscopy technique has been also performed under high pressure using a diamond anvil cell (DAC) [2][3][4][5][6][7][8][9][10][11][12][13] since the strongly correlated materials show many interesting physical properties under high pressure. In a DAC, a pair of diamond anvils and a thin metal gasket are used to seal a sample and a pressure transmitting medium [6].…”

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

“…96 The Yb valence increases with pressure, from 2 at am-bient pressure to about 2.6 at 20 GPa. 97,98 The R(ω) of elemental Yb at room temperature in the 0.03-1.1 eV range was measured under high pressure up to 18 GPa by Okamura et al 94 R(ω) developed a dip at approximately 0.1 eV with increasing pressure up to 4 GPa. This suggested a decrease in the density of states near E F , which corresponds well to an increase in resistivity.…”