Infrared spectroscopy under multiextreme conditions: Direct observation of pseudogap formation and collapse in CeSb

Infrared reflectivity measurements of CeSb under multiextreme conditions (low temperatures, high pressures, and high magnetic fields) were performed. A pseudogap structure, which originates from the magnetic band folding effect, responsible for the large enhancement in the electrical resistivity in the single-layered antiferromagnetic structure (AF-1 phase) was found at a pressure of 4 GPa and at temperatures of 35–50 K. The optical spectrum of the pseudogap changes to that of a metallic structure with increasing magnetic field strength and increasing temperature. This change is the result of the magnetic phase transition from the AF-1 phase to other phases as a function of the magnetic field strength and temperature. This result is an important optical observation of the formation and collapse of a pseudogap under multiextreme conditions.

Figure 1

(Color) (a) Temperature dependence of the reflectivity spectrum of CeSb at $P=4\mathrm{GPa}$ and $B=0\mathrm{T}$. Successive curves are offset by 0.25 for clarity. The spectra at 10 and 70 K are also plotted by dotted and dot-dashed lines, respectively, in the intermediate temperature range as a guide. (b) The difference $\left(\Delta \right)$ between the reflectivity at 0.2 eV and that at 0.4 eV as a function of temperature. The hatched areas are the phase transition temperatures. The phase names of AF-1A, AF-1, and P follow the same notation as for neutron scattering.[3]

Figure 2

(Color) Temperature dependence of the optical conductivity spectrum $\left[\sigma \left(\omega \right)\right]$ of CeSb at $P=4\mathrm{GPa}$ and $B=0\mathrm{T}$. The direct current conductivities at $T=35\mathrm{K}$ (엯), 45 K (▵), and 50 K (◻) are also plotted. Since the measured energy range was above 0.65 eV, the $R\left(\omega \right)$ in the energy range below 0.65 eV was extrapolated using Hagen-Ruben’s function at 10 and 70 K or the modified Hagen-Ruben’s function at 35, 45, and 50 K for the Kramers-Kronig analysis. See the text for detail. Inset: The effective electron number $\left[N_{\mathrm{eff}}\left(\omega \right)\right]$ derived from $\sigma \left(\omega \right)$.

Figure 3

(Color) Optical conductivity $\left[\sigma \left(\omega \right)\right]$ spectra of CeSb at $P=4\mathrm{GPa}$. (a) Magnetic field dependence at $T=10\mathrm{K}$. (b) Magnetic field dependence at $T=40\mathrm{K}$. (c) Temperature dependence at $B=14\mathrm{T}$. (d) Magnetic field–temperature $(B,T)$ phase diagram of CeSb at $P=4\mathrm{GPa}$ derived from $\sigma \left(\omega \right)$ spectra under multiextreme conditions. The first-order- and second-order-like spectral changes are shown by lines and hatchings, respectively. The arrows indicate the scans (a)–(c).