Infrared Spectroscopy under Multiextreme
Conditions: Direct Observation of Pseudogap
Formation and Collapse in CeSb
Cerium monoantimonide (CeSb) has manymagnetic phases with complex magnetic structures
at low temperatures, high pressures and highmagnetic fields [1]. The magnetic structure affects
the transport property through the hybridization
between Ce 4f and Sb 5p orbitals, referred to as pf
mixing [2]. One of the characteristic properties is thatat pressures (P) of several GPa [3] the electricalresistivity at a temperature (T) of around 30 K
increases by one full order over that at ambientpressure. Then, we investigated the origin of thisenhancement of the electric resistivity using theinfrared spectroscopy under multiextreme conditionsof low temperatures, high pressures and highmagnetic fields.
The experiment was performed at the infrared
magneto-optical imaging beamline BL43IR. To
perform infrared spectroscopy under multiextreme
conditions, a diamond anvil pressure cell with asample was set at the sample position located at thecenter of a superconducting magnet. The sample
conditions were T  4 K, P = 4 GPa and magneticfields of B  14T.
The temperature dependence of the opticalconductivity [σ(ω)] spectrum derived from theKramers-Kronig analysis of the reflectivity spectrum at
P = 4 GPa and B = 0 T is shown in Fig. 1. Below
T = 30 K and above 60 K, the spectra are typicallymetallic because σ(ω) increases with decreasing
photon energy. At intermediate temperatures, the σ(ω)
spectrum displays a strong temperature dependence.
At 30 K, the spectrum changes drastically. In
particular, σ(ω) below 0.1 eV decreases withdecreasing photon energy. This means that themetallic character is suppressed at intermediatetemperatures. The σ(ω) spectrum at 35 K has a peakwith a center energy of 65 meV. The peak structure
indicates the pseudogap appears under the given
condition.
The pseudogap collapses not only by increasing
temperature but also by applying magnetic field
higher than 6.5 ± 0.5 T. The magnetic field temperature (B-T ) phase diagram at 4 GPa resulting
from changes in the σ(ω) spectrum is shown in Fig. 2.
In the phase named AF-1, an antiferromagnetic spinaligns directly along the magnetic field. The Sb 5p

CeSb P = 4GPa

T=70 K
65 K
60 K
55 K
50 K
45 K
40 K
35 K
430 K
25 K
220 K
10 K

00.0

0.2

0.4

Fig. 1. Temperature dependence of opticalconductivity (σ(ω)) spectrum of CeSb at P = 4
GPa and B = 0 T. Successive curves are offset
by 103 for clarity. Same phases are shown by
the same colors.

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band is modulated by the magnetic structure and thenthe energy gap opens at the Fermi level. In the figure,two phase diagrams of P-T at B = 0 T [1] and B-T at
ambient pressure [4] are also plotted. The phase
diagram at 4 GPa is simpler than that at ambientpressure. In particular, the complex magneticstructure at ambient pressure disappears at 4 GPa.
At ambient pressure, since the pf mixing competes
with other magnetic interactions and crystal field

splitting, such complex magnetic phases andstructures appear. With increasing pressure, the pf
mixing is enhanced and then predominates among
these interactions. The enhancement of pf mixing
also results in a simpler magnetic phase diagram at 4GPa. This is the plausible result of the pf mixing
enhancement due to applied pressure. This is the first
optical observation of the magnetic field induced
nonmetal-metal phase transition at high pressures [5].

14Magnetic Field (T)1210864210203040Temperature (K)

650

ssu

4Pre00re(G2Pa)

060

Fig. 2. Magnetic field - temperature (B-T ) phase diagram of CeSb at P = 4 GPa derived
from σ(ω) spectra under multiextreme conditions. The phase diagrams of P-T at 0 T (bluelines) [1] and B-T at ambient pressure (green lines) [5] are also plotted for reference. AF-1
and AF-1A are the names of the phases. Only AF-1 phase has a pseudogap structure.

Shin-ichi Kimura

References

UVSOR Facility, Institute for Molecular Science[1] T. Osakabe et al.: Acta Phys. Pol. 34 (2003) 1469.
[2] H. Takahashi and T. Kasuya: J. Phys. C: Solid
State Phys. 18 (1985) 2697; ibid 2709; ibid 2721; ibid2731; ibid 2745; ibid 2755.
[3] N. Môri et al.: JJAP Series 8 (1993) 182.
[4] S. Kimura et al.: J. Phys. Soc. Jpn. 71 (2002)
2299.
[5] T. Nishi, S. Kimura, T. Takahashi, Y. Mori, Y.S.
Kwon, H.J. Im and H. Kitazawa: Phys. Rev. B 71
(2005) 220401(R).

E-mail: kimura@ims.ac.jp

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