TOPOLOGICAL CHANGE OF FERMI SURFACE IN BISMUTH UNDER HIGH PRESSURE

Abstract

Bismuth (Bi) is one of the most studied materials for specific electrical property and capability of various applications. For instance, Bi shows very small electron masses and high carrier mobility. Bi represents complicate phase diagram [1], the various experiments under high pressure have been reported. Bi is semimetal under ambient pressure at room temperature (RT), there is a band-crossing of 32-38 meV between the valence band at T point and the conduction band at L point [2,3]. As a pressure increases, the band-crossing gradually decreases [4], structural phase transition occurs from rhombohedral into base centered monocrinic Bi (II) at 2.5 GPa (at RT). Bi shows semimetal-semiconductor transition at this pressure and the band-crossing is released. Abrupt reduction of carrier density occurs at phase transition, it is expected that the change of the Fermi surface appears. However, there have not been detailed reports on effects of the carrier density change caused by the pressure. In this work, we perform reflection-type time-resolved pump and probe spectroscopy in Bi single crystal under high pressure, in order to study the effects of the carrier density change on carrier and phonon dynamics. As a result from our experiment, we observe drastic changes of coherent phonons and electronic responses through phase transition pressure. The single crystal of Bi prepared by the zone melting method is used. After trigonal face is cleaved, the sample is polished to be reduced its thickness less than 100 mm. The pressure generation and measurement technique have been described in detail in elsewhere [5]. The screw type of diamond anvil cell (DAC) is used in this experiment as a pressure source. The sample is loaded in gasketed DAC together with a ruby chip. The diameter of the sample chamber is about 600 mm and its depth is about 150 mm at the beginning of the experiments. A microscope pump and probe system designed for observation of coherent phonons and electronic responses is used in here. A mode-locked Ti:sapphire laser is operated with pulse duration of 130 fs and a repetition rate of 80 MHz. Central wavelength is 790 nm. The beams are focused in the diameter of 10 mm. The excitation power density is 56 mm/cm. Figure 1 shows pressure dependence of the transient reflectivity changes DR/R with the pump and probe measurement at pressure from atmosphere to 3.0 GPa. The pump and probe beams pass diamond and pressure transmitting medium, however, large influences of the DAC on the signals are not observed. The background components of the electronic response are observed superimposed by the oscillation components of the coherent phonon. The frequency of the coherent phonons at ambient pressure is 2.92 THz (at RT) and it agrees well with phonon frequency of A1g mode obtained by Raman scattering measurements [6]. Coherent phonons and electronic responses are clearly observed as a transient reflectivity changes below 2.5 GPa.