Shunya Yamamoto

In this work, we performed transient reflection measurements using the pump–probe protocol to investigate the influence of lattice defects induced by 380 keV argon (Ar) ions on the phonon dynamics of interlayer shearing (E2g1) and in-plane carbon stretching (E2g2) modes in highly oriented pyrolytic graphite (HOPG). The frequency and decay rate of E2g1- and E2g2-mode phonons were changed significantly at the fluence of 1×1014 ions/cm2. This indicated that significant structural changes in HOPG due to 380 keV Ar-ion irradiation were expressed as changes in the frequency and decay rate of phonons. In HOPG irradiated with 380 keV Ar+ at the fluence of 1×1014 ions/cm2, our measurement results indicated that compressive and tensile stresses contributed to in-plane and interlayer graphite structures, respectively.

TaAs single crystals were grown by a standard chemical vapor transport method. The single-crystallinity and homogeneous distribution of elements were confirmed by transmission electron microscope and x-ray diffraction observations. Positron annihilation measurements revealed that the atomic vacancy concentration was kept below 10−5 at. %. However, inductively coupled plasma analysis showed an As-deficient (7–9 at. %) off-stoichiometry. First-principles calculations implied that the off-stoichiometry could be compensated for with excess Ta antisite defects, thereby inducing metallic states. Nevertheless, excellent semimetallic transport properties of a well-suppressed carrier density (≲1018 cm−3), ultrahigh carrier mobility (≳106 cm2/V/s), and large transverse magnetoresistance (>200000% at 9 T) with the quantum oscillation were obtained at 1.7 K. This indicated the robustness of semimetallic transport properties against the off-stoichiometric disorder and the quenching of metallic conduction associated with excess Ta atoms. The negative longitudinal magnetoresistance, which is considered evidence of a Weyl semimetal (chiral anomaly), was not observed. These data were discussed with theoretical calculations.

Mechanism of the ion track formation in crystalline silicon (c-Si) is discussed, particularly under 1–9 MeV C₆₀ ion irradiation. In this energy region, the track formation was not expected because the energy E was much lower than the threshold of E _th = 17 MeV determined by extrapolation from higher energy data in the past literature. The track formation is different between irradiations of C₆₀ ions and of monoatomic ions: The tracks were observed under 3 MeV C₆₀ ion irradiation but not under 200 MeV Xe ions, while both the irradiations have the same electronic stopping (S _e) of 14 keV nm⁻¹ but much higher nuclear stopping (S _n) for the former ions. The involvement of S _n is suggested for the C₆₀ ions. While the inelastic thermal spike (i-TS) calculations predict that the high energy monoatomic ion irradiation forms the tracks, the tracks have never been experimentally detected, suggesting quick annihilation of the tracks by highly enhanced recrystallization in c-Si. Exceptions are C₆₀ ions of 1–9 MeV, where the track radii are well reproduced by the i-TS theory with assuming the melting transition. Collisional damage induced by the high S _n from C₆₀ ions obstructs the recrystallization in c-Si. Then the tracks formed by the melting transition survive against the recrystallization. This is a new type of the synergy effect between S _e and S _n, different from the already-known mechanisms, i.e., the pre-damage effect and the unified thermal spike. While c-Si was believed as a radiation-hard material in the S _e regime with high S _e threshold, this study suggests that c-Si has a low S _e threshold but with efficient recrystallization.