2020同步年報

Physics and Materials Science 019 H A2 − H A1 remains almost unchanged, keeping the accessible field small, despite the great expansion in ΔT. This observation is in qualitative agreement with previous reports at low pressures up to 1.4 GPa by Wu et al . 2 and up to 5.7 GPa by Sidorov et al ., 3 who raised T A2 from ∼ 56 to 60.5 K, and T c to 75 K, respectively. It is worth mentioning that, while T A2 and Δ T are observed to increase smoothly with pres- sure, they exhibit a drastic increase at 7.9 GPa, indicating a possible pressure- induced structural transition. This effect was explored carefully using high- pressure X-ray diffraction experiments at TLS 01C2 of NSRRC, as discussed in the following. Figure 2 shows synchrotron X-ray diffraction (XRD) measurements under high pressure that were undertaken to investigate pressure-induced structural tran- sitions in Cu 2 OSeO 3 . Because of the small volume ( ～ 0.003 mm 3 ) of the samples in a high-pressure diamond anvil cell, the authors decided to implement the structural study using synchrotron XRD. Room-temperature synchrotron XRD with wavelength 0.6889 Å (18 keV) was performed; the patterns were analyzed. As shown in Fig. 2(a) , they display cubic phase P2 1 3 with lattice parameter a = 8.9193 Å, consistent with a previous report. 4 The same crystal structure persists as pressure is increased to 3.96 GPa. However, at 5.28 GPa, new Bragg reflection signals emerge, indicating the breaking of crystal symmetry. This pattern was indexed within an orthorhombic phase of space group P2 1 2 1 2 1 (losing the three- fold rotational symmetry) with lattice parameters a = 8.7988 Å, b = 8.7790 Å, c = 8.7409 Å. At ∼ 7.01 GPa, Cu 2 OSeO 3 underwent a second structural transition to a monoclinic phase with space group P2 1 (losing the 2 1 screw axis symmetry). A schematic diagram of relevant pressure-induced structural phases was con- Fig. 2 : Pressure dependence of XRD patterns. (a) Evolution of room-temperature synchrotron XRD patterns for a polycrystalline Cu 2 OSeO 3 sample under large quasi-hydrostatic pressure up to 10.47 GPa, indicating multiple structural phase transitions. (b) Schematic diagram represent- ing the pressure-induced structural phase transitions in Cu 2 OSeO 3 . It should be noted that 1) initial cubic phase P2 1 3 transforms into orthorhombic phase P2 1 2 1 2 1 at 5.28 GPa and 2) a second structural transition from orthorhombic P2 1 2 1 2 1 to monoclinic P2 1 phase occurs at 7.01 GPa. [Reproduced from Ref. 1] sequently established, as shown in Fig. 2(b) , in which phases cubic P2 1 3, orthorhombic P2 1 2 1 2 1 and monoclinic P2 1 are marked in gray, blue and red, respectively. The results thus add two additional structural phases below 11 GPa, the limit of the present synchrotron XRD experiments, that can host the skyrmions. The authors concluded that the observation of additional structures suggests that the skyrmion state might be insensitive to the underlying crystal structure. This work is expected to stimulate research to find new skyrmion materials with varied crystal structures while retaining the skyrmion state under ambient temperature and pressure conditions. (Reported by Ashish Chainani) This report features the work of Ching- Wu Chu, H.-D. Yang and their co-work- ers published in Proc. Natl. Acad. Sci. USA 117 , 8783 (2020). TLS 01C2 SWLS − X-ray Powder Diffraction • High-pressure XRD • Materials Science, Condensed-matter Physics References 1. L. Deng, H.-C. Wu, A. P. Litvinchuk, N. F. Q. Yuan, J.-J. Lee, R. Dahal, H. Berger, H.-D. Yang, C.-W. Chu, Proc. Natl. Acad. Sci. USA 117 , 8783 (2020). 2. H. C. Wu, K. D. Chandrasekhar, T. Y. Wei, K. J. Hsieh, T. Y. Chen, H. Berger, H. D. Yang, J. Phys. D Appl. Phys. 48 , 475001 (2015). 3. V. A. Sidorov, A. E. Petrova, P. S. Berdonosov, V. A. Dolgikh, S. M. Stishov, Phys. Rev. B 89 , 100403 (2014). 4. H. C. Wu, T. Y. Wei, K. D. Chandrasekhar, T. Y. Chen, H. Berger, H. D. Yang, Sci. Rep. 5 , 13579 (2015).