The superconducting pyrochlore oxide Cd2Re2O7 is revisited with a particular emphasis on the sample-quality issue. The compound has drawn attention as the only superconductor (Tc = 1.0 K) that has been found in the family of {\alpha}-pyrochlore oxides since its discovery in 2001. Moreover, it exhibits two characteristic structural transitions from the cubic pyrochlore structure, with the inversion symmetry broken at the first one at 200 K. Recently, it has attracted increasing attention as a candidate spin-orbit coupled metal (SOCM), in which specific Fermi liquid instability is expected to lead to an odd-parity order with spontaneous inversion-symmetry breaking [L. Fu, Phys. Rev. Lett. 115, 026401 (2015)] and parity-mixing superconductivity [V. Kozii and L. Fu: Phys. Rev. Lett. 115 (2015) 207002; Y. Wang et al., Phys. Rev. B 93 (2016) 134512]. We review our previous experimental results in comparison with those of other groups in the light of the theoretical prediction of the SOCM, which we consider meaningful and helpful for future progress in understanding this unique compound.Comment: 15 pages, 11 figures, J. Phys. Soc. Jpn, in pres

The successive phase transitions of the pyrochlore oxide Cd 2 Re 2 O 7 are studied by polarizing microscopy and magnetic susceptibility measurements. The formation of twin domains is visualized in the polarizing images of a pristine (111) crystal surface upon cooling through the cubic-to-tetragonal transition at T s1 ~ 200 K. Moreover, a dramatic change in the twinning pattern is observed at T s2 ~120 K, which suggests that the tetragonal c axis flips as the strain changes its direction at the tetragonal-to-tetragonal transition. Magnetic susceptibility measurements reveal significant domain alignment upon cooling across T s1 and T s2 in a magnetic field of 7 T, which are due to ~10% anisotropy in the magnetic susceptibility for the lowtemperature phases. Interestingly, the anisotropy is reversed at T s2 : χ c < χ a above T s2 and vice versa below T s2 .

Cd2Re2O7 is the only superconductor (Tc = 1.0 K) among many pyrochlore oxides with the chemical formula A2B2O7. [1][2][3][4] Upon cooling, it shows two structural phase transitions from Fd-3m (phase I) to I-4m2 (phase II) at Ts1 = 200 K, and then to I4122 (phase III) at Ts2 = 120 K. 5) Inversion symmetry breaking (ISB) occurs at the first transition. The origin of the first transition is suggested to be the band Jahn-Teller effect that lifts the degeneracy of the Fermi surface, because a large decrease in the density-of-states is observed by magnetic susceptibility and nuclear magnetic resonance measurements below Ts1. [6][7][8] A recent theory by L. Fu suggests that the ISB transition of Cd2Re2O7 is driven by a Fermi liquid instability for the spin-orbit-coupled metal (SOCM), which is a metal with a centrosymmetric crystal structure and a strong spin-orbit coupling. 9) The SOCM possesses three kinds of Fermi liquid instabilities induced by spin-orbit coupling toward gyrotropic, multipolar, and ferroelectric orders in terms of symmetry. The irreducible representation of each order has odd parity and can be coupled with an odd-parity phonon at the  point, which results in a parity-breaking transition. The soft phonon of the successive transitions of Cd2Re2O7 is a Eu mode in the Oh point group, 10,11) which coincides with the irreducible representation of the multipolar order. Therefore, Cd2Re2O7 is considered to be an SOCM candidate with a multipolar order. 9) However, the recent second harmonic generation study suggests that the order parameter of the ISB transition belongs to a T2u multipolar order, which eventually induces a Eu distortion. [12][13][14] Thus, the mechanism of the ISB transition of Cd2Re2O7 is still under debate.In noncentrosymmetric metals, the spin splitting of Fermi surface occurs owing to antisymmetric spin-orbit coupling (ASOC), which has been intensively studied in the field of spintronics. 15) Moreover, exotic superconductivity with the mixing of s-and p-wave symmetries can occur in the absence of inversion symmetry, 16) which appears to be the case for Cd2Re2O7. [17][18][19] Spin splitting due to ASOC has been experimentally observed by means of the de Haas-van Alphen (dHvA) oscillation 20,21) and spin-/angle-resolved photoemission spectroscopy. 22) The ISB transition of Cd2Re2O7 should involve the deformation and spin-splitting of the Fermi surface but has not yet been directly observed; previous transport measurements suggested certain changes in the Fermi surface structure at Ts1 as well as at Ts2. 4) In this study, to clarify the electronic states of Cd2Re2O7 and to obtain insight into a possible multipolar order, we perform dHvA oscillation measurements by means of magnetic torque on high-quality crystals.Crystals of Cd2Re2O7 were grown by a chemical vapor transport technique, the details of which have been reported elsewhere. 23) The residual-resistivity ratio (RRR), defined as 300 K)/3 K) in the present study, was at most 40 in previous reports, 4) while we have obtained hig...

We realized a pseudo-microgravity environment for dusty plasmas in a ground-based experiment, using the field-emitting regime of a surface dielectric barrier discharge in high-pressure carbon dioxide (CO2) including supercritical conditions. Using the high and adjustable density of high-pressure CO2, the balance between gravitational force and buoyancy was controlled. When changing the density of CO2 in the range of 0.234 g/cm3 to 0.668 g/cm3, i.e., smaller and larger than that of the particles (0.5 g/cm3), a particle arrangement in the direction of the gravitational force was formed only when the density of CO2 was in the range of ±0.17 g/cm3 with respect to that of the particles. This experimentally demonstrates that the pseudo-microgravity that emerges due to the buoyancy from the high-pressure CO2 contributes to the particle arrangement in the gravitational direction, and hence, it compensates the gravity-induced anisotropy.

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