Pyrochlore Oxide Superconductor Cd₂Re₂O₇Revisited

Abstract

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

Variation in Superconducting Symmetry Against Pressure on Noncentrosymmetric Superconductor Cd2Re2O7 Revealed by 185/187Re Nuclear Quadrupole Resonance

“…The double SC transition seems to be the evidence that phase VII is the mixtures of phase IV and VIII as discussed in Ref. 11. In addition to the peculiar pressure dependence of T c at zero fields, H c2 enhancement at 4.7 GPa was reproduced through ac susceptibility measurements.…”

Formation and Control of Twin Domains in the Pyrochlore Oxide Cd2Re2O7

“…Images were recorded using a monochrome CCD camera (ST-402ME, SBIG). A pristine (111) crystal surface was observed in a crossed-Nicols setup with a polarizer along [1][2][3][4][5][6][7][8][9][10] and with an analyzer along in most observations; if not mentioned specifically, the direction is based on the cubic structure of phase I. A crystal was attached on a sapphire plate with varnish and cooled slowly from room temperature to 8 K in vacuum.…”

“…The surface may have been degraded by polishing, which has actually been observed by various measurements in previous studies. 2) We are currently attempting to obtain a crystal with a (001) facet via fine tuning of the crystal growth conditions. Magnetic susceptibility was measured in a magnetic property measurement system 3 (Quantum Design, MPMS3).…”

“…The pyrochlore oxide Cd 2 Re 2 O 7 has attracted great attention as a candidate spin-orbit-coupled metal (SOCM). 1,2) An SOCM is a metal with a centrosymmetric crystal structure and a large spin-orbit coupling. It possesses a specific Fermi liquid instability that causes spontaneous inversion-symmetry breaking and leads to either multipolar, ferroelectric, or gyrotropic order.…”

Magnetoelectric Response in Electric Octupole State: Possible Hidden Order in Cuprate Superconductors

“…Experimental study traces back to magnetic monopole order in Cr 2 O 3 , 64) and recent studies found magnetic toroidal order in LiCoPO 4 , 65,66) magnetic quadrupole order in Sr 2 IrO 4 67,68) and Ba(TiO)Cu 4 (PO 4 ) 4 , 69) and odd-parity electric order in Cd 2 Re 2 O 7 . 70,71) Recent theoretical study has also identified magnetic hexadecapole order in BaMn 2 As 2 . 57) Furthermore, a comprehensive group theoretical classification has identified more than 100 compounds as odd-parity magnetic multipole states.…”