Superconductivity and Rattling under High Pressure in the β-Pyrochlore Oxide RbOs₂O₆

Abstract

Rattling-induced superconductivity in the β-pyrochlore oxide RbOs 2 O 6 is investigated under high pressures up to 6 GPa. Resistivity measurements in a high-quality single crystal show that the superconducting transition temperature T c increases gradually from 6.3 K at ambient pressure to 8.8 K at 3.5 GPa, surprisingly remains almost constant at 8.8 ± 0.1 K in a wide pressure range between 3.5 (P o ) and 4.8 GPa, and suddenly drops to 6.3 K at P s = 4.9 GPa, followed by a gradual decrease with further pressure increase. Two anomalies in the temperature dependence of the normal-state resistivity are observed at P o < P < P s and P > P s , revealing the presence of two high-pressure phases corresponding to the changes in T c . The rattling of the Rb ion inside a cage made of Os and O atoms may be slightly and seriously modified in these high-pressure phases that probably have cages of reduced symmetry, respectively, so that electron-rattler interactions that govern the superconducting and transport properties of β-RbOs 2 O 6 are significantly affected. KEYWORDS: pyrochlore oxide, RbOs 2 O 6 , resistivity, high pressure, superconductivity, rattling to form a three-dimensional skeleton. 10) A virtual size mismatch between the guest A ion and the cage made of the octahedra allows the guest ion to move almost freely with an unusually large atomic excursion in an anharmonic potential inside the cage. 11) Since the mismatch becomes large from Cs to K with decreasing ionic radius of the A ions, the intensity of rattling is enhanced accordingly towards KOs 2 O 6 . Evidence of rattling in β-pyrochlore oxides has been obtained from structural analyses showing large atomic displacement parameters 10,12) or from heat capacity and spectroscopic measurements that showed Einstein-like modes with low energies of 2-7 meV. 5,6,[13][14][15][16][17] The rattling apparently affects the electronic properties of β-pyrochlores. A high resistivity and its anomalous temperature dependence, shown by a concave-downward curvature in a wide temperature range, have been observed and ascribed to a strong scattering of electrons by rattling. 18) Moreover, the increase observed in the spin-lattice relaxation rate of A-nucleus NMR arises from a strong electron-lattice coupling of the same origin. 18,19) Because of this large electron-rattler interaction, β-pyrochore oxides undergo superconducting transitions at relatively high temperatures of T c = 9.6, 6.3, and 3.3 K for A = K, Rb, and Cs, respectively. Nagao et al. suggested that superconductivity is induced by the rattling itself, because the estimated average frequency of phonons mediating Cooper pairing coincides with the energy of rattling for each of the three compounds. 7) Hattori and Tsunetsugu have investigated the role of rattling in the mechanism of superconductivity in the framework of a strong coupling theory of superconductivity and successfully reproduced the observed T c s. 20)Unique chemical trends for various parameters are observed in the series of β-pyrochlore oxides: the rattl...

OptimalTcfor Electron-Doped Cuprate Realized under High Pressure

“…Resistivity under high pressure was measured by a four-probe method using a cubic-anvil-press apparatus equipped with sintered diamond anvils. [25][26][27] Daphne 7474 oil was used as a pressure-transmitting medium. known underdoped high-T c cuprates.…”