High-Pressure Study on the Superconducting Pyrochlore Oxide Cd₂Re₂O₇

Abstract

Very recently superconductivity with a critical temperature T c ~ 1 K was found in the pyrochlore oxide Cd 2 Re 2 O 7 . [1][2][3] It is the first superconductor in the family of pyrochlore oxides which possess a unique tetrahedral network of transition metal ions. The following experimental efforts to elucidate the mechanism of the superconductivity are now being done extensively. Specific heat and Re NQR measurements have revealed that the superconducting gap is almost isotropic. 4,5) In contrast, µSR measurements have pointed out a possibility of an anisotropic superconducting order parameter. 6)Besides superconductivity, Cd 2 Re 2 O 7 exhibits an interesting phase transition at high temperature of T s = 200 K, which may reflect the peculiar electronic structure of the compound and must be closely related to the occurrence of superconductivity at low temperature. 1,7,8) It is believed that, as temperature decreases, the crystal structure changes from the ideal cubic pyrochlore structure with space group Fd3m to another slightly distorted cubic structure with space group F43m.7) The transition is of the second order and the cell volume increases gradually with decreasing temperature below T s . The structural refinements of the low-temperature (LT) phase has not yet been completed. However, a consideration on the change of space groups from Fd3m to F43m which involves the lack of inversion symmetry at the Re site leads us to assume that each Re tetrahedron can expand or shrink alternatingly without distortions, resulting in a kind of breathing mode frozen in the three-dimensional network.7) It is considered, in other words, that all the bonds of the Re tetrahedral network are identical forming the diamond structure in the high-temperature (HT) phase, while they exhibit an alternation in the LT phase forming the zinc-blende structure. 9)On the other hand, this structural transition is accompanied by various, dramatic changes in physical properties:1) The resistivity is almost independent of temperature above T s , while it shows a good metallic behavior below T s with an apparent kink at T s . Correspondingly, magnetic susceptibility exhibits a sudden decrease below T s , with the value at T = 5 K reduced by about 35% compared with that at room temperature.1) These facts mean that a crucial change in the electronic structure occurs at the transition, suggesting that the origin is purely electronic and attributable to an inherent instability in the electronic structure of the pyrochlore oxide.Since the cell volume is larger in the LT phase, we have expected that the application of high pressure can suppress the transition and thus can stabilize the HT structure. This is the motivation of the present high-pressure (HP) study. Here we report a structural study and resistivity measurements on Cd 2 Re 2 O 7 under high pressure. An intriguing P-T phase diagram has been obtained, which illustrates a rich physics in the itinerant electron system on the pyrochlore lattice. §2. ExperimentalSingle crystal and polycrystalli...

Probing lattice dynamics ofCd2Re2O7pyrochlore: Thermal transport and thermodynamics study

“…32,33 As is shown in Fig. 9, the onset of the power law resistivity behavior is close to where the thermal conductivity undergoes the slope change in absence of a phase transition.…”

“…The small magnitude of H is a reflection of the close values of the electron and hole mobilities. 33 The associated temperature variations in the resistivity, thermopower, Hall coefficient, and Hall mobility constitute sufficient evidence for a metallic normal state that is essentially governed by the band structure. Besides, the system is paramagnetic in the temperature range studied.…”

“…35,36 The results of the pressure dependence studies also corroborated a rigid lattice to which electrons couple only weakly above T ‫ء‬ . 11,33 At this point, it is instructive to refer to TiS 2 , which exhibits a power-law resistivity behavior in the range well above room temperature. [45][46][47][48] Thompson proposed that the electron-electron scattering mechanism could dominate the electron-phonon interaction at high temperatures if "E F is small," and it yields a T 2 -dependent resistivity in TiS 2 .…”

Superconductivity and Structural Phase Transitions in Caged Compounds RT2Zn20 (R = La, Pr, T = Ru, Ir)

“…These results strongly suggested the relationship between the superconductivity and the structural phase transitions. 12) We next focus on the magnetic properties of the Pr ions in PrRu 2 Zn 20 and PrIr 2 Zn 20 . As is shown in Fig.…”

Metallic “Ferroelectricity” in the PyrochloreCd2Re2O7

“…The normal state properties of Cd 2 Re 2 O 7 are also intriguing [9][10][11][12][13][14][15], particularly the C-T transition at 200 K that profoundly affects the elec-tronic structure, transport, and magnetic susceptibility of this material. Evidence from resistivity, specific heat, nuclear quadrupole resonance, and x-ray diffraction [9,12,[15][16][17] indicates that the 200 K C-T transition is continuous.…”