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3D-2D structural phase transition is artificially induced to invoke giant electronic modulation in nonequilibrium (Pb1−xSnx)Se., Material properties depend largely on the dimensionality of the crystal structures and the associated electronic structures. If the crystal-structure dimensionality can be switched reversibly in the same material, then a drastic property change may be controllable. Here, we propose a design route for a direct three-dimensional (3D) to 2D structural phase transition, demonstrating an example in (Pb1−xSnx)Se alloy system, where Pb2+ and Sn2+ have similar ns2 pseudo-closed shell configurations, but the former stabilizes the 3D rock-salt-type structure while the latter a 2D layered structure. However, this system has no direct phase boundary between these crystal structures under thermal equilibrium. We succeeded in inducing the direct 3D-2D structural phase transition in (Pb1−xSnx)Se alloy epitaxial films by using a nonequilibrium growth technique. Reversible giant electronic property change was attained at x ~ 0.5 originating in the abrupt band structure switch from gapless Dirac-like state to semiconducting state.

5d pyrochlore oxides with all-in-all-out magnetic order are prime candidates for realizing strongly correlated, topological phases of matter. Despite significant effort, a full understanding of all-in-all-out magnetism remains elusive as the associated magnetic excitations have proven difficult to access with conventional techniques. Here we report a Raman spectroscopy study of spin dynamics in the all-in-all-out magnetic state of the 5d pyrochlore Cd2Os2O7. Through a comparison between the two-magnon scattering and spin-wave theory, we confirm the large single ion anisotropy in this material and show that the Dzyaloshinskii–Moriya and exchange interactions play a significant role in the spin-wave dispersions. The Raman data also reveal complex spin–charge–lattice coupling and indicate that the metal–insulator transition in Cd2Os2O7 is Lifshitz-type. Our work establishes Raman scattering as a simple and powerful method for exploring the spin dynamics in 5d pyrochlore magnets. Pyrochlore 5d transition metal oxides are expected to have interesting forms of magnetic order but are hard to study with conventional probes. Here the authors show that Raman scattering can be used to measure magnetic excitations in Cd2Os2O7 and that it exhibits complex spin-charge-lattice coupling.

We investigate the ultrafast dynamics of ${\mathrm{Cd}}_{2}{\mathrm{Os}}_{2}{\mathrm{O}}_{7}$, a prototype material showing a Lifshitz-type transition as a function of temperature. In the paramagnetic metallic state, the photoreflectivity shows a subpicosecond relaxation, followed by a featureless small offset. In the antiferromagnetic state slightly below ${T}_{N}$, however, the photoreflectivity resurges over hundreds of picoseconds, which goes beyond the usual realm of the effective-temperature model. Our observations are consistent with the Lifshitz phase transition of ${\mathrm{Cd}}_{2}{\mathrm{Os}}_{2}{\mathrm{O}}_{7}$ driven by the evolution of the local magnetic moment.

The superconducting pyrochlore oxide Cd2Re2O7 ( T c = 1 K ), which is now considered as a candidate of the spin-orbit-coupled metal, shows an inversion-symmetry-breaking structural transition at T s 1 = 200 K . T s 1 decreases with increasing pressure and disappears at around P c = 4.2 GPa , where at least four high-pressure phases with tiny structural distortions are suggested by means of powder X-ray diffraction [Yamaura PRB 2017]. We have carried out Raman scattering experiments to investigate changes in the crystal symmetry under high pressures up to 4.8 GPa. A structural transition at 1.9–3.0 GPa and the recovery of inversion symmetry above P c are observed at 12 K.

5d pyrochlore oxides with all-in-all-out magnetic order are prime candidates for realizing strongly correlated, topological phases of matter. Despite significant effort, a full understanding of all-in-all-out magnetism remains elusive as the associated magnetic excitations have proven difficult to access with conventional techniques. Here we report a Raman spectroscopy study of spin dynamics in the all-in-all-out magnetic state of the 5d pyrochlore Cd2Os2O7. Through a comparison between the two-magnon scattering and spin-wave theory, we confirm the large single ion anisotropy in this material and show that the Dzyaloshinskii–Moriya and exchange interactions play a significant role in the spin-wave dispersions. The Raman data also reveal complex spin–charge–lattice coupling and indicate that the metal–insulator transition in Cd2Os2O7 is Lifshitz-type. Our work establishes Raman scattering as a simple and powerful method for exploring the spin dynamics in 5d pyrochlore magnets., Pyrochlore 5d transition metal oxides are expected to have interesting forms of magnetic order but are hard to study with conventional probes. Here the authors show that Raman scattering can be used to measure magnetic excitations in Cd2Os2O7 and that it exhibits complex spin-charge-lattice coupling.

The $5d$-transition-metal pyrochlore oxide $\mathrm{C}{\mathrm{d}}_{2}\mathrm{R}{\mathrm{e}}_{2}{\mathrm{O}}_{7}$, which was recently suggested to be a prototype of the spin-orbit-coupled metal [Phys. Rev. Lett. 115, 026401 (2015)], exhibits an inversion-symmetry-breaking (ISB) transition at 200 K and a subsequent superconductivity below 1 K at ambient pressure. We study the crystal structure at high pressures up to 5 GPa by means of synchrotron x-ray powder diffraction. A rich structural phase diagram is obtained, which contains at least seven phases, and is almost consistent with the electronic phase diagram determined by previous resistivity measurements. Interestingly, the ISB transition vanishes at $\ensuremath{\sim}4\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ where the enhancement of the upper critical field was observed in resistivity. Moreover, it is shown that the point groups at 8 K, probably kept in the superconducting phases, sequentially transform into piezoelectric, ferroelectric, and centrosymmetric structures on the application of pressure.

The first Cr-dominant amphibole, ehimeite, ideally NaCa2Mg4CrSi6Al2O22(OH)2, has been found in a chromitite deposit in the Akaishi Mine, Higashi-Akaishi Mountain, Ehime Prefecture, Japan. Ehimeite occurs as prismatic crystals of up to 1.5 cm in length and 0.5 cm in width and is found in association with chromite, kammererite (Cr-rich clinochlore), Cr-poor clinochlore, phlogopite, and uvarovite. It is transparent, emerald green to pale green in color with pale green streaks, and has a vitreous luster. Optically, it is biaxial positive with α = 1.644(2), β = 1.647(2), γ = 1.659(2), and 2Vcalc. = 53°. It has a Mohs’ hardness of 6 and densities of 3.08(3) g/cm3 (measured using heavy liquids) and 3.121 g/cm3 (calculated from powder diffraction data and the empirical formula). The empirical formula is (Na0.88K0.07)Σ0.95(Ca1.89Na0.02Mg0.09)Σ2.00(Mg4.03Cr0.62Al0.19Fe3+0.07Fe2+0.07Ti0.03)Σ5.00(Si6.14Al1.86)Σ8.00O22(OH)2 on the basis of O = 22 and OH = 2, and ehimeite mainly forms a solid solution, NaCa2Mg4(Cr, Al)Si6Al2O22(OH)2, with pargasite. It has a monoclinic unit cell with a = 9.9176(14) A, b = 18.0009(12) A, c = 5.2850(7) A, β = 105.400(7)°, V = 909.6 (17) A3, and Z = 2, and it belongs to the space group C2/m, as refined from powder XRD data. The eight strongest lines in the powder XRD pattern [d (A), I/I0, hkl] are (3.370, 58, 150), (2.932, 43, 221), (2.697, 81, 151), (2.585, 50, 061), (2.546, 100, 202), (2.346, 42, 351), (2.156, 35, 261), and (1.514, 55, 263). The crystal structure has been refined to R1 = 0.0488 using single-crystal XRD data. It has been concluded that ehimeite in the Akaishi Mine was formed by the reaction of chromitite and the metamorphic fluid in the retrograde stage of serpentinization during the Sanbagawa metamorphism.

We investigated the metal-insulator transition (MIT) driven by all-in-all-out (AIAO) antiferromagnetic ordering in the 5d pyrochlore Cd(2)Os(2)O(7) using optical spectroscopy and first-principles calculations. We showed that the temperature evolution in the band-gap edge and free carrier density were consistent with rigid upward (downward) shifts of electron (hole) bands, similar to the case of Lifshitz transitions. The delicate relationship between the band gap and free carrier density provides experimental evidence for the presence of an AIAO metallic phase, a natural consequence of such MITs. The associated spectral weight change at high energy and first-principles calculations further support the origin of the MIT from the band shift near the Fermi level. Our data consistently support that the MIT induced by AIAO ordering in Cd(2)Os(2)O(7) is not close to a Slater type but instead to a Lifshitz type.

We report the results of the low-temperature specific heat measurements of the single crystalline β -pyrochlore oxide superconductors AOs2O6 (A=K, Rb, and Cs) under high pressure up to 13 GPa. We find that superconducting transition temperature (Tc) monotonically increases for CsOs2O6 and RbOs2O6, while the one for KOs2O6 decreases by applying the pressure. With further increasing the pressure, Tc is suddenly suppressed at the same lattice volume for all compounds, concomitant with the first-order structural phase transition.

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 press

The crystal structure of the β -pyrochlore oxide superconductor KOs2O6 is re-examined. A single-crystal X-ray diffraction (XRD) analysis at room temperature first revealed that the compound crystallizes in a cubic structure with the centrosymmetric space group F d 3 m , as in conventional pyrochlore oxides. Later, however, Schuck et al. claimed a different non-centrosymmetric F 4 3 m structure based on their single-crystal XRD analysis. To unambiguously determine the true crystal structure of KOs2O6, we carried out high-resolution synchrotron powder X-ray and convergent-beam electron diffraction measurements at room temperature. The space group was determined with high reliability to be centrosymmetric F d 3 m , not F 4 3 m . This confirms the importance of the K atom location in a high-symmetry site, which causes unusually large rattling of the K atom.

To elucidate the superconducting gap structure and the influence of rattling motion on quasiparticle dynamics in the superconducting state of KOs 2 O 6 , thermal conductivity and microwave surface impedance were measured at low temperatures. The magnetic field dependence of thermal conductivity at the lowest temperature and the temperature dependence of penetration depth strongly indicate a fully gapped superconductivity in KOs 2 O 6 . The quasiparticle scattering time is strongly enhanced in the superconducting state, indicating a strong electron–electron inelastic scattering in the normal state. These results highlight that KOs 2 O 6 is unique among superconductors, which is characterized by the coexistence of fully gapped superconductivity and strong electron correlations.

Last five years we have mainly investigated phonons in the following cage compounds by Raman scattering: RT 4 X 12 , A 8 Ga 16 Ge 30 , La 3 Pd 20 Ge 6 , RB 6 , KOs 2 O 6 . From this systematic investigation we have clarified the following dynamical properties of the guest ion trapped in the large cages. The vibration of the guest ion is highly anharmonic due to weak atomic interaction between the guest ion and cage atom. This anharmonicity universally decreases the vibration energy of the guest ion with decreasing temperature. The existence of carriers decreases the interaction between the guest atoms and changes the dispersion curve of the guest ion mode to flat, which is the experimental evidence of the weak interaction between the guest atoms. For thermoelectricity, it is found in A 8 Ga 16 Ge 30 that the off-center rattling decreases lattice thermal conductivity.

The effect of magnetic field on S = 3 2 Heisenberg antiferromagnets on pyrochlore lattice was investigated. Chromium spinels CdCr 2 O 4 and HgCr 2 O 4 exhibit first order field-induced transitions to magnetization plateau states with one-half moment of Cr 3 + . One-half moment is reasonably understood as formation of collinear three-up and one-down spin configuration for each tetrahedron, which is a building block of pyrochlore lattice. The effect of lattice distortions to stabilize this spin ordering state makes these magnetization plateau phases appear within wide range of magnetic field. It is quite likely that field-induced collinear three-up and one-down state is universal for antiferromagnetic Heisenberg spin systems on pyrochlore lattice, however, such pure spin system seems to be limited to chromium spinel oxides at present.

We investigate the metal-insulator transition (MIT) of the osmium pyrochlore oxide Cd2Os2O7 through transport and magnetization measurements. The MIT and a magnetic transition to the all-in/all-out (AIAO) order occur simultaneously at 227 K. We propose a mechanism based on a Lifshitz transition induced by the AIAO magnetic order probably via strong spin-orbit couplings in the specific semimetallic band structure. It is suggested, moreover, that two observed puzzles, a finite conductivity near T = 0 and an emergence of weak ferromagnetic moments, are not bulk properties but originate at magnetic domain walls between two kinds of AIAO domains., 11 pages, 8 figures, to be published in APL Materials

We performed the X-ray diffraction measurements at low temperature under ambient pressure for the pressure-induced superconductor β'-Cu 0.65 V 2 O 5 . Satellite reflections were observed with an in...

Pyrochlore oxides have the general chemical formula A2B2O7 or A2B2O6O', where A is a larger cation and B is a smaller transition metal cation. The ideal pyrochlore structure contains a tetrahedral network called the pyrochlore lattice, which has been studied extensively in order to elucidate the effect of geometrical frustration on the properties of localized spin and itinerant electron systems. Recently, superconductivity was found in Cd2Re2O7 at Tc= 1.0 K for the first time in the family of pyrochlore oxides. Although the mechanism of the superconductivity appears to be conventional, it exhibits unique successive structural phase transitions at high temperatures accompanied by large changes in physical properties. Very recently, new pyrochlore oxides AOs2O6 with A = K, Rb, and Cs were discovered, which show superconductivity at 9.6 K, 6.3 K, and 3.3 K, respectively. Although the nature of superconductivity on these β-pyrochlore superconductors is not clear at the moment, we believe that interesting physics is involved on the basis of electron correlations on the pyrochlore lattice.

We report a structural phase transition found at Ts = 200 K in a pyrochlore oxide Cd2Re2O7 which shows superconductivity at Tc = 1.0 K. X-ray diffractionexperiments indicate that the phase transition is of the second order, from a high-temperature phase with the ideal cubic pyrochlore structure (space group Fd-3m) to a low-temperature phase with another cubic structure (space group F-43m). It is accompanied by a dramatic change in the resistivity and magnetic susceptibility and thus must induce a significant change in the electronic structure of Cd2Re2O7., 4 pages, 4figures, proceeding for ISSP8

Long-range non-collinear all-in/all-out magnetic order has been directly observed for the first time in real space in the pyrochlore Cd$_2$Os$_2$O$_7$ using resonant magnetic microdiffraction at the Os L$_3$ edge. Two different antiferromagnetic domains related by time-reversal symmetry could be distinguished and have been mapped within the same single crystal. The two types of domains are akin to magnetic twins and were expected - yet unobserved so far - in the all-in/all-out model. Even though the magnetic domains are antiferromagnetic, we show that their distribution can be controlled using a magnetic field-cooling procedure., Comment: Main text: 11 pages, 4 figures ; Supplementary materials: 9 pages, 5 figures

The title compounds are prepared from mixtures of Rh or Ir, BaS or BaSe, Ge, and S or Se (h-BN capsule, 5 GPa, 1473 K, 2 h).

Structure and chemical features of layered compounds BaM2Ge4Ch6 (M = Rh, Ir; Ch = S, Se) synthesized under high-pressure and high-temperature method are systematically studied. These compounds crystallize in an orthorhombic phase with space group Pbca (No. 61). The remarkable structural feature is to have the M-Ge-Ch pyrite-type building units, stacking with Ba-Ch layers alternatively along c axis. It is very rare and novel that pyrite-type subunits as the building blocks in the layered compounds. Theoretical calculations and experimental results indicate that there are strongly polarized covalent bonds between Ge and Ch atoms, forming heteromolecule-like anions in these compounds. Moreover, Ge atoms in this structure exhibit unusual valence state (~ +1) due to the tetrahedral coordination environment of Ge atoms along with M and Ch atoms simultaneously., 24 pages, 12 figures, 5 tables

To elucidate the superconducting gap structure and the influence of rattling motion on quasiparticle dynamics in the superconducting state of KOs 2 O 6 , the thermal conductivity and microwave surface impedance were measured at low temperatures. The magnetic field dependence of thermal conductivity and temperature dependence of penetration depth demonstrate full-gap superconductivity in KOs 2 O 6 . The quasiparticle scattering time is strongly enhanced in the superconducting state, indicating a strong electron inelastic scattering in the normal state. These results highlight that KOs 2 O 6 is unique among superconductors with strong electron correlations.

V 51 NMR measurements have been made on a single crystal to study the spin gap in the vanadium bronze η - Na 1.286 V 2 O 5 . We observe four sets of V 51 NMR spectra coming from the nonmagnetic V1 and V2 sites above T 0 ∼ 110 K and eight sets below T 0 due to the doubling of the V1 and V2 sites accompanied by the structural transition. The V 51 Knight shift measurements for the V1 and V2 sites indicate that the magnetic vanadium sites show the spin-gap behavior with the gap energy dependent on the vanadium sites below T 0 .

For the first time, large single crystals of lithium divanadate spinel, LiV2O4, showing heavy-fermion behavior down to 20 K, have been grown by a flux method using an effective LiCl–Li2MoO4–LiBO2 solvent system. LiV2O4 belongs to the normal spinel structure type. The V atoms (16d) have octa­hedral coordination [V—O = 1.9709 (7) A] and are arranged in a pyrochlore-type sublattice [V⋯V = 2.9147 (4) A]. The Li atoms (8a) have tetra­hedral coordination [Li—O = 1.9487 (5) A] and are located in channels running parallel to [110].

Rattling dynamics have been identified as fundamental to superconductivity in defect pyrochlore osmates and aluminium vanadium intermetallics, as well as low thermal conductivity in clathrates and filled skutterudites. Combining inelastic neutron scattering (INS) measurements and ab initio molecular dynamics (MD) simulations, we use a new approach to investigate rattling in the Al-doped defect pyrochlore tungstates: AAl0.33W1.67O6 (A = K, Rb, Cs). We find that although all the alkali metals rattle, the rattling of the K atoms is unique, involving local diffusion within a cage as well as vibrations at a range of frequencies. We show that the novel K rattling is driven by the strong anharmonicity and anisotropy of the local potentials around the K atoms, both of which are a consequence of the small size of the K rattler. Our findings may open new possibilities for phonon engineering in thermoelectric materials., Comment: 17 pages, 12 figures and 3 tables

The chemical composition, crystal structure, and magnetic properties of a manganese zinc alloy with an ideal composition of YMn2Zn20, which comprises a pyrochlore lattice made of Mn atoms, are reported. The compound is stable only when In or Al is partially substituted for Zn. We have determined the actual chemical formula as YMn2+dZn20-x-dMx, with M = In or Al, and have identified the characteristic preferences with which the incorporated M and excess Mn atoms occupy the three crystallographic sites for Zn atoms. The Mn atoms in the pyrochlore lattice possess small magnetic moments that interact with each other antiferromagnetically but exhibit no long-range order above 0.4 K, probably owing to the geometrical frustration of the pyrochlore lattice. As a result, the effective mass of the conduction electrons is considerably enhanced, as observed in the related pyrochlore-lattice compounds (Y,Sc)Mn2 and LiV2O4. However, the presence of excess Mn atoms with large localized magnetic moments comparable to spin 5/2 tends to mask the inherent magnetism of the pyrochlore Mn atoms. It is suggested that "YMn2Zn18In2" with neither excess Mn atoms nor site disorder would be an ideal compound for further study., Comment: 19 pages, 18 figures, accepted for publication in J. Solid State Chem

Evidence for another phase transition at 120 K in the metallic pyrochlore oxide Cd2Re2O7, following the structural transition at 200 K and followed by the superconducting transition at 1.0 K, is given through resistivity, magnetoresistance, specific heat, and X-ray diffraction measurements. The results indicate unique successive structural and electronic transitions occurring in the pyrochlore compound, revealing an interesting interplay between the crystal and electronic structures on the itinerant electron system in the pyrochlore lattice.

Cd${}_{2}$Re${}_{2}$O${}_{7}$ exhibits a unique structural transition characterized by a loss of the inversion symmetry in a metal. Here, we report on structural and electronic properties for ${A}_{2}$Re${}_{2}$O${}_{7}$ ($A=\text{Ca}$ and Pb), and S-substituted Cd${}_{2}$Re${}_{2}$O${}_{7}$. On the basis of material characteristics, we show that the covalency of particular bonds stabilizes the low-temperature phase without the inversion symmetry. We also observed an incoherent charge transport with a weakly temperature-dependent resistivity on the verge of structural transition. This phenomenon is argued in terms of the polaron formation.

Rattling-induced superconductivity in the beta-pyrochlore oxide RbOs2O6 is investigated under high pressures up to 6 GPa. Resistivity measurements in a high-quality single crystal show that the superconducting transition temperature Tc increases gradually from 6.3 K at ambient pressure to 8.8 K at 3.5 GPa, surprisingly remains almost constant at 8.8 \pm 0.1 K in a wide pressure range between 3.5 (Po) and 4.8 GPa, and suddenly drops to 6.3 K at Ps = 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 Po < P < Ps and P > Ps, revealing the presence of two high-pressure phases corresponding to the changes in Tc. 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 beta-RbOs2O6 are significantly affected., arXiv admin note: text overlap with arXiv:1009.0355

The superconducting properties of two β-pyrochlore oxides, CsOs 2 O 6 and RbOs 2 O 6 , are studied by thermodynamic and transport measurements using high-quality single crystals. It is shown that the character of superconductivity changes systematically from weak coupling for CsOs 2 O 6 to moderately strong coupling for RbOs 2 O 6 , and finally to extremely strong coupling with BCS-type superconductivity for KOs 2 O 6 , with increasing T c . Strong-coupling correction analyses of the superconducting properties reveal that a low-energy rattling mode of the alkali metal ions is responsible for the mechanism of the superconductivity in each compound. The large enhancement of T c from Cs to K is attributed to the increase in the electron–rattler coupling with decreasing characteristic energy of the rattling and with increasing anharmonicity. The existence of weak anisotropy in the superconducting gap or in the electron–rattler interactions is found for the Cs and Rb compounds.

A miniature Hall-sensor array was used to detect magnetic induction locally in the vortex states of the beta-pyrochlore superconductor KOs2O6. Below the first-order transition at Tp~8 K, which is associated with a change in the rattling motion of K ions, the lower critical field and the remanent magnetization both show a distinct decrease, suggesting that the electron-phonon coupling is weakened below the transition. At high magnetic fields, the local induction shows an unexpectedly large jump at Tp whose sign changes with position inside the sample. Our results demonstrate a novel redistribution of vortices whose energy is reduced abruptly below the first-order transition at Tp.

The electronic structure near the Fermi level (E F ) of the β-pyrochlore superconductor KOs 2 O 6 is studied using laser-excited ultrahigh-resolution photoemission spectroscopy. The superconducting gap clearly opens across the superconducting transition (T c = 9.6 K), with the strong electron-phonon coupling value of 2Δ(0)/k B Τ c ≥ 4.56. A fitting analysis identifies clear anomalies at T P = 7.5 K in the temperature dependencies of the superconducting gap size and the quasiparticle relaxation lifetime. These anomalies and the fine spectral structures arising from phonons suggest that the existence of the rattling behavior of K ions significantly affects the superconductivity in KOs 2 O 6 .

Superconducting and normal-state properties of the $\ensuremath{\beta}$-pyrochlore oxide $\mathrm{K}{\mathrm{Os}}_{2}{\mathrm{O}}_{6}$ are studied by means of thermodynamic and transport measurements. It is shown that the superconductivity is of conventional $s$-wave type and lies in the extremely strong-coupling regime. Specific heat and resistivity measurements reveal that there are characteristic low-energy phonons that give rise to unusual scattering of carriers due to strong electron-phonon interactions. The entity of the low-energy phonons is ascribed to the heavy rattling of the K ion confined in an oversized cage made of $\mathrm{Os}{\mathrm{O}}_{6}$ octahedra. It is suggested that this electron-rattler coupling mediates the Cooper pairing, resulting in the extremely strong-coupling superconductivity.

Microwave penetration depth $\ensuremath{\lambda}$ and surface resistance at 27 GHz are measured in high quality crystals of ${\mathrm{KOs}}_{2}{\mathrm{O}}_{6}$. Firm evidence for fully gapped superconductivity is provided from $\ensuremath{\lambda}(T)$. Below the second transition at ${T}_{p}\ensuremath{\sim}8\text{ }\text{ }\mathrm{K}$, the superfluid density shows a steplike change with a suppression of effective critical temperature ${T}_{c}$. Concurrently, the extracted quasiparticle scattering time shows a steep enhancement, indicating a strong coupling between the anomalous rattling motion of K ions and quasiparticles. The results imply that the rattling phonons help to enhance superconductivity, and that K sites freeze to an ordered state with long quasiparticle mean free path below ${T}_{p}$.

We report on measurements of the temperature dependence of the magnetic penetration depth down to 0.04 K in a high-quality sample of the $\ensuremath{\beta}$-pyrochlore ${\mathrm{KOs}}_{2}{\mathrm{O}}_{6}$ (${T}_{c}=9.65\text{ }\text{ }\mathrm{K}$) with a spin-frustrated lattice. We observe temperature-independent behavior below $T\ensuremath{\simeq}0.3{T}_{c}$, which is firm evidence for the presence of an isotropic superconducting gap in this material. In the whole temperature range the superfluid density is very well described, without the need of adjustable parameters, by a strong-coupling extension of the BCS model for an isotropic gap. Thus, the penetration depth results indicate that ${\mathrm{KOs}}_{2}{\mathrm{O}}_{6}$ is a strong-coupling superconductor with a fully developed energy gap. No effect of the second phase transition taking place at ${T}_{p}=7.5\text{ }\text{ }\mathrm{K}$ was observed on the penetration depth, which suggests that the Cooper pairs remain unperturbed across this transition.

Chemical trends of fundamental superconducting parameters and normal-state properties are described for a family of pyrochlore oxide superconductors. Particularly, the change of Tc from 1.0 K for alpha-pyrochlore Cd2Re2O7 to 3.3 K (A = Cs), 6.3 K (Rb), and 9.6 K (K) for beta-pyrochlore AOs2O6 is discussed on the basis of the conventional BCS scheme. Enhanced Tc and anomalous features observed for KOs2O6 are ascribed to low-energy phonons probably coming from the rattling of the K cations., 8 pages, to be published in the Proceedings of M2S-HTSC2006 (Physica C)

Resistivity and specific heat have been measured on a single crystalline sample of the beta-pyrochlore oxide superconductor, KOs2O6. It is found that a second peak in specific heat, which may evidence an unknown phase transition, appears around Tp ~ 7.5 K below the superconducting transition temperature Tc = 9.53 K. Applying magnetic fields up to 14 T, Tc is reduced gradually down to 7.1 K, while Tp is raised a little and becomes even higher than Tc at 14 T, which implies that the second anomaly is not associated directly with the superconductivity. It is demonstrated, however, that there is significant communication between the two anomalies, suggesting that they come from the same electrons. It is also reported that the Sommerfeld coefficient ? in KOs2O6 is possibly much larger than in other members of beta-pyrochlore oxide superconductors, RbOs2O6 (Tc = 6.3 K) and CsOs2O6 (Tc = 3.3 K)., Comment: This paper has been withdrawn by the author due to a crucial error in the magnitude of magnetic fields used in the experiments

We report the X-ray study for the pyrochlore oxide Cd2Re2O7. Two symmetry-lowering structural transitions were observed at Ts1=200K and Ts2=120K. The former is of the second order from the ideal cubic pyrochlore structure with space group Fd-3m to a tetragonally distorted structure with I-4m2, while the latter is of the first order likely to another tetragonal space group I4122. We discuss the feature of the lattice deformation., Comment: 4 pages, 4 figures

We report the first pyrochlore oxide superconductor ${\mathrm{Cd}}_{2}{\mathrm{Re}}_{2}{\mathrm{O}}_{7}$. Resistivity, magnetic susceptibility, and specific heat measurements on single crystals evidence a bulk superconductivity at 1 K. Another phase transition found at 200 K suggests that a peculiar electronic structure lies behind the superconductivity.

High-pressure resistivity measurements are performed on a high-quality single crystal of the β -pyrochlore oxide KOs2O6 in the pressure range of 1.0 to 5.0 GPa. The superconducting transition temperature T c increases slightly from 9.6 K at ambient pressure to 9.8 K at 1.0 GPa, decreases gradually with increasing pressure, and suddenly drops from 6.5 K to 3.2 K across P s = 3.6 GPa . The drop of T c at P s is likely to be related to a change in the rattling vibration associated with a symmetry-breaking structural transition.

We report the results of the low temperature specific heat of the β -pyrochlore oxide superconductors RbOs 2 O 6 and KOs 2 O 6 under high pressure. The superconducting transition temperature T c of RbOs 2 O 6 monotonically increases with increasing pressure and then abruptly vanishes at 6.0 GPa. Similar abrupt vanishment is also observed in KOs 2 O 6 at 5.2 GPa. Under high pressure above 5.8 GPa, other two anomalies are found instead of that of the superconductivity for RbOs 2 O 6 . It is suggested that the abrupt destruction of the superconductivity and the appearance of anomalies under high pressure are important to understand the origin of the superconductivity in this system.

Complex conductivity is investigated at 19 GHz and 44 GHz in the normal state in β -pyrochlore materials, AOs 2 O 6 (A = Cs, Rb and K). In Cs material, large enhancement of the quasiparticle (QP) scattering time, τ , is observed at low temperatures, whereas there is no such enhancement in Rb and K materials. This indicates that rattling motion is absent in Cs material, whether in K and Rb materials it plays the role as a scatterer for QPs. In Rb materials, we find charge excitation possibly originated from the rattling motion.

Vibrations of alkaline guest atom A in β -pyrochlore oxides AOs2O6 (A = K, Rb, and Cs) are analyzed by a simple interaction model, where interatomic force constants between A atoms k ( AA ) and between A atom and cage k ( AC ) are employed. The obtained coefficients show that A atoms vibrate in highly anharmonic potential with strong interactions between A atoms. Especially for K atoms in KOs2O6, its potential can be approximately described only by fourth-order term.

We develop a physical understanding for how anhamonicity of lattice vibration affects transition temperature, \(T_{\text{c}}\), of superconductivity mediated by phonons, via specific heat measurements on β-pyrochlore oxide superconductors under high pressure and in magnetic fields. We find that \(T_{\text{c}}\) demonstrates unimodal dependence on pressure, where \(T_{\text{c}}\) increases more than three times by pressure. On the other hand, an electron–phonon coupling constant, λ, is monotonically enhanced as pressure is applied, leading to the enhancement of effective mass of conduction electrons. From the analysis based on the McMillan equation, moreover, an average phonon frequency, ωlog, is found to reflect a downward trend on all the pressure regions, which is consistent with the results of the previous inelastic neutron scattering and specific heat measurements. The downward trend of ωlog means that the application of pressure enhances a magnitude of anhamonicity, contrary to a common sense of pres...

The 5d transition metal oxide Cd2Os2O7 having a pyrochlore lattice of Os atoms exhibits a metal-insulator transition with a magnetic ordering at 227 K, but the magnetic structure in the ordered state and the origin of the transition have not yet been specified. The commensurate magnetic reflection with the propagation vector k = (0, 0, 0) was first observed in the resonant X-ray scattering using a high-quality single crystal. We also clarified the only one kind of magnetic moment via the synchrotron radiation-based Mossbauer effect. We propose that a strong spin-orbit interaction is notably apparent in the spin arrangement and the insulating mechanism in this compound.

In this review, we present a summary of experimental and theoretical investigations on the rattling and superconductivity of the β-pyrochlore osmium oxides AOs 2 O 6 (A = Cs, Rb, and K), after these oxides were found in 2004. First, we provide the background of our research, a brief history of rattling, and the characteristics of cage compounds thus far studied in terms of rattling. Then we show the structural properties of β-pyrochlore oxides, focusing on the rattling of an A ion in an oversized cage made of oxide ions. After the chemical trends of the normal and superconducting states are summarized, the role of rattling, which is a local, essentially anharmonic oscillation with a large atomic excursion, is discussed. Furthermore, two recent topics, i.e., an isomorphic structural transition and peculiar behavior under high pressure, are addressed. We conclude that rattling vibrations markedly affect the structural and electronic properties of β-pyrochlore oxides and even induce or enhance superconductiv...

We prepared a series of high-quality single crystals of the filled skutterudite R Os 4 Sb 12 ( R = La, Ce, Pr, Nd, Sm) and performed precise X-ray structural analyses to investigate the relation between the crystal structure and rattling vibration of the guest R atoms. The results revealed that in a cage made of Sb, with increasing guest free space from La to Sm, the thermal displacement parameter of the R ion increases and the Einstein temperature, which represents the characteristic energy of the rattling, decreases. Anisotropic distribution of the electron density was observed, in particular for small R ions, which indicates that there is large anharmonicity along the [111] direction that reflects the specific shape of the Sb cage.

We report the results of the low-temperature specific heat measurements on the β-pyrochlore oxide superconductor RbOs 2 O 6 ( T c =6.3 K) in magnetic field under high pressure. We investigated the upper critical field H c2 at several pressures and found that the slope of H c2 at T c monotonically increases with increasing pressure in low-pressure superconducting phase SC1, indicating that the effective mass m * is enhanced by the pressure. Moreover, the jump of the specific heat at T c is also enhanced by the application of the pressure in SC1, suggesting that the pressure gives rise to the enhancement of the electron–phonon coupling and consequently enhances T c .

We report the result of the specific heat and the powder X-ray diffraction measurements on the single crystalline β-pyrochlore oxide superconductors A Os 2 O 6 under high pressure. We found from the specific heat measurements that the superconducting phase in the low pressure region (SC1) suddenly vanishes at the critical pressure P * and an alternative superconducting phase (SC2) with lower T c compared to that of SC1 appears above P * . In addition, we found from the X-ray diffraction measurements that a first-order phase transition of the crystal structure takes place at ∼ P * , accompanied with reduction of the crystal symmetry. This result suggests that the sudden suppression of T c arises from the structural phase transition.