arXiv subjects

Y. Shimono

Publications and source records attributed to Y. Shimono.

3 recordsLinked to original sources

arXiv2007

Effects of Rattling Phonons on the Quasiparticle Excitation and Dynamics in the Superconducting $β$-Pyrochlore KOs$_2$O$_6$

Microwave penetration depth $λ$ and surface resistance at 27 GHz are measured in high quality crystals of KOs$_2$O$_6$. Firm evidence for fully-gapped superconductivity is provided from $λ(T)$. Below the second transition at $T_{\rm p}\sim 8$ K, the superfluid density shows a step-like change with a suppression of effective critical temperature $T_{\rm c}$. 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 $T_{\rm p}$.

cond-mat.supr-con↗

arXiv2006

Uncommonly High Upper Critical Field in the Superconducting KOs$_2$O$_6$ Pyrochlore

The entire temperature dependence of the upper critical field $H_{\rm c2}$ in the $β$-pyrochlore KOs$_2$O$_6$ is obtained from high-field resistivity and magnetic measurements. Both techniques identically give $H_{\rm c2}(T \simeq 0 {\rm K})$ not only surprisingly high ($\sim 33$ T), but also the approach to it unusually temperature-\emph{linear} all the way below $T_{\rm c}$ (= 9.6 K). We show that, while $H_{\rm c2}(0)$ exceeds a simple spin-singlet paramagnetic limit $H_{\rm P}$, it is well below an $H_{\rm P}$ enhanced due to the missing spatial inversion symmetry reported recently in KOs$_2$O$_6$, ensuring that the pair-breaking here is executed by orbital degrees. {\it Ab initio} calculations of orbital $H_{\rm c2}$ show that the unusual temperature dependence is reproduced if dominant s-wave superconductivity resides on the smaller closed Fermi surfaces.

cond-mat.supr-con↗

arXiv2006

Thermal Conductivity of the Pyrochlore Superconductor KOs2O6: Strong Electron Correlations and Fully Gapped Superconductivity

To elucidate the nature of the superconducting ground state of the geometrically frustrated pyrochlore KOs2O6 (Tc=9.6K), the thermal conductivity was measured down to low temperatures (~Tc/100). We found that the quasiparticle mean free path is strikingly enhanced below a transition at Tp=7.5K, indicating enormous electron inelastic scattering in the normal state. In a magnetic field the conduction at T ->0K is nearly constant up to ~0.4Hc2, in contrast with the rapid growth expected for superconductors with an anisotropic gap. This unambiguously indicates a fully gapped superconductivity, in contrast to the previous studies. These results highlight that KOs2O6 is unique among superconductors with strong electron correlations.

cond-mat.supr-con↗