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تسجيل مستخدم جديدLarge mass enhancement in RbOs2O6
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Heat capacity measurements on the recently discovered geometrically frustrated beta-pyrochlore superconductor RbOs2O6 (Tc=6.4 K) yield a Sommerfeld coefficient of 44 mJ/(molf.u. K^2). This is about 4 times larger than the one found in band structure calculations. In order to specify the enhancement due to electron-electron interactions, we have measured the electron-phonon enhancement. By a suitable analysis, an electron-phonon coupling constant lambda_ep = 1 +/- 0.1 is derived from the specific heat jump at Tc. This leaves a significant additional lambda_add = 2.1 +/- 0.3 for enhancement due to other mechanisms, possibly related to the triangular lattice. To arrive at these results, an appropriate analysis method for bulk thermodynamic data based on the condensation energy was applied.
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We have determined the Fermi surface in KOs$_2$O$_6$ ($T_c$ = 9.6 K and $B_{c2} sim$ 32 T) via de Haas-van Alphen (dHvA) oscillation measurements and a band structure calculation. We find effective masses up to 26(1) $m_e$ ($m_e$ is the free electron
mass), which are unusually heavy for compounds where the mass enhancement is mostly due to electron-phonon interactions. Orbit-resolved mass enhancement parameters $lambda_{dHvA}$ are large but fairly homogeneous, concentrated in the range 5 -- 8. We discuss origins of the large homogeneous mass enhancement in terms of rattling motion of the K ions.
Single crystals of RbOs2O6 have been grown from Rb2O and Os in sealed quartz ampoules. The crystal structure has been identified at room temperature as cubic with the lattice constant a = 10.1242(12) A. The anisotropy of the tetrahedral and octahedra
l networks is lower and the displacement parameters of alkali metal atoms are smaller than for KOs2O6, so the rattling of the alkali atoms in RbOs2O6 is less pronounced. Superconducting properties of RbOs2O6 in the mixed state have been well described within the London approach and the Ginzburg-Landau parameter kappa(0) = 31 has been derived from the reversible magnetization. This parameter is field dependent and changes at low temperatures from kappa = 22 (low fields) to kappa = 31 at H_{c2}. The thermodynamic critical field H_{c}(0) = 1.3 kOe and the superconducting gap 2delta/k_{B}T_{c} = 3.2 have been estimated. These results together with slightly different H_{c2}(T) dependence obtained for crystals and polycrystalline RbOs2O6 proof evidently that this compound is a weak-coupling BCS-type superconductor close to the dirty limit.
Two beta-pyrochlore oxide superconductors, CsOs2O6 and RbOs2O6, are studied thermodynamically by measuring specific heat on polycrystalline samples. It is found that a Sommerfeld coefficient ? is nearly equal, 20 mJ/K2 mol Os, in the two oxides with
different superconducting transition temperatures; Tc = 3.3 K and 6.3 K, respectively. This suggests that the density of states at the Fermi level is not a crucial parameter to determine the Tc of the beta-pyrochlore oxide superconductors, which is incompatible with the general expectation for a conventional BCS-type superconductor. Anomalous lattice contributions to specific heat at low temperature are also reported, which may come from nearly localized phonon modes associated with the rattling of the alkali metal ions weakly bound in an oversized cage formed by OsO6 octahedra.
To assess electron correlation and electron-phonon coupling in the recently discovered beta-pyrochlores KOs2O6 and RbOs2O6, we have performed specific heat measurements in magnetic fields up to 14 T. We present data from high quality single crystalli
ne KOs2O6, showing that KOs2O6 is a strong coupling superconductor with a coupling parameter lambda_ep approx 1.0 to 1.6 (RbOs2O6: lambda_ep approx 1). The estimated Sommerfeld coefficient of KOs2O6, gamma=76 to 110 mJ/(mol K^2), is twice that of RbOs2O6 [gamma=44 mJ/(mol K^2)]. Using strong-coupling corrections, we extract useful thermodynamic parameters of KOs2O6. Quantifying lambda_ep allows us to determine the mass enhancement over the calculated band electronic density of states. A significant contribution in addition to the electron-phonon term of lambda_c=1.7 to 4.3 is deduced. In an effort to understand the origin of the enhancement mechanism, we also investigate an unusual energetically low-lying phonon. There are three phonon modes per RbOs2O6, suggestive of the phonon source being the rattling motion of the alkali ion. This dynamic instability of the alkali ions causes large scattering of the charge carriers which shows up in an unusual temperature dependence of the electrical resistivity.
Metamagnetic transitions in the novel spin-triplet superconductor UTe$_2$ were investigated through the newly developed simultaneous measurements of magnetization and sample temperature for the field along the orthorhombic $b$-axis and close to the [
011] direction, where reentrant superconductivity (RSC) is detected below and above the first-order metamagnetic transition field $H_{rm m}$. Combining the Maxwells relation and the Clausius-Clapeyron equation, we obtained the field dependence of Sommerfeld coefficient $gamma$ through the first-order metamagnetic transition. A significant enhancement of the effective mass toward $H_{rm m}$ were detected for both field directions. On the other hand, above $H_{rm m}$, the effective mass discontinuously decreases for $H~||~b$, while it discontinuously increases for $H~||~sim [011]$, which seems to make a crucial role for the RSC.
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