Session J11: Focus Session: MgB2-like: Novel Non-Boride Superconductors

Muon spin rotation study of perturbation of the crystalline-electric-field induced by interstitial muon in PrOs$_4$Sb$_{12}$ and PrRu$_{4}$Sb$_{12}$

Muon spin rotation measurements of the temperature dependence of the positive muon Knight shift in single crystals of PrOs$\rm_{4}$Sb$\rm_{12}$ and PrRu$\rm_{4}$Sb$\rm_{12}$ reveal a linear scaling of the Knight shift with the bulk magnetic susceptibility at high temperatures. A small deviation from the linear relation appears in PrOs$\rm_{4}$Sb$\rm_{12}$ below $6.3$ K. However, a large magnitude of deviation is observed in PrRu$\rm_{4}$Sb$\rm_{12}$ below $32$ K. The deviation can be explained by the positive muon induced modification of the susceptibility of neighboring Pr$^{3+}$ ions due to a change of the crystalline-electric-field (CEF) splitting. The data indicate that this modification is much smaller in PrOs$\rm_{4}$Sb$\rm_{12}$ than in PrRu$\rm_{4}$Sb$\rm_{12}$. A model calculation based on CEF theory is in progress.   

Interplay of superconductivity and rattling phenomena in $\beta $-pyrochlore KOs$_{2}$O$_{6}$ studied by photoemission spectroscopy

The electronic structure near Fermi level of KOs$_{2}$O$_{6}$ is studied by a laser-excited photoemission spectroscopy. The superconducting(SC) gap clearly opens across the SC transition at 9.6 K, with the strong electron-phonon coupling value of 2$\Delta $(0)/k$_{B}$T$_{c} \quad \ge $ 4.56. Fitting analysis identifies clear anomalies at 7.5 K in the temperature dependences of the SC 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$_{6}$.   

Controlling physical parameters of layer-structured nitride-halide superconductors

Metal-intercalation into band insulators sometimes affords superconductors, well-known examples of which are carbon-based materials, such as fullerides and graphite. Layer-structured nitride-halide Li$_x$ZrNCl and Li$_xM_y$HfNCl ($M$ denotes molecule) belong to another class of intercalation-induced superconductors with relatively high $T_c$, in which doping level and interlayer distance (and hence interlayer hopping interaction) can independently be controlled by changing Li concentration and the size of the co- intercalated molecule. The controllability provides a unique and interesting opportunity to investigate the effect of the two important physical parameters on $T_c$ in a single system. Recent progress in the synthesis technique enabled us to obtain for the first time a series of single-phase samples of Li$_x$ZrNCl with finely controlled doping-levels which were notoriously difficult to prepare. Using these samples, we have established[1] an electronic phase diagram to find anomalous doping evolution of $T_c$, which takes a maximum value on the verge of superconductor- insulator transition. Based on this phase diagram and the results of systematic Raman scattering and transport measurements, we will discuss possible roles in producing relatively high $T_c$ played by charge fluctuation and reduced disorder scattering in the layered structure reminiscent of modulation-doped semiconductors. We will also briefly refer to our very recent results on the Hf-based materials in which both of the doping level and interlayer distance were varied. \newline [1] Y. Taguchi {\it et al.}, Phys. Rev. Lett. {\bf 97}, 107001 (2006)   

Independent control of carrier concentration and interlayer spacing in Li$_x$HfNCl layered superconductors

Alkali-metal and organic molecule co-intercalated HfNCl is a new class of layered superconductors with relatively high transition temperature ($T_c$) of 25.5 K. Recently, we have succeeded in synthesis of single phase samples of Li$_x$(molecule)$_y$HfNCl with a wide range of doping concentration of 0.10 $\leq$ $x$ $\leq$ 0.50, where we are able to, continuously and independently, control the carrier density and interlayer distance \textit{d} between the conducting Hf-N layers by means of co-intercalation of Li and organic molecule. Without any molecule, superconductivity appears at $x$ $\sim$ 0.15 and $T_c$ is almost constant against $x$ above this critical value. Furthermore, we found that $T_c$ is enhanced ($\sim$30$\%$) from 20 K to 25.5 K with increasing of \textit{d}.   

Unconventional superconductivity in single crystal Lu$_{2}$Fe$_{3}$Si$_{5}$

Dynamic magnetic susceptibility for a single crystal of the ternary superconductor Lu$_{2}$Fe$_{3}$Si$_{5}$ has been measured using a tunnel diode resonator (TDR) technique. The London penetration depth exhibits non-exponential temperature dependence. We analyze the obtained superfluid density by comparing models of two-gap superconductivity, a gap with nodes or a highly anisotropic gap. The upper critical field is highly anisotropic and is unusually large. Furthermore, hysteresis in the susceptibility implies unusually strong temperature dependence of the critical current. The results are discussed in terms of possible unconventional behavior of this low-T$_{c}$ superconductor.   

Two-Gap Superconductivity in Lu$_{2}$Fe$_{3}$Si$_{5}$

Lu$_{2}$Fe$_{3}$Si$_{5}$ is a superconductor with $T_{c}\sim $ 6 K containing nonmagnetic irons. Anomalous temperature dependence of specific heat in the superconducting state has been reported in polycrystalline samples; reduced specific heat jump at $T_{c}$ and apparent residual $T$-linear term in the limit of $T$=0 K. We have successfully grown high-quality single crystals of Lu$_{2}$Fe$_{3}$Si$_{5}$ using the floating-zone technique, and characterized its superconducting and normal state properties. The anomalies of the specific heat reported in polycrystalline samples are reproduced in the single crystals. In addition, we find a second drop of the specific heat below 1 K. We can fit the temperature dependence of the specific heat by assuming two superconducting gaps as in the case of MgB$_{2}$. Temperature dependence of Hall coefficient is nonmonotonic, and also suggests the presence of multiple bands in this compound.   

Point-contact Andreev reflection tunneling spectroscopy (PCARTS) of the superconducting gap structure in LuNi$_{2}$B$_{2}$C

The PCARTS technique is employed to investigate the gap anisotropy and proposed existence of point-nodes in LuNi$_{2}$B$_{2}$C (T$_{C}$ $\sim$ 16 K). Differential conductance spectra are taken from two different sets of single crystal samples along three major orientations: [001], [110], and [100]. Analyzing using the single-gap Blonder-Tinkham-Klapwijk (BTK) model reproducibly shows the gaps along these directions are 2.4, 2.6, and 2.3 meV, respectively, for one set of samples and 2.4, 2.8, and 2.7 meV, respectively, for the other set. This is smaller than the gap anisotropy reported by other groups[1]. At low temperatures, the single-gap BTK model does not satisfactorily fit our data. Models employing an anisotropic gap are being investigated, as are experiments parameterizing the tunneling cone effect.\newline [1] Y.G. Naidyuk, et al,condmat/0609769(2006); N.L.Bobrov, et al,PRB 71, 014512 (2005); S. Mukhopadhyay, et al,PRB 72, 014545 (2005).   

Anisotropic properties of aligned weak-ferromagnetic superconductor RuSr$_2$GdCu$_{2}$O$_{8}$

The $\it{ab}$-plane aligned powder in epoxy matrix for the tetragonal RuSr$_{2}$GdCu$_{2}$O$_{8}$ weak-ferromagnetic superconductor was achieved using a field powder alignment method with $\it{ab}$-plane parallel to the applied magnetic field. The $\it{c}$-axis aligned powder can also be obtained using the field-rotation method where $\it{c}$-axis is perpendicular to the applied magnetic field and along the rotation axis. The temperature dependence of magnetic moment m (T) for the aligned powder provides the desired anisotropic properties where larger magnetic moment along the $\it{ab}$- plane was observed. The field-cooled (FC) and zero-field-cooled (ZFC) data in low applied field (1 G) for both directions indicate a weak-ferromagnetic (canted-antiferromagnetic) transition of Ru moment at T$_N$(Ru) = 131 K and a superconducting transition in the CuO$_{2}$ plane at T$_{c}$ = 39 K. The low temperature antiferromagnetic ordering of the rare earth Gd moment is observed at T$_{N}$(Gd) = 2.5 K. Diamagnetic superconducting shielding signal is much weaker than bulk sample due to small powder diameter (1-10 $\mu$m), long penetration depth $\lambda$ and the two-dimensional (2D) character of CuO$_{2}$ plane. Low temperature, low field magnetization data m(B$_{a}$,T) will be discussed.   

Order parameter suppression and structure of the surface states in non-centrosymmetric superconductors

We consider the structure of the surface states at the pairbreaking boundaries of non-centrosymmetric superconductors. In the region of the order parameter suppression multiple Andreev reflections significantly modify the energy and the intragap density of states due to bound states. We elucidate the physics behind this modification by considering a simple model of gap suppression, and comparing it with a fully self- consistent microscopic calculation. We emphasize the experimentally relevant consequences of the lack of inversion symmetry for the surface states. As the discontinuity in the spin-orbit coupling at the boundary makes the interface spin-active, we analyse the resulting spin structure of the bound states.   

Superconductivity in Sn(1-x-d)In(x)Te and Pb(1-y)Tl(y)Te

Recent evidence for a charge-Kondo effect in superconducting samples of Pb$_{1-y}$Tl$_y$Te [PRL \textbf{94}, 157002 (2005)] raises the possibility that systems of degenerate semiconductors doped with valence skipping elements may be an ideal realm in which to study suggested negative U electronic pairing mechanisms in superconductors [PRL \textbf{61}, 2713 (1988)]. However, questions of exactly how the proposed charge-Kondo behavior relates to superconductivity in Pb$_{1-y}$Tl$_y$Te remain. In this work, we study the relationship between the DOS at the Fermi level and Tc in the related superconductor Sn$_{1-x-d}$In$_x$Te, where enhancement of Tc is found above a critical concentration $x_c > d/2$, where the Fermi level is pinned in the impurity band and the DOS is enhanced [Sov. Phys. Solid State \textbf{28}, 612 (1986)]. We find that the elevated DOS is insufficient to explain the enhanced Tc for these samples, suggesting an additional pairing mechanism is involved, such as the negative U mechanism mentioned above.   

Novel superconductivity in a new noncentrosymmetric superconductor

Low temperature specific, resistivity and magnetization are measured in a newly fabricated superconductor. It is found that this material has no central inversion symmetry. Specific heat measurement show that the major part of the system has a s-wave symmetry and the superconducting gap is thus derived. However, when the superconductivity is suppressed by the magnetic field, a further drop of specific heat coefficient is observed just at T$_{c}$. This unexpected behavior remains to a very high magnetic field and without any obvious shift of the transition temperature. It is tempting to argue that this drop of specific heat coefficient may be induced by the spin triplet pairs.   

Superconductivity in the new Platinum Germanides APt$_4$Ge$_{12}$ (A=Sr,Ba,La,Pr)

New germanium-platinum compounds with the filled-skutterudite crystal structure were synthesized. Magnetic susceptibility, specific heat, and electrical resistivity measurements find superconductivity in LaPt$_4$Ge$_{12}$ and PrPt$_4$Ge$_{12}$ below ca.\ 8\,K. The parameters of the normal and superconducting states were established. Strong electron-phonon coupling and a crystal electric field singlet groundstate is found for the Pr compound. Electronic structure calculations show a large density of states at the Fermi level, predominantly due to Ge $4p$ orbitals. Similar behavior, albeit with lower $T_c$, was observed for SrPt$_4$Ge$_{12}$ and BaPt$_4$Ge$_{12}$.