Yohei Uemura

The muon-spin-relaxation rate $\ensuremath{\sigma}$ has been measured in sixteen specimens of high-${T}_{c}$ cuprate superconductors (the 2:1:4, 1:2:3, 2:2:1:2, and 2:2:2:3 series). This has allowed us to study the magnetic field penetration depth $\ensuremath{\lambda}$ and thus the superconducting carrier density ${n}_{s}$ divided by the effective mass ${m}^{*}(\ensuremath{\sigma}\ensuremath{\propto}\frac{1}{{\ensuremath{\lambda}}^{2}}\ensuremath{\propto}\frac{{n}_{s}}{{m}^{*}})$. A universal linear relation between ${T}_{c}$ and $\ensuremath{\sigma}(T\ensuremath{\rightarrow}0)\ensuremath{\propto}\frac{{n}_{s}}{{m}^{*}}$ has been found with increasing carrier doping. In heavily doped samples, however, ${T}_{c}$ shows saturation and suppression with increasing $\frac{{n}_{s}}{{m}^{*}}$. This saturation starts at different values of $\frac{{n}_{s}}{{m}^{*}}$ for materials with different multiplicities of CuO planes.

The muon-spin relaxation rate $\ensuremath{\sigma}$ has been measured in the high-${T}_{c}$ superconductors $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{x}$ for $x=6.66, 6.95, 7.0$, and ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$Cu${\mathrm{O}}_{4}$ in transverse external magnetic fields \ensuremath{\sim}1-4 kG. We find a simple relation which connects the transition temperature ${T}_{c}$, the magnetic-field penetration depth ${\ensuremath{\lambda}}_{L}$, the carrier concentration ${n}_{s}$, and the effective mass ${m}^{*}$ as ${T}_{c}\ensuremath{\propto}\ensuremath{\sigma}\ensuremath{\propto}\frac{1}{{\ensuremath{\lambda}}_{L}^{2}}\ensuremath{\propto}\frac{{n}_{s}}{{m}^{*}}$. The linear dependence ${T}_{c}\ensuremath{\propto}\frac{{n}_{s}}{{m}^{*}}$ suggests a high-energy scale for the coupling between superconducting carriers.

Paramagnetic spin fluctuations in the weak itinerant-electron ferromagnet MnSi have been measured by using both unpolarized and polarized neutron scattering, and the scattering intensity was put on an absolute value by the simultaneous measurements of phonon cross sections. The magnetic excitation spectra over the whole Brillouin zone, for a wide energy range from 0 to 20 meV (\ensuremath{\Elzxh}\ensuremath{\omega}/${\mathrm{kT}}_{c}$=7.9) and up to temperatures (T/${T}_{c}$=10) well above ${T}_{c}$, follow closely the Moriya-Kawabata self-consistent-renormalization theory for the weak itinerant-electron ferromagnet. Based on these experimental results, the origin of the increase in the amplitude of the local magnetic moment 〈${M}_{L}^{2}$〉 above ${T}_{c}$ is attributed to the thermal excitations of the high-energy spin fluctuations in the Stoner continuum.

The magnetic-field penetration depth \ensuremath{\lambda} in the organic superconductors \ensuremath{\kappa}-(BEDT-TTF${)}_{2}$ Cu(NCS${)}_{2}$ and \ensuremath{\kappa}-(BEDT-TTF${)}_{2}$ Cu[N(CN${)}_{2}$]Br has been measured over a wide temperature region 20 mK\ensuremath{\le}T\ensuremath{\le}15 K via the muon-spin-relaxation technique. Linear variation of \ensuremath{\lambda} with T at low temperatures, found in both systems, is consistent with anisotropic superconducting pairings with line nodes in the energy gap. In the latter compound, we observe flux depinning at T\ensuremath{\sim}5 K, well below ${\mathit{T}}_{\mathit{c}}$=12 K.

By muon spin-relaxation measurements on single-crystal specimens, we show that superconductivity in the $A{\text{Fe}}_{2}{\text{As}}_{2}$ $(A=\text{Ca},\text{Ba},\text{Sr})$ systems, in both the cases of composition and pressure tunings, coexists with a strong static magnetic order in a partial volume fraction. The superfluid response from the remaining paramagnetic volume fraction of $({\text{Ba}}_{0.5}{\text{K}}_{0.5}){\text{Fe}}_{2}{\text{As}}_{2}$ exhibits a nearly linear variation in $T$ at low temperatures, suggesting an anisotropic energy gap with line nodes and/or multigap effects.