Uemura Line Research Articles

Superconducting fluctuation effect in CaFe0.88Co0.12AsF

Out-of-plane angular dependent torque measurements were performed on CaFe0.88Co0.12AsF single crystals. Superconducting fluctuations, featured by magnetic field enhanced and exponential temperature dependent diamagnetism, are observed above the superconducting transition temperature Tc, which is similar to that of cuprate superconductors, but less pronounced. In addition, the ratio of Tc versus superfluid density follows well the Uemura line of high-Tc cuprates, which suggests the exotic nature of the superconductivity in CaFe0.88Co0.12AsF.

Muon spin rotation investigation of the pressure effect on the magnetic penetration depth in YBa2Cu3Ox

The pressure dependence of the magnetic penetration depth $\ensuremath{\lambda}$ in polycrystalline samples of YBa${}_{2}$Cu${}_{3}$O${}_{x}$ with different oxygen concentrations $x=6.45$, 6.6, 6.8, and 6.98 was studied by muon spin rotation ($\ensuremath{\mu}$SR). The pressure dependence of the superfluid density ${\ensuremath{\rho}}_{s}\ensuremath{\propto}1/{\ensuremath{\lambda}}^{2}$ as a function of the superconducting transition temperature ${T}_{\mathrm{c}}$ is found to deviate from the usual Uemura line. The ratio $(\ensuremath{\partial}{T}_{\mathrm{c}}/\ensuremath{\partial}P)/(\ensuremath{\partial}{\ensuremath{\rho}}_{s}/\ensuremath{\partial}P)$ is smaller by a factor of $\ensuremath{\simeq}$2 than that of the Uemura relation. In underdoped samples, the zero-temperature superconducting gap ${\ensuremath{\Delta}}_{0}$ and the BCS ratio ${\ensuremath{\Delta}}_{0}/{k}_{B}{T}_{\mathrm{c}}$ both increase with increasing external hydrostatic pressure, implying an increase of the coupling strength with pressure. The relation between the pressure effect and the oxygen isotope effect on $\ensuremath{\lambda}$ is also discussed. In order to analyze reliably the $\ensuremath{\mu}$SR spectra of samples with strong magnetic moments in a pressure cell, a special model was developed and applied.

Field and Temperature Dependence of the Superfluid Density inLaFeAsO1−xFxSuperconductors: A Muon Spin Relaxation Study

We present zero field and transverse field muon spin relaxation experiments on the recently discovered Fe-based superconductor LaFeAsO1-xFx (x=0.075 and x=0.1). The temperature dependence of the deduced superfluid density is consistent with a BCS s-wave or a dirty d-wave gap function, while the field dependence strongly evidences unconventional superconductivity. We obtain the in-plane penetration depth of lambda ab(0)=254(2) nm for x=0.1 and lambda ab(0)=364(8) nm for x=0.075. Further evidence for unconventional superconductivity is provided by the ratio of Tc versus the superfluid density, which is close to the Uemura line of high-Tc cuprates.

Muon-Spin-Rotation Measurements of the Penetration Depth of the Infinite-Layer Electron-DopedSr0.9La0.1CuO2Cuprate Superconductor

Muon-spin-rotation (muSR) measurements of the in-plane penetration depth lambda(ab) have been performed in the infinite-layer electron-doped Sr0.9La0.1CuO2 high-T(c) superconductor (HTS). Absence of the magnetic rare-earth ions in this compound allowed us to measure for the first time the absolute value of lambda(ab)(0) in electron-doped HTSs using muSR. We found lambda(ab)(0)=116(2) nm. The zero-temperature depolarization rate sigma(0) proportional, variant 1/lambda(2)(ab)(0)=4.6(1) micros(-1) is more than 4 times higher than expected from the Uemura line. Therefore, this electron-doped HTS does not follow the Uemura relation found for hole-doped HTSs.

μSR study of charge neutrally doped Nd 1−2 xCa xTh xBa 2Cu 3O 7− δ

Equal amounts of Ca and Th were doped into the Nd 1−2 x Ca x Th x Ba 2Cu 3O 7− δ superconductor with 0≤ x≤0.10. X-ray diffraction showed a single orthorhombic phase for all samples except at x=0.10, suggesting a solubility limit below x=0.10. For increasing doping concentration x, the superconducting T c and the electrical resistivity, ρ at 275 K both vary strongly, with a linear decrease of T c and an increase of ρ. The muon spin relaxation rate, σ, follows the Uemura line for large x, and deviates in the direction of increasing σ-values for decreasing x. The approach of the data to the Uemura line was compared between different alloy systems by plotting the normalized distance to the Uemura line as a function of doping concentration. Ca–Th doped samples were found to approach the Uemura line about three times faster than for Ca–Pr doping. The same replacements in Y-123 confirm this result. The strong doping concentration dependence of σ in Ca–Th substituted samples is explained in terms of electronic disorder effects.

Muon spin relaxation in Re-substitutedHgA2Can−1CunO2n+2+x(A=Sr,Ba;n=2,3)superconductors

The muon relaxation rate \ensuremath{\sigma} for the Re-substituted ${\mathrm{HgSr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{6+y}$ and ${\mathrm{HgBa}}_{2}{\mathrm{Ca}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{8+y}$ superconductors has been measured as a function of temperature. The low-temperature values of \ensuremath{\sigma} fall close to the universal Uemura line ${T}_{c}(\ensuremath{\sigma}),$ as observed for other layered superconductors; therefore our data show no evidence of a reduced anisotropy, i.e., interlayer metallization, induced by Re. The in-plane penetration depth values obtained for these 1212 and 1223 compounds are found to be significantly different $(\ensuremath{\lambda}=1665\AA{}$ and $\ensuremath{\lambda}=1310\AA{},$ respectively). The roles of the inserted ${\mathrm{CuO}}_{2}$ planes, the Ba/Sr substitution and the character of the spacing layers are considered to discuss the observed features.

Ab-plane optical properties of Ni-doped Bi 2Sr 2CaCu 2O 8+ δ

The ab-plane optical reflectance of single crystal of Bi 2Sr 2Ca(Cu 1− x Ni x ) 2O 8+ δ , for x=0.0215, has been measured from 80 to 40 000 cm −1 (10 meV–5 eV) and at temperatures between 20 and 300 K. Through Ni substitution, the optical conductivity shows a broad peak at about 400 cm −1, which grows in intensity and shifts to lower frequency as the temperature is reduced. We suggest that this peculiar low-frequency feature is associated with the significant disorder induced by Ni impurities. Analysis of the conductivity within the framework of the two-component and the one-component pictures indicates that the main effect of Ni substitution is to increase impurity scattering rate. There is no evidence of a rapid drop in the free carrier scattering rate near T c. An estimate of the low-frequency spectral weight shows that Ni reduces the carrier density in the CuO 2 plane: each Ni removes one carrier. Below T c, a sum-rule evaluation finds the superconducting condensate contains about a fifth of the total doping-induced, or two thirds of the free-carrier oscillator strength in the normal state. The strength of the condensate is horizontally offset from the Uemura line, with a smaller superfluid density than implied by T c. This decreased superfluid density in the CuO 2 plane could be connected with the effect of impurity-induced disorder.

OPTICAL CONDUCTIVITY OF Nd 1.85Ce 0.15CuO 4: STRENGTH OF THE CONDENSATE

The ab-plane reflectance of a Nd 1.85Ce 0.15CuO 4 single crystal ( T c =23 K) has been measured from ≈30 to 9500 cm −1 above and below T c , and the optical properties calculated from a Kramers–Kronig analysis. A rich phonon spectrum is observed with some new spectral features appearing below ≈100 K. The normal-state optical conductivity may be described by a Drude-like component and an overdamped mid infrared component. Below T c the plasma frequency of the condensate is ω pS ≈10 000 cm −1. The London penetration depth is determined to be λ ab =1600±100 Å, which places it well off the Uemura line. Estimates of the electron–phonon coupling from normal-state transport measurements yield λ tr <0.5. The small values for the penetration depth and the electron–phonon coupling constant suggest that the superconductivity in this material is not due to the electron–phonon mechanism, and is different to that in other hole-doped superconducting cuprates.

Optical properties ofNd1.85Ce0.15CuO4

The ab-plane reflectance of a Nd{sub 1.85}Ce{sub 0.15}CuO{sub 4} single crystal (T{sub c}=23 K) has been measured from {approx}35 to 9500 cm{sup {minus}1} at temperatures above and below T{sub c}, and the optical properties calculated from a Kramers-Kronig analysis. A rich phonon spectrum is observed, and there are a number of c-axis infrared and Raman modes that are observed at low temperature which are believed to be activated by disorder; several of these modes show evidence for electron-phonon coupling. The normal-state optical conductivity may be described by a Drude-like component and an overdamped midinfrared component. The Drude-like component narrows rapidly; at 30 K the mean free path is estimated to be {approx}750 {Angstrom}. Below T{sub c} the Drude carriers collapse into the condensate; the plasma frequency of the {delta} function is determined to be {omega}{sub pS}{approx}10000 cm{sup {minus}1}. The small coherence length ({xi}{sub 0}{approx}70 {Angstrom}) places this material into the clean limit ({xi}{sub 0}/l{lt}1). The London penetration depth is determined to be {lambda}{sub ab}=1600{plus_minus}100 {Angstrom}, which places it well off the Uemura line. Estimates of the electron-phonon coupling from normal-state transport measurements of {lambda}{sub tr}{lt}0.5, and the absence of Holstein sidebands below T{sub c}, indicate that the carriers that participatemore » in superconductivity are weakly coupled to the phonons. The small values for the penetration depth and the electron-phonon coupling constant suggest that the superconductivity in this material is not due to the electron-phonon mechanism, and is different than in other hole-doped superconducting cuprates. {copyright} {ital 1997} {ital The American Physical Society}« less