Muon-spin rotation measurements of the penetration depth of the Mo_3Sb_7 superconductor

Measurements of the magnetic field penetration depth \lambda in superconductor Mo_3Sb_7 (T_c~2.1 K) were carried out by means of muon-spin-rotation. The absolute values of \lambda, the Ginzburg-Landau parameter \kappa, the first H_{c1} and the second H_{c2} critical fields at T=0 are \lambda(0)=720(100)nm, \kappa(0)=55(9), \mu_0H_{c1}(0)=1.8(3)mT, and \mu_0H_{c2}(0)=1.9(2)T. The zero temperature value of the superconducting energy gap \Delta(0) was found to be 0.35(1)meV corresponding to the ratio 2\Delta(0)/k_BT_c=3.83(10). At low temperatures \lambda^{-2}(T) saturates and becomes constant below T~0.3T_c, in agreement with what is expected for s-wave BCS superconductors. Our results suggest that Mo_3Sb_7 is a BCS superconductor with the isotropic energy gapComment: 5 pages, 4 figure

Pressure effect on the in-plane magnetic penetration depth in YBa_2Cu_4O_8

We report a study of the pressure effect (PE) on the in-plane magnetic field penetration depth lambda_{ab} in YBa_2Cu_4O_8 by means of Meissner fraction measurements. A pronounced PE on lambda_{ab}^{-2}(0) was observed with a maximum relative shift of \Delta\lambda^{-2}_{ab}/\lambda^{-2}_{ab}= 44(3)% at a pressure of 10.2 kbar. It arises from the pressure dependence of the effective in-plane charge carrier mass and pressure induced charge carrier transfer from the CuO chains to the superconducting CuO_2 planes. The present results imply that the charge carriers in YBa_2Cu_4O_8 are coupled to the lattice.Comment: 4pages 3 figure

Comment on ``Muon-spin rotation studies of the superconducting properties of Mo_3Sb_7'', Phys.Rev.B 78, 172505 (2008)

In a recent article Tran et al. [Phys. Rev.B 78, 172505 (2008)] report on the result of the muon-spin rotation (\muSR) measurements of Mo_3Sb_7 superconductor. Based on the analysis of the temperature and the magnetic field dependence of the Gaussian relaxation rate \sigma_{sc} they suggest that Mo_3Sb_7 is the superconductor with two isotropic s-wave like gaps. An additional confirmation was obtained from the specific heat data published earlier by partly the same group of authors in [Acta Mater. 56, 5694 (2008)]. The purpose of this Comment is to point out that from the analysis made by Tran et al. the presence of two superconducting energy gaps in Mo_3Sb_7 can not be justified. The analysis of \muSR data does not account for the reduction of \sigma_{sc} with increasing temperature, and, hence, yields inaccurate information on the magnetic penetration depth. The specific heat data can be satisfactory described within the framework of the one-gap model with the small residual specific heat component. The experimental data of Tran et al., as well as our earlier published \muSR data [Phys. Rev. B 78, 014502 (2008)] all seem to be consistent with is the presence of single isotropic superconducting energy gap in Mo_3Sb_7.Comment: Comment on Phys.Rev.B 78, 172505 (2008

Pressure-induced electronic phase separation of magnetism and superconductivity in CrAs

The recent discovery of pressure induced superconductivity in the binary helimagnet CrAs has attracted much attention. How superconductivity emerges from the magnetic state and what is the mechanism of the superconducting pairing are two important issues which need to be resolved. In the present work, the suppression of magnetism and the occurrence of superconductivity in CrAs as a function of pressure ($p$) were studied by means of muon spin rotation. The magnetism remains bulk up to $p\simeq3.5$~kbar while its volume fraction gradually decreases with increasing pressure until it vanishes at $p\simeq$7~kbar. At 3.5 kbar superconductivity abruptly appears with its maximum $T_c \simeq 1.2$~K which decreases upon increasing the pressure. In the intermediate pressure region ($3.5\lesssim p\lesssim 7$~kbar) the superconducting and the magnetic volume fractions are spatially phase separated and compete for phase volume. Our results indicate that the less conductive magnetic phase provides additional carriers (doping) to the superconducting parts of the CrAs sample thus leading to an increase of the transition temperature ($T_c$) and of the superfluid density ($\rho_s$). A scaling of $\rho_s$ with $T_c^{3.2}$ as well as the phase separation between magnetism and superconductivity point to a conventional mechanism of the Cooper-pairing in CrAs.Comment: 9 pages, 8 figure

Magnetic field dependence of the oxygen isotope effect on the magnetic penetration depth in hole-doped cuprate superconductors

The magnetic field dependence of the oxygen-isotope (^{16}O/^{18}O) effect (OIE) on the in-plane magnetic field penetration depth \lambda_{ab} was studied in the hole-doped high-temperature cuprate superconductors YBa_2Cu_4O_8, Y_0.8Pr_0.2Ba_2Cu_3O_7-\delta, and Y_0.7Pr_0.3Ba_2Cu_3O_7-\delta. It was found that \lambda_ab for the ^{16}O substituted samples increases stronger with increasing magnetic field than for the ^{18}O ones. The OIE on \lambda_ab decreases by more than a factor of two with increasing magnetic field from \mu_0H=0.2 T to \mu_0H=0.6 T. This effect can be explained by the isotope dependence of the in-plane charge carrier mass m^\ast_{ab}.Comment: 4 pages, two figure

Oxygen-isotope effect on the in-plane penetration depth in underdoped Y_{1-x}Pr_xBa_2Cu_3O_{7-delta} as revealed by muon-spin rotation

The oxygen-isotope (^16O/^18O) effect (OIE) on the in-plane penetration depth $\lambda_{ab} (0)$ in underdoped Y_{1-x}Pr_xBa_2Cu_3O_{7-delta} was studied by muon-spin rotation. A pronounced OIE on $\lambda_{ab}^{-2}(0)$ was observed with a relative isotope shift of $\Delta\lambda^{-2}_{ab}/\lambda^{-2}_{ab}$=-5(2)% for x =0.3 and -9(2)% for x=0.4. It arises mainly from the oxygen-mass dependence of the in-plane effective mass $m_{ab}^{\ast}$. The OIE exponents of T_{c} and of $\lambda_{ab}^{-2}(0)$ exhibit a relation that appears to be generic for cuprate superconductors.Comment: 4 pages, 4 eps figures, RevTex

Correlation between oxygen isotope effects on the transition temperature and the magnetic penetration depth in high-temperature superconductors close to optimal doping

The oxygen-isotope (^{16}O/^{18}O) effect (OIE) on the in-plane magnetic penetration depth \lambda_{ab}(0) in optimally-doped YBa_2Cu_3O_{7-\delta} and La_{1.85}Sr_{0.15}CuO_4, and in slightly underdoped YBa_2Cu_4O_8 and Y_{0.8}Pr_{0.2}Ba_2Cu_3O_{7-\delta} was studied by means of muon-spin rotation. A substantial OIE on \lambda_{ab}(0) with an OIE exponent \beta_O=-d\ln\lambda_{ab}(0)/d\ln M_O\approx - 0.2 (M_O is the mass of the oxygen isotope), and a small OIE on the transition temperature T_c with an OIE exponent \alpha_O=-d\ln T_{c}/d \ln M_O\simeq0.02 to 0.1 were observed. The observation of a substantial isotope effect on \lambda_{ab}(0), even in cuprates where the OIE on T_c is small, indicates that lattice effects play an important role in cuprate HTS.Comment: 6 pages, 4 figure

Muon-Spin-Rotation Measurements of the Penetration Depth in the Infinite-Layer Electron-Doped Cuprate Superconductor Sr0.9La0.1CuO2

Muon spin rotation (mSR) measurements of the in-plane penetration depth lambda_ab have been performed in the electron-doped infinite layer high-Tc superconductor (HTS) Sr0.9La0.1CuO2. Absence of the magnetic rare-earth ions in this compound allowed to measure for the first time the absolute value of lambda_ab(0) in electron-doped HTS using mSR. We found lambda_ab(0)=116(2) nm. The zero-temperature depolarization rate sigma(0)?1/lambda_ab(0)^2=4.6(1) MHz is more than four times higher than expected from the Uemura line. Therefore this electron-doped HTS does not follow the Uemura relation found for hole-doped HTS.Comment: to be published in Physical Review Letter