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 is
sues 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 $psimeq3.5$~kbar while its volume fraction gradually decreases with increasing pressure until it vanishes at $psimeq$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 ($3.5lesssim plesssim 7$~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.
The magnetic penetration depth ($lambda$) as a function of applied magnetic field and temperature in SrPt$_3$P($T_csimeq8.4$ K) was studied by means of muon-spin rotation ($mu$SR). The dependence of $lambda^{-2}$ on temperature suggests the existence
of a single $s-$wave energy gap with the zero-temperature value $Delta=1.58(2)$ meV. At the same time $lambda$ was found to be strongly field dependent which is the characteristic feature of the nodal gap and/or multi-gap systems. The multi-gap nature of the superconduicting state is further confirmed by observation of an upward curvature of the upper critical field. This apparent contradiction would be resolved with SrPt$_3$P being a two-band superconductor with equal gaps but different coherence lengths within the two Fermi surface sheets.
A study of the temperature and field dependence of the penetration depth lambda of the superconductor RbFe_2As_2 (T_c=2.52 K) was carried out by means of muon-spin rotation measurements. In addition to the zero temperature value of the penetration de
pth lambda(0)=267(5) nm, a determination of the upper critical field B_c2(0)=2.6(2) T was obtained. The temperature dependence of the superconducting carrier concentration is discussed within the framework of a multi-gap scenario. Compared to the other 122 systems which exhibit much higher Fermi level, a strong reduction of the large gap BCS ratio 2Delta/k_B T_c is observed. This is interpreted as a consequence of the absence of interband processes. Indications of possible pair-breaking effect are also discussed.
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.
Using muon-spin rotation, we studied the in-plane (lambda_ab) and the out of plane (lambda_c) magnetic field penetration depth in SrFe_1.75Co_0.25As_2 (T_c=13.3 K). Both lambda_ab(T) and lambda_c(T) are consistent with the presence of two superconduc
ting gaps with the gap to T_c ratios 2Delta/k_BT_c=7.2 and 2.7. The penetration depth anisotropy gamma_lambda=lambda_c/lambda_ab increases from gamma_lambda=2.1 at T_c to 2.7 at 1.6 K. The mean internal field in the superconducting state increases with decreasing temperature, just opposite to the diamagnetic response seen in magnetization experiments. This unusual behavior suggests that the external field induces a magnetic order which is maintained throughout the whole sample volume.
We investigate the magnetic penetration depth lambda in superconducting Ba_1-xK_xFe_2As_2 (T_csimeq32K) with muon-spin rotation (muSR) and angle-resolved photoemission (ARPES). Using muSR, we find the penetration-depth anisotropy gamma_lambda=lambda_
c/lambda_{ab} and the second-critical-field anisotropy gamma_{H_c2} to show an opposite T-evolution below T_c. This dichotomy resembles the situation in the two-gap superconductor MgB_2. A two-gap scenario is also suggested by an inflection point in the in-plane penetration depth lambda_ab around 7K. The complementarity of muSR and ARPES allows us to pinpoint the values of the two gaps and to arrive to a remarkable agreement between the two techniques concerning the full T-evolution of lambda_ab. This provides further support for the described scenario and establishes ARPES as a tool to assess macroscopic properties of the superconducting condensate.
We report on measurements of the in-plane magnetic penetration lambda_{ab} in the optimally doped cuprate superconductor (BiPb)_2(SrLa)_2CuO_6+delta (OP Bi2201) by means of muon-spin rotation (muSR). We show that in unconventional $d-$wave supercondu
ctors (like OP Bi2201), muSR experiments conducted in various magnetic fields allow to evaluate the zero-field magnetic penetration depth lambda_0, which relates to the zero-field superfluid density in terms of rho_sproptolambda_0^-2.
The in-plane magnetic penetration depth lambda_{ab} in optimally doped (BiPb)_2(SrLa)_2CuO_{6+delta} (OP Bi2201) was studied by means of muon-spin rotation. The measurements of lambda_{ab}^{-2}(T) are inconsistent with a simple model of a d-wave orde
r parameter and a uniform quasiparticle weight around the Fermi surface. The data are well described assuming the angular gap symmetry obtained in ARPES experiments [Phys. Rev. Lett {bf 98}, 267004 (2007)], where it was shown that the superconducting gap in OP Bi2201 exists only in segments of the Fermi surface near the nodes. We find that the remaining parts of the Fermi surface, which are strongly affected by the pseudogap state, do not contribute significantly to the superconducting condensate. Our data provide evidence that high temperature superconductivity and pseudogap behavior in cuprates are competing phenomena.
Measurements of the in-plane magnetic field penetration depth lambda_{ab} in Fe-based superconductors with the nominal composition SmFeAsO_0.85 (T_csimeq52K) and NdFeAsO_0.85 (T_csimeq51K) were carried out by means of muon-spin-rotation. The absolute
values of lambda_{ab} at T=0 were found to be 189(5)nm and 195(5)nm for Sm and Nd substituted samples, respectively. The analysis of the magnetic penetration depth data within the Uemura classification scheme, which considers the correlation between the superconducting transition temperature T_c and the effective Fermi temperature T_F, reveal that both families of Fe-based superconductors (with and without fluorine) falls to the same class of unconventional superconductors.
We report on measurements of the in-plane magnetic penetration depth lambda_{ab} in the infinite-layer electron-doped high-temperature cuprate superconductor Sr_0.9La_0.1CuO_2 by means of muon-spin rotation. The observed temperature and magnetic fiel
d dependences of lambda_{ab} are consistent with the presence of a substantial s-wave component in the superconducting order parameter in good agreement with the results of tunneling, specific heat, and small-angle neutron scattering experiments.
