I. M. Savić

Detection of muonated free radicals through the effects of avoided level crossing. Theory and analysis of spectra

Abstract A treatment is developed for the analysis of μSR spectra of muonium-substituted free radicals detected under conditions of avoided level crossing in high longitudinal magnetic fields. Approximate analytical results are compared with those of numerical calculations, and an experimental example is presented.

Magnetic properties of RuSr2RECu2O8 (RE = Gd, Eu) and Ru1-xSr2GdCu2+xO8-y superconductors

Abstract Synthesis in 1% oxygen at 930°C leads to non-superconducting RuSr 2 GdCu 2 O 8 in which superconductivity subsequently can be induced with oxygen annealings without observable stoichiometric changes. Detailed ac susceptibility and zero-field muon-spin rotation experiments reveal that the superconducting compounds always exhibit a lower temperature of the magnetic transition and larger values of the internal magnetic field in the ordered state of the Ru sublattice. The same annealings in oxygen, however, lead to non-superconducting RuSr 2 EuCu 2 O 8 . The magnetic properties of a newly discovered series of Ru 1− x Sr 2 GdCu 2+ x O 8− y superconductors (maximum T c =72 K for x =0.4) are reported. Low temperature behavior of the magnetization shows a significant contribution of the paramagnetic system of Gd ions to the magnetic response below the superconducting transition. Muon-spin rotation experiments reveal the presence of the magnetic transitions at low temperatures below the superconducting transition. XANES CuK absorption edge data show an increase in Cu valence with x .

Site-selective oxygen isotope effect on the magnetic-field penetration depth in underdoped Y0.6Pr0.4Ba2Cu3O7-δ

We report site-selective oxygen isotope ( 1 6 O/ 1 8 O) effect (OIE) measurements on the in-plane penetration depth λ a b in underdoped Y 0 . 6 Pr 0 . 4 Ba 2 Cu 3 O 7 - δ , using the muon-spin rotation technique. A pronounced OIE on the transition temperature T c as well as on λ a b 2(0) was observed, which mainly arises from the oxygen sites within the superconducting CuO 2 planes (100% within error bar). The values of the corresponding relative isotope shifts were found to be ΔT c /T c =-3.7(4)% and Δλ - 2 ab(0)/λ - 2 ab(0)=-6.2(1.0)%. Our results imply that in this compound the phonon modes involving the movement of planar oxygen are dominantly coupled to the electrons.

Magnetic and electronic properties of YBa2Cu3Ox (6.5 < x < 7.0) studied by muon spin rotation

Abstract A systematic study of the local magnetic field distribution p(B) in the perovskite-type superconductor YBa2Cu3Ox is presented. Special attention is given to the second moment 〈 ΔB2 〉 of p(B) which is closely related to the superconducting carrier density ns and the effective mass m ∗ of the superconducting carriers. The influence of temperature, anisotropy and hole doping are investigated in detail. It is found that the temperature dependence of the magnetic penetration depth in YBa2Cu3Ox is in agreement with conventional s-wave pairing and suggests strong coupling. The anisotropic behavior of p(B) is well described by a theory for uniaxial type II superconductors, with an anisotropy ratio γ ∼- 5(1). Moreover, the zero-temperature values λab(0) = 130(10) nm and λc(0) ∼- 500–800 nm (penetration depth for screening currents flowing in the ab-planes and along the c-axis, respectively) were extracted from our data. μSR experiments performed on oxygen deficient samples indicate that the structural and electronic properties of YBa2Cu3Ox are strongly correlated via the oxygen content x.

μSR in oxygen deficient YBa2Cu3O x (6.5⩽x⩽7.0)

A systematic μSR study of the local magnetic field distribution in a series of oxygen deficient YBa2Cu3Ox samples with 6.5⩽x⩽7.0 is reported. Special attention was given to perform the experiments under the same conditions, so that the oxygen content of the measured samples was the only parameter varied. The behavior of the depolarization rate σ as a function of the oxygen contentx was found to have strong similarities with the behavior of the critical temperatureTc as a function ofx. In particular, two step-like increases of σ were observed abovex=6.7 and 6.9. The temperature dependence of the normalized depolarization rate σ(T)/σ(0) is well described by the two-fluid model forx⩾6.781(1) and clearly deviates from this behavior forx⩽6.704(1). Our results are compared to those obtained by other groups.

Unconventional angular dependence of the microscopic magnetic field distribution in single crystal Bi2Sr2CaCu2O8 from muon-spin rotation studies

Abstract The angular dependence of the microscopic magnetic field distribution in single-crystal Bi 2 Sr 2 CaCu 2 O 8 has been measured for a range of applied fields using the muon-spin rotation technique. The results show significant deviations from the predictions of the anisotropic 3D model, which may reflect the extremely anisotropic, quasi-2D nature of the flux structure. The temperature dependence of the field distribution at 400 mT has also been measured for a range of orientations, showing disagreement with the predictions of a simple two-fluid model.

Muon-spin rotation measurements of the penetration depth of theMo3Sb7superconductor

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 gap

Muon-spin rotation (μSR) study of the temperature dependence of the London penetration depth in copper oxide superconductors

Abstract A μSR study of the temperature dependence of the London penetration depth λ in sintered samples of YBa 2 Cu 3 O z (with various oxygen contents x ), YBa 2 Cu 4 O 8 and Bi 2 Sr 2 CaCu 2 O 8 is presented. It is found that the temperature behavior of λ of all these cuprate superconductors is consistent with conventional s -wave pairing. However, there are significant differences concerning the exact temperature dependence of λ in these materials. In YBa 2 Cu 3 O z with high x , the behavior of λ( T ) is well described by the two-fluid model (strong coupling), whereas λ( T ) in YBa 2 Cu 3 O z with low x , YBa 2 Cu 4 O 8 and Bi 2 Sr 2 CaCu 2 O 8 is in better agreement with weak-coupling BCS theory. Possible reasons for the different temperature behavior of λ in these materials are discussed.

Correlation between oxygen isotope effects on transition temperature and 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.

Temperature dependence of muon-decay positron channeling in semiconductors

Planar channeling data ofμ+-decay positrons in various semiconductors are reported. Together with the extensive spectroscopic data supplied by transverse μSR, the location of the different states of the hydrogen pseudo-isotopeμ+ e− (muonium) can be identified by means of planar simulations. In high purity silicon as well as in gallium arsenide a thermally activated site transition is observed which can be assigned to a transition between different muonium states.