Possibility of a s-wave pairing in heavily Zn-doped YBa₂Cu₃O₇₋δ based on magnetic field effect on Andreev reflection spectra

When the dx₂₋y₂ -wave pairing is suppressed by Zn-doping in YBa₂Cu₃O₇₋δ some of the Andreev reflection spectra were found to be similar to the s-wave spectra of conventional superconductors. The energy gap is rather reproducible (2.3–3.0 meV). It is suppressed by low magnetic field (HcPC = 120–270 mT) in great contrast to the d-wave spectra (HcPC > 3 T) with the similar order of gap magnitude. We suppose that the s-wave pairing occurs near the Zn impurities

Evidence of a s-wave subdominant order parameter in YBa₂Cu₃O₇₋δ from break-junction tunneling spectra

The tunneling spectra of YBa₂Cu₃O₇₋δ break-junctions have been investigated for the tunneling direction close to the node one. The behavior of the zero-bias conductance peak (ZBCP) and Josephson current have been studied with temperature and magnetic field. The observed deep splitting of ZBCP which starts at TS<20–30 K is in agreement with the theory for the dx²–y²± is order parameter [Y. Tanuma, Y. Tanaka, and S. Kashiwaya, Phys. Rev. B64, 214519 (2001)]. We also observed that a low (0.04 T) magnetic field significantly depresses such splitting. The 1/T temperature dependence of maximum Josephson current that goes to saturation at T<TS also confirms the mixed order parameter formation

Extraction of Kaon Formfactors from K^- -> mu^- nu_mu gamma Decay at ISTRA+ Setup

The radiative decay K->mu nu gamma has been studied at ISTRA+ setup in a new kinematical region. About 22K events of K^- -> mu^- nu_mu gamma have been observed. The sign and value of Fv-Fa have been measured for the first time. The result is Fv-Fa=0.21(4)(4).Comment: 11 pages, 21 figures, submitted to Phys. Lett.

Search for Heavy Neutrino in K->mu nu_h(nu_h-> nu gamma) Decay at ISTRA+ Setup

Heavy neutrino nu_h with m_h < 300MeV/c^2 can be effectively searched for in kaon decays. We put upper limits on mixing matrix element |U_mu_h}|^2 for radiatively decaying nu_h from K->mu nu_h (nu_h -> nu gamma) decay chain in the following parameter region: 30MeV/c^2 < m_h < 80MeV/c^2; 10^{-11}sec < tau_h < 10^{-9}sec. For the whole region |U_{mu h}|^2 < 5 x 10^{-5} for Majorana type of nu_h and | U_{\mu h}|^2 < 8 x 10^{-5} for the Dirac case.Comment: Published in Phys. Lett.