Scott L. Stubbs

Hole-doping dependence of the magnetic penetration depth and vortex core size in YBa2Cu3Oy : Evidence for stripe correlations near 1/8 hole doping

We report on muon spin rotation measurements of the internal magnetic field distribution n(B) in the vortex solid phase of YBa2Cu3Oy (YBCO) single crystals, from which we have simultaneously determined the hole doping dependences of the in-plane Ginzburg-Landau (GL) length scales in the underdoped regime. We find that Tc has a sublinear dependence on 1/lambda_{ab}^2, where lambda_{ab} is the in-plane magnetic penetration depth in the extrapolated limits T -> 0 and H -> 0. The power coefficient of the sublinear dependence is close to that determined in severely underdoped YBCO thin films, indicating that the same relationship between Tc and the superfluid density is maintained throughout the underdoped regime. The in-plane GL coherence length (vortex core size) is found to increase with decreasing hole doping concentration, and exhibit a field dependence that is explained by proximity-induced superconductivity on the CuO chains. Both the magnetic penetration depth and the vortex core size are enhanced near 1/8 hole doping, supporting the belief by some that stripe correlations are a universal property of high-Tc cuprates.

Signatures of new d-wave vortex physics in overdoped Tl2Ba2CuO(6+x) revealed by TF-µ(+)SR.

The spontaneous expulsion of applied magnetic field, the Meissner effect, is a defining feature of superconductors; in Type-II superconductors above the lower critical field, this screening takes the form of a lattice of magnetic flux vortices. Using implanted spin-1/2 positive muons, one can measure the vortex lattice field distribution through the spin precession and deduce key parameters of the superconducting ground state, and thereby fundamental properties of the superconducting pairing. Muon spin rotation/relaxation (µSR) experiments have indeed revealed much interesting physics in the underdoped cuprates, where superconductivity is closely related to, or coexistent with, disordered or fluctuating magnetic and charge excitations. Such complications should be absent in overdoped cuprates, which are believed to exhibit conventional Fermi liquid behaviour. These first transverse field (TF)-µ+SR experiments on heavily-overdoped single crystals reveal a superfluid density exhibiting a clear inflection point near 0.5Tc , with a striking doping-independent scaling. This reflects hitherto unrecognized physics intrinsic to d-wave vortices, evidently generic to the cuprates, and may offer fundamentally new insights into their still-mysterious superconductivity.

Localization of muonium atoms in KCl at low temperature

Muonium (μ+e− or Mu) atoms have long been believed to exhibit band-like propagation at low temperatures in ionic insulators and semiconductors. However, new muon spin relaxation measurements in transverse magnetic field reveal strong localization of Mu atoms at low temperatures in the ionic insulator KCl, similar to that observed in van der Waals cryocrystals. This discrepancy with previous results challenges our understanding of muonium quantum dynamics in solids: can Mu atoms ever truly delocalize at low temperature, or is low-temperature Mu localization a universal phenomenon?

Spin-singlet state of electrons in filled skutterudites

Muon spin rotation experiments have been performed on filled skutterudites PrOs4Sb12 and REPt4Ge12 (RE = La, Pr, Nd, Eu) over a temperature range 0.025–300 K in magnetic fields up to 7 T. An electron bound state is detected spectroscopically in filled skutterudites containing magnetic ions, both below and above the superconducting transition temperature Tc and both below and above the corresponding upper critical magnetic field Hc2, including the pseudogap region. This state is proposed to be a spin-singlet —an electron pair confined around the positive muon.