Neutron scattering, muon spin relaxation, and dc susceptibility studies have been carried out on polycrystalline Tb 2 Ti 2 O 7 , a pyrochlore antiferromagnet in which the Tb 31 moments reside on a network of corner-sharing tetrahedra. Unlike other geometrically frustrated systems, Tb 2 Ti 2 O 7 remains paramagnetic down to ϳ0.07 K, rather than ordering into a conventional Néel or spin-glass-like state, despite the fact that short-range antiferromagnetic correlations (AFC) develop at ϳ50 K. At the first AFC wave vector, its low-lying, relatively flat magnetic excitation spectrum softens partially below 30 K.

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A novel high magnetic field (8 T) spectrometer for muon spin rotation (µSR) has been used to measure the temperature dependence of the in-plane magnetic penetration depth λ ab in YBa2Cu3O6.95. At low H and low T , λ ab exhibits the characteristic linear T -dependence associated with the energy gap of a d x 2 −y 2 -wave superconductor. However, at higher fields λ ab is essentially temperature independent at low T . We discuss possible interpretations of this surprising new feature in the low-energy excitation spectrum. 74.25.Nf, 74.72.Bk, 76.75.+i In a superconductor, the resistance to the flow of electric current drops to an unmeasurably small value below a certain critical temperature T c . This remarkable characteristic is due to the formation of pairs of electrons (or holes), called "Cooper pairs", which link together and carry the charge through the sample with virtually no opposition. To break apart the pairs, an additional energy is needed to excite individual electrons above an energy gap which exists at the Fermi surface in the superconducting state. The nature of these elementary excitations, known as "quasiparticles" (QPs), is directly related to the size and symmetry of the energy gap. The gap itself reflects the symmetry of the pair wave function (or order parameter), knowledge of which is essential to understanding the physics of the underlying mechanism responsible for superconductivity.A major breakthrough in the study of high-T c cuprate superconductors (HTSCs) came when it was realized that the symmetry of the energy gap was different from that in conventional low-T c materials. In particular, the energy gap was found to vanish along certain directions in momentum space. These so-called "nodes" serve as a conduit for extreme low-energy QP excitations. One of the key early experiments providing evidence for the existence of gap nodes was microwave measurements by Hardy et al.[1] of the in-plane penetration depth change ∆λ ab = λ ab (T )−λ ab (1.35 K) in the Meissner state of highpurity YBa 2 Cu 3 O 6.95 . In this phase, magnetic field is partially screened from the interior by "supercurrents" circulating around the sample perimeter. These supercurrents constitute the response of the superconductor to the applied field. The penetration depth λ is the characteristic length scale over which the field decays in from the surface, and the quantity λ −2 is proportional to the density of Cooper pairs, i.e. "superfluid density", n s . Because thermal energy can excite QPs, λ −2 decreases with increasing T . In a conventional superconductor, this temperature dependence is typically weak at low T because the isotropic energy gap exponentially cuts off the QP excitations as T → 0 K. In Ref.[1], however, λ −2 ab was found to decrease sharply upon raising the temperature above 1.35 K-the lowest temperature reached in the experiment. This suggested that the minimum gap size was very small. Moreover, at low temperatures ∆λ ab was observed to be proportional to T , which is characteristic of a superconducting o...