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We present a study of thermal conductivity, κ, in undoped and doped strontium titanate in a wide temperature range (2-400 K) and detecting different regimes of heat flow. In undoped SrTiO_{3}, κ evolves faster than cubic with temperature below its peak and in a narrow temperature window. Such behavior, previously observed in a handful of solids, has been attributed to a Poiseuille flow of phonons, expected to arise when momentum-conserving scattering events outweigh momentum-degrading ones. The effect disappears in the presence of dopants. In SrTi_{1-x}Nb_{x}O_{3}, a significant reduction in lattice thermal conductivity starts below the temperature at which the average inter-dopant distance and the thermal wavelength of acoustic phonons become comparable. In the high-temperature regime, thermal diffusivity becomes proportional to the inverse of temperature, with a prefactor set by sound velocity and Planckian time (τ_{p}=(ℏ/k_{B}T)).

A quantum spin liquid state has long been predicted to arise in spin-1/2 Heisenberg square-lattice antiferromagnets at the boundary region between Néel (nearest-neighbor interaction dominates) and columnar (next-nearest-neighbor interaction dominates) antiferromagnetic order. However, there are no known compounds in this region. Here we use d10–d0 cation mixing to tune the magnetic interactions on the square lattice while simultaneously introducing disorder. We find spin-liquid-like behavior in the double perovskite Sr2Cu(Te0.5W0.5)O6, where the isostructural end phases Sr2CuTeO6 and Sr2CuWO6 are Néel and columnar type antiferromagnets, respectively. We show that magnetism in Sr2Cu(Te0.5W0.5)O6 is entirely dynamic down to 19 mK. Additionally, we observe at low temperatures for Sr2Cu(Te0.5W0.5)O6—similar to several spin liquid candidates—a plateau in muon spin relaxation rate and a strong T-linear dependence in specific heat. Our observations for Sr2Cu(Te0.5W0.5)O6 highlight the role of disorder in addition to magnetic frustration in spin liquid physics.

By muon spin-relaxation measurements on single-crystal specimens, we show that superconductivity in the AFe 2 As 2 ͑A =Ca,Ba,Sr͒ systems, in both the cases of composition and pressure tunings, coexists with a strong static magnetic order in a partial volume fraction. The superfluid response from the remaining paramagnetic volume fraction of ͑Ba 0.5 K 0.5 ͒Fe 2 As 2 exhibits a nearly linear variation in T at low temperatures, suggesting an anisotropic energy gap with line nodes and/or multigap effects.

We have performed transverse field muon spin rotation measurements of single crystals of Ba͑Fe 0.926 Co 0.074 ͒ 2 As 2 with the applied magnetic field along the ĉ direction. Fourier transforms of the measured spectra reveal an anisotropic line-shape characteristic of an Abrikosov vortex lattice. We have fit the SR spectra to a microscopic model in terms of the penetration depth and the Ginzburg-Landau parameter . We find that as a function of temperature, the penetration depth varies more rapidly than in standard weak-coupled BCS theory. For this reason we first fit the temperature dependence to a power law where the power varies from 1.6 to 2.2 as the field changes from 0.02 to 0.1 T. Due to the surprisingly strong field dependence of the power and the superfluid density we proceeded to fit the temperature dependence to a two-gap model, where the size of the two gaps is field independent. From this model, we obtained gaps of 2⌬ 1 = 3.77k B T C and 2⌬ 2 = 1.57k B T C , corresponding to roughly 6 and 3 meV, respectively.