We investigate the effects of quantum (zero-temperature) long-wavelength fluctuations of free-standing crystalline membranes, which are two-dimensional objects embedded into three-dimensional space. The fluctuations produce logarithmic renormalization of elasticity and bending moduli of the membranes. We find one-loop RG equations to demonstrate that the system is in the “asymptotic freedom” regime; that is, the quantum
fluctuations destabilize the flat membrane phase.
The magnetoresistance (MR) ρ/ρ of the cage-glass compound HoxLu1−xB12 with various concentrations of magnetic holmium ions (x 0.5) has been studied in detail concurrently with magnetization M(T) and Hall effect investigations on high-quality single crystals at temperatures 1.9–120 K and in magnetic field up to 80 kOe. The undertaken analysis of ρ/ρ allows us to conclude that the large negative magnetoresistance (nMR) observed in the vicinity of the N´eel temperature is caused by scattering of charge carriers on magnetic clusters of Ho3+ ions, and that these nanosize regions with antiferromagnetic (AF) exchange inside may be considered as short-range-order AF domains. It was shown that the Yosida relation −ρ/ρ ∼ M2 provides an adequate description of the nMR effect for the case of Langevin-type behavior of magnetization. Moreover, a reduction of Ho-ion effective magnetic moments in the range 3–9 μB was found to develop both with temperature lowering and under the increase of holmium content. A phenomenological description of the large positive quadratic contribution ρ/ρ ∼ μ2 DH2 which dominates in HoxLu1−xB12 in the intermediate temperature range 20–120 K allows us to estimate the drift mobility exponential changes μD ∼ T −α with α = 1.3–1.6 depending on Ho concentration. An even more comprehensive behavior of magnetoresistance has been found in the AF state of HoxLu1−xB12 where an additional linear positive component was observed and attributed to charge-carrier scattering on the spin density wave (SDW). High-precision measurements of ρ/ρ = f (H,T ) have allowed us also to reconstruct the magnetic H-T phase diagram of Ho0.5Lu0.5B12 and to resolve its magnetic structure as a superposition of 4f (based on localized moments) and 5d (based on SDW) components.
Transport measurements are presented on thin-film superconducting spin-valve systems, where the controlled noncollinear arrangement of two ferromagnetic Co layers can be used to influence the superconducting state of Nb.We observe a very clear oscillation of the superconducting transition temperature with the relative orientation of the two ferromagnetic layers. Our measurements allow us to distinguish between the competing influences of domain averaging, stray dipolar fields, and the formation of superconducting spin triplets. Domain averaging is shown to lead to a weak enhancement of transition temperature for the antiparallel configuration of exchange fields, while much larger changes are observed for other configurations, which can be attributed to drainage currents due to spin triplet formation.
We study Josephson junctions with weak links consisting of two parallel disordered arms with magnetic properties: ferromagnetic, half-metallic, or normal with magnetic impurities. In the case of long links, the Josephson effect is dominated by mesoscopic fluctuations. In this regime, the system realizes a φ0 junction with sample-specific φ0 and critical current. Cooper pair splitting between the two arms plays a major role and leads to 2Φ0 periodicity of the current as a function of flux between the arms. We calculate the current and its flux and polarization dependence for the three types of magnetic links.