Thin Ohmic or superconducting strip with an applied ac electric current

The complex impedance, currents, and electric and magnetic fields are calculated as functions of resistivity and frequency or London depth for a long thin strip with applied ac current. Both Ohmic and superconducting strips are considered. While the inductance per unit length of the strip depends on the strip length logarithmically, the sheet current, magnetic field, resistance, and magnetic susceptibility are independent of this length. It is found that the enhancement of resistance by the skin effect in thin Ohmic strips is much weaker (logarithmic) than in thick wires.Comment: 4 pages, 3 figures, for Phys. Rev.

Composition-tuned magneto-optical Kerr effect in L10-MnxGa films with giant perpendicular anisotropy

We report the large polar magnetooptical Kerr effect in L10-MnxGa epitaxial films with giant perpendicular magnetic anisotropy in a wide composition range. The Kerr rotation was enhanced by a factor of up to 10 by decreasing Mn atomic concentration, which most likely arises from the variation of the effective spin-orbit coupling strength, compensation effect of magnetic moments at different Mn atom sites, and overall strain. The Kerr ellipticity and the magnitude of the complex Kerr angle is found to have more complex composition-dependence that varies with the photon energy. These L10-MnxGa films show large Kerr rotation of up to 0.10o, high reflectivity of 35%-55% in a wide wavelength range of 400~850 nm, and giant magnetic anisotropic field of up to 210 kOe, making them an interesting material system for emerging spintronics and terahertz modulator applications

Buckling instability in type-II superconductors with strong pinning

We predict a novel buckling instability in the critical state of thin type-II superconductors with strong pinning. This elastic instability appears in high perpendicular magnetic fields and may cause an almost periodic series of flux jumps visible in the magnetization curve. As an illustration we apply the obtained criteria to a long rectangular strip.Comment: Submitted to Phys. Rev. Let

Turbulent channel flow of dense suspensions of neutrally-buoyant spheres

Dense particle suspensions are widely encountered in many applications and in environmental flows. While many previous studies investigate their rheological properties in laminar flows, little is known on the behaviour of these suspensions in the turbulent/inertial regime. The present study aims to fill this gap by investigating the turbulent flow of a Newtonian fluid laden with solid neutrally-buoyant spheres at relatively high volume fractions in a plane channel. Direct Numerical Simulation are performed in the range of volume fractions Phi=0-0.2 with an Immersed Boundary Method used to account for the dispersed phase. The results show that the mean velocity profiles are significantly altered by the presence of a solid phase with a decrease of the von Karman constant in the log-law. The overall drag is found to increase with the volume fraction, more than one would expect just considering the increase of the system viscosity due to the presence of the particles. At the highest volume fraction here investigated, Phi=0.2, the velocity fluctuation intensities and the Reynolds shear stress are found to decrease. The analysis of the mean momentum balance shows that the particle-induced stresses govern the dynamics at high Phi and are the main responsible of the overall drag increase. In the dense limit, we therefore find a decrease of the turbulence activity and a growth of the particle induced stress, where the latter dominates for the Reynolds numbers considered here.Comment: Journal of Fluid Mechanics, 201

Specific heat and thermal conductivity in the mixed state of MgB2

The specific heat C and the electronic and phononic thermal conductivities kappa_e and kappa_{ph} are calculated in the mixed state for magnetic fields H near H_{c2}. The effects of supercurrent flow and Andreev scattering of the Abrikosov vortex lattice on the quasiparticles are taken into account. The resulting function C(H) is nearly linear while kappa_e(H) exhibits an upward curvature near H_{c2}. The slopes decrease with impurity scattering which improves the agreement with the data on MgB_2. The ratio of phonon relaxation times tau_n/tau_s = g(omega_0,H) for phonon energy omega_0, which is nearly a step function at omega_0 = 2Delta for the BCS state, is smeared out and tends to one for increasing H. This leads to a rapid reduction of kappa_{ph}(H) in MgB_2 for relatively small fields due to the rapid suppression of the smaller energy gap.Comment: 8 pages, 4 figures, accepted for publication in Phys. Rev. Letter