Change In Critical Field Research Articles

Gravity effects on mixing with magnetic micro-convection in microfluidics

Mixing remains an important problem for the development of successful microfluidic and lab-on-a-chip devices, where simple and predictable systems are particularly interesting. One is magnetic micro-convection, an instability happening on the interface of miscible magnetic and non-magnetic fluids in a Hele-Shaw cell under applied field. Previous work proved that the Brinkman model quantitatively explains the experiments. However, a gravity-caused convective motion complicated the tests. Here we first improve the experimental system to exclude the parasitic convection. Afterwards, we experimentally observe the magnetic micro-convection, by finding and quantifying how gravity and laminar flow stabilizes the perturbations that create it. Accordingly, we improve our theoretical model for a zero-flow condition and perform a linear analysis. Two dimensionless quantities --magnetic and gravitational Rayleigh numbers-- are used to compare the experimental observations and theoretical predictions for the critical field of instability and the characteristic size of the emerging pattern. Finally, we discuss the conditions at which gravity plays an important role in microfluidic systems.

3-D Branching of Magnetic Domains on Compressed Si-Fe Steel With Goss Texture

The influence of an applied compressive stress on the hysteresis curve and domain structure in conventional (110) [001] Fe-3%Si steel cut parallel to the rolling direction was studied. Quasi-static hysteresis loops under compressive stress up to 70 MPa were measured. The magnetic domains and magnetization processes were observed by longitudinal Kerr microscopy at different levels of stress. With increasing compressive stress, the domain structure in the demagnetized state evolves from initial simple 180° slab-like domains along the surface-parallel easy axis into stress pattern I, checkerboard pattern, and finally into stress pattern II at high stresses. The checkerboard pattern, observed at the intermediate stress level between 10 and 25 MPa, has not been reported before such thin sheets. The applied magnetic field makes the surface-parallel [001] domains with magnetization in field direction grow into the bulk at the expense of all other domains for all stress levels. The magnetization curves at stress above 10 MPa have a constricted shape with a sudden change of magnetization at a critical fields. The averaged critical field changes linearly with stress with a slope close to theoretical predictions.

Effect of Disorder on Fermi Surface in Heavy Electron Systems

The Kondo lattice model with substitutional disorder is studied with attention to the size of the Fermi surface and the associated Dingle temperature. The model serves for understanding heavy-fermion Ce compounds alloyed with La according to substitution Ce{x}La{1-x}. The Fermi surface is identified from the steepest change of the momentum distribution of conduction electrons, and is derived at low enough temperature by the dynamical mean-field theory (DMFT) combined with the coherent potential approximation (CPA). The Fermi surface without magnetic field increases in size with decreasing x from x=1 (Ce end), and disappears at such x that gives the same number of localized spins as that of conduction electrons. From the opposite limit of x=0 (La end), the Fermi surface broadens quickly as x increases, but stays at the same position as that of the La end. With increasing magnetic field, a metamagnetic transition occurs, and the Fermi surface above the critical field changes continuously across the whole range of x. The Dingle temperature takes a maximum around x=0.5. Implication of the results to experimental observation is discussed.

Critical Field and Shubnikov-de Haas Oscillations of ?-(BEDT-TTF)2Cu(NCS)2under Pressure

We have used a novel experimental method to study the crossover of an anisotropic superconductor from a possible Pauli limited superconducting state to a vortex limited superconducting state by applying pressure. The new apparatus combined a tuned tank circuit with a nonmetallic diamond anvil cell to measure the change in critical field with angle in κ-(BEDT-TTF)2Cu(NCS)2 at pressures up to 1.75 kbar and at temperatures down to 70 mK. The critical fields (in the perpendicular or parallel orientation to the conducting planes) have been found to decrease by more than 90% within less than 2 kbar of pressure. In the parallel orientation, at 1.75 kbar, we have seen a clear change from the ambient pressure behavior of the critical field with temperature at low temperatures. Up to P=1.75 kbar, the Hc2θ phase diagram is in good agreement with the theoretical prediction for weakly coupled layered superconductors. We have also succeeded in measuring oscillations in the resistivity of the normal state at higher magnetic field which could be used to find the effective quasi-particle mass. The α-orbit Shubnikov-de Haas frequency was found to increase at a rate of 44T/kbar. Our experiment opens the possibility for further investigations of the effective mass with pressure, especially because the setup is suitable for pulsed fields as well.

Anisotropy of the upper critical field in a heavy-fermion superconductor.

We study the upper critical field for various possible order parameters of a heavy-fermion superconductor. We pay particular attention to the question of how the anisotropy of the upper critical field changes by effects of paramagnetism, impurity scattering, and Fermi-surface anisotropy in the presence of strong spin-orbit coupling. The paramagnetic limit can have a dramatic effect on the anisotropy in odd-parity superconductors, and provide important information on the symmetry of underlying order parameters. We also discuss the effect of impurity scattering.

Flux Pinning in A15 Superconductors

Global pinning force is influenced greatly by microstructures like grain size and third elements, when Nb3 Sn and V3 Ga superconductors are prepared by various different techniques. The structure dependence of flux pinning has been investigated in details. The main flux pinning center was attributed to the grain boundary. Accordinng to the modified shear model proposed by Evetts and Plummer (1985), the global pinning force has been analysed. It was found to have the same magnitude for both NB3Sn and V3Ga. The third elements influenced the specific force as well as the upper critical field. In and Ga made the pinning force increase, while the pinning force decreased by the addition of Ti or Zr. Generally, increasing the critical field resulted in the decrease of specific pinning force. This apparent result is suggested to relate with the change of the thermodynamic critical field. The effect of strain on the pinning force was not direct, but appeared through the change of critical field.

Upper Critical Field Changes in Electron‐Irradiated Niobium

AbstractMeasurements are reported of the dependence of the upper critical field Hc 2 on the concentration and configuration of Frenkel defects in niobium. The defects were produced by low‐temperature irradiation with 3 MeV electrons and subsequent annealing treatments. The results qualitatively confirm a theory by Eilenberger which predicts that Hc 2 not only depends on the electronic mean free path but also on the ratio between s‐ and p‐wave scattering of the defects.

Superconducting Metals in Porous Glass as Granular Superconductors

The critical fields of superconductors in porous glass are found to have the same temperature dependence as the critical field of a homogeneous dirty alloy. An effective mean free path for electrons in the normal state can be deduced; it is proportional to the pore diameter but is much smaller. This is most easily explained if the superconductor consists of grains separated by tunneling barriers, for this gives a mean free path equal to the grain size multiplied by an average of the transmission coefficient of the barriers. This model can also give a plausible explanation of the large change in critical field produced by chemical treatment of the glass before impregnation with metal. Parmenter's theory of granular superconductors shows that vortices can exist in such materials, and the critical current should be determined by vortex pinning. The measured critical current for indium in porous glass is less than that calculated from a Parmenter formula which assumes that the fields in the sample are uniform.

Effect of Plastic Deformation and Annealing Temperature on Superconducting Properties

Single crystals and polycrystals of Nb and Re were deformed in tension at room temperature. The critical field was found in both cases to increase with deformation and in the case of Nb the increase was as large as 150%. The change in critical field is related to the increase in plastic deformation. Annealing the plastically deformed sample restored the original critical field. The transition temperature in Nb was not affected either by the deformation or the annealing treatment. These experimental results can be explained theoretically if one assumes that in a hard superconductor the current is carried by filaments. The filaments may be identified with dislocations.The transition temperature of Re was found to depend on the annealing temperature and in some cases on plastic deformation. The transition temperature is higher, the higher the annealing temperature before plastic deformation. Plastic deformation does not affect the transition temperature in the cases where the transition is sharp. However, when the transition is broad, plastic deformation raises the transition temperature and sharpens the transition region. These results can be interpreted in terms of the equilibrium number of vacancies at the annealing temperature.