Parallel Field Geometry Research Articles

Magnetic moment jumps in flat and nanopatterned Nb thin-walled cylinders

Penetration of magnetic flux into hollow superconducting cylinders is investigated by magnetic moment measurements. The magnetization curves of a flat and a nanopatterned thin-walled superconducting Nb cylinders with a rectangular cross section are reported for the axial field geometry. In the nanopatterned sample, a row of micron-sized antidots (holes) was milled in the film along the cylinder axis. Magnetic moment jumps are observed for both samples at low temperatures for magnetic fields not only above Hc1, but also in fields lower than Hc1, i. e., in the vortex-free regime. The positions of the jumps are not reproducible and they change from one experiment to another, resembling vortex lattice instabilities usually observed for magnetic fields larger than Hc1. At temperatures above 0.66Tc and 0.78Tc the magnetization curves become smooth for the patterned and the as-prepared sample, respectively. The magnetization curve of a reference flat Nb film in the parallel field geometry does not exhibit jumps in the entire range of accessible temperatures.

Dc and ac magnetic properties of thin-walled Nb cylinders with and without a row of antidots

dc and ac magnetic properties of two thin-walled superconducting Nb cylinders with a rectangular cross-section are reported. Magnetization curves and the ac response were studied on as-prepared and patterned samples in magnetic fields parallel to the cylinder axis. A row of micron-sized antidots (holes) was made in the film along the cylinder axis. Avalanche-like jumps of the magnetization are observed for both samples at low temperatures for magnetic fields not only above Hc1, but in fields lower than Hc1 in the vortex-free region. The positions of the jumps are not reproducible and they change from one experiment to another, resembling vortex lattice instabilities usually observed for magnetic fields larger than Hc1. At temperatures above and the magnetization curves become smooth for the patterned and the as-prepared samples, respectively. The magnetization curve of a reference planar Nb film in the parallel field geometry does not exhibit jumps in the entire range of accessible temperatures. The ac response was measured in constant and swept dc magnetic field modes. Experiment shows that ac losses at low magnetic fields in a swept field mode are smaller for the patterned sample. For both samples the shapes of the field dependences of losses and the amplitude of the third harmonic are the same in constant and swept field near Hc3. This similarity does not exist at low fields in a swept mode.

Behaviour of iron oxide (Fe3O4) Janus particles in overlapping external AC electric and static magnetic fields

The assembly behaviour of Fe3O4-capped Janus particles with magnetically and electrically anisotropic properties is investigated in overlapping external AC electric and static magnetic fields. Generally, Fe3O4 Janus particles have been shown to form double and staggered chains in external static magnetic and AC electric fields, respectively. Here, the reaction of Fe3O4 Janus particles to sequentially applied AC electric and static magnetic fields in parallel and perpendicular field geometry is studied in order to elucidate to what extend the asymmetric nature of the Janus particle can be addressed independently by each field during assembly. A parallel field geometry leads to contraction of the Janus particle chains, whereas a perpendicular field geometry leads first to the rotation of the Janus particle chains towards the more dominant field followed by rhombohedral stacking of the chains into two-dimensional, two-particle thick sheets as a result of the weaker field. The Janus particle assembly behaviour in each field geometry tested is parameterized by the characteristic angle, θ, and the inter-particle distance, d. The observed response of the Fe3O4 Janus particles is shown to be driven by the interaction of electric and magnetic dipoles generated in the caps by the external AC electric and static magnetic fields, respectively, as well as the polarization of the polystyrene particle. Reversibility of the assembly behaviour correlates strongly with the fact that the electric dipoles disappear immediately after the AC electric field is turned off, whereas the magnetic dipoles remain permanently in the Fe3O4 caps even after the static magnetic field is removed. As a result, the Fe3O4 Janus particle assembly behaviour is responsive to both the electric and the magnetic fields and their behaviour can be manipulated by the sequence and relative orientation of the external fields.

A comparison between the response of compound semiconductor detectors in single and back-to-back configuration

CdTe detectors (3 × 5 mm 2 electrode area and 1 mm thick), mounted in a back-to-back configuration with common anode, have been characterized, aiming at determining the actual regions of these devices giving the best and almost constant spectroscopic performance. This configuration ensures that the maximum paths of the charge carriers correspond to the inter-electrode distances and consequently the spectroscopic performance is essentially the same as for the single coupled detectors, but allows one: • to double the useful absorbing thickness when irradiated in the classical planar parallel field geometry; • to double the pixel area when irradiated in the planar transverse field (PTF) geometry, while the absorbing thickness can be freely adjusted according to the experimental requirements. The tests were performed in PTF geometry by using well collimated photon beams (10–300 keV energy range) obtained by a 20 mm thick tungsten collimator having a 0.1 × 1 or a 0.3 × 1 mm 2 collimating channel. Experimental results concerning the spectroscopic behaviour of the coupled detectors have been compared with those obtained by single devices having the same thickness as the single components or the same total thickness as the coupled device. The results show that the back-to-back configuration does not introduce further significant noise with respect to 1 mm thick single detectors and their spectroscopic performance is similar or better than the one shown by 2 mm thick single devices for fine and wide collimator measurements, and mostly in the uncollimated case.

Aharonov-Bohm spectral features and coherence lengths in carbon nanotubes

The electronic properties of carbon nanotubes are investigated in the presence of disorder and a magnetic field parallel or perpendicular to the nanotube axis. In the parallel field geometry, the $\phi_{0}(=hc/e)$-periodic metal-insulator transition (MIT) induced in metallic or semiconducting nanotubes is shown to be related to a chirality-dependent shifting of the energy of the van Hove singularities (VHSs). The effect of disorder on this magnetic field-related mechanism is considered with a discussion of mean free paths, localization lengths and magnetic dephasing rate in the context of recent experiments.

Spin-dependent transport in a magnetic two-dimensional electron gas

Magneto-transport and magneto-optical probes are used to interrogate spin-dependent transport in magnetic heterostructures wherein a two dimensional electron gas (2DEG) is exchange-coupled to local moments. At low temperatures, the significant s–d exchange-enhanced spin splitting in these “magnetic” 2DEGs is responsible for the observation of unusual transport properties such as a complete spin polarization of the gas at large Landau level filling factors and a pronounced, non-monotonic background magneto-resistance. Magneto-transport measurements of gated samples performed in a parallel field geometry are used to systematically study the variation of the magneto-resistance with sheet concentration, yielding new insights into the dependence of spin transport on the Fermi energy of the majority spin carriers.

Neutron standing wave resonator to measure the flux line lattice in thin film superconducting samples (abstract)

Flux line lattices in superconductors have been extensively studied in bulk samples by a variety of techniques including neutron diffraction. We present a technique that is useful for studying the flux lattice in a thin film in the configuration where the field (and hence the flux lines) is parallel to the sample surface. This configuration provides information about the in-plane order and is not accessible by other means (e.g., scanning tunnel microscope, Bitter decoration). A 5000-Å-thick niobium film epitaxially grown on an MgO substrate is studied by polarized neutron reflectometry (PNR) in this parallel field geometry. In order to enhance the signal, a neutron standing wave resonator is created in the superconducting film by spin casting a thin layer of deuterated polystyrene on the Nb. The increased scattering caused by the presence of flux lines in the sample leads to a depletion of the reflected neutron beam at the resonant wave vector. Our results show the expected depletion in the superconducting state for fields at which flux lines are expected to be present. The field and temperature dependence of the data are consistent with scattering arising from flux lines. We discuss plans to prove that the source of the increased scattering is indeed the flux lines. The most conclusive evidence for a well-ordered flux lattice in the sample is the presence of off-specular diffraction, strongly enhanced by the standing wave in the resonator. Measurements to see this off-specular signal have proved unsuccessful, leading us to conclude that we may not have a well-ordered lattice in our superconducting sample.

Electromodulation in Poly(Para-Phenylene)-Type Ladder Polymer

We report studies on the electronic structure of the methyl substituted poly(para-phenylene)-type ladder polymer (m-LPPP) using electromodulation-techniques. Electric fields from 10 kV/cm up to a few hundred kV/cm were applied to the samples either parallel or perpendicular to the incident light. The lineshape of the electroabsorption(EA)-spectra follows very closely the first derivative of the unperturbed absorption-spectra. Comparing EA-measurements with perpendicular and parallel field geometry provides further insight into the nature of ordering of the polymer-chains in films of different thickness.

Structural and giant magnetoresistance characterization of Ag [sbnd]Co multilayers

Ag Co multilayers were prepared on various substrates (Si, polyimide and glass) by e-beam evaporation under ultra high vacuum. X-ray diffraction and high resolution electron microscopy studies showed a deterioration of multilayer structure upon reducing the individual Co-layer thickness to 0.5 nm. Furthermore, the saturation field in the parallel field geometry increases, as SQUID magnetometry revealed, while magnetoresistance reaches 16% at room temperature and exceeds 30% at 30 K. Magnetoresistance values were found to depend strongly on individual layer thicknesses as well as on the total film thickness.

Resonant photorefractive AlGaAs/GaAs multiple quantum wells grown by molecular beam epitaxy at low temperature

The resonant photorefractive devices using low temperature AlGaAs/GaAs multiple-quantum-well structures in a parallel field geometry are demonstrated. The samples are semi-insulating as grown. The AsGa-related defects incorporated into the samples during low temperature growth provide the required deep centers. No proton implantation, Cr doping, or annealing is needed for device fabrication. In the photorefractive wave mixing experiment, an output diffraction efficiency higher than 0.84% and a two-wave-mixing gain of more than 3000 cm−1 are obtained under a dc electric field of 15 kV/cm.

Surface erosion issues and analysis for dissipative divertors

Erosion/redeposition is examined for the sidewalls of a dissipative divertor using coupled impurity transport, charge exchange, and sputtering codes, applied to plasma solutions for the ITER design. A key issue for this regime is possible runaway self-sputtering, due to the effect of a low boundary density and nearly parallel field geometry on redeposition parameters. Net erosion rates, assuming finite self-sputtering, vary with wall location, boundary conditions, and plasma solution, and are roughly of the following order. 200–2000 Å/s for beryllium, 10–100 Å/s for vanadium, and 0.3–3 Å/s for tungsten.

Model of resonant electrooptical effect near exciton peak for MQW structures

An electrooptical response model of MQW structures is presented at resonant conditions near the exciton absorption peak for both perpendicular and parallel field geometry. We show that the effect can be treated as a quadratic one for a perpendicular field while in parallel geometry the electrooptic coefficient demonstrates an exponential field dependence.

Magnetic properties of YBaCuO thick films prepared by LPE method

Single-crystalline YBaCuO thick films have been grown on NdGaO 3 substrates by liquid phase epitaxial technique. With the magnetic field H parallel to the c-axis, critical currents of up to 5 MA.cm −2 at 4,2K from magnetization measurements are obtained. With H perpendicular to the c-axis, various magnetization measurements with field cooling in 30 kOe indicated strong pinning effect with critical currents more much higher than that observed in the parallel field geometry.

Exactly solvable models for the radiofrequency size effect. I. Line shape

The radiofrequency size effect (RFSE) is investigated for two model “metals” having Fermi surfaces in the form of a cube and a corrugated sheet. In the parallel-field geometry, and with the Fermi surface oriented suitably with respect to the surfaces of the plate, the nonlocal conductivities reduce to a series of delta functions, thereby facilitating the calculation (in closed form) of the response of the plate to various modes of excitation. The calculations show explicitly the structure of the field “splash” within the plate as well as the structure of the fundamental and second RFSE resonances in the rf surface impedance and transmission coefficient. The effects of multiple revolutions and secondary excitation of trajectories are exhibited, and the electron mean free path dependence of the resonances is examined.