Localization length at the conductivity minima of the quantum Hall effect

The quantum localization is known to be responsible for the deep conductivity minima of the quantum Hall effect. In this paper we calculate the localization length as a function of magnetic field at such minima for several models of disorder (“white-noise”, short-range, and long-range random potentials). We find that with the exponent between one and , depending on the model. In particular, for the “white-noise” random potential roughly coincides with the classical cyclotron radius. Our results are in agreement with available experimental data.     

Curved two-dimensional electron gases

A method is introduced which makes it possible to fabricate non-planar two-dimensional electron gases. Making use of the “epitaxial lift-off” process, patterned, gated and contacted heterostructures are transferred from their crystalline substrate onto small glass tubes with diameters of a few millimeters. The transport properties of two-dimensional electron gases inside these curved semiconductor films are characterized by low-temperature magnetoresistance measurements. In the longitudinal resistance we find weak Shubnikov–de Haas oscillations, which are periodic in B⁻¹. The transverse resistance exhibits well-developed quantum Hall plateaus at low magnetic fields. At high fields, we observe a pronounced breakdown of the Hall effect. The experimental results are compared with simple theoretical models.     

Screening model of magnetotransport hysteresis observed in bilayer quantum Hall systems

We report on theoretical and experimental investigations of a novel hysteresis effect that has been observed on the magnetoresistance of quantum Hall bilayer systems. Extending to these system a recent approach, based on the Thomas–Fermi–Poisson nonlinear screening theory and a local conductivity model, we are able to explain the hysteresis as being due to screening effects such as the formation of “incompressible strips”, which hinder the electron density in a layer within the quantum Hall regime to reach its equilibrium distribution.     

K–K excitations on as “primary” superfields

We show that the K–K spectrum of IIB string on is described by “twisted chiral” superfields, naturally described in “harmonic superspace”, obtained by taking suitable gauge singlets polynomials of the D3-brane boundary superconformal field theory.To each p-order polynomial is associated a massive K–K short representation with states. The quadratic polynomial corresponds to the “supercurrent multiplet” describing the “massless” bulk graviton multiplet.     

The symmetries of the Manton superconductivity model

The symmetries and conserved quantities of Manton’s modified superconductivity model with non-relativistic Maxwell–Chern–Simons dynamics (also related to the Quantized Hall Effect) are obtained in the “Kaluza–Klein type” framework of Duval et al.     

Detonability limits in thin annular channels

In this paper, detonability limits in two-dimensional annular channels are investigated. Since the channel heights are small in comparison to the tube diameter, curvature effects can be neglected and the annular channels can be considered to be essentially two-dimensional. Mixtures that are highly diluted with argon are used since previous investigations seem to indicate that detonations in such mixtures are “stable” in that cellular instabilities play minor roles on the propagation of the detonation. For stable detonations where the ZND structure is valid, boundary layer effects can be modeled as a flow divergence term in the conservation of mass equation following the pioneering work of Fay [J.A. Fay, Phys. Fluids 2(3) (1959) 283–289]. Expansion due to flow divergence in the reaction zone results in a velocity deficit. There exists a maximum deficit when an eigenvalue detonation velocity can no longer be found, which can be taken as the onset of the detonability limits. Experimentally, it was found that unlike “unstable” detonations, the detonability limits for “stable” detonations are well-defined. No unstable near-limit phenomena (e.g., galloping detonations) was observed. Good agreement is found between the theoretical predictions and the experimentally obtained velocity deficits and limits in the two channel heights of 2.2 and 6.9 mm for hydrogen–oxygen and acetylene–oxygen mixtures diluted with over 50% argon. It may be concluded that at least for these special mixtures where the detonation is “stable,” the failure mechanism is due to flow divergence caused by the negative displacement thickness of the boundary layer behind the leading shock front of the detonation wave.     

A mesoscopic approach to the “negative” viscosity effect in ferrofluids

We present a mesoscopic approach to analyse the dynamics of a single magnetic dipole under the influence of an oscillating magnetic field, based on the formulation of a Fokker–Planck equation. The dissipated power and the viscosity of a suspension of such magnetic dipoles are calculated from non-equilibrium thermodynamics of magnetized systems. By means of this method we have found a non-monotonous behaviour of the viscosity as a function of the frequency of the field which has been referred to as the “negative” viscosity effect. Moreover, we have shown that the viscosity depends on the vorticity field thus exhibiting non-Newtonian behaviour. Our analysis is complemented with numerical simulations which reproduce the behaviour of the viscosity we have found and extend the scope of our analytical approach to higher values of the magnetic field.     

Beyond nanosizing: an approach to shape analysis of fluorescent nanostructures by SMI-microscopy

Using spatially modulated illumination (SMI) light microscopy it is possible to measure the sizes of fluorescent structures that have an extension far below the conventional optical resolution limit (“subresolution size”). Presently, the sizes are determined as the object extension along the optical axis of the SMI microscope. For this, however, “a priori” assumptions on the fluorochrome distribution (“shape”) within the examined fluorescent structure have to be made. Usually it is assumed that the fluorochrome follows a Gauss-distribution or a spherical distribution. In this report we overcome the necessity to make an assumption on the shape of the fluorochrome distribution. We introduce two new experimentally obtained parameters which allow the determination of a shape measure to describe the spatial distribution of the fluorescent dye. This becomes possible by independent measurements with different excitation wavelengths. As an example, we present shape parameter measurements on individual fluorescent microspheres with a nominal geometrical diameter (“size”) of 190 nm. In the case investigated, the experimental shape correlated well with a homogeneous fluorochrome distribution (“spherical shape”) but not with a variety of other “shapes”.     

Experimental study of flame stabilization in low Reynolds and Dean number flows in curved mesoscale ducts

Flame stabilization during non-premixed combustion in curved ducts with a diameter of the order of magnitude of the premixed flame thickness of the reactants was investigated experimentally, since it has been established that this is a configuration with potential advantages in the context of “micro”-combustion. It was shown that, in such “mesoscale” tubes, a stable flame oscillation including extinction/re-ignition phenomena can be established for steady boundary conditions. These oscillations lead, under appropriate conditions, to sound emission in the 50–350 Hz range. This was a mode of stabilization in addition to the “classical” steady flamelet, stabilized through thermal losses to the duct walls at higher values of the Reynolds number. Curvature of the duct was shown to have minimal effect on reactant mixing, which was diffusion-controlled, but significantly affected flame thickness and stabilization. To probe the fuel-oxidizer mixing in the U-shaped, optically accessible tubes, laser induced fluorescence of acetone fuel dopant was used, and the flame structure was studied using OH PLIF. The various stabilization regimes are discussed in terms of the Reynolds and Dean numbers of the tube flow.     

Anomalous Hall effect in the mixed state of high-temperature superconductors

The Hall effect in the mixed state of high-T_c superconductors (HTSC) is of an anomalous nature: near the transition there is a range of temperatures and of magnetic fields where the sign of the Hall effect is opposite to that in the normal state. The universality of the phenomenon in question is indicative of its connection with some general properties of the mixed state of type-II superconductors, namely, with peculiarities of motion of magnetic flux vortex lines (vortices) in these superconductors. This work puts forward a model accounting for a number of vortex motion specific features and providing a possibility to obtain the characteristics of the anomalous Hall effect.

The work is based on the phenomenologically generalized results of Bardeen-Stephen and Nozieres-Vinen, supplemented with an allowance for a new mechanism of vortex “friction” associated with Andreev electron reflection on the interface between the normal core and the superconducting periphery of a vortex. Within the framework of the model suggested, magnetic field (and temperature) dependences of the longitudinal and Hall resistances of a mixed state superconductor have been calculated at temperatures nearing T_c. At certain quite realistic parameters which define the forces acting on the vortices, there is a range of magnetic fields and temperatures where the sign of the Hall effect is opposite to that in the normal state. The lower limit of this range is the irreversibility line and the upper critical field.     

Stable bonds and unstable bonds in Y(Pr)-123 system

The Pr content dependence of the bond lengths of Y–Cu, Y–O, O–O, Cu–O, Ba–Cu and Ba–O in PrBa₂Cu₃O_y are analyzed and discussed in detail. The different valence bonds can be divided into two groups: stable bonds and unstable bonds. Since the bond lengths and angles between every two ions among O2, Cu2 and O3 are very stable, we conclude that the three ions form an unchangeable triangle when the Pr doping changes. The triangle is called “fixed triangle”. This “fixed triangle” can slightly rotate around the Cu2 ions. It is just this rotation that leads to the unstable bonds. As the increase of the Pr content, the bond lengths between the two Cu(2)–O planes becomes larger and larger, the Cu(2)–O planes bend towards the Ba–O plane. The bonds lengths between the Cu(2)–O and Ba–O planes vary oppositely from those of Cu(2)–O, and become shorter and shorter. These changes connected with the superconductivity are discussed.     

Symmetric behavior of vortex states of superconducting networks in a magnetic field

We present magnetic field dependence of phase transition temperature and vortex configuration of superconducting networks based on theoretical study. The applied magnetic field is called “filling field” that is defined by applied magnetic flux (in unit of the flux quantum) per unit loop of the superconducting network. If a superconducting network is composed of very thin wires whose thicknesses are less than coherence length, the de Gennes–Alexander (dGA) theory is applicable. We have already shown that field dependences of transition temperature curves have symmetric behavior about the filling field of 1/2 by solving the dGA equation numerically in square lattices, honeycomb lattices, cubic lattices and those with randomly lack of wires networks. Many experimental studies also show the symmetric behavior. In this paper, we make an explicit theoretical explanation of symmetric behaviors of superconducting network respect to the applied field.     

Electrical and magnetic properties in Co-P amorphous alloys

Different anomalies in the Hall effect, electrical resistivity, and magnetic susceptibility of simple Co-P amorphous alloys have been observed. We show that the anomalies detected are due to a transition from “para” to “ferromagnetic” order. The transition, which is not well defined, suggests a large compositional inhomogeneity in the samples with high content in P.     

Deformed “commutative” Chern–Simons’ system

Noncommutative Chern–Simons’ system is non-perturbatively investigated at a full deformed level. A deformed “commutative” phase space is found by a non-canonical change between two sets of deformed variables of noncommutative space. It is explored that in the “commutative” phase space all calculations are similar to the case in commutative space. Spectra of its energy and angular momentum of the Chern–Simons’ system are obtained at the full deformed level. The noncommutative–commutative correspondence is clearly showed. Formalism for the general dynamical system is briefly presented. Some subtle points are clarified.     

Vertical magneto-tunneling through a quantum dot and the density of states of small electronic systems

One-electron tunneling through a quantum dot with a strong magnetic field in the direction of the current is studied. The linear magneto-conductance is computed for a model parabolic dot with seven electrons in the intermediate states and for different values of the magnetic field. It is shown that the dot density of states at low excitation energies can be extracted from a precise measurement of the conductance at the upper edge of the Coulomb blockade diamond. We parametrized the density of states with a single “temperature” parameter (in the so called “constant temperature approximation”), and found that this parameter depends very weakly on the magnetic field.     

Nucleation and growth mechanism for flame synthesis of MoO2 hollow microchannels with nanometer wall thickness

The growth and morphological evolution of molybdenum-oxide microstructures formed in the high temperature environment of a counter-flow oxy-fuel flame using molybdenum probes is studied. Experiments conducted using various probe retention times show the sequence of the morphological changes. The morphological row begins with micron size objects exhibiting polygonal cubic shape, develops into elongated channels, changes to large structures with leaf-like shape, and ends in dendritic structures. Time of probe–flame interaction is found to be a governing parameter controlling the wide variety of morphological patterns; a molecular level growth mechanism is attributed to their development. This study reveals that the structures are grown in several consecutive stages: material “evaporation and transportation”, “transformation”, “nucleation”, “initial growth”, “intermediate growth”, and “final growth”. XRD analysis shows that the chemical compositions of all structures correspond to MoO₂.     

Three-dimensional diffuse optical imaging of hand joints: System description and phantom studies

In this paper we describe a three-dimensional (3D) continuous wave (CW) diffuse optical tomography (DOT) system and present 3D volumetric reconstruction studies using this DOT system with simple phantom models that simulate hand joints. The CCD-based DOT system consists of 64×64 source/detector fiber optic channels, which are arranged in four layers, forming a cylindrical fiber optic/tissue interface. Phantom experiments are used to evaluate system performance with respective to axial spatial resolution, optical contrast and target position for detection of osteoarthritis where cartilage is the primary target region of interest. These phantom studies suggest that we are able to quantitatively resolve a 2 mm thick “cartilage” and qualitatively resolve a 1 mm thick “cartilage” using our 3D reconstruction approach. Our results also show that optical contrast of 3:1–7:1 between the “disease cartilage” and normal cartilage can be quantitatively recovered. Finally, the target position along axial direction on image reconstruction is studied. All the images are obtained using our 3D finite-element-based reconstruction algorithm.