Aperiodic Fluctuations Research Articles

Detection of Nanoscale Magnetic Activity Using a Single Carbon Nanotube

The ultimate conductometric sensor for ferromagnetic activity of nanoscale magnetic materials could be a single carbon nanotube. We show that the electrical conductance of an individual carbon nanotube is sensitive to magnetic transitions of nanoscale magnets embedded inside it. To establish this, multiwall carbon nanotubes were impregnated with cobalt nanoclusters. Temperature dependence of conductance (5 K < T <300 K) of these nanotubes shows the usual Lüttinger-liquid power law behavior at higher temperatures and an onset of Coulomb blockade at lower temperatures. At the lowest temperature (T approximately 6 K), the differential conductance (dI/dV versus V) develops aperiodic fluctuations under an external magnetic field B, the rms amplitude of which grows with the magnitude of the field itself. Low-temperature magnetoconductance, studied as function of temperature and bias, can be interpreted in terms of weak antilocalization effects due to the presence of the magnetized clusters. The temperature dependence of magnetoconductance further presents a "peak"-like feature and slow dynamics around T =55 K, which depend on the magnitude and history of the applied B field. These observations indicate a sensitivity of electronic transport in the multiwall nanotubes to the dynamics of nanoscale magnets at low temperature.

Resistance Fluctuations and Aharonov–Bohm-Type Oscillations in Antidot Arrays in the Quantum Hall Regime

Resistance fluctuation phenomenon in antidot lattices in the quantum Hall regime are studied. Magnetoresistance of finite antidot array systems in the quantum Hall plateau transition regime exhibits two types of oscillatory effect. One is the aperiodic resistance fluctuations (RFs) and the other is the Aharonov-Bohm (AB)-type oscillations. Their dependences on the magnetic field and the gate voltage are quite distinct. While the aperiodic RFs are attributed to the complex evolution of the conducting network of compressible channels, the AB-type oscillations are interpreted in terms of edge states formed around individual antidots. The self-consistent screening effect is important for the both phenomenon, whereas, the single electron charging effect plays a minor role in the present case.

Unsteady Aperiodic Behavior of a Periodically Disturbed Tip-Leakage Vortex

unsteadyperiodicvorticalinflow representativeof anarrayofstator necklace vortices.Thedownstream tip-leakage vortex is disturbed periodically by the vortical inflow shed by those vortex generators. Phase-averaged two-point correlations are measured near the trailing edge of the cascade. The measurement results clearly show that organized, intense, large-scale aperiodic motion exists at each phase time within the tip-leakage-vortex region. The aperiodic fluctuations obtained by linear stochastic estimation have a magnitude of less than � 0:15U1 in all three velocity directions and appear to be organized large-scale structures. The streamwise fluctuations appear as coexisting “rush” and “retard” regions with different shapes, and the crossflow fluctuations appear to be clockwise/ anticlockwise or doubletlike rotating vortical structures with different strengths and center positions. These aperiodic fluctuations make the estimated instantaneous velocity field significantly different from the phaseaveraged periodic velocity field.

Spontaneous emission of Weibel fluctuations by anisotropic distributions

Recently [Yoon, Phys. Plasmas 14, 064504 (2007)], the spontaneous emission of magnetic field fluctuations in isotropic particle distribution functions was investigated. Here, the question is addressed as to how these fluctuations develop for an anisotropic distribution function that supports the Weibel instability. It is shown that, depending on the parameters, either electromagnetic or aperiodic magnetic fluctuations are dominant.

Manifestations of phase-coherent transport in graphene

The electronic transport properties of graphene exhibit pronounced differences from those of conventional two dimensional electron systems investigated in the past. As a consequence, well established phenomena such as the integer quantum Hall effect and weak localization manifest themselves differently in graphene. Here we present an overview of recent experiments that we have performed to probe phase coherent transport. In particular, we have investigated in great detail Josephson supercurrent and superconducting proximity effect in junctions consisting of a graphene layer in between superconducting electrodes. We have also used the same devices to measure aperiodic conductance fluctuations and weak localization. The experimental results clearly indicate that lowtemperature transport in graphene is phase coherent on a ?1 ?m length scale, irrespective of the position of the Fermi level. We discuss the different behavior of Josephson supercurrent and weak localization in terms of the unusual properties of the electronic states in graphene upon time reversal symmetry.

The Relativistic Filamentation Instability in Magnetized Plasmas

The instability of perpendicular aperiodic fluctuations in a magnetized overall-neutral two-stream distribution function with relativistic speeds is investigated. It is demonstrated that there exists a critical magnetic field strength, above which the filamentation instability is not excited. The theory is applied to relativistic jets in AGNs, microquasars, and radio lobes in large radio galaxies.

Turbulence Measurements and Analysis in a Multistage Axial Turbine

This paper presents turbulence measurements and detailed flow analysis in an axial turbine stage. Fast response aerodynamic probes were used to resolve aperiodic fluctuations along the three directions. Assuming incompressible flow, the effective turbulence level and Reynolds stress are retrieved by evaluating the stochastic velocity component out of the measured time-resolved pressure and flow angle fluctuations along the streamwise, radial, and circumferential direction. A comparison between turbulence intensity and measured total pressure shows that flow structures with higher turbulence level are identified in the region of loss cores at the exit of the second stator passage. Turbulence intensity is evaluated under isotropic and nonisotropic assumption in order to quantify the departure from isotropic conditions. The measurements show that locally the streamwise fluctuating component can be twice bigger than the radial and tangential component. The current analysis shows that multisensor fast response aerodynamic probes can be used to provide information about the mean turbulence levels in the flow and the Reynolds stress tensor, in addition to the measurements of unsteady total pressure loss.

Plasma Instabilities in Gamma‐Ray Bursts: Neutral Points and the Effect of Mode Coupling

The method of determining neutral points, corresponding to vanishing frequency in wavenumber space, is applied to the case of gamma-ray bursts (GRBs) to determine low-frequency instabilities. If a particular particle distribution function permits the existence of neutral points, instability is guaranteed on one side or the other of the neutral point. By using the rest frame of the GRB jets, only perpendicular particle motion has to be taken into account. The discussion is limited to the cases of a monoenergetic electron-proton plasma with stationary protons and also to a symmetric electron-positron plasma. In the limiting case of wave propagation perpendicular to a homogeneous background magnetic field, taken to be parallel to the streaming direction of the jet, it is shown that to lowest order in the wave frequency, unstable aperiodic fluctuations occur that grow very rapidly on timescales comparable to or smaller than the characteristic times of the GRB, leading to the conclusion that the jets most likely consist mainly of electron-proton plasmas for the cases investigated. Furthermore, high electron Lorentz factors, as well as the coupling effect between longitudinal and transverse modes, are shown to stabilize the system.

Simulation of transport processes during Czochralski growth of YAG crystals

Numerical simulation of transport phenomena in the solid, liquid and gaseous phases of a Czochralski process is reported. The Czochralski domain comprises a YAG melt, crystal and gas within the enclosure. The mathematical model is axisymmetric in space and unsteady in time. The governing equations are those of conservation of mass, momentum and energy. The simulation includes a bulk radiation model to account for the semi-transparency of the YAG melt and the growing crystal. Results have been obtained for thermal boundary conditions that do not change with time, a constant diameter growing crystal for which the pull velocity changes with time. Buoyant convection in the melt is seen to produce a melt–crystal interface that is convex into the melt. When the crystal is given rotation, centrifugal forces drive a clockwise roll that counteracts the thermally driven motion. At a specific rotation rate, the interface shape changes from convex to concave. The critical rotation rate for interface inversion has been obtained in the study as a function of the radius ratio and the aspect ratio. Marangoni convection has an effect of strengthening buoyancy-driven flow. Unsteadiness in the YAG melt is observed at high Grashof numbers. The introduction of crystal rotation at high Grashof numbers is found to change the periodic oscillations to aperiodic high amplitude fluctuations. Simulation that includes the crystal and the gas phases along with the melt reveals the possibility of superheating of the crystal beyond its melting point. Similarly, the possibility of subcooling of the melt near the crystal edge below the melting point of YAG is indicated for a certain range of parameters. The internal absorption of radiation in the crystal increases thermal losses from the melt, steepens temperature gradients and is found to create deeply convex melt–crystal interface towards the melt. Additionally, the bulk of the melt is found to become cooler. Scattering is found to have an opposite, though minor influence on the melt–crystal interface, strength of natural convection and pull velocity.

M31 Globular Cluster X‐Ray Sources:XMM‐NewtonandChandraResults

We present the results of an M31 globular cluster (GC) X-ray source survey, based on the data of XMM-Newton and Chandra observations covering ~6100 arcmin2 of M31. We detected 43 X-ray sources coincident with GC candidates from various optical surveys. The inferred isotropic X-ray luminosities of GC sources lie between ~1035 and ~1039 ergs s-1 in the 0.3-10 keV energy band. The spectral properties of the 31 brightest sources in our sample were found to be similar to those of the low-mass X-ray binaries (LMXBs) located in the bulge and the GCs of the Milky Way. The spectral distribution of the M31 GC X-ray sources is consistent with that derived for the bulge of M31 and other nearby galaxies of different morphological type. Several sources demonstrate a correlation between the level of X-ray flux and the hardness of their energy spectrum reminiscent of the Galactic Z and atoll sources. We found that ~80% of the M31 GC sources with multiple flux measurements available show significant variability on timescales from days to years. The X-ray source RX J0043.2+4127, coincident with GC Bo 163, has been found to show recurrent transient outbursts with peak luminosities of ~1038 ergs s-1. Several sources in our sample show significant variability on a timescale of individual observations, ranging from aperiodic fluctuations to regular dipping. The X-ray luminosity function of GC sources is found to be significantly different from that of the point sources in the bulge and disk of M31. The luminosity distribution of M31 GC sources has ~10 times higher peak luminosity and a much higher fraction of bright sources than the Milky Way GC distribution. Six persistent sources in our sample (or ~14% of the total number) have luminosities exceeding 1038 ergs s-1 during all observations, and three other sources occasionally exceed that luminosity level. Our observations indicate that GC sources make the dominant contribution to the bright source counts in the areas of M31 covered by the survey: ~40% of the total number of sources with luminosities above 1037 ergs s-1 reside in GCs, with the fraction of GC sources rising to 67%-90% for luminosities above 1038 ergs s-1. The contribution of the GC sources to the total number of bright sources found in M31 is much higher than in the Milky Way but surprisingly close to that of early-type galaxies. We found that the brightest M31 GC sources tend to reside at large galactocentric distances outside the central bulge. We found that GCs hosting bright X-ray sources are optically brighter and more metal-rich than the rest of the M31 GCs, in agreement with previous studies. The brightest sources, with luminosities above ~1038 ergs s-1, show a tendency to reside in more metal-poor clusters. The remarkable similarities between the properties of the M31 GC X-ray sources and those of Galactic neutron star LMXBs lead us to expect most of the persistent M31 GC X-ray sources to be LMXB systems with neutron star primaries. However, the current X-ray spectral and timing data cannot rule out the possibility of finding active accreting black holes in our GC source sample.

Steady, annual, and monthly recharge implied by deep unconfined aquifer flow

We consider the response of a deep unconfined horizontal aquifer to steady, annual, and monthly recharge. A groundwater divide and a zero head reservoir constrain the aquifer, so that sinusoidal monthly and aperiodic annual recharge fluctuations create transient specific discharge near the reservoir and an unsteady water table elevation inland. One existing and two new long-term data sets from the Plymouth-Carver Aquifer in southeastern Massachusetts calibrate and confirm hydraulic properties in a set of analytical models. [Geohydrology and simulated groundwater flow, 1992] data and a new power law for tritiugenic helium to tritium ratios calibrate the steady recharge that drives the classical parabolic model of steady hydraulics [Applied Hydrogeology, 2001]. Observed water table and gradient fluctuations calibrate the transient recharge models. In the latter regard, monitoring wells within 1 km of Buttermilk Bay exhibit appreciable specific discharge and reduced water table fluctuations. We apply [Trans Am Geophys Union 32(1951)238] periodic model to the monthly hydraulics and a recharge convolution integral [J Hydrol 126(1991)315] to annual flow. An infiltration fraction of 0.79 and a consumptive use coefficient of 1.08×10 −8 m/s °C relate recharge to precipitation and daylight weighted temperature across all three time scales. Errors associated with this recharge relation decrease with increasing time scale.

Quantum transport properties in a ferromagnetic nanoring

Abstract Low-temperature transport properties in a ferromagnetic Fe 19 Ni 81 nanoring were studied. In the nanoring, resistance oscillation due to the Aharonov–Bohm effect of a conduction electron was observed. However, the reproducibility of resistance with magnetic field, which is usually observed in non-magnetic metals, was not observed. This result suggests that some kinds of degrees of freedom, which persist even at low temperature break in part the reproducibility of the aperiodic fluctuation in magnetoresistance.

ＡＦＭ酸化を用いて作製したＩｎＡｓ表面反転層およびＩｎＡｓ／ＡｌＧａＳｂ量子井戸ナノスケール開放型ドット中での電子波干渉効果

Electron interference effects at 4.2K were studied in nano-scale open dots fabricated in InAs surface inversion layers and in an InAs/AlGaSb quantum well. These structures were fabricated using AFM oxidation. We have observed periodic oscillations and aperiodic fluctuations on both systems. We investigated oscillation period using Fast Fourier Transform analysis. We found that for the InAs surface inversion layers the oscillation periods correspond to that of Aharonov-Bohm effect (AB effect) in agreement with the dot size. In contrast, for the InAs/AlGaSb quantum well, the deduced oscillation period did not correspond to the dot size. These results suggest that the observed AB oscillations are caused by the channel formed along the dot periphery due to the electron transfer from the InAs oxide to the InAs. Aperiodic fluctuations were characterized using correlation field analysis. For InAs/AlGaSb quantum well, correlation field was increased due to smaller skipping radius, as magnetic field increased. For InAs inversion layers, the correlation fields were not changed because the dot size is smaller than the cyclotron radius. The estimated electron phase breaking times tf were 1 psec and 10 psec for the InAs surface inversion layers and quantum well, respectively. The longer tf in InAs than that of GaAs systems at 4.2K indicates the importance of electron interference in InAs even at higher temperatures.

Quantum transport properties in ferromagnetic nanorings at low temperature

The low-temperature magnetoresistance in ferromagnetic Ni and Fe19Ni81 nanorings were studied. Although the aperiodic fluctuation and the periodic oscillation were observed in Fe19Ni81 nanoring, the periodic oscillation disappears in Ni nanoring. The estimated phase coherence length in Ni nanoring is about 80 nm, which is much smaller than that in Fe19Ni81 nanoring (∼500 nm). These results imply that there exists a mechanism, such as coupling between conduction electron and local magnetic anisotropy.

Low temperature magnetoresistance in ferromagnetic nanoring system

Quantum interference effect of conduction electrons in a ferromagnetic mesoscopic sized ring has been investigated. With decreasing temperature below 4.2 K, fine structures appear in the magnetoresistance. The observed aperiodic fluctuation of resistance with changing magnetic field can be attributed to the interference effect in the diffusive metals.

Magneto-transport properties of InAs/AlGaSb open quantum dot structures

We report on the magneto-transport properties of InAs/AlGaSb open quantum dot structures. The open dot structures have been fabricated by using electron beam lithography. The open dot structures are characterized by magnetoresistance measurements at 4.2 K. In addition to the Shubnikov-de Hass oscillations, aperiodic magnetoresistance fluctuations have been observed at low magnetic fields below 1 T. The amplitude of the fluctuations was quite large even at 4.2 K in comparison with the case of the GaAs/AlGaAs system. Electron-wave interference effect and phase-breaking times in InAs mesoscopic structures are discussed from the analyses using correlation functions of magneto-conductance fluctuations.

Comment on “Magnetoresistance and differential conductance in mutliwalled carbon nanotubes”

Jeong-O Lee et al. [Phy. Rev. B, 61, R16 362 (2000)] reported magnetoresistance and differential conductance measurements of multiwalled carbon nanotubes. The observed aperiodic conductance fluctuations and the negative magnetoresistance was interpreted to originate exclusively from changes in the density of states at the Fermi energy. We show that this interpretation is questionable and not supported by their measurements.

Decadal Variability of Two Oceans and an Atmosphere

A model of the midlatitude, large-scale interaction between the upper ocean and the troposphere is used to illustrate possible mechanisms of connection between the decadal variability in the North Atlantic and in the North Pacific. The two ocean basins are connected to each other only through their coupling to the common, zonally averaged atmosphere. The ocean–atmosphere coupling takes place via wind-driven torques and heat fluxes at the air–sea interface. In this formulation, the decadal variability in each ocean basin consists of ocean–atmosphere modes and arises from a delayed feedback of the upper-ocean heat content onto the wind-driven flow mediated by the atmosphere through the requirements of global heat and momentum balances. The presence of two ocean basins leads to three basic kinds of coupling-induced behavior: phase locking, oscillation death, and chaos. In the phase-locked regime, the western boundary currents of the two basins oscillate in synchrony, with the narrower basin following the wider basin by a small time lag. In the oscillation death solutions, a steady solution is reached, even though each ocean basin, when uncoupled, would have sustained oscillations. In the chaotic regime, the interbasin coupling induces aperiodic fluctuations in both basins characterized by variability at centennial, as well as decadal, timescales.

Three-dimensional flow field downstream of an embedded stator in a multistage axial flow compressor: Part 1: Steady and unsteady flow fields

A comprehensive investigation of the three-dimensional unsteady flow and thermal field downstream of an embedded stator in a multistage compressor, acquired with a high-response hot-film probe and aspirating probe, is presented and analysed. Some of the earlier data (from a five-hole probe and a high-response Kulite probe) from the same compressor is used with the present data to provide an integrated interpretation of the flow and thermal fields. Part 1 includes a brief description of the facility, the development of the hot-film technique for the multistage flow field measurement and an interpretation of the various flow field features, such as the hub clearance flow and the suction surface-casing end-wall corner region. Part 2 covers velocity-velocity and velocity-temperature correlations and the assessment of their magnitudes in the average-passage equations. The results presented in this part of the paper (Part 1) indicate that major blockage in the stator hub end wall is caused by leakage flow and its possible subsequent roll-up into a vortex. Similar features exist in the suction surface casing end-wall corner due to secondary flow and its interaction with the upstream rotor end-wall flow. These are also the regions with the highest levels of unsteadiness. The transport of rotor wake towards the pressure side of the stator is confirmed. Both the revolution and the blade periodic velocity fluctuations are found to be significantly greater than the aperiodic fluctuations.

Emergence of log-periodic oscillations in periodic and aperiodic Ising models

This work analyzes the emergence of log-periodic oscillations in thermodynamic functions of Ising models on hierarchical lattices. Several situations, where the exchange interactions are periodic or aperiodic, are taken into account. High precision values for the thermodynamic functions are numerically obtained with the method of transfer matrices. Fitting the curves close to the critical temperature leads to the values of the critical exponents and to the period and amplitude of the oscillations. The first two quantities are found to agree with the results predicted by the renormalization group. The amplitude of oscillations, which are minute for both periodic systems and those with aperiodic irrelevant fluctuations, are significantly enhanced for systems with aperiodic relevant fluctuations. Distinct morphologies of the oscillating pattern are discussed, where oscillations are respectively sinusoidal and with a significant contribution of higher order harmonics.