Two-dimensional Localization Research Articles

Metabolism-enhanced tumor localization by fluorescence imaging: in vivo animal studies.

We present a high-sensitivity near-infrared optical imaging system for noninvasive cancer detection and localization based on molecularly labeled fluorescent contrast agents. This frequency-domain system utilizes the interferencelike pattern of diffuse photon density waves to achieve high detection sensitivity and localization accuracy for the fluorescent heterogeneity embedded inside the scattering media. A two-dimensional localization map is obtained through reflectance probe geometry and goniometric reconstruction. In vivo measurements with a tumor-bearing mouse model by use of the novel Cypate-mono-2-deoxy-glucose fluorescent contrast agent, which targets the enhanced tumor glycolysis, demonstrate the feasibility of detection of a 2-cm-deep subsurface tumor in the tissuelike medium, with a localization accuracy within 2-3 mm.

LOCALIZATION OF NORTH ATLANTIC RIGHT WHALE SOUNDS IN THE BAY OF FUNDY USING A SONOBUOY ARRAY

AbstractA free‐drifting 14‐sonobuoy array was used to localize North Atlantic right whales (Eubalaena glacialis) in the Grand Manan Basin area of the Bay of Fundy. This area is a primary summer/autumn right whale habitat and overlaps an international shipping lane. The three‐hour deployment on a single day provided two‐dimensional localization of 94 right whale sounds based on arrival time differences determined from spectrogram cross‐correlation analysis. The sounds were of two distinct types: tonal and gunshot. Maximum detection distances were about 30 km for both types of sound. The mean RMS location error was 1.8 km for tonal‐type sounds and 2.5 km for gunshot‐type sounds. The average RMS error was 20% of the average distance from the receiving hydrophones, the primary source of error being uncertainty in the sonobuoy positions.

Large negative magnetoresistance and two-dimensional weak localization in carbon nanofiber fabricated using electrospinning

The carbon nanofibers used in this work were derived from polyacrylonitrile/N,N-dimethyl formamide precursor solution using electrospinning and vacuum pyrolysis techniques. Their conductivity σ was measured at temperatures between 1.9 and 300 K and transverse magnetic field between −9 and 9 T. Zero magnetic field conductivity σ(0, T) was found to increase monotonically with the temperature with a convex σ(0, T) vs T curve. The conductivity increases with the external transverse magnetic field, revealing negative magnetoresistance at temperatures between 1.9 and 10 K, with a maximum magnetoresistance of −75% at 1.9 K and 9 T. The magnetic field dependence of the conductivity can be explained using the two-dimensional (2D) weak localization effect. The temperature dependence of σ(0, T) can be explained using a modified simple two-band model with temperature-dependent carrier mobility, and a correction introduced by 2D weak localization.

About wave localization in two-dimensional random media

In this Letter, we wish to discussion some basic questions pertinent to the phenomenon of Anderson localization of classical waves in two-dimensional random media. Although a definite answer to the two-dimensional localization is not yet found, a common consensus has been reached based upon the scaling analysis Abraham et al. [Phys. Rev. Lett. 43 (1979) 679]. That is, all waves are localized in two dimensions for any given amount of disorders. This view has been prevalent for more than two decades. Here, we explain some recent results and considerations that tend to be contrary to this view or the consequences of it.

Kondo effect in underdopedn-type superconductors

We present high-field magnetotransport properties of high-quality single-crystalline thin films of heavily underdoped nonsuperconducting $(\mathrm{L}\mathrm{a},\mathrm{C}\mathrm{e}{)}_{2}{\mathrm{CuO}}_{4},$ $(\mathrm{P}\mathrm{r},\mathrm{C}\mathrm{e}{)}_{2}{\mathrm{CuO}}_{4},$ and $(\mathrm{N}\mathrm{d},\mathrm{C}\mathrm{e}{)}_{2}{\mathrm{CuO}}_{4}.$ All three materials show identical behavior. They are metallic at high temperatures and show an insulating ``upturn'' at low temperatures. The insulating upturn has a $\mathrm{log}T$ dependence, but saturates toward the lowest temperatures. Notably, the insulating upturn tends to be suppressed by applying magnetic fields. This negative magnetoresistance has a $\mathrm{log}B$ dependence, and its anisotropy shows a nonsimple behavior. We discuss these findings from the viewpoints of Kondo scattering and also two-dimensional weak localization, and demonstrate Kondo scattering as a more plausible explanation. The Kondo scatters are identified as ${\mathrm{Cu}}^{2+}$ spins in the ${\mathrm{CuO}}_{2}$ planes.

Two-Dimensional Localization Detected by the Imaginary Vector Potential

In non-interacting disordered systems with the timereversal symmetry, all the states are believed to be localized in two or lower dimension. The role of the nonHermitian effect such as the relaxation processes breaks the phase coherence, and provides a gradual crossover into metallic behavior. An exception to the rule is the non-Hermitian term coupled with the first-order differential, i.e., the imaginary vector potential. It has been recognized subsequently that sufficiently large imaginary vector potential always induces a delocalization transition (not crossover) with complex eigen energies. The purpose of the paper is to establish a new way of probing the localization property by using this nonHermitian effect as a probe. Unlike previous attempts to estimate the localization length from the complex energy spectrum, our procedure is on principle applicable to any dimension. We confirm the validity by examining the 2D systems numerically. It is shown that the breakdown of the imaginary gauge transformation can characterize the inverse localization length near the band center, though we need to clarify its meaning. The non-Hermitian delocalization process is far from complete understanding in the twoand higher-dimensions, e.g., whether the transition is continuous or abrupt. On this regard, we also briefly mention our view based on our numerical results. The Hamiltonian considered for numerical calculations is the 2D non-Hermitian tight-binding model,

Two-dimensional electronic system and weak localization of the conducting LB films of BO and fatty acid

The conducting Langmuir–Blodgett (LB) films of BEDO-TTF (BO) and stearic acid show a weakly localized conduction based on the two-dimensional electronic system (2DES). We prepared the mixed LB films of BO and fatty acid with different molecular length. The LB films of BO with behenic acid or parmitic acid show the negative magnetoresistance indicating the existence of the two-dimensional weak localization with the electronic length of 667 and 580 Å, respectively. From these results, the formation of 2DES is a common feature in the BO and fatty acid LB films.

A detection system for three-dimensional visualization of cosmic muons trajectories

A detection system for measuring the direction of cosmic muons is presented. The detector is composed by two multiwire proportional chambers 55 mm apart from each other. These chambers operate as two independent subdetectors in coincidence. Each subdetector provides two-dimensional (2D) localization of ionizing events making use of a single X and Y cathode structure for position readout. 2D localization is achieved by associating one delay line to each spatial coordinate. The trajectory is reconstructed from the two pairs of coordinates measured in the subdetector planes. Signals related to the X and Y coordinates of ionizing events are electronically processed and recorded by a data acquisition interface connected to a host personal computer (PC). A user interface software shows the trajectories of ionizing events in a three-dimensional graphic window. Cosmic particles trajectories are visualized in real time. Results are shown that characterize the system performance. The angular and length distributions for measured and simulated muon trajectories are also presented. The obtained results show that, besides the educational purpose of showing real time cosmic ray trajectories, the presented system is also suitable for studying physical properties of cosmic muons.

Orientation dependence of magneto-resistance behaviour in a carbon nanotube rope

The orientation dependence of magneto-resistance behaviour for a single-walled carbon nanotube (SWCN) rope is reported. A clear delineation of behaviours is observed between applying a magnetic field perpendicular or parallel to the rope axis. For a perpendicular field, monotonic negative magneto-resistance is observed due to two-dimensional weak localisation within the rope. By contrast, for a parallel field, complicated oscillatory behaviour is observed due to the Altshuler–Aronov–Spivak effect around closed electron trajectories on discrete cylinders within the SWCN rope. A dominant oscillatory mode can be identified which corresponds to closed paths around the outer circumference of the SWCN rope. However, due to the composite filamentary nature of the rope, the overall oscillatory behaviour is rather complicated and can be classified as universal conductance fluctuations. With a backgate voltage applied to the sample, Coulomb blockade peaks are observed in the transport current with additional peak structure superimposed due to resonant tunnelling. We find an interesting effect whereby these peaks are suppressed in the presence of a magnetic field.

2D localization using resistive strips associated to the Micromegas structure

An attractive method of two-dimensional localization based on charge division with Ni–Cr resistive strips has been adapted to a Micromegas detector taking advantage of its high gain. Along the strips, depending on the signal to noise ratio, 1.05 mm – 300 μm spatial resolutions (respectively, 0.80–0.23% of the strip length) are obtained with an 3 H source. For trajectory reconstruction in magnetic fields, these accuracies could match most of the localization constraints requested for measurements of the non-bending coordinate. In the direction perpendicular to the strips, localizations better than 100 μm could be achieved using the center of gravity of the charges.

Magnetic-field-induced superconductor–metal-insulator transitions in bismuth metal graphite

Bismuth-metal graphite (MG) has a unique layered structure where Bi nanoparticles are encapsulated between adjacent sheets of nanographites. The superconductivity below $T_{c}$ (= 2.48 K) is due to Bi nanoparticles. The Curie-like susceptibility below 30 K is due to conduction electrons localized near zigzag edges of nanographites. A magnetic-field induced transition from metallic to semiconductor-like phase is observed in the in-plane resistivity $\rho_{a}$ around $H_{c}$ ($\approx$ 25 kOe) for both $H$$\perp$$c$ and $H$$\parallel$$c$ ($c$: c axis). A negative magnetoresistance in $\rho_{a}$ for $H$$\perp$$c$ (0$<H\leq$3.5 kOe) and a logarithmic divergence in $\rho_{a}$ with decreasing temperature for $H$$\parallel$$c$ ($H$ $>$ 40 kOe) suggest the occurrence of two-dimensional weak localization effect.

Observation of two-dimensional weak localization as a sign of coherent carrier transport in the conducting Langmuir–Blodgett films of BEDO-TTF and stearic acid

Abstract We report the DC electrical transport properties in the conducting Langmuir–Blodgett (LB) films formed by a molecular association of the electroactive donor molecule bis(ethylenedioxy)tetrathiafulvalene (BEDO-TTF) and stearic acid CH 3 (CH 2 ) 16 COOH. The DC resistivity is 1.25×10 −2 Ωcm at 300 K and the temperature variation is metallic. A logarithmic increase of the DC resistivity and the negative magnetoresistance are observed below 120 K. They are interpreted by the weak localization (WL) of a two-dimensional (2D) electronic system confined within the homogeneous conducting layer with molecular size thickness of BEDO-TTF. The existence of the WL indicates that the system is essentially in the coherent regime. A mesoscopic scale of the electronic length with phase memory provides evidence of the 2D coherent charge transport in the conducting LB films.

Particle-hole symmetric localization in two dimensions

We revisit two-dimensional particle-hole symmetric sublattice localization problem, focusing on the origin of the observed singularities in the density of states $\ensuremath{\rho}(E)$ at the band center $E=0.$ The most general system of this kind [R. Gade, Nucl. Phys. B 398, 499 (1993)] exhibits critical behavior and has $\ensuremath{\rho}(E)$ that diverges stronger than any integrable power law, while the special random vector potential model of Ludwig et al. [Phys. Rev. B 50, 7526 (1994)] has instead a power-law density of states with a continuously varying dynamical exponent. We show that the latter model undergoes a dynamical transition with increasing disorder---this transition is a counterpart of the static transition known to occur in this system; in the strong-disorder regime, we identify the low-energy states of this model with the local extrema of the defining two-dimensional Gaussian random surface. Furthermore, combining this ``surface fluctuation'' mechanism with a renormalization group treatment of a related vortex glass problem leads us to argue that the asymptotic low-E behavior of the density of states in the general case is $\ensuremath{\rho}(E)\ensuremath{\sim}{E}^{\ensuremath{-}1}{e}^{\ensuremath{-}c|\mathrm{ln}E{|}^{2/3}},$ different from earlier prediction of Gade. We also study the localized phases of such particle-hole symmetric systems and identify a Griffiths ``string'' mechanism that generates singular power-law contributions to the low-energy density of states in this case.

Formation of two-dimensional weak localization in conducting Langmuir-Blodgett films

We report the magnetotransport properties up to 7 T in the organic highly conducting Langmuir-Blodgett(LB) films formed by a molecular association of the electroactive donor molecule bis(ethylendioxy)tetrathiafulvalene (BEDO-TTF) and stearic acid CH$_3$(CH$_2$)$_{16}$COOH. We show the logarithmic decrease of dc conductivity and the negative transverse magnetoresistance at low temperature. They are interpreted in the weak localization of two-dimensional (2D) electronic system based on the homogeneous conducting layer with the molecular size thickness of BEDO-TTF. The electronic length with phase memory is given at the mesoscopic scale, which provides for the first time evidence of the 2D coherent charge transport in the conducting LB films.

Laplace spectrum for exponential decomposition and pole–zero estimation

A novel Laplace transform power spectrum is proposed as a basic tool for complex-exponential decomposition of finite-duration continuous functions. The weighted spectrum leads to a direct decomposition in terms of exponential damping function content in addition to the usual decomposition in terms of sinusoidal circular content produced by Fourier and Laplace transforms. The weighting results in a two-dimensional spectral peak on the Laplace s-plane identifying the exponential and frequency content of finite-duration functions. An algorithm for two-dimensional generalised spectral analysis of finite duration functions is described. It is shown that the proposed spectral weighting eliminates the exponential divergence of spectra of usual transforms which masks poles along one dimension of the complex plane. As an application, an algorithm for mathematical modelling pole-zero estimation through a direct two-dimensional localisation of pole-zero peaks of weighted spectra is described.

Development of high gain GEM detectors

We describe systematic measurements, carried out with single and double GEM detectors with printed circuit read-out. The maximum safe operating gain has been measured at increasing radiation flux, and under exposure to heavily ionizing tracks. Detection efficiency, localization accuracy and cluster size have been measured in a minimum ionizing particle beam. With a suitably configured readout electrode, fast two-dimensional localization of radiation is demonstrated.

Effects of geometric and refractive index disorder on wave propagation in two-dimensional photonic crystals

The effects of disorder in the geometry and refractive index on the transmittance of two-dimensional photonic crystals composed of dielectric circular cylinders are considered, including randomness of radii, positions of the cylinder centers, and thickness of each layer of the photonic crystal. The effects of combinations of different types of strong disorder are also considered. The localization and homogenization properties of disordered photonic crystals are investigated. Analytical expressions for the two-dimensional localization length in the form of integrals are presented for both polarizations. It is shown numerically that the slope of the exponential divergence of the localization length in two dimensions is proportional to the inverse of the square of randomness for strong disorder and proportional to the inverse of the randomness for weak disorder. The effective dielectric constants for both polarizations in the case of strong disorder are also found. The transition from localization to homogenization is discussed and the terms responsible for this transition are identified and investigated.

Transport and NMR study of the scandium boron carbide compoundSc2B1.1C3.2with a boron and carbon mixed graphitelike layer

Transport properties and nuclear magnetic resonance (NMR) of the new scandium boron carbide compound ${\mathrm{Sc}}_{2}{\mathrm{B}}_{1.1}{\mathrm{C}}_{3.2}$ were investigated. ${\mathrm{Sc}}_{2}{\mathrm{B}}_{1.1}{\mathrm{C}}_{3.2}$ has a trigonal crystal structure $[a=b=23.710(9)A,$ $c=6.703(2)A,$ $P3m1]$ and is composed of alternate $[{\mathrm{B}}_{1/3}{\mathrm{C}}_{2/3}{]\ensuremath{-}\mathrm{S}\mathrm{c}\ensuremath{-}\mathrm{C}\ensuremath{-}\mathrm{S}\mathrm{c}\ensuremath{-}[\mathrm{B}}_{1/3}{\mathrm{C}}_{2/3}]$ layers, with the boron and carbon mixed layer $[{\mathrm{B}}_{1/3}{\mathrm{C}}_{2/3}]$ forming a very rare graphitelike structure. Physical properties similar to graphite intercalation compounds (GIC) were observed. The temperature dependence of the resistivity showed a large anisotropy. The in-plane resistivity showed a metallic quadratic dependence, also observed in some GIC's while the resistivity along the c axis, perpendicular to the layers, increased with decreasing temperature. From magnetic susceptibility and specific-heat measurements, the orbital susceptibility was indicated to take a paramagnetic value. At low temperatures an increase of the in-plane resistivity with $\mathrm{log}T$ dependence and negative magnetoresistance was observed. Two-dimensional Anderson localization was indicated, possibly originating from disorder in the $[{\mathrm{B}}_{1/3}{\mathrm{C}}_{2/3}]$ graphitelike layer. ${}^{11}\mathrm{B}$ magic angle spinning (MAS) nuclear magnetic resonance (NMR) results indicate a large distribution of the chemical shift of the boron nuclei, which is consistent with the existence of disorder within the graphitic layer.

Two-dimensional localization of excitons in QWs formed by II–VI solid solutions

We have studied the localization of two-dimensional exciton states in II–VI heterostructures by compositional fluctuations of solid solution, forming the quantum well. It is shown that due to the two-dimensional character of exciton motion in quantum wells, the effect of compositional fluctuation on exciton states is much stronger than in three-dimensional solutions of the same composition. The method for calculation of the density of fluctuation states below the edge of two-dimensional exciton band and the spectral density of exciton transitions is developed. The classification of states with respect to its migration properties and contribution to the luminescence processes has been carried out using the continuum percolation theory. As a result, the shape of emission band, its relative shift with respect to absorption band and the position of mobility edge are calculated. The results of calculations are in good agreement with the experimental PL and PLE spectra of ZnSe superlattices with submonolayer insertions of CdSe.

Imaging and Tracking of Single GFP Molecules in Solution

Visualization and tracking of single fluorescent molecules is a recent development in optical microscopy holding great promise for the study of cell biological processes. However, all experimental strategies realized so far confined the observation to extremely thin interfacial layers. The detection and characterization of single molecules in three-dimensionally extended systems such as living cells has yet to be accomplished. We show, here, for the first time that single protein molecules can be visualized and tracked in three-dimensional (3D) samples at room temperature. Using a wide-field fluorescence microscope equipped with an Ar +-laser and a low-light-level CCD camera, single molecules of the green fluorescent protein (GFP) were detected in gels and viscous solutions at depths of up to ∼10 μm from the interface. A time resolution of 5 ms was achieved by a high-speed framing mode. The two-dimensional localization accuracy was determined to be ∼30 nm. The number of photons emitted by single GFP molecules before photodestruction was found to be ≤ 410 5. Freely diffusing GFP molecules could be tracked over up to nine images acquired at a frame rate of ∼80 Hz. From the trajectories, the diffusion coefficients of single GFP molecules were derived and found to agree well with expectation and microphotolysis measurements. Our results imply that the visualization and tracking of single molecules in living cells is possible.