Magnetic shielding properties of GdBCO bulks with different crystal orientation

AbstractHigh-temperature bulk superconductors have significant potential for superconductivity applications. For this paper, the magnetic shielding properties of GdBCO bulk with different crystal orientations were investigated at different temperatures for the purpose of determining its application as field concentrators. Four discs with a diameter of 20mm and thickness of 5mm were cut from the GdBCO single domain. In two discs, the c-axis of crystal was parallel to the disc radius, and, in the others, the c-axis was perpendicular to it. The magnetic shielding effects in a couple of bulks with a gap of 2mm were measured in background fields up to 11 T. The magnetic fields were measured at the center and edge points between the two bulks at LN2, LHe, and temperatures controlled with a cryocooler. In LHe, the discs whose c-axes were parallel to the external magnetic fields maintained a zero field up to 11 T. Even in LN2, the field was shielded to 1 T. The results confirmed the strong magnetic shield effects of GdBCO bulk and can be used for the design of a field concentrator

Effects of nonlinear sweep in the Landau-Zener-Stueckelberg effect

We study the Landau-Zener-Stueckelberg (LZS) effect for a two-level system with a time-dependent nonlinear bias field (the sweep function) W(t). Our main concern is to investigate the influence of the nonlinearity of W(t) on the probability P to remain in the initial state. The dimensionless quantity epsilon = pi Delta ^2/(2 hbar v) depends on the coupling Delta of both levels and on the sweep rate v. For fast sweep rates, i.e., epsilon << l and monotonic, analytic sweep functions linearizable in the vicinity of the resonance we find the transition probability 1-P ~= epsilon (1+a), where a>0 is the correction to the LSZ result due to the nonlinearity of the sweep. Further increase of the sweep rate with nonlinearity fixed brings the system into the nonlinear-sweep regime characterized by 1-P ~= epsilon ^gamma with gamma neq 1 depending on the type of sweep function. In case of slow sweep rates, i.e., epsilon >>1 an interesting interference phenomenon occurs. For analytic W(t) the probability P=P_0 e^-eta is determined by the singularities of sqrt{Delta ^2+W^2(t)} in the upper complex plane of t. If W(t) is close to linear, there is only one singularity, that leads to the LZS result P=e^-epsilon with important corrections to the exponent due to nonlinearity. However, for, e.g., W(t) ~ t^3 there is a pair of singularities in the upper complex plane. Interference of their contributions leads to oscillations of the prefactor P_0 that depends on the sweep rate through epsilon and turns to zero at some epsilon. Measurements of the oscillation period and of the exponential factor would allow to determine Delta, independently.Comment: 11 PR pages, 12 figures. To be published in PR

Photoacoustic Signal Enhancement by Localized Surface Plasmon of Gold Nanoparticles

Photoacoustic imaging has been widely studied as a deep biological tissue imaging modality combining optical absorption and ultrasonic detection. It enables multi-scale high resolution imaging of optical absorbing intrinsic molecules as well as exogenous molecules. Gold nanoparticles have the primary advantages of large absorption cross section and bioconjugation capability for the imaging contrast agents. In order to design the photoacoustic imaging agents for enhancing the contrast with high specificity to targeted molecules and / or cell, we measured and analyzed time-of-flight photoacoustic signals of aqueous solutions of various shapes and sizes of gold nanoparticles. The signal intensities were sensitive to the shapes and sizes of the gold nanoparticles. We found a strong photoacoustic signal of the polyhedral gold nanoparticle due to the localized surface plasmon resonance. The experimental results derive the strategy of designing the optimum photoacoustic contrast agents. When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3543

Direct evidence for ferromagnetic spin polarization in gold nanoparticles

We report the first direct observation of ferromagnetic spin polarization of Au nanoparticles with a mean diameter of 1.9 nm using X-ray magnetic circular dichroism (XMCD). Owing to the element selectivity of XMCD, only the gold magnetization is explored. Magnetization of gold atoms estimated by XMCD shows a good agreement with the results obtained by conventional magnetometry. This result is evidence of intrinsic spin polarization in nano-sized gold.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let

Polarizations and Nullcone of Representations of Reductive Groups

The paper starts with the following simple observation. Let V be a representation of a reductive group G, and let f_1,f_2,...,f_n be homogeneous invariant functions. Then the polarizations of f_1,f_2,...,f_n define the nullcone of k 0} h(t) x = 0 for all x in L. This is then applied to many examples. A surprising result is about the group SL(2,C) where almost all representations V have the property that all linear subspaces of the nullcone are annihilated. Again, this has interesting applications to the invariants on several copies. Another result concerns the n-qubits which appear in quantum computing. This is the representation of a product of n copies of $SL_2$ on the n-fold tensor product C^2 otimes C^2 otimes ... otimes C^2. Here we show just the opposite, namely that the polarizations never define the nullcone of several copies if n <= 3. (An earlier version of this paper, distributed in 2002, was split into two parts; the first part with the title ``On the nullcone of representations of reductive groups'' is published in Pacific J. Math. {bf 224} (2006), 119--140.

Breakdown of the Two-Step Model in K-Shell Photoemission and Subsequent Decay Probed by the Molecular-Frame Photoelectron Angular Distributions of CO_2

We report results of measurements and of Hartree-Fock level calculations of molecular-frame photoelectron angular distributions (MFPADs) for C 1s photoemission from CO2. The agreement between the measured and calculated MFPADs is on average reasonable. The measured MFPADs display a weak but definite asymmetry with respect to the O+ and CO+ fragment ions at certain energies, providing evidence for an overlap of gerade and ungerade final ionic states giving rise to a partial breakdown of the two-step model of core-level photoionization and its subsequent Auger decay

Transient response of a quantum wave to an instantaneous potential step switching

The transient response of a stationary state of a quantum particle in a step potential to an instantaneous change in the step height (a simplified model for a sudden bias switch in an electronic semiconductor device) is solved exactly by means of a semianalytical expression. The characteristic times for the transient process up to the new stationary state are identified. A comparison is made between the exact results and an approximate method.Comment: 8 pages, 8 figures, Revtex