Ideal Dipole Research Articles

Quantum demixing in binary mixtures of dipolar bosons

Quantum Monte Carlo simulations of a two-component Bose mixture of trapped dipolar atoms of identical masses and dipole moments, provide numerical evidence of de-mixing at low finite temperatures. De-mixing occurs as a consequence of quantum statistics, which results in an effective attraction between like bosons. Spatial separation of two components takes place at low temperature with the onset of long exchanges of identical particles, underlying Bose-Einstein condensation of both components. Conversely, at higher temperature the system is miscible due to the entropy of mixing. Exchanges are also found to enhance de-mixing in the case of mixtures of non-identical and distinguishable species.

Magnetic black box

This paper introduces an experiment involving two identical small dipole magnets. One is hidden with arbitrary orientation in a black box. The second is suspended on a string above the box. By studying the horizontal and vertical oscillations of the suspended magnet, it is possible to determine the magnetic moment and the orientation of the hidden magnet, and the horizontal component of the Earth’s magnetic field.

A dipole loudspeaker with a balanced directivity pattern

Analytical equations describing radiation characteristics of an oscillating ring in a circular finite baffle are derived, including the limiting case of a dipole point source at the center. An oscillating sphere would represent the ideal dipole source, having a constant directivity pattern at all frequencies, but would be inconvenient to realize especially in portable devices. It is found that a planar piston with uniform surface velocity but variable phase arranged to emulate the sphere does not have such a smooth on-axis response as the sphere. Instead a planar piston with the same phase distribution but uniform pressure represents an ideal planar source with a smooth on-axis response and near constant directivity. The surface velocity is plotted and it is then shown that a similar response can be achieved using a finite number of concentric rings based on this velocity distribution.

Vertically Polarized Dipoles and Monopoles, Directivity, Effective Height and Antenna Factor

The effective length (Le) of linear antennas like dipoles and the effective height (He) of monopoles shorter than a wavelength are determined using a transmitting Hertz Dipole as a reference. Effective Receiving Area (AeR) has been calculated in the receiving mode and hence the Antenna Receiving Directivity (DR) or Gain (GR)) can be determined. From these calculations, it can be seen that the Received Power (WR) in a link between two dipole antennas in free space or between two monopole antennas over a perfect ground is of the same value in the far field region. As a result, the directivity or gain of the ideal dipole antenna in free space has the same value in transmission and reception; but, a different value for monopole and dipole antennas installed over a perfect ground plane. This result is fulfilled in the ideal as well as the real case where losses and mismatchings are involved.

Scattering of the near field of an electric dipole by a single-wall carbon nanotube

The use of carbon nanotubes as optical probes for scanning near-field optical microscopy requires an understanding of their near-field response. As a first step in this direction, we investigated the lateral resolution of a carbon nanotube tip with respect to an ideal electric dipole representing an elementary detected object. A Fredholm integral equation of the first kind was formulated for the surface electric current density induced on a single-wall carbon nanotube (SWNT) by the electromagnetic field due to an arbitrarily oriented electric dipole located outside the SWNT. The response of the SWNT to the near field of a source electric dipole can be classified into two types, because surface-wave propagation occurs with (i) low damping at frequencies less than ~ 200-250 THz and (ii) high damping at higher frequencies. The interaction between the source electric dipole and the SWNT depends critically on their relative location and relative orientation, and shows evidence of the geometrical resonances of the SWNT in the low-frequency regime. These resonances disappear when the relaxation time of the SWNT is sufficiently low. The far-field radiation intensity is much higher when the source electric dipole is placed near an edge of SWNT than at the centroid of the SWNT. The use of an SWNT tip in scattering-type scanning near-field optical microscopy can deliver a resolution less than ~ 20 nm. Moreover, our study shows that the relative orientation and distance between the SWNT and the nanoscale dipole source can be detected.

Mutual Coupling Effects on Pattern Diversity Antennas for MIMO Femtocells

Diversity antennas play an important role in wireless communications. However, mutual coupling between multiple ports of a diversity antenna has significant effects on wireless radio links and channel capacity. In this paper, dual-port pattern diversity antennas for femtocell applications are proposed to cover GSM1800, UMTS, and WLAN frequency bands. The channel capacities of the proposed antennas and two ideal dipoles with different mutual coupling levels are investigated in an indoor environment. The relation between mutual coupling and channel capacity is observed through investigations of these antennas.

Novel Multilayer Dipoles for Wireless Inter-/Intraconnects

A novel multi-metal-layer dipole antenna, in which some identical dipoles are deployed on different metal layers and effectively connected by the feeding and shorting vias, is proposed for wireless inter-/intraconnects. An accurate equivalent model is also presented for better understanding of its physical fundamentals. Simulations show that a six-layer dipole typically has a size reduction of 13.0% and a bandwidth enhancement of 35.3% compared with the corresponding one-layer dipole. An example chip carrying one- and six-layer dipoles was also fabricated through a 0.18-?m mixed-mode process and on-wafer characterized with full two-port measurements. The experimental results show that further size reduction and bandwidth enhancement could be obtained due to the loadings of lossy vias in six-layer dipoles, and the transmission coefficients of multilayer dipoles could also be improved for the in-phase additive contributions of dipoles on different layers. The propagation mechanisms of wireless intraconnects were evaluated and discussed with the assistance of path-loss coefficient ?, and it is revealed that the propagation is mostly through the surface waves when the dipole operate far below its resonant frequency, while the space-wave propagation would dominate when approaching the resonant frequency.

ANALYSIS AND DESIGN OF AN UHF RFID METAL TAG USING MAGNETIC COMPOSITE MATERIAL AS SUBSTRATE

Using magnetic composite material as the substrate for RFID metal tag has several advantages over conventional metal tags, such as flexibility and miniaturized size. In this paper, the radiation intensity contributed by a half-wave dipole is derived based on the result of an ideal Hertzian dipole, which leads to a simple relation for thin substrate. Later on, the material constants of two materials are measured and the one capable of generating greater radiation intensity is used in the course of antenna design. A primitive pattern design demonstrates the metal tag has a satisfactory 2.7m reading range, and a dimension of 80× 22× 2mm3.

Fretting About FRET: Breakdown of the Ideal Dipole Approximation

Fluorescence-detected resonance energy transfer (FRET) experiments have been a useful tool in structural biology for four decades and have enjoyed resurgence in the last several years due to improved fluorescent labeling techniques and the rapid growth of single-molecule methods. As modern experiments examine a variety of complex systems, the validity of the assumptions that underlie analysis of FRET data is unclear. In this talk I will examine one of these, the ideal dipole approximation (IDA). Calculations showing the breakdown of the IDA in several commonly-used FRET probes (e.g. Fluorescein, AlexaFluor 488 and 594, Cy3, Cy5) will be presented and connections will be drawn to the impact on FRET experiments. In particular, breakdown of the IDA exacerbates problems due to limited sampling of dye orientations (i.e. the kappa squared problem). Guidelines will be suggested for planning a FRET experiment to avoid potential issues with the IDA and other assumptions employed in analysis of FRET data.

Performances of the scanning system for the CNAO center of oncological hadron therapy

In hadron therapy one of the most advanced methods for beam delivery is the active scanning technique which uses fast scanning magnets to drive a narrow particle beam across the target. The Centro Nazionale di Adroterapia Oncologica (CNAO) will treat tumours with this technique. The CNAO scanning system includes two identical dipole magnets for horizontal and vertical beam deflection, each one connected to a fast power supply. The dose delivery system exploits a set of monitor chambers to measure the fluence and position of the beam and drives the beam during the treatment by controlling the sequence of currents set by the power supplies. A test of the dynamic performance of the scanning system has been performed using a Hall probe to measure the field inside the magnet and the results are presented in this paper.

Fretting about FRET: Failure of the Ideal Dipole Approximation

With recent growth in the use of fluorescence-detected resonance energy transfer (FRET), it is being applied to complex systems in modern and diverse ways where it is not always clear that the common approximations required for analysis are applicable. For instance, the ideal dipole approximation (IDA), which is implicit in the Förster equation, is known to break down when molecules get “too close” to each other. Yet, no clear definition exists of what is meant by “too close”. Here we examine several common fluorescent probe molecules to determine boundaries for use of the IDA. We compare the Coulombic coupling determined essentially exactly with a linear response approach with the IDA coupling to find the distance regimes over which the IDA begins to fail. We find that the IDA performs well down to roughly 20 Å separation, provided the molecules sample an isotropic set of relative orientations. However, if molecular motions are restricted, the IDA performs poorly at separations beyond 50 Å. Thus, isotropic probe motions help mask poor performance of the IDA through cancellation of error. Therefore, if fluorescent probe motions are restricted, FRET practitioners should be concerned with not only the well-known κ 2 approximation, but also possible failure of the IDA.

Ideal dipole approximation fails to predict electronic coupling and energy transfer between semiconducting single-wall carbon nanotubes

The electronic coupling values and approximate energy transfer rates between semiconductor single-wall carbon nanotubes are calculated using two different approximations, the point dipole approximation and the distributed transition monopole approximation, and the results are compared. It is shown that the point dipole approximation fails dramatically at tube separations typically found in nanotube bundles ( approximately 12-16 A) and that the disagreement persists at large tube separations (>100 A, over ten nanotube diameters). When used in Forster resonance energy transfer theory, the coupling between two point transition dipoles is found to overestimate energy transfer rates. It is concluded that the point dipole approximation is inappropriate for use with elongated systems such as carbon nanotubes and that methods which can account for the shape of the particle are more suitable.

Modelling line-of-sight coupled MIMO systems with generalised scattering matrices and spherical wave translations

A model to rigorously characterise line-of-sight MIMO systems is introduced. It is based on the generalised scattering matrix of each antenna, considered as isolated, and the rotation and translation coefficients of spherical modes. The resulting channel matrix rigorously includes the exact spherical vector nature of electromagnetic propagation (which may exert a significant influence over short-range links), mutual coupling effects and real antenna reflection, transmission, reception and scattering features. Numerical results are presented for MIMO systems made up of ideal dipoles.

Response of juvenile scalloped hammerhead sharks to electric stimuli

Sharks can use their electrosensory system to detect electric fields in their environment. Measurements of their electrosensitivity are often derived by calculating the voltage gradient from a model of the charge distribution for an ideal dipole. This study measures the charge distribution around a dipole in seawater and confirms the close correspondence with the model. From this, it is possible to predict how the sharks will respond to dipolar electric fields comprised of differing parameters. We tested these predictions by exposing sharks to different sized dipoles and levels of applied current that simulated the bioelectric fields of their natural prey items. The sharks initiated responses from a significantly greater distance with larger dipole sizes and also from a significantly greater distance with increasing levels of electric current. This study is the first to provide empirical evidence supporting a popular theoretical model and test predictions about how sharks will respond to a variety of different electric stimuli.

Optimal Single-Port Matching Impedance for Capacity Maximization in Compact MIMO Arrays

A complete multiple-input multiple-output (MIMO) system model with compact arrays at both link ends containing arbitrary matching networks is presented based on a Z-parameter approach. The complete channel matrix including the coupling effect is also presented. Utilizing this system model, the optimum single-port matching impedance for capacity maximization is derived for a 2 times 2 MIMO system with coupling at the receivers only. A closed-form result for the optimum matching impedance in high signal-to-noise ratio scenarios is given and proved to be equal to the input impedance of the receive end. Simulation of ideal dipoles verifies our analytical results and demonstrates the superiority of the optimum matching to other matching conditions in improving MIMO system performance. Experimental data for monopoles is also presented to further confirm our numerical findings and validate the accuracy of our derivation.

Low-energy resonances and bound states of aligned bosonic and fermionic dipoles

The low-energy scattering properties of two aligned identical bosonic and identical fermionic dipoles are analyzed. Generalized scattering lengths are determined as functions of the dipole moment and the scattering energy. Near resonance, where a new bound state is being pulled in, all nonvanishing generalized scattering lengths diverge, with the ${a}_{00}$ and ${a}_{11}$ scattering lengths being dominant for identical bosons and identical fermions, respectively. Implications for the energy spectrum and the eigenfunctions of trapped two-dipole systems and for pseudopotential treatments are discussed.

Beamforming correction for dipole measurement using two-dimensional microphone arrays

In this paper, a beamforming correction for identifying dipole sources by means of phased microphone array measurements is presented and implemented numerically and experimentally. Conventional beamforming techniques, which are developed for monopole sources, can lead to significant errors when applied to reconstruct dipole sources. A previous correction technique to microphone signals is extended to account for both source location and source power for two-dimensional microphone arrays. The new dipole-beamforming algorithm is developed by modifying the basic source definition used for beamforming. This technique improves the previous signal correction method and yields a beamformer applicable to sources which are suspected to be dipole in nature. Numerical simulations are performed, which validate the capability of this beamformer to recover ideal dipole sources. The beamforming correction is applied to the identification of realistic aeolian-tone dipoles and shows an improvement of array performance on estimating dipole source powers.

Optical solitary waves in three-level media: effects of different dipole moments

The exact electric polarization operator of the Maxwell-Bloch equations for a three-level Λ-type system is derived without assuming equality of the transition dipole moments. Numerical solution of the resulting solitary wave equations yields a richer spectrum than found earlier for identical dipole moments [Hioe and Grobe, Phys. Rev. Lett.73, 2559 (1994)]. The two components of the new solitary waves each have a zero-pulse area and take the form of modulated oscillations.

Assessment of quantum chemical methods and basis sets for excitation energy transfer

The validity of several standard quantum chemical approaches and other models for the prediction of exciton energy transfer is investigated using the HOMO–LUMO excited states of benzene dimer as an example. The configuration interaction singles (CIS), time-dependent Hartree-Fock (TD-HF), time dependent density functional theroy (TD-DFT), and complete-active-space self-consistent-field (CASSCF) methods are applied with a supermolecule approach and compared to the previously established monomer transition density method and the ideal dipole approximation. Strong and physically incorrect admixture of charge-transfer states makes TD-DFT inappropriate for investigations of potential energy surfaces in such dimer systems. CIS, TD-HF and CASSCF perform qualitatively correct. TD-HF seems to be a particularly appropriate method due to its general applicability and overall good performance for the excited state and for transition properties. Double-zeta basis sets with polarisation functions are found to be sufficient to predict transfer rates of dipole allowed excitations. Efficient excitation energy transfer is predicted between degenerate excited states while avoided curve crossings of nearly spaced π-aggregates are identified as a possible trapping mechanism.

Calculations of the exciton coupling elements between the DNA bases using the transition density cube method

Excited states of the double-stranded DNA model (A)12.(T)12 were calculated in the framework of the Frenkel exciton theory. The off-diagonal elements of the exciton matrix were calculated using the transition densities and ideal dipole approximation associated with the lowest energy pipi* excitations of the individual nucleobases as obtained from time-dependent density functional theory calculations. The values of the coupling calculated with the transition density cubes (TDC) and ideal dipole approximation (IDA) methods were found to be significantly different for the small interchromophore distances. It was shown that the IDA overestimates the coupling significantly. The effects of structural fluctuations of the DNA chain on the magnitude of dipolar coupling were also found to be very significant. The difference between the maximum and minimum values was as large as 1000 and 300 cm(-1) for the IDA and TDC methods, respectively. To account for these effects, the properties of the excited states were averaged over a large number of conformations obtained from the molecular dynamics simulations. Our calculations using the TDC method indicate that the absorption of the UV light creates exciton states carrying the majority of the oscillator strength that are delocalized over at least six DNA bases. Upon relaxation, the excitation states localize over at least four contiguous bases.