Wolff Model Research Articles

A broad-band self-consistent modelling of the X-ray spectrum of 4U 1626−67

The accretion-powered X-ray pulsar 4U 1626--67 is one of the few highly magnetized pulsars that accretes through Roche-lobe overflow from a low-mass companion. The characteristics of its broadband spectrum are similar to those of X-ray pulsars hosted in a high-mass X-ray binary systems, with a broad resonant cyclotron scattering feature (CRSF) at $\sim$37 keV. In this work, we examine the pulse-resolved and the pulse-averaged broadband spectrum using data from NuSTAR and Swift. We use the Becker & Wolff model of bulk+thermal Comptonization to infer key physical parameters of the accretion column flow and a broadband model for the disk reflected spectrum. In the softer X-ray band, we need to add a soft black-body component with $kT_{\rm bb}$ $\sim$0.5 keV, whose characteristics indicate a possible origin from the neutron star surface. Residuals suggest that the shape of the cyclotron line could be more satisfactorily fitted using a narrow core and broader wings and, at higher energies, a second harmonic could be present at $\sim$61 keV.

Interplay between magnetic and superconducting order in the non-stoichiometric iron pnictide [formula omitted

It is shown that As vacancies in the LaFeAsO superconductor induce a strong enhancement of the paramagnetic response both in the normal and the superconducting state. Although As vacancies themselves are non-magnetic defects, they are able to form localized magnetic moments due to strong covalent bonding with neighboring Fe ions. The microscopic theory of magnetic moment formation in semi-metallic state of non-stoichiometric LaFeAsO is constructed in a framework of appropriately modified Anderson–Wolff model. Experimental manifestations of enhanced paramagnetism and the influence of magnetic fluctuations on superconducting pairing are briefly discussed.

Spin-Polarization in Magneto-Optical Conductivity of Dirac Electrons

A mechanism is proposed based on the Kubo formula to generate a spin-polarized magneto-optical current of Dirac electrons in solids which have strong spin-orbit interactions such as bismuth. The ac current response functions are calculated in the isotropic Wolff model under an external magnetic field, and the selection rules for Dirac electrons are obtained. By using the circularly polarized light and tuning its frequency, one can excite electrons concentrated in the spin-polarized lowest Landau level when the chemical potential locates in the band gap, so that spin-polarization in the magneto-optical current can be achieved.

Wiedemann-Franz law in the SU(N) Wolff model

We study the electrical and thermal transport through the SU$(N)$ Wolff model with the use of bosonization. The Wilson ratio reaches unity as $N$ grows to infinity. The electric conductance is dominated by the charge channel, and decreases monotonically with increasing interaction. The thermal conductivity enhances in the presence of a local Hubbard $U$. The Wiedemann-Franz law is violated; the Lorentz number depends strongly on the interaction parameter, which can be regarded as a manifestation of spin-charge separation.

Bosonization study of the SU(N) Wolff model

We have studied the low temperature properties of the SU(N) Wolff impurity model via Abelian bosonization. The path integral treatment of the problem allows for an exact evaluation of low temperature properties of the model. The single particle Green's function enhances due to the presence of local correlation. The basic correlation function such as the charge or spin correlator are also affected by the presence of impurity, and show local Fermi liquid behaviour.

Multilayer Cavity Model for Microstrip Rectangular and Circular Patch Antenna

We report an improved form of the cavity model called the modified Wolff model (MWM) to compute the resonance frequency, input impedance, bandwidth, efficiency, directivity, and radiation pattern of a multilayer rectangular and circular patch antenna in the lossy iso/anisotropic medium. We compare the MWM against the experimental results obtained from the several sources and also against the computed results of the method-of-moments–based Ensemble. The MWM is revealed as an accurate, fast, and reliable model for computation of all these parameters.

Quantum Coherence Tunnelings: Multi-Kondo Peaks and Anomalous Coulomb Oscillations**The project supported by the National University of Singapore under Grant R151000009-112 and the Huazhong University of Science and Technology under Grant No. 200201-01

On the basis of a multi-level Anderson–Wolff model, an electronic tunneling is studied numerically in the self-consistent field approximation. It is shown that in the intermediate coupling regime, conductance and magnetization could display universal fluctuations. In particular, new anomalous Coulomb oscillations assisted by the multi-Kondo peaks are predicted theoretically.

Accurate Determination of Input Parameters of Rectangular Microstrip Patch Antenna on Thick Substrate and Comparision of Several Models

Theoretical models like standard cavity model, multiport cavity model (MCM), MOM-based commercial software, and Ensemble do not provide accurate results on the resonance frequency, input impedance, and bandwidth for the rectangular patch on the thick substrate (h > 0.05 λ g ). This paper reports an improved form of the cavity model called the modified Wolff model (MWM) that computes accurately these parameters of the rectangular patch on the thick substrate up to the substrate thickness 0.229 λ g . We also compare all of these models against the experimental results obtained from the several sources. The MWM comes out as the most accurate model for the computation of input parameters of the rectangular patch both on the thin and thick substrates also having anisotropy.

Determination of dielectric constant and loss‐tangent of substrate sheet using microstrip patch resonator

AbstractThis paper presents a new microstrip rectangular patch resonator based method to determine the dielectric constant and the loss tangent of a dielectric material in sheet form. Using an improved form of the cavity model, called the modified Wolff model (MWM), the present method extracts these two parameters from the measured resonance frequency and Q‐factor. The MWM takes care of all kind of losses in the patch, resulting in accurate determination of the dielectric constant and loss tangent of the substrate. © 2002 Wiley Periodicals, Inc. Microwave Opt Technol Lett 35: 175–179, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.10549

Analysis of circular microstrip antennas on low‐ and high‐relative‐permittivity substrates

AbstractA simple and accurate cavity model called the modified Wolff model (MWM) is presented to compute the resonance frequency and input impedance of the circular microstrip antenna on the PTFE, alumina, and GaAs substrates. Results of the MWM show better agreement with the experimental results as compared to results of the commercial software; namely, the standard cavity model adopted in the PCAAD, the multiport cavity model (MCM), and the method‐of‐moments– (MOM) based Ensemble. The MWM works with high accuracy on both the thin and thick substrates and from microwave to millimeter‐wave range. © 2002 Wiley Periodicals, Inc. Microwave Opt Technol Lett 34: 75–80, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.10377

Resonance frequency and bandwidth of rectangular microstrip antenna on thick substrate

We report an improved cavity model called modified Wolff model (MWM) to compute the resonance frequency of a rectangular patch antenna on the thick PTFE substrate (0.037/spl lambda//sub g/-0.229/spl lambda//sub g/). The MWM accounts for the effect of anisotropy and total losses on the resonance frequency, therefore it has a maximum deviation 2% against the experimental results. The previous cavity model, the multiport cavity model, and MOM based commercial software, Ensemble, compute resonance frequency with deviation between 4%-36%. Results on the bandwidth computed by these models have also been compared against the experimental results.

Input impedance of rectangular microstrip patch antenna with iso/anisotropic substrate-superstrate

The modified Wolff model (MWM), which is an improved version of the cavity model, is presented to compute the resonance frequency and input impedance of a rectangular microstrip patch antenna under the isotropic/anisotropic substrate-superstrate configuration. The model has accuracy better than 1.5% for both the resonance frequency and resonant resistance, as compared against the results of the method of moments (MOM) and experimental results.

Input impedance of probe‐fed multilayer rectangular microstrip patch antenna using the modified Wolff model

AbstractThe modified Wolff model (MWM)‐based cavity model is presented for the simple and accurate computation of the input impedance of a probe‐fed multilayer rectangular microstrip patch antenna. Results of the MWM are compared with both the experimental data and data of the MoM, with agreement within 1.5% for resonance frequency and within 0.15 for normalized input impedance. Further results are presented on the input impedance of a suspended rectangular microstrip patch antenna with a two‐layer superstrate. © 2001 John Wiley & Sons, Inc. Microwave Opt Technol Lett 31: 237–239, 2001.

Resonance frequency of anisotropicmultilayer rectangular microstrip patch using MWM

The resonance frequency of a multilayer anisotropic microstrip rectangular patch is determined by using the modified Wolff model (MWM formulation) of the cavity model. The model has a deviation of 1-2%, against the results obtained using spectral domain analysis (SDA).

Numerical singularities in many-body moments expansions

It is well known that connected moments expansions when used to approximate the ground-state energy of Hamiltonian systems may suffer from extraneous singularities in various regions of parameter space. In this brief Letter a numerical investigation is presented to study an approximation scheme developed by Ullah to avoid the singularities found in moments expansions. Specifically, we study the single-impurity Wolff model as well as the 32-site Anderson lattice.

Magnetic properties of a magnetic impurity in a strongly correlated electronic system

With use of a quantum Monte Carlo method, we investigate the magnetic properties of a substitutional magnetic impurity in a strongly correlated electronic system, i.e., the Wolff model with an on-site repulsive interaction between conduction electrons. It is found that the magnetic susceptibility at the impurity-site decreases at low temperatures with increase in the interaction strength. Furthermore, dependence of the equal-time density correlation functions with the aligned and the opposite spins on the interaction strength indicates that conduction electrons only with the opposite spins to the magnetic impurity increase around the impurity at low temperatures as the interaction is increased. The equal-time spin correlation function is also calculated.

Local moment formation at interstitial impurity sites in metals

We have calculated the electronic structure and related properties around an interstitial impurity in several metallic hosts. This was done using the real-space linear muffin tin orbital scheme (RS-LMTO-ASA), a first-principles, self-consistent approach implemented directly in real space. We show that interstitial Fe does not develop a local moment in trivalent and tetravalent Sc, Y, Ti and Zr hosts. In divalent Ba, Ca, Sr and Yb we find that the appearance of local moment is extremely dependent on nearest neighbor relaxation, while in alkali metals such as K, we expect the interstitial Fe impurity to be magnetic. We show that these trends can be qualitatively understood using simple ideas based on the Wolff model and the Stoner criterium. We also consider Fe impurities in Gd, a trivalent magnetic host. We find an unusually large induced local moment at the interstitial Fe site and discuss the origin of this effect. Finally, we compare our results with TDPAD and in beam Mossbauer experiments in these systems.

Quantum Monte Carlo study of a magnetic impurity in a strongly correlated electron system

Using a quantum Monte Carlo method, we study the magnetic properties of a substitutional magnetic impurity in a strongly correlated electron system, i.e. the Wolff model with an on-site repulsive interaction between conduction electrons. We find that the magnetic susceptibility at the impurity site decreases at low temperatures with increasing the interaction between conduction electrons. The local moment and the equal-time spin correlation function are also calculated. The results are compared with those of the Hubbard model.

Correlation effects in the transport through a few quantum levels

An extended version of the Wolff model for the Kondo impurity is applied to tunnelling phenomena through a small region with a few quantized levels. In particular, the magnetic properties of the small region, which will affect the transport, are studied by using the quantum Monte Carlo (QMC) method.

Fermi-liquid analysis on a Wolff model for resonant tunneling through a single quantum level.

Fermi-liquid analysis on a Wolff model for resonant tunneling through a single quantum level.