Hole Bandwidth Research Articles

Relation between Anharmonicity of Free-Energy Profile and Spectroscopy in Solvation Dynamics: Differences in Spectral Broadening and Peak Shift in Transient Hole-Burning Spectroscopy Studied by Equilibrium Molecular Dynamics Simulation

Solvation dynamics is used to monitor the time-dependent fluctuation of solvents, which plays an essential role in chemical reactions in solution. Transient hole-burning spectroscopy, in which a ground-state depletion (hole) formed by a laser pulse is observed, can be used to monitor solvation dynamics. Previous experiments demonstrated that the hole bandwidth relaxes an order of magnitude slower than the hole peak shift in organic solute-solvent systems. However, the detailed mechanisms behind this are still unclear. In this study, we developed a methodology to calculate transient hole spectra using equilibrium molecular dynamics simulation, in which a series of time-dependent system ensembles is accumulated to derive the appropriate dynamic properties. The simulated transient hole spectra adequately reproduced previous spectroscopic results. The different hole bandwidth and peak shift dynamics are ascribed to a non-Gaussian property or anharmonicity of the free energy profile with respect to the solvation coordinate.

Joint Power, Original Bandwidth, and Detected Hole Bandwidth Allocation for Multi-Homing Heterogeneous Networks Based on Cognitive Radio

In this paper, we investigate a joint resource allocation problem based on cognitive radio (CR) techniques for user equipment with multi-homing capabilities. We consider a heterogeneous wireless medium where users in overlapping coverage areas simultaneously communicate with different base stations and access points. Currently, existing works assume that the working frequency bands of different networks are separated. Unlike these works, this paper focuses on the multi-homing networks, which can share spectrum resources of each other to enhance the resource utilization efficiency. Based on spectrum sensing and spectrum sharing techniques in CR, we propose and then formulate an uplink joint original bandwidth, detected hole bandwidth and power allocation method. Specifically, the formulated optimization problem is a mixed integer nonlinear optimization problem. We adopt the continuity relaxation method to further transform it into a convex optimization problem and then solve it by Lagrange dual solution. A suboptimal method is further proposed with a reduced system overhead. Simulation results demonstrate the significantly improved performance of our proposed methods (both optimal and suboptimal) in terms of system throughput and energy efficiency over a joint resource allocation benchmark. Our results also indicate that the suboptimal strategy can indeed reduce the system overhead remarkably.

Design and Fabrication of the Novel Miniaturized Microstrip Coupler 3dB Using Stepped Impedance Resonators for the ISM Applications

In this work, a novel miniaturized compact coupler using the shunt-stubs artificial transimission lines with high and low impedances is presented. Design of the proposed coupler is accomplished by modifying the length and impedance of the branch lines in the conventional structure with the planar resonators in order to achieve branch line coupler with compact size and improvement of the performances. First part of this work is focusing on the theorical study of the proposed resonators where the equations are obtained. Secondly, the proposed coupler is designed on FR4 susbtrate, and simulated by using the EM Solver (ADS from Agilent technologies and CST microwave studio) in order to operate in the ISM band. The obtained results show good agreement with the simulations and the coupler shows a good perfo6rmance in the hole bandwidth. The size of the proposed coupler is reduced around 50% compared to the conventional design. The last part conerns the fabrication and test of the proposed coupler. The measurement and simulation results are in good agreements.

Spin dynamics in the diluted ferromagnetic Kondo lattice model

The interplay of disorder and competing interactions is investigated in the carrier-inducedferromagnetic state of the Kondo lattice model within a numerical finite-size study in whichimpurity positional disorder is treated exactly. Competition between impurityspin couplings with increasing hole doping results in a strong optimization ofTc at , highlighting the importance of compensation in diluted magnetic semiconductors. The estimatedTc is in good agreement with experimental results forGa1−xMnxAs for corresponding impurity concentration, hole bandwidth, and compensation. Dilution isshown to result in a sharply enhanced density of low-energy magnon modes extended overthe majority bulk spins and high-energy magnon modes localized over impurity spinclusters. The effects of these magnon properties on finite-temperature spin dynamics arestudied quantitatively using a locally self-consistent magnon renormalization scheme.The large enhancement in the density of low-energy magnetic excitations resultsin a rapid thermal decay of magnetization, which fits well with the Bloch formM0(1−BT3/2) at lowtemperature, with B of the same order of magnitude as obtained in recent superconductingquantum interference device (SQUID) magnetization measurements onGa1−xMnxAs samples. Furthermore, strong cluster couplings and correlations are shownto slow down the magnetization decay and prolong magnetic ordering nearTc, as is also observed in magnetization measurements.

Spin dynamics in cuprate perovskites

Results obtained with the use of the t-J model of Cu–O planes and Mori’s projection operator formalism are compared with data of neutron scattering experiments in lanthanum and yttrium cuprates. This comparison allows us to interpret the intensive peak at the antiferromagnetic wave vector observed in yttrium cuprates as a manifestation of excitations of localized Cu spins. The high-frequency incommensurability detected both in lanthanum and yttrium cuprates is connected with the dispersion of these excitations, while the low-frequency incommensurability arises due to a dip in the spin-excitation damping at the antiferromagnetic wave vector. For moderate doping the dip stems from the weakness of the interaction between the spin excitations and holes near hot spots. It is conjectured that the dissimilarity of the susceptibility frequency dependencies in yttrium and lanthanum cuprates may be connected with different values of the hole bandwidth and damping in these crystals.

Magnon and hole excitations in the two-dimensional half-filled Hubbard model

Spin and hole excitation spectra and spectral weights are calculated for the half-filled Hubbard model, as a function of $t/U$. We find that the high energy spin spectra are sensitive to charge fluctuations. The energy difference $\Delta(\pi,0)- \Delta(\pi/2,\pi/2)$, which is negative for the Heisenberg model, changes sign at a fairly small $t/U\approx 0.053(5)$. The hole bandwidth is proportional to $J$, and considerably larger than in the t-J models. It has a minimum at ($\pi/2,\pi/2$) and a very weak dispersion along the antiferromagnetic zone boundary. A good fit to the measured spin spectra in La$_2$CuO$_4$ at $T=10K$ is obtained with the parameter values $U=3.1{\rm eV}$, $t=0.35{\rm eV}$.

INCOMMENSURATE SPIN DYNAMICS IN UNDERDOPED CUPRATE PEROVSKITES

The incommensurate magnetic response observed in normal-state cuprate perovskites is interpreted based on the projection operator formalism and the t–J model of Cu-O planes. In agreement with experiment the calculated dispersion of maxima in the susceptibility has the shape of two parabolas with upward and downward branches which converge at the antiferromagnetic wave vector. The maxima are located at the momenta (½, ½ ± δ), (½ ± δ, ½) and at (½ ± δ, ½ ± δ), (½ ± δ, ½ ∓ δ) in the lower and upper parabolas, respectively. The upper parabola reflects the dispersion of magnetic excitations of the localized Cu spins, while the lower parabola arises due to a dip in the spin-excitation damping at the antiferromagnetic wave vector. For moderate doping this dip stems from the weakness of the interaction between the spin excitations and holes near the hot spots. The frequency dependence of the susceptibility is shown to depend strongly on the hole bandwidth and damping and varies from the shape observed in YBa 2 Cu 3 O 7-y to that inherent in La 2-x Sr x CuO 4.

Effects of acoustic plasmons in photoemission from coupled layered systems

We have studied photoemission in a quasi-two-dimensional solid consisting of coupled layers adopting the calculation in the limit of narrow hole band width. Unlike ordinary three-dimensional systems, gapless acoustic plasmons can be excited in the low energy regime. Combining the dielectric response of the coupled layers and the random phase approximation for a single layer, we can explicitly turn on or off the acoustic plasmon excitation. Through the comparison between the on- and off-acoustic calculations, acoustic plasmons in the photoemission spectra are found to suppress the quasiparticle weight and lead to an asymmetric broadening of the quasiparticle peak. Such tendencies are strengthened as the interlayer dielectric coupling increases. Further, the model is applied to the high-temperature superconductors and found to give additional asymmetric broadening to the pure two-dimensional spectra, which means that a significant part of the anomalously large inelastic background may be attributed to multiple intrinsic losses by acoustic plasmons.

Proper reduction scheme to an extended t-J model and the hole dispersion inSr2CuO2Cl2

The reduction of the three-band p-d model for the ${\mathrm{CuO}}_{2}$ plane in cuprates with the standard set of parameters as derived by Hybertsen et al. [Phys. Rev. B 41, 11 068 (1990)] to an effective t-${\mathrm{t}}^{|\mathrm{I}\mathrm{H}}$-${\mathrm{t}}^{|\mathrm{I}\mathrm{J}}$-J model with a three-site hopping term is discussed in detail in the framework of the cell-perturbation method. The reduction procedure is formulated to avoid any ambiguity in obtaining the correct signs of the hopping parameters in the effective model, and the contributions of first and second order resulting from the intercell hopping are distinguished. Based on the so-defined extended t-J model, the dispersion of a single hole in an antiferromagnetic background is calculated both by a variational ansatz and within the self-consistent Born approximation (SCBA). An extension of the variational method allows us to investigate the influence of a three-site hopping term both on the mean-field level and beyond this approximation. The latter corrections are shown to be small. Both methods give the coinciding result that the additional hopping terms lead to a more isotropic minimum of the dispersion at the point (\ensuremath{\pi}/2,\ensuremath{\pi}/2) in k space and to a slightly increased bandwidth in comparison to the pure t-J model. These results are compared with recent photoemission measurements for ${\mathrm{Sr}}_{2}$${\mathrm{CuO}}_{2}$${\mathrm{Cl}}_{2}$ [Wells et al., Phys. Rev. Lett. 74, 964 (1995)], showing an improvement against the pure t-J model, but no complete agreement. In particular, a small anisotropy in the calculated effective masses remains. The lifetime of the hole quasiparticles is investigated within the SCBA, and it is shown that the upper part of the hole spectrum loses its quasiparticle character due to a strong increase of damping if the bare hole bandwidth exceeds a certain threshold value of roughly 2J.

Single-hole state in the half-filled Hubbard model. The coupled-cluster method

We have investigated the ground state of a single hole in the half-filled Hubbard model on a 2D square lattice using the coupled- cluster method. In the strong-coupling limit we have obtained an analytical expression of the hole energy dispersion ϵ( k ) with several of its features consistent with earlier studies of the Hubbard model and t-J model. An appreciable discrepancy on the hole bandwidth is, however, found between the Hubbard model and the t-J model. We believe that this discrepancy is due to the absence of the three-site hopping term in the t-J model.

Hole motion in a quantum Néel state.

The spin-wave expansion is used to motivate a simple trial wave function for the N\'eel ground state of a spin-1/2 Heisenberg antiferromagnet on a square lattice; the wave function yields an upper bound on the ground-state energy per bond of (-0.3317\ifmmode\pm\else\textpm\fi{}0.0002)J, where J is the exchange constant. The wave function is easily generalizable to the case where a single hole, with hopping matrix element t, is present. The hole energy-momentum relationship is determined for J/t>0.25; the minimum hole energy is always on the zone boundary at k=(\ensuremath{\pi}/2,\ensuremath{\pi}/2). The hole bandwidth, W, has a maximum value of 1.24t at J=0.73t. The small parameter which makes all of these calculations possible is 1/(2Z-2), where Z=4 is the coordination number of the square lattice.