Showing papers on "Energy (signal processing) published in 2000"
TL;DR: N nanoscopic semiconductor quantum rings in which the electronic states are in the true quantum limit are realized, using self-assembly techniques and complementary spectroscopic techniques.
Abstract: Making use of self-assembly techniques, we realize nanoscopic semiconductor quantum rings in which the electronic states are in the true quantum limit. We employ two complementary spectroscopic techniques to investigate both the ground states and the excitations of these rings. Applying a magnetic field perpendicular to the plane of the rings, we find that, when approximately one flux quantum threads the interior of each ring, a change in the ground state from angular momentum $\ensuremath{\ell}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0$ to $\ensuremath{\ell}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\ensuremath{-}1$ takes place. This ground state transition is revealed both by a drastic modification of the excitation spectrum and by a change in the magnetic-field dispersion of the single-electron charging energy.
718 citations
TL;DR: In this paper, functional renormalization-group equations were derived to describe the depinning transition at zero temperature and a creep regime at finite temperature and slow drive f. Since they hold at finite velocity v, they allow to remedy some shortcomings of the previous approaches to zero-temperature depinning.
Abstract: Elastic systems driven in a disordered medium exhibit a depinning transition at zero temperature and a creep regime at finite temperature and slow drive f. We derive functional renormalization-group equations which allow us to describe in detail the properties of the slowly moving states in both cases. Since they hold at finite velocity v, they allow us to remedy some shortcomings of the previous approaches to zero-temperature depinning. In particular, they enable us to derive the depinning law directly from the equation of motion, with no artificial prescription or additional physical assumptions, such as a scaling relation among the exponents. Our approach provides a controlled framework to establish under which conditions the depinning regime is universal. It explicitly demonstrates that the random potential seen by a moving extended system evolves at large scale to a random field and yields a self-contained picture for the size of the avalanches associated with the deterministic motion. At finite temperature $Tg0$ we find that the effective barriers grow with length scale as the energy differences between neighboring metastable states, and demonstrate the resulting activated creep law $v\ensuremath{\sim}\mathrm{exp}(\ensuremath{-}{\mathrm{Cf}}^{\ensuremath{-}\ensuremath{\mu}}/T)$ where the exponent \ensuremath{\mu} is obtained in a $\ensuremath{\epsilon}=4\ensuremath{-}D$ expansion $(D$ is the internal dimension of the interface). Our approach also provides quantitatively an interesting scenario for creep motion as it allows us to identify several intermediate length scales. In particular, we unveil a ``depinninglike'' regime at scales larger than the activation scale, with avalanches spreading from the thermal nucleus scale up to the much larger correlation length ${R}_{V}.$ We predict that ${R}_{V}\ensuremath{\sim}{T}^{\ensuremath{-}\ensuremath{\sigma}}{f}^{\ensuremath{-}\ensuremath{\lambda}}$ diverges at small drive and temperature with exponents $\ensuremath{\sigma},\ensuremath{\lambda}$ that we determine.
348 citations
TL;DR: In this article, the authors derived the pre-exponential factor for the superparamagnetic relaxation time in the presence of an electric field gradient in accordance with the Blume-Tjon model from simultaneous fitting to ac and dc magnetization curves.
Abstract: The magnetic properties of hematite $(\ensuremath{\alpha}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3})$ particles with sizes of about 16 nm have been studied by use of M\"ossbauer spectroscopy, magnetization measurements, and neutron diffraction. The nanoparticles are weakly ferromagnetic at temperatures at least down to 5 K with a spontaneous magnetization that is only slightly higher than that of weakly ferromagnetic bulk hematite. At $T\ensuremath{\gtrsim}100\mathrm{K}$ the M\"ossbauer spectra contain a doublet, which is asymmetric due to magnetic relaxation in the presence of an electric field gradient in accordance with the Blume-Tjon model. Simultaneous fitting of series of M\"ossbauer spectra obtained at temperatures from 5 K to well above the superparamagnetic blocking temperature allowed the estimation of the pre-exponential factor in N\'eel's expression for the superparamagnetic relaxation time, ${\ensuremath{\tau}}_{0}=(6\ifmmode\pm\else\textpm\fi{}4)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}\mathrm{s}$ and the magnetic anisotropy energy barrier, ${E}_{\mathrm{bm}}{/k=590}_{\ensuremath{-}120}^{+150}\mathrm{K}.$ A lower value of the pre-exponential factor, ${\ensuremath{\tau}}_{0}{=1.8}_{\ensuremath{-}1.3}^{+3.2}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}\mathrm{s},$ and a significantly lower anisotropy energy barrier ${E}_{\mathrm{bm}}^{\mathrm{magn}}/k=305\ifmmode\pm\else\textpm\fi{}20\mathrm{K}$ was derived from simultaneous fitting to ac and dc magnetization curves. The difference in the observed energy barriers can be explained by the presence of two different modes of superparamagnetic relaxation which are characteristic of the weakly ferromagnetic phase. One mode involves a rotation of the sublattice magnetization directions in the basal (111) plane, which gives rise to superparamagnetic behavior in both M\"ossbauer spectroscopy and magnetization measurements. The other mode involves a fluctuation of the net magnetization direction out of the basal plane, which mainly affects the magnetization measurements.
341 citations
01 May 2000
TL;DR: The design and use of the SimplePower framework is presented that includes a transition-sensitive, cycle-accurate datapath energy model that interfaces with analytical and transition sensitive energy models for the memory and bus subsystems, respectively.
Abstract: With the emergence of a plethora of embedded and portable applications, energy dissipation has joined throughput, area, and accuracy/precision as a major design constraint. Thus, designers must be concerned with both optimizing and estimating the energy consumption of circuits, architectures, and software. Most of the research in energy optimization and/or estimation has focused on single components of the system and has not looked across the interacting spectrum of the hardware and software. The novelty of our new energy estimation framework, SimplePower, is that it evaluates the energy considering the system as a whole rather than just as a sum of parts, and that it concurrently supports both compiler and architectural experimentation.We present the design and use of the SimplePower framework that includes a transition-sensitive, cycle-accurate datapath energy model that interfaces with analytical and transition sensitive energy models for the memory and bus subsystems, respectively. We analyzed the energy consumption of ten codes from the multidimensional array domain, a domain that is important for embedded video and signal processing systems, after applying different compiler and architectural optimizations. Our experiments demonstrate that early estimates from the SimplePower energy estimation framework can help identify the system energy hotspots and enable architects and compiler designers to focus their efforts on these areas.
321 citations
TL;DR: It is shown that the results of the Super-Kamiokande atmospheric neutrino experiment, interpreted in terms of nu(mu)<-->nu(tau) flavor transitions, can probe possible decoherence effects induced by new physics with high sensitivity, supplementing current laboratory tests based on kaon oscillations and on neutron interferometry.
Abstract: It is shown that the results of the Super-Kamiokande atmospheric neutrino experiment, interpreted in terms of ${\ensuremath{
u}}_{\ensuremath{\mu}}\ensuremath{\leftrightarrow}{\ensuremath{
u}}_{\ensuremath{\tau}}$ flavor transitions, can probe possible decoherence effects induced by new physics (e.g., by quantum gravity) with high sensitivity, supplementing current laboratory tests based on kaon oscillations and on neutron interferometry. By varying the (unknown) energy dependence of such effects, one can either obtain strong limits on their amplitude or use them to find an unconventional solution to the atmospheric $\ensuremath{
u}$ anomaly based solely on decoherence.
217 citations
TL;DR: Chabrier and Potekhin this paper improved the fit of the electron screening contribution in the free energy of the Coulomb liquid at high densities where the electrons are relativistic.
Abstract: The analytic equation of state of nonideal Coulomb plasmas consisting of pointlike ions immersed in a polarizable electron background [G. Chabrier and A. Y. Potekhin, Phys. Rev. E 58, 4941 (1998)] is improved, and its applicability range is considerably extended. First, the fit of the electron screening contribution in the free energy of the Coulomb liquid is refined at high densities where the electrons are relativistic. Second, we calculate the screening contribution for the Coulomb solid (bcc and fcc) and derive an analytic fitting expression. Third, we propose a simple approximation to the internal and free energy of the liquid one-component plasma of ions, accurate within the numerical errors of the most recent Monte Carlo simulations. We obtain an updated value of the coupling parameter at the solid-liquid phase transition for the one-component plasma: ${\ensuremath{\Gamma}}_{m}=175.0\ifmmode\pm\else\textpm\fi{}0.4(1\ensuremath{\sigma}).$
192 citations
TL;DR: In this paper, the electron-phonon energy relaxation rate in a conductor with the vibrating and static \ensuremath{delta}-correlated random electron-scattering potentials was calculated.
Abstract: Employing the Keldysh diagram technique, we calculate the electron-phonon energy relaxation rate in a conductor with the vibrating and static \ensuremath{\delta}-correlated random electron-scattering potentials. If the scattering potential is completely dragged by phonons, this model yields the Schmid's result for the inelastic electron-scattering rate ${\ensuremath{\tau}}_{e\ensuremath{-}\mathrm{ph}}^{\ensuremath{-}1}.$ At low temperatures the effective interaction decreases due to disorder, and ${\ensuremath{\tau}}_{e\ensuremath{-}\mathrm{ph}}^{\ensuremath{-}1}\ensuremath{\propto}{T}^{4}l$ (l is the electron mean-free path). In the presense of the static potential, quantum interference of numerous scattering processes drastically changes the effective electron-phonon interaction. In particular, at low temperatures the interaction increases, and ${\ensuremath{\tau}}_{e\ensuremath{-}\mathrm{ph}}^{\ensuremath{-}1}\ensuremath{\propto}{T}^{2}/l.$ Along with an enhancement of the interaction, which is observed in disordered metallic films and semiconducting structures at low temperatures, the suggested model allows us to explain the strong sensitivity of the electron relaxation rate to the microscopic quality of a particular film.
181 citations
TL;DR: A new approach for calculating dynamical properties within the numerical renormalization group is introduced, which provides a unifying framework for calculating dynamics at any temperature and represents the correct extension of Wilson's original thermodynamic calculation.
Abstract: In this paper we introduce a new approach for calculating dynamical properties within the numerical renormalization group. It is demonstrated that the method previously used fails for the Anderson impurity in a magnetic field due to the absence of energy scale separation. The problem is solved by evaluating the Green function with respect to the reduced density matrix of the full system, leading to accurate spectra in agreement with the static magnetization. The new procedure provides a unifying framework for calculating dynamics at any temperature and represents the correct extension of Wilson's original thermodynamic calculation.
178 citations
TL;DR: Molecular dynamics simulations are used to generate an ensemble of saddles of the potential energy of a Lennard-Jones liquid, classifying all extrema by their potential energy u and number of unstable directions k, and a well-defined relation k(u) is revealed.
Abstract: Molecular dynamics simulations are used to generate an ensemble of saddles of the potential energy of a Lennard-Jones liquid. Classifying all extrema by their potential energy $u$ and number of unstable directions $k$, a well-defined relation $k(u)$ is revealed. The degree of instability of typical stationary points vanishes at a threshold potential energy ${u}_{\mathrm{th}}$, which lies above the energy of the lowest glassy minima of the system. The energies of the inherent states, as obtained by the Stillinger-Weber method, approach ${u}_{\mathrm{th}}$ at a temperature close to the mode-coupling transition temperature ${T}_{c}$.
175 citations
TL;DR: This work introduces a framework which makes the triple coincidence inevitable, and presents a simple model, where a false vacuum energy yields a cosmological constant of this form.
Abstract: Cosmological data suggest that we live in an interesting period in the history of the universe when ${\ensuremath{\rho}}_{\ensuremath{\Lambda}}\ensuremath{\sim}{\ensuremath{\rho}}_{M}\ensuremath{\sim}{\ensuremath{\rho}}_{R}$. The occurrence of any epoch with such a ``triple coincidence'' is puzzling, while the question of why we happen to live during this special epoch is the ``Why now?'' problem. We introduce a framework which makes the triple coincidence inevitable; furthermore, the ``Why now?'' problem is transformed and greatly ameliorated. The framework assumes that the only relevant mass scales are the electroweak scale ${M}_{\mathrm{EW}}$, and the Planck scale ${M}_{\mathrm{Pl}}$ and requires ${\ensuremath{\rho}}_{\ensuremath{\Lambda}}^{1/4}\ensuremath{\sim}{M}_{\mathrm{EW}}^{2}/{M}_{\mathrm{Pl}}$ parametrically. Assuming that the true vacuum energy vanishes, we present a simple model, where a false vacuum energy yields a cosmological constant of this form.
TL;DR: In this article, a Monte Carlo approach using the event generator SOPHIA for the simulation of hadronic nucleon-photon interactions and a realistic integration of the particle trajectories in a random extragalactic magnetic field is presented.
Abstract: We present a new calculation of the propagation of protons with energies above ${10}^{19} \mathrm{eV}$ over distances of up to several hundred Mpc The calculation is based on a Monte Carlo approach using the event generator SOPHIA for the simulation of hadronic nucleon-photon interactions and a realistic integration of the particle trajectories in a random extragalactic magnetic field Accounting for the proton scattering in the magnetic field affects noticeably the nucleon energy as a function of the distance to their source and allows us to give realistic predictions on arrival energy, time delay, and arrival angle distributions and correlations as well as secondary particle production spectra
TL;DR: In this article, the free energy of an oxide/metal interface to the free energies of surfaces and the work of separation of the interface were derived, which is useful for first-principles calculations.
Abstract: We derive a formula, useful for first-principles calculations, which relates the free energy of an oxide/metal interface to the free energies of surfaces and the work of separation of the interface. We distinguish the latter mechanical quantity from the thermodynamic work of adhesion, and we describe explicitly how both may be calculated. Our formulas for interfacial and surface energies are cast in terms of quantities which can be calculated or looked up in tables, and include as additional parameters the ambient temperature and partial pressure of oxygen ${P}_{{\mathrm{O}}_{2}}.$ From total-energy calculations for the $\mathrm{Nb}(111)/\ensuremath{\alpha}\ensuremath{-}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ (0001) interface, free Nb and ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ surfaces, we obtain firstly numerical estimates of the works of separation, which are independent of ${P}_{{\mathrm{O}}_{2}}.$ We then obtain surface energies, interfacial energies, and the equilibrium work of adhesion as a function of ${P}_{{\mathrm{O}}_{2}}.$
TL;DR: The fundamental limit on signal energy transfer during a binary switching transition is E/sub s/(min)=(ln2)kT as mentioned in this paper, which is based on two entirely distinct physical models, one ideal MOSFET operating in a CMOS inverter circuit at the limit of its capacity for binary signal discrimination.
Abstract: The fundamental limit on signal energy transfer during a binary switching transition is E/sub s/(min)=(ln2)kT. Derivation of precisely this result based on two entirely distinct physical models confirms its validity. The first model is an ideal MOSFET operating in a CMOS inverter circuit at the limit of its capacity for binary signal discrimination. The second is an isolated interconnect treated as a noisy communication channel. Both models are relevant to modern terascale integration.
TL;DR: In this paper, the authors studied the spectrum and average mass composition of cosmic rays with primary energies between 10−17 eV and 10−18 eV using a hybrid detector consisting of the High Resolution Fly's Eye (HiRes) prototype and the MIA muon array.
Abstract: We study the spectrum and average mass composition of cosmic rays with primary energies between 10^{17} eV and 10^{18} eV using a hybrid detector consisting of the High Resolution Fly's Eye (HiRes) prototype and the MIA muon array. Measurements have been made of the change in the depth of shower maximum as a function of energy. A complete Monte Carlo simulation of the detector response and comparisons with shower simulations leads to the conclusion that the cosmic ray intensity is changing f rom a heavier to a lighter composition in this energy range. The spectrum is consistent with earlier Fly's Eye measurements and supports the previously found steepening near 4 \times 10^{17} eV .
TL;DR: In this paper, an analytic theoretical description of transport processes based on the concept of transport energy is suggested for disordered organic solids, which gives not only the natural explanation of experimental data but also accounts for the results of computer simulations considered so far puzzling.
Abstract: An analytic theoretical description of transport processes based on the concept of transport energy is suggested for disordered organic solids. It gives not only the natural explanation of experimental data but also accounts for the results of computer simulations considered so far puzzling. In particular, this approach accounts for the strong difference between the temperature dependence of the carrier drift mobility and that of the relaxation time. Experimental data for the low-field drift mobility display the temperature dependence in the form $\ensuremath{\mu}\ensuremath{\propto}\mathrm{exp}{\ensuremath{-}{(T}_{0}{/T)}^{2}}.$ It is believed that the characteristic temperature ${T}_{0}$ is determined solely by the scale of the energy distribution of localized states, and such a temperature dependence of \ensuremath{\mu} is widely used to determine this energy scale from experimental data for various materials. We show that this temperature dependence is not universal and that parameter ${T}_{0}$ depends also on the concentration of localized states and on the decay length of the carrier wave function in the localized states. The suggested theory provides a general basis for the treatment of transport processes in disordered organic media.
Patent•
07 Apr 2000
TL;DR: In this paper, the authors present an optical system that includes a plurality of optical processing nodes in optical communication via at least one signal varying device and a pump energy source for providing pump energy to stimulate Raman scattering/gain in the optical fiber within the signal wavelength range.
Abstract: Optical systems of the present invention include a plurality of optical processing nodes in optical communication via at least one signal varying device. The signal varying devices includes an optical fiber suitable for facilitating Raman scattering/gain in a signal wavelength range and a pump energy source for providing pump energy in a plurality of pump wavelengths. The pump source provides sufficient pump energy in each pump wavelength to stimulate Raman scattering/gain in the optical fiber within the signal wavelength range. The pump wavelengths are selected such that the combined Raman gain resulting from the pump energy supplied by each pump wavelength produces a desired signal variation profile in the signal wavelength range. In addition, the pump energy supplied by at least one of the pump wavelengths can be varied to produce a controlled signal intensity variation profile over the signal wavelength range in the optical fiber.
TL;DR: In this article, a virtual crystal approximation (VCA) method was proposed for modeling disordered solid solutions, based on the VCA on a ferroelectric oxide, determining the energy differences between the high-temperature rhombohedral, low-tetragonal, and tetragonal phases.
Abstract: We present a method for modeling disordered solid solutions, based on the virtual crystal approximation (VCA). The VCA is a tractable way of studying configurationally disordered systems; traditionally, the potentials which represent atoms of two or more elements are averaged into a composite atomic potential. We have overcome significant shortcomings of the standard VCA by developing a potential which yields averaged atomic properties. We perform the VCA on a ferroelectric oxide, determining the energy differences between the high-temperature rhombohedral, low-temperature rhombohedral, and tetragonal phases of $\mathrm{Pb}({\mathrm{Zr}}_{1\ensuremath{-}x}{\mathrm{Ti}}_{x}){\mathrm{O}}_{3}$ at $x=0.5$ and comparing these results to superlattice calculations and experiment. We then use our method to determine the preferred structural phase at $x=0.4.$ We find that the low-temperature rhombohedral phase becomes the ground state at $x=0.4,$ in agreement with experimental findings.
TL;DR: In this article, a set of high-resolution x-ray diffraction and time-resolved optical spectroscopy (OSO) spectrograms were used to study the decay time of the photoluminescence peak (Stokes shift) and the intrinsic radiative lifetimes.
Abstract: We apply photoluminescence, photoluminescence excitation, and time-resolved optical spectroscopy for studying a set of ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}/\mathrm{G}\mathrm{a}\mathrm{N}$ periodic structures, which were characterized by high-resolution x-ray diffraction including x-ray mapping in reciprocal space. We found that the energy differences between the absorption edge and the photoluminescence peak (Stokes shift), and the photoluminescence decay time drastically increase with the ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ layer thickness. The decay time strongly increases with the sample temperature. We were able to quite accurately determine the radiative and nonradiative decay times of excitons in these structures by measuring the temperature dependence of the decay times, the integrated photoluminescence intensities, and the photoluminescence intensities immediately after the picosecond excitation pulse. The intrinsic radiative lifetimes, which are inversely proportional to the exciton oscillator strengths, were then calculated from the temperature dependence of the radiative lifetimes. These experimental findings are analyzed using an eight-band $\mathbf{k}\ensuremath{\cdot}\mathbf{P}$ model, which quantitatively explains both the Stokes shifts and the intrinsic radiative lifetimes. Their strong dependence on the quantum well width is due to a large (\ensuremath{\sim}1 MV/cm) lattice-mismatch strain-induced piezoelectric field along the growth axis.
24 Apr 2000
TL;DR: The proposed control scheme based on wave variables is modified by introducing an energy input/output balance monitoring mechanism, which limits the energy that the system can generate.
Abstract: In previous work, we (1999) have proposed a control scheme based on wave variables. Unlike other methods, the proposed method could minimize the performance degradation due to the fluctuation of time delay. With this method however the system may potentially generate infinite energy in some special situations and the system stability could not be ensured rigorously. In this paper, we modify the method by introducing an energy input/output balance monitoring mechanism, which limits the energy that the system can generate. We conducted some simulation studies with a one DOF system. Simulation results show the validity of the proposed scheme.
Patent•
11 Aug 2000TL;DR: In this paper, a beam of x-rays is used for scanning the enclosure and for identifying areas of suspect material, and the beam is subsequently coherently scattered off suspect materials, during the course of a single pass of the enclosure past the beam, for uniquely discriminating innocuous from contraband substances.
Abstract: A system and method for inspecting an enclosure. A beam of x-rays is used for scanning the enclosure and for identifying areas of suspect material. The beam is subsequently coherently scattered off suspect materials, during the course of a single pass of the enclosure past the beam, for uniquely discriminating innocuous from contraband substances. One or more energy dispersive detectors measure radiation coherently scattered by an identified volume of suspect material. Absorption effects of the energy distribution of the coherently scattered radiation are compensated by means of a fiducial reference disposed between the interrogated object and the detectors.
Patent•
27 Apr 2000
TL;DR: In this article, a method and apparatus of measuring a predetermined parameter having a known relation to the transit time of movement of an energy wave through a medium, by transmitting from a first location in the medium a cyclicallyrepeating energy wave, receiving the cyclically-repeating EH wave at a second location in a medium and detecting a predetermined fiducial point in the EHW received at the second location.
Abstract: A method and apparatus of measuring a predetermined parameter having a known relation to the transit time of movement of an energy wave through a medium, by transmitting from a first location in the medium a cyclically-repeating energy wave; receiving the cyclically-repeating energy wave at a second location in the medium; detecting a predetermined fiducial point in the cyclically-repeating energy wave received at the second location; continuously changing the frequency of transmission of the cyclically-repeating energy wave from the first location to the second location in accordance with the detected fiducial point of each received cyclically-repeating energy wave received at the second location such that the number of waves received at the second location from the first location is a whole integer; and utilizing the change in frequency to produce a measurement of the predetermined parameter.
TL;DR: In this article, the authors present the universal theory of localized impurities in a confining paramagnetic state of two-dimensional antiferromagnets with global SU(2) spin symmetry.
Abstract: We present the universal theory of arbitrary, localized impurities in a confining paramagnetic state of two-dimensional antiferromagnets with global SU(2) spin symmetry. The energy gap of the host antiferromagnet to spin-1 excitations, \ensuremath{\Delta}, is assumed to be significantly smaller than a typical nearest neighbor exchange. In the absence of impurities, it was argued in earlier work [Chubukov et al., Phys. Rev. B 49, 11 919 (1994)] that the low-temperature quantum dynamics is universally and completely determined by the values of \ensuremath{\Delta} and a spin-wave velocity c. Here we establish the remarkable fact that no additional parameters are necessary for an antiferromagnet with a dilute concentration of impurities, ${n}_{\mathrm{imp}}$---each impurity is completely characterized by a integer/half-odd-integer valued spin S which measures the net uncompensated Berry phase due to spin precession in its vicinity. We compute the impurity-induced damping of the spin-1 collective mode of the antiferromagnet: the damping occurs on an energy scale $\ensuremath{\Gamma}{=n}_{\mathrm{imp}}(\ensuremath{\Elzxh}{c)}^{2}/\ensuremath{\Delta},$ and we predict a universal, asymmetric line shape for the collective mode peak. We argue that, under suitable conditions, our results apply unchanged (or in some cases, with minor modifications) to d-wave superconductors, and compare them to recent neutron-scattering experiments on ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}$ by Fong et al. [Phys. Rev. Lett. 82, 1939 (1999)]. We also describe the universal evolution of numerous measurable correlations as the host antiferromagnet undergoes a quantum phase transition to a N\'eel ordered state.
TL;DR: Renormalized statistical theory is used to calculate the interactions between short scales and long scales in the Hasegawa-Mima model of drift-wave turbulence, and rigorous and heuristic derivations demonstrate that the results do not depend on, and cannot be derived from, properties of linear normal modes; they are intrinsically nonlinear.
Abstract: Renormalized statistical theory is used to calculate the interactions between short scales (wave vector k) and long scales (wave vector q<
TL;DR: In this article, a comprehensive study of the optical properties of epilayers with large Al mole fraction was presented, showing that the anomalous temperature-induced emission shift is attributed to energy tail states due to alloy potential inhomogeneities in the epilayer.
Abstract: We present a comprehensive study of the optical characteristics of ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}\mathrm{}$ epilayers $(0l~xl~0.6)$ by means of photoluminescence (PL), PL excitation, and time-resolved PL spectroscopy. For ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}\mathrm{}$ with large Al content, we observed an anomalous PL temperature dependence: (i) an ``S-shaped'' PL peak energy shift (decrease-increase-decrease) and (ii) an ``inverted S-shaped'' spectral width broadening (increase-decrease-increase) with increasing temperature. We observed that the thermal decrease in integrated PL intensity was suppressed and the effective lifetime was enhanced in the temperature region showing the anomalous temperature-induced emission behavior, reflecting superior luminescence efficiency by suppressing nonradiative processes. All these features were enhanced as the Al mole fraction was increased. From these results, the anomalous temperature-induced emission shift is attributed to energy tail states due to alloy potential inhomogeneities in the ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}\mathrm{}$ epilayers with large Al content.
TL;DR: The long-range asymptotic behavior of the exchange-correlation Kohn-Sham (KS) potential and its relation to the exchange correlation energy E{sub xc} are considered using various approaches.
Abstract: The long-range asymptotic behavior of the exchange-correlation Kohn-Sham (KS) potential {nu}{sub xc} and its relation to the exchange-correlation energy E{sub xc} are considered using various approaches The line integral of {nu}{sub xc}([{rho}];r) yielding the exchange-correlation part {Delta}E{sub xc} of a relative energy {Delta}E of a finite system, shows that a uniform constant shift of {nu}{sub xc} never shows up in any physically meaningful energy difference {Delta}E {nu}{sub xv} may thus be freely chosen to tend asymptotically to zero or to some nonzero constant Possible choices of the asymptotics of the potential are discussed with reference to the theory of open systems with a fractional number of electrons The authors adhere to the conventional choice {nu}{sub xc}({infinity}) = 0 for the asymptotics of the potential leading to {epsilon}{sub N} = {minus}I{sub p} for the energy {epsilon}{sub N} of the highest occupied orbital A statistical average of orbital dependent model potentials is proposed as a way to model {nu}{sub xc} An approximate potential {nu}{sub xco}{sup SAOP} with exact {minus}1/r asymptotics is developed using the statistical average of, on the one hand, a model potential {nu}{sub xc{sigma}}{sup Ei} for the highest occupied KS orbital {psi}{sub N{sigma}} and, on the other hand, a modelmore » potential {nu}{sub xc}{sup GLB} for other occupied orbitals It is demonstrated for the well-studied case of the Ne atom, that calculations with the new model potential can, in principle, reproduce perfectly all energy characteristics« less
TL;DR: In this article, the results of electronic Raman-scattering experiments in differently doped single crystals of the same type are described. But the authors do not find a direct relation between the pseudogap and the superconducting gap.
Abstract: We describe the results of electronic Raman-scattering experiments in differently doped single crystals of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6+x}$ and ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}({\mathrm{Ca}}_{x}{\mathrm{Y}}_{1\ensuremath{-}x}){\mathrm{Cu}}_{2}{\mathrm{O}}_{8}.$ The data in antiferromagnetic insulating samples suggest that at least the low-energy parts of the spectra of metallic samples originate predominantly from excitations of free carriers. We therefore propose an analysis of the data in terms of a memory function approach which has been introduced earlier for the current response. Dynamical scattering rates $\ensuremath{\Gamma}(\ensuremath{\omega})=1/\ensuremath{\tau}(\ensuremath{\omega})$ and mass-enhancement factors $1+\ensuremath{\lambda}(\ensuremath{\omega}{)=m}^{*}(\ensuremath{\omega})/m$ of the carriers are obtained. It is found that a strong polarization dependence of the carrier lifetime develops towards low doping. In ${B}_{2g} (\mathrm{xy})$ symmetry selecting predominantly electrons with momenta along the diagonals of the ${\mathrm{CuO}}_{2}$ planes the Raman data compare well with the results obtained from dc and dynamical transport. In ${B}_{1g} {(x}^{2}\ensuremath{-}{y}^{2})$ symmetry projecting out momenta along the Cu-O bonds the dc scattering rates of underdoped materials become temperature independent and considerably larger than in ${B}_{2g}$ symmetry. This increasing anisotropy is accompanied by a loss of spectral weight in ${B}_{2g}$ symmetry in the range between the superconducting transition at ${T}_{c}$ and a characteristic temperature ${T}^{*}$ of the order of room temperature which compares well with the pseudogap temperature found in other experiments. The energy range affected by the pseudogap is doping and temperature independent. The integrated spectral loss is approximately 25% in underdoped samples and becomes much weaker towards higher carrier concentration. In underdoped samples, superconductivity-related features in the spectra can be observed only in ${B}_{2g}$ symmetry. The peak frequencies scale with ${T}_{c}.$ We do not find a direct relation between the pseudogap and the superconducting gap.
TL;DR: Numerical and exact analytic results show that the transition is second order in modified models where the self-avoidance is partially or completely neglected, inspired by a model introduced long ago by Poland and Sheraga for the denaturation transition in DNA.
Abstract: We study pairs of interacting self-avoiding walks ${{\mathit{\ensuremath{\omega}}}^{1},{\mathit{\ensuremath{\omega}}}^{2}}$ on the $3d$ simple cubic lattice. They have a common origin ${\mathit{\ensuremath{\omega}}}_{0}^{1}={\mathit{\ensuremath{\omega}}}_{0}^{2},$ and are allowed to overlap only at the same monomer position along the chain: ${\mathit{\ensuremath{\omega}}}_{i}^{1}\ensuremath{
e}{\mathit{\ensuremath{\omega}}}_{j}^{2}$ for $i\ensuremath{
e}j,$ while ${\mathit{\ensuremath{\omega}}}_{i}^{1}={\mathit{\ensuremath{\omega}}}_{i}^{2}$ is allowed. The latter overlaps are indeed favored by an energetic gain \ensuremath{\epsilon}. This is inspired by a model introduced long ago by Poland and Sheraga [J. Chem. Phys. 45, 1464 (1966)] for the denaturation transition in DNA where, however, self avoidance was not fully taken into account. For both models, there exists a temperature ${T}_{m}$ above which the entropic advantage to open up overcomes the energy gained by forming tightly bound two-stranded structures. Numerical simulations of our model indicate that the transition is of first order (the energy density is discontinuous), but the analog of the surface tension vanishes and the scaling laws near the transition point are exactly those of a second-order transition with crossover exponent $\ensuremath{\varphi}=1.$ Numerical and exact analytic results show that the transition is second order in modified models where the self-avoidance is partially or completely neglected.
TL;DR: In this paper, features of exciton-polariton eigenmodes in a series of light-coupled semiconductor semiconductor multiple quantum wells with varying number of quantum wells from 1 to 100, and with various periodicities (Bragg, near-Bragg and anti-bragg), are studied in linear measurements of reflection, transmission, and absorption.
Abstract: Features of exciton-polariton eigenmodes in a series of light-coupled ${\mathrm{In}}_{0.04}{\mathrm{Ga}}_{0.96}\mathrm{A}\mathrm{s}/\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}$ semiconductor multiple quantum wells with varying number of quantum wells $N$ from 1 to 100, and with various periodicities (Bragg, near-Bragg, and anti-Bragg), are studied in linear measurements of reflection, transmission, and absorption. At Bragg periodicity (period $d={\ensuremath{\lambda}}_{x}/2),$ a photonic band-gap mode grows in amplitude and increases linearly in linewidth with increasing $N.$ The $N$ times increased radiative damping rate is seen to arise from the light character of the eigenmode being swept out of a photonic band-gap structure. The slope of linewidth versus $N$ gives the radiative linewidth of the exciton. Away from Bragg periodicity two branches of energy levels can be resolved in absorption, corresponding to the $N$ exciton-polariton normal modes in the multiple-quantum-well structure. Signatures of individual modes becoming optically active are observed in the rich structure of reflection spectra for changing quantum-well periodicity. Antireflection coating of the samples is shown to be an effective way of thus isolating the multiple-quantum-well response.
Patent•
21 Apr 2000
TL;DR: In this article, a transducer array is used to transmit ultrasonic energy towards an acoustic reflector, such as a planar air mirror, and the received energy is reflected off of the reflector using a sensing element.
Abstract: Systems and methods for testing the performance of a focused ultrasound transducer array include transmitting ultrasonic energy from the transducer array towards an acoustic reflector, such as a planar air mirror, and receiving ultrasonic energy reflected off of the acoustic reflector using a sensing element. A characteristic of the reflected ultrasonic energy, such as amplitude and phase, is measured by processing circuitry, for example, by comparing the characteristic of the received ultrasonic energy to a corresponding characteristic of the transmitted ultrasonic energy to obtain an actual gain and phase shift for the received ultrasonic energy. A controller compares the actual gain and phase shift of the received ultrasonic energy to an expected gain and phase shift of the received ultrasonic energy. This information is used to calibrate the transducer array and/or to declare a system failure if the comparison indicates an error.