Showing papers in "Physical Review Letters in 1994"
TL;DR: By finding measurements that optimally resolve neighboring quantum states, this work uses statistical distinguishability to define a natural Riemannian metric on the space of quantum-mechanical density operators and to formulate uncertainty principles that are more general and more stringent than standard uncertainty principles.
Abstract: By finding measurements that optimally resolve neighboring quantum states, we use statistical distinguishability to define a natural Riemannian metric on the space of quantum-mechanical density operators and to formulate uncertainty principles that are more general and more stringent than standard uncertainty principles.
2,481 citations
TL;DR: Luminescent measurements show that the efficiency increases with decreasing size of the particles, as expected within the framework of an electron-hole localization theory, suggesting that doped nanocrystals are indeed a new class of materials heretofore unknown.
Abstract: We report for the first time that doped nanocrystals of semiconductor can yield both high luminescent efficiencies and lifetime shortening at the same time. Nanocrystals of Mn-doped ZnS with sizes varying from 3.5 to 7.5 nm were prepared by a room temperature chemical process. These nanosized particles have an external photoluminescent quantum efficiency as high as 18% at room temperature and a luminescent decay at least 5 orders of magnitude faster than the corresponding ${\mathrm{Mn}}^{2+}$ radiative transition in the bulk crystals. Luminescent measurements show that the efficiency increases with decreasing size of the particles, as expected within the framework of an electron-hole localization theory. These results suggest that doped nanocrystals are indeed a new class of materials heretofore unknown.
1,855 citations
TL;DR: An algorithmic proof that any discrete finite-dimensional unitary operator can be constructed in the laboratory using optical devices is given, and optical experiments with any type of radiation exploring higher-dimensional discrete quantum systems become feasible.
Abstract: An algorithmic proof that any discrete finite-dimensional unitary operator can be constructed in the laboratory using optical devices is given. Our recursive algorithm factorizes any N\ifmmode\times\else\texttimes\fi{}N unitary matrix into a sequence of two-dimensional beam splitter transformations. The experiment is built from the corresponding devices. This also permits the measurement of the observable corresponding to any discrete Hermitian matrix. Thus optical experiments with any type of radiation (photons, atoms, etc.) exploring higher-dimensional discrete quantum systems become feasible.
1,699 citations
TL;DR: It is found that at the first stage of reheating the classical inflation field rapidly decays into particles or into other bosons due to a broad parametric resonance, which implies that the inflation field can be a dark matter candidate.
Abstract: The theory of reheating of the Universe after inflation is developed. We have found that typically at the first stage of reheating the classical inflation field $\ensuremath{\varphi}$ rapidly decays into $\ensuremath{\varphi}$ particles or into other bosons due to a broad parametric resonance. Then these bosons decay into other particles, which eventually become thermalized. Complete reheating is possible only in those theories where a single particle $\ensuremath{\varphi}$ can decay into other particles. This imposes strong constraints on the structure of inflationary models, and implies that the inflation field can be a dark matter candidate.
1,571 citations
TL;DR: This model is reexamine and it is found that hot, warm, and cold dark matter are all possibilities and the number density of sterile neutrinos is proportional to [mu][sup 2]/M so that the energy density today is independent of M, which is crucial in determining the large scale structure of the Universe.
Abstract: The simplest model that can accommodate a viable nonbaryonic dark matter candidate is the standard electroweak theory with the addition of right-handed (sterile) neutrinos. We consider a single generation of neutrinos with a Dirac mass [mu] and a Majorana mass [ital M] for the right-handed component. If [ital M][much gt][mu] (standard hot dark matter corresponds to [ital M]=0), then sterile neutrinos are produced via oscillations in the early Universe with energy density independent of [ital M]. However, [ital M] is crucial in determining the large scale structure of the Universe; for [ital M][similar to]100 eV, sterile neutrinos make an excellent warm dark matter candidate.
1,378 citations
TL;DR: A macroscopic Coulomb crystal of solid particles in a plasma has been observed, and strongly coupled plasma theory predicts that the particles should organize in a Coulomb solid, in agreement with the observations.
Abstract: A macroscopic Coulomb crystal of solid particles in a plasma has been observed Images of a cloud of $7\ensuremath{-}\ensuremath{\mu}m$ "dust" particles, which are charged and levitated in a weakly ionized argon plasma, reveal a hexagonal crystal structure The crystal is visible to the unaided eye The particles are cooled by neutral gas to 310 K, and their charge is $g9800e$, corresponding to a Coulomb coupling parameter $\ensuremath{\Gamma}g20 700$ For such a large $\ensuremath{\Gamma}$ value, strongly coupled plasma theory predicts that the particles should organize in a Coulomb solid, in agreement with our observations
1,296 citations
TL;DR: Using self-consistent field theory, microphases of diblock copolymers are examined and a stable gyroid phase is found which occurs between the lamellar and hexagonal ones and terminates at a triple point.
Abstract: Using self-consistent field theory, we examine microphases of diblock copolymers and find, in addition to lamellar, hexagonal, and cubic phases, a stable gyroid phase which occurs between the lamellar and hexagonal ones. It terminates at a triple point, with a lamellar to hexagonal transition occurring in the weak-segregation limit. Other phases of experimental interest are studied, and we describe the regions in which they are most nearly stable.
1,235 citations
TL;DR: In this paper, the scaling laws of fully developed turbulence are described in terms of scalings of a sequence of moment ratios of the energy dissipation field at inertial-range scale l. These moment ratios form a hierarchy of structures.
Abstract: The inertial-range scaling laws of fully developed turbulence are described in terms of scalings of a sequence of moment ratios of the energy dissipation field ${\mathrm{\ensuremath{\epsilon}}}_{\mathit{l}}$ coarse-grained at inertial-range scale l. These moment ratios ${\mathrm{\ensuremath{\epsilon}}}_{\mathit{l}}^{(\mathit{p})}$=〈${\mathrm{\ensuremath{\epsilon}}}_{\mathit{l}}^{\mathit{p}+1}$〉/〈${\mathrm{\ensuremath{\epsilon}}}_{\mathit{l}}^{\mathit{p}}$〉(p=0, 1, 2,...,) form a hierarchy of structures. The most singular structures ${\mathrm{\ensuremath{\epsilon}}}_{\mathit{l}}^{(\mathrm{\ensuremath{\infty}})}$ are assumed to be filaments, and it is argued that ${\mathrm{\ensuremath{\epsilon}}}_{\mathit{l}}^{(\mathrm{\ensuremath{\infty}})}$\ensuremath{\sim}${\mathit{l}}^{\mathrm{\ensuremath{-}}2/3}$. Furthermore, a universal relation between scalings of successive structures is postulated, which leads to a prediction of the entire set of the scaling exponents: 〈${\mathrm{\ensuremath{\epsilon}}}_{\mathit{l}}^{\mathit{p}}$〉\ensuremath{\sim}${\mathit{l}}_{\mathit{p}}^{\mathrm{\ensuremath{\tau}}}$, ${\mathrm{\ensuremath{\tau}}}_{\mathit{p}}$=-2/3p+2[1-( 2) / 3 ${)}^{\mathit{p}}$] and 〈\ensuremath{\delta}${\mathit{v}}_{\mathit{l}}^{\mathit{p}}$〉\ensuremath{\sim}${\mathit{l}}_{\mathit{p}}^{\mathrm{\ensuremath{\zeta}}}$, ${\mathrm{\ensuremath{\zeta}}}_{\mathit{p}}$=p/9+2[1-(2/3${)}^{\mathit{p}/3}$].
1,217 citations
TL;DR: In this article, the Coulomb crystals and liquids were directly observed for the first time using an optical microscope and the system parameters, hexagonal, fcc and bcc crystal structures and solids with coexisting different crystal structures can be formed.
Abstract: The strongly coupled dusty plasmas are formed by suspending negatively charged ${\mathrm{SiO}}_{2}$ fine particles with 10 \ensuremath{\mu}m diameter in weakly ionized rf Ar discharges. The Coulomb crystals and liquids are directly observed for the first time using an optical microscope. By properly controlling the system parameters, hexagonal, fcc and bcc crystal structures and solids with coexisting different crystal structures can be formed. Increasing the rf power causes the transition to the more disordered liquid state.
1,032 citations
TL;DR: Through the reduction of the number of emitting dots in small mesa structures, it is shown narrow lines in the spectra, each associated with a single InAs dot, indicate short capture and relaxation times into the dots.
Abstract: We present photoluminescence data on InAs quantum dots grown by molecular beam epitaxy on GaAs. Through the reduction of the number of emitting dots in small mesa structures, we evidence narrow lines in the spectra, each associated with a single InAs dot. Beyond the statistical analysis allowed by this technique, our results indicate short capture and relaxation times into the dots. This approach opens the route towards the detailed optical study of high quality easily fabricated single semiconductor quantum dots.
948 citations
TL;DR: In this article, the statistics of an ensemble of images taken in the woods were studied and it was shown that local quantities such as contrast and power spectra exhibit scaling with a nontrivial exponent.
Abstract: We study the statistics of an ensemble of images taken in the woods. Distributions of local quantities such as contrast are scale invariant and have nearly exponential tails. Power spectra exhibit scaling with a nontrivial exponent. These data limit the information content of natural images and point to the importance of gain-control strategies in visual processing.
TL;DR: The problem of separating n linearly superimposed uncorrelated signals and determining their mixing coefficients is reduced to an eigenvalue problem which involves the simultaneous diagonalization of two symmetric matrices whose elements are measureable time delayed correlation functions.
Abstract: The problem of separating n linearly superimposed uncorrelated signals and determining their mixing coefficients is reduced to an eigenvalue problem which involves the simultaneous diagonalization of two symmetric matrices whose elements are measureable time delayed correlation functions. The diagonalization matrix can be determined from a cost function whose number of minima is equal to the number of degenerate solutions. Our approach offers the possibility to separate also nonlinear mixtures of signals.
TL;DR: A well-defined crossover is found between a L\'evy and a Gaussian regime, and that the crossover carries information about the relevant parameters of the underlying stochastic process.
Abstract: We introduce a class of stochastic process, the truncated L\'evy flight (TLF), in which the arbitrarily large steps of a L\'evy flight are eliminated. We find that the convergence of the sum of $n$ independent TLFs to a Gaussian process can require a remarkably large value of $n$---typically $n\ensuremath{\approx}{10}^{4}$ in contrast to $n\ensuremath{\approx}10$ for common distributions. We find a well-defined crossover between a L\'evy and a Gaussian regime, and that the crossover carries information about the relevant parameters of the underlying stochastic process.
TL;DR: Using detailed plane-wave ab initio pseudopotential local density functional (LDA) calculations, it is found that the electronic properties of small tubes are significantly altered from those obtained in previous tight-binding calculations.
Abstract: Hybridization of the ${\mathrm{\ensuremath{\sigma}}}^{\mathrm{*}}$ and ${\mathrm{\ensuremath{\pi}}}^{\mathrm{*}}$ states of the graphene network is shown to be as important as band-folding effects in determining the metallicity of small radius carbon nanotubes. Using detailed plane-wave ab initio pseudopotential local density functional (LDA) calculations, we find that the electronic properties of small tubes are significantly altered from those obtained in previous tight-binding calculations. Strongly modified low-lying conduction band states are introduced into the band gap of insulating tubes because of strong ${\mathrm{\ensuremath{\sigma}}}^{\mathrm{*}\mathrm{\ensuremath{-}}}$${\mathrm{\ensuremath{\pi}}}^{\mathrm{*}}$ hybridization. As a result, the LDA gaps of some tubes are lowered by more than 50%, and a tube previously predicted to be semiconducting is shown to be metallic.
TL;DR: Feynman path integrals are used to describe the two hydrogen atoms and a few of the surface atoms, thereby including quantum effects such as tunneling and zero point energy, as well as thermal averaging.
Abstract: We have evaluated the sticking probability and activation energy for dissociation of ${\mathrm{H}}_{2}$ molecules on a Cu(110) surface by a reversible work formulation of quantum transition state theory. Feynman path integrals are used to describe the two hydrogen atoms and a few of the surface atoms, thereby including quantum effects such as tunneling and zero point energy, as well as thermal averaging. At a temperature below 600 K an onset of a quantum regime is observed as the activation energy drops by 30%.
TL;DR: It is shown that a sizable areal density of midgap states exists on a {110} surface of a d-wave superconductor, which can either have vacuum or an insulator at the surface, or be separated from vacuum or a insulator by a clean, size-quantized, normal metal overlayer.
Abstract: It is shown that a sizable areal density of midgap states exists on a {110} surface of a ${\mathit{d}}_{\mathit{x}\mathit{a}}^{2}$-${\mathit{x}}_{\mathit{b}}^{2}$-wave superconductor, which can either have vacuum or an insulator at the surface, or be separated from vacuum or an insulator by a clean, size-quantized, normal metal overlayer. These ``midgap'' states have many observable consequences---some of which are briefly discussed here---which can be used as a clear signature to distinguish between d-wave and anisotropic s-wave superconductors.
TL;DR: Even if the net force is always zero, flow is induced by a fluctuation of the energy barrier, but only at flipping times roughly in between the adiabatic adjustment times on the left and right of the barrier.
Abstract: The motion of a heavily damped Brownian particle in a periodic potential subject to a dichotomously fluctuating perturbation is considered. We show that even if the net force is always zero, flow is induced by a fluctuation of the energy barrier, but only at flipping times roughly in between the adiabatic adjustment times on the left and right of the barrier. Predictions of our model are consistent with recent experimental data obtained by Svoboda et al. [Nature (London) 365, 721\char21{}727 (1993)] for a single kinesin molecule moving along a biopolymer.
TL;DR: The production of He[sup +] and He[Sup 2+] by a 160 fs, 780 nm laser has been measured over an unprecedented 12 orders of magnitude in counting range and the NS yield is found to scale with the ac-tunneling rate for the neutral, even when tunneling is not the dominant ionization pathway.
Abstract: The production of ${\mathrm{He}}^{+}$ and ${\mathrm{He}}^{2+}$ by a 160 fs, 780 nm laser has been measured over an unprecedented 12 orders of magnitude in counting range. Enhanced double electron emission, called nonsequential (NS) ionization, was observed over an intensity range where the single ionization dynamics is evolving from multiphoton to pure tunneling. The NS yield is found to scale with the ac-tunneling rate for the neutral, even when tunneling is not the dominant ionization pathway. A rescattering mechanism fails to predict the observed NS threshold or magnitude.
TL;DR: Anomalously large dynamical effective charges give rise to very strong mixing of the mode eigenvectors on going from the TO to the LO case, resulting in a ``giant LO-TO splitting'' in the sense that the soft TO mode is most closely related to the hardest LO modes.
Abstract: We perform a first-principles investigation of the role of Coulomb interactions in eight AB${\mathrm{O}}_{3}$ cubic perovskite compounds. The predicted spontaneous polarization and the LO and TO phonon frequencies are found to be in good agreement with experiment. Anomalously large dynamical effective charges give rise to very strong mixing of the mode eigenvectors on going from the TO to the LO case, resulting in a ``giant LO-TO splitting'' in the sense that the soft TO mode is most closely related to the hardest LO modes. The results help explain the extreme sensitivity of these compounds to electrostatic boundary conditions.
IBM1
TL;DR: Flux quantization in superconducting rings with 0, 2, and 3 grain-boundary Josephson junctions is used to test the pairing symmetry in high-${T}_{c}$ superconductors and results are consistent with $d$-wave pairing symmetry.
Abstract: We have used the concept of flux quantization in superconducting $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ rings with 0, 2, and 3 grain-boundary Josephson junctions to test the pairing symmetry in high-${T}_{c}$ superconductors. The magnetic flux threading these rings at 4.2 K is measured by employing a scanning superconducting quantum interference device microscope. Spontaneous magnetization of a half magnetic flux quantum, $\frac{{\ensuremath{\Phi}}_{0}}{2}=\frac{h}{4e}$ has been observed in the 3-junction ring, but not in the 2-junction rings. These results are consistent with $d$-wave pairing symmetry.
TL;DR: In this article, a method for performing electronic structure calculations without the explicit use of a basis is presented. But this method requires the use of supercell geometries and no artifacts such as supercells need be introduced for localized systems.
Abstract: We present a method for performing electronic structure calculations without the explicit use of a basis. We combine a finite-difference approach with ab initio pseudopotentials. In contrast to methods which use a plane wave basis, our calculations are performed completely in ``real space.'' No artifacts such as supercell geometries need be introduced for localized systems. Although this approach is easier to implement than one with a plane wave basis, no loss of accuracy occurs. The electronic structure of several diatomic molecules, ${\mathrm{Si}}_{2}$, ${\mathrm{C}}_{2}$, ${\mathrm{O}}_{2}$, and CO, are calculated to illustrate the utility of this method.
IBM1
TL;DR: It is shown that strained epitaxial layers can relax by two competing mechanisms; this work discusses how temperature, surfactants, or compositional grading change and so control the mode of relaxation.
Abstract: We show that strained epitaxial layers can relax by two competing mechanisms. At large misfit, the surface becomes rough, allowing easy nucleation of dislocations. However, strain-induced surface roughening is thermally activated, and the energy barrier increases very rapidly with decreasing misfit \ensuremath{\varepsilon}. Thus below some misfit ${\mathrm{\ensuremath{\varepsilon}}}_{\mathit{c}}$, the strain relaxes by nucleation of dislocations at existing sources before the surface has time to roughen. Relaxation via surface roughening is technologically undesirable; we discuss how temperature, surfactants, or compositional grading change ${\mathrm{\ensuremath{\varepsilon}}}_{\mathit{c}}$ and so control the mode of relaxation.
TL;DR: In this paper, the magnetic anisotropy energy constant increases with decreasing particle size, presumably because of the influence of surface anisotropic properties, such as magnetic hyperfine fields.
Abstract: Nanoparticles of metallic iron on carbon supports have been studied insitu by use of M\"ossbauer spectroscopy. The magnetic anisotropy energy constant increases with decreasing particle size, presumably because of the influence of surface anisotropy. Chemisorption of oxygen results in formation of a surface layer with magnetic hyperfine fields similar to those of thicker passivation layers, and with a ferromagnetic coupling to the spins in the core of the particles. In contrast, thicker passivation layers have a noncollinear spin structure.
TL;DR: Self-assembled lens-shaped InGaAs quantum dots in a suitably designed field-effect-type GaAs/AlAs heterostructure allows us to charge the lowest discrete quantum levels in the dots with single electrons, extracting energy spacings from capacitance as well as infrared transmission spectroscopy at low temperatures and under application of high magnetic fields.
Abstract: Imbedding self-assembled lens-shaped InGaAs quantum dots in a suitably designed field-effect-type GaAs/AlAs heterostructure allows us to charge the lowest discrete quantum levels in the dots with single electrons. Because of their small diameters of about 20 nm the Coulomb charging energy is significantly smaller than the quantization energies. We extract energy spacings of about 41 meV between the $s$-like ground state and the first excited $p$-like state from capacitance as well as infrared transmission spectroscopy at low temperatures and under application of high magnetic fields.
TL;DR: An extension to multivariate time series of the phase-randomized Fourier-transform algorithm for generating surrogate data that mimic not only the autoncorrelations of each of the variables in the original data set, but also the cross-correlations between all the variables as well.
Abstract: We propose an extension to multivariate time series of the phase-randomized Fourier-transform algorithm for generating surrogate data. Such surrogate data sets must mimic not only the autocorrelations of each of the variables in the original data set, they must mimic the cross correlations between all the variables as well. The method is applied both to a simulated example (the three components of the Lorentz equations) and to data from a multichannel electroencephalogram.
TL;DR: This work presents a new class of inflationary models based on realistic supersymmetric grand unified theories which do not have the usual `fine tuning' problems and is found to be proportional to the GUT (Planck) scale.
Abstract: We explore constraints on the spectral index $n$ of density fluctuations and the neutrino energy density fraction ${\ensuremath{\Omega}}_{\ensuremath{
u}}$ from observations of large scale structure. The best fits imply $n\ensuremath{\approx}1$ and ${\ensuremath{\Omega}}_{\ensuremath{
u}}\ensuremath{\approx}0.1\ensuremath{-}0.3$, for Hubble constants 40-60 km ${\mathrm{s}}^{\ensuremath{-}1}$ ${\mathrm{Mpc}}^{\ensuremath{-}1}$. We present a new class of inflationary models based on realistic supersymmetric grand unified theories (GUTs) which do not have the usual "fine tuning" problems. The amplitude of primordial density fluctuations is found to be $\ensuremath{\propto}{(\frac{{M}_{X}}{{M}_{P}})}^{2}$, where ${M}_{X}$ (${M}_{P}$) denote the GUT (Planck) scale. The spectral index $n=0.98$, in excellent agreement with the observations.
TL;DR: The predictions of mode-coupling-theory on the existence of a von Schweidler law are found to hold for a molecular dynamics simulation of a supercooled binary Lennard-Jones mixture and the exponents in these two power-laws are very close to satisfying the exponent relationship predicted by the mode-Coupling theory.
Abstract: We have performed molecular dynamics simulations of a supercooled atomic liquid. The self-intermediate-scattering function in the $\ensuremath{\beta}$-relaxation regime has a power-law time dependence and temperature dependence consistent with the mode-coupling-theory prediction of a von Schweidler law, with exponents that are very close to satisfying the exponent relationship predicted by the theory. The diffusion constants have a power-law dependence on temperature with the same critical temperature. The exponents for diffusion differ from those of the relaxtion time, a result that is in disagreement with the theory.
TL;DR: Steady-state planar spatial solitons are predicted for photorefractive materials when the diffraction of an optical beam is exactly compensated by nonlinear self-defocusing as mentioned in this paper.
Abstract: Steady-state dark (bright) planar spatial solitons are predicted for photorefractive materials when the diffraction of an optical beam is exactly compensated by nonlinear self-defocusing (focusing), due to the screening field set up around a dark notch (or a bright beam) in a photorefractive material to which an external field is applied. These screening solitons appear in steady state and differ from previously observed spatial solitons in their properties and physical origin.
TL;DR: Findings suggest that the electronic and structural properties ofVO may be more bandlike than correlated, and rigid criteria for distinguishing correlated from band insulators are not available.
Abstract: The electronic and structural properties of ${\mathrm{VO}}_{2}$ across its metal-insulator transition are studied using the local-density approximation. Band theory finds a monoclinic distorted ground state in good agreement with experiment, and an almost open gap to charge excitations. Although rigid criteria for distinguishing correlated from band insulators are not available, these findings suggest that ${\mathrm{VO}}_{2}$ may be more bandlike than correlated.