Richard N. Silver

Quantum Breathers in Electron-Phonon Systems

Quantum breathers are studied numerically in several electron-phonon coupled finite chain systems, in which the coupling results in intrinsic nonlinearity but with varying degrees of nonadiabaticity. As for quantum nonlinear lattice systems, we find that quantum breathers can exist as eigenstates of the system Hamiltonians. Optical responses are calculated as signatures of these coherent nonlinear excitations. We propose a new type of quantum nonlinear excitation, {open_quotes}breather excitons,{close_quotes} which are bound states of breathers and excitons, whose properties are clarified with a minimal exciton-phonon model. {copyright} {ital 1998} {ital The American Physical Society}

The Los Alamos Neutron Scattering Center

Abstract The Los Alamos Neutron Scattering Center (LANSCE) is a new high intensity pulsed spallation neutron source for research in the structure and dynamics of condensed matter. With first operation in 1985, LANSCE is expected to produce the worlds highest peak thermal flux of 1.7 × 10 16 n⧸cm 2 · s by 1987. A description of the LANSCE facility and source characteristics is presented. Some experimental highlights of research at the low intensity (peak thermal flux of 6 × 10 13 n⧸cm 2 · s) WNR predecessor to LANSCE are described which illustrate the scientific potential and instrumentation of LANSCE. Future plans for the LANSCE facility and research program are discussed.

Application of maximum entropy to neutron tunneling spectroscopy

Abstract We demonstrate the maximum entropy method for the deconvolution of high-resolution tunneling data acquired with a quasi-elastic spectrometer. Given a precise characterization of the instrument resolution function, a maximum entropy analysis of lutidine data obtained with the IRIS spectrometer at ISIS results in an effective factor-of-three improvement in resolution.

Spin correlation functions in random-exchange s=1/2 XXZ chains

The decay of (disorder‐averaged) static spin correlation functions at T=0 for the one‐dimensional spin‐1/2 XXZantiferromagnet with uniform longitudinal coupling JΔ and random transverse coupling Jλ i is investigated by numerical calculations for ensembles of finite chains. At Δ=0 (XXmodel) the calculation is based on the Jordan‐Wigner mapping to free lattice fermions for chains with up to N=100 sites. At Δ≠0 Lanczos diagonalizations are carried out for chains with up to N=22 sites. The longitudinal correlation function 〈S z 0 S z r 〉 is found to exhibit a power‐law decay with an exponent that varies with Δ and, for nonzero Δ, also with the width of the λ i ‐distribution. The results for the transverse correlation function 〈S x 0 S x r 〉 show a crossover from power‐law decay to exponential decay as the exchange disorder is turned on.

Applications of Maximum Entropy and Bayesian Methods in Computational Many-Body Physics

Computer simulations of many-body systems can present difficult data analysis problems. An example is the determination of dynamical properties from quantum Monte Carlo (QMC) simulations. Data on imaginary time Matsubara Greens’ functions implicitly contain information about real time behavior. However, the spectral representation which relates imaginary time to real time is similar to a Laplace transform, and the numerical inversion of such transforms is notoriously unstable in the presence of statistical noise. Another example is the determination of the densities of states and the thermodynamic functions from imprecise knowledge of a finite number of moments. Data analysis problems in computer simulations are similar in many respects to those found in experimental research. They are often inverse problems, they can be ill-posed, the data may be incomplete, and they may be subject to statistical and systematic errors. Fortunately, the recent conceptual and algorithmic advances in maximum entropy (Max-Ent) and Bayesian data analysis methods1 are equally applicable to computer simulations. We illustrate such applications by calculating the dynamical properties of the Anderson model for dilute magnetic alloys and the densities of states of the 2-D Heisenberg model.

Bayesian Methods, Maximum Entropy, and Quantum Monte Carlo

We heuristically discuss the application of the method of maximum entropy to the extraction of dynamical information from imaginarytime, quantum Monte Carlo data. The discussion emphasizes the utility of a Bayesian approach to statistical inference and the importance of statistically well-characterized data.

Applications of Maxent to Quantum Monte Carlo

We consider the application of maximum entropy methods to the analysis of data produced by computer simulations. The focus is the calculation of the dynamical properties of quantum many-body systems by Monte Carlo methods, which is termed the “Analytic Continuation Problem.” For the Anderson model of dilute magnetic impurities in metals, we obtain spectral functions and transport coefficients which obey “Kondo Universality.”

Dynamics of the Anderson Model for Dilute Magnetic Alloys: A Quantum Monte Carlo and Maximum Entropy Study

The single-impurity Anderson model1 was invented nearly thirty years ago to describe dilute magnetic impurities in metallic hosts, including the formation and properties of itinerant local moments in alloys. The model is a prerequisite for the understanding of mixed valent and heavy-fermion phenomena.2 A limit of the model is the famous Kondo model3 which describes resistivity minima and saturation. The model is simple to state and its properties are readily measureable in the laboratory.4 However, the solution of the model is a difficult many-body problem. In recent years considerable progress in understanding the static properties of the model has been achieved using non-perturbative methods such as the Bethe ansatz,5 the renormalization group,6 and quantum Monte Carlo (QMC).7 Nevertheless, the understanding of the dynamical properties has remained elusive. These include the spectral density of the impurity state, the transport coeffecients, and the dynamical magnetic susceptibility. The spectral density has been calculated reliably only for large orbital degeneracy8 or for expansion parameters below the range of most physical interest.9 It has been obtained only at zero temperature by the renormalization group method,10 and it is extremely difficult to calculate reliably from QMC.11-15

Bose Condensate in Superfluid 4He and Momentum Distributions by Deep Inelastic Scattering

In 1938 London [1,2] offered an explanation of the observation earlier that year of superfluid behavior in liquid 4He when it is cooled below a critical temperature of 2.17 °K. He argued that the superfluid transition was analogous to the Bose condensation of an (ideal) gas of non-interacting atoms obeying the same Bose-Einstein spin-statistics relation as 4He atoms. This relation requires the many-atom wave function to be completely symmetric in the atomic coordinates, resulting in a preference for the atoms to occupy the same single-particle states. For a finite system of atoms the momenta are quantized in spacings proportional to the inverse of the system size. At high temperatures the fraction of atoms occupying any one of the momentum states also scales as the inverse of the size. However, as the temperature is reduced below a critical Bose condensation temperature a significant fraction of the atoms, independent of the system size, begins to occupy the zero-momentum state. The Bose condensate fraction of an ideal gas approaches one at zero temperature. For 4He, by analogy, at high temperatures in the normal fluid the condensate fraction should be zero, but as temperatures are reduced below the superfluid transition temperature the condensate fraction should rise to a non-zero value. The effect of the strong interactions among the (non-ideal) 4He atoms is to deplete the zero temperature condensate fraction from one in an ideal gas to a value much less than one for 4He. While the analogy between superfluidity and Bose condensation is imperfect, the concept of a Bose condensate in the superfluid phase has survived. A variety of increasingly sophisticated many-body calculations have predicted a condensate fraction of about 10 % at zero temperature in superfluid 4He at SVP. Because of the importance of superfluidity and the related phenomenon of superconductivity to condensed matter physics, this simple prediction has motivated a more than twenty year effort involving up to one hundred scientists to measure the Bose condensate fraction in 4He.

Final state effects in quantum fluids

The extraction of momentum distributions from high energy scattering experiments depends on the validity of the impulse approximation (IA), which has been extensively discussed in the workshop overview [1].