12,093 research outputs found
Spectrum of Neutral Helium in Strong Magnetic Fields
We present extensive and accurate calculations for the excited state spectrum
of spin-polarized neutral helium in a range of magnetic field strengths up to
G. Of considerable interest to models of magnetic white dwarf stellar
atmospheres, we also present results for the dipole strengths of the low lying
transitions among these states. Our methods rely on a systematically saturated
basis set approach to solving the Hartree--Fock self-consistent field
equations, combined with an ``exact'' stochastic method to estimate the
residual basis set truncation error and electron correlation effects. We also
discuss the applicability of the adiabatic approximation to strongly magnetized
multi-electron atoms.Comment: 19 pages, 7 figures, 10 table
Ferrotoroidic Moment as a Quantum Geometric Phase
We present a geometric characterization of the ferrotoroidic moment in terms
of a set of Abelian Berry phases. We also introduce a fundamental complex
quantity which provides an alternative way to calculate the ferrotoroidic
moment and its moments, and is derived from a second order tensor. This
geometric framework defines a natural computational approach for density
functional and many-body theories
Zero Temperature Phases of the Electron Gas
The stability of different phases of the three-dimensional non-relativistic
electron gas is analyzed using stochastic methods. With decreasing density, we
observe a {\it continuous} transition from the paramagnetic to the
ferromagnetic fluid, with an intermediate stability range () for the {\it partially} spin-polarized liquid. The freezing
transition into a ferromagnetic Wigner crystal occurs at . We
discuss the relative stability of different magnetic structures in the solid
phase, as well as the possibility of disordered phases.Comment: 4 pages, REVTEX, 3 ps figure
Optimal Quantum Measurements of Expectation Values of Observables
Experimental characterizations of a quantum system involve the measurement of
expectation values of observables for a preparable state |psi> of the quantum
system. Such expectation values can be measured by repeatedly preparing |psi>
and coupling the system to an apparatus. For this method, the precision of the
measured value scales as 1/sqrt(N) for N repetitions of the experiment. For the
problem of estimating the parameter phi in an evolution exp(-i phi H), it is
possible to achieve precision 1/N (the quantum metrology limit) provided that
sufficient information about H and its spectrum is available. We consider the
more general problem of estimating expectations of operators A with minimal
prior knowledge of A. We give explicit algorithms that approach precision 1/N
given a bound on the eigenvalues of A or on their tail distribution. These
algorithms are particularly useful for simulating quantum systems on quantum
computers because they enable efficient measurement of observables and
correlation functions. Our algorithms are based on a method for efficiently
measuring the complex overlap of |psi> and U|psi>, where U is an implementable
unitary operator. We explicitly consider the issue of confidence levels in
measuring observables and overlaps and show that, as expected, confidence
levels can be improved exponentially with linear overhead. We further show that
the algorithms given here can typically be parallelized with minimal increase
in resource usage.Comment: 22 page
Quantum Phase Diagram of the t-Jz Chain Model
We present the quantum phase diagram of the one-dimensional - model
for arbitrary spin (integer or half-integer) and sign of the spin-spin
interaction , using an {\it exact} mapping to a spinless fermion model
that can be solved {\it exactly} using the Bethe ansatz. We discuss its
superconducting phase as a function of hole doping . Motivated by the new
paradigm of high temperature superconductivity, the stripe phase, we also
consider the effect the antiferromagnetic background has on the - chain
intended to mimic the stripe segments.Comment: 4 pages, 2 figure
The dimerized phase of ionic Hubbard models
We derive an effective Hamiltonian for the ionic Hubbard model at half
filling, extended to include nearest-neighbor repulsion. Using a spin-particle
transformation, the effective model is mapped onto simple spin-1 models in two
particular cases. Using another spin-particle transformation, a slightly
modified model is mapped into an SU(3) antiferromagnetic Heisenberg model whose
exact ground state is known to be spontaneously dimerized. From the effective
models several properties of the dimerized phase are discussed, like
ferroelectricity and fractional charge excitations. Using bosonization and
recent developments in the theory of macroscopic polarization, we show that the
polarization is proportional to the charge of the elementary excitations
Generalized Coherent States as Preferred States of Open Quantum Systems
We investigate the connection between quasi-classical (pointer) states and
generalized coherent states (GCSs) within an algebraic approach to Markovian
quantum systems (including bosons, spins, and fermions). We establish
conditions for the GCS set to become most robust by relating the rate of purity
loss to an invariant measure of uncertainty derived from quantum Fisher
information. We find that, for damped bosonic modes, the stability of canonical
coherent states is confirmed in a variety of scenarios, while for systems
described by (compact) Lie algebras stringent symmetry constraints must be
obeyed for the GCS set to be preferred. The relationship between GCSs,
minimum-uncertainty states, and decoherence-free subspaces is also elucidated.Comment: 5 pages, no figures; Significantly improved presentation, new
derivation of invariant uncertainty measure via quantum Fisher information
added
Effects of Backflow Correlation in the Three-Dimensional Electron Gas: Quantum Monte Carlo Study
The correlation energy of the homogeneous three-dimensional interacting
electron gas is calculated using the variational and fixed-node diffusion Monte
Carlo methods, with trial functions that include backflow and three-body
correlations. In the high density regime the effects of backflow dominate over
those due to three-body correlations, but the relative importance of the latter
increases as the density decreases. Since the backflow correlations vary the
nodes of the trial function, this leads to improved energies in the fixed-node
diffusion Monte Carlo calculations. The effects are comparable to those found
for the two-dimensional electron gas, leading to much improved variational
energies and fixed-node diffusion energies equal to the release-node energies
of Ceperley and Alder within statistical and systematic errors.Comment: 14 pages, 5 figures, submitted to Physical Review
Exactly Solvable Hydrogen-like Potentials and Factorization Method
A set of factorization energies is introduced, giving rise to a
generalization of the Schr\"{o}dinger (or Infeld and Hull) factorization for
the radial hydrogen-like Hamiltonian. An algebraic intertwining technique
involving such factorization energies leads to derive -parametric families
of potentials in general almost-isospectral to the hydrogen-like radial
Hamiltonians. The construction of SUSY partner Hamiltonians with ground state
energies greater than the corresponding ground state energy of the initial
Hamiltonian is also explicitly performed.Comment: LaTex file, 21 pages, 2 PostScript figures and some references added.
To be published in J. Phys. A: Math. Gen. (1998
Performance of the star‐shaped flyer in the study of brittle materials: Three dimensional computer simulations and experimental observations
A three dimensional finite element computer simulation has been performed to assess the effects of release waves in normal impact soft‐recovery experiments when a star‐shaped flyer plate is used. Their effects on the monitored velocity‐time profiles have been identified and their implications in the interpretation of wave spreading and spall signal events highlighted. The calculation shows that the star‐shaped flyer plate indeed minimizes the magnitude of edge effects. The major perturbation to the one‐dimensional response within the central region of the target plate results from spherical waves emanating from the corners of the star‐shaped plate. Experimental evidence of the development of a damage ring located in coincidence with the eight entrant corners of the flyer plate is reported. Microscopy studies performed in the intact recovered samples revealed that this damage ring eliminates undesired boundary release waves within the central region of the specimen. Consequently, the observed damage in compression and tension within this region can be attributed primarily to the conditions arising from a state of uniaxial strain.
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