6,794 research outputs found
Many-body Green's function theory for electron-phonon interactions: ground state properties of the Holstein dimer
We study ground-state properties of a two-site, two-electron Holstein model
describing two molecules coupled indirectly via electron-phonon interaction by
using both exact diagonalization and self-consistent diagrammatic many-body
perturbation theory. The Hartree and self-consistent Born approximations used
in the present work are studied at different levels of self-consistency. The
governing equations are shown to exhibit multiple solutions when the
electron-phonon interaction is sufficiently strong whereas at smaller
interactions only a single solution is found. The additional solutions at
larger electron-phonon couplings correspond to symmetry-broken states with
inhomogeneous electron densities. A comparison to exact results indicates that
this symmetry breaking is strongly correlated with the formation of a bipolaron
state in which the two electrons prefer to reside on the same molecule. The
results further show that the Hartree and partially self-consistent Born
solutions obtained by enforcing symmetry do not compare well with exact
energetics, while the fully self-consistent Born approximation improves the
qualitative and quantitative agreement with exact results in the same symmetric
case. This together with a presented natural occupation number analysis
supports the conclusion that the fully self-consistent approximation describes
partially the bipolaron crossover. These results contribute to better
understanding how these approximations cope with the strong localizing effect
of the electron-phonon interaction.Comment: 9 figures, corrected typo
Observation of quantum spin noise in a 1D light-atoms quantum interface
We observe collective quantum spin states of an ensemble of atoms in a
one-dimensional light-atom interface. Strings of hundreds of cesium atoms
trapped in the evanescent fiel of a tapered nanofiber are prepared in a
coherent spin state, a superposition of the two clock states. A weak quantum
nondemolition measurement of one projection of the collective spin is performed
using a detuned probe dispersively coupled to the collective atomic observable,
followed by a strong destructive measurement of the same spin projection. For
the coherent spin state we achieve the value of the quantum projection noise 40
dB above the detection noise, well above the 3 dB required for reconstruction
of the negative Wigner function of nonclassical states. We analyze the effects
of strong spatial inhomogeneity inherent to atoms trapped and probed by the
evanescent waves. We furthermore study temporal dynamics of quantum
fluctuations relevant for measurement-induced spin squeezing and assess the
impact of thermal atomic motion. This work paves the road towards observation
of spin squeezed and entangled states and many-body interactions in 1D spin
ensembles
Proceedings from the 1984 Tax Institute Symposium: Cash or Deferred Arrangements Under I.R.C. Section 401(k)
This article examines the reasons why an employer might consider a cash or deferred profit sharing or stock bonus plan. It looks at the rules for a cash or deferred arrangement (CODA) under Section 401(k) of the Internal Revenue Code. It covers the four primary requirements under the I.R.C., and then fifth, and biggest requirement, nondiscrimination
A new proof that alternating links are non-trivial
We use a simple geometric argument and small cancellation properties of link
groups to prove that alternating links are non-trivial. This proof uses only
classic results in topology and combinatorial group theory.Comment: Minor changes. To appear in Fundamenta Mathematica
Adjustable thermal ''tree''
Tree mounts 10 thermocouples on extensible arms to provide a reliable heat profile of conditions within heat treating devices, such as ovens and autoclaves, and within environmental test chambers
Formalizing Size-Optimal Sorting Networks: Extracting a Certified Proof Checker
Since the proof of the four color theorem in 1976, computer-generated proofs
have become a reality in mathematics and computer science. During the last
decade, we have seen formal proofs using verified proof assistants being used
to verify the validity of such proofs.
In this paper, we describe a formalized theory of size-optimal sorting
networks. From this formalization we extract a certified checker that
successfully verifies computer-generated proofs of optimality on up to 8
inputs. The checker relies on an untrusted oracle to shortcut the search for
witnesses on more than 1.6 million NP-complete subproblems.Comment: IMADA-preprint-c
Bose-Einstein condensation of alkaline earth atoms: {Ca}
We have achieved Bose-Einstein condensation of Ca, the first for an
alkaline earth element. The influence of elastic and inelastic collisions
associated with the large ground state s-wave scattering length of Ca
was measured. From these findings, an optimized loading and cooling scheme was
developed that allowed us to condense about atoms after laser
cooling in a two-stage magneto-optical trap and subsequent forced evaporation
in a crossed dipole trap within less than 3 s. The condensation of an alkaline
earth element opens novel opportunities for precision measurements on the
narrow intercombination lines as well as investigations of molecular states at
the S--P asymptotes
Generation and detection of a sub-Poissonian atom number distribution in a one-dimensional optical lattice
We demonstrate preparation and detection of an atom number distribution in a
one-dimensional atomic lattice with the variance dB below the Poissonian
noise level. A mesoscopic ensemble containing a few thousand atoms is trapped
in the evanescent field of a nanofiber. The atom number is measured through
dual-color homodyne interferometry with a pW-power shot noise limited probe.
Strong coupling of the evanescent probe guided by the nanofiber allows for a
real-time measurement with a precision of atoms on an ensemble of some
atoms in a one-dimensional trap. The method is very well suited for
generating collective atomic entangled or spin-squeezed states via a quantum
non-demolition measurement as well as for tomography of exotic atomic states in
a one-dimensional lattice
The exact Hohenberg-Kohn functional for a lattice model
Trabajo presentado al: "Deutsche Physikalische Gesellschaft Spring Meeting" celebrado en Dresden (Alemania) del 30 de marzo al 4 de Abril de 2014.Standard local exchange-correlation and semi-local functionals in ground-state density functional theory are known for their shortcomings in describing correct charge transfer, dissociation energies of
molecular ions, and barriers of chemical reactions.
To understand the failures of approximate functionals and to gain insight into the behavior of the exact functional, we investigate the exact solution of the many-body Schrödinger equation for a lattice
model. Using exact diagonalization, we explicitely construct the exact Hohenberg-Kohn functional and the mapping from densities to wavefunctions. Besides the normal inter-system derivative discontinuity
widely discussed in the density-functional theory community, we observe a new feature of the exact functional in the low-density
limit. This "intra-system derivative discontinuity" resembles the inter-system derivative discontinuity, but is within the system.
The description of many physical phenomena linked to charge-transfer processes (both in the static and dynamical regimes) require a proper account of this "intra-system derivative discontinuity".Peer reviewe
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