6,232 research outputs found
Frequency multiplication in high-energy electron beams Semiannual progress report, Oct. 1, 1966 - Apr. 1, 1967
Frequency multiplication in high energy electron beam
An exactly solvable model of a superconducting to rotational phase transition
We consider a many-fermion model which exhibits a transition from a
superconducting to a rotational phase with variation of a parameter in its
Hamiltonian. The model has analytical solutions in its two limits due to the
presence of dynamical symmetries. However, the symmetries are basically
incompatible with one another; no simple solution exists in intermediate
situations. Exact (numerical) solutions are possible and enable one to study
the behavior of competing but incompatible symmetries and the phase transitions
that result in a semirealistic situation. The results are remarkably simple and
shed light on the nature of phase transitions.Comment: 11 pages including 1 figur
Generation of Entangled Photon Holes using Quantum Interference
In addition to photon pairs entangled in polarization or other variables,
quantum mechanics also allows optical beams that are entangled through the
absence of the photons themselves. These correlated absences, or ``entangled
photon holes'', can lead to counter-intuitive nonlocal effects analogous to
those of the more familiar entangled photon pairs. Here we report an
experimental observation of photon holes generated using quantum interference
effects to suppress the probability that two photons in a weak laser pulse will
separate at an optical beam splitter.Comment: 4 pages, color figures, submitted to Phys. Rev.
Spin dependent photoelectron tunnelling from GaAs into magnetic Cobalt
The spin dependence of the photoelectron tunnel current from free standing
GaAs films into out-of- plane magnetized Cobalt films is demonstrated. The
measured spin asymmetry (A) resulting from a change in light helicity, reaches
+/- 6% around zero applied tunnel bias and drops to +/- 2% at a bias of -1.6 V
applied to the GaAs. This decrease is a result of the drop in the photoelectron
spin polarization that results from a reduction in the GaAs surface
recombination velocity. The sign of A changes with that of the Cobalt
magnetization direction. In contrast, on a (nonmagnetic) Gold film A ~ 0%
Optical and transport properties of heavy fermions: theory compared to experiment
Employing a local moment approach to the periodic Anderson model within the
framework of dynamical mean-field theory, direct comparison is made between
theory and experiment for the dc transport and optical conductivities of
paramagnetic heavy fermion and intermediate valence metals. Four materials,
exhibiting a diverse range of behaviour in their transport/optics, are analysed
in detail: CeB6, YbAl3, CeAl3 and CeCoIn5. Good agreement between theory and
experiment is in general found, even quantitatively, and a mutually consistent
picture of transport and optics results.Comment: 21 pages, 10 figures; Replacement with minor style changes made to
avoid postscript file error
Vector coherent state representations, induced representations, and geometric quantization: I. Scalar coherent state representations
Coherent state theory is shown to reproduce three categories of
representations of the spectrum generating algebra for an algebraic model: (i)
classical realizations which are the starting point for geometric quantization;
(ii) induced unitary representations corresponding to prequantization; and
(iii) irreducible unitary representations obtained in geometric quantization by
choice of a polarization. These representations establish an intimate relation
between coherent state theory and geometric quantization in the context of
induced representations.Comment: 29 pages, part 1 of two papers, published versio
Hermitian boson mapping and finite truncation
Starting from a general, microscopic fermion-to-boson mapping that preserves
Hermitian conjugation, we discuss truncations of the boson Fock space basis. We
give conditions under which the exact boson images of finite fermion operators
are also finite (e.g., a 1+2-body fermion Hamiltonian is mapped to a 1+2-body
boson Hamiltonian) in the truncated basis. For the most general case, where the
image is not necessarily exactly finite, we discuss how to make practical and
controlled approximations.Comment: 12 pages in RevTex with no figures, Los Alamos preprint #
LA-UR-94-146
Pairing effect on the giant dipole resonance width at low temperature
The width of the giant dipole resonance (GDR) at finite temperature T in
Sn-120 is calculated within the Phonon Damping Model including the neutron
thermal pairing gap determined from the modified BCS theory. It is shown that
the effect of thermal pairing causes a smaller GDR width at T below 2 MeV as
compared to the one obtained neglecting pairing. This improves significantly
the agreement between theory and experiment including the most recent data
point at T = 1 MeV.Comment: 8 pages, 5 figures to be published in Physical Review
Nonabelian density functional theory
Given a vector space of microscopic quantum observables, density functional
theory is formulated on its dual space. A generalized Hohenberg-Kohn theorem
and the existence of the universal energy functional in the dual space are
proven. In this context ordinary density functional theory corresponds to the
space of one-body multiplication operators. When the operators close under
commutation to form a Lie algebra, the energy functional defines a Hamiltonian
dynamical system on the coadjoint orbits in the algebra's dual space. The
enhanced density functional theory provides a new method for deriving the group
theoretic Hamiltonian on the coadjoint orbits from the exact microscopic
Hamiltonian.Comment: 1 .eps figur
Scalar ground-state observables in the random phase approximation
We calculate the ground-state expectation value of scalar observables in the
matrix formulation of the random phase approximation (RPA). Our expression,
derived using the quasiboson approximation, is a straightforward generalization
of the RPA correlation energy. We test the reliability of our expression by
comparing against full diagonalization in 0 h-bar omega shell-model spaces. In
general the RPA values are an improvement over mean-field (Hartree-Fock)
results, but are not always consistent with shell-model results. We also
consider exact symmetries broken in the mean-field state and whether or not
they are restored in RPA.Comment: 7 pages, 3 figure
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