1,115 research outputs found
Nonclassicality filters and quasiprobabilities
Necessary and sufficient conditions for the nonclassicality of bosonic
quantum states are formulated by introducing nonclassicality filters and
nonclassicality quasiprobability distributions. Regular quasiprobabilities are
constructed from characteristic functions which can be directly sampled by
balanced homodyne detection. Their negativities uncover the nonclassical
effects of general quantum states. The method is illustrated by visualizing the
nonclassical nature of a squeezed state.Comment: Significantly revised version, more emphasis on practical applicatio
Kowalevski's analysis of the swinging Atwood's machine
We study the Kowalevski expansions near singularities of the swinging
Atwood's machine. We show that there is a infinite number of mass ratios
where such expansions exist with the maximal number of arbitrary constants.
These expansions are of the so--called weak Painlev\'e type. However, in view
of these expansions, it is not possible to distinguish between integrable and
non integrable cases.Comment: 30 page
Defect-induced spin-glass magnetism in incommensurate spin-gap magnets
We study magnetic order induced by non-magnetic impurities in quantum
paramagnets with incommensurate host spin correlations. In contrast to the
well-studied commensurate case where the defect-induced magnetism is spatially
disordered but non-frustrated, the present problem combines strong disorder
with frustration and, consequently, leads to spin-glass order. We discuss the
crossover from strong randomness in the dilute limit to more conventional glass
behavior at larger doping, and numerically characterize the robust short-range
order inherent to the spin-glass phase. We relate our findings to magnetic
order in both BiCu2PO6 and YBa2Cu3O6.6 induced by Zn substitution.Comment: 6 pages, 5 figs, (v2) real-space RG results added; discussion
extended, (v3) final version as publishe
Kondo behavior in the asymmetric Anderson model: Analytic approach
The low-temperature behavior of the asymmetric single-impurity
Anderson model is studied by diagrammatic methods resulting in analytically
controllable approximations. We first discuss the ways one can simplify parquet
equations in critical regions of singularities in the two-particle vertex. The
scale vanishing at the critical point defines the Kondo temperature at which
the electron-hole correlation function saturates. We show that the Kondo
temperature exists at any filling of the impurity level. A quasiparticle
resonance peak in the spectral function, however, forms only in almost
electron-hole symmetric situations. We relate the Kondo temperature with the
width of the resonance peak. Finally we discuss the existence of satellite
Hubbard bands in the spectral function.Comment: REVTeX4, 11 pages, 5 EPS figure
Verifying continuous-variable entanglement in finite spaces
Starting from arbitrary Hilbert spaces, we reduce the problem to verify
entanglement of any bipartite quantum state to finite dimensional subspaces.
Hence, entanglement is a finite dimensional property. A generalization for
multipartite quantum states is also given.Comment: 4 page
Anisotropic superexchange of a 90 degree Cu-O-Cu bond
The magnetic anisotropy af a rectangular Cu-O-Cu bond is investigated in
second order of the spin-orbit interaction. Such a bond is characteristic for
cuprates having edge sharing CuO_2 chains, and exists also in the Cu_3O_4 plane
or in ladder compounds. For a ferromagnetic coupling between the copper spins
an easy axis is found perpendicular to the copper oxygen plaquettes in
agreement with the experimental spin structure of Li_2CuO_2. In addition, a
pseudo-dipolar interaction is derived. Its estimation in the case of the
Cu_3O_4 plane (which is present for instance in Ba_2Cu_3O_4Cl_2 or
Sr_2Cu_3O_4Cl_2) gives a value which is however two orders of magnitude smaller
than the usual dipole-dipole interaction.Comment: 6 pages, 2 figures, improved referenc
Is Weak Pseudo-Hermiticity Weaker than Pseudo-Hermiticity?
For a weakly pseudo-Hermitian linear operator, we give a spectral condition
that ensures its pseudo-Hermiticity. This condition is always satisfied
whenever the operator acts in a finite-dimensional Hilbert space. Hence weak
pseudo-Hermiticity and pseudo-Hermiticity are equivalent in finite-dimensions.
This equivalence extends to a much larger class of operators. Quantum systems
whose Hamiltonian is selected from among these operators correspond to
pseudo-Hermitian quantum systems possessing certain symmetries.Comment: published version, 10 page
Partial Disorder in the Periodic Anderson Model on a Triangular Lattice
We report our theoretical results on the emergence of a partially-disordered
state at zero temperature and its detailed nature in the periodic Anderson
model on a triangular lattice at half filling. The partially-disordered state
is characterized by coexistence of a collinear antiferromagnetic order on an
unfrustrated honeycomb subnetwork and nonmagnetic state at the remaining sites.
This state appears with opening a charge gap between a noncollinear
antiferromagnetic metal and Kondo insulator while changing the hybridization
and Coulomb repulsion. We also find a characteristic crossover in the
low-energy excitation spectrum as a result of coexistence of magnetic order and
nonmagnetic sites. The result demonstrates that the partially-disordered state
is observed distinctly even in the absence of spin anisotropy, in marked
contrast to the partial Kondo screening state found in the previous study for
the Kondo lattice model.Comment: 4 pages, 4 figures, accepted for publication in J. Phys. Soc. Jp
Weak commutation relations of unbounded operators and applications
Four possible definitions of the commutation relation [S,T]=\Id of two
closable unbounded operators are compared. The {\em weak} sense of this
commutator is given in terms of the inner product of the Hilbert space \H
where the operators act. Some consequences on the existence of eigenvectors of
two number-like operators are derived and the partial O*-algebra generated by
is studied. Some applications are also considered.Comment: In press in Journal of Mathematical Physic
Expanding Semiflows on Branched Surfaces and One-Parameter Semigroups of Operators
We consider expanding semiflows on branched surfaces. The family of transfer
operators associated to the semiflow is a one-parameter semigroup of operators.
The transfer operators may also be viewed as an operator-valued function of
time and so, in the appropriate norm, we may consider the vector-valued Laplace
transform of this function. We obtain a spectral result on these operators and
relate this to the spectrum of the generator of this semigroup. Issues of
strong continuity of the semigroup are avoided. The main result is the
improvement to the machinery associated with studying semiflows as
one-parameter semigroups of operators and the study of the smoothness
properties of semiflows defined on branched manifolds, without encoding as a
suspension semiflow
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