8,924 research outputs found
Zinc calixarene complexes for the ring opening polymerization of cyclic esters
Reaction of Zn(CâFâ
)â·toluene (two equivalents) with 1,3-dipropoxy-p-tert-butyl-calix[4]arene (LÂčHâ) led to the isolation of the complex [{Zn(CâFâ
)}âLÂč] (1), whilst similar use of Zn(Me)â resulted in the known complex [{Zn(Me)}âLÂč] (2). Treatment of LÂčHâ with in situ prepared Zn{N(SiMeâ)â}â in refluxing toluene led to the isolation of the compound [(Na)ZnN(SiMeâ)âLÂč] (3). The stepwise reaction of LÂčHâ and sodium hydride, followed by ZnClâ and finally NaN(SiMeâ)â yielded the compound [Zn{N(SiMeâ)â}âLÂč] (4). The reaction between three equivalents of Zn(CâFâ
)â·toluene and oxacalix[3]arene (LÂČHâ) at room temperature formed the compound {[Zn(CâFâ
)]âLÂČ} (5); heating of 5 in acetonitrile caused the ring opening of the parent oxacalix[3]arene and rearrangement to afford the complex [(LÂČ)Znâ(CâFâ
)(R)(RH)OH]·5MeCN R = CâFâ
CHâ-(p-á”BuPhenolate-CHâOCHââ)ââp-á”BuPhenolate-CHâOâ»)Âłâ» (6). The molecular structures of the new complexes 1, 3 and 6, together with that of the known complex 2, whose solid state structure has not previously been reported, have been determined. Compounds 1, 3â5 have been screened for the ring opening polymerization (ROP) of Δ-caprolactone (Δ-CL) and rac-lactide. Compounds featuring a ZnâCâFâ
fragment were found to be poor ROP pre-catalysts as they did not react with benzyl alcohol to form an alkoxide. By contrast, compound 4, which contains a zinc silylamide linkage, was the most active of the zinc-based calix[4]arene compounds screened and was capable of ROP at ambient temperature with 65% conversion over 4 h
Quantal interferometry with dissipative internal motion
In presence of dissipation, quantal states may acquire complex-valued phase
effects. We suggest a notion of dissipative interferometry that accommodates
this complex-valued structure and that may serve as a tool for analyzing the
effect of certain kinds of external influences on quantal interference. The
concept of mixed-state phase and concomitant gauge invariance is extended to
dissipative internal motion. The resulting complex-valued mixed-state
interference effects lead to well-known results in the unitary limit and in the
case of dissipative motion of pure quantal states. Dissipative interferometry
is applied to fault-tolerant geometric quantum computation.Comment: Slight revision, journal reference adde
Shaped nozzles for cryogenic buffer gas beam sources
Cryogenic buffer gas beams are important sources of cold molecules. In this
work we explore the use of a converging-diverging nozzle with a buffer-gas
beam. We find that, under appropriate circumstances, the use of a nozzle can
produce a beam with improved collimation, lower transverse temperatures, and
higher fluxes per solid angle
Beyond the hybrid library : libraries in a Web 2.0 world
Considers the development of social networking and the concept of Web 2.0. Looks at the implications for libraries and how traditional competences remain relevant. Explores what libraries are doing and must do to remain relevan
Non-linear Microwave Surface Impedance of Epitaxial HTS Thin Films in Low DC Magnetic Fields
We have carried out non-linear microwave (8 GHz) surface impedance
measurements of three YBaCuO thin films in dc magnetic fields
(parallel to c axis) up to 12 mT using a coplanar resonator technique. In zero
dc field the three films, deposited by the same method, show a spread of
low-power residual surface resistance, and penetration depth,
(T=15 K) within a factor of 1.9. However, they exhibit dramatically
different microwave field, dependences of the surface resistance,
, but universal dependence. Application of a dc field was
found to affect not only absolute values of and , but the functional
dependences and as well. For some of the samples
the dc field was found to decrease below its zero-field low-power value.Comment: 4 pages, 4 figures. To be published in IEEE Trans. Appl. Supercond.,
June 199
Lower Bounds on the Ground State Entropy of the Potts Antiferromagnet on Slabs of the Simple Cubic Lattice
We calculate rigorous lower bounds for the ground state degeneracy per site,
, of the -state Potts antiferromagnet on slabs of the simple cubic
lattice that are infinite in two directions and finite in the third and that
thus interpolate between the square (sq) and simple cubic (sc) lattices. We
give a comparison with large- series expansions for the sq and sc lattices
and also present numerical comparisons.Comment: 7 pages, late
Noncyclic geometric changes of quantum states
Non-Abelian quantum holonomies, i.e., unitary state changes solely induced by
geometric properties of a quantum system, have been much under focus in the
physics community as generalizations of the Abelian Berry phase. Apart from
being a general phenomenon displayed in various subfields of quantum physics,
the use of holonomies has lately been suggested as a robust technique to obtain
quantum gates; the building blocks of quantum computers. Non-Abelian holonomies
are usually associated with cyclic changes of quantum systems, but here we
consider a generalization to noncyclic evolutions. We argue that this open-path
holonomy can be used to construct quantum gates. We also show that a structure
of partially defined holonomies emerges from the open-path holonomy. This
structure has no counterpart in the Abelian setting. We illustrate the general
ideas using an example that may be accessible to tests in various physical
systems.Comment: Extended version, new title, journal reference adde
Determination of characteristic muon precession and relaxation signals in FeAs and FeAs2, possible impurity phases in pnictide superconductors
We report muon-spin relaxation measurements of highly homogeneous samples of
FeAs and FeAs2, both previously found as impurity phases in some samples of
recently synthesized pnictide superconductors. We observe well defined muon
precession in the FeAs sample with two precession frequencies of 38.2(3) and
22.7(9) MHz at 7.5 K, with the majority of the amplitude corresponding to the
lower frequency component. In FeAs2 we confirm previous measurements showing
that no long-ranged magnetic order occurs above 2 K and measure the muon spin
relaxation rate, which increases on cooling. Our results exclude the
possibility that previous muon-spin relaxation measurements of pnictide
superconductors have been measuring the effect of these possible impurities.Comment: 4 pages, 3 figures, corrected Figure
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