6,730 research outputs found
Thermal expansion of the spin-1/2 Heisenberg-chain compound Cu(CHN)(NO)
Compounds containing magnetic subsystems representing simple model spin
systems with weak magnetic coupling constants are ideal candidates to test
theoretical predictions for the generic behavior close to quantum phase
transitions. We present measurements of the thermal expansion and
magnetostriction of the spin-1/2-chain compound copper pyrazine dinitrate
Cu(CHN)(NO). Of particular interest is the low-temperature
thermal expansion close to the saturation field ,
which defines a quantum phase transition from the gapless Luttinger liquid
state to the fully saturated state with a finite excitation gap. We observe a
sign change of the thermal expansion for the different ground states, and at
the quantum critical point the low-temperature expansion approaches a
divergence. Thus, our data agree very well with the expected
quantum critical behaviour.Comment: 4 pages, 3 figures; to appear in the proceedings of the ICM 09 held
in Karlsruhe, German
Signature of nearly icosahedral structures in liquid and supercooled liquid Copper
A growing body of experiments display indirect evidence of icosahedral
structures in supercooled liquid metals. Computer simulations provide more
direct evidence but generally rely on approximate interatomic potentials of
unproven accuracy. We use first-principles molecular dynamics simulations to
generate realistic atomic configurations, providing structural detail not
directly available from experiment, based on interatomic forces that are more
reliable than conventional simulations. We analyze liquid copper, for which
recent experimental results are available for comparison, to quantify the
degree of local icosahedral and polytetrahedral order
Assessing Learning Outcomes in Middle-Division Classical Mechanics: The Colorado Classical Mechanics/Math Methods Instrument
Reliable and validated assessments of introductory physics have been
instrumental in driving curricular and pedagogical reforms that lead to
improved student learning. As part of an effort to systematically improve our
sophomore-level Classical Mechanics and Math Methods course (CM 1) at CU
Boulder, we have developed a tool to assess student learning of CM 1 concepts
in the upper-division. The Colorado Classical Mechanics/Math Methods Instrument
(CCMI) builds on faculty consensus learning goals and systematic observations
of student difficulties. The result is a 9-question open-ended post-test that
probes student learning in the first half of a two-semester classical mechanics
/ math methods sequence. In this paper, we describe the design and development
of this instrument, its validation, and measurements made in classes at CU
Boulder and elsewhere.Comment: 11 pages, 6 figures, 1 tabl
Ginzburg-Landau theory of crystalline anisotropy for bcc-liquid interfaces
The weak anisotropy of the interfacial free-energy is a crucial
parameter influencing dendritic crystal growth morphologies in systems with
atomically rough solid-liquid interfaces. The physical origin and quantitative
prediction of this anisotropy are investigated for body-centered-cubic (bcc)
forming systems using a Ginzburg-Landau theory where the order parameters are
the amplitudes of density waves corresponding to principal reciprocal lattice
vectors. We find that this theory predicts the correct sign,
, and magnitude, , of this anisotropy in good agreement
with the results of MD simulations for Fe. The results show that the
directional dependence of the rate of spatial decay of solid density waves into
the liquid, imposed by the crystal structure, is a main determinant of
anisotropy. This directional dependence is validated by MD computations of
density wave profiles for different reciprocal lattice vectors for
crystal faces. Our results are contrasted with the prediction of the reverse
ordering from an earlier formulation of
Ginzburg-Landau theory [Shih \emph{et al.}, Phys. Rev. A {\bf 35}, 2611
(1987)].Comment: 9 pages, 5 figure
Out of Focus and Into the Frame: Information Skills Benchmarking at the University of Queensland Library
This paper describes the University of Queensland Library's 1999 Information Skills benchmarking project. Particular reference is made to focus group methodology and findings. Customer based criteria for the design, and redesign, of Information Skills Programs in academic libraries are discussed
Viscoelasticity and metastability limit in supercooled liquids
A supercooled liquid is said to have a kinetic spinodal if a temperature Tsp
exists below which the liquid relaxation time exceeds the crystal nucleation
time. We revisit classical nucleation theory taking into account the
viscoelastic response of the liquid to the formation of crystal nuclei and find
that the kinetic spinodal is strongly influenced by elastic effects. We
introduce a dimensionless parameter \lambda, which is essentially the ratio
between the infinite frequency shear modulus and the enthalpy of fusion of the
crystal. In systems where \lambda is larger than a critical value \lambda_c the
metastability limit is totally suppressed, independently of the surface
tension. On the other hand, if \lambda < \lambda_c a kinetic spinodal is
present and the time needed to experimentally observe it scales as
exp[\omega/(\lambda_c-\lambda)^2], where \omega is roughly the ratio between
surface tension and enthalpy of fusion
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