2,634 research outputs found
Preparation and Mechanical Properties of Continuous Carbon Nanotube Networks Modified C f
Continuous carbon nanotube (CNT) networks were formed in Cf/SiC composites via freeze-drying method. Composites were fabricated by precursor infiltration and pyrolysis (PIP) process afterwards. The different distribution morphologies of CNTs in the preforms originating from the different CNT contents were analyzed while the influence of the distribution of CNTs was discussed in detail. Compared to composites without CNTs, the interfacial shear strength (ILSS) and the flexural strength of Cf/1%CNTs/SiC were increased by 31% and 27%, respectively, but the values of Cf/2.5%CNTs/SiC decreased as a result of lots of defects caused by excess CNTs. With the analysis of ILSS, the flexural strengths, and the fracture morphologies, CNTs effectively improved the weak interfacial strength between T700SC carbon fibers and SiC matrix
Local geometry and quantum geometric tensor of mixed states
The quantum geometric tensor (QGT) is a fundamental concept for
characterizing the local geometry of quantum states. After casting the geometry
of pure quantum states and extracting the QGT, we generalize the geometry to
mixed quantum states via the density matrix and its purification. The
gauge-invariant QGT of mixed states is derived, whose real and imaginary parts
are the Bures metric and the Uhlmann form, respectively. In contrast to the
imaginary part of the pure-state QGT that is proportional to the Berry
curvature, the Uhlmann form vanishes identically for ordinary physical
processes. Moreover, there exists a Pythagorean-like equation that links
different local distances and reflect the underlying fibration. The Bures
metric reduces to the Fubini-Study metric as temperature approaches zero if the
eigenvalues of the density matrix do not change during the process,
establishing a correspondence between pure and mixed states. We also present
two examples with contrasting local geometries and discuss experimental
implications.Comment: 22 pages, 3 figure
Geometric phases of mixed quantum states: A comparative study of interferometric and Uhlmann phases
Two geometric phases of mixed quantum states, known as the interferometric
phase and Uhlmann phase, are generalizations of the Berry phase of pure states.
After reviewing the two geometric phases and examining their parallel-transport
conditions, we specify a class of cyclic processes that are compatible with
both conditions and therefore accumulate both phases through their definitions,
respectively. Those processes then facilitate a fair comparison between the two
phases. We present exact solutions of two-level and three-level systems to
contrast the two phases. While the interferometric phase exhibits
finite-temperature transitions only in the three-level system but not the
two-level system, the Uhlmann phase shows finite-temperature transitions in
both cases. Thus, using the two geometric phases as finite-temperature
topological indicators demonstrates the rich physics of topology of mixed
states.Comment: 12 pages, 2 figures, submitte
Uhlmann phase of coherent states and the Uhlmann-Berry correspondence
We first compare the geometric frameworks behind the Uhlmann and Berry phases
in a fiber-bundle language and then evaluate the Uhlmann phases of bosonic and
fermionic coherent states. The Uhlmann phases of both coherent states are shown
to carry geometric information and decrease smoothly with temperature.
Importantly, the Uhlmann phases approach the corresponding Berry phases as
temperature decreases. Together with previous examples in the literature, we
propose a correspondence between the Uhlmann and Berry phases in the
zero-temperature limit as a general property except some special cases and
present a conditional proof of the correspondence.Comment: 23 pages, no figur
3-tert-Butyl-4-oxo-3,4-dihydrophthalazin-1-yl 3,5-dimethylbenzoate
The title compound, C21H22N2O3, was synthesized by the reaction of tert-butylhydrazine with phthalic anhydride and further O-benzoylation of the resulting intermediate by 3,5-dimethylbenzoyl chloride. Intermolecular C—H⋯O=C interactions link the molecules into layers
Large intrinsic anomalous Hall effect in both NbFeB and TaFeB with collinear antiferromagnetism
It is rarely reported that collinear antiferromagnetic (AFM) metals can have
anomalous Hall effect (AHE). In this letter, based on symmetry analysis and the
first-principles electronic structure calculations, we predict that two
existing collinear antiferromagnets NbFeB and TaFeB,
whose N\'eel temperatures are above room temperature, have very large AHE with
anomalous Hall conductance (AHC) -100 cm and
cm, respectively. We further complete the symmetry
resquirements for realizing the AHE in collinear antiferromagnetism
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