177 research outputs found
Stress distribution and the fragility of supercooled melts
We formulate a minimal ansatz for local stress distribution in a solid that
includes the possibility of strongly anharmonic short-length motions. We
discover a broken-symmetry metastable phase that exhibits an aperiodic,
frozen-in stress distribution. This aperiodic metastable phase is characterized
by many distinct, nearly degenerate configurations. The activated transitions
between the configurations are mapped onto the dynamics of a long range
classical Heisenberg model with 6-component spins and anisotropic couplings. We
argue the metastable phase corresponds to a deeply supercooled non-polymeric,
non-metallic liquid, and further establish an order parameter for the
glass-to-crystal transition. The spin model itself exhibits a continuous range
of behaviors between two limits corresponding to frozen-in shear and uniform
compression/dilation respectively. The two regimes are separated by a
continuous transition controlled by the anisotropy in the spin-spin
interaction, which is directly related to the Poisson ratio of the
material. The latter ratio and the ultra-violet cutoff of the theory determine
the liquid configurational entropy. Our results suggest that liquid's fragility
depends on the Poisson ratio in a non-monotonic way. The present ansatz
provides a microscopic framework for computing the configurational entropy and
relaxational spectrum of specific substances.Comment: 11 pages, 5 figures, Final version published in J Phys Chem
Gap-dependent quasiparticle dynamics and coherent acoustic phonons in parent iron pnictide CaFe2As2 across the spin density wave phase transition
We report ultrafast quasiparticle (QP) dynamics and coherent acoustic phonons
in undoped CaFe_2As_2 iron pnictide single crystals exhibiting spin-density
wave (SDW) and concurrent structural phase transition at temperature TSDW ~ 165
K using femtosecond time-resolved pump-probe spectroscopy. The contributions in
transient differential reflectivity arising from exponentially decaying QP
relaxation and oscillatory coherent acoustic phonon mode show large variations
in the vicinity of T_SDW. From the temperature-dependence of the QP
recombination dynamics in the SDW phase, we evaluate a BCS-like temperature
dependent charge gap with its zero-temperature value of ~(1.6+/-0.2)k_BT_SDW,
whereas, much above T_SDW, an electron-phonon coupling constant of ~0.13 has
been estimated from the linear temperature-dependence of the QP relaxation
time. The long-wavelength coherent acoustic phonons with typical time-period of
~100 ps have been analyzed in the light of propagating strain pulse model
providing important results for the optical constants, sounds velocity and the
elastic modulus of the crystal in the whole temperature range of 3 K to 300 K.Comment: Revised version (to appear as Full Paper in Journal of Physical
Society of Japan (2013)); http://jpsj.ipap.jp/link?JPSJ/82/044715
Wettability Modification of Nanomaterials by Low-Energy Electron Flux
Controllable modification of surface free energy and related properties (wettability, hygroscopicity, agglomeration, etc.) of powders allows both understanding of fine physical mechanism acting on nanoparticle surfaces and improvement of their key characteristics in a number of nanotechnology applications. In this work, we report on the method we developed for electron-induced surface energy and modification of basic, related properties of powders of quite different physical origins such as diamond and ZnO. The applied technique has afforded gradual tuning of the surface free energy, resulting in a wide range of wettability modulation. In ZnO nanomaterial, the wettability has been strongly modified, while for the diamond particles identical electron treatment leads to a weak variation of the same property. Detailed investigation into electron-modified wettability properties has been performed by the use of capillary rise method using a few probing liquids. Basic thermodynamic approaches have been applied to calculations of components of solid–liquid interaction energy. We show that defect-free, low-energy electron treatment technique strongly varies elementary interface interactions and may be used for the development of new technology in the field of nanomaterials
Expression of Bcl-2 and Bax in Mouse Renal Tubules during Kidney Development
Bcl-2 and Bax play an important role in apoptosis regulation, as well as in cell adhesion and migration during kidney morphogenesis, which is structurally and functionally related to mitochondria. In order to elucidate the role of Bcl-2 and Bax during kidney development, it is essential to establish the exact location of their expression in the kidney. The present study localized their expression during kidney development. Kidneys from embryonic (E) 16-, 17-, 18-day-old mouse fetuses, and postnatal (P) 1-, 3-, 5-, 7-, 14-, 21-day-old pups were embedded in Epon. Semi-thin serial sections from two E17 kidneys underwent computer assisted 3D tubule tracing. The tracing was combined with a newly developed immunohistochemical technique, which enables immunohistochemistry on glutaraldehyde fixated plastic embedded sections. Thereby, the microstructure could be described in detail, and the immunochemistry can be performed using exactly the same sections. The study showed that Bcl-2 and Bax were strongly expressed in mature proximal convoluted tubules at all time points, less strongly expressed in proximal straight tubules, and only weakly in immature proximal tubules and distal tubules. No expression was detected in ureteric bud and other earlier developing structures, such as comma bodies, S shaped bodies, glomeruli, etc. Tubules expressing Bcl-2 only were occasionally observed. The present study showed that, during kidney development, Bcl-2 and Bax are expressed differently in the proximal and distal tubules, although these two tubule segments are almost equally equipped with mitochondria. The functional significance of the different expression of Bcl-2 and Bax in proximal and distal tubules is unknown. However, the findings of the present study suggest that the mitochondrial function differs between mature proximal tubules and in the rest of the tubules. The function of Bcl-2 and Bax during tubulogenesis still needs to be investigated
Status of QUBIC, the Q&U Bolometer for Cosmology
The Q&U Bolometric Interferometer for Cosmology (QUBIC) is a novel kind of
polarimeter optimized for the measurement of the B-mode polarization of the
Cosmic Microwave Back-ground (CMB), which is one of the major challenges of
observational cosmology. The signal is expected to be of the order of a few
tens of nK, prone to instrumental systematic effects and polluted by various
astrophysical foregrounds which can only be controlled through multichroic
observations. QUBIC is designed to address these observational issues with a
novel approach that combines the advantages of interferometry in terms of
control of instrumental systematics with those of bolometric detectors in terms
of wide-band, background-limited sensitivity.Comment: Contribution to the 2022 Cosmology session of the 33rd Rencontres de
Blois. arXiv admin note: substantial text overlap with arXiv:2203.0894
- …