57 research outputs found
Harmonic behavior of metallic glasses up to the metastable melt
In two amorphous alloys ZrTiCuNiBe and ZrAlNiCu coherent neutron scattering has been measured over five decades in energy, including measurements in the metastable melt of a metallic alloy more than 80 K above Tg. In the vibrational spectra a pronounced "boson" peak is found: Even in crystallized samples the density of states exceeds the Debye ω2 model, and in the amorphous state low-frequency vibrations are further enhanced. The peak position shows no dispersion in q, while intensities are strongly correlated with the static structure factor. Over the full energy range the temperature dependence is strictly harmonic. From high-energy resolution measurements we establish lower bounds for the temperatures at which structural α and fast β relaxation become observable
Self-Diffusion in Random-Tiling Quasicrystals
The first explicit realization of the conjecture that phason dynamics leads
to self-diffusion in quasicrystals is presented for the icosahedral Ammann
tilings. On short time scales, the transport is found to be subdiffusive with
the exponent , while on long time scales it is consistent
with normal diffusion that is up to an order of magnitude larger than in the
typical room temperature vacancy-assisted self-diffusion. No simple finite-size
scaling is found, suggesting anomalous corrections to normal diffusion, or
existence of at least two independent length scales.Comment: 11 pages + 2 figures, COMPRESSED postscript figures available by
anonymous ftp to black_hole.physics.ubc.ca directory outgoing/diffuse (use bi
for binary mode to transfer), REVTeX 3.0, CTP-TAMU 21/9
Dynamics of liquid 4He in Vycor
We have measured the dynamic structure factor of liquid 4He in Vycor using
neutron inelastic scattering. Well-defined phonon-roton (p-r) excitations are
observed in the superfluid phase for all wave vectors 0.3 < Q < 2.15. The p-r
energies and lifetimes at low temperature (T = 0.5 K) and their temperature
dependence are the same as in bulk liquid 4He. However, the weight of the
single p-r component does not scale with the superfluid fraction (SF) as it
does in the bulk. In particular, we observe a p-r excitation between T_c =
1.952 K, where SF = 0, and T_(lambda)=2.172 K of the bulk. This suggests, if
the p-r excitation intensity scales with the Bose condensate, that there is a
separation of the Bose-Einstein condensation temperature and the superfluid
transition temperature T_c of 4He in Vycor. We also observe a two-dimensional
layer mode near the roton wave vector. Its dispersion is consistent with
specific heat and SF measurements and with layer modes observed on graphite
surfaces.Comment: 3 pages, 4 figure
Integrative and perturbation-based analysis of the transcriptional dynamics of TGFβ/BMP system components in transition from embryonic stem cells to neural progenitors
Cooperative actions of extrinsic signals and cell-intrinsic transcription factors alter gene regulatory networks enabling cells to respond appropriately to environmental cues. Signaling by transforming growth factor type β (TGFβ) family ligands (eg, bone morphogenetic proteins [BMPs] and Activin/Nodal) exerts cell-type specific and context-dependent transcriptional changes, thereby steering cellular transitions throughout embryogenesis. Little is known about coordinated regulation and transcriptional interplay of the TGFβ system. To understand intrafamily transcriptional regulation as part of this system's actions during development, we selected 95 of its components and investigated their mRNA-expression dynamics, gene-gene interactions, and single-cell expression heterogeneity in mouse embryonic stem cells transiting to neural progenitors. Interrogation at 24 hour intervals identified four types of temporal gene transcription profiles that capture all stages, that is, pluripotency, epiblast formation, and neural commitment. Then, between each stage we performed esiRNA-based perturbation of each individual component and documented the effect on steady-state mRNA levels of the remaining 94 components. This exposed an intricate system of multilevel regulation whereby the majority of gene-gene interactions display a marked cell-stage specific behavior. Furthermore, single-cell RNA-profiling at individual stages demonstrated the presence of detailed co-expression modules and subpopulations showing stable co-expression modules such as that of the core pluripotency genes at all stages. Our combinatorial experimental approach demonstrates how intrinsically complex transcriptional regulation within a given pathway is during cell fate/state transitions
Nonlinear Bogolyubov-Valatin transformations and quaternions
In introducing second quantization for fermions, Jordan and Wigner
(1927/1928) observed that the algebra of a single pair of fermion creation and
annihilation operators in quantum mechanics is closely related to the algebra
of quaternions H. For the first time, here we exploit this fact to study
nonlinear Bogolyubov-Valatin transformations (canonical transformations for
fermions) for a single fermionic mode. By means of these transformations, a
class of fermionic Hamiltonians in an external field is related to the standard
Fermi oscillator.Comment: 6 pages REVTEX (v3: two paragraphs appended, minor stylistic changes,
eq. (39) corrected, references [10]-[14], [36], [37], [41], [67]-[69] added;
v4: few extensions, references [62], [63] added, final version to be
published in J. Phys. A: Math. Gen.
4f-spin dynamics in La(2-x-y)Sr(x)Nd(y)CuO(4)
We have performed inelastic magnetic neutron scattering experiments on
La(2-x-y)Sr(x)Nd(y)CuO(4) in order to study the Nd 4f-spin dynamics at low
energies. In all samples we find at high temperatures a quasielastic line
(Lorentzian) with a line width which decreases on lowering the temperature. The
temperature dependence of the quasielastic line width Gamma/2(T) can be
explained with an Orbach-process, i.e. a relaxation via the coupling between
crystal field excitations and phonons. At low temperatures the Nd-4f magnetic
response S(Q,omega) correlates with the electronic properties of the
CuO(2)-layers. In the insulator La(2-y)Nd(y)CuO(4) the quasielastic line
vanishes below 80 K and an inelastic excitation occurs. This directly indicates
the splitting of the Nd3+ ground state Kramers doublet due to the static
antiferromagnetic order of the Cu moments. In La(1.7-x)Sr(x)Nd(0.3)CuO(4) with
x = 0.12, 0.15 and La(1.4-x)Sr(x)Nd(0.6)CuO(4) with x = 0.1, 0.12, 0.15, 0.18
superconductivity is strongly suppressed. In these compounds we observe a
temperature independent broad quasielastic line of Gaussian shape below T about
30 K. This suggests a distribution of various internal fields on different Nd
sites and is interpreted in the frame of the stripe model. In
La(1.8-y)Sr(0.2)Nd(y)CuO(4) (y = 0.3, 0.6) such a quasielastic broadening is
not observed even at lowest temperature.Comment: 8 pages, 10 figures included, to appear in Phys. Rev.
Variation in 12 porcine genes involved in the carbohydrate moiety assembly of glycosphingolipids does not account for differential binding of F4 Escherichia coliand their fimbriae
Erythrocyte and Porcine Intestinal Glycosphingolipids Recognized by F4 Fimbriae of Enterotoxigenic Escherichia coli
Enterotoxigenic F4-fimbriated Escherichia coli is associated with diarrheal disease in neonatal and postweaning pigs. The F4 fimbriae mediate attachment of the bacteria to the pig intestinal epithelium, enabling an efficient delivery of diarrhea-inducing enterotoxins to the target epithelial cells. There are three variants of F4 fimbriae designated F4ab, F4ac and F4ad, respectively, having different antigenic and adhesive properties. In the present study, the binding of isolated F4ab, F4ac and F4ad fimbriae, and F4ab/ac/ad-fimbriated E. coli, to glycosphingolipids from erythrocytes and from porcine small intestinal epithelium was examined, in order to get a comprehensive view of the F4-binding glycosphingolipids involved in F4-mediated hemagglutination and adhesion to the epithelial cells of porcine intestine. Specific interactions between the F4ab, F4ac and F4ad fimbriae and both acid and non-acid glycosphingolipids were obtained, and after isolation of binding-active glycosphingolipids and characterization by mass spectrometry and proton NMR, distinct carbohydrate binding patterns were defined for each fimbrial subtype. Two novel glycosphingolipids were isolated from chicken erythrocytes, and characterized as GalNAcα3GalNAcß3Galß4Glcß1Cer and GalNAcα3GalNAcß3Galß4GlcNAcß3Galß4Glcß1Cer. These two compounds, and lactosylceramide (Galß4Glcß1Cer) with phytosphingosine and hydroxy fatty acid, were recognized by all three variants of F4 fimbriae. No binding of the F4ad fimbriae or F4ad-fimbriated E. coli to the porcine intestinal glycosphingolipids occurred. However, for F4ab and F4ac two distinct binding patterns were observed. The F4ac fimbriae and the F4ac-expressing E. coli selectively bound to galactosylceramide (Galß1Cer) with sphingosine and hydroxy 24:0 fatty acid, while the porcine intestinal glycosphingolipids recognized by F4ab fimbriae and the F4ab-fimbriated bacteria were characterized as galactosylceramide, sulfatide (SO3-3Galß1Cer), sulf-lactosylceramide (SO3-3Galß4Glcß1Cer), and globotriaosylceramide (Galα4Galß4Glcß1Cer) with phytosphingosine and hydroxy 24:0 fatty acid. Finally, the F4ad fimbriae and the F4ad-fimbriated E. coli, but not the F4ab or F4ac subtypes, bound to reference gangliotriaosylceramide (GalNAcß4Galß4Glcß1Cer), gangliotetraosylceramide (Galß3GalNAcß4Galß4Glcß1Cer), isoglobotriaosylceramide (Galα3Galß4Glcß1Cer), and neolactotetraosylceramide (Galß4GlcNAcß3Galß4Glcß1Cer)
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