44 research outputs found
Critical exponents and scaling invariance in the absence of a critical point
The paramagnetic-to-ferromagnetic phase transition is believed to proceed
through a critical point, at which power laws and scaling invariance,
associated with the existence of one diverging characteristic length scale --
the so called correlation length -- appear. We indeed observe power laws and
scaling behavior over extraordinarily many decades of the suitable scaling
variables at the paramagnetic-to-ferromagnetic phase transition in ultrathin Fe
films. However, we find that, when the putative critical point is approached,
the singular behavior of thermodynamic quantities transforms into an analytic
one: the critical point does not exist, it is replaced by a more complex phase
involving domains of opposite magnetization, below as well as the
putative critical temperature. All essential experimental results are
reproduced by Monte-Carlo simulations in which, alongside the familiar exchange
coupling, the competing dipole-dipole interaction is taken into account. Our
results imply that a scaling behavior of macroscopic thermodynamic quantities
is not necessarily a signature for an underlying second-order phase transition
and that the paramagnetic-to-ferromagnetic phase transition proceeds, very
likely, in the presence of at least two long spatial scales: the correlation
length and the size of magnetic domains.Comment: 10 pages, added figure 1, rearranged section
Scale invariance of a diodelike tunnel junction
We measure the current vs voltage (I-V) characteristics of a diodelike tunnel
junction consisting of a sharp metallic tip placed at a variable distance d
from a planar collector and emitting electrons via electric-field assisted
emission. All curves collapse onto one single graph when I is plotted as a
function of the single scaling variable Vd^{-\lambda}, d being varied from a
few mm to a few nm, i.e., by about six orders of magnitude. We provide an
argument that finds the exponent {\lambda} within the singular behavior
inherent to the electrostatics of a sharp tip. A simulation of the tunneling
barrier for a realistic tip reproduces both the scaling behavior and the small
but significant deviations from scaling observed experimentally.Comment: 6 pages, 6 figures. Accepted for publication in Physical Review
Hallmark of quantum skipping in energy filtered lensless scanning electron microscopy
We simulate the electronic system of ejected electrons arising when a tip, positioned few 10 amp; 8201;nm away from a surface, is operated in the field emission regime. We find that, by repeated quantum reflections quantum skipping , electrons produced at the nanoscale primary site are able to reach the macroscopic environment surrounding the tip surface region. We observe the hallmark of quantum skipping in an energy filtered experiment that detects the spin of the ejected electron
Rad51 Polymerization Reveals a New Chromatin Remodeling Mechanism
Rad51 protein is a well known protagonist of homologous recombination in eukaryotic cells. Rad51 polymerization on single-stranded DNA and its role in presynaptic filament formation have been extensively documented. Rad51 polymerizes also on double-stranded DNA but the significance of this filament formation remains unclear. We explored the behavior of Saccharomyces cerevisiae Rad51 on dsDNA and the influence of nucleosomes on Rad51 polymerization mechanism to investigate its putative role in chromatin accessibility to recombination machinery. We combined biochemical approaches, transmission electron microscopy (TEM) and atomic force microscopy (AFM) for analysis of the effects of the Rad51 filament on chromatinized templates. Quantitative analyses clearly demonstrated the occurrence of chromatin remodeling during nucleoprotein filament formation. During Rad51 polymerization, recombinase proteins moved all the nucleosomal arrays in front of the progressing filament. This polymerization process had a powerful remodeling effect, as Rad51 destabilized the nucleosomes along considerable stretches of DNA. Similar behavior was observed with RecA. Thus, recombinase polymerization is a powerful mechanism of chromatin remodeling. These remarkable features open up new possibilities for understanding DNA recombination and reveal new types of ATP-dependent chromatin dynamics
Phase-resolved pulsed precessional motion at a Schottky barrier
The precessional motion of the magnetization is excited by an ultrashort magnetic-field pulse at a Schottky barrier containing a ferromagnetic film. The precessional frequency, measured as a function of a bias field of variable strength and direction by time resolved Kerr microscopy, is accurately reproduced by a model based on the theory of ferromagnetic resonance. The spatially resolved precessional phase reveals a jump related to the chiral character of the exciting magnetic-field pulse