372 research outputs found
Renal and extrarenal regulation of potassium
The ISN Forefronts in Nephrology Symposium took place 8–11 September 2005 in Kartause Ittingen, Switzerland. It was dedicated to the memory of Robert W. Berliner, who died at age 86 on 5 February 2002. Dr Berliner contributed in a major way to our understanding of potassium transport in the kidney. Starting in the late 1940s, without knowledge of how potassium was transported across specific nephron segments and depending only on renal clearance methods, he and his able associates provided a still-valid blueprint of the basic transport properties of potassium handling by the kidney. They firmly established that potassium was simultaneously reabsorbed and secreted along the nephron; that variations in secretion in the distal nephron segments play a major role in regulating potassium excretion; and that such secretion is modulated by sodium, acid–base factors, hormones, and diuretics. These conclusions were presented in a memorable Harvey Lecture some forty years ago, and they have remained valid ever since. The concepts have also provided the foundation and stimulation for later work on single nephrons, tubule cells, and transport proteins involved in potassium transport
Singular electrostatic energy of nanoparticle clusters
The binding of clusters of metal nanoparticles is partly electrostatic. We
address difficulties in calculating the electrostatic energy when high charging
energies limit the total charge to a single quantum, entailing unequal
potentials on the particles. We show that the energy at small separation
has a strong logarithmic dependence on . We give a general law for the
strength of this logarithmic correction in terms of a) the energy at contact
ignoring the charge quantization effects and b) an adjacency matrix specifying
which spheres of the cluster are in contact and which is charged. We verify the
theory by comparing the predicted energies for a tetrahedral cluster with an
explicit numerical calculation.Comment: 17 pages, 3 figures. Submitted to Phys Rev
Scattering fingerprints of two-state dynamics
Particle transport in complex environments such as the interior of living cells is often (transiently) non-Fickian or anomalous, that is, it deviates from the laws of Brownian motion. Such anomalies may be the result of small-scale spatio-temporal heterogeneities in, or viscoelastic properties of, the medium, molecular crowding, etc. Often the observed dynamics displays multi-state characteristics, i.e. distinct modes of transport dynamically interconverting between each other in a stochastic manner. Reliably distinguishing between single- and multi-state dynamics is challenging and requires a combination of distinct approaches. To complement the existing methods relying on the analysis of the particle's mean squared displacement, position- or displacement-autocorrelation function, and propagators, we here focus on 'scattering fingerprints' of multi-state dynamics. We develop a theoretical framework for two-state scattering signatures—the intermediate scattering function and dynamic structure factor—and apply it to the analysis of simple model systems as well as particle-tracking experiments in living cells. We consider inert tracer-particle motion as well as systems with an internal structure and dynamics. Our results may generally be relevant for the interpretation of state-of-the-art differential dynamic microscopy experiments on complex particulate systems, as well as inelastic or quasielastic neutron (incl. spin-echo) and x-ray scattering probing structural and dynamical properties of macromolecules, when the underlying dynamics displays two-state transport
Unusual electronic ground state of a prototype cuprate: band splitting of single CuO_2-plane Bi_2 Sr_(2-x) La_x CuO_(6+delta)
By in-situ change of polarization a small splitting of the Zhang-Rice singlet
state band near the Fermi level has been resolved for optimum doped (x=0.4)
BiSrLaCuO at the (pi,0)-point (R.Manzke et al.
PRB 63, R100504 (2001). Here we treat the momentum dependence and lineshape of
the split band by photoemission in the EDC-mode with very high angular and
energy resolution. The splitting into two destinct emissions could also be
observed over a large portion of the major symmetry line M, giving the
dispersion for the individual contributions. Since bi-layer effects can not be
present in this single-layer material the results have to be discussed in the
context of one-particle removal spectral functions derived from current
theoretical models. The most prominent are microscopic phase separation
including striped phase formation, coexisting antiferromagnetic and
incommensurate charge-density-wave critical fluctuations coupled to electrons
(hot spots) or even spin charge separation within the Luttinger liquid picture,
all leading to non-Fermi liquid like behavior in the normal state and having
severe consequences on the way the superconducting state forms. Especially the
possibilty of observing spinon and holon excitations is discussed.Comment: 5 pages, 4 figure
Structural behavior of PbBiSrLaCuO for 0<y<0.53
In the Bi cuprates, the presence of a near 15 superstructure is well
known. Usually, this superstructure is suppressed by the substitution of lead,
but there have been reports of a phase separation in so called {\alpha} and
{\beta} phases. This paper shows in high detail time how and why the phase
separation develops and what happens to the quasi-15 superstructure
upon lead substitution. For this purpose, the lanthanum- and lead-substituted
single-layered superconductor BiSrCuO has been
investigated by scanning tunneling microscopy and low-energy electron
diffraction. The La content was kept constant at slightly under-doped
concentration while the Pb content was changed systematically. Thermodynamic
considerations show that a phase mixture of {\alpha} and {\beta} phases is
inevitable.Comment: 17 pages, 4 figure
Chiral sedimentation of extended objects in viscous media
We study theoretically the chirality of a generic rigid object's
sedimentation in a fluid under gravity in the low Reynolds number regime. We
represent the object as a collection of small Stokes spheres or stokeslets, and
the gravitational force as a constant point force applied at an arbitrary point
of the object. For a generic configuration of stokeslets and forcing point, the
motion takes a simple form in the nearly free draining limit where the
stokeslet radius is arbitrarily small. In this case, the internal hydrodynamic
interactions between stokeslets are weak, and the object follows a helical path
while rotating at a constant angular velocity about a fixed axis. This
is independent of initial orientation, and thus constitutes a chiral
response for the object. Even though there can be no such chiral response in
the absence of hydrodynamic interactions between the stokeslets, the angular
velocity obtains a fixed, nonzero limit as the stokeslet radius approaches
zero. We characterize empirically how depends on the placement of the
stokeslets, concentrating on three-stokeslet objects with the external force
applied far from the stokeslets. Objects with the largest are aligned
along the forcing direction. In this case, the limiting varies as the
inverse square of the minimum distance between stokeslets. We illustrate the
prevalence of this robust chiral motion with experiments on small macroscopic
objects of arbitrary shape.Comment: 35 pages, 10 figures; Section VII.A redone and other edits made for
clarity. Accepted by Phys. Rev.
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