The Absence of Vortex Lattice Melting in a Conventional Superconductor

The state of the vortex lattice extremely close to the superconducting to normal transition in an applied magnetic field is investigated in high purity niobium. We observe that thermal fluctuations of the order parameter broaden the superconducting to normal transition into a crossover but no sign of a first order vortex lattice melting transition is detected in measurements of the heat capacity or the small angle neutron scattering (SANS) intensity. Direct observation of the vortices via SANS always finds a well ordered vortex lattice. The fluctuation broadening is considered in terms of the Lowest Landau Level theory of critical fluctuations and scaling is found to occur over a large H_{c2}(T) range

Strong polarized relations for the continuum

We prove that the strong polarized relation for the continuum holds for $\aleph_0$ and for every supercompact cardinal. We use iteration of Mathias forcing.Comment: 9 page

Luminal Mg2+, A Key Factor Controlling RYR2-mediated Ca2+ Release: Cytoplasmic and Luminal Regulation Modeled in a Tetrameric Channel

In cardiac muscle, intracellular Ca2+ and Mg2+ are potent regulators of calcium release from the sarcoplasmic reticulum (SR). It is well known that the free [Ca2+] in the SR ([Ca2+]L) stimulates the Ca2+ release channels (ryanodine receptor [RYR]2). However, little is known about the action of luminal Mg2+, which has not been regarded as an important regulator of Ca2+ release

Luminal Ca2+–regulated Mg2+ Inhibition of Skeletal RyRs Reconstituted as Isolated Channels or Coupled Clusters

In resting muscle, cytoplasmic Mg2+ is a potent inhibitor of Ca2+ release from the sarcoplasmic reticulum (SR). It is thought to inhibit calcium release channels (RyRs) by binding both to low affinity, low specificity sites (I-sites) and to high affinity Ca2+ sites (A-sites) thus preventing Ca2+ activation. We investigate the effects of luminal and cytoplasmic Ca2+ on Mg2+ inhibition at the A-sites of skeletal RyRs (RyR1) in lipid bilayers, in the presence of ATP or modified by ryanodine or DIDS. Mg2+ inhibits RyRs at the A-site in the absence of Ca2+, indicating that Mg2+ is an antagonist and does not simply prevent Ca2+ activation. Cytoplasmic Ca2+ and Cs+ decreased Mg2+ affinity by a competitive mechanism. We describe a novel mechanism for luminal Ca2+ regulation of Ca2+ release whereby increasing luminal [Ca2+] decreases the A-site affinity for cytoplasmic Mg2+ by a noncompetitive, allosteric mechanism that is independent of Ca2+ flow. Ryanodine increases the Ca2+ sensitivity of the A-sites by 10-fold, which is insufficient to explain the level of activation seen in ryanodine-modified RyRs at nM Ca2+, indicating that ryanodine activates independently of Ca2+. We describe a model for ion binding at the A-sites that predicts that modulation of Mg2+ inhibition by luminal Ca2+ is a significant regulator of Ca2+ release from the SR. We detected coupled gating of RyRs due to luminal Ca2+ permeating one channel and activating neighboring channels. This indicated that the RyRs existed in stable close-packed rafts within the bilayer. We found that luminal Ca2+ and cytoplasmic Mg2+ did not compete at the A-sites of single open RyRs but did compete during multiple channel openings in rafts. Also, luminal Ca2+ was a stronger activator of multiple openings than single openings. Thus it appears that RyRs are effectively “immune” to Ca2+ emanating from their own pore but sensitive to Ca2+ from neighboring channels

Exploring the fragile antiferromagnetic superconducting phase in CeCoIn5

CeCoIn5 is a heavy fermion Type-II superconductor which exhibits clear indications of Pauli-limited superconductivity. A variety of measurements give evidence for a transition at high magnetic fields inside the superconducting state, when the field is applied either parallel to or perpendicular to the c axis. When the field is perpendicular to the c axis, antiferromagnetic order is observed on the high-field side of the transition, with a magnetic wavevector of (q q 0.5), where q = 0.44 reciprocal lattice units. We show that this order remains as the magnetic field is rotated out of the basal plane, but the associated moment eventually disappears above 17 degrees, indicating that the anomalies seen with the field parallel to the c axis are not related to this magnetic order. We discuss the implications of this finding.Comment: Accepted Physical Review Letters, September 2010. 4 pages, 4 figure