120 research outputs found
Unconventional Pairing in Heavy Fermion Metals
The Fermi-liquid theory of superconductivity is applicable to a broad range
of systems that are candidates for unconventional pairing. Fundamental
differences between unconventional and conventional anisotropic superconductors
are illustrated by the unique effects that impurities have on the
low-temperature transport properties of unconventional superconductors. For
special classes of unconventional superconductors the low-temperature transport
coefficients are {\it universal}, i.e. independent of the impurity
concentration and scattering phase shift. The existence of a universal limit
depends on the symmetry of the order parameter and is achieved at low
temperatures , where is the bandwidth
of the impurity induced Andreev bound states. In the case of UPt thermal
conductivity measurements favor an or ground state.
Measurements at ultra-low temperatures should distinguish different pairing
states.Comment: 8 pages in a LaTex (3.0) file plus 5 Figures in PostScript. To appear
in the Proceedings of the XXI International Conference on Low Temperature
Physics held in Prague, 8-14 August 199
model of superconducting UPt
The phase diagram of superconducting UPt is explained in a
Ginzburg-Landau theory starting from the hypothesis that the order parameter is
a pseudo-spin singlet which transforms according to the representation
of the point group. We show how to compute the positions of the phase
boundaries both when the applied field is in the basal plane and when it is
along the c-axis. The experimental phase diagrams as determined by longitudinal
sound velocity data can be fit using a single set of parameters. In particular
the crossing of the upper critical field curves for the two field directions
and the apparent isotropy of the phase diagram are reproduced. The former is a
result of the magnetic properties of UPt and their contribution to the free
energy in the superconducting state. The latter is a consequence of an
approximate particle-hole symmetry. Finally we extend the theory to finite
pressure and show that, in contrast to other models, the model
explains the observed pressure dependence of the phase boundaries.Comment: RevTex, 29 pages, 18 PostScript figures in a uuencoded, gzipped tar
file. PostScript version of paper, tar file of PostScript figures and
individual PostScript figures are also available via anonymous ftp at
ftp://nym.physics.wisc.edu/anonymou/papers/upt3
Influence of a magnetic field on the antiferromagnetic order in UPt_3
A neutron diffraction experiment was performed to investigate the effect of a
magnetic field on the antiferromagnetic order in the heavy fermion
superconductor UPt_3. Our results show that a field in the basal plane of up to
3.2 Tesla, higher than H_c2(0), has no effect: it can neither select a domain
nor rotate the moment. This has a direct impact on current theories for the
superconducting phase diagram based on a coupling to the magnetic order.Comment: 7 pages, RevTeX, 3 postscript figures, submitted to Phys. Rev.
Superconductivity in heavy-fermion U(Pt,Pd)3 and its interplay with magnetism
The effect of Pd doping on the superconducting phase diagram of the
unconventional superconductor UPt3 has been measured by (magneto)resistance,
specific heat, thermal expansion and magnetostriction. Experiments on single-
and polycrystalline U(Pt1-xPdx)3 for x<= 0.006 show that the superconducting
transition temperatures of the A phase, Tc+, and of the B phase, Tc-, both
decrease, while the splitting DTc increases at a rate of 0.30(2)K/at.%Pd. We
find that DTc(x) correlates with an increase of the weak magnetic moment m(x)
upon Pd doping. This provides further evidence for Ginzburg-Landau scenarios
with magnetism as the symmetry breaking field, i.e. the 2D E representation and
the 1D odd parity model. Only for small splittings DTc is proportional to
m^2(Tc+) (DTc<= 0.05 K) as predicted. The results at larger splittings call for
Ginzburg-Landau expansions beyond 4th order. The tetracritical point in the B-T
plane persists till at least x= 0.002 for B perpendicular to c, while it is
rapidly suppressed for B||c. Upon alloying the A and B phases gain stability at
the expense of the C phase.Comment: 25 pages text (PS), 8 pages with 14 figures (PS), submitted to
Phys.Rev.
Contour models of cellular adhesion
The development of traction-force microscopy, in the past two decades, has
created the unprecedented opportunity of performing direct mechanical
measurements on living cells as they adhere or crawl on uniform or
micro-patterned substrates. Simultaneously, this has created the demand for a
theoretical framework able to decipher the experimental observations, shed
light on the complex biomechanical processes that govern the interaction
between the cell and the extracellular matrix and offer testable predictions.
Contour models of cellular adhesion, represent one of the simplest and yet most
insightful approach in this problem. Rooted in the paradigm of active matter,
these models allow to explicitly determine the shape of the cell edge and
calculate the traction forces experienced by the substrate, starting from the
internal and peripheral contractile stresses as well as the passive restoring
forces and bending moments arising within the actin cortex and the plasma
membrane. In this chapter I provide a general overview of contour models of
cellular adhesion and review the specific cases of cells equipped with
isotropic and anisotropic actin cytoskeleton as well as the role of bending
elasticity.Comment: 24 pages, 9 figures. arXiv admin note: text overlap with
arXiv:1304.107
From 2D to 3D: novel nanostructured scaffolds to investigate signalling in reconstructed neuronal networks
To recreate in vitro 3D neuronal circuits will ultimately increase the relevance of results from cultured to whole-brain networks and will promote enabling technologies for neuro-engineering applications. Here we fabricate novel elastomeric scaffolds able to instruct 3D growth of living primary neurons. Such systems allow investigating the emerging activity, in terms of calcium signals, of small clusters of neurons as a function of the interplay between the 2D or 3D architectures and network dynamics. We report the ability of 3D geometry to improve functional organization and synchronization in small neuronal assemblies. We propose a mathematical modelling of network dynamics that supports such a result. Entrapping carbon nanotubes in the scaffolds remarkably boosted synaptic activity, thus allowing for the first time to exploit nanomaterial/cell interfacing in 3D growth support. Our 3D system represents a simple and reliable construct, able to improve the complexity of current tissue culture models
Thin Polymer Brush Decouples Biomaterial's Micro-/Nano-Topology and Stem Cell Adhesion
Surface morphology and chemistry of polymers used as biomaterials, such as tissue engineering scaffolds, have a strong influence on the adhesion and behavior of human mesenchymal stem cells. Here we studied semicrystalline poly(ε-caprolactone) (PCL) substrate scaffolds, which exhibited a variation of surface morphologies and roughness originating from different spherulitic superstructures. Different substrates were obtained by varying the parameters of the thermal processing, i.e. crystallization conditions. The cells attached to these polymer substrates adopted different morphologies responding to variations in spherulite density and size. In order to decouple substrate topology effects on the cells, sub-100 nm bio-adhesive polymer brush coatings of oligo(ethylene glycol) methacrylates were grafted from PCL and functionalized with fibronectin. On surfaces featuring different surface textures, dense and sub-100 nm thick brush coatings determined the response of cells, irrespective to the underlying topology. Thus, polymer brushes decouple substrate micro-/nano-topology and the adhesion of stem cells
Expanding Functionality of Recombinant Human Collagen Through Engineered Non-Native Cysteines
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