282 research outputs found
Enhancement of superconductivity near the ferromagnetic quantum critical point in UCoGe
We report a high-pressure single crystal study of the superconducting
ferromagnet UCoGe. Ac-susceptibility and resistivity measurements under
pressures up to 2.2 GPa show ferromagnetism is smoothly depressed and vanishes
at a critical pressure GPa. Near the ferromagnetic critical point
superconductivity is enhanced. Upper-critical field measurements under pressure
show attains remarkably large values, which provides solid evidence
for spin-triplet superconductivity over the whole pressure range. The obtained
phase diagram reveals superconductivity is closely connected to a
ferromagnetic quantum critical point hidden under the superconducting `dome'.Comment: 4 pages, 3 figures; accepted for publication in PR
Algebraic and analytic Dirac induction for graded affine Hecke algebras
We define the algebraic Dirac induction map \Ind_D for graded affine Hecke
algebras. The map \Ind_D is a Hecke algebra analog of the explicit
realization of the Baum-Connes assembly map in the -theory of the reduced
-algebra of a real reductive group using Dirac operators. The definition
of \Ind_D is uniform over the parameter space of the graded affine Hecke
algebra. We show that the map \Ind_D defines an isometric isomorphism from
the space of elliptic characters of the Weyl group (relative to its reflection
representation) to the space of elliptic characters of the graded affine Hecke
algebra. We also study a related analytically defined global elliptic Dirac
operator between unitary representations of the graded affine Hecke algebra
which are realized in the spaces of sections of vector bundles associated to
certain representations of the pin cover of the Weyl group. In this way we
realize all irreducible discrete series modules of the Hecke algebra in the
kernels (and indices) of such analytic Dirac operators. This can be viewed as a
graded Hecke algebra analogue of the construction of discrete series
representations for semisimple Lie groups due to Parthasarathy and
Atiyah-Schmid.Comment: 37 pages, revised introduction, updated references, minor correction
First Observation of Quantum Oscillations in the Ferromagnetic Superconductor UCoGe
We succeeded in growing high quality single crystals of the ferromagnetic
superconductor UCoGe and measured the magnetoresistance at fields up to 34T.
The Shubnikov-de Haas signal was observed for the first time in a U-111 system
(UTGe, UTSi, T: transition metal). A small pocket Fermi surface (F~1kT) with
large cyclotron effective mass 25m0 was detected at high fields above 22T,
implying that UCoGe is a low carrier system accompanyed with heavy
quasi-particles. The observed frequency decreases with increasing fields,
indicating that the volume of detected Fermi surface changes nonlinearly with
field. The cyclotron mass also decreases, which is consistent with the decrease
of the A coefficient of resistivity.Comment: 5 pages, 5 figures, accepted for publication in J. Phys. Soc. Jp
Upper critical field in superconductors near ferromagnetic quantum critical points; UCoGe
We study the strong-coupling superconductivity near ferromagnetic quantum
critical points, mainly focusing on the upper critical fields . Based
on our simple model calculations, we discuss experimentally observed unusual
behaviors of in a recently discovered ferromagnetic superconductor
UCoGe. Especially, the large anisotropy between -axis and
-axis, and the strong-coupling behaviors in
are investigated. We also examine effects of
non-analytic corrections in the spin susceptibility on the superconductivity,
which can arise from effective long range interactions due to particle-hole
excitations.Comment: Proceedings of ICHE2010, Toky
Diverse foraging strategies by a marine top predator : sperm whales exploit pelagic and demersal habitats in the Kaikōura submarine canyon
Support for this work was provided from contributions by Ray Dalio to the WHOI Access to the Sea Fund.The submarine canyon off Kaikōura (New Zealand) is an extremely productive deep-sea habitat, and an important foraging ground for male sperm whales (Physeter macrocephalus). We used high-resolution archival tags to study the diving behaviour of sperm whales, and used the echoes from their echolocation sounds to estimate their distance from the seafloor. Diving depths and distance above the seafloor were obtained for 28 dives from six individuals. Whales foraged at depths between 284 and 1433 m, targeting mesopelagic and demersal prey layers. The majority of foraging buzzes occurred within one of three vertical strata: within 50 m of the seafloor, mid-water at depths of 700-900 m, and mid-water at depths of 400-600 m. Sperm whales sampled during this study performed more demersal foraging than that reported in any previous studies – including at Kaikōura in further inshore waters. This suggests that the extreme benthic productivity of the Kaikōura Canyon is reflected in the trophic preferences of these massive top predators. We found some evidence for circadian patterns in the foraging behaviour of sperm whales, which might be related to vertical movements of their prey following the deep scattering layer. We explored the ecological implications of the whales’ foraging preferences on their habitat use, highlighting the need for further research on how submarine canyons facilitate top predator hotspots.PostprintPeer reviewe
Parabolically induced representations of graded Hecke algebras
We study the representation theory of graded Hecke algebras, starting from
scratch and focusing on representations that are obtained with induction from a
discrete series representation of a parabolic subalgebra. We determine all
intertwining operators between such parabolically induced representations, and
use them to parametrize the irreducible representations.Comment: In the second version several new results have been added to prove
some claims from the last page of the first version. In the third version the
introduction has been extended and we determine the global dimension of a
graded Hecke algebr
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