150 research outputs found
Multiband superconductivity with unexpected deficiency of nodal quasiparticles in CeCu2Si2
Superconductivity in the heavy-fermion compound CeCu2Si2 is a prototypical
example of Cooper pairs formed by strongly correlated electrons. For more than
30 years, it has been believed to arise from nodal d-wave pairing mediated by a
magnetic glue. Here, we report a detailed study of the specific heat and
magnetization at low temperatures for a high-quality single crystal.
Unexpectedly, the specific-heat measurements exhibit exponential decay with a
two-gap feature in its temperature dependence, along with a linear dependence
as a function of magnetic field and the absence of oscillations in the field
angle, reminiscent of multiband full-gap superconductivity. In addition, we
find anomalous behavior at high fields, attributed to a strong Pauli
paramagnetic effect. A low quasiparticle density of states at low energies with
a multiband Fermi-surface topology would open a new door into electron pairing
in CeCu2Si2.Comment: 5 pages, 4 figures (main text) + 5 pages, 6 figures (supplemental
material), published in Phys. Rev. Let
Fans and polytopes in tilting theory II: -fans of rank 2
The -fan of a finite dimensional algebra is a fan in its real Grothendieck
group defined by tilting theory. We give a classification of complete -fans
of rank 2. More explicitly, our first main result asserts that every complete
sign-coherent fan of rank 2 is a -fan of some finite dimensional algebra.
Our proof is based on three fundamental results, Gluing Theorem, Rotation
Theorem and Subdivision Theorem, which realize basic operations on fans in the
level of finite dimensional algebras. For each of 16 convex sign-coherent fans
of rank 2, our second main result gives a characterization of algebras
of rank 2 satisfying . As a by-product of our method, we
prove that for each positive integer , there exists a finite dimensional
algebra of rank 2 such that the Hasse quiver of the poset of 2-term silting
complexes of has precisely connected components.Comment: 37 pages, v2: Fixed typos, updated references and added section
Fans and polytopes in tilting theory
For a finite dimensional algebra over a field , the 2-term silting
complexes of gives a simplicial complex called the
-simplicial complex. We give tilting theoretic interpretations of the
-vectors and Dehn-Sommerville equations of . Using -vectors of
2-term silting complexes, gives a nonsingular fan in
the real Grothendieck group called the
-fan. For example, the fan of -vectors of a cluster algebra is given by
the -fan of a Jacobian algebra of a non-degenerate quiver with potential. We
give several properties of including idempotent reductions,
sign-coherence, Jasso reductions and a connection with Newton polytopes of
-modules. Moreover, gives a (possibly infinite and non-convex)
polytope in called the -polytope
of . We call -convex if is convex. In this case, we show that
it is a reflexive polytope, and that the dual polytope is given by the 2-term
simple minded collections of .
We give an explicit classification of -convex algebras of rank . We
classify algebras whose -polytopes are smooth Fano. We classify classical
and generalized preprojective algebras which are -convex, and also describe
their -polytope as the dual polytopes of short root polytopes of type
and . We also classify Brauer graph algebras which are -convex, and
describe their -polytopes as root polytopes of type and .Comment: 70 page
Thermodynamic study of gap structure and pair-breaking effect by magnetic field in the heavy-fermion superconductor CeCu2Si2
This paper presents the results of specific-heat and magnetization
measurements, in particular their field-orientation dependence, on the first
discovered heavy-fermion superconductor CeCuSi (
K). We discuss the superconducting gap structure and the origin of the
anomalous pair-breaking phenomena, leading e.g., to the suppression of the
upper critical field , found in the high-field region. The data
show that the anomalous pair breaking becomes prominent below about 0.15 K in
any field direction, but occurs closer to for . The
presence of this anomaly is confirmed by the fact that the specific-heat and
magnetization data satisfy standard thermodynamic relations. Concerning the gap
structure, field-angle dependences of the low-temperature specific heat within
the and planes do not show any evidence for gap nodes. From
microscopic calculations in the framework of a two-band full-gap model, the
power-law-like temperature dependences of and , reminiscent of nodal
superconductivity, have been reproduced reasonably. These facts further support
multiband full-gap superconductivity in CeCuSi.Comment: 10 pages, 8 figures, published in Phys. Rev.
High-performance, adiabatically nanotapered fibre-chip couplers in silicon at 2 microns wavelength
Fibre optic technology connects the world through the Internet, enables
remote sensing, and connects disparate functional optical devices. Highly
confined silicon photonics promises extreme scale and functional integration.
However, the optical modes of silicon nanowire waveguides and optical fibres
are very different, making efficient fibre-chip coupling a challenge. Vertical
grating couplers, the dominant coupling method today, have limited optical
bandwidth and are naturally out-of-plane. Here we demonstrate a new method that
is low-loss, broadband, easily manufacturable, and naturally planar. We
adiabatically couple a tapering silicon nanowire waveguide to a conic
nanotapered optical fibre, measuring transmission between 2.0 and 2.2 micron
wavelength. The silicon chip is fabricated at a commercial foundry and then
post-processed to release the tapering nanowires. We estimate an optimal
per-coupler transmission of -0.48 dB (maximum; 95% confidence interval [+0.46,
-1.68] dB) and a 1-dB bandwidth of 295 nm . With automated measurements, we
quantify the device tolerance to lateral misalignment, measuring a flat
response within +/- 0.968 micron. This design can enable low-loss modular
systems of integrated photonics irrespective of material and waveband.Comment: 6 pages, 3 figure
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