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Preventing Zombie Lending
Because of limited liability, insolvent banks have an incentive to continue lending to insolvent borrowers, in order to hide losses and gamble for resurrection, even though this is socially inefficient. We suggest a scheme that regulators could use to solve this problem. The scheme would induce banks to reveal their bad loans, which can then be dealt with. Bank participation in the scheme would be voluntary. Even though banks have private information on the quantity of bad loans on their balance sheets, the scheme avoids creating windfall gains for bank equity holders. In addition, some losses can be imposed on debt holders
Connectivity in bridge-addable graph classes: the McDiarmid-Steger-Welsh conjecture
A class of graphs is bridge-addable if given a graph G in the class, any graph obtained by adding an edge between two connected components of G is also in the class. We prove a conjecture of McDiarmid, Steger, and Welsh, that says that if is any bridge-addable class of graphs on n vertices, and is taken uniformly at random from , then is connected with probability at least , when n tends to infinity. This lower bound is asymptotically best possible since it is reached for forests. Our proof uses a “local double counting” strategy that may be of independent interest, and that enables us to compare the size of two sets of combinatorial objects by solving a related multivariate optimization problem. In our case, the optimization problem deals with partition functions of trees relative to a supermultiplicative functional.Postprint (author's final draft
Phase relations in K_xFe_{2-y}Se_2 and the structure of superconducting K_xFe_2Se_2 via high-resolution synchrotron diffraction
Superconductivity in iron selenides has experienced a rapid growth, but not
without major inconsistencies in the reported properties. For
alkali-intercalated iron selenides, even the structure of the superconducting
phase is a subject of debate, in part because the onset of superconductivity is
affected much more delicately by stoichiometry and preparation than in cuprate
or pnictide superconductors. If high-quality, pure, superconducting
intercalated iron selenides are ever to be made, the intertwined physics and
chemistry must be explained by systematic studies of how these materials form
and by and identifying the many coexisting phases. To that end, we prepared
pure K_2Fe_4Se_5 powder and superconductors in the K_xFe_{2-y}Se_2 system, and
examined differences in their structures by high-resolution synchrotron and
single-crystal x-ray diffraction. We found four distinct phases: semiconducting
K_2Fe_4Se_5, a metallic superconducting phase K_xFe_2Se_2 with x ranging from
0.38 to 0.58, an insulator KFe_{1.6}Se_2 with no vacancy ordering, and an
oxidized phase K_{0.51(5)}Fe_{0.70(2)}Se that forms the PbClF structure upon
exposure to moisture. We find that the vacancy-ordered phase K_2Fe_4Se_5 does
not become superconducting by doping, but the distinct iron-rich minority phase
K_xFe_2Se_2 precipitates from single crystals upon cooling from above the
vacancy ordering temperature. This coexistence of metallic and semiconducting
phases explains a broad maximum in resistivity around 100 K. Further studies to
understand the solubility of excess Fe in the K_xFe_{2-y}Se_2 structure will
shed light on the maximum fraction of superconducting K_xFe_2Se_2 that can be
obtained by solid state synthesis.Comment: 12 pages, 16 figures, supplemental materia
Intrinsic Localized Modes Observed in the High Temperature Vibrational Spectrum of NaI
Inelastic neutron measurements of the high-temperature lattice excitations in
NaI show that in thermal equilibrium at 555 K an intrinsic mode, localized in
three dimensions, occurs at a single frequency near the center of the spectral
phonon gap, polarized along [111]. At higher temperatures the intrinsic
localized mode gains intensity. Higher energy inelastic neutron and x-ray
scattering measurements on a room-temperature NaI crystal indicate that the
creation energy of the ground state of the intrinsic localized mode is 299 meV.Comment: 17 pages, 5 figures Revised version; final versio
Magnetism and unconventional superconductivity in CeMIn heavy-fermion crystals
We review magnetic, superconducting and non-Fermi-liquid properties of the
structurally layered heavy-fermion compounds CeMIn (M=Co, Rh,
Ir). These properties suggest d-wave superconductivity and proximity to an
antiferromagetic quantum-critical point.Comment: submitted 23rd International Conference on Low Temperature Physics
(LT-23), Aug. 200
Novel Coexistence of Superconductivity with Two Distinct Magnetic Orders
The heavy fermion Ce(Rh,Ir)In5 system exhibits properties that range from an
incommensurate antiferromagnet on the Rh-rich end to an exotic superconductor
on the Ir-rich end of the phase diagram. At intermediate composition where
antiferromagnetism coexists with superconductivity, two types of magnetic order
are observed: the incommensurate one of CeRhIn5 and a new, commensurate
antiferromagnetism that orders separately. The coexistence of f-electron
superconductivity with two distinct f-electron magnetic orders is unique among
unconventional superconductors, adding a new variety to the usual coexistence
found in magnetic superconductors.Comment: 3 figures, 4 page
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