1,735 research outputs found
Electron localization and a confined electron gas in nanoporous inorganic electrides
The nanoporous main group oxide 12CaO.7Al(2)O(3) (C12A7) can be transformed from a wide-gap insulator to an electride where electrons substitute anions in cages constituting a positive frame. Our ab initio calculations of the electronic structure of this novel material give a consistent explanation of its high conductivity and optical properties. They show that the electrons confined in the inert positive frame are localized in cages and undergo hopping between neighboring cages. The results are useful for the understanding of behavior of confined electron gas of different topology and electron-phonon coupling, and for designing new transparent conductors, electron emitters, and electrides
Exceptional String: Instanton Expansions and Seiberg-Witten Curve
We investigate instanton expansions of partition functions of several toric
E-string models using local mirror symmetry and elliptic modular forms. We also
develop a method to obtain the Seiberg--Witten curve of E-string with arbitrary
Wilson lines with the help of elliptic functions.Comment: 71 pages, three Wilson line
Non-Supersymmetric Stringy Attractors
In this paper we examine the stability of non-supersymmetric attractors in
type IIA supergravity compactified on a Calabi-Yau manifold, in the presence of
sub-leading corrections to the N=$ pre-potential. We study black hole
configurations carrying D0-D6 and D0-D4 charges. We consider the O(1)
corrections to the pre-potential given by the Euler number of the Calabi-Yau
manifold. We argue that such corrections in general can not lift the zero modes
for the D0-D6 attractors. However, for the attractors carrying the D0-D4
charges, they affect the zero modes in the vector multiplet sector. We show
that, in the presence of such O(1) corrections, the D0-D4 attractors can either
be stable or unstable depending on the geometry of the underlying Calabi-Yau
manifold, and on the specific values of the charges they carry.Comment: corrected typos, minor modification
Some combinatorial problems
AbstractThere are many interesting and sophisticated problems posed in the IMO, Putnam and domestic Olympiads. Some of these problems have deep mathematical background, nice generalizations, and lead to new areas of research in combinatorics. We investigate several topics in this category and mention some results and open problems
Origin of critical-temperature enhancement of an iron-based high-T_c superconductor, LaFeAsO_{1-x}F_{x} : NMR study under high pressure
Nuclear magnetic resonance (NMR) measurements of an iron (Fe)-based
superconductor LaFeAsO_{1-x}F_x (x = 0.08 and 0.14) were performed at ambient
pressure and under pressure. The relaxation rate 1/T_1 for the overdoped
samples (x = 0.14) shows T-linear behavior just above T_c, and pressure
application enhances 1/T_1T similar to the behavior of T_c. This implies that
1/T_1T = constant originates from the Korringa relation, and an increase in the
density of states at the Fermi energy D(E_F) leads to the enhancement of T_c.
In the underdoped samples (x = 0.08), 1/T_1T measured at ambient pressure also
shows T-independent behavior in a wide temperature range above T_c. However, it
shows Curie-Weiss-like T dependence at 3.0 GPa accompanied by a small increase
in T_c, suggesting that predominant antiferromagnetic fluctuation suppresses
development of superconductivity or remarkable enhancement of T_c. The
qualitatively different features between underdoped and overdoped samples are
systematically explained by a band calculation with hole and electron pockets
Seiberg-Witten prepotential for E-string theory and random partitions
We find a Nekrasov-type expression for the Seiberg-Witten prepotential for
the six-dimensional non-critical E_8 string theory toroidally compactified down
to four dimensions. The prepotential represents the BPS partition function of
the E_8 strings wound around one of the circles of the toroidal
compactification with general winding numbers and momenta. We show that our
expression exhibits expected modular properties. In particular, we prove that
it obeys the modular anomaly equation known to be satisfied by the
prepotential.Comment: 14 page
Proximity fingerprint of s+- superconductivity
We suggest a straightforward and unambiguous test to identify possible
opposite signs of superconducting order parameter in different bands proposed
for iron-based superconductors (s+- state). We consider proximity effect in a
weakly coupled sandwich composed of a s+- superconductor and thin layer of
s-wave superconductor. In such system the s-wave order parameter is coupled
differently with different s+- gaps and it typically aligns with one of these
gaps. This forces the other s+- gap to be anti-aligned with the s-wave gap. In
such situation the aligned band induces a peak in the s-wave density of states
(DoS), while the anti-aligned band induces a dip. Observation of such
contact-induced negative feature in the s-wave DoS would provide a definite
proof for s+- superconductivity.Comment: 4 pages, one figur
Angle-resolved photoemission spectroscopy of Co-based boride superconductor LaCo1.73Fe0.27B2
We have performed angle-resolved photoemission spectroscopy of Co-based
boride superconductor LaCo1.73Fe0.27B2 (Tc = 4.1 K), which is isostructural to
the 122-type Fe-pnictide superconductor with the pnictogen atom being replaced
with boron. We found that the Fermi level is located at a dip in the density of
states (DOS) in contrast to Co-pnictide ferromagnets. This reduction in DOS
together with the strong Co 3d-B 2p covalent bonding removes the ferromagnetic
order and may cause the superconductivity. The energy bands near the Fermi
level show higher three dimensionality and a weaker electron-correlation effect
than those of Fe pnictides. The Fermi surface topology is considerably
different from that of Fe pnictides, suggesting the difference in the
superconducting mechanism between boride and pnictide superconductors.Comment: 5 pages, 4 figure
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