1,716 research outputs found
^{75}As NMR study of the growth of paramagnetic-metal domains due to electron doping near the superconducting phase in LaFeAsO_{1-x}F_{x}
We studied the electric and magnetic behavior near the phase boundary between
antiferromagnetic (AF) and superconducting (SC) phases for a prototype of
high-T_c pnictides LaFeAsO_{1-x}F_{x} by using nuclear magnetic resonance, and
found that paramagnetic-metal (PM) domains segregate from AF domains. PM
domains grow in size with increasing electron doping level and are accompanied
by the onset of superconductivity, and thus application of pressure or
increasing the doping level causes superconductivity. The existence of PM
domains cannot be explained by the existing paradigm that focuses only on the
relationship between superconductivity and antiferromagnetism. Based on orbital
fluctuation theory, the existence of PM domains is evidence of the
ferroquadrupole state.Comment: 5 figure
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
On the least redundancy problem of the queries of order two in combinatorial filing scheme
The paper concerns a least redundancy problem of queries of order two in a combinatorial file organization scheme. Every record will be assumed to have m attributes, each of them having n levels, and the queries of order two will be identified with edges of a complete m-partite graph Km(n,…, n). S. Yamamoto, S. Tazawa, K. Ushio, and H. Ikeda have proved that if c ⩽ (m − 1), then the graph, termed “claw with degree c,” has the least redundancy among all the graphs consisting of c edges over Km(n,…, n), and they presented a file organization scheme realizing the least redundancy. S. Tazawa and S. Yamamoto have proved that the claw with degree c has the least redundancy even in the case of c ⩽ n(m − 1). The purpose of this paper is to introduce some transformations of graphs over Km(n,…, n) and to prove that a graph termed “complete normal form” has the least redundancy in any case of c > 0. In mathematical language, the problem here is stated as follows: Let V be an n-dimensional lattice point space {1,…, m} × … × {1,…, m}. For fixed i, j (i ≠ j), p, p′, we define a subset V(i,j,p,p′) = {v} ∈ V; vi = p}, vj = p′} ⊂ V. For a given possible integer c, how should we select c mutually different V(i,j, p, p′) such that the number of lattice points contained in the union of them is minimum. The solution is Theorem 5, and Theorem 7 gives a formula for finding the minimum number
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
Magnetic order in CaFe1-xCoxAsF (x = 0, 0.06, 0.12) superconductor compounds
A Neutron Powder Diffraction (NPD) experiment has been performed to
investigate the structural phase transition and magnetic order in CaFe1-xCoxAsF
superconductor compounds (x = 0, 0.06, 0.12). The parent compound CaFeAsF
undergoes a tetragonal to orthorhombic phase transition at 134(3) K, while the
magnetic order in form of a spin-density wave (SDW) sets in at 114(3) K. The
antiferromagnetic structure of the parent compound has been determined with a
unique propagation vector k = (1,0,1) and the Fe saturation moment of 0.49(5)uB
aligned along the long a-axis. With increasing Co doping, the long range
antiferromagnetic order has been observed to coexist with superconductivity in
the orthorhombic phase of the underdoped CaFe0.94Co0.06AsF with a reduced Fe
moment (0.15(5)uB). Magnetic order is completely suppressed in optimally doped
CaFe0.88Co0.12AsF. We argue that the coexistence of SDW and superconductivity
might be related to mesoscopic phase separation.Comment: 4pages, 4figure
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
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