^{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