An alternative construction of B-M and B-T unitals in Desarguesian planes

We present a new construction of non-classical unitals from a classical unital $U$ in $PG(2,q^2)$. The resulting non-classical unitals are B-M unitals. The idea is to find a non-standard model $\Pi$ of $PG(2,q^2)$ with the following three properties: 1. points of $\Pi$ are those of $PG(2,q^2)$; 2. lines of $\Pi$ are certain lines and conics of $PG(2,q^2)$; 3. the points in $U$ form a non-classical B-M unital in $\Pi$. Our construction also works for the B-T unital, provided that conics are replaced by certain algebraic curves of higher degree.Comment: Keywords: unital, desarguesian plane 11 pages; ISSN: 0012-365

Control of vegetative growth of grapevine shoots by ethylene-releasing substances Conditions and sites of action

Grapevine shoots of the cvs. Muscat Hamburg and Perlette were treated with two commercial ethylene-releasing substances, Ethrel (2-chloroethylphosphonic acid) and Alsol (2Tchloroethyl-tris-(2-methoxyethoxy)silane) at acid and neutral pH. Ethrel caused significant shoot growth inhibition, Alsol had no effect on the growth rate. Ethrel was more active at pH 6.9 than at pH 2.2, and its uptake was significantly higher at the low pH. In many cases the apical meristem of Ethrel-treated shoots was damaged. The relation of uptake to growth response was determined. Leaf blade treatments alone did not inhibit the growth of the apex.Die Kontrolle des vegetativen Wachstums von Rebtrieben durch Äthylen-liefernde Substanzen Einwirkungsbedingungen und -orteTriebe der Rebsorten Muskat Hamburg und Perlette wurden mit zwei Äthylen-freisetzenden Handelspräparaten behandelt: Ethrel (2-Chloräthylphosphonsäure) und Alsol (2-Chloräthyl-tris-(2-methoxyäthoxy)silan) bei saurem und neutralem pH. Ethrel verursachte eine signifikante Hemmung des Triebwachstums, Alsol hatte keinen Einfluß auf die Wachstumsgeschwindigkeit. Ethrel war bei pH 6,9 wirksamer als bei pH 2,2; im niedrigen pH-Bereich war seine Aufnahme signifikant erhöht. In den behandelten Sprossen war vielfach das apikale Meristem geschädigt. Die Beziehung zwischen Substanzaufnahme und Wachstumsreaktion wurde bestimmt. Bei alleiniger Blattbehandlung war das Wachstum der Triebspitze nicht gehemmt

Nesting symmetries and diffusion in disordered d-wave superconductors

The low-energy density of states (DOS) of disordered 2D d-wave superconductors is extremely sensitive to details of both the disorder model and the electronic band structure. Using diagrammatic methods and numerical solutions of the Bogoliubov-de Gennes equations, we show that the physical origin of this sensitivity is the existence of a novel diffusive mode with momentum close to $(\pi,\pi)$ which is gapless only in systems with a global nesting symmetry. We find that in generic situations, the DOS vanishes at the Fermi level. However, proximity to the highly symmetric case may nevertheless lead to observable non-monotonic behavior of the DOS in the cuprates

Are there nodes in LaFePO, BaFe2_2(AsP)2_2, and KFe2_2As2_2 ?

We reexamined the experimental evidences for the possible existence of the superconducting (SC) gap nodes in the three most suspected Fe-pnictide SC compounds: LaFePO, BaFe$_2$(As$_{0.67}$P$_{0.33}$)$_2$, and KFe$_2$As$_2$. We showed that while the $T$-linear temperature dependence of the penetration depth $\lambda(T)$ of these three compounds indicate extremely clean nodal gap superconductors, the thermal conductivity data $\lim_{T,H \rightarrow 0} \kappa_S (H,T)/T$ unambiguously showed that LaFePO and BaFe$_2$(As$_{0.67}$P$_{0.33}$)$_2$ are extremely dirty, while KFe$_2$As$_2$ can be clean. This apparently conflicting experimental data casts a serious doubt on the nodal gap possibility on LaFePO and BaFe$_2$(As$_{0.67}$P$_{0.33}$)$_2$.Comment: 11 pages, 5 figures A new section "4. Remark on the quantum oscillation (QO) experiments" is adde

Finite Density of States in a Mixed State of d_x^2-y^2+id_xy Superconductor

We have calculated the density of states of quasiparticles in a d_x^2-y^2+id_xy superconductor, and show that in the mixed state the quasiparticle spectrum remains gapless because of the Doppler shift by superflow. It was found that if the d_{xy} order gap $\Delta_1\propto \sqrt{H}$ as suggested by experiments, then thermal conductivity $\kappa \propto \sqrt{H}$ in accord with experimental data at lowest temperatures. This is an appended version of the paper published in Phys. Rev. {\bf B 59}, 6024, (1999). We now also discuss the disorder effects and analyze the H log H crossover at small fields. We argue that H log H regime is present and disorder effect is dominant as the field-induced seconary gap is small at small fields.Comment: This is an appended version of the paper published in Phys. Rev. {\bf B 59}, 6024, (1999). We now also discuss the disorder effects and analyze the H log H crossover at small fields. 3 pages, Latex file with 2 eps figure file