First-principles structural optimization and electronic structure of the superconductor picene for various potassium doping levels

We theoretically explore the crystal structures of K$_x$picene, for which a new aromatic superconductivity has recently been discovered for $x=3$, by systematically performing first-principles full structural optimization covering the concentration range $x=1$-4. The crystal symmetry (space group) of the pristine picene is shown to be preserved in all the optimized structures despite significant deformations of each picene molecule and vast rearrangements of herringbone array of molecules. For K$_x$picene ($x=1$-4) optimization indicates that (i) multiple structures exist in some cases, and (ii) dopants can enter not only in the interlayer region between the stack of herringbone structures, but also in the intralayer region. In the electronic structure obtained with the local density approximation for the optimized structures, the dopants are shown to affect the electronic properties not only through the rearrangement and distortion of molecules, but also molecule-metal atom hybridization. In other words, the rigid-band approximation is invalidated for multifold reasons. As a consequence the resultant Fermi surface exhibits a variety of multiband structures which take diverse topology for K$_1$picene and K$_3$picene.Comment: 7 figures, to be published in Phys. Rev.

Effect of high annealing temperature on giant tunnel magnetoresistance ratio of CoFeB/MgO/CoFeB magnetic tunnel junctions

We report tunnel magnetoresistance (TMR) ratios as high as 472% at room temperature and 804% at 5 K in pseudo-spin valve (SV) CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) annealed at 450oC, which is approaching the theoretically predicted value. By contrast, the TMR ratios for exchange-biased (EB) SV MTJs with a MnIr antiferromagnetic layer are found to drop when they are annealed at 450oC. Energy dispersive X-ray analysis shows that annealing at 450oC induces interdiffusion of Mn and Ru atoms into the MgO barrier and ferromagnetic layers in EB-SV MTJs. Mechanisms behind the different annealing behavior are discussed.Comment: 13 pages, 5 figure

Electronic structure of solid coronene: differences and commonalities to picene

We have obtained the first-principles electronic structure of solid coronene, which has been recently discovered to exhibit superconductivity with potassium doping. Since coronene, along with picene, the first aromatic superconductor, now provide a class of superconductors as solids of aromatic compounds, here we compare the two cases in examining the electronic structures. In the undoped coronene crystal, where the molecules are arranged in a herringbone structure with two molecules in a unit cell, the conduction band above an insulating gap is found to comprise four bands, which basically originate from the lowest two unoccupied molecular orbitals (doubly-degenerate, reflecting the high symmetry of the molecular shape) in an isolated molecule but the bands are entangled as in solid picene. The Fermi surface for a candidate of the structure of K$_x$coronene with $x=3$, for which superconductivity is found, comprises multiple sheets, as in doped picene but exhibiting a larger anisotropy with different topology.Comment: 5 pages, to be published in Phys. Rev.

Improved subjective symptoms of conjunctivochalasis using bipolar diathermy method for conjunctival shrinkage

Tomoyuki Kashima1,2, Hideo Akiyama1, Fumihide Miura2, Shoji Kishi11Department of Ophthalmology, Gunma University School of Medicine, Gunma, Japan; 2Department of Ophthalmology, Saku Central Hospital, Nagano, JapanPurpose: To evaluate the improvement in subjective symptoms of conjunctivochalasis after bipolar coagulation.Methods: Forty-three eyes of 26 patients (average age, 75.7 ± 8.4 years) were included in this study. The inferior conjunctivas were cauterized between April 2009 and June 2010. Surgery involved pinching the excess conjunctiva and performing bipolar cauterization after subconjunctival injection of a local anesthetic agent. Patients were asked to describe the postoperative foreign-body sensation and change in subjective symptoms 1 month postoperatively, with the preoperative symptom score defined as 10.Results: Twenty-two patients (84.6%) reported symptom relief immediately after the procedure, though all patients had a mild gritty sensation for 1–2 weeks postoperatively. The mean postoperative symptom score obtained from the questionnaire was 3.27 ± 3.31, which was significantly lower than the preoperative score (P < 0.0001). No patients had experienced recurrent symptoms at the end of the follow-up period.Conclusion: Thermal cauterization can achieve conjunctival shrinkage and adherence to the subconjunctival tissues. Operation time is only a few minutes, postoperative pain is not severe, and the procedure can be performed in an outpatient clinic, all of which represent benefits to the patients.Keywords: coagulation, cauterization, conjunctival injection&nbsp

Dependence of Giant Tunnel Magnetoresistance of Sputtered CoFeB/MgO/CoFeB Magnetic Tunnel Junctions on MgO Barrier Thickness and Annealing Temperatur

We investigated the dependence of giant tunnel magnetoresistance (TMR) on the thickness of an MgO barrier and on the annealing temperature of sputtered CoFeB/MgO/CoFeB magnetic tunnel junctions deposited on SiO2/Si wafers. The resistance-area product exponentially increases with MgO thickness, indicating that the quality of MgO barriers is high in the investigated thickness range of 1.15-2.4 nm. High-resolution transmission electron microscope images show that annealing at 375 C results in the formation of crystalline CoFeB/MgO/CoFeB structures, even though CoFeB electrodes are amorphous in the as-sputtered state. The TMR ratio increases with annealing temperature and is as high as 260% at room temperature and 403% at 5 K.Comment: 12 pages, 5 figure